GB2391263A

GB2391263A - Reciprocating piston engine

Info

Publication number: GB2391263A
Application number: GB0217655A
Authority: GB
Inventors: Julian George Daniel Branston
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-07-30
Filing date: 2002-07-30
Publication date: 2004-02-04
Also published as: GB0217655D0

Abstract

A reciprocating piston engine, which may run on a two-stroke cycle, comprises a piston 18, a connecting rod 17, a crank 16, a crankcase 14 and a cylinder 20 with an exhaust 25 and inlet 23, 36. The crankcase defines a crankcase chamber 14 which has a varying pressure in response to the reciprocating piston. The crankcase 14 is connected to a pump 24 by a connecting port 23 and the pump consists of a divided chamber 28 divided by a moving member 26 which responds to the pressure variations in the crankcase 14. The first side of the divided chamber is connected to the connecting port 23 and the second side has inlet 30 and outlet pods 32 and unidirectional valve means so that motion of the moving member 26 causes flow through the second side of the chamber. The pump 24 may be used with pressure reservoir 34 in order to introduce new mixture or charge into the combustion chamber 22 through inlet 36. An engine so arranged has the advantage of increasing the engine efficiency due to the elimination of the transfer port.

Description

1 239 1 263

r7n7C36cs Reciprocating Piston Engine The present invention relates to an engine and in particular. although not exclusively, to a two-stroke reciprocating piston engine.

5 It is known to provide a crankcase-scavenged two-stroke engine comprising a piston which reciprocates in a cylinder, the cylinder having a transfer port from the crankcase, and an exhaust port. The top of the exhaust port is located higher up the cylinder than the transfer port, so as to permit most of the combustion gases to escape before a new charge enters the cylinder via the transfer port. A subsequent charge enters the 10 crankcase on the upstroke of the piston, and is pushed into the cylinder when the transfer port reopens on the next down stroke of the piston.

In a typical crankcase-scavenged two-stroke engine the piston typically displaces 20% of the total swept volume between bottom dead centre and when the transfer port 15 closes. This volume represents a lost pumping opportunity.

Several problems are associated with the prior art crankcase-scavenged two-stroke

engines. The requirement for the transfer port represents an inefficiency of the induction cycle since little or no work can be obtained from the piston displacement 2() when the transfer port is open. The transfer port design of the prior art two-stroke

engine may therefore reduce the maximum power output from the engine.

Secondly the pumping efficiency of the piston in transferring the crankcase charge into the cylinder is reduced due to the volume of the transfer passages and ports, and the 25 swept volume of the piston when uncovering the transfer port. Furthermore the crankcase charge may become contaminated with crankcase oil, which then enters the combustion chamber, and results in increased emissions from the engine.

I'hese problems are all well known, and numerous solutions have been proposed to deal 30 with engine efficiency, charge contamination and pollution reduction due to unburned t'uel leaving the exhaust port.

(.. 2 What is required is an improved engine which can overcome the aforementioned problems, and maximise the opportunity for crankcase pumping.

According to the invention there is provided a reciprocating piston engine assembly including a cylinder with an inlet and an exhaust' a crankcase, a crank, a connecting rod and a piston, the crankcase comprising a closed chamber having a connecting port in a wall thereof' and the assembly further comprising a pump having a divided chamber therein. one side of said chamber being connected to said crankcase via said connecting 10 port, and the other side of said chamber having an inlet port, an outlet port, and valve means to ensure unidirectional flow therethrough, wherein said chamber is divided by a moving member responsive to variations in pressure in said crankcase to cause flow through said other side of said chamber.

