GB2154280A - I.c. engine-driven positive displacement superchargers - Google Patents

Abstract

A freewheel 2, 3 or fluid coupling permits the charger 1 to exceed its input drive speed in response to engine suction. A heat exchanger downstream of the charger 1 may be a mass of steel wool or include passages for intake air, supplying the charger, or vehicle passenger compartment air. The charger 1 may be fed by a turbocharger compressor. A valve 11 may control the proportions of unheated and exhaust or radiator heated air supplied to the charger 1 through a rotatable throttle valve 6. <IMAGE>

Description

SPECIFICATION Positive displacement superchargers for in ternal combustion engines The invention relates to combustion air control systems for internal combustion engines, and has the general object of utilising the energy which is otherwise lost at the throttle valve of conventional systems.

It is known for internal combustion engines, of the type having their power output deter mined by the quantity of air involved in each cycle, e.g. spark ignition (S.l) types, to have a supercharger system in which a variable throughput compressor is used to control the air flow and so the engine power output.

Such a compressor may be driven from the engine by a constant ratio drive, as disclosed for instance by British Patent 665424.

The situation in need of improvement is the rate of change of air supply to the engine during sudden increase of engine load.

A major fault with pressure charging by an exhaust driven turbocharger is that on decreasing engine load, the turbocharger speed drops, and with it the charge pressure. Thus when a higher load is required, the engine accelerates in a low pressure charge condition and the response to the demand is delayed.

Using an engine-driven supercharger, the charge pressure ratio can be maintained if the supercharger draws its air through a conventional air throttle valve. The opening of this valve gives a full charge of air to the supercharger and so superior acceleration can be obtained. The supercharger takes power at the low engine loads in proportion to the pressure increase across it, which increases the engine fuel consumption under cruise and light load conditions. Therefore, it is not economic to use the high supercharge pressure ratio wanted for peak acceleration.

Ideally what is required is control of the quantity of air delivered to the engine without the losses associated with the pressure rise through the supercharger brought about by the use of a throttle valve type power control.

The use of a "variable" throughput type supercharger typified by British Patent 665484, permits operation without a throttle valve, since by having air ports which can be adjusted during a time period instead of throttling, the supercharger can be made to deliver any required quantity of air, and so suit any engine load. On changing the setting to increase engine load, a rapid response is still achieved, compared with turbocharging, but without the high engine fuel consumption of the throttled system at light loads. A limitation with all supercharging systems, is set by the ratio of the volumetric throughputs of the supercharger and the engine, and the volume of the engine intake system, which must be pressurised before there is full supercharge at the engine ports.

The rate at which an engine's intake volume is filled depends on the supercharger's capacity and speed and is normally fixed for any one application by the choice of the supercharger speed ratio.

The present invention proposes an internal combustion engine supercharging system in which an engine driven supercharger is coupled so as to run at a greater rotational speed than that provided by the drive from the engine at maximum supercharge.

This is possible when a variable throughput type of supercharger is used. At light engine loads there is a partial vacuum in the engine's intake system resulting from the throughput control setting which causes the engine to rotate the supercharger by application of reduced pressure thereto so that the supercharger, in effect becomes an air motor. Because of the small mass flow under these conditions, there is little air energy available for conversion to power. It will, however, still be greater than the friction losses of a well designed machine and it is further proposed in accordance with the invention to fit an overrun or freewheel device between the engine and the supercharger so that the latter can be sucked round faster than it would normally be driven.

Thus at light load the supercharger can run faster than it would if driven, and then, on a sudden demand for load, increasing the variable flow setting causes a greater than normal supply of air from the faster running supercharger until the kinetic energy of the free running parts has been used up. This gives improved engine acceleration, at low cost and with no worsening of the full load engine performance. Preferably, the overrun device is a free-wheel of the type having rollers in inclined slots, but the invention is not limited to this type. For large engines a fluid coupling, which could be emptied, and rapidly refilled, would be suitable and allow one component to serve two purposes.

The supply of air is determined by the pressure and temperature at which it enters the engine and this can most suitably be increased according to thermodynamic laws, by lowering temperature rather than by increasing pressure. It is a unique feature of running the supercharger as an air motor that it causes a temperature drop of the air supplied. It is proposed to use this to create a cold-source.

In one preferred form this cold-source is used to cool the charge between the supercharger and the engine during burst of acceleration to high load. In another form, the cold-source is used to cool the air before it enters the supercharger.

