GB2264333A

GB2264333A - Compound expansion i.c.piston engine.

Info

Publication number: GB2264333A
Application number: GB9223576A
Authority: GB
Inventors: Ian Nicholas Harper
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-02-06
Filing date: 1992-11-09
Publication date: 1993-08-25
Also published as: GB9223576D0

Abstract

Four-stroke i.c. cylinders containing pistons 2 and 3 alternately receive charge compressed by a piston 1 and alternately discharge exhaust gas to an expansion cylinder containing a piston 4. The compressed charge passes through an intercooler (16, Fig. 2) and the cylinder centrelines may be offset from the crankshaft centreline. The length of the charge inlet pipe may be varied dependent upon engine speed (Figs. 3 and 3a). The cylinder valve seats may be recessed (Fig. 4). <IMAGE>

Description

COMPOUND PISTON COMPRESSION IGNITION ENGINE This invention relates to a compound compression ignition engine.

This application is an extension of applications no 92()2487.6.and 9219196.4

Engines are well known devices for converting combustible matcrial, mainly petrochemicals such as petrol or diesel, into mechanical energy. When used in cars, small generators ctc. they also need other characteristics: quietness, smoothness, high power to weight and volume ratios. low emissions of pollutants, high reliability and durability, and low construction costs.

There are many different solutions to meet these criteria, but all are a compromise between the various conflicting objectives.

Two power strokes per revolution is regarded as the minimum for smoothness, which cquates to four cylinders operating on the four stroke cycle.

Diesels are more eftdent under many circumstances, but in comparison with petrol engines have higher noise levels, lack of power and flexibility, greater weight and expense. For maximum efficiency they should have direct injection and low combustion chamber surtace- to-volume ratios, which is difficult to achieve for small engines. They are heavier and more expensive partly because of components designed to withstand the higher pressures, and also because of the injection equipment which has to meter very accurately high pressure fuel to four cylinders at tightly controlled rates. They have high compression ratios to permit easy starting. but do not need such high ratios when running, and the high ratios lead to greater forces and weight, and NOX emissions.

With small diesels, two-stage compression makes the high compression stage smaller, with longer combustion times, lower forces and hence weight. The combustion chamber is more compact. losing less heat and efficiency. A high pressure cylinder operating on the four-stroke cycle gives one power pulse per two revolutions, but if the burnt gases expand in a further stage giving the same power as the high pressure stage, and separated by equal amounts of time, the result is one pulse per revolution. Two high pressure cylinders fed by one low pressure compression cylinder and exhausting to one low pressure expansion cylinder would be equivalent to a four-cylinder four stroke engine. High gas velocity in the passage between the low and high pressure cylinders enables high swirl. Together with the compact combustion chamber, this permits direct injection for higher efficiency.

Direct injection with greater combustion times enable higher RPM and power levels. The large valves possible in the low pressure cylinder enable much better breathing and thus power output maintained over higher speeds. In addition, the expansion cylinder can be made bigger than the compression cylinder, giving greater efficiency as more power is extracted from the exhaust stroke. The low pressure compression cylinder piston conrod and rings have relatively low pressures and temperatures, so any or all could be made of plastic or some other lightweight material.

According to the present invention there is a compression ignition four-cylinder compound cnginc. There is one low pressure compression stage, feeding an intercooler, from which coolcd high pressure gas passes alternately to two high pressure four stroke power cylinders, which in turn exhaust to a single low pressure expansion piston. The expansion stages produce similar power pulses at approximately opposite strokes. The low pressure piston compresses frcsh air and passes it alternately to the high pressure cylinders through a passage controlled by valves in the high pressure cylinders.Exhaust gas passes alternately from each high pressure cylinder, via exhaust valves into the low pressure expansion stage which operates on a twostroke cycle, and is of larger dimensions than the compression stage to allow greater efficiency. Balancing shafts may be provided. Low heat conducting materials could be used for the combustion chamber and piston crowns to increase efficiency.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings.

Figure 1 shows the engine longitudinally in section.

Figure 2 shows a plan view from above of a cylinder layout.

Figure 3 shows the inlet tract arrangement Figure 4 shows the valve seat arrangement Operation.

The four pistons 1,2,3,4 are provided with rings in the normal manner, and are drawn up and down by connecting rods connected to a crankshaft 5, in the normal manner.

