WO2018142097A1

WO2018142097A1 - Improvements to operation of engines

Info

Publication number: WO2018142097A1
Application number: PCT/GB2018/000016
Authority: WO
Inventors: Peter Martin
Original assignee: Combustion Order Ltd
Priority date: 2017-02-02
Filing date: 2018-01-30
Publication date: 2018-08-09
Also published as: GB2559361A; GB201701707D0

Abstract

A method is provided for improving the efficiency of an internal combustion engine (1) having a piston (15) configured for reciprocating movement within a cylinder (16). The rate of movement of the piston (15) along the cylinder (16) with respect to a combustion chamber is at least reduced during at least a part of the first 90 degrees of each combustion stroke. The movement of the piston (15) is reduced for a continuous period starting after the top dead centre of the piston (15) within the cylinder (16) and finishing not later than 90 degrees after top dead centre. The reduction is applied to each cycle of the engine such that the path of the piston (15) during each cycle of the engine is cyclic but not completely sinusoidal for at least part of the first 90 degrees of the combustion stroke.

Description

Improvements to Operation of Engines

The present invention relates to improvements in the method of operation of internal combustion engines in order to achieve improved performance, fuel efficiency and exhaust emissions.

Combustion is the burning of a fuel compound in air. Combustion is initiated by free radicals, normally produced by heating and aided by an increase in pressure. Combustion itself is characterised by chain branching reactions which generate further radicals. This can lead to rapid propagation of the chain branching reactions under favourable conditions, which is normally the desired result.

The chemical pathways in combustion are extremely complex. The radicals are very short lived species and it is only recently that the long predicted QOOH radical has been identified. Combustion is, in reality, a general term that covers a wide spectrum of reactions. These range from smouldering, through traditional combustion (deflagration) to explosion (detonation). Typically, the reactivity of the fuel determines where in the combustion spectrum the reaction occurs, for similar combustion conditions.

Combustion of hydrocarbon based fuel is at the heart of the internal combustion engine, of which the reciprocating engine is the dominant type by market volume. The reciprocating engine has particular characteristics. A piston moves in a sinusoidal mode inside an engine cylinder, connected at an offset position to a crankshaft by a connecting rod. Thus, each up and down stroke of the piston is identical to the next, regardless of what stroke it is in the engine cycle, the down stoke typically being the power stroke and the upstroke the return or exhaust stroke in a 2-stroke engine. Similarly, the down stroke would correspond with either the power stroke or the intake stroke in a 4-stroke engine.

The piston undergoes a speed difference on the power stroke when the combustion of the fuel generates the expanding gasses which drive the piston down the cylinder bore and thus rotate the crank to generate rotational mechanical energy. Typically, this generates an acceleration of the crankshaft rotation even at steady state (constant rpm) conditions. Likewise, there could be speed differences, both positive and negative on the other strokes of the engine.

There are known other engine types (swash plate, crank-less, opposing piston etc.) where the piston movement can be different for each stroke type. Typically, these utilise a cam mechanism to control the piston movement. However, the power stroke will be the same between consecutive engine cycles.

There are also known, some engine types with adjustability in the crankshaft or connecting rod mechanism that allows the piston stroke to be changed in response to operational conditions, albeit, with a relatively slow time constant.

There are also known engine types which change the valve timings which have a similar effect to altering the stroke of the engine (e.g. the Atkinson cycle engine).

In an internal combustion engine the combustion of the fuel occurs in a closed mechanical system. The restrictions imposed by this constraint have resulted in the dominance of two specific fuels that have characteristics best suited to this application; the first is gasoline (petrol), the second is diesel.

Furthermore, two specific engine types have evolved to exploit the different characteristics of each fuel type. The gasoline (petrol) engine typically operates with a compression ratio of approximately 10:1 , uses spark ignition and requires a stoichiometric ratio of air to fuel ratio of 14:1 for optimal combustion. When ignition occurs, combustion occurs rapidly. This conforms to the Otto thermodynamic cycle.

The diesel engine typically operates with a compression ratio of 20:1 , uses compression ignition and is not strictly stoichiometric, being able to combust with much higher ratios of air. When ignition occurs, combustion progresses less rapidly. This conforms to the diesel thermodynamic cycle.

These two thermodynamic cycles are ideal and thus unlikely to be seen in the real world. There also exists a hybrid cycle, sometimes known as the Trinkler cycle which is a combination of both.

It has been a long standing goal of engine designers to combine the beneficial aspects of both types of cycle. Homogenous Charge Compression Ignition (HCCI), Activated Radical Combustion (ARC), Reactivity Controlled Compression Ignition (RCCI) are examples of this. In particular, there is a general drive to reduce the harmful emissions from, in particular, compression ignition engines, which generally emit higher levels of polluting gases and particulates than their equivalent spark ignition engines. There is also a general ongoing aim to increase the efficiency of both types of engine.

With those aims in mind, it is an established practice for additives to be used in compression ignition engines to generate an improvement in engine efficiency and a reduction in emissions. It is not possible for an additive to change the chemical composition of the primary fuel and hence its combustion characteristics. The term primary fuel used above and hereinafter is referring to the fuel which is the majority component of the fuel supplied to the engine by molecular volume and whose combustion provides the main power for the engine. This contrasts with a secondary fuel, which is the minority fuel by molecular volume and is used in addition to the primary fuel, typically with different combustion characteristics, the secondary fuel being consumed during the combustion, unlike a catalyst.

