WO1994028299A1

WO1994028299A1 - Exhaust gas recirculation in two stroke internal combustion engines

Info

Publication number: WO1994028299A1
Application number: PCT/AU1994/000288
Authority: WO
Inventors: David Richard Worth
Original assignee: Orbital Engine Company (Australia) Pty. Limited
Priority date: 1993-06-02
Filing date: 1994-05-31
Publication date: 1994-12-08
Also published as: CN1120362A; JPH09504848A; CA2162037A1; EP0704021A1; EP0704021A4; KR960702056A; BR9406493A; TW289783B

Abstract

A method of operating a two stroke cycle crankcase scavenged internal combustion engine comprising selectively delivering exhaust gas from a location (6) downstream of an engine exhaust port (5) to a crankcase (7) associated with at least one engine cylinder (1) and into an induction system of the engine upstream of an entrance (4) to the crankcase. An engine with means to use this method is also claimed.

Description

EXHAUST GAS RECIRCULATION- IN TWO STROKE INTERNAL COMBUSTION ENGINES

This invention relates to internal combustion engines operating on the two stroke cycle and to the management of the combustion process thereof to control the level of contaminants in the exhaust emissions.

It has in the past been recognised that conventional two stroke cycle engines exhibit poor performance in the area of fuel consumption and also in the area of the level of harmful emissions in the engine exhaust. However, there are substantial benefits to be obtained by wider use of engines operating on the two stroke cycle, firstly, because of their relatively simple construction, and secondly, because of their relatively small physical size and resultant high power to weight ratio. There has accordingly been considerable development in recent years directed to the control of the combustion process of two stroke cycle engines in a manner to reduce the level of emissions in the exhaust, and/or reduce the fuel consumption.

It has been recognised that the introduction of exhaust gas back into the fuel/air mixture prior to the ignition thereof can contribute to a reduction in the production of NO_x (oxides of nitrogen) during the combustion process, as the presence of exhaust gas in the fuel/air mixture reduces the resultant temperature and pressure in the engine cylinder resulting from the combustion, which is contrary to the high temperature and pressure conditions that promote the creation of NO_x. This process of mixing exhaust gas with the fuel/air mixture is commonly referred to as exhaust gas recirculation (EGR) and is typically achieved by bleeding a controlled quantity of exhaust gas from the exhaust system into the air induction manifold of the engine.

Although this procedure has been used successfully in four stroke cycle engines, it is not as effective when applied to two stroke cycle engines, due principally to the low level of vacuum existing downstream from the conventional throttle in the air induction system thereof. This would typically result in a low rate of intake of exhaust gas and in the case of a multi-cylinder engine, a poor distribution of the exhaust gas in the induced air. Further, particularly in the case of a crankcase scavenged engine, there is a significant time lag in the response by the engine to the introduction of the exhaust gas to the induction system due to the distance it is required to travel before entering the combustion chamber, particularly at low loads. Also, where exhaust gas is introduced into the induction system of a crankcase scavenged two stroke cycle engine, as the air capacity of the crankcase is greater than that of the cylinder, there is a further time lag in a change in the rate of exhaust gas supply to the crankcase being seen in the engine cylinder.

It has been previously proposed in publications such as International Patent Publication WO 79/00757 by J.P. Soubis and United States Patent No. 3581719 by Gau to recycle exhaust gas into the combustion chamber for the purpose of returning thereto any unburnt fuel that may be in the exhaust gas. It has long been recognised that carburetted two stroke cycle engines exhibit the problem that part of the air and fuel charge that enters the combustion chamber passes out through the exhaust port prior to the commencement of combustion. The above referred to prior patents each are directed to overcoming this problem by re-directing the fuel rich portion of the exhaust gas back into the air intake or crankcase for recycling into the combustion chamber. As the recycled portion of the exhaust gas is primarily fuel and fresh air, it will not have a significant effect in the control of the generation of NO_x, the problem the present invention is directed to overcoming.