15 In such an arrangement a rise and fall in crankcase pressure is an inevitable result of piston reciprocation, and the effect in the pump is to cause movement of the moving member, with consequent cyclical variation of the volume of said other side. The valve means ensure that unidirectional flow is a result, and consequently the pump can be arranged to provide a supply of fresh clean air, for example to the combustion chamber 20 of the engine. It will be appreciated that the moving member is a barrier to crankcase oil mist. Furthermore, in a two- stroke engine? the connection of the transfer passages to the crankcase may be eliminated so that the full displacement of the piston is used to 25 generate a cyclical pressure variation in the crankcase, which can be transferred to the pump. In addition, an engine so arranged reduces the unpowered displacement of the piston stroke, due to the transfer port being open in the prior art design, which may provide an

30 increase in the power output from the engine. Correspondingly the engine may have improved overall engine efficiency.

C 3 In the preferred embodiment the engine assembly comprises a single cylinder engine.

However a multi-cylinder engine may also benefit from the current invention provided that the crankcase is divided into substantially sealed chambers, one for each piston/connecting rod assembly.

I'referably the pump provides air under pressure to the engine, where it may be admitted to the combustion chamber by for example a cam operated poppet valve. More sophisticated valving is of course possible, including variable valve timing of the cylinder inlet and outlet, and such an arrangement is particularly effective in scavenging 10 of a two-stroke engine. In conjunction with an air inlet valve, the engine preferably uses fuel injection to ensure an accurate feel/air ratio over a wide range of operating . conditions. Air under pressure *om the pump may also be mixed with fuel upstream of the engine, 15 for example in a carburettor or indirect injection system.

In a further refinement of a two-stroke engine, air from the pump may be introduced into the exhaust as a pulse to both urge burnt gases down the exhaust tract, and to prevent a fresh fuel/air charge from passing to exhaust before combustion' thereby 20 mirroring the characteristics of prior art exhaust expansion chambers. Alternatively

addition combustion gases may be provided to the combustion chamber after the exhaust closes and before ignition.

Preferably the pump has a first plenum chamber downstream thereof. This allows the 25 fluid to be supplied for example to the exhaust or the combustion chamber on demand and without pressure pulsing due to the cyclical nature of pump operation.

In the alternative embodiment pressure pulsing of the pump may be used to advantage in a tuned inlet tract, so as to maximise the volume of air admitted to the cylinder on 30 each suction stroke.

An' 4 The pump may be arranged separately from, immediately adjacent or integrated in the crankcase. The separate location of the pump from the engine has the advantage that a cooler and thereby denser charge is provided to the cylinder than prior art engines using

a convention transfer port design. Any kind of moving member is possible, but 5 preferably a low friction member is preferred so as to reduce parasitic losses to a minimum. In the preferred embodiment the moving member comprises a bellows.

Advantageously the inlet port of the pump is in fluid communication with an air box, the air box being open to atmosphere. In an alternative embodiment the inlet port has a 10 venturi with a fuel supply to provide a charge for the combustion chamber.

A tan may be included upstream of the pump inlet port in order to increase inlet pressure' and thereby outlet pressure.

15 In accordance with another embodiment there is provided a second plenum chamber downstream of said first plenum chamber. The second plenum chamber operating at a higher pressure to introduce clean air into the inlet or exhaust at a higher pressure than the first plenum chamber.

20 In accordance with another aspect there is provided a reciprocating piston engine assembly having a flywheel, wherein the flywheel includes a cam profile thereon adapted to operate a reciprocating pump. Such a pump may be used to supply clean air under pressure, for example to the first or second plenum chamber.

25 Other features of the invention will be apparent from the following description of a

preferred embodiment shown by way of example only in the accompanying drawing, in which; - Figure I is a schematic representation of an engine according to the present invention.

30 - Figure 2 is a schematic representation of an engine according to a second embodiment of the present invention.

:' s Referring firstly to Figure I there is shown a schematic representation of a two stroke engine according to the present invention? generally designated 10. The engine 10 comprises a crankcase 12 which dehmes a crankcase chamber 14. The crankcase 12 houses a crank 16, the crank 16 being connected via a connecting rod 17 to a piston 18 5 which reciprocates in a cylinder 20. The piston 18 and the cylinder 20 together define a combustion chamber 22. The combustion chamber 22 has a fuel inlet 23, a fresh air inlet 36 and an exhaust 25.