On the down stroke of low compression piston 1, the valve 7 is opened (All the valves are actuated by a camshaft by some means, the camshaft being driven by some means from the crankshaft at half crankshaft speed, in the normal manner, not shown) and air is drawn in from inlet 6. On the upstroke of piston 1 valve 7 is closed, air is compressed, and then at about 45 degrees before top dead centre valve 8 opens and air flows into passage 9 leading to the intercooler 16.

When the first high pressure piston, 2 falls, valve 11 is open to allow air from the intercooler to flow via passage 10 into the cylinder.

On the upstroke of piston 2, valve 11 is closed and the gas is being compressed to ignition temperatures and pressures. At the same time, piston 1 is descending to draw air in to the low pressure cylinder ready to be fed into the second high pressure cylinder. At around TDC of piston 2, a fuel injector (not shown) starts introducing fuel into the combustion chamberwhich is of a design to promote efficient combustion such as Ricardo Comet. Piston 1 then falls, producing power. At the bottom of it's stroke, the exhaust valve 12 opens, and on the upstroke exhaust gas is pushed past the valve 12 and into exhaust passage 13 to the low pressure expansion cylinder, where piston 4 is falling and providing power to crankshaft 5. At the bottom of the stroke, valves 14 open and piston 9 rises pushing exhaust gas out into exhaust outlet 15.The cycle for the second high pressure cylinder 3 is the same, but 360 degrees later.

Further Details The high pressure pistons 7 and 6 may be made of iron, or may have a separate iron or ceramic crown to reduce heat loss. The cylinder head may also have an insulating layer or inscrt at the top of the high pressure cylinders to reduce heat loss. The compression cylinder could be positioned opposed to any of the other cylinders, or any angle in between and/ol with it's big end on one of the other big end journals, or in a fork-and-blade configuration with another conrod. If it were about 90 degrees round the crankshaft centrelinefrom the expansion cylinderwith it's big end sharing the expansion cylinder journal, then this would greatly aid balanceof the compress on cylinder.

There could be several groups of four cylinders driving the same crankshaft, and they could be in line. or opposed, or at angles in between.

The engine as shown has only three main bearings: because of the low pressures and forces on piston 1 an end bearing may not be needed with a consequent saving on cost and friction.

More main bearings could be provided: there could be an additional one between the two high pressure cylinders, and one on the compression end. The big end bearing for the compression piston could be aball or roller bearing.

There could be only one compression ring for each of the low pressure cylinders, because of the relatively low pressures, thus reducing friction. Additionally, oil control rings may not be necessary for the high pressure cylinders, because their pressure is always positive and their size is small, or for the exhaust cylinder, where there is high pressure on each down stroke.

In comparison with a conventional diesel, much lower swirl and /or squish levels are necessary, because of the greater combustion time available. In turn, this helps reduce conduction losses into the cylinders and pistons, thus improving efficiency whilst reducing thermal stresses on the piston.

The valve 8 from the compression cylinder to the intercooler could be self-acting, rather than camshaft operated. The valves in the high pressure cylinder need to have strong valve springs to resist high back pressures from the intcrcooler and expansion stages.

The engine may be provided with one or more balance shafts to smooth out the reciprocating forces. The inlet valves, 11,12 and the corresponding exhaust valves 14 will require means of sealing the passage of medium pressure air past their stems, such as rings bearing on the valve stems and held in the valve guide. There may be more than two valves to each high pressure cylinder, to improve breathing, and more than one valve in the compression and exhaust cylinders. The valves could be indented into the cylinder head by around .05 to .1 inch, as shown in figure 4, with a small clearance 21 between the circumference of the valve and the cyl inder head, 19. This clearance, shown enlarged for clarity, is as small as possible so that the valve is effectively closed when it cntcrs the indentation.This enables the piston to be flat topped: it also enables the camshaft to open earlier and close later because the initial valve movement which has to be slow ttl reduce acceleration and deceleration forces, takes place when the valve is effectively closed. This greater opening duration will enable greater lift, to more than offset the .05-.1 inch lift lost.

The cylinder centrelines of one or more cylinders could be offset from the crankshaft cen reline. This increases the angle between the conrod and the cylinder in one stroke, which also increases the sidethrust. And on the other stroke it reduces the angle, thus reducing side thrust.

