GB2125891A

GB2125891A - Compression ignition engine

Info

Publication number: GB2125891A
Application number: GB08317679A
Authority: GB
Inventors: James Sleator Johnston
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-08-19
Filing date: 1983-06-29
Publication date: 1984-03-14
Also published as: GB8317679D0

Abstract

Prior to t.d.c. of the piston 3, e.g. 15 DEG before, air at a pressure greater than that produced by the piston at t.d.c. in the main combustion space 24 is delivered through a passage 23 to cause ignition of injected fuel. The fuel is injected into the passage 23, the space 24 or the cylinder 7. The inlet 21 to the passage 23 may be uncovered by a piston 11 which lifts in response to increasing pressure in the compression space 26 above the piston 8. Vanes 10 on the piston stem 9 provide air swirl in the space 24. The air transfer passage (22a, b, c, d Figs. 6 and 7) may extend through the piston 8 and the stem 9. <IMAGE>

Description

SPECIFICATION Compression ignition engines This invention relates to so-called compression ignition engines in which there is used, in the combustion space bounded by the piston and cylinder, a compression ratio which is sufficiently high to cause ignition of the fuel-air mixture in the combustion space without the need for ignition means such as sparking plugs and the like.

Although thermodynamically similar to the spark-ignition engine, the compression ignition piston engine has advantages, including the fact that it will operate more efficiently than the sparkignition engine on higher flash-point fuels of lower cost.

A full charge of air (without fuel content) is drawn into the combustion space, bounded in the cylinder bore by the piston, during an induction stroke. The compression ratio used is made high enough to raise the temperature of the compressed air to well above the flash-point of the fuel at the point of highest compression, i.e. at the end of the compression stroke. Just before the piston reaches the end of the compression stroke i.e. at top-dead-centre, fuel is sprayed into the combustion space, and the hot air starts the ignition which continues as a combustion process serving to generate expansion and thereby drive the piston.

In current engines, it is usual to commence injection of fuel at about 150 before top-deadcentre, and to continue it to about 80 after topdead-centre. However, it is only very shortly before top-dead-centre that the temperature of the air becomes sufficient to cause ignition of the fuel, and accordingly combustion can only proceed from top-dead-centre onwards. This results in loss of transfer of power from the combustion process to the piston, because of the so-cailed "delayperiod". Moreover, as the entire fuel-air charge then tends to burn substantially instantaneously at or shortly after top-dead-centre, an extremely high explosion (rather than continued burning) pressure is generated in the combustion space and gives rise to noisy running, straining of the engine structure and wastage of a significant amount of the heat generated.

It is accordingly an object of the present invention to provide an improved arrangement wherein combustion of the entire fuel-air charge may be commenced smoothly and completed by adjustment as required around top-dead-centre, such that optimum expansion pressure is applied to the piston.

Another object of the invention is to provide an arrangement to enhance turbulence of the air charge, and subsequently of the fuel-air charge, in the combustion space in such a manner as to ensure that the pressure of combustion is distributed evenly over substantially the whole of the area of the piston.

According to the present invention a method, of operating a compression ignition internal combustion engine having a given compression ratio determined by its combustion space and piston and cylinder characteristics, comprises the steps of, prior to top-dead-centre: (i) as a result of subjecting it to a higher compression ratio, generating compressed air having a temperature suitable for ignition of injected fuel, (ii) transferring a charge of said compressed air to the combustion space to cause ignition of injected fuel therein.

Where the engine is of the kind in which injection occurs within the combustion space itself, the charge of compressed air may be arranged to cause ignition of fuel already injected into that combustion space.

Where the engine is of the kind in which injection occurs externally of the combustion space, the step of transfer of the charge of compressed air to the combustion space may include carrying of said injected fuel into the combustion space.

It is usual, in combustion ignition engines, to commence injection well before top-dead-centre, say at about 1 50 of crank rotation before topdead-centre. In accordance with a preferred feature of the invention, the generation of the charge of compressed air, or at least its transfer towards the combustion space, is commenced prior to commencement of the injection of fuel.