It is proposed in United States Patent No. 4213431 by Onishi to recycle exhaust gas into the air induction system of a two stroke cycle engine with the intent of controlling the generation of NO_x during the combustion process. In this proposal, the exhaust gas is introduced directly into the air induction passage upstream of the carburettor. A valve is provided in the duct conveying the exhaust gas to the induction passage, the valve being temperature activated.

There is disclosed in the applicant's copending Patent Application No. PCT/AU94/00009 a method of introducing exhaust gas into the combustion chamber of a two stroke cycle engine whereby effective control of the amount and distribution of the exhaust gas can provide beneficial results in the management of exhaust emissions. In this prior application, the disclosed method of operating a two stroke cycle crankcase scavenged internal combustion engine comprises selectively delivering exhaust gas from a location downstream of the engine exhaust port directly into the engine crankcase to be delivered together with air in the crankcase to an engine combustion chamber, and controlling the quantity of exhaust gas delivered to the crankcase each engine cycle in accordance with engine operating conditions.

As the admission of the exhaust gas to the crankcase is principally for the control of exhaust emissions, it is not necessary, and in some circumstances can be undesirable, to admit exhaust gas to the crankcase under all operating conditions. Accordingly, in the invention of this prior application, it is desirable to be selective in the introduction of the exhaust gas to the crankcase and also in controlling the rate of supply of exhaust gas. This control of the exhaust gas supply to the engine can be achieved by an ECU managed control valve provided between the exhaust system of the engine and the engine crankcase to control the exhaust gas flow to the crankcase. The ECU managing the control valve preferably receives inputs regarding engine operating conditions and in particular, engine load, speed and temperature conditions and from these inputs determines when exhaust gas is required to be introduced to the crankcase and the quantity thereof required. Preferably, the control valve incorporates a position feedback means to indicate to the ECU the actual position of the valve to thereby permit comparison of the actual position thereof with the required position thereby enhancing the accuracy of the control of the rate of supply of exhaust gas to the crankcase. Alternatively, the ECU can be programmed to determine the actual mass of exhaust gas delivered to the crankcase each cycle and to compare that mass with the required mass of exhaust gas for the existing engine operating conditions. That is, the mass of air and exhaust gas in the crankcase can be determined by measuring the temperature and pressure therein at a preset point in the engine cycle when the volume of the space in the crankcase is known.The difference between this calculated gas mass and the mass of air entering the crankcase determined by, for example, an air flow sensor in the intake manifold, would give the mass of recirculated exhaust gas in the crankcase. Any correction required can then be effected by adjustment of the rate of supply of exhaust gas to the crankcase via the ECU controlled valve. Further, the accuracy of this measurement process can be checked by comparing the calculated air mass in the crankcase with the air mass as determined by the air flow sensor when no exhaust gas is present in the crankcase.

Notwithstanding the above, it has been found that during medium to high load operation, use of the invention of the above mentioned prior application, under certain operating conditions, may result in insufficient exhaust gas being delivered to respective individual cylinder crankcases of a multi- cylinder engine within the time interval available per cylinder cycle. This is due in part to the pressure differential promoting the flow of exhaust gas into the crankcase of the respective cylinders which tends to be reduced as engine load increases. Also, due to the corresponding time limitation, it is difficult to achieve effective distribution of the exhaust gas throughout the air charge in the crankcase at high engine speeds.

It is the object of the present invention to alleviate or avoid one or more of the above described problems.

With this object in view, the present invention provides a method of operating a two stroke cycle crankcase scavenged internal combustion engine comprising selectively delivering exhaust gas from a location downstream of an engine exhaust port to a crankcase associated with at least one engine cylinder and into an induction system of the engine upstream of an entrance to the crankcase associated with at least one engine cylinder.

