GB2219345A - Engine crankshaft arrangement - Google Patents

Abstract

The crankshaft axis is offset from the plane containing the axis of the or each i.c. engine cylinder 1 or the axis of one or each cylinder of a Stirling engine (Fig. 8) by 40 to 70%, e.g. 50%, of the piston stroke. The engine valve timing is adjusted to compensate for the stroke characteristics provided by the offset. Engine dimensions are given in the specification. <IMAGE>

Description

IMPROVED RECIPROCATING PISTON COMBUSTION ENGINE The invention relates to internal and external combustion engines and in particular those that comprise a cylinder, a piston reciprocable in the cylinder, and means for translating reciprocable linear motion of the piston into rotary action, usually a crank-shaft or eccentric.

Ongoing research worldwide into reciprocating internal combustion engines has and is concentrated on obtaining improved specific fuel consumption and combustion with lower levels of toxic elements. Two main approaches have been the development of the catalyst treatment of exhaust, and the development of improved combustion - the so called "lean-burn" approach. The catalyst idea burns more fuel to clean up the exhaust,using very expensive materials and has a poor durability record. Lean-burn engines, while in production, are still being developed. This involves radical re-design of major components, particularly the cylinder head and piston crown.

It is against this background of a long-standing international search for improvements in engine efficiency, involving great efforts by many of the world's largest and best-equipped engineering companies, that the present invention should be considered.

Most reciprocating engines are built with a single plane passing through the longitudinal axis of the piston and the axis of rotation of the crank-shaft. This conventional symmetric configuration will be referred to as "normal", having no offset.

However, engines have been built with a small degree of lateral offset between a plane through the longitudinal axis of the cylinder and a parallel plane passing through the axis of rotation of the crank-shaft. This latter layout is known as a "desaxe" configuration.

The desaxe configuration is credited to the prolific British inventor Turner and is described in British Patent 1270 dated 1879. Turner found that there was a advantage to be gained in such configurations as the friction between the piston and the cylinder walls due to the angularity of the connecting rod would be reduced. He suggested that the degree of offset required was one-eighth, or in percentage terms twelve and a half percent of the length of the piston stroke.

By the early part of the twentieth century a significant number of engine manufactures had adopted the desaxe configuration.

These included amongst others Brasier, Mors, Duryea and Ailsa Craig.

The latter company, Ailsa Craig, once owned by the inventor's family, is credited with the actual introduction of the desaxe principle into production and continued to produce engines of this type for well over forty years. Their reason for adopting the desaxe principle was the reduction in friction during the working stroke in the cycle. Their engines incorporated an offset amounting to 11 percent of the piston stroke.

Lancia of Italy developed the desaxe principle as a means of achieving very compact narrow-Vee engines, initially for aircraft use as their British Patents No. 149301 of 1915 and 179163 of 1919 refer to. Lancia continued to use a desaxe configuration in their narrow-vee automobile engines until quite recently, and their stated reason for continuing to use this configuration was compactness. The amount offset in their production engines varied but was generally up to 25 percent of the piston stroke.

The desaxe configuration has also been used on two-stroke reciprocating piston engines to provide improved scavenging over bottom dead centre. It has also been used to reduce piston slap noise. In both cases this is normally achieved by offsetting the gudgeon pin within the limits of the boss.

However despite this very extensive utilisation of the desaxe principle earlier this century, over the past twenty-five years it has fallen out of favour mainly due to the opinion of the motor industry that such small reductions in engine friction did not warrant the extra complication that its utilisation required.

Therefore in the light of recent requirements during the past fifteen years or so to improve combustion efficiency in such internal combustion engines it appears that no previous research has appreciated the potential significance in improving such combustion efficiency by promoting a substantial extent ion in the desaxe offset, whereby such an offset in fact approaches, equals or even exceeds half the stroke of the piston member.

The fact is that when such a comparatively large offset is introduced, it results in substantially reduced piston speed during down-stroke with a correspondingly more rapid up-stroke.