In a third form the cold-source is used to provide air-conditioning of other air, for example that from a passenger compartment.

In all these systems the overrun device, by permitting the air motor to run faster, requires the flow control valve to be closed further and so create a high expansion ratio and lower temperatures. This creates a more effective system.

When the supercharger has a high efficiency as an air motor, then running free could cause overspeeding. Power could then be taken from it by driving an auxiliary machine, or by feeding some power back to the engine to control the speed. In this latter case a variable speed drive should be used, from the engine to the supercharger, but this would be changed in the opposite sense to that used for known forms of supercharging, in that its speed ratio would increase with reducing engine load.

In the system of the Invention, work is done by the supercharger when running as an expanding machine. This lowers the temperature of the air charge, and under humid conditions icing may occur which could cause blockage of flow area, or sticking of moving parts.

It is within the scope of the invention to fit an air heater to avoid this overcooling. Such a heater can be, on a motor vehicle a small part of the vehicle radiator, or a jacket positioned so that the heat is taken from around the exhaust pipe. Either of these can be positioned in advance of air entry to the supercharger, and there may be by-pass ducts to permit the degree of heating to be kept to a minimum, as dictated by the ice formation or fuel condensation limits acceptable to the engine.

Such a system is best suited to engines which run for long periods at very light loads, for example emergency generating sets.

In vehicle applications, where load changes are frequent and rapid, with very few periods of sustained top power, advantage can be taken of the transient conditions to further improve the engine performance, by putting a heat exchangerbetween the supercharger and the engine. In its simplest form this may be a "passive" heat exchanger, that is one without a secondary fluid for example a bundle of steel wool. When running at light load this will become very cold and then act as an air cooler when the engine output is increased.

This will raise its temperature so that it becomes an air heater when the load is next reduced. Both these processes improve the engine's fuel consumption and extend the time over which the supercharger provides "free" supercharge, whilst free-wheeling down to the 'drive' speed.

According to a further feature of the invention a heat exchanger may be fitted after the supercharger, to form either a hot or cold source, depending upon the temperature of the charge leaving the supercharger.

Thus there may be a passive heat-exchanger between the supercharger and the engine.

The provision of air-condition in vehicles is restricted by the high cost of providing a cold source, for compartment air cooling. The use of a fluid heat-exchanger between the supercharger and the engine provides such a source, and with valving to limit the operation to periods when the supercharger delivery is below ambient provides a solution to the problem of providing air-conditioning.

According to a further feature of the invention there may be a secondary fluid heatexchanger between the supercharger and the engine. In one form this second fluid is used for air-conditioning.

For conditions which demand high acceleration for short periods, the cold source is used to cool the charge of air before it enters the supercharger. By this means the mass flow of the supercharger is increased, and it is chiefly on mass flow that the engine's torque is dependant. Mechanical efficiency is important too, and this increases as the cooler charge reduces the work per unit of mass flow. Uses include bull-dozers and city buses for example.

The invention may further provide a secondary fluid heat-exchanger between the engine and the supercharger in which the secondary fluid is the charge air before it enters the supercharger.

The invention is hereinafter described by reference to the accompanying drawings of a preferred embodiment and a modification thereof. In these drawings: Figure 1 is a simplified end view of a supercharger installation; Figure 2 shows the air intake and heat exchanger arrangement of Fig. 1; whilst Figure 3 shows a modified air intake and heat exchanger arrangement.

Referring to Figs. 1 and 2 there is shown a supercharger 1 driven by an engine through a belt 20 from a pulley 2 attached to an extension of the engine crankshaft by a free-wheel hub 3. The hub, of known type, contains rollers on inclined planes, which wedge against an outer ring, in the drive direction, and drop back into slots in the hub when the ring turns faster than the hub.

The supercharger shown is of the variable throughput type as described in British Patent 665484 and has interconnected rotors 4 and 5. In operation, air entering the rotor 5 via a rotatably adjustable valve 6 may, depending on the position of adjustment of this valve, wholly or partially fill the working space. If partially filled, then the air charge is expanded to the full volume later to be re-compressed to a volume matching that of the engine, and delivered through an exchanger 8, to the engine cylinder(s) 9. Fuel may be injected into the air charge, before, during or after the supercharger without departing from the desired effects of the invention.