It is, then, known that an additive can be used to change the combustion conditions prior to the main injection of primary fuel. Additives which have been demonstrated in the prior art to be effective with diesel engines include liquid petroleum gas (LPG), hydrogen and Nitrous Oxide.

These require that the compression ignition engine operates with a pilot injection of primary fuel, which is employed to raise the in cylinder temperature and pressure to effectively activate the additive prior to the main injection of primary fuel. The net effect is that combustion of the main injection of primary diesel fuel exhibits characteristics similar to those of gasoline (petrol). These include low ignition delay, fast combustion propagation and more complete combustion of the fuel. This results in lower particulate emissions (soot & unburnt fuel) and a significant reduction in the oxides of Nitrogen (NOX). An example of the magnitudes which might be involved, a 40% increase in fuel efficiency at idle (approx. 800rpm) for a heavy diesel truck engine (6 cylinder - 12 litres) indicates that a significant change to the combustion chemistry has occurred.

A consistent explanation, mechanism or formula for creating and controlling these enhanced combustion conditions has not, however, previously been realised. The earliest diesel engines were based on the "hot bulb" principle of operation, whereby constant combustion was maintained in a small vessel, the contents of which was then distributed to each cylinder at an appropriate time to ignite the main fuel charge. By maintaining constant combustion the availability of free radicals was ensured. This early technology was surpassed with advances in engine design, facilitating compression ignition and direct injection technology (including pilot and multiple injection strategies) allowing the engine to better utilise each combustion cycle individually.

It is then known, from the prior art, to try to maintain a concentration of free radicals immediately prior to the main combustion event. Examples are gaseous feedback from the prior combustion cycle of another cylinder, to the blending of fuels in an RCCI engine in an attempt to effectively elongate the combustion event in time.

Gasoline (petrol) engines are generally less efficient that diesel engines due to the lower compression ratios. However, because gasoline combusts rapidly with low ignition delay (following spark ignition) they are well suited to high speed applications. Diesel cycle engines on the other hand can typically either be low or high speed. An example of a low speed diesel engine is a marine diesel that uses heavy oil as a fuel. Typically, these operate by heating the fuel (to reduce viscosity) and with a maximum speed of a few hundred rpm. An example of a high speed diesel engine is found in a passenger car engine. Typically, these operate on a single fuel (pump diesel) and have a maximum speed only slightly less than a gasoline (petrol) engine. Because of the combustion characteristics of the diesel fuel compared to gasoline these require exhaust after treatments to deal with particulates and NOX.

The fundamental difference in combustion characteristics between diesel and gasoline are the auto ignition characteristics, the speed of the initiation and rate of propagation of radicals and the complete speed of the combustion process. In a gasoline engine this produces a period of constant or pseudo constant volume combustion, whereby the downwards movement of the piston significantly lags the increase in cylinder pressure and temperature due to the combustion. This is because the speed of the combustion is much greater than the mechanical response time of the engine. This effect is amplified the nearer combustion occurs to the TDC position as this is where the moment (torque) on the crankshaft is a minima. In a similarly sized diesel engine, despite the heavier construction, this period of constant or pseudo constant volume combustion is minimal and thus the fuel is not fully combusted.

Moreover, for both fuel types, the standard reciprocating engine motion minimises the time that the gasses are constrained on the power stroke, due to the sinusoidal motion of the piston and the acceleration of the piston during the power stroke.

The piston speed within an internal combustion engine is never constant. The piston will experience acceleration during the power (i.e. combustion) stroke (due to the generation and rapid expansion of the combustion gases) and deceleration on the other strokes, most noticeably on the compression stroke. Thus, it is only during the compression stroke where the engine is generating energy, during other times it is consuming energy.

Moreover, for a given engine design, the amount of energy generated will depend on the quantity of fuel and air supplied to the engine cylinder prior to combustion and the efficiency and speed of the combustion and the peak pressure achieved during combustion. Likewise, the energy consumed during the other cycles of the engine will depend on the amount of air in the engine, the pressure inside the engine cylinder and the pumping efficiency of the engine. This will be dependent on the design of the engine, but factors will include the compression ratio, the friction losses, the valve timings and any restrictions in the intake and exhaust systems. Thus, the piston speed will also change dependent on the operational conditions of the engine.

Likewise, the load applied to the engine will also affect the piston speed as this is the desired consumer of the energy output from the engine.

The inventor for the present invention has realised that an engine design whereby the power stroke is of increased time duration, or is subject to a high load (thus slowing the rate of crankshaft rotation), compared to a standard reciprocating engine design, may address these drawbacks and generate longer periods of actual or pseudo constant volume combustion with the aim of improving combustion completeness and efficiency. A change in piston speed (i.e. reducing the rate of piston acceleration during all or part of the power stroke) is a side effect of slowing the rate of increase of the combustion volume after ignition with the objective of deliberately inducing constant or pseudo constant volume combustion conditions.