Further according to the invention a single or multi-cylinder compression ignition internal combustion engine comprises in each cylinder a main piston and cylinder assembly bounding a combustion space and having a given air compression ratio, an auxiliary piston and cylinder assembly arranged to generate compressed air at a higher compression ratio having a temperature suitable for ignition of injected fuel, means for actuation of the auxiliary piston and cylinder assembly in timed relationship with respect to the actuation of the main piston and cylinder assembly such that the auxiliary piston and cylinder assembly generates said compressed air prior to achievement of top-dead-centre by the main piston, and means for transfer of a charge of said compressed air to said combustion space, prior to top-dead-centre, to cause ignition of injected fuel therein prior to achievement of topdead-centre by the main piston.

The means for transfer of said charge of compressed air to the combustion space may comprise passage means leading from said auxiliary cylinder to said combustion space, with fuel injector means being positioned intermediate along said passage means and operated in timed relationship with the auxiliary piston and cylinder assembly such that transfer of said charge along said passage means causes carrying of injected fuel into the combustion space.

In a preferred form, the auxiliary piston and cylinder assembly includes a compression control piston movable in the auxiliary cylinder and coacting with air outlet means in said cylinder to form an outlet valve for said charge of compressed air. The compression control piston may be spring urged towards closed condition of said outlet valve, or may be movable, to open anc close said outlet valve, under the control of a timing means relative to movements of the main piston.

The compression control piston may include a valve for inlet of normal or supercharged atmospheric air to the auxiliary cylinder, and such valve may be spring-urged towards closed condition.

In a preferred embodiment, the auxiliary piston is carried by, and moves in unison with, the main piston. By way of example, the auxiliary piston may be carried on a stem projecting, in the direction of movement of the main piston, from the main piston, and such stem may have vane means, such as helical vanes, adapted to cause or enhance turbulence in the air of the combustion space as the main piston travels towards topdead-centre on a compression stroke.

In order that the nature of the invention may be readily ascertained, some embodiments of compression ignition engine constructed and operative in accordance therewith are hereinafter particularly described with reference to the figures of the accompanying drawings, wherein: Fig. 1 is a schematic cross-section of part of the cylinder block and cylinder head of a first embodiment of engine; Fig. 2 is a schematic plan view to show relationship of parts of the engine to the conventional inlet and outlet valves; Fig. 3 is an elevation to show an early combustion transfer angled slot; Fig. 4 is an elevation to shown an early combustion straight slot; Fig. 5 is a partial cross-section taken on the line V-V in Fig. 1.

Fig. 6 is a view, corresponding to that of Fig. 1, of a second embodiment; Fig. 7 is a view, corresponding to that of Fig. 1, of a third embodiment.

Referring to Fig. 1 the cylinder block 1 has a main bore 2 within which is received a piston 3 having the usual coupling to a connecting rod (not shown) coupled to a crankshaft.

On top of the cylinder block is provided a cylinder head 4 which includes openings 5, 6 (see Fig. 2) for the usual normal or supercharged atmospheric air-inlet or exhaust valves which may be conventional and actuated in conventional manner.

In the cylinder head 4 there is provided a bore 7 which receives an auxiliary piston 8 carried on a stem 9 which is mounted on the head end of the piston 3, such that the main piston 3 and the auxiliary piston 8 always move in unison. The stem 9 is provided with a plurality of externally projecting helical vanes 10 the purpose of which is explained later herein.

In the upper end of the bore 7 there is provided a compression-control piston 11 which, as illustrated, is urged downwardly by a compression spring 1 2 acting between a stop 13 on the cylinder head and a flange 14 on the piston 11.

Within the compression-control piston 11 there is provided an air inlet valve 1 5 urged towards closed condition on its seating 1 6 by a compression spring 1 7 acting between a flange 18 of the piston 11 and a washer 19 on the stem 20 of the valve.