The crankcase chamber 14 is in fluid communication with a pump 24 via a connecting 10 port 23. The pump 24 has a membrane 26 that reciprocates in a pump chamber 28. The pump 24 has an inlet 30 and an outlet 32. The inlet 30 is in fluid communication with an air box (not shown) having an air filter (not shown). The air box is open to atmosphere to provide a supply of clean and fresh air to the pump 24. Each of the inlet 30 and the outlet 32 has a one way valve (not shown) such as a reed valve. The outlet IS 32 from the pump 24 is in fluid communication with a plenum chamber or pressure reservoir 34. An electronic control valve 33 may also be provided between the pump 24 and the pressure reservoir 34. The pressure reservoir 34 is in fluid communication with an inlet 36 to the combustion chamber 22 and an inlet 38 to the exhaust 25. The inlet 36 to the combustion chamber 22 and the inlet to the exhaust 25 may also be provided with 20 electronic control valves 40 to control the flow of fresh air according to the timing of the engine.

As the piston 18 reciprocates in the cylinder 20 the pressure within the crankcase chamber 14 varies in a cyclic manner. This cyclic pressure change causes the membrane 25 26 to reciprocate within the pump chamber 28. The one way valves 30, 32 of the pump 24 allow the pump 24 to pump fresh air in response to the varying pressure within the crankcase chamber 14. The membrane 26 acts to separate the volume of gas in the crank case chamber 14 from the fresh air being pumped by the pump 24. This allows the oil contaminated gasses within the chamber 14 to be separated from the fresh air 30 being pumped by the pump 24. The pressure reservoir 34 acts as a source of pressurised fresh air which can be supplied on demand by the control valves 40 to either of the inlet 36 to the combustion chamber or the inlet 38 to the exhaust 25.

(A 6 second embodiment of the present invention is presented in Figure 2. Features common to the embodiment of Figure I are shown with like reference numerals. In this embodiment there is provided a second plenum chamber or pressure reservoir 35 5 between the pressure reservoir 34 and the inlet 38 to the exhaust 25. The second reservoir 35 may have an additional inlet 37 which is connected to a second pump (not shown)' tor example an electric pump or an engine driven pump such as a cam driven pump. The additional inlet 37 and the inlet 38 to the exhaust 25 may have electronic control valves 40. The second reservoir 35 is intended to operate at lower volume and 10 higher pressure than the first pressure reservoir 34 It will be appreciated that the second pump of the embodiment shown in Figure 2 may be used with the embodiment illustrated in Figure 1. In this instance the second pump my be used to increase the pressure of the pressure reservoir (34).

The inlet 30 to the pump 24 of Figures 1 and 2 may alternatively be connected to an indirect fuel injection system or a carburettor in order to pump a fuel/air mixture. lathe carburettor or indirect fuel injection may alternatively be located downstream of the pressure reservoir 34 or 35 on either or both of the inlet 36 to the combustion chamber 20 or the inlet 38 to the exhaust.

The control valves 40 of the embodiments illustrated in Figures I and 2 maintain an optimum pressure within the combustion chamber 22 depending on the engine load or the engine speed. For example, the control valve 40 on the inlet 36 may be used to 25 provide additional combustion gases to the combustion chamber 22 after the exhaust closes and before ignition. The control valve 40 on the exhaust inlet 38 may be used to push unburned combustion gases back into the combustion chamber when the exhaust 25 is open to the combustion chamber 22. The inlets 36. 38 may also be aimed or introduced into the combustion chamber more effectively to assist with purging the 30 unburned gasses.