In a tradition engine design the load is applied approximately constantly, nominally across all engine cycles. Thus, the energy output of the engine generated during the power stroke is required both to drive the applied constant load and service the energy requirements of the engine itself at all other times (i.e. all cycles except the power stroke). This scenario will require significant piston acceleration during the power stroke. With the application of a constant load it is not possible to reduce the piston acceleration sufficiently to generate constant volume or pseudo volume combustion conditions necessary for this invention without stalling the engine as there is insufficient work energy available to drive the load and service the requirements of the engine itself at times other than on the power stroke.

This remains true for multi-cylinder engines and two stroke engines compared to four stroke engines (1 power stroke every 2 cycles and 1 power stroke every four cycles respectively) which simply have additional power strokes to reduce the speed variations of a common crankshaft.

In the present invention it has been recognised that, by applying an additional load only during all or part of the power stroke, a significant reduction in piston acceleration can be achieved. Thus, slowing the rate of increase of the combustion volume after ignition with the objective of inducing constant or pseudo constant volume combustion conditions without stalling the engine.

The inventor has further realised that a concentration of free radicals generated immediately prior to the main combustion event can be maintained with combustion enhancement techniques, and that the only logical explanation for the efficacy of combustion enhancement is that it is related to a change in the free radical environment in the engine cylinder prior to the main combustion event.

According to the first aspect of the present invention there is provided a method of improving the efficiency of an internal combustion engine having a piston configured for reciprocating movement within a cylinder, characterised by at least reducing the rate of movement of the piston along the cylinder with respect to the combustion chamber during at least a part of the first 90 degrees of each combustion stroke, the movement of the piston being reduced for a continuous period starting after the top dead centre of the piston within the cylinder and finishing not later than 90 degrees after top dead centre, and the reduction being applied each cycle of the engine such that the path of the piston during each cycle of the engine is cyclic but not completely sinusoidal for at least part of the first 90 degrees of the combustion stroke.

In the method in accordance with the first aspect of the invention the movement of the piston with respect to the combustion chamber is retarded or arrested during the first 90 degrees of each combustion stroke, the restriction being applied to each cycle of the engine so that the piston still executes a periodic movement but the path is not sinusoidal for at least part of the first 90 degrees of the combustion stroke.

This has the advantage that, by at least slowing the rate of increase of the volume of the combustion chamber, a period of constant or pseudo constant volume combustion is achieved, which increases combustion efficiency and moves the combustion reaction along the combustion spectrum. By pseudo constant volume, it is meant that the rate of increase of the volume of the combustion chamber is reduced as compared with a piston which is undergoing a normal combustion induced sinusoidal movement. Accordingly, the timing of the combustion is controlled in a manner which changes the combustion conditions so as to change the combustion characteristics by moving the combustion along the combustion spectrum rather than by changing the reactivity of the fuel. In this way, the combustion of the diesel fuel is made to be more characteristic of petrol, so that the particulate and NOX emissions are reduced whilst maintaining the advantage of the higher fuel efficiency associated with diesel fuel, or the combustion of petrol fuel is shifted further to provide even less emissions, more power, or both.

Slowing the rate of increase of the combustion volume after ignition with the objective of inducing constant or pseudo constant volume combustion conditions will result in an increase in peak cylinder pressure. This increased peak cylinder pressure is effectively stored energy. This can be utilised later, in a more advantageous position, than in a standard engine. The advantage of this is that as the piston travels away from the TDC (top dead centre) on the power stroke the moment exerted on the crankshaft by the connecting rod (con rod) increases until it is a maximum at 90 degrees after TDC.

The movement of the piston may be controlled during just a part of, or for the whole of the first 90 degrees of rotation of the crankshaft after TDC of the piston, but it is important that it controlled on every engine cycle. The portion during which the movement of the piston is controlled or retarded may, though, be varied between cycles to achieve optimum engine performance at different engine speeds and loads.

In a particularly preferred embodiment, the movement of the piston is completely stopped for at least part of the first 90 degrees of the combustion stroke so as to keep the volume of the combustion chamber constant for said at least part of the first 90 degrees of the combustion stroke. This optimises the movement of the combustion along the combustion spectrum and accordingly optimises the reduction in emissions of the engine by ensuring that the constant or pseudo constant volume conditions are maintained for sufficient time to allow the maximum possible shift of the combustion along the combustion spectrum.

The movement of the piston may be controlled by mechanical means. This may be by, for example, controlling the length of the rod which connects the piston to the crankshaft, using opposed pistons, dual crankshafts in the engine, a geared arrangement such as planetary gears for the connecting rod and crankshaft, a camming arrangement between the connecting rod and the crankshaft configured to reduce the rate of or even stop the movement of the piston during a period of the rotation of the crankshaft.

In a standard engine that has piston connected via a fixed connecting rod to an offset position of a crank via a single bearing at each end of the connecting rod, the movement of the piston will be sinusoidal. Because of the acceleration and deceleration of the piston due to the engine cycles this sinusoid will be compressed or elongated in time at various points along the time (X) axis.

If the piston in this arrangement is connected via a connecting rod whose effective length can be changed (via hydraulic extension, provision of an additional pivot point etc.) then the motion of the piston need no longer be sinusoidal. This is also true of alternative engine designs where the connecting rod is of fixed length but acts on a cam rather than the crankshaft, such as the swash plate engine. However, although the piston movement can be non-sinusoidal, it is always cyclic and follows a repetitive cycle, although in some designs this can be varied over time. Thus, the magnitude and shape of the excursion on the displacement (Y) axis can be customised in addition to changes along the time (X) axis. The purpose of this is generally to change the compression ratio of the engine. This is desirable because of the direct relationship between compression ratio and thermodynamic efficiency of the engine. The aim being to change the compression ratio of the engine to match the operational conditions to optimise the engine efficiency or emissions performance, or both. Other schemes are known which utilise changes to the cylinder valve timings to generate similar effects (e.g. Atkinson cycle).