In hot climates the air intake to the supercharger is by way of an air filter 10, a flap valve 11 being in the 'B' position indicated by the dashed line in Fig. 1. In cold climates the valve 11 is turned to the 'A' position indicated by the full line and air is drawn from an exhaust operated heater 1 2. By means of a suitable control the flap valve 11 may be partially open to adjust the temperature of the air at 8 to suit the requirement of the mo ment. This would be determined by a lower limit set by the formation of ice to a dangerous extent, and an upper limit set by the detonation or power derating of the engine.

It is within the scope of the invention for the heat exchanger 8 to have as its heating fluid the air from the passenger compartment 1 3 of a vehicle, thereby providing a degree of refrigeration in hot climates. A fan 1 4 circulates air when its temperature is suitable for air-conditioning.

In operation of the system on a road vehicle it will be found that there are many periods when the power output of the engine is fluctuating rapidly, that is in terms of time in seconds, not hours. During such periods the air from the supercharger requires heating at light loads and cooling at high loads, in order to get the best performance.

It is within the scope of the invention for the heat exchanger 8 after the supercharger to have a high thermal capacity such that much or all of its working is without the supply of an external source of heat-exchange fluid. In practice the extent of external fluid heating, or cooling would be determined by the air-conditioning requirement of the vehicle.

Fig. 3 shows a form of the invention in which the secondary fluid of the heat-exchanger 8 is the working fluid itself. The air charge, after leaving the filter passes via a bypass 1 6 through the heat-exchanger 8 giving up heat at light loads. It then passes to the intake 1 7 of the supercharger, where it is expanded and cooled further before being warmed by passing through the heat-exchanger, and then to the engine. On a sudden demand for power the valve 6 (Fig. 1) is opened fully and the intake space filled with very cold air. It does, therefore, pass a greater than normal quantity of air for a given power taken, as determined by the pressure rise across the supercharger. This can be only a transient condition, until the heat-exchanger is warmed up by the supercharge air, and so is suitable for intermittent loads.When continuous high load is required a valve 1 5 is used to isolate the heat-exchanger and open the supercharger intake 1 7 to the air filter.

It is within the scope of the invention to use this superharging system for engines of other than spark ignition type. Diesel engines, when highly turbocharged have fuel systems which vary the fuel in proportion to the air flow quantity, as does the spark ignition engine.

Such engines can take advantage of this supercharging system when it is used in addition to the turbocharger, to give a greater rate of response to a demand for increased load.

In such an installation the turbocharger's compressor would feed the supercharger intake, and be adjusted to over-supercharge the engine, this being reduced to the correct value by the expansion through the supercharger.

On a sudden demand for power the intake valve 6 would be opened fully and a high supercharge pressure obtained for a short period during which time the increased air mass has also helped accelerate the turbocharging at a greater than normal rate.

Claims (11)

1. An internal combustion engine supercharging system wherein an engine-driven variable throughput supercharger is coupled so as to run at a greater rotational speed than that provided by the engine when running at maximum supercharge.

2. A system in accordance with Claim 1 wherein the coupling between the supercharger and the engine is an overrun device and the supercharger can be rotated more rapidly by air induction derived from the engine than by the mechanical engine drive.

3. A system in accordance with Claim 2 wherein the overrun device is a freewheel.

4. A system in accordance with Claim 2 wherein the overrun device is a fluid coupling.

5. A system in accordance with Claim 1 wherein there is a variable speed drive controlled so the speed of the supercharger increases as engine load decreases.

6. A system in accordance with any of Claims 1 to 5 including a heat exchanger between the output of the supercharger and the engine for optimising the temperature of the fluid delivered by the supercharger.

7. A system in accordance with Claim 5 wherein the heat exchanger is a passive heat exchanger.

8. A system in accordance with Claim 5 wherein the heat exchanger operates with a secondary fluid.

9. A system in accordance with Claim 8 wherein the secondary fluid is the charge air before it enters the supercharger.

10. A system in accordance with Claim 8 wherein the secondary fluid is air from a passenger compartment of a vehicle in which the system is installed.

11. A system in accordance with any of the preceding claims which includes valve controlled means for controlling the temperature of the air which is admitted to the supercharger.

1 2. A system in accordance with any of the preceding claims and including an engine turbocharger wherein the compressor of the turbocharger feeds the supercharger intake.

1 3. An internal combustion engine supercharging system substantially as hereinbefore described with reference to, and as shown in Figs. 1 and 2 or Figs. 1 and 3 of the accompanying drawings.