It is also proposed by the present invention to provide, in a crankcase scavenged two stroke cycle engine, means to selectively deliver exhaust gas from a location downstream of the exhaust port into the crankcase associated with at least one engine cylinder and into the induction system of the engine upstream of an entrance to the crankcase.

Preferably, the means to deliver exhaust gas is arranged so that the exhaust gas is delivered to the induction system in the proximity of the conventional throttle valve, and preferably at or adjacent to the upstream side of the throttle valve and downstream of any air flow sensor in the induction system. Preferably, a control valve is provided in a line conveying the exhaust gas to the induction system and that valve is ECU controlled in response to inputs related to engine operating conditions such as load, speed, crankcase pressure, and temperature so that the supply of exhaust gas to the induction system can be terminated if desired and/or the rate of supply of exhaust gas varied.

Conveniently, the amount of exhaust gas being recirculated may be calculated by including an air flow sensor at the entrance to the induction system and by calculating the total mass of air and exhaust gas in the crankcase at a selected point in the engine cycle. The crankcase gas mass can be determined by measuring temperature and pressure values in the crankcase at a particular point in the engine cycle.

It has been found that the introduction of the exhaust gas into the induction system in the vicinity of the throttle valve results in effective mixing of the exhaust gas with the air entering the induction system. This is particularly advantageous at medium to high engine speeds when relatively short cycle times exist, reducing the effectiveness of delivery of the exhaust gas directly into the crankcase of each engine cylinder. In this regard, it is believed that, such short cycle times, together with the low pressure differential driving the exhaust gas and the rapid changes in the flow direction in the exhaust gas supply and induction air supply to the respective crankcases, result in insufficient exhaust gas being mixed with the air in the crankcase to be supplied to the combustion chambers.

The introduction of exhaust gas into the induction system in the vicinity of the throttle valve, where at medium to high engine speeds there is substantial turbulence in the air flow, achieves improved distribution of the exhaust gas in the induced air, and hence more even distribution of the exhaust gas between the respective crankcases of each engine cylinder and hence between the combustion chambers thereof. As a consequence, improved control of the combustion process is achieved with a resultant improved control of exhaust gas emissions.

In a multi-cylinder two stroke cycle crankcase scavenged engine, where each cylinder typically has an individual crankcase compartment, a plenum chamber may be provided which communicates individually with the crankcase of each cylinder, with exhaust gas from one or more of the engine cylinders being provided to the plenum chamber. Under normal conditions, it may only be necessary to supply exhaust gas from one or some of the cylinders to the plenum chamber, even where a greater number of cylinders are supplied with exhaust gas from the plenum chamber. The exhaust gas for admission to the induction system may also be supplied from the same plenum chamber.

In part, the exhaust gas performs the emission control function by reducing the overall cycle temperature and pressure in the engine cylinder during combustion, as the exhaust gas has a higher specific heat than air and hence will reduce the overall cycle temperature of the gases in the combustion chamber. As high temperature is one of the requirements for the production of NO_x, this reduction in overall cycle temperature contributes to a reduction in the production of NO_x. It is also to be noted that, in stratified charge engines, the exhaust gas of a two stroke cycle engine typically has a higher oxygen content than that of a four stroke cycle engine and therefore more exhaust gas is required to be recycled in a two stroke cycle engine to receive a comparable level of NO_x control. Further, the cooler the exhaust gas before entering the combustion chambers of the engine, the greater the quantity thereof that can be put in. Accordingly, the use of a plenum chamber in a multi-cylinder engine can readily be made to contribute to a reduction in the temperature of exhaust gas delivered to the combustion chamber and, in addition, provision can be made to enhance the dissipation of heat from the plenum chamber to achieve a further temperature reduction. Also a heat exchanger can be incorporated in the path of the exhaust gas to the engine crankcase and/or induction system to further contribute to a reduction of the temperature of the exhaust gas prior to admission to the crankcase and/or induction system. Conveniently, the duct conveying the exhaust gas to the cylinder or cylinders or to the plenum chamber can be cooled by external fins or by liquid or water cooling, such as from the engine cooling system.

