GB2156432A

GB2156432A - Compression ignition engine

Info

Publication number: GB2156432A
Application number: GB08507392A
Authority: GB
Inventors: James Sleator Johnston
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-03-22
Filing date: 1985-03-21
Publication date: 1985-10-09
Also published as: GB8507392D0; GB8407518D0

Abstract

An auxiliary piston 13 operates in unison with the main piston 3 and provides a higher compression ratio than the main piston. The auxiliary chamber 17 communicates through a restriction 20 with an ignition chamber 17b normally communicating with the main chamber. During a compression stroke, at a point shortly before TDC, the ignition chamber 17b is cut off from the main combustion chamber 18 by the enlargement 14 at the and of the auxiliary piston connecting rod 15. Fuel is injected into the 17b ignition chamber through the passage 16 and commences to burn. At a corresponding point after TDC, the ignition chamber 17b is again placed into communication with the chamber 18, and ignited fuel-air mixture passes rapidly into the main combustion chamber. <IMAGE>

Description

SPECIFICATION Compression ignition engines This invention relates to compression-ignition or so-called Diesel engines which are in a broad way thermodynamically similar to spark-ignitiion engines. The cycle for both types includes induction, compression, addition of heat, expansion and exhaust. The combustion process and the method of power control in the compression-ignition engine are however very different from those in the spark ignitiion engine.

In the C.I. engine usually a full unthrottled charge of air is drawn in during the induction stroke. A compression ratio of up to say 20 to 1 is used so that the temperature of the air towards the end of the compression stroke is quite high. Just before top dead centre (TDC) fuel is injected into the combustion chamber space. Owing to the high temperature of the air the fuel ignites' and burns almost as soon as introduced. The combustion products are expanded and exhausted in the usual way.

Stages of combusion The combustion process in a C.I. engine has been described as having three stages, the delay period, rapid combustion and controlled combustion. The accompanying pressure-crank-angle diagram (Fig. 5) shows the typical stages. The delay period starts at about the angle of injection, say 15 degress before TDC and lasts to about 5 degrees before TDC; rapid combustion takes only about 7 degrees around TDC at the highest compression temperatures, and controlled combustion lasts until say 13 degrees after TDC.

It has been known for many years that control over the three stages of combustion in the shallow high-pressure air sandwich of a compression ignition engine combustion chamber at around TDC is impossible. In consequence various kinds of small capacity chambers external to the main combustion chamber have been devised. Generally the function of these so called pre-combustion chambers has been to receive through a small passage some of the hot high-pressure air from the main combustion chamber at before TDC and contain this air together with an injection of fuel. The hope was that this mixture would be held for such time that the delay period might be sufficiently completed before the pressure reduction in the main cylinder after TDC would allow emission from the pre-combustion chamber into the increasing volume of the main-combustion chamber.Generally however the reduction of explosive pressure around TDC has not been sufficient to significantly reduce the need for structurally heavier and noisy engines.

The reason for the massive construction of C.I.

engines is because the highest pressure due to exploding fuel occurs more or less close to TDC. Precisely at TDC a very high loading would be absorbed by designing the main cylinder wall heavy enough to absorb high tensile strain (physi cal stretching) and the piston rod heavy enough to absorb compressive strain (physical shortening).

Therefore the nearer the highest combustion pres sure to TDC the more massive the engine structure. The objectionable noise is partly due to intense vibrations and exhaust reverberations.

It is therefore an object of the present invention to provide a method and means for supplying to the, or each of the, main engine cylinders a mass of fuel-air mixture already taken separately in an auxiliary chamber through the three stages of the combusion process as heretofore described, and to make this mass available for use in the main com bustion chamber as from say 15 degrees after TDC or from the earliest torque effective crank angle after TDC.

Another object of the present invention is to provide simple means of mechanical adjustment to match local commercial grades of fuel.

A further object of the present invention is to provide intense turbulence during preparation of fuel-air mixtures during pre-combustion, and of main cylinder air during the entire expansion stroke.