Alternatively, the cylinder may be allowed to move in the direction of rotation of the crankshaft for the required portion of the combustion stroke or a part thereof so as to compensate for the rotation of the crankshaft and thereby reduce the relative movement of the piston in the cylinder or even stop it, the cylinder then being rotated back to its original position during at least a part of the remainder of the engine cycle. In the embodiment where the cylinder is rotated in the same direction as the crankshaft the increase in peak cylinder pressure is the only energy storage mechanism. The cylinder moving mechanism will require energy to operate it during each power stroke. This can be considered akin to an active engine mount. A standard passive engine mount (traditionally rubber) is deflected when the crank shaft accelerates on the power stroke. This is because the engine cylinder has a corresponding rotation in the opposite direction to the crankshaft, as described by Newton's third law of motion. This is effectively wasted energy which only serves to vibrate or compress and thus, heat up the engine mount material. Utilisation of an active engine mount may then reduce these losses.

In the present invention A standard engine arrangement is maintained with a fixed connecting rod with a single bearing at each end connecting to the piston and an offset position on the crankshaft respectively. Thus, the motion of the piston remains both sinusoidal and cyclic. The purpose of the invention is intended to alter the sinusoidal characteristic of such a standard engine in the time domain only. By changing the characteristics of the compression and elongation due to the various engine cycles in time seen at various points along the time (X) axis, optimal engine operation can be achieved.

Specifically, inducing constant or pseudo constant volume combustion conditions will elongate part of the sine wave (where it was previously compressed) on the time (X) axis for the power stroke.

Similarly, applying stored energy to generate rotational force on the crankshaft could also be used to compress part of the sine wave (where it was previously elongated) on the time (X) axis for the compression stroke.

The invention does not change the compression ratio of the engine, or the valve timing.

In an alternative approach, the movement of the piston may be controlled by controlling the load on the engine which is transmitted to the piston back through the crankshaft. In particular, this may be achieved by, for example, varying an electrical load as might be applied by an alternator, the power from which may be used to charge on-board batteries for powering an alternative drive means such as in a hybrid vehicle. This approach has the advantage that it could be retro applied to existing engine designs by appropriate reprogramming of the engine management system rather than requiring a completely new design for implementation. Such loading may be applied as one continuous load or may be applied in multiple bursts, the latter being more suitable for power generation or providing power for an electric drive system.

In one embodiment, a variable load is applied to the engine that is synchronised with the rotational position of a reciprocating engine and the position in the engine cycle.

Measurement of cylinder pressure may also be used to control the application of the variable load to the engine in a closed loop control system.

A variable load may advantageously be applied after combustion of the primary fuel charge has started in a compression ignition engine. Primary combustion occurs at different positions depending on the operational conditions (such as rpm), and the implementation, in some embodiments, may be better linked to the combustion event. In the embodiment where an electrical load is applied the crankshaft to slow the rate of increase of the combustion volume after ignition with the objective of inducing constant or pseudo constant volume combustion conditions, energy can additionally be stored as electrical charge. This could typically be in a capacitor or a battery. Thus, a proportion of the energy employed to increase the combustion efficiency of the engine is available to be re-used as electrical energy. In the embodiment where a mechanical load is applied electrically to the crankshaft to slow the rate of increase of the combustion volume after ignition with the objective of inducing constant or pseudo constant volume combustion conditions, energy can be stored in a flywheel type device. Thus, a proportion of the energy used to increase the combustion efficiency of the engine is available to be re-used as kinetic energy.

Energy stored during the combustion stroke, whether electrical or in another form, may be used to increase the rate of piston movement during the compression stroke. Traditionally this is the cycle where the piston decelerates most because of the work required to compress the air in the cylinder to raise the temperature and pressure to a point where auto ignition of the fuel will occur in a compression ignition engine. Therefore, to assist this movement using stored energy will reduce the piston speed fluctuations in the engine. Thus, it makes it possible to make a smoother running single cylinder engine, thus reducing the need for multi cylinder engines in some applications. This is particularly advantageous when used in a motor/generator type implementation whereby the same electrical unit is used as a generator during the combustion stroke and as a motor during the compression stroke.

The stored energy may alternatively or additionally be used to increase the rate of piston movement during the exhaust stroke. This has the advantage of potentially improving emissions.

Both the start point and the end point of the retardation of the piston during the first 90 degrees after top dead centre may be varied depending on operating conditions. It is not necessary that the retardation starts at top dead centre nor that it stops at a fixed period or fixed angle after it starts. So, for example, the mechanical or load based retardation might start at 10 degrees after top dead centre and stop at 30 degrees at 2000 rpm, but might be applied from 15 degrees to 55 degrees at 4000 rpm. The exact requirements will be different for different engine designs and requirements. A key feature of the method of the first aspect of the invention is that energy is not wasted, as it is effectively stored up in the combustion chamber in the form or increased pressure during the combustion, which, when the retardation is released, will result in faster acceleration of the piston and therefore greater torque delivered to the crankshaft for the same fuel consumption. In one embodiment, the retardation is released at 90 degrees so as to provide the maximum moment arm on the crankshaft and hence the maximum torque. Equally, for the load based control, energy can also be recovered through the load applying means, such as by storing as electrical energy as set out above.