Where the pressure is much higher with one stroke than the other, overall friction may be reduced if the cylinder is offset to reduce the angle when pressure is higher. With the compression and expansion cylinders, pressure is much higher in one direction than another, thus offsetting them will reduce friction. In the case of the expansion cylinder, the offset is arranged to reduce angles on the downstroke: for the compression cylinder the opposite is needed. Figure 2 shows the cylinder arrangementwith offsets.

The engine could be made with higher than necessary compression ratios, and with the means of varying the injection timing. The iirlliii3 could be advanced for starting, when iiigl: compress sion is needed, and delayed for low RPM to reduce both the duration of combustion, and the temperatures, and so limit NOX emission and thermal conduction losses, and also friction losses. The injection timing could be advanced for high RPM when combustion durations are shorter. or when the volume of air entering is reduced.

Because there is a single inlet tract into the low pressure compression cylinder, the intake effi ciencv can be maximised by varying the length of the inlet tract to make use of the air inertia to overfill the cylinder at different RPM. Additionally, the high pressure generated at the end of of the intake period can be used by adding a longer, wider pipe to the tract, tuned so that it provides an additional pressure wave one revolution later, using the reflection of the wave from the 'open" pipe end. The fundamental frequency or overtones could be used to achieve this effect. This tract can be made variable to maximise gains.

In the same way, the exhaust tract serving only one cylinder can be tuned to maximise power, and could even be made variable in the same way as the inlet tract.

Figure 3 and 3a show one way in which the inlet tract and the organ effect pipe may be altered in length at the same time.

There is a short inlet tract 3, connected to a wider pipe 14, which may be connected to an air filter/induction noise suppressor (not shown). There is a valve 16 which rotates about a pivot 15. At high RPM the valve is in the position shown, blocking the air from the alternative path 17/18 shown in figure 3a, and leaving a clear passage to pipe 14.

When tilC RPM is low. a longer tract and pipe is needed, so an actuator, not shown rotates valve 16 to cover pipe 14 and frce the passage of air through 17,18 which arc longer.

The expansion stage may be sized so that under normal conditions it does not require a silencer because it expands the gas almost to atmospheric pressure, and it mav them be useful to have no exhaust pipe either.

To reduce the temperatures, a generous oil supply to the high pressure cylinders could be provided, and oil cooling may be useful. The fuel injected might also be limited to around 65 of the stochiometric, to reduce the thermal load. Because the efficiency of diescls reduces at approaching the stochiometric level, this would not reduce power by that much, but would lead to very low emissions.

The engine could be turbocharged, or supercharged by the usual means, with or without intercoolers. The larger expansion cylinder would enable more energy to be extracted from the higher exhaust pressures than is usually the case, thus improving efficiency.

Valve timing.

The inlet valve of the compression cylinder can close well after bottom dead centre, to allow gas inertia to force more gas into the cylinder. The HP inlet valve can open well before TDC, and should shut perhaps 20 degrees after BDC.

The HP outlet valve should open around BDC, and can close after TDC.

The expansion cylinder outlet valve can open before BDC, and needs to shut before TDC (perhaps by 30 degrees) to retain sufficient gas to compress into the transfer passages 13 to meet the pressure of the gases coming from the HP cylinder.

The valves in the high pressure cylinder could be tilted. If the exhaust valves were tilted so their tops were leaning towards the power cylinder and the combustion chamber became wedge or semicircular shaped, this would help increase the space available to use larger valves and shorten thetransfer passage.

There could be a selfoperatingvalve between the compression cylinder and the medium pressure transfer passage to the high pressure cylinder. This self acting/ spring loaded valve opens automatically when sufficient pressure has been generated, and closes when the pressure in the compression cylinder falls below the pressure in the transfer passage. In this way, all the air from the compression stage passes to the intercoolers.

Starting In order to improve starting, the timing of the closing of the exhaust valve of the high pressure cylinders could be altered/ advanced, so that an amount of the gas within the cylinder is retained. and added to the fresh charge forced in by the compression cylinder. This also means altering the timing of the HP and compression inlet valves so that they are not both open together. otherwise the extra retained gas will simply leave through the inlet. Advancing the closure of the HP exhaust valve by around 60 degrees will rctain 25aSs of the volume, and raise the effective compression ratio at starting by 25%. Similarly, delaying the opening of the HP inlet valve by 30 degrees will ensure that the entrapped gas cannot escape past the compression inlet valve.