According to the present invention there is provided a method, of operating a compression-ignition engine having: (a) a main piston and cylinder bounding a main chamber and providing therein a main compression ratio, (b) an auxiliary piston and cylinder bounding an auxiliary chamber communicating through a restriction with a pre-ignition chamber, said auxiliary piston and cylinder providing a simultaneously higher auxiliary compression ratio, said method comprising steps of:: (i) during a compression stroke of the main and auxiliary pistons, and prior to achievement of a selected position before top-dead-centre, compressing air in said main chamber, and in said auxiliary chamber and pre-ignition chamber, whilst maintaining said pre-ignition chamber in communication with said main chamber, (ii) at a selected position and thereafter until achievement of a corresponding position after topdead-centre, maintaining said pre-ignition chamber out of communication with said main chamber, (iii) at a point in between the achievement of said selected position and the achievement of topdead-centre, injecting fuel into the pre-ignition chamber so as to obtain ignition therein due to heating result from said higher auxiliary compression ratio, and (iv) upon achievement of said corresponding position during the subsequent expansion stroke, re-establishing communication between the pre-ignition chamber and and the main chamber for passage of ignited fuel-air mixture at a higher pressure from the pre-ignition chamber into the main chamber.

Further according to the invention there is provided compression ignition engine comprising: (i) a main piston and cylinder bounding a main chamber and providing therein a main compression ratio, (ii) an auxiliary piston and cylinder bounding an auxiliary chamber communicating through a restriction with a pre-ignition chamber, said auxiliary piston and cylinder providing a higher auxiliary compression ratio, said auxiliary piston and cylinder being operable simultaneously with said main piston and cylinder during induction, compression, expansion and exhaust strokes, (iii) passage means to provide communication between said pre-ignition chamber and said main chamber, (iv) valve means to close said passage means during motion from a selected position prior to top-dead-centre to a corresponding position after top-dead-centre, but otherwise to open said passage means, (v) injection means arranged to inject fuel into said pre-ignition chamber at a point in time between achievement of said selected position and achievement of top-dead-centre.

In a preferred arrangement, the auxiliary piston is carried by the main piston. The passage means may include an opening in the auxiliary piston.

Said valve means may be constituted by a seating in the auxiliary cylinder, and a valve element carried by the main piston and which enters and leaves said seating respectively at said selected position and at said corresponding position.

The dimensions, and thus the compression ratio, of the auxiliary chamber may be adjustable, and in a preferred arrangement an air inlet valve assembly for the auxiliary chamber is adjustable as a whole axially in the auxiliary cylinder.

In a preferred form the air inlet valve assembly includes an axially adjustable compression ratio control piston. The compression ratio control piston may include the inlet valve for admission of normal or supercharged atmospheric air to the auxiliary cylinder and such valve element may be spring-loaded towards closed condition.

In a preferred embodiment the auxiliary piston is carried by 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. At the attachment of the auxiliary piston to the main piston there may be one or more shims or distance pieces which may be increased or decreased in number for adjustment of axial separation of the auxiliary and main pistons, e.g. in unison with any adjustment of compression ratio. Modification of the extent of engagement of the valve element into the auxiliary cylinder to increase or decrease the holding time required for preparatory processing of the fuel may also be made by adjusting the shims. Either or both adjustments may be made in conjunction with adjustment of the compression ratio control piston.

In a preferred form the auxiliary piston is hollow having top and bottom heads with two oval rods between.

In a preferred arrangement, the selected position and the corresponding position are respectively 15 before TDC, and 15 after TDC.

Advantageously, the injection of the fuel into the auxiliary chamber occurs at, or very shortly after, the achievement of the selected position.

Conveniently, the main compression ratio is at or near to 20:1 and the auxiliary compression ratio is at or near to 40:1.

In order that the nature of the invention may be readily ascertained, an embodiment of a compression ignition having a 90 mm bore and 100 mm stroke, constructed and operative in accordance therewith is hereinafter particularly described with reference to the figures of the accompanying drawings wherein: Figure 1 is a schematic cross-section of a main piston and auxiliary piston at the end of a compression stroke; Figure 2 is a schematic cross-section of part of the main cylinder and piston and the auxiliary piston, the main piston and the auxiliary piston being shown halfway down on an induction stroke; Figure 3 is a plan view to indicate the relative location of the auxiliary cylinder to the main engine inlet and exhaust valves; Figure 4 shows a plan view and a series of cross- sections, of the auxiliary piston, taken on lines A-A, B-B, C-C, and D-D.

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

On top of the main cylinder block is provided a cylinder head 4 which includes openings 5 and 6 (see Fig. 3) for the usual normal or supercharged atmospheric air-inlet and exhaust valves which may be conventional and actuated in conventional manner.