Combustion enhancement according to this first aspect of the invention produces faster, or more intense combustion, both in terms of auto ignition delay and combustion propagation. The net effect is that for the same ignition timing the combustion event is advanced in time. This corresponds with either a smaller cylinder volume, or a lower rate of increase of cylinder volume, or both, because of the position of the piston relative to TDC. This results in higher engine cylinder pressure and temperature being achieved. Additionally, the piston has an increased resistance to movement, which slows the increase in cylinder volume, as the turning moment on the crankshaft is reduced the closer the piston is to TDC in a reciprocating engine.

In a reciprocating engine the movement of the piston relative to a fixed engine cylinder is sinusoidal. This results in sinusoidal variation in the effective volume of the combustion chamber. The method according to the invention enables this sinusoid movement to be modified to, for example, a flat topped sinusoid relationship (X direction change) or a gapped sinusoid relationship (Y direction change), or a combination of both.

In the invention, the method employed to slow the rate of increase of the combustion volume after ignition, with the objective of inducing constant or pseudo constant volume combustion conditions, must be variable and individually controlled for each engine cycle. This ensures that optimum combustion conditions are achieved for each engine cycle regardless of the operational state of the engine.

The constant or pseudo constant volume combustion conditions change the nature of the combustion by moving it along the combustion spectrum. This shift from deflagration towards detonation produces faster, more complete combustion that generates a different emission profile.

A necessary characteristic of constant or pseudo constant volume combustion conditions is increased peak cylinder pressure during the power stroke. This is achieved for each individual engine cycle using a variable mechanism. A closed loop system to implement the control for this would preferably use cylinder pressure as the feedback signal. An open loop system would preferably use the operating conditions of the engine to predict the requirement for the next engine cycle based on the current cycle. A preferred system would most likely use elements of both to provide redundancy and fail safe operation.

According to another aspect of the present invention, there is provided a method of enhancing the combustion in a cylinder of an internal combustion engine comprising providing a primary hydrocarbon based fuel for igniting in the combustion chamber of an engine in order to drive a piston along a cylinder to provide drive to an engine, providing a secondary hydrocarbon based fuel into the combustion chamber, the LFL of the secondary fuel being higher than the LFL of the primary fuel, the concentration of the secondary fuel being less than or equal to its LFL relative to the amount of air in the cylinder, whereby during the combustion of the primary fuel, the temperature and pressure of the secondary fuel is increased such that the secondary fuel experiences chemical decomposition and generates free radicals.

A method in accordance with the further aspect of the invention has the advantage that, by selection of the secondary fuel to have a higher LFL than the primary fuel, the secondary fuel does not combust during an initial combustion of the primary fuel and instead has its temperature and pressure increased by the heat and vapour gas expansion resulting from the combustion of the primary fuel until it exceeds its auto ignition temperature, whereupon it will generate radicals and experience chemical decomposition, which is similar to the onset of combustion but without the rapid propagation, due to the low concentration of the secondary fuel (i.e. below the LFL).

The higher LFL of the secondary fuel ensures that the secondary fuel experiences the combustion of the primary fuel when the secondary fuel has exceeded its LFL and, therefore, no longer exhibits the normal rate of combustion. The chemical decomposition of the secondary fuel will result in the generation of radicals which enhance the combustion of the primary fuel with the overall aim being to move the combustion along the combustion spectrum.

Preferably there is a difference between the LFLs of the primary and second fuels of at least one under standard conditions (atmospheric pressure, room temperature).

In a preferred embodiment of the invention, a pilot combustion of the primary fuel occurs prior to a main injection of the primary fuel, the secondary fuel being present in the combustion chamber during the pilot combustion such that it experiences the increased temperature and pressure resulting from the pilot combustion. In particular, therefore, injection of the primary fuel into the combustion chamber is separated into a pilot injection and a main injection, the secondary fuel being injected prior to or during the pilot injection.

The purpose of the secondary fuel is to generate radicals which will enhance the combustion of the main injection of the primary fuel, so that the radicals must therefore persist in the short time between the pilot and main injections of the primary fuel.

The pilot injection of primary fuel should be of sufficient volume to combust normally. This is because the concentration will, by definition, be between the LFL and UFL limits for the primary fuel. The combustion of the primary fuel will increase the temperature and pressure in the engine cylinder. This is in addition to the increase in pressure and temperature in the engine cylinder generated by the reduction in cylinder volume induced during the compression stroke of the engine.

Preferably, the secondary fuel is introduced into the cylinder prior to the end of the compression stroke of the piston, and, in particular, substantially at the start of the compression stroke.

Preferably, the secondary fuel is fed into an air intake of the engine at a concentration which is less than its LFL.

Increased pressure is known to reduce the auto ignition temperature of a combustant. Furthermore, it is also known that increased pressure can change the flammability limit of a combustant. It is therefore likely that some of the products of decomposition of the secondary fuel could be experiencing traditional combustion under the increased pressure and temperature conditions in the engine cylinder and thus generating radicals, during the period between the pilot and main injections of the primary fuel.