To permit those trained in this art to understand this novel invention and the several advantages accruing from its employment, in the accompanying drawings forming a part of this specification, a desirable and preferred embodiment of the invention has been illustrated in detail of which: Figure 1 is a vertical section through an internal-combustion engine equipped with the new features of construction; Figure 2 is a graph depicting the variation in slopes between an offset cylinder engine according to the invention and a normal cylinder engine, and where the piston height above crank is plotted against advancing crank angle for each engine; Figure 3 is a graph depicting the variation in slopes between an offset cylinder engine according to the invention and a normal cylinder engine, and where the piston side force is plotted against advancing crank angle for each engine;; Figure 4 represents a reciprocating piston engine where the longitudinal axis of the cylinder passes through the rotary axis of the crank-shaft; Figure 5 represents a reciprocating piston engine where the longitudinal axis of the cylinder is offset to one side of the rotary axis of the crank-shaft by an amount equal to 67.8 percent of the piston stroke; Figure 6 is a graph of piston travel against increasing degrees of cylinder offsets; Figure 7 is a graph depicting the variations in slopes between an offset cylinder engine according to the invention and a normal cylinder engine, and where the mechanical advantage is plotted against advancing crank angle for each engine; Figure 8 is a schematic view of a Stirling engine incorporating offset cylinders for both the displacer and working chambers of such external heat engines.

By reference to the drawing Figure 1, it will be perceived that apart from the unusual lateral position of the cylinder in relation to the crank-shaft, the internal-combustion engine is of a conventional form and consequently its well-known structural elements and characteristics need not be described except in so far as they concern the present invention.

The motor cylinder is characterised 1, its crank-case 2, the crank-shaft 3 and the piston 4, the latter being reciprocable in the cylinder 1 with crank means 3 for translating reciprocal linear motion of the piston 4 into rotary motion of the drive-shaft 5. A con-rod 7 attaches the piston 4 to the crank means 3 by way of small-end 8 and big-end 9 bearings.

A spark plug 14 is provided for the purpose of igniting the fuel/air mixture, and valve means 15 such as the well known poppet type operated by cam shaft 16 is arranged to open and close both the induction and exhaust portingxfor the engine.

(Only one valve 15 is shown in Figure 1).

The operational rotation of the engine of Figure 1 is arranged to be anti-clockwise and therefore the cylinder's 1 longitudinal axis is purposely offset 6 to the left side of the rotational axis of the drive-shaft 5 by an amount approximately equal to half the crank 3 throw.

Therefore by comparing a plane drawn through the longitudinal axis of the cylinder 1 with a plane through the rotational axis of the translational crank means 3, a lateral offset 6 is produced by an amount approaching, equal to or exceeding half the piston 4 stroke in the cylinder 1, so that the translational crank means 3 is in advance of the cylinder 1 and wherein the piston 4 takes longer to travel from top dead centre 10 to bottom dead centre 11 than in the reverse direction.

The major point of the invention follows from the understanding that combustion takes time, and that therefore the reduced rate of piston decent during the working stroke provided by such a substantial cylinder offset results in more time and thus permits improved and optimum combustion.

Figure 2, a graph of piston height over crank against advancing crank angle compares the difference in time available for fuel combustion between a normal cylinder engine and an offset-cylinder engine according to the invention.

Curve 21 shows that with an extended offset, the piston 4 travels more slowly from top dead centre 10 to bottom dead centre 11 when compared to curve 20 for a normal cylinder engine, for which top dead centre is shown as 10' and bottom dead centre as 12.

As a result, the crankshaft rotation of the offset-cylinder is substantially in excess of the ususal 180 degrees in a normal cylinder engine. This extention in crank-shaft angle is shown as "t" in Figure 2.

It should be added that in order to correctly show the effect of delayed bottom dead centre 11 for the extended offset cylinder engine, top dead centre 10 has been retarded on the graph Figure 2 to zero degrees advancing crank-shaft angle so that for the purpose of making a direct graphical comparison between the engines it occurs at the same point as for a normal cylinder engine, i.e. zero degrees of advancing crank angle.

In practical terms, as the pressure force due to combustion acting on a piston of an extended offset cylinder engine will be transmitted through a longer crank-shaft angle it follows that a higher output power is achievable than in a comparable normal cylinder engine using the same diameter of piston.