In the cylinder head 4 there is provided an auxiliary cylinder 7, which has a bore 8 and an air-inlet valve 9 incorporating a pintle 21 and with compression spring 10 housed in a compression ratio control piston 12 adjustable axially by its screwed cap 11. The auxiliary cylinder 7 receives an auxiliary piston 7a which is mounted on the head of main piston 3 such that the main piston 3 and the auxiliary piston always move in unison.

The auxiliary piston 7a is hollow and has a top head 13 and bottom head 14 connected by two oval-section rods 15. A fuel injector (not shown) opens into the auxiliary cylinder space 17,through the passage 16 shown in Fig.1. A plan view and some cross-sections of the auxiliary piston 7a are shown in Fig. 4. Combustion chamber 17 for the auxiliary cylinder has a clearance 17a at TDC, and combustion chamber 18 for the main cylinder has clearance at 18a. Combustion ratio adjusting spacers are shown at 19. In the position shown in Fig. 1, the compression ratio (CR) of the main cylinder is, for example, about 20:1, and for the auxiliary cylinder about 40:1. The auxiliary piston top head 13 has a hole 20 drilled through it to provide access of air to the hollow space below.

The operation is as follows: It is assumed that, in the position illustrated in Fig. 1, the main piston 3 is at top dead centre (TDC) so that the volume of the combustion space 18a is minimum. As rotation of the crankshaft con tinues the pistons 3 and 7a will commence to move downwardly in their respective bores 2 and 8. The main piston 3 will carry out an induction stroke, as shown in Fig. 2 and will draw in air through the main inlet valve opening 5 to chamber 18. The auxiliary piston 7a will simultaneously draw in air as a result of opening air-inlet valve 9 against its spring-loading. When the pistons have passed through bottom dead centre they begin to rise again in a compression stroke.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, e.g. about 15" before top-dead-centre to about 8" after top-deadcentre. In a conventional engine the temperaure increase, due to compression of the air in the space 18 of bore 2, becomes high enough to heat the fuel to ignition only just before top-dead-centre, 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.

Reference is now made to the engine as illustrated in the drawings, and referring to the pressure-crank-angle diagram shown at Fig. 5.

Maximum expansion pressure occurs at about 8" after TDC. At this angle the main piston has dropped about 1 mm from top and the crank shaft has rotated about 5mm from the top centre line.

Applying the highest working pressure at this angle explains the need for massive construction as described heretofore.

It is clear that if, without loss of efficiency in fuel usage, the high pressure might be used at a more torque-effective angle after TDC, the compression ignition engine could be superior in smoothness of power delivery to the spark ignition engine.

This invention avoids a high percentage of this excess pressure being applied around TDC on the main piston, and serves to apply it smoothly later in the firing stroke. The method applying this operation is as follows: Reference is made to Fig. 1 on the accompanying drawings, but starting with pistons 3 and 7a at bottom dead centre with maximum volume of air in chambers 18 and 17.

Pistons 3 and 7a will rise and compress the air above them, and auxiliary piston head 13 will also carry the common air in chamber 18 upwards, causing brisk turbulence in the whole chamber space above main piston 3. Piston head 13 in the auxiliary cylinder 7 generates a higher compression ratio (CR) than main piston 3, so that fresh atmospheric air will pass preferentially downwardly through hole 20 into a pre-ignition chamber 17b defined in the auxiliary cylinder below the head 13, and then continuing downwards to enhance the oxygen content in the main combustion chamber 18. Some of the higher pressure air will continue to pass through hole 20 and when piston head 13 rises to 15 before TDC the bottom auxiliary piston bottom head 14 will engage the lower end of the auxiliary cylinder 7 and seal off chamber 17.Also at 15 before TDC the fuel injector 16 will deliver a fine spray into pre-ignition chamber 17b. During transporting of piston head 13 from 15 degrees before TDC to TDC, the high pressure very hot air streaming through hole 20 will cause intense turbulence in the fuel/air mixture. At TDC, the pressure in space 17a will have risen to say 40:1 CR. At 15" after TDC, the finely-divided fuellair mixture will have been contained in pre-ignition chamber 17b through 30 of crank travel and will travel and will have been processed through the so-called three stages of combustion.

At this point in the operation, the main piston 3 will have passed TDC and the air in the main cylinder chamber 18 will have been heated to the equivalent of 20:1 CR, before the main piston drops through 15 crank angle degrees after TDC, and the air in chamber 18 increases by 40% in volume.