An engine so described herein reduces the unpowered displacement of the piston stroke, due to the transfer port, which may provide an increase in the power output from the engine. Correspondingly the engine may have improved overall engine efficiency, power output, petrol consumption and exhaust emissions. Furthermore, since there is no 5 engine oil mist introduced into the charge the engine emissions may be reduced when compared to the prior art crankcase-scavenged two-stroke engine. lithe full displacement

of the piston is utilised in the pump 24. Furthermore the air or fuel/air mixture may also be thermally insulated from the engine to provide a cooler and, therefore, denser charge.

10 An engine assembly so constructed may also be cheaper to manufacture since the required casting of the cylinder 20 and internal transfer ports is reduced.

Correspondingly the tooling to manufacture the cylinder 20 is cheaper.

Whilst a preferred embodiment for the device has been described it will be appreciated 15 that many other designs of the engine exist that would have the desired effect of the invention with the proviso that the crank case is used to pump atmospheric air into the combustion chamber.

Claims

(A 8 Claims

1. According to the invention there is provided a reciprocating piston engine assembly including a cylinder with an inlet and an exhaust, a crankcase, a crank, a 5 connecting rod and a piston, the crankcase comprising a closed chamber having a connecting port in a wall thereof, and the assembly further comprising a pump having a divided chamber therein' one side of said chamber being connected to said crankcase via said connecting port, arid the other side of said chamber having an inlet port, an outlet port. and valve means to ensure unidirectional flow therethrough, wherein said 10 chamber is divided by a moving member responsive to variations in pressure in said crankcase to cause flow through said other side of said chamber.

2. An assembly according to claim I and adapted to connect said outlet port in {luid communication with the inlet of said cylinder.

3. An assembly according to claim I or claim 2 and adapted to connect said outlet port in fluid communication with the exhaust of said cylinder.

4. An assembly according to claim 3 wherein air from the pump is introduced into 20 the exhaust as a pulse.

S. An assembly according to any preceding claim wherein said moving member is a bellows.

25 6. An assembly according to claim 5 wherein the capacity of said bellows is substantially equal to the swept volume of said piston.

7. An assembly according to any of claims 1 - 4 wherein said moving; member is a diaphragm.

X. An assembly according to any preceding claim wherein said inlet port is connected to an air box.

it? 9 9. An assembly according to any preceding claim wherein said pump is connected to a first plenum chamber downstream thereof; said first plenum chamber having an inlet and an outlet.

5 1(). An assembly according to claim 9 and further including a second pump, said second pump having an inlet connected to an air box upstream thereof, and an outlet connected to the inlet of said first plenum chamber.

11. An assembly according to claim 10 wherein said second pump is an engine 10 driven pump.

12. An assembly according to claim 11 wherein said engine further comprises a flywheel, wherein said flywheel includes a cam profile thereon adapted to operate said second pump.

13. An assembly according to claim 9 and further including a second plenum chamber downstream of said first plenum chamber, wherein the second plenum chamber operates at a higher pressure than said first plenum chamber.

20 14. An assembly according to claim 13 and further including a second pump, said second pump having an inlet connected to an air box upstream thereof, and an outlet connected to an inlet of said second plenum chamber.

15. An assembly according to claim 14 wherein said second pump is an engine 25 driven pump.

16. An assembly according to claim 15 wherein said engine further comprises a flywheel, wherein said flywheel includes a cam profile thereon adapted to operate said second pump.

17. An assembly according to any preceding claim wherein the engine has a fuel injection system.

f 18. An assembly according to any preceding claim wherein air under pressure is mixed with fuel upstream of the cylinder.

5 19. An assembly according to any preceding claim wherein a fan is included upstream of the pump inlet port in order to increase inlet pressure' and thereby outlet pressure. 20. An assembly according to any preceding claim wherein the engine has a two 10 stroke combustion cycle.

21. In combination an engine assembly according to any preceding claim and a vehicle. 15 22. An engine assembly as herein before described with reference to one of figure I and figure 2.