At a position adjacent to the upper end of piston 8 and the lower end of piston 11 the portion of the cylinder head bounding the bore 7 is provided with an air outlet slot 21 opening into a curved passage 22 which leads to a passage 23 receiving the outlet nozzle (not shown) of a conventional fuel injection device and opening at its lower end into the combustion space 24 defined between the piston 3 and the cylinder head, or in a variant (not shown) into an annular space defined between the stem 9 and the bore 7.

The fuel injector has its centre line at 25.

The operation is as follows:- It is assumed that, in the position illustrated, the piston 3 is at top-dead-centre, so that the volume of combustion space 24 is minimum. As rotation of the crankshaft continues, the pistons 3 and 8 will commence to move downwardly in the respective bores 2 and 7. The main piston 3 will carry out a suction stroke and will draw in air through the inlet valve 5.

As the piston 8 moves downwardly, the piston 11 is permitted to move downwardly under the action of its loading spring 12, and thereby covers and seals the slot 21. The downward movement of the piston 8 accordingly causes a suction stroke during which air enters the space 26 as a result of opening of the valve 1 6 against its spring-loading.

When the pistons have passed through bottomdead-centre, they begin to rise again in a compression stroke. The main piston 3 in its bore 2 might be designed, for example, to produce a compression ratio of 20:1. Compression continues until the piston 3 again reaches topdead-centre. It is usual practice in compression ignition engines to commence forced injection of fuel at a few degrees (of crank rotation) before top-dead-centre and to continue it to a few degrees after top-dead-centre, e.g. about 1 50 before top-dead-centre to about 80 after topdead-centre. In a conventional engine the temperature increase, due to compression of the air in the cylinder 2, only becomes high enough to heat the fuel to ignition just before top-deadcentre, so that it is only at about top-dead-centre, and later, that combustion can proceed. The whole fuel injection charge then burns substantially instantaneously and, because an extremely high explosion pressure is generated, there is caused the well-known knocking noise which is characteristic of compression-ignition engines, together with straining of the engine structure and wastage of a significant quantity of heat.

A compression ratio of 20:1, in a conventional engine, will maintain combustion of fuel in a running engine, but will not usually generate enough heat to start a cold engine. Such engines may have a "hot bulb" to facilitate cold starting.

Moreover, at low running speeds, a compression ratio of 20:1 will produce an objectionable "rattle, and at high speeds will cause rough running noises.

Reverting now to the engine as illustrated in Fig. 1 of the drawings, the auxiliary piston 8 carries out its own compression stroke in the cylinder bore 7, and is arranged to provide a maximum compression ratio, at about 1 80 before top-dead-centre, of say 28 :1. This causes the compression-control piston 11 to be moved upwardly, against its spring 12, so as to clear the outlet slot 21. As a result, the very hot air at very high pressure from the space 26 will commence streaming through the outlet slot 21 along the passage 22 and past the fuel injector 25 in passage 23. During this same period, the main piston 3 will have generated at, say, 100 before top-dead-centre air compressed to 18:1.The fuel injector commences its injection of fuel at about 15 before top-dead-centre and terminates injection at about 80 after top-dead-centre. Thus, at about the same time as the fuel injector commences to inject fuel into the (main) compressed air volume of the cylinder space 24, which is very turbulent and is at a compression ratio of about 18:1 to 20:1 ,the very hot air travelling down the passages 22 and 23 and past the injector, at a pressure in the region of 28:1 compression ratio, preferentially pumps burning fuel and air into that main compressed air volume before top-dead-centre, which results in the appropriate correct explosion pressure being obtained at the correct time for smooth expansion.

For the action so far described, with moderate compression ratios and moderate fuel flashpoints, and utilising a spring-loaded compression-control piston 1 lithe outlet slot 21 would be the angled version seen in Fig. 3. Where higher compression ratios and higher fuel flashpoints are concerned, it is preferable to have the compression-control piston 10 actuated by a timing means in fixed (but adjustable) relationship with rotation of the crankshaft, e.g. the usual timing camshaft of the engine, and the outlet slot 21 is then preferably the straight version seen in Fig. 4.