Two chemicals that could be produced depending on the secondary fuel employed could include Hydrogen (H2) and Acetylene (C2H2). These are characterised by incredibly low ignition energy requirements and hence can combust readily and rapidly.

In the method of the further aspect of the invention, the primary fuel combusts following the pilot injection pulse and radicals are generated as part of the normal combustion process. However, the secondary fuel, may not start combustion at all, or alternatively stops traditional combustion before the primary fuel, as the secondary fuel has exceeded its LFL. The remaining secondary fuel is still experiencing the standard combustion of the primary fuel, because the primary fuel has a lower LFL, which raises the temperature and / or pressure in the engine cylinder.

This method sustains the combustion and chemical decomposition cycles and therefore the availability of free radicals following the pilot injection of the primary fuel until the main injection of the primary fuel. The availability and distribution of free radicals prior to the main injection of primary fuel ensures optimal combustion results. Hydrogen itself would also serve as a secondary fuel but does not require the pyrolysis stage to be effective.

In one embodiment, the secondary fuel is injected into the engine cylinder throughout the compression stroke to ensure that the secondary fuel is not consumed prior to the main injection of primary fuel.

In another embodiment of the invention, a secondary fuel is injected into the engine cylinder between the pilot and main combustion events, the concentration of the co- combustant being above its LFL, as modified by the engine cylinder conditions. This has the advantage that it sustains the generation of radicals via traditional combustion.

It will be still further understood that the method according to the first aspect of the invention may advantageously be used in conjunction with the method according to the second aspect of the invention. In order that the invention may be well understood, there will now be described some embodiments thereof, given by way of example, reference being made to the accompanying drawings, in which:

Figure 1 is a schematic view of an engine utilising a combustion enhancement system.

Figures 2A and 2B are schematic side views of a cylinder of an engine to which the present invention may be applied with the piston shown in two different positions;

Figures 3A and 3B are schematic side views of the cylinders of Figures 2A and 2B with rotation applied thereto in order to produce a period of constant volume of the combustion chamber in accordance with an aspect of the invention;

Figure 4 is a schematic view of a first embodiment of an electrical energy storage system for use in conjunction with the present invention; and

Figure 5 is a schematic view of a second embodiment of a mechanical energy storage system for use in conjunction with the present invention.

Referring first to Figure 1 , there is shown a schematic view of a combustion enhancement system for a combustion ignition diesel engine according to the invention. An engine 1 has an electronic control unit (ECU) 2 connected to it by means of which operation of the engine 1 , such as fuel delivery to the engine 1 , may be controlled. The system includes a turbo 3 by means of which the pressure of the air which is mixed with the fuel may be increased. A motor 4, which is controlled by the ECU 2, is provided to control the delivery of a secondary fuel to the cylinders of the engine 1 for combustion.

In the illustrated example, the secondary fuel is Liquefied Petroleum Gas (LPG) which is stored under pressure in a canister 5 that is preferably replaceable or refillable. Other secondary fuels may also be used within the scope of the invention, the important thing being that it is hydrocarbon based and has a higher LFL (Lower Flammability Limit) than the primary fuel, which in the case of the illustrated example is diesel fuel. The secondary fuel is used as an additive to generate a co-combustant as explained in more detail below.

The outlet of the canister 5 is connected to the engine air inlet 6 at a point 7 upstream of the turbocharger 3 so that the injection of the secondary fuel into the engine air inlet 6 prior to the turbocharger 3 occurs nominally at atmospheric pressure. The flow rate of the secondary fuel into the air inlet 6 is controlled so that the concentration of the secondary fuel in the air inlet 6 is below the LFL of the secondary fuel and hence the mixture of air and secondary fuel in the air inlet is inflammable.

Delivery of the secondary fuel to the air inlet is metered using a pump 8 so that a uniform concentration can be maintained. The pump 8 is preferably a peristaltic pump. The pump 8 is controlled by the motor 4 which receives commands from the ECU 2. This controls the pump speed to ensure that the secondary fuel concentration is maintained within acceptable parameters. This control loop nominally updates at least once per engine cycle.

In the illustrated embodiment, a single ECU 2 controls the whole engine system and the communication between the ECU 2 and the pump motor 4 is via a CANbus 9. The motor 4 driving the peristaltic pump 8 is preferably a stepper motor.

The ECU 2 also monitors the gas pressure on the input to the pump using pressure sensors 10 to check for leaks and also to alert the system when the secondary fuel supply is getting low. A gas valve 11 , such as an electric gas valve, is also provided in the outlet feed from the canister 5 which is operable to isolate the secondary fuel canister 5 is also controlled in a similar fashion. A pressure regulator 12 for the gas is used to ensure that an accurately metered amount can be delivered to the injector 7, via the pump 8.

Referring next to Figures 2A and 2B, there is shown a single cylinder 15 which houses a reciprocating piston 16 therein connected to a crankshaft 17 by a connecting rod 18 as a basic example of a reciprocating engine. When the crankshaft 17 rotates, the piston 16 moves within the engine cylinder 15 due to the connection via the connecting rod 18. For one complete revolution of the crankshaft 17 the piston 16 describes both an up and a down stroke. The piston speed is at a minimum at the end of the stroke when it changes direction. These points are defined as TDC (when the piston is at the top of its stroke) and BDC (when the piston is at the bottom of its stroke).

