The invention relates to power units comprising an internal combustion engine having variable-volume combustion chambers supercharged by a turbocompressor unit having a turbine receiving the engine exhaust gases and comprising a bypass pipe through which the air not drawn by the engine can travel from the compressor to the turbine with a pressure drop which, if appreciable, is substantially independent of the flow rate and increases with the compressor outlet pressure.
While the invention is suitable for use with spark-ignited as well as with compression-ignited engines, with reciprocating piston as well as with rotary engines, it is particularly advantageous in the case of a power unit comprising a Diesel engine having a low volumetric ratio (below 12 and typically between 6 and 10) supercharged by a turbocompressor unit having a high compression ratio. The term "turbocompressor unit" should be interpreted as covering the case where there are several elements and/or stages, the air being cooled if necessary between successive compression elements.
Usually the compression ratio of the compressor is given a value in inverse proportion to the engine volumetric ratio, so as to obtain acceptable combustion pressures for the engine. For example, for an engine having a volumetric ratio of 7, the compression ratio of the compressor will normally be between 6 and 8.
Power units of the kind defined hereinbefore are described in French Pat No. 2,179,310 and in the corresponding U.S. Pat. No. 3,988,894 assigned to the assignee herein. In such power units, the turbocompressor unit can operate like a gas turbine near the surge line, and therefore with high efficiency. If an auxiliary combustion chamber is provided upstream of the turbine, the compressor can operate independently of the engine irrespective of the engine operating conditions (more particularly when the engine is at a stop) and the supercharging pressure can be adjusted by metering the fuel flow rate into the auxiliary chamber.
It is an object of the invention to provide an improved power unit of the above type. It is another object to provide a power unit having a reduced specific consumption, adapted to operate satisfactorily under different conditions (inter alia during idling and low-power operation). It is a more specific object to provide a power unit wherein suitable matching of a turbo-compressor selected so as to be well matched to the engine for operation under the rated conditions (i.e. at high supercharging pressure) is retained at low supercharging pressures.
Power units are known which comprise a super-charged internal combustion engine (without a bypass pipe which is open during on-load operation) and arrangement for facilitating operation at low power (U.S. Pat. No. 2,633,698). That arrangement provides for heating the air entering the engine when starting up at low ambient temperature. Air can be heated before entering the engine by burning fuel, but this method is neither efficient nor economic, since the heating must be intense at low ambient temperatures and the available torque is also very low, since the air intaken by the engine has a lower density and is impoverished in oxygen. A fraction of the engine exhaust gas or of the combustion gases leaving the auxiliary chamber can be recycled to the engine intake, but this method is complex and requires permanent maintainance, since devices for recyling hot gases tend to clog up and it is difficult to build valves which operate satisfactorily if subjected for long time periods to gases at a high temperature. Finally, the air can be heated by compression on starting or during low-power operation, simply by supplying the auxiliary combustion chamber with fuel at a rate sufficient for the supercharging pressure not to fall below a given .music-flat.threshold" value. However, the "threshold" pressure must be high if the motor has a low volumetric ratio and if the power unit has to operate at a low ambient temperature. Consequently, this method is uneconomic, since it results in high fuel consumption in the auxiliary chamber. In addition, the engine starter has to be made larger so as to crank the engine at a high supercharging pressure.
According to an aspect of the invention, there is provided a power unit comprising an internal combustion engine having variable volume combustion chambers, a supercharging turbocompressor unit having a compressor and a turbine, the inlet of said turbine being connected to receive the exhaust gas of said engine, pipe means connecting the outlet of the compressor to the intake of the engine, permanently open bypass conduit means having an inlet and an outlet connected to flow the air delivered by said compressor and not drawn to the engine to the inlet of said turbine with a pressure drop which, if appreciable, is substantially independent of the flow rate in the bypass conduit and increases with the output pressure of the compressor, and heat exchanger means between pressurized air flow from the compressor to the engine and bypass conduit means and the gas flow from the turbine outlet to atmosphere.
The aforementioned arrangement combines the known advantage of a heat exchanger in regenerative operation, i.e. reduced fuel consumption in the auxiliary combustion chamber and silencing of the turbine exhaust, with heating of the air intaken by the engine, without the need to immerse valve means in hot exhaust gases.
The heat exchanger may be swept by the entire flow of air travelling from the compressor to the engine and the bypass pipe. An air cooler provided with control and regulating means may be disposed in the path of the air travelling from the heat exchanger to the engine intake manifold. By modulating the heat transfer in the air cooler, e.g. by varying the flow rate of cooling fluid (air or liquid) through it, the temperature of air entering the engine can be maintained at a level above the self-ignition threshold in the engine combustion chambers, but can be kept low enough to avoid the harmful effect of reducing the air density and overheating the engine at high power. The modulation can be made without using components exposed to high temperature, and the system is largely self-regulating.
It can be seen that the arrangement according to the invention is paradoxical, since the air leaving the compressor first circulates through a heat exchanger for heat exchange with the turbine exhaust gases (a process used in prior art installations for heating the air) and then circulates through an air cooler before it is drawn by the engine. The arrangement will undoubtly be considered as erroneous by a technician in the relevant field, who at first sight would reject it. However, surprising advantages of the invention can be determined by attentively examining operation under all conditions and by taking account of the fact that, at low power, heat transfers in the cooler are reduced to a minimum, whereas at high power the air is hardly heated at all or is even cooled in the exchanger, whose function in the power unit of this invention is appreciably different from the function of an air heater supplied by a fraction of the gases leaving the turbine in a conventional installation.
To obtain such advantages, control and regulating means are typically provided which:
put the cooler out of action during starting, idling and low-load operation (e.g. as long as the pressure of the air intaken by the engine is below a predetermined value) and
subsequently give the cooler an efficiency which is substantially proportional to the engine speed and the pressure of the air intaken by the engine.
The invention is of particular interest in the case of a power unit comprising a turbocompressor having a high rated pressure ratio (about 6) and a high overall efficiency (the overall efficiency being equal to the product of the isentropic efficiencies of the compressor and turbine, and of the mechanical and volumetric efficiency, and should be considered high when it exceeds approx. 0.6), associated with an engine having a low volumetric ratio. Then, as will be shown hereunder, the heat exchanger acts as an air heater during low-power operation (corresponding to low supercharging pressures, where it is necessary to heat air entering the engine for the purpose of self-ignition) but operates as a cooler when air enters the engine at high power, since the temperature of the turbine exhaust gases is below the temperature of the air leaving the compressor, so that, in order to obtain maximum power, the air from the compressor must be cooled before entering the engine. Consequently, the air cooler, which is disposed upstream of the engine intake manifold, can be designed to have a lower transfer power than would be required from a cooler acting alone, i.e. capable of lowering the temperature of the air delivered by the compressor to a temperature acceptable for the engine under the most unfavourable conditions, i.e. at high power. Finally, by an effect which may be termed "thermal" obstruction of the engine intake, the invention ensures that the compressor remains matched with the engine, which constitutes a positive displacement machine, down to low values of the supercharging pressure.