Respective valves may be provided to control the supply of exhaust gas to the engine crankcase or individual crankcase compartments and to the engine induction system. The control valves may be arranged in parallel to independently control the supply of exhaust gas to the crankcase or crankcase compartments and induction system or in series with the induction system control valve typically located downstream of the crankcase control valve. In the latter arrangement, exhaust gas is only supplied to the induction system while it is being supplied to the crankcase or crankcase compartments.

In one embodiment, a piston controlled port can be provided in the lower portion of the engine cylinder wall to communicate with the crankcase of that particular cylinder and also with the supply of exhaust gas, such as via the plenum chamber. Preferably, the port communicates with the exhaust system of the engine via an ECU controlled valve as previously described. The port is located so that it will be exposed whilst the piston is within a limited extent of movement on either side of the top dead centre position thereof which represents the period of minimum pressure (sub-atmospheric) in the crankcase. Conveniently, the port can be located so as to be open for approximately 40° to 60° of crankshaft rotation to each side of the top dead centre position of the piston stroke.

It may be convenient in some engine configurations to provide two or more such piston controlled ports to provide a relatively large flow area for the exhaust. gas into the crankcase. Preferably, the port or ports have the major dimension thereof in the circumferential direction of the cylinder to provide the maximum open port area during the restricted port open period. It has been found that the use of a piston controlled port to control the timing of admission of the exhaust gas results in improved equitable cylinder to cylinder distribution of the exhaust gas.

Preferably, the ECU managing the degree of opening of the valve or valves as previously referred to is programmed to control the valve(s) by reference to a speed/load based look-up map. The load may be plotted on a FPC (fuel per cycle) basis. In addition, the ECU preferably also responds to engine temperature since, for example, at some cold start conditions, the addition of exhaust gas can be detrimental to the engine operating stability. Usually at low loads, such as below about one quarter of the maximum fuelling rate, exhaust gas would not be provided to the combustion chambers of the engine as NOχ is not usually present in the combustion gases, and if present, is in insignificant quantities. Further, the inclusion of exhaust gas in the air charge in the low load range of engine operation will tend to promote instability and thus increase hydrocarbons (HC) at a period in engine operation when HC control is critical.

Still further, exhaust gas is typically added to the air charge as a function of the operating temperature of the engine. That is, exhaust gas is generally not recirculated whilst the engine is cold, such as on start up, but rather once the engine has warmed up.

However, even at low temperatures, exhaust gas may be added to the air charge at high loads to control emission.

In an engine where a catalyst unit is provided in the exhaust system, the exhaust gas can be taken from the exhaust system either upstream or downstream of the catalyst unit.

The method and apparatus as disclosed herein may be used exclusively to recycle combusted gas to the combustion chambers of an engine for the purposes of emission control or may be used in conjunction with other forms of exhaust gas recirculation such as, for example, those disclosed in the applicant's co-pending Patent Application No. PL 9163.

The invention will be more fully understood from the following description of two alternative arrangements of crankcase scavenged two stroke cycle engines. In the drawings:

Figure 1 is a diagrammatic representation of one embodiment of an exhaust gas recirculation system;

Figure 2 is a more detailed drawing of an alternative embodiment of an exhaust gas recirculation system; and Figure 3 is a longitudinal sectional plan view of a portion of a three cylinder engine incorporating the exhaust gas recirculation system as shown in Figures 1 and 2. Referring now to Figure 1 , there is illustrated diagrammatically a typical crankcase scavenged two stroke cycle engine having a cylinder 1 and a piston 2 connected in the conventional manner to a crankshaft (not shown) located in a crankcase 3. An air induction passage 4 communicates with the crankcase 3 via a reed valve assembly 11 in the conventional manner, and conventional transfer porting and passages are provided in the piston 2 and cylinder 1 for the transfer of air or an air/fuel mixture from the crankcase 3 to a combustion chamber 12 above the piston 2. An exhaust port 5 is provided in the wall of the cylinder 1 which communicates the combustion chamber 12 with the exhaust pipe 14 when the piston 2 has moved downwardly in the cylinder 1 a distance sufficient to uncover the exhaust port 5. The exhaust pipe 14 conveys exhaust gases to a suitable discharge location.