In the situation where the piston 16 is at the TDC position following the compression stroke (Figure 2A), following ignition which occurs around TDC, the combustion of the primary fuel will generate expanding gasses which act to drive the piston 16 down the cylinder 15 during the power stroke. In a conventional engine, the cylinder remains stationary and the piston 18 moves downwards in the cylinder 15 as shown in Figure 2B. However, in accordance with one embodiment of the present invention, at the start of or during this power stroke, the engine cylinder 15 may be rotated in the same direction as the crankshaft 17 about a common axis as shown in Figures 3A and 3B. Depending on the relative speeds of the rotation this would have the effect either of maintaining a constant combustion volume above the cylinder 16 for the time when the cylinder 15 is actually rotating, as shown in Figure 3A, or reducing the rate of increase of the downward movement of the piston 16 in the cylinder 15 resulting from the combustion above the piston 16, as shown in Figure 3B. This will, then, produce a period of constant volume or pseudo constant volume combustion sufficient to change the combustion environment and hence combustion characteristics.

Once the rotation of the engine cylinder 15 stops, the engine operation returns to normal. This rotation of the cylinder 15 in the same direction as the crankshaft 17 would only occur during the power stroke. Following completion of the forward rotation, the cylinder 15 will then be rotated back to its start position ready for commencement of the next combustion event. This could occur slowly over the remainder of the engine cycle. This could be completed during the return stroke of the piston 16 so that once the rotation in the direction of the cylinder 15 has been completed during the power stroke, the cylinder remains stationary until the piston reaches bottom dead centre. It may even be possible, in certain applications, to allow the engine cylinder to move incrementally and thus rotate fully around the crankshaft after successive engine cycles but this might present practical mechanical issues.

In an alternative approach which is also within the scope of the present invention, the engine could instead be loaded during the power stroke in order to apply some resistance to the rotation of the crankshaft 17 during the power stroke in order to reduce the acceleration of the piston during the combustion event and thus produce a period of pseudo constant volume combustion sufficient to change the combustion environment and hence combustion characteristics. By pseudo constant volume, it is meant that the acceleration of the piston downwards in the cylinder is reduced so that the rate of increase of the volume of the combustion chamber above the piston is correspondingly reduced. Conceptually, this is akin to applying a brake to the crankshaft and thus retarding the normal rotation of the crankshaft relative to the engine cylinder.

This loading to force the engine to do additional work could be achieved in a number of different ways which are within the knowledge of the skilled. This can be achieved with selective mechanical, fluid, magnetic or electrical coupling.

Rather than wasting the energy associated with retarding the engine by back loading, this energy is instead stored, either in mechanical form or electrical form. This stored energy is then available to be used when required so as to deliver increased system efficiency in addition to combustion efficiency of the engine.

The loading could be applied for a continuous period during the first 90 degrees after TDC, and could be applied as a constant load or a load which varies during the period for which it is applied to match changing engine conditions. The load could be applied during part of or the whole 90 degrees as indicated above. Alternatively, the load could be applied in a pulsed manner during the period of application rather than in a single burst, which could make it more suitable for electric power generation or power supply to an electric drive system.

In order to synchronise the application of the load to the engine with the engine cycle and also timing the release of the stored energy, an electronic controller is provided, which may be separate to or integrated into the standard engine ECU.

Figure 4 illustrates an electrical based system for loading the engine according to one embodiment of the invention. An electrical generator 20, which could take the form of the existing alternator or an additional generator that could be used in electric propulsion, has its field windings 21 connected by a switching controller 22 (using components such as via FET switches or thyristors) when the additional engine load is required and the field winding 21 open circuited at other times. The synchronisation of the switching controller is achieved using sensors 23 which monitor the rotational position of the engine and the engine cycle. Thus the generator provides pulses of electrical power. These are converted using an electronic power supply 24 to generate DC power, which can be used for electrical power 25, for battery charging 26 as in a standard or hybrid automobile, or the like. Additionally, the pulses of power, especially if at a high voltage, could be used for direct drive of electric actuators or motors 27, or be used to energise an electrical accumulator 28.

Figure 5 illustrates an alternative embodiment in which an electrical winding 30 is used with the same electronic switching arrangement. The electrical winding 30 acts on a mass 31 that rotates. The mass is connected to the crankshaft using a flexible coupling 32 (e.g. spring or torsion bar) that is designed to store energy when the mass is retarded. When the retardation of the mass is released (by electronic switching of the winding) the stored energy is released to generate an acceleration of the crankshaft directly.

It will be understood that the concepts described above may be used together, so that the secondary fuel may be used in conjunction with the constant or pseudo constant volume control system.

Claims

1. A method of improving the efficiency of an internal combustion engine having a piston configured for reciprocating movement within a cylinder, characterised by at least reducing the rate of movement of the piston along the cylinder with respect to the combustion chamber during at least a part of the first 90 degrees of each combustion stroke, the movement of the piston being reduced for a continuous period starting after the top dead centre of the piston within the cylinder and finishing not later than 90 degrees after top dead centre, and the reduction being applied each cycle of the engine such that the path of the piston during each cycle of the engine is cyclic but not completely sinusoidal for at least part of the first 90 degrees of the combustion stroke.