The fuel injector need not necessarily be located in a passage 23 leading to the combustion space 24 but could have its outlet situated within that space 24, as is already known in the art.

It is useful to have the compressed air, contained in the space 24, in a state of cyclonic movement and high turbulence, and these effects should reach their highest intensity just before top-dead-centre. Reference is now made to the helical vanes 10 on the stem 9. When the main piston 3 and the auxiliary piston 8 are commencing to rise from bottom-dead-centre towards top-dead-centre, the helical vanes 10 swirl the otherwise static air in a clockwise motion until, just before top-dead-centre, the motion is at its most intense and the compressed air in the space 24 is best adapted to receive the input stream of burning fuel-air mixture. The arrows in Fig. 2 indicate the issuing of the fuel-air mixture from the passage 23 into the space 24, and the arrows 28 indicate the swirling of the compressed air in the space 24.

The following is an example of pressures and timings which might be obtained in practice: At 1 50 before top-dead-centre, i.e. the usual start of the injection and delay period, the pressure in the combustion space 24 might be, for example, about 400 psia. (27 Kg/cm2). At the same instant, the pressure in the space 26 above the auxiliary piston might be about 500 psia (34 Kg/cm2). At this instant, the auxiliary piston will have its upper end disposed just below the outlet slot 21 and accordingly, in operation, as the piston rises further it starts to cover the slot 21 and hot air, with a pressure rising to about 600 psia (41 Kg/cm2) due to restriction of flow through the slot by the piston, will pass through the remaining lessening slot area and will enter the passage 22.

Simultaneously the pressure in the main cylinder combustion space 24 will have risen to about 500 psia (34 Kg/cm2) at top-dead-centre. Thus, during passage of the pistons from 1 50 before top-dead-centre to 80 after top-dead-centre, there will be (a) injection of fuel and (b) streaming of air at 600 psia (41 Kg/cm2) down the passage 22.

Movement down the passage 22 will be relatively slow, say approximately 0.005 seconds to traverse from one end to the other, such that there will be time to accommodate both the delay period and the rapid combustion period, and to deliver into the combustion space 24, at about top-dead-centre, burning fuel-air mixture which will combine smoothly by cyclonic turbulence with the maximum volume of air, to control combustion over the whole area of the piston and cylinder head, and which will expand and act on the main piston 3 without violent explosion.

In four stroke engines, the helical vanes 10 will swirl the air on the firing stroke, and on the exhaust stroke the piston 8 and the vanes 10 will greatly facilitate completely scavenging of the space 24.

Referring to Fig. 6 there is shown a second embodiment in which the passage 22 of Fig. 1 is eliminated and replaced by a passage 22a which passes through the piston 8 and stem 9 and emerges at outlet 22b into the combustion space 24. The fuel injector, represented schematically by its centre line 25, feeds fuel in via a passage 23a opening into the combustion space 24. In a variant (not shown), the passage 23a may open into the annular space defined between the cylinder wall 7 and the stem 9, below the piston 8.

In this construction, the angled slot 21 of Fig. 1 is omitted, and the temperature and flow rate of the high temperature air, from the space above the piston 8, would be controlled by the dimensions of the orifice 21 a and of the passage 22a.

Fig. 6 shows the auxiliary piston 8 on a compression stroke at a compression ratio of, say, about 40:1 , with a space 26 of about 2 mm in height. The crank angle at this point will be about 250 before TDC, and compressed air at rapidly increasing temperature will already have flowed through orifice 21 a, passage 22a, and outlet 22b.

The piston 8 will then carry the compression control piston 11 on a cushion of compressed air to TDC. The fuel injector 25 will have injected fuel at, say, 1 50 before TDC across the very high temperature injected air, forming a nucleus of combustion which spreads over the whole combustion space.