One way in which this could be done is to have hydraulic self adjusting tappet fulcrums for the high pressure cylinder inlet and exhaust valves, arranged so that when the engine is not running and the lubricating oil pressure is nil, they allow the valve clearance to increase by around .25 inches. A spring will need to be provided to ensure that the adjusting fulcrums shrink even if the engine does not come to rest with the cam pressing the tappet. This will delay opening and closing of the valves until the oil pressure rises and reduces the gap.

Provision will have to be made to ensure that only small quantities of fuei are injected until clearance is reduced, otherwise excessive pressures might be generated during the start-up phase.

Claims

1 A piston engine for converting chemical energy into mechanical energy, comprising a crankshaft, a plurality of pistons and cylinders and associated connecting rods: one two-stroke cylinder which takes in and compresses air and passes it alternately at medium pressure to two four stroke high pressure cylinders, which burn fuel and air and pass the burnt gas toa single low pressure expansion cylinder to extract further energy, a piston moves in said expansion cylinder in an opposite direction to the high pressure pistons, to provide four power strokes per two revolutions ofthe crankshaft as in a conventional four cylinder 4 stroke.

2 An engine as claimed in claim 1 where the cylinders lie with their axes parallel and on the same side of the crankshaft and the two high pressure cylinders lie between the compression and expansion cylinders.

3 An engine as claimed in claim 1 where the high pressure and expansion cylinders lie with their axes parallel and on the same side of the crankshaft and the two high pressure cylinders lie on opposite sides of the expansion cylinder.

4 An engine as claimed in claim 3 where the compression cylinder may be located at any angle around the crankshaft axis and/or where the associated conrod bears on the same journal as the expansion cylinder.

5 An engine as claimed in any preceding claim where any or all of the valves are poppet valves in the compression cylinder and/or two exhaust valves in the expansion cylinder.

6 An engine as claimed in any preceding claim where the compression cylinder and/or piston is made ofplastic, optionally reinforced.

7 An engine as claimed in any preceding claim where the compression piston and cylinder and conrod is made of plastic, optionally reinforced.

8 An engine as claimed in any preceding claim in which the expansion cylinder has one compression ring, with optionally an oil scraper ring.

9 An engine as claimed in any preceding claim in which the compression cylinder has one compression ring.

10 An engine as claimed in any preceding claim in which the high pressure pistons have no oil scraper ring.

11 An engine as claimed in any preceding claim in which the poppet valves for the power cylinders have a ring bearing on their stems or any other means in which gas leakage in the gap between valve andvalveguide maybe resistedwhenthevalve is open and closed.

12 An engine as claimed in any preceding claim in which there is a means for using the inertia of incoming gases down the inlet tract to fill the compression cylinder to a maximum pressure.

13 An engine as claimed in any preceding claim in which there is a provided a means for utilising the high pressure in an inlet tract when the inlet valve or valves have closed on one stroke, to increase the pressure to which the compression cylinder is filled two strokes (360 degrees) later.

14 Any piston engine in which there is a means for utilising the high pressure in the inlet tract when the inlet valve or valves have closed on one stroke, to increase the pressure and fill in the same cylinder inlet duringthe next induction stroke, by reflecting pressures from the end of a tuned inlet pipe using the fundamental resonance or overtones of the inlet pipe.

15 An engine as claimed in claim 14 or 13 in which there is means for utilising both the inertia of gases on the same stroke, and a means for utilising the high pressure in the inlet tractwhen the inlet valve or valves have closed on one stroke to increase the pressure in the compression cylinder during the next induction stroke.

16 An engine as claimed in claim 15 in which there is an inlet tract consisting of two lengths of pipe of different dimensions with the smaller diameter pipe connected directly to the cylinder head so that the smaller diameter pipe is the right length to maximise gas inertia pressure increase, and the larger pipe connects to the other end of the short pipe (With some means for making a smooth air passage between the two such as a venturi) and is the right length to reflect the pressure wave generated by the valve closing on one stroke to amplify the cylinder pressure during the next induction stroke.

17 An engine as claimed in claim 16 in which there is a means of varying the length of both pipes simultaneously, to enable cylinder fill maximisation to occur at two or more different en ginespeeds.

18 An engine as claimed in claim 17 in which the means ofvarying the length of both pipes simultaneously is a butterfly valve operated by control means, which in one position allows air to come down a short dual pipe arrangement only, and in the other position allows air to come down a long dual pipe arrangement only.

19 An engine as in claim 12 in which there is a means of varying the length of a single inlet pipe to enable pressure maximisation to occur at two or more different engine speeds.

