GB2118621A - Two stage i.c. engine supercharging - Google Patents

Abstract

A supercharger (10) has a centrifugal low pressure compressor (18) in flow series with a high pressure centrifugal compressor (24). The low pressure compressor (18) is driven by the engine exhaust gases via a turbine (20), and the high pressure compressor (24) is driven from the engine crank-shaft (16) via a speed increasing gearbox (26). Control of turbine and compressor inlet guide vanes 28, 30, 32 provides for a substantially constant boost pressure over at least the upper half of the engine speed range. <IMAGE>

Description

SPECIFICATION A compound supercharger for piston engines This invention relates to superchargers for internal combustion piston engines, and is more particularly concerned with superchargers for the engines of high performance motor cars.

High performance piston engines have depended upon raising the pressure of the fuel/air mixture at entry to the cylinders to increase the mass flow of charge induced at the maximum number of cycles per second that mechanical stresses will allow.

Early in the history of supercharging the induction pressure was raised by the ROOTS type displacement compressor. This was satisfactory for low boost pressures but inefficient for higher boost pressures because the Roots compressor does not carry out any internal compression within itself.

For this reason the Roots compressor tended to be overtaken by the Zoller type vane compressor which did compress the charge to a limited extent within the casing before delivery to the induction pipe of the engine. However, the Zoiler compressor is not a robust unit. Not only is the internal compression limited but it suffers from air leakage between the pumping vanes and the casing, difficulties of lubrication, and mechanical failure at high boost pressures.

To overcome these limitations designers went as far as placing Roots and Zoller compressors in series. This had the disadvantage of making a bulky and heavy installation. This was critical in the case of the aero engine which must be as light as possible, and also inconvenient in racing car installations.

The centrifugal type of compressor is not only very much smaller and lighter for a given airflow (even allowing for the bulk and weight of the increasing gear), but is quite capable of providing the maximum boost pressure that a piston engine can withstand and, furthermore, at a significantly higher efficiency. It does, however, have one fundamental disadvantage and that is that the boost pressure that it generates increases roughly as the cube of the speed.

The consequence of this is that when an engine with a centrifugal supercharger reduces in rotational speed (at wide open throttle setting) the boost pressure and hence the engine torque and power fall away very rapidly. This is not much of a handicap to the aero engine which takes off, climbs and cruises within a limited range of crankshaft speeds, but it is a serious disadvantage to the racing car. The Roots and Zoller compressors, limited and inefficient as they were, did deliver a substantially constant boost pressure to the cylinder over a large part of the engine speed range.

The other method of driving the compressor is by a direct drive turbine placed in the exhaust from the cylinders and is known as a turbocharger.

This arrangement eliminates the mechanical drive from the engine crankshaft and increasing gear, and the features incorporated to avoid drive shock failure, and is called a "turbocharger".

Turbochargers were first developed for (a) aircraft engines to maintain power at altitude where the atmospheric pressure is reduced, (b) to increase the thermal efficiency of piston engines, especially the large oil or Diesel engines, by capturing waste energy in the exhaust.

Over the years these units have been developed and are used extensively on Diesel engines for electrical power generation, marine propulsion and they are now appearing as very small high speed units for vehicle engines. They were not very successful when applied to the aircraft piston engine because the return in additional power developed did not compensate for the increase in bulk, weight and complexity of the power plant installation. Also the improvement in installed specific fuel consumption at the cruising speed in flight was negligible when the gain in propulsion power from the ejector exhaust of a straight through exhaust system engine, such as the Rolls Royce Merlin V1 2 aero engine was properly allowed for.

Of course, thermal efficiency apart (and the theoretical potential gain is seldom attained), the practical attraction of the turbocharger is that it is a self-contained unit that can be adapted to existing engines to increase power output, whereas without it, a manufacturer may have to make more expensive design changes to increase power output to meet a customer's requirements (e.g. boring out the cylinders, if that were indeed possible). The turbocharger unit is a valuable "power matching device" with some potential for improved brake thermal efficiency especially in the case of two-stroke oil engines since these suffer from the disadvantage of the limitation of expansion ratio within the cylinder.

The adaptability of the turbocharger to existing engines is undoubtedly a major factor in their adoption, but there are problems and some of these are very relevant to high performance racing car engines: (a) When the turbocharger is properly matched to the high power requirements at maximum engine speed (e.g. as it is for the cruising speed of marine Diesels) it provides the increase in engine power and the best chance of increasing the brake thermal efficiency at this condition. When the engine is throttled back, however, the exhaust pressure from the cylinders falls, and the turbine speed and hence the turbocharger unit decreases in speed. The compressor pressure reduces and hence the engine boost pressure falls much as it does in the case of a mechanically driven centrifugal compressor.