A bypass passage 15 communicates with the exhaust pipe 14 at a junction 6 to convey exhaust gas from the exhaust pipe 14 to the exhaust gas recirculation (EGR) plenum chamber 7 and/or an air induction system 20 of the engine. A valve 8, in the bypass passage 15, is operable to control the delivery of the exhaust gas from the exhaust pipe 14 to the EGR chamber 7, which is communicable with the crankcase 3 via a reed valve assembly 9. Thus, when the control valve 8 is open, exhaust gas will be supplied to the EGR chamber 7 and when the pressure of the gas in the chamber 7 is greater than that in the crankcase 3, exhaust gas will pass from the chamber 7 into the crankcase 3. The bypass passage 15 is preferably located to communicate with the exhaust pipe 14 at a high pressure area such as upstream of a main catalyst (not shown) in the exhaust pipe 14. Also, the valve 8 is preferably located close to the crankcase 3 to reduce lag in the response of the engine to adjustment of the valve 8.

The air induction system 20 incorporates a conventional throttle valve 21 to control the rate of air supply to the engine, that is to all cylinders 1 of the engine. The passage 22 communicates with the bypass passage 15 upstream of the valve 8 and with the air induction system 20 immediately upstream of the throttle valve 21. A valve 23 controls the supply of exhaust gas to the air induction system 20. The amount of exhaust gas admitted to the crankcase 3 and the air induction system 20 can be controlled by the valves 8 and 23 respectively and, subject to engine operating conditions, the control valves 8 and 23 may be open, closed, or occupy any intermediate position therebetween. Preferably, the operation of the control valves 8 and 23 is managed by an ECU 17 which receives input signals indicative of various operating conditions of the engine, and in accordance with a preset strategy, the quantity of exhaust gas admitted to the crankcase 3 and/or the air induction system 20 respectively per cylinder cycle is controlled. This control is conveniently achieved by way of a look-up map stored in the ECU.

Referring now to Figure 2 of the drawings, there is shown a partial cross section of a crankcase scavenged two stroke cycle engine with the crankcase and cylinder block in cross section and the cylinder head and injector equipment in full outline. For the purposes of clarity, the piston, crankshaft and connecting rod are not shown in Figure 2. Further, the cylinder 30 as shown in

Figure 2 may be considered as being representative of a single cylinder engine or one cylinder of a multi-cylinder engine. The engine is basically of conventional construction having a cylinder 30 in which a piston (not shown) reciprocates and is connected by a connecting rod (not shown) to a crankshaft shown diagrammatically at 31. The cylinder 30 has an exhaust port 32 and a plurality of transfer ports, two of which are shown at 33 and 34, to provide communication between a crankcase 35 and the cylinder 30, subject to the position of the piston within the cylinder 30 as per the conventional two stroke cycle principle. The exhaust port 32 communicates with an exhaust passage 36 which in turn communicates with an exhaust pipe 37 in the mouth of which are located conventional exhaust catalyst elements 38. Downstream of the catalyst elements 38, the exhaust pipe 37 communicates with a branch passage 39 which leads to a cavity 40 in the cylinder block. In Figure 2, the branch passage 39 communicates with the exhaust pipe 37 downstream of the exhaust catalyst elements 38. However, branch passage 39 may alternatively communicate with the exhaust pipe 37 upstream of the catalyst elements 38 where, typically, the pressure of the exhaust gas is higher, the temperature is lower and the oxygen content therein is lower.