The bottom auxiliary piston head 14 will also have dropped through 15 crank angle degrees and, an emerging from the auxiliary cylinder 7, will open at 360" outlet for the emission of a higherpressure umbrella of already igniting gas into the chamber 18. Dispersion of the gas will take place very rapidly indeed from such a thin film blown into the mass of hot air, in a turbulent state due to the descent of piston 7a.

The crank angle diamgram Fig. 6 shows the same motoring curve as in Fig. 5 but, with the closing-off of the auxiliary cylinder at 15 before TDC as shown, the pressure in the main cylinder will simply be maintained through the top pressure region motoring curve until 15" after TDC. From 15" after TDC the combustion pressure being generated in the auxiliary cylinder will flow smoothly into the main cylinder to create increased pressure as shown on the firing curve.

A very important function of the auxiliary piston 7a is to act as an outrigger bearing to control main piston slap at the start of a power stroke.

Claims

1. A method of operating a compression-ignition engine having: (a) a main piston and cylinder bounding a main chamber and providing therein a main compression ratio, (b) an auxiliary piston and cylinder bounding an auxiliary chamber communicating through a restriction with a pre-ignition chamber, said auxiliary piston and cylinder providing simultaneously a higher auxiliary compression ratio, said method comprising steps of:: (i) during a compression stroke of the main and auxiliary pistons, and prior to achievement of a selected position before top-dead-centre, compressing air in said main chamber, and in said auciliary chamber and pre-ignition chamber, whilst maintaining said pre-ignition chamber in communication with said main chamber, (ii) at a selected position and thereafter until achievement of a corresponding position after topdead-centre, maintaining said pre-ignition chamber out of communication with said main chamber, (iii) at a point in time between the achievement of said selected position and the achievement of top-dead-centre, injecting fuel into the pre-ignition chamber so as to obtain ignition therein due to heating result from said higher auxiliary compression ratio, and (iv) upon achievement of said corresponding position during the subsequent expansion stroke, re-establishing communication between the pre-ignition chamber and the main chamber for passage of ignited fuel-air mixture at a higher pressure from the pre-ignition chamber into the main chamber.

2. A compression ignition engine comprising: (i) a main piston and cylinder bounding a main chamber and providing therein a main compression ratio, (ii) an auxiliary piston and cylinder bounding an auxiliary chamber communicating through a restriction with a pre-ignition chamber, said auxiliary piston and cylinder providing a higher auxiliary compression ratio, said auxiliary piston and cylinder being operable simultaneously with said main piston and cylinder during induction, compression and exhaust strokes, (iii) passage means to provide communication between said pre-ignition chamber and said main chamber, (iv) valve means to close said passage means during motion from a selected position prior to top-dead-centre, but otherwise to open said passage means, (v) injection means arranged to inject fuel into said pre-ignition chamber at a point in time between achievement of said selected position and achievement of top-dead-centre.

3. A compression-ignition engine, as claimed in claim 2, wherein said auciliary piston is carried by said main piston.

4. A compression-ignition engine, as claimed in either of claims 2 and 3, wherein said valve means is constituted by a seating in the auxiliary cylinder, and a valve element carried by the main piston and which enters and leaves said seating respectively at said selected position and at said corresponding position.

5. A compression-ignition engine, as claimed in any one of claims 2 to 4, wherein said restriction is formed in the auxiliary piston.

6. A compression-ignition engine, as claimed in any one of claims 2 to 5, wherein the dimensions of the auxiliary chamber are adjustable.

7. A compression-ignition engine, as claimed in claim 6, wherein the auxiliary chamber comprises an air inlet valve assembly which is axially adjustable in the auxiliary chamber.

8. A compression-ignition engine, as claimed in claim 7 wherein the air inlet valve assembly includes an axially adjustable compression ratio control piston.

9. A compression-ignition engine, as claimed in claim 8, wherein the compression ratio control piston includes a spring-loaded air inlet valve element.

10. A compression-ignition engine, as claimed in any one of claims 2 to 9, wherein the auxiliary piston is carried by 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 from the main piston.

12. A compression-ignition engine, as claimed in claim 11, having means for adjustment of axial separation of the auxiliary and main pistons.

13. The method of operating a compression-ignition engine, as claimed in claim 1, substantially as described herein with reference to the accompanying drawings.

14. A compression-ignition engine substantially as described herein with reference to the accompanying drawings.