(b) For engines that are required to operate over a range of crank RPM, particularly in the case of the motor vehicle engine, the turbocharger is matched to the engine at some fraction of the maximum crank RPM (e.g. 75 to 80%) to give the maximum permissible induction manifold boost pressure at that condition. Above that speed the boost pressure is kept substantially constant by means of a boost pressure sensitive control that opens a port in the engine exhaust ahead of the turbine in order to let down the exhaust pressure by by-passing a fraction of the exhaust gas past the turbine direct into the turbine exhaust or straight to atmosphere. This is called the waste gate.

It prevents the turbine increasing in speed as the engine crank RPM increases from, say 75% speed, to 100% speed to maintain substantially constant boost pressure. How effective this is in practice depends very much on the effective flow matching of the compressor and turbine with the cylinder inlet and outlet flows. Unless these units operate at working conditions on their pressureflow characteristics at efficient points, power is wasted.

(c) The inefficiencies introduced by wastegating to regulate boost pressure must cause some reduction in any gain in thermal efficiency that the turbocharger gave at the chosen matched condition (see (b) above).

(d) For high power output engines requiring high boost pressures the turbine driving power of the turbocharger can only be obtained if the cylinder exhaust back pressure is raised. This reduces the power developed in the cylinder which, of course, theoretically should be recovered by the turbine. However, due to the pressure losses between the cylinder at the end of expansion through the exhaust valves, ports and transfer exhaust ducting it is doubtful whether this power loss is recovered in the turbine, particularly with wastegate operation.

(e) High cylinder exhaust back pressure may inhibit scavenging effectiveness and cause cylinder and valve overheating. It discourages the attainment of the highest air flow through the cylinder required to give the highest possible net power output.

(f) For engines required to vary power quickiy involving rapid changes in crank rotational speed the turbocharger is known to lag on acceleration and lead on deceleration due to the rotational inertia. This not only introduces a compressor/engine flow-pressure matching problem (which could cause the compressor to choke or stall), but produces a situation of uncontrollable power over a small interval of time.

This phenomenon is known as "turboshaft lag".

The present invention seeks to provide a supercharging arrangement which avoids at least some of the limitations of the existing supercharging systems and offers unique advantages which are not present independently in the existing systems.

Accordingly, the present invention provides a compound supercharger for use with a piston engine, comprising a relatively high pressure compressor arranged to be mechanically driven by the engine, and a relatively low pressure compressor arranged to be driven by turbine means, the turbine means being driven by the engine exhaust gases, the compressors being arranged in flow series to receive atmospheric air and to discharge compressor air to the inlets of the engine cylinders, the performance and matching characteristics of the compressors being such that the pressure ratio of the supercharger remains substantially constant above a predetermined fraction of the engine maximum speed.

The compressors are preferably of the centrifugal type having a plurality of guide vanes at their respective inlets to control the angle and velocity of the incoming air.

The high pressure compressor can be driven from the engine crankshaft via a speed increasing gear box, and the low pressure compressor can have a vaneless diffuser.

The present invention will now be more particularly described with reference to the accompanying drawings in which, Figure 1 is a diagrammatic layout of one form of compound supercharger according to the present invention, and Figures 2, 3, 4 are performance curves of the compound supercharger of the invention, also showing the performance of existing superchargers.

Referring to the Figures, a compound supercharger 10 is shown in conjunction with an internal combustion piston engine 12, represented by a piston and cylinder 14 and a crankshaft 1 6.

The supercharger 10 comprises a low pressure (LP) centrifugal compressor 1 8 driven by a radial or axial turbine 20 via a shaft 22, and a high pressure (HP) centrifugal compressor 24 which is driven by the engine crankshaft 16 via a speed increasing gear box 26.

The supercharger delivers compressor air to the inlets of the cylinders 1 2 of the engine and the turbine 20 is driven by the exhaust gases of the engine.

A ring of variable guide vanes 28 is provided at the inlet to the turbine 20, and the inlets to the low and high pressure compressors both have a ring of guide vanes 30 and 32 respectively, all the guide vanes being provided to vary the velocity and direction of the gases entering the respective turbine and compressors.