Communication between the passage 39 and the cavity 40 is under the control of a valve element 41 being part of a solenoid valve mechanism 42. The cavity 40 communicates with a plenum chamber constituted by internal duct 43 provided in the cylinder block which, in a multi-cylinder engine as shown in Figure 3, communicates with each cylinder 30 of the engine individually through an individual exhaust gas recirculation port 45. The ports 45 are located within the wall of the cylinders 30 so that, during a selected portion of each cylinder cycle, the ports 45 are uncovered by the pistons to permit exhaust gas to flow from the cavity 40 into the respective crankcase 35 of each cylinder of the engine.

As previously indicated, the preferred timing of the opening and closing of the EGR port 45 is within a range of 40° to 60° before and 40° to 60° after the top dead centre position of the piston in the respective cylinder 30. It will be appreciated that the communication between the crankcase 35 and the EGR port 45 may be determined by the location of an appropriate aperture in the skirt of the piston. This is common technology in relation to the control of the flow of gases through the transfer ports of two stroke cycle engines such as the transfer ports 33 and 34 as shown in Figure 2. Further, it is preferred that the port 45 is located on the side of the cylinder 30 against which the piston is thrust so as to effectively seal the port 45 when required.

The cavity 40 also communicates via the passage 46 with the air induction passage 47 through which air is drawn into the respective crankcases 35 of each cylinder 30. The passage 46 delivers exhaust gas to the air induction passage 47 immediately upstream of the throttle valve 49 that controls the rate of air intake to the engine. The passage 46 is provided with a valve element 50 which controls, in association with valve element 41 , the operation of which will be described hereunder, the quantity of exhaust gas supplied to the air induction passage 47. Valve element 50 may be controlled in a similar manner as valve element 41.

It is also to be understood that the air induction passage 47 may connect directly with the exhaust pipe 37 or branch passage 39 and independently of the cavity 40. An independent control valve may be provided in the air induction passage 47 of such an independent construction. This construction would enable exhaust gas to be supplied in a controlled manner to the crankcase 35 and the induction system 47 individually or in combination.

As referred to previously in the specification, it is desirable to control the delivery of exhaust gas through the EGR port 45 and to the air induction passage 47 in accordance with variations in engine operating conditions and for this purpose, the solenoid unit 42 which controls the operation of the valve element 41 is under the control of a programmed ECU incorporating an appropriate look-up map. Usually, the map is arranged so that the valve element 41 controlling the exhaust gas flow to the crankcase 35 and air induction passage 47 is ramped rapidly from closed to open once the fuelling rate increases above a selected level. The solenoid unit 42 is typically provided with a valve element position sensor which provides feedback information to the

ECU to facilitate the accurate control of the position of the valve element 41 to achieve the required rate of supply of exhaust gas to the engine. As carbon or other particle-like material may build up on the valve element 41 that will influence the movement of the valve element 41 , known compensation procedures can be incorporated in the ECU programme to take account of such influences.

As an alternative to the use of a look-up map to control the rate of supply of exhaust gas, the ECU can be programmed to determine the mass of air entering each crankcase 35 and to determine the combined exhaust gas and air mass in the crankcase 35 at a point in the cycle of that engine cylinder 30.

The mass of air entering the crankcase 35 can be determined by the conventional hot wire air flow meter in the air induction passage 47, and the mass of air and exhaust gas can be determined by measuring the temperature and pressure in the crankcase 35 at a preset point in the engine cycle where the volume of the space in the crankcase 35 is known. From these two mass determinations the actual mass of exhaust gas can be determined and compared with the required amount of exhaust gas, thus determining if adjustment is required to be made to the rate of supply of the exhaust gas. This method of determining the exhaust gas content within the crankcase 35 can be used in conjunction with other forms of control of EGR than that described herein. Other engine operating parameters that can be controlled, in conjunction with the supply of exhaust gas to the crankcase 35 and air induction system 47, include advance of the ignition spark to improve stability, and the use of a back pressure valve in the exhaust system, downstream of the point of exhaust gas take-off to control the rate of exhaust gas available for supply to the combustion chamber. The greater the back pressure in the exhaust system, the greater the pressure and hence the rate of supply of exhaust gas for admission to the crankcase 35.