Furthermore in the offset cylinder engine, if the opening of the exhaust valve 15 is delayed by an amount approximately corresponding to the increase in crank-shaft angle due to offset, here shown opening at 22 on curve 21, it has been 'found that a further increase in output power is forthcoming by way of extracting further useful work from the gases during the working stroke than would otherwise be possible.

Point 23 signifies the point on curve 20 where the exhaust valve would open in a normal cylinder engine and is approximately fifteen degrees earlier than the offset cylinder engine using the same dimensioned piston.

It is the opinion of the inventors that a significant proportion of the extra output power measured from an experimental extended offset cylinder engine that is not forthcoming from an identical but normal cylinder engine is gained from the combination of extending the combustion period, delayed exhaust opening and the longer period during which the crank approximates to ninety degrees with the con rod..

In the identical but normal cylinder engine no such extra output power can be extracted as any remaining pressurised gas in the cylinder is vented to exhaust as the exhaust valve opens before bottom dead centre. This loss in energy can be observed in current "normal" internal combustion engines when the exhaust manifold is removed for inspection, where a flame is seen passing through the annular gap as the exhaust valve opens to vent the exhaust gases to atmosphere . This is fuel burning to waste.

With the extended offset cylinder, the greater crankshaft angle during the downward stroke is compensated for by a correspondingly shorter crank-shaft angle from bottom dead centre 11 to top dead centre 10 on the up-stroke. The shorter time taken in the exhaust stroke results in a more rapid clearance of the products of combustion. As these gases are very hot and tend to destroy cylinder wall lubricants, it is an advantage to expel such gases as rapidly as possible.

similarly the compression stroke is likewise affected. In addition to greater turbulence a more rapid compression action results in lower leakage losses past the piston rings so that compression pressure and mixture temperature can be optimised prior to ignition. This is of particular interest when cold starting small diesels under 0.5 litres cylinder capacity.

Furthermore as induction of the fuel/air mixture occurs over an extended crank-angle for the extended offset cylinder engine, and therefore the time taken for the induction down-stroke in the piston travel is substantially slower than in normal engines, it is considered that cylinder fill will be more complete, this will improve the volumetric efficiency of the engine and also improve the atomisation of the fuel mixture itself by heat transfer. Any such extention in time may also help to keep the valve area cooler than heretherto was possible.

During the all-important working-expansion strokeucycle, the side thrust acting on the piston is minimised for the extended offset cylinder as can be seen in Figure 3 by comparing curves 25 and 27 and where curve 27 is for the extended offset cylinder. Figure 3 represents a graph of piston side force against advancing crank angle.

Curve 25 depicts the piston side load for a normal non-offset cylinder, and shows that the force increases in magnitude as the crank approaches ninety degrees after piston top dead centre.

Curve 27 indicates the substantial reduction in side load that is possible when an extended offset of half the piston stroke is used. Over the period of highest combustion pressure the con rod is rapidly approaching vertical ninety degrees crank angle), at which point there is no side thrust. As a result the extended offset cylinder has an advantage in reduced piston side loading of over sixty percent when compared to a non-offset cylinder engine.

An extended offset cylinder engine according to the invention also has the advantage over the normal type in that the volumetric capacity of the engine is increased assuming equal dimensions for piston diameter and con rod centres. This is due to the greater difference between the gudgeon pin height at top dead centre and bottom dead centre by way of geometric considerations than that obtainable with a standard normal cylinder engine configuration.

For example, when a normal cylinder with a stroke of 46.8 mm and con-rod centres 89 mm is offset by half the stroke, the piston stroke is increased to 48.7 mm which results in a 4 percent increase in volumetric capacity of the engine. Furthermore, an offset by half the bore increases the piston stroke to 50.4 mm and the volumetric capacity by 7.7 percent. This can be seen when comparing Figs. 4 and 5.

Due to the more efficient combustion of the fuel mixture in the engine according to this invention, preliminary indications suggest worthwhile reductions in some exhaust gas constituents such as hydrocarbons, carbon monoxide and possible inhibition of certain nitrous oxides as a consequence of extending the time interval during the down-stroke phase of combustion.

Any limitations and resulting disadvantages in applying this feature of extended cylinder offset can be taken into account and solved without undue complication and difficulty.

