TWO AND FOUR STROKE COMPOUND DIESEL ENGINES WITH CVT DRIVE

Abstract: This invention relates to compound power units. A power plant or unit (10) comprises a reciprocating engine (11), having an output shaft (12); a turbocharging unit generally indicated at (13); a continuously variable transmission (CVT) (14) coupled between low pressure turbine (18) of the unit (13) and the shaft (12); and a control unit (15) for operating for CVT (14) to transfer power between the turbine (18) and the shaft (12) in dependence on the operating conditions of the plant (10).

Description: Two and Four Stroke Compound Diesel Engines with CVT Drive This invention relates to compound power units of the type wherein a turbocharged internal combustion engine has its turbocharger or lowest pressure turbocharger coupled to the output shaft of the engine via a continuously variable transmission (CVT) to supply excess power from the turbocharger to the output shaft. Although most proposals in this field are largely theoretical and it is recognised that such arrangements will improve the efficiency of such power units, they -s little to change the overall torque characteristics of the engine and particularly the substantial fall off in available torque at low engine running speeds. Further the major practical construction has required a separate power turbine, which is driven by the turbocharger and then itself drives the output shaft by means of a CVT. From ne aspect the invention consists in a compound power unit including a reciprocating internal combustion engine, having an output shaft, and at least one turbocharger connected to the engine output shaft through a continuously variable transmission means and means for controlling the continuously variable transmission means to transfer power between the turbocharger and the engine output shaft, in either sense, in dependence on the operating conditions of the unit. In a preferred embodiment the unit comprises two or UBSTITUTESHEET more turbochargers in series and the CVT means is coupled to the lowest pressure turbocharger. The control means may be responsive to the engine boost pressure, the engine speed or the output shaft speed or any combination of these. The control means may also control a variable valve timing mechanism in order to prevent excessive cylinder pressure. Compared with the equivalent conventional compound engine scheme in which, as already stated, a further separate power turbine is required, the scheme has the following advantages: 1. The third turbine is eliminated, leading to considerable mechanical simplification, and a much more compact layout. 2. In the conventional compound scheme, reduction of engine speed at full fuelling generally leads to a loss of boost pressure and a consequent rapid reduction of the power contribution of the separate power turbine, with a corresponding loss of thermodynamic efficiency and of output torque. In the proposed scheme the use of the CVT connecting the LP turbocharger to the engine output shaft enables the unit to maintain a high rotational speed, thus ensuring that boost pressure is maintained at a high level over the full engine speed range, and enabling the unit to deliver a high and continuously increasing torque as engine speed is reduced. Thus surprisingly using power from the engine to drive the turbocharger, at appropriate operating conditions, leads to an increase in the engine's efficiency rather than the decrease which would normally be expected. 3. The CVT performs the twofold function of either transmitting excess power from the LP turbocharger to the engine output shaft, or alternatively when there is a power deficit in the LP turbocharger, to transmit power in the reverse direction. However, in either case the magnitude of the power thus transmitted is small in relation to the power devel-*-ed in the LP turbine, so that a small and comp«- .ε CVT with correspondingly low losses can be used. 4. The scheme offers much greater operational flexibility than either turbocharged or conventional compound schemes with a separate power turbine as a result of the higher and continuously increasing torque level with reducing engine speed. This will result in simplification of the transmission system with fewer gear ratios being required. 5. The ability to vary LP turbocharger speed relative to engine speed will result in improved transient response and easier engine starting. 6. The ability to vary LP turbocharger speed relative to engine speed also provides a means of SUBSTITUTESHEET controlling air-fuel ratio in such a manner as to reduce or minimise emissions compared with a turbocharged or conventional compound engine in which such a facility does not exist. From another aspect the invention consists in a compound power unit, an internal combustion engine and at least one turbocharger connected to the engine output shaft through a continously variable transmission (CVT) , so that power can be transferred between the engine and the turbocharger in either sense. The invention may be performed in various ways and one specific example of a possible embodiment will now be described with reference to the accompanying drawings, in which:- Figure 1 is a diagrammatic layout; Figure 2 is a torque-speed curve illustrating the performance of such a power plant; and Figure 3 is a notional control schedule for the LP turbocharger CVT. A power plant is generally indicated at 10 in Figure 1 and comprises a reciprocating engine 11, which may be two stroke or four stroke, having an output shaft 12; a gearbox 12a; a turbocharging unit generally indicated at 13; a continuously variable transmission 14 coupled between the turbocharging unit 13 and the shaft 12; and a control unit 15. The turbocharging unit 13 receives exhaust from the engine 11 via pipe 16 and this drives a high pressure turbine 17 and then a series connected low pressure turbine 18. Each turbine has an associated compressor 19,20 which it drives via a respective shaft 21,22 such that air is drawn through an inlet 23, into the low pressure compressor 19, and then passed through an intercooler 24 to the high pressure compressor 20. This in turn feeds the pressurised air through an after cooler 25 into the engine 11. The pressure of the air entering the engine is monitored by a pressure transducer 26. The intercooler 24 and after cooler 25 are provided to give greater thermodynamic efficiency and reduced thermal loading. A speed transducer 27 is provided to monitor the speed of the output shaft 12 and the output of both this transducer and the pressure transducer 26 are fed to the control unit 15. The unit 15, which is conveniently a microprocessor, produces a control signal for the CVT 14 on line 28. The control unit 15 controls the CVT 14 to transfer power between the low pressure turbine 18 and the output shaft 12 in either sense in αependence on the operational conditions indicated by the transducers 26,27 and in accordance, for example, with a control schedule -uch as described below in connection with Figure 3. Referring to the output shaft torque-speed curves shown in Figure 2, curve A is typical of a 2 stage turbocharged engine giving approximately 25% torque back-up at 60% rated speed. The 'conventional 1 compound torque characteristic, curve B, shows a rather higher level of torque back-up at a TUTESHEET similar fraction of rated speed. The compound engine with CVT drive to the low pressure turbocharger and which is the subject of this Application has a greatly improved torque curve, C, with approximately 50% torque back-up achieved at minimum governed engine speed rather than at some intermediate speed level between minimum and rated speed. Figure 3 shows a notional control schedule for the CVT and valve mechanism, the former showing the progressive decease in CVT speed ratio defined as output speed to engine crankshaft sprocket input speed from turbocharger sprocket with reduction in engine speed, and also as a function of load. The control schedule for the valve mechanism, in this case for a four stroke engine, shows the progressive advance in inlet valve closing as a function of reducing engine speed only. For four stroke engines a variable valve timing mechanism 29 may be provided under the control of the control unit 15. This variable inlet closing prevents the build up of excessive cylinder pressure at high boost pressures. Although an essentially mechanical CVT is illustrated it is to be understood that hydraulic or electrical equivalents, which are equally applicable, are to be included in the description CVT. Equally the arrangement described is viable with a compound engine only having a single turbocharger. T