Referring to the embodiment of Fig. 7, the passage 22tis made shorter so as to open into the annular space, between cylinder 7 and stem 9, just below the piston 8, at opening 22d. The fuel injector 25 feeds in fuel through a passage 23b which opens into the same annular space, and directs the incoming fuel charge onto the hot stem 9. The combination of hot air, being ejected before fuel injection and picking up the flash vapour from the film of fuel on the hot stem 9, causes the most complete combination of burning material with the quantity of compressed intake air in the combustion space 24.

This provides a shorter time delivery of hot air to mix with and ignite the fine droplets of fuel sprayed across the piston stem 9. The stem 9 becomes very hot during operation and gives up heat to the fuel/air mixture, and thus provides an important secondary function as a fuel film evaporator. The combination of hot air and fuel vapour delivered into the main combustion space 24, at a selected crank angle position before TDC, will blend smoothly and rapidly with the main air charge to provide controlled combustion after TDC.

Claims

1. A method, of operating a compression ignition internal combustion engine having a given compression ratio determined by its combustion space and piston and cylinder characteristics, comprising the steps of, prior to top-dead-centre: (i) as a result of subjecting it to a higher compression ratio, generating compressed air having a temperature suitable for ignition of injected fuel, and (ii) transferring a charge of said compressed air to the combustion space to cause ignition of injected fuel therein.

2. The method claimed in claim 1 wherein said fuel is injected, prior to top-dead-centre, into the combustion space.

3. The method claimed in claim 1 wherein said charge of compressed air carries said injected fuel into the combustion space.

4. A single or multi-cylinder compression ignition internal combustion engine comprising a main piston and cylinder assembly bounding a combustion space and having a given compression ratio, an auxiliary piston and cylinder assembly arranged to generate compressed air at a higher compression ratio having a temperature suitable for ignition of injected fuel, means for actuation of the auxiliary piston and cylinder assembly in timed relationship with respect to the actuation of the main piston and cylinder assembly such that the auxiliary piston and cylinder assembly generates said compressed air prior to achievement of top-dead-centre by the main piston, and means for transfer of a charge of said compressed air to said combustion space, prior to top-dead-centre, to cause ignition of injected fuel therein prior to achievement of topdead-centre by the main piston.

5. A compression ignition engine, as claimed in claim 1, wherein said means for transfer of said charge comprise passage means leading from said auxiliary cylinder to said combustion space, and wherein fuel injector means are positioned intermediately along said passage means and are operated in timed relationship with said auxiliary piston and cylinder assembly such that transfer of said charge along said passage means causes carrying of injected fuel into the combustion space.

6. A compression ignition engine, as claimed in either of claims 4 and 5, wherein said auxiliary piston and cylinder assembly includes a compression control piston movable in said auxiliary cylinder and coacting with air outlet means in said cylinder to form an outlet valve for said charge of compressed air.

7. A compression ignition engine, as claimed in claim 6, wherein said compression control piston is spring-urged towards closed condition of said outlet valve.

8. A compression ignition engine, as claimed in claim 6, wherein said compression control piston is movable, to open and close said outlet valve, under the control of a timing means relative to movements of the main piston.

9. A compression ignition engine, as claimed in any one of claims 6 to 8, wherein said compression control piston includes a valve for inlet of air to the auxiliary cylinder.

10. A compression ignition engine, as claimed in any one of claims 4 to 9, wherein the auxiliary piston is carried by and moves in unison with the main piston.

11. A compression ignition engine, as claimed in claim 10, wherein the auxiliary piston is carried on a stem projecting, in the direction of movement of the main piston, from the main piston.

12. A compression ignition engine, as claimed in claim 11 , wherein said stem has vane means adapted to cause or enhance turbulence of air in said combustion space as said main piston travels towards top-dead-centre on a compression stroke.

13. The method of operating a compression ignition internal combustion engine substantially as described herein with reference to Figs. 1-5, or Fig. 6 or Fig. 7 of the accomplanying drawings.

14. A compression ignition internal combustion engine substantially as described herein with reference to Figs. 1-5 or Fig. 6 or Fig. 7 of the accompanying drawings.