In the specification and drawings P1 indicates the low pressure compressor inlet pressure, P2 the high pressure compressor outlet pressure, P3 the high pressure compressor outlet pressure and P4 the boost pressure at the engine cylinder inlets.

The two compressors 1 8, 24 are designed with pressure-flow characteristics that can operate at efficient working points without the need for "flow spillage", as is sometimes provided for in automobile engines.

In use, as engine crank RPM increases to increase air flow through the engine and increase power output the pressure ratio (PJP1) of the LP compressor 1 8 decreases and the driving turbine nozzle area will increase reducing the back pressure in the engine. This is beneficial to increasing engine charge flow and easing the temperature and pressure condition in the cylinder and valves during the scavenging interval.

The product of the pressure ratios of the two compressors in series (HP - PP2) and (LP - P2/P1) gives the overall pressure ratio P3/P into an after-cooler 34 and PdP, in the induction manifold (deducting cooler pressure loss (P3/P4)).

This is illustrated in Fig. 2, the air being to produce a substantially constant boost pressure ratio PdP, over at least the upper half of the crank speed range. Curves for the conventional gear driven centrifugal compressor and a two-stage type with variable inlet whirl and special boost regulating method are illustrated for comparison.

Although not shown, the exhaust turbine nozzle area variation is linked directly to the variable inlet whirl vanes 30 of the LP compressor 18, and these whirl vanes are considered as the main engine inlet throttle. A boost control unit (not shown) is provided to operate the vanes 28, 30 of the turbine 20 and compressor 1 8.

When the engine is running at a crank RPM equal to STAGE 1, but throttled, and increased power is required, the LP compressor inlet whirl vanes 30 are moved in the opening direction and the engine draws in increased air flow and corresponding fuel flow, from a boost pressure regulated carburettor or fuel injector (not shown).

The LP compressor 18 and driving turbine 20 increase in speed reaching STATE 1 at full throttle.

Engine crank RPM increases above STATE 1 and as the HP compressor pressure ratio increases (Fig. 3) the LP compressor 1 8 is regulated to decrease speed and pressure ratio by means of the variable vanes 28 in the axial turbine 20 which move progressively into settings of increased throat area allowing engine back pressure to reduce.

The compound supercharger according to the invention can be applied to almost any form of piston engine, e.g. single bank in line, multiple "flat twin" or a V cylinder arrangement.

In the latter case two liquid after coolers are envisaged, one to each bank of cylinders receiving compressor air from the two half volutes of the HP compressor, and a long driving shaft can be provided between the exhaust turbine and the LP compressor which are located at opposite ends of the engine.

The use of a compound supercharger of the present invention can provide a racing car which has a substantially constant full throttle engine torque output over a wide range of engine RPM which is particularly beneficiai for racing circuits.

Claims (9)

1. A compound supercharger for use with a piston engine, comprising a relatively high pressure compressor arranged to be mechanically driven by the engine, and a relatively low pressure compressor arranged to be driven by turbine means, the turbine means being driven by the engine exhaust gases, the compressors being arranged in flow series to receive atmospheric air and to discharge compressor air to the inlet of the engine cylinders, the performance and matching characteristics of the compressors being such that the pressure ratio of the supercharger remains substantially constant above a predetermined fraction of the engine maximum speed.

2. A compound supercharger as claimed in claim 1 having a compressor after-cooler located between the outlet of the high pressure compressor and the inlets to the engine cylinders.

3. A compound supercharger as claimed in claim 1 in which both compressors are of the centrifugal type and both have means at their inlets to vary the angle and velocity of the incoming air.

4. A compound supercharger as claimed in claim 3 in which the means to vary the angle and velocity of the incoming air comprises a number of variable angle guide vanes at each compressor inlet.

5. A compound supercharger as claimed in clairn 1 in which the high pressure compressor is driven from the engine crankshaft via a speed increasing gearbox.

6. A compound supercharger as claimed in claim 1 in which the low pressure compressor has a vaneless diffuser.

7. A compound supercharger as claimed in claim 1 in which the low pressure compressor speed is regulated by the turbine drive by means of variable throat area nozzle guide vanes.

8. A compound supercharger as claimed in claim 6 in which the turbine nozzle area is varied in combination with the variable inlet vanes to the low pressure compressor to maintain a substantially constant delivery pressure to the engine above a predetermined engine crankshaft speed.

9. A compound supercharger for use with a piston engine, constructed and arranged for use and operation substantially as herein described, and with reference to the accompanying drawings.