The asymmetric motion of a multi-cylinder engine according to the invention indicates the need to carefully balance dynamically and to counter-balance the reciprocating and rotating elements and this can readily be achieved by several known means. The con rods remains in nearly the same relationship to each other when the cylinder is moved to offset, therefore only a small amount of alteration is required and this is readily accomplished by standard bobweight practice.

Increasing the offset further than fifty percent of the piston stroke increases the crank-shaft angle traversed between top dead centre and bottom dead centre. Therefore the degree of offset used tends to be limited by the corresponding increase in piston side thrust.

With extended offset engines according to the invention, the larger the piston diameter, the easier it is to accommodate the con rod. The con rod should be kept as short as practicable in the interests of weight and compactness. A very short skirted piston or a slot in the piston skirt on the crank-shaft side may be necessary to accommodate the con rod. This is acceptable as the side thrust acts on the piston on the side away from the crankshaft, principally during compression and is minimised during the working stroke.

In a working prototype built and tested according to the invention, with geometry placing the offset at 68 percent of the stroke compared to normal, has shown a twenty percent improvement in specific fuel consumption over an identical engine with a non-offset cylinder.

Figures 4 and 5 illustrate how the crank angle covered from top dead centre to bottom dead centre increases from the standard 180 degrees on the normal engine (Figure 4) to 193 degrees on the extended offset engine (Figure 5). This results in a gain in time of around seven percent in favour of extended offset for both the combustion and induction phases of the engine cycle with a corresponding reduction of seven percent in time for the exhaust and compression phases.

While such an extended cylinder offset has shown substantial improvements it is recommended that the offset be limited to about fifty percent of the stroke in order to obtain the maximum benefits of the invention. This will however, reduce the time available for combustion and therefore an ideal compromise between extending the offset beyond fifty percent of stroke to lengthen combustion time must be weighted against corresponding increased piston side thrust frictional losses.

It is believed that in each new design of engine all the various parameters must be weighed up together and taken into account in order to find the ideal degree of extended offset that will provide maximum benefit for that particular design. For example, it has been found that to derive the greatest benefit of extended cylinder offset it is necessary to keep within the following design parameter: That the ratio of the piston stroke to cylinder bore should be less than 1.25 and be ideally 0.75 so to minimise the loss of piston skirt material when providing clearance for the connecting rod at the point when the crank-shaft is at 270 degrees from the vertical position.

This is the angle of maximum con-rod angularity for all offsets and it occurs during compression and exhaust, i.e. during periods of relatively low cylinder pressure and therefore is not unduly serious.

At ignition of a cylinder with a fifty percent offset of the piston stroke, the con-rod is, during the early phase of combustion, rapidly approaching a vertical alignment with the longitudinal axis of the cylinder. At this point there is no side thrust between the piston and the cylinder wall.

Fig.6 is a graph of piston travel against increasing cylinder offsets. Curve 30 shows that the earliest point where any benefit is gained from extended cylinder offset is well beyond the twenty-five percent offset which has up to now been greatest degree of offset used in desaxe.

At around fifty percent cylinder offset a worthwhile gain in the extension of burning time is achieved which is not forthcoming from using much smaller degrees of offset as have been previously used in desaxe.

It is estimated that the upper limit should be about seventy percent as beyond this point the con-rod is subjected to very steep angles of inclination which may cause failure. Therefore it is believed that the area where the engine designer can achieve maximum benefit from the extended offset is in the region of forty to seventy percent offset of the piston stroke.

Figure 7 is a graph of mechanical advantage against advancing crank angle and shows two curves, the first parts of which are superimposed up to about thirty-eight degrees of advancing crank angle. Beyond thirty-eight degrees the curves diverge and overall in terms of mechanical advantage, curve 40 for the extended offset cylinder shows a significant advantage over curve 41 for the normal cylinder.

By calculation, there may be a small reduction of about two percent advantage in terms of mechanical advantage up to ninety degrees of crank angle for the extended offset cylinder, but this in fact is more than compensated for by an increase in such mechanical advantage over the remaining phase of the combustion cycle where there is a gain of over twenty percent mechanical advantage in favour of the extended offset cylinder.