Claims: CLAIMS 1. A compound power unit including an internal combustion engine, having an output shaft, and at least one turbocharger connected to the output shaft through a continuously variable transmission means and means for controlling the continuously variable transmission (CVT) means to transfer the turbocharger and the engine output shaft, in either sense, in dependence on the operating conditions of the unit. 2. A unit as claimed in Claim 1, further comprising two or more turbochargers connected in series and wherein the CVT means is coupled to the lowest pressure turbocharger. 3. A unit as claimed in Claim 1 or Claim 2, wherein the control means is responsive to engine boost pressure, engine speed or output shaft speed or any combination of these. 4. A unit as claimed in any one of the preceding claims, further comprising a variable valve timing mechanism and wherein the control means controls the mechanism in a sense to control cylinder pressure. 5. A compound power unit wherein the CVT is electrical or electrically operable. 6. A compound power unit, an internal combustion engine and at least one turbocharger connected to the engine output shaft through a continuously variable transmission (CVT) , so that power can be transferred between the engine ITUTESHEET and the turbocharger in either sense. 7. A compound power unit substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: WALLACE FRANK JULIUS, WALLACE, FRANK, JULIUS
Applicants: F J WALLACE & ASSOCIATES LTD, F.J. WALLACE & ASSOCIATES LTD.
Application year: 1991
Main classification: F02B37
IPC codes: F02B3, F02B37, F02B41, F02B75