2. A method according to claim 1 , wherein the reduction of the movement of the piston is finished before 90 degrees after top dead centre.

3. A method according to claim 1 or claim 2, wherein the period for which the movement of the piston is reduced is variable so as to enable the period of reduction to be changed for different strokes of the piston within the cylinder to suit different operating conditions of the engine.

4. A method according to any of the preceding claims, wherein the movement of the piston within the cylinder is completely stopped for said at least part of the first 90 degrees of the combustion stroke such that the volume of the combustion chamber is kept constant for said at least part of the first 90 degrees of the combustion stroke.

5. A method according to any of the preceding claims, comprising controlling the movement of the piston by mechanical means.

6. A method according to claim 5, comprising controlling the movement of the piston by means of a mechanical linkage between the piston and a crankshaft of the engine.

7. A method according to claim 6, comprising controlling the movement of the piston by means of a cam arrangement provided between the piston and the crankshaft.

8. A method according to claim 6 or claim 7, comprising controlling the movement of the piston by means of controlling a variable linkage provided between the piston and the crankshaft.

9. A method according to any of claims 5 to 8, comprising rotating the cylinder in the direction of rotation of the crankshaft during said at least part of the first 90 degrees of rotation of the crankshaft in order to at least partially counteract the rotation of the crankshaft.

10. A method according to claim 9, comprising the further step of counter-rotating the cylinder after completion of the combustion stroke in order to return the cylinder to its starting position.

11. A method according to any of the preceding claims, comprising the further step of applying a load to a crankshaft in order to inhibit the rotation of the crankshaft during said at least part of the first 90 degrees of the combustion stroke and thereby inhibit the movement of the piston.

12. A method according to claim 11 , wherein the load applied to the crankshaft is a variable load that is synchronised with the rotational position of the engine and the position in the engine cycle.

13. A method according to claim 12, wherein the variable load is applied after combustion of a primary fuel charge has started in a compression ignition engine.

14. A method according to claim 12 or claim 13, comprising the further step of measuring pressure within a cylinder of the engine and controlling the application of the variable load based on the measured cylinder pressure.

15. A method according to claim 11 , wherein the load is applied as a single, continuous load during said at least part of the first 90 degrees of the combustion stroke.

16. A method according to any of claims 11 to 15, wherein the step of applying a load comprises connecting an electrical load to the engine.

17. A method according to any of claims 11 to 14, wherein the load is applied in a pulsed manner during said at least part of the first 90 degrees of the combustion stroke.

18. A method according to any of claims 11 to 17, comprising the further step of storing energy generated by said load.

19. A method according to claim 18, comprising the further step of using said energy stored during the combustion stroke to increase the rate of piston movement during the compression stroke.

20. A method according to claim 18 or claim 19, comprising the further step of using said energy stored during the combustion stroke to increase the rate of movement of the piston during the exhaust stroke.

21. An engine comprising a cylinder, a piston mounted in the cylinder for longitudinal reciprocating movement within the cylinder, a crankshaft, and connection means connecting the piston to the crankshaft such that longitudinal reciprocating movement of the piston effects rotation of the crankshaft about its longitudinal axis, wherein the connection means is configured to at least inhibit the reciprocating movement of the piston within the cylinder for a continuous period for at least a part of a 90 degree portion of the cycle of the piston starting after the top dead centre of the piston within the cylinder and finishing not later than 90 degrees after top dead centre in order to at least reduce the rate of movement of the piston within the cylinder.

22. An engine according to claim 21 , wherein the connection means includes a cam and follower which is configured to include a variable response in at least part of the 90 degree portion of the cycle of the piston.,

23. An engine according to claim 21 or claim 22, wherein said connection means includes a variable linkage between the piston and the crankshaft which is adjustable during the stroke of the piston in order to vary the movement thereof within the cylinder.

24. A method according to any of the preceding claims further comprising enhancing the combustion in the cylinder of the internal combustion engine by providing a primary hydrocarbon based fuel for igniting in the combustion chamber of an engine in order to drive a piston along a cylinder to provide drive to an engine, providing a secondary hydrocarbon based fuel into the combustion chamber, the LFL of the secondary fuel being higher than the LFL of the primary fuel, the concentration of the secondary fuel being less than or equal to its LFL relative to the amount of air in the cylinder, whereby during combustion of the primary fuel, the temperature and pressure of the secondary fuel is increased such that the secondary fuel experiences chemical decomposition.

25. A method according to claim 24, wherein there is a difference between the LFLs of the primary and second fuels of at least one at atmospheric pressure and room temperature.

26. A method according to claim 24 or claim 25, comprising the further steps of carrying out a pilot combustion of the primary fuel prior to a main injection of the primary fuel, the secondary fuel being introduced into the combustion chamber so that it is present during the pilot combustion such that it experiences the increased temperature and pressure resulting from the pilot combustion.

27. A method according to claim 26, wherein injection of the primary fuel into the combustion chamber is separated into a pilot injection and a main injection, the secondary fuel being injected prior to or during the pilot injection.

28. A method according to claim 26 or claim 27, wherein, during the pilot combustion, the secondary fuel is heated to above its auto ignition temperature.

29. A method according to any of claims 24 to 28, wherein the secondary fuel is fed into an air intake of the engine at a concentration which is less than its LFL.