Improvements related and here described to internal combustion engines may also advantageously be applied to external combustion engines of the Stirling type.

The Stirling engine illustrated in Figure 8 has external combustion by any means, the heat being applied to one end of a closed cylinder 51 shown as chamber 50, the other opposing chamber 52 being cooled . Within this cylinder 51 a loose fitting displacer 53 with its operating rod 54 passing through a gland 55 moves the working fluid (air or gas, sometimes pressurised) from one chamber 50 to the other chamber 52. The end of the operating rod 54 is linked by connecting rod 56 and crank 57 to the crank-shaft 58 of the Stirling engine.

In operation, heat is given up to the surface of the displacer 53 and recovered during the return stoke. This process is vital and referred to as "regeneration".

Adjacent to the displacer cylinder 51 is a working cylinder 60, and a gas tight piston 61 so that the reciprocating action of the piston 61 can be converted into rotary motion on the crank-shaft 58 by means of a co-operating crank 62 and connecting rod 63.

As will be seen from the illustration, crank-shaft 58 is common for both the displacer cylinder and working cylinder of the Stirling heat engine. There is purposely an angle of approximately ninety degrees between the cranks 57 and 62, such that the displacer crank 57 is leading the working crank 62 in rotation.

This phase lag allows time for the working fluid to change volume when moving from chamber 50 to chamber 52 in the displacer cylinder 51.

The displacer 53 comprising the regenerator absorbs heat as it moves the working fluid from hot chamber 50 to cold chamber 52, and then gives up this stored heat to the cooled working fluid as it passes back to the hot chamber 50.

Preliminary work with unpressureized Stirling engines indicates that advantages can be obtained when the cylinder assemblies are offset from the crank-shaft 58.

The resulting extension during the down strokes appear to be responsible for improved performance of the Stirling engine by allowing longer time for such critical energy transfer.

The working cylinder 60 may be offset while the displacer cylinder remains normal for some improvements to be gained in the working cycle of the Stirling engine.

There is potential for considerable exploratory work to improve the performance of the reciprocating Stirling engine by incorporating cylinder offset to the displacer cylinder and/or the working cylinder to utilise the asymmetric motions resulting from this application in conjunction with each other and the crank phase angle whether in pressurised or unpressurised form.

The rotation direction has, of course, to be taken into account when -examining the combination of these factors. The illustration of Fig. 8 shows both the displacer and working cylinders offset by an amount equal to half the piston stroke.

For purposes of clarification the cylinders 51 and 60 are shown side-by-side, but in practice cylinder 60 would really be positioned behind cylinder 51 in the drawing, both connecting rods 56 and 63 being attached through their respective crank links 57 and 62 to the common crank-shaft 58.

The advantages accruing from the employment of a structure according to the invention has the benefits that engine manufacturers are required to implement only minimal tooling alterations in order to change current normal cylinder engines to extended offset cylinder engines, with the added benefit that no extra components are required.

Claims (5)

1. A reciprocating piston engine comprising at least one piston and co-operating cylinder, the piston attached to a crank-shaft by means of a connecting-rod for the purpose of translating reciprocating motion into rotary motion, and where the longitudinal axis of the cylinder is laterally offset from a plane through the axis of rotation of the crank-shaft by an amount approaching, equal to or exceeding the half the piston stroke.

2. A reciprocating piston engine according to Claim 1 wherein the range of useful lateral offset is defined as being between forty percent to seventy percent of the piston stroke.

3. A reciprocating piston engine according to Claims 1 and 2 wherein the lateral offset of the longitudinal axis of the cylinder lies in advance of a plane passing through the rotary axis of the crank-shaft in the direction of rotation of the crank-shaft.

4. A reciprocating piston engine according to Claims 1 to 3 wherein the longitudinal plane of the con rod is substantially parallel with the longitudinal axis of the cylinder through the period about ninety degrees of crank movement after piston top dead centre during the power phase of the engine cycle.

5. A reciprocating piston engine according to Claims 1 to 4 wherein the valve operational period for the engine can be delayed and extended in accordance with the uneven cycle timing for the purpose of improved engine efficiency.