Also, the exhaust gas to be delivered to the engine cylinder 30 can be passed through a heat exchanger or other cooling means prior to entry to the crankcase 35 in order to increase the density thereof whereby a greater mass of exhaust gas would then be available for delivery to the crankcase 35.

In Figure 2, the branch passage 39 communicates with the exhaust pipe 37 downstream of the catalyst element 38. However, it may alternatively communicate with the exhaust pipe 37 upstream of the catalyst element where the pressure of the exhaust gas is higher and the temperature lower.

In practice, it has been found convenient to provide multiple delivery locations for the exhaust gas to the plenum chamber to assist in achieving substantial uniform distribution of the exhaust gas from a common plenum chamber into the respective crankcase 35 of each cylinder 30. Also, in the higher range of operating speeds, the rate of rise of the crankcase pressure, after top dead centre position of the piston, is such that a reverse flow of air from each respective crankcase 35 through the ports 45 can occur. This can lead to a dilution of the exhaust gas in the plenum chamber and the possibility of unequal distribution of exhaust gas to the respective engine cylinders 30. This problem can be at least reduced by suitable selection of the length of the ports 45 between the plenum chamber constituted by internal duct 43 and the crankcase chambers, so that any reverse flow of gas from the crankcase 35 is substantially retained within the port 45 associated with that crankcase chamber 35 and not passed into the plenum chamber 43.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1. A method of operating a two stroke cycle crankcase scavenged internal combustion engine comprising selectively delivering exhaust gas from a location downstream of an engine exhaust port to a crankcase associated with at least one engine cylinder and into an induction system of said engine upstream of an entrance to said crankcase.

2. A method as claimed in claim 1 wherein said exhaust gas is delivered into said induction system of said engine in the proximity of a throttle valve.

3. A method as claimed in claim 1 or 2 wherein said exhaust gas is delivered from an exhaust system downstream of said engine exhaust port.

4. A method as claimed in claim 2 or 3 wherein said exhaust gas is delivered into the induction system at or adjacent an upstream side of said throttle valve.

5. A method as claimed in any one of claims 1 to 4 wherein the rate of supply of said exhaust gas to at least one of said induction system and said crankcase is controlled by a control valve located in a line conveying said exhaust gas to at least one of said induction system and said crankcase.

6. A method as claimed in claim 5 wherein said control valve is controlled with reference to an engine operating condition.

7. A method as claimed in claim 6 wherein said engine operating condition is selected from the group consisting of engine load, engine speed, crankcase pressure and crankcase temperature.

8. A method as claimed in any one of claims 1 to 7 wherein exhaust gas is delivered from said location downstream of the exhaust port of at least one engine cylinder to a plenum chamber from where said exhaust gas is supplied to at least one of said induction system and said crankcase.

9. A method as claimed in any one of claims 1 to 8 wherein said exhaust gas is cooled prior to introduction to at least one of said crankcase and said induction system.

10. A method as claimed in claim 8 or 9 wherein said exhaust gas is cooled while resident in said plenum chamber.

11. A method as claimed in any one of claims 3 to 10 wherein said exhaust gas is cooled while resident in a line conveying exhaust gas from said exhaust system to at least one of said induction system and said crankcase.

12. A method as claimed in any one of claims 1 to 11 wherein the timing of the delivery of said exhaust gas to said crankcase is controlled by the location of a port that is opened and closed in response to movement of a piston in said at least one engine cylinder.

13. A method as claimed in any one of claims 1 to 12 wherein the delivery of said exhaust gas to said crankcase commences at between 60° and 40° before the top dead centre point in the combustion chamber cycle.

14. A method as claimed in any one of claims 1 to 13 wherein exhaust gas is delivered to said induction system and said crankcase at medium to high loads of engine operation.