20 An engine as claimed in any preceding claim in which the cylinder centreline of the compression cylinder is at right angles to, but offset from the crankshaft centreline to reduce sidethrust and hence friction in the up/compression stroke (while incrcasing it on the other).

21 An engine as claimed in any preceding claim in which the cylinder centreline of the high pressure and/ or the expansion cylinders is at right anglers to, but offset from the crankshaft centreline to reduce sidethrust and hence friction in the power/down stroke (while increasing it on the other).

22 An engine as claimed in any preceding claim in which the big end journal for the expansion cylinder is substantially bigger than the big end journals for the other pistons.

23 An engine as claimed in any preceding claim in which the big end journal for the compression cylinder is substantially smaller than the big end journals for the other pistons.

24 An engine as claimed in any preceding claim in which the pistons of the power cylinders are made of ferrous material, or stainless steel, or chrom'ium plated, or polished: or any other means for reducing heat conductance.

25 An engine as claimed in any preceding claim in which the pistons of the power cylinders are made of ferrous material, or stainless steel, or chromium plated, or polished: or any other means for reducing heat conductance.

26 An engine as claimed in any preceding claim in which the pistons of the expansion cylinder is made of ferrous material, or stainless steel, or chromium plated, or polished: or any other means for reducing heat conductance.

27 An engine as claimed in any preceding claim in which there is an intercooler between compression cylinder and the power cylinders.

28 An engine as claimed in any preceding claim in which there is a valve, controlling the passage from the compression cylinder to the power cylinders, situated in or adjacent the cylinder head of the compression cylinder. The valve controlling the passage between the compression cylinder and the power cylinders maybe operated by a camshaft or self operating.

29 An engine as claimed in any preceding claim in which the expansion cylinder is sized to expand the exhaust gases to below a pressure which generates acceptable exhaust noise, wherby there is no necessity for a silencer, or silencer and exhaust pipe.

30 An engine as claimed in any preceding claim in which the valves are poppet and some or all ofthem when closed are recessed into the head, with a small clearance between the outer diameter of the poppet head and the cylinder, so that the valve is effectively closed before the poppet closes onto the valve seat. This increases the speed with which the valve is effectively opened or closed. It allows greater times between the camshaft starting to lift the valve, and fi- nails leaving the valve on its seat, which enables higher effective lift. ..

31 An engine as claimed in any preceding claim in which the high pressure cylinders are sized to produce approximately the same power as the expansion cylinder, to give four roughly equal power strokes per two revolutions.

32 An engine as in claim 1 where a supercharger is provided to compress gas into the compression cylinder.

33 An engine as in claim 1 where there is a turbocharger provided to feed increased pressure air into the first cylinder, with or without an intercooler between the turbocharger and the compression cylinder, andwith the turbine fed from the exhaust from the fourth cylinder.

34 An engine as claimed in any preceding claim in which the poppet valves for the power cylinders have a ring bearing on their stems or any other means in which gas leakage in the gap between valve and valve guide may be resisted.

35 An engine as claimed in any preceding claim in which the fuel burnt is any combustible liquid, or any fuel made of a mixture of liquids, or consisting of a liquid carrying finely ground combustible solids.

36 An engine as claimed in any preceding claim in which the fuel is premixed with air before combustion and is then ignited by a spark from a spark plug protruding into the combustion chamber. The spark is produced and timed by some means.

37 An engine as claimed in the preceding claim in which the fuel is injected by any means into the passage between the compression cylinder and the high pressure cylinders.

38 An engine as claimed in any preceding claim in which there is provideda means for varying the timing of fuel injection or spark ignition means, such that the timing might be advanced at starting, and delayed for low speed operation and advanced for high speed operation.

39 An engine as claimed in any preceding claim where there are two or more sets of four cylinders operating off the same crankshaft.

40 An engine as claimed in any preceding claimwhere the valve or valves in the expansion cylinder that control the exhaausting of gas to air are arranged to close significantly before top dead centre, whereby the gas that remains is compressed to near the pressure ofthe gas incoming from the high pressure cylinder.

41 An engine as claimed in any preceding claim where the valve or valves in thehigh pressure cylinders that control the transfer of compressed air into the high pressure cylinders, are arranged to close significantly before top dead centre, whereby the gas that remains is compressed to near the pressure of the air incoming from the compression cylinder;