15. A method as claimed in any one of claims 8 to 14 wherein exhaust gas is supplied from said plenum chamber to said crankcase when said plenum chamber pressure exceeds the pressure in said crankcase.

16. A method as claimed in any one of claims 3 to 15 wherein the back-pressure in said exhaust system is controlled to vary the rate of supply of exhaust gas to at least one of said induction system and said crankcase.

17. A method as claimed in any one of claims 1 to 16 wherein exhaust gas is delivered into said induction system downstream of an air flow sensor located therein.

18. In a crankcase scavenged two stroke cycle engine, means to selectively deliver exhaust gas from a location downstream of an exhaust port of the engine into a crankcase associated with at least one engine cylinder and into an induction system of the engine upstream of an entrance to said crankcase.

19. The combination of claim 18 wherein said means to deliver exhaust gas is arranged so that exhaust gas is delivered into said induction system in the proximity of a throttle valve.

20. The combination of claim 18 or 19 wherein said means to deliver exhaust gas is arranged to deliver exhaust gas from an exhaust system downstream of said engine exhaust port.

21. The combination of any one of claims 19 to 20 wherein said means to deliver exhaust gas is arranged so that the exhaust gas is delivered into said induction system at or adjacent to an upstream side of the throttle valve.

22. The combination of any one of claims 18 to 21 wherein said means to deliver exhaust gas is arranged so that exhaust gas is delivered into said induction system downstream of an air flow sensor located therein.

23. The combination of any one of claims 18 to 22 wherein a control valve to control the rate of supply of exhaust gas is provided in a line conveying the exhaust gas to at least one of said induction system and said crankcase.

24. The combination as claimed in claim 23 wherein said control valve is controlled with reference to an engine operating condition.

25. The combination as claimed in claim 24 wherein said engine operating condition is selected from the group consisting of engine load, engine speed, crankcase pressure and crankcase temperature.

26. The combination as claimed in any one of claims 18 to 25 wherein a sensor means is located in said crankcase which, in association with a control unit, calculates the total mass of air and exhaust gas in the crankcase at a selected point in a cylinder stroke.

27. The combination as claimed in any one of claims 18 to 26 wherein a plenum chamber is provided to supply exhaust gas directly to at least one of said induction system and said crankcase.

28. The combination as claimed in claim 27 wherein said plenum chamber is provided with cooling means to cool exhaust gas resident in said plenum chamber.

29. The combination as claimed in any one of claims 20 to 28 wherein a line conveying exhaust gas from the exhaust system to at least one of said crankcase and said induction system is provided with cooling means to cool exhaust gas resident in said line.

30. The combination as claimed in any one of claims 18 to 29 wherein a port for entry of exhaust gas to said crankcase is included in each engine cylinder which is cyclically opened and closed in response to reciprocation of a piston in said cylinder.

31. The combination as claimed in claim 30 wherein said piston and said port are relatively arranged to commence opening of the port at between 60° and 40° before the top dead centre position of the piston in said cylinder.

32. The combination as claimed in claim 30 or 31 wherein a further port for entry of exhaust gas to said crankcase is included in said engine cylinder.

33. The combination as claimed in any one of claims 30 to 32 wherein each port has its major dimension in the circumferential direction of the engine cylinder.

34. The combination as claimed in any one of claims 30 to 33 wherein a control valve is associated with each port to control the rate of supply of exhaust gas to said crankcase.

35. The combination as claimed in any one of claims 23 to 34 wherein control valves associated with the crankcase and induction system are arranged in parallel.

36. The combination as claimed in any one of claims 23 to 34 wherein control valves associated with the crankcase and induction system are arranged in series.

37. The combination as claimed in any one of claims 20 to 36 further including back-pressure control means for controlling the back-pressure in said exhaust system thereby controlling the rate of supply of exhaust gas to at least one of said induction system and said crankcase.