GB2163486A

GB2163486A - Fuel injection i.c. engine

Info

Publication number: GB2163486A
Application number: GB08520900A
Authority: GB
Inventors: John Heath Greenhough
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-08-23
Filing date: 1985-08-21
Publication date: 1986-02-26
Also published as: GB8421445D0; GB2163486B; GB8520900D0

Abstract

A combustion chamber 15 is formed in the cylinder head 11 and an injector 16 is arranged to discharge into it. The chamber 15 is provided with a lining 13 of ceramic material, to enhance heat retention. The chamber can be in register with one or a group of exhaust valves 12. The engine is preferably operated in accordance with a method wherein injection ceases at a point near T.D.C. and the point of initiation of injection is varied from a position having a low angular spacing from T.D.C. at idling to a position having a high angular spacing before T.D.C. at maximum power. The fuel injection may produce charge rotation (Fig. 1) or be directed at the exhaust valve (Fig. 3). <IMAGE>

Description

SPECIFICATION Internal combustion engine This invention relates to a fuel-injected twostroke or four-stroke cycle internal combustion engine.

In a known pre-mixed spark ignited engine, British Patent No. 1536791, a substantially homogeneous combustible charge is accelerated into a combustion chamber formed round the exhaust valve. This is achieved by forming a trough-like depression extending from the cylinder inlet valve to half way down the precombustion chamber. Near the end of the compression stroke the mixture is accelerated by piston displacement along this trough shaped guide channel, to cause a substantial vortex flow past the spark plug, so that an even rate of ignition is achieved.

Although this is advantageous for the premixed engine cycle, it is contrary to the requirements of the direct injected engine. This engine requires minimum air swirl at low speed and load, so that the small quantity of fuel injected will form an ignitable mixture and will maintain it long enough for spontaneous ignition to take place, before it is diluted by air motion below the limit of flammability.

At high speed and load, because of the limited time available, the highly concentrated fuel requires high velocity air, so that the fuel and air will be mixed well enough for good combustion to be achieved.

At the start of injection if too high a vortex is created in the combustion chamber the concentrated fuel droplets are flung by centrifugal force on to the wall of the combustion chamber, causing a reduction in the mixing efficiency.

With high swirl ratios, both fuel economy and lean iimits are adversely affected. Therefore it is of great importance to control the rate of air movement.

In an existing compression ignition cycle engine, ie in a two or four stroke diesel engine, because temperature is a very important factor, injection is delayed as late as possible in the comporession stroke, so that the fuel will ignite more readily. There will always be a delay period after the start of injection, until the fuel has absorbed enough heat to vaporize and self-ignite. Because of these limiting factors the time taken for injection is very short and will have an injection duration of 25 crank angle at full load.

By concentrating the injection of fuel in the short time available, perfect mixing of all the fuel and air is not possible. This will limit the peak cylinder combustion temperature that would be attainable if all the fuel and air was burnt. Because the maximum cylinder temperature is reduced, the overall cylinder temperature will fall more rapidly. When the adiabetic flame temperature drops below a critical value, any lean region of unburnt gas left in the cylinder will cease to propogate flame, if the temperature drops below this value, thus limiting the power transmitted to the crank shaft and causing unburnt fuel to be released in the exhaust gases.

-The direct injection engine often has a combustion chamber formed in the centre of the piston crown. This is normally part-spherically shaped and the fuel will be injected vertically into the cavity.

The standard injection sequence used, will start injection from a fixed position before T.D.C. The fuel injected will be progressively extended from this fixed point towards T.D.C.

to match engine requirements. This causes the internal cylinder temperature to vary in proportion to the amount of fuel burned. This makes the piston crown combustion chamber subject to wide temperature fluctuations. This causes a reduction in the thermal efficiency of the engine at low to medium load. Also when the engine is run for periods at low load (low temperature) products of partial combustion are released with the exhaust gases.

It is a known factor that swirl is necessary for good combustion and in most modern engines it is standard practice to modify the geometry of the inlet air-passage to create swirl. This has the added advantage of increasing the swirl ratio in proportion to engine speed so that improved combustion will be achieved with high speed and load. The disadvantages are that a low load when the temperatures are low the fuel is dissipated too quickly causing products of partial combustion to be released with the exhaust gases. Although this problem can be partially overcome by increasing injector pressure to assist vaporization, the engine efficiency is still impaired.

The present invention is a means by which a more ideal method of combustion can be attained. This is achieved by controlling the degree of mixing in the first stage of injection to allow the rapidly vaporizing fuel to remain in sufficient concentration for chemical reaction and rapid ignition to take place. Thereafter the rate of burning is rapidly increased by intense motion.

In the present invention, to improve engine efficiency a combustion chamber is provided as an insulated pocket formed in the cylinder head, for example around the exhaust valve(s).

By forming a combustion chamber in an insulated pocket round the exhaust valve the heat from combustion will be retained by the combustion chamber wall. This will be further enhanced by the passage of the hot exhaust gases. The retention of heat by the combustion chamber wall is a means of raising the compressed air temperature to smooth out large temperature variations caused by fluctuating load without raising the overall intake air temperature. This will reduce the delay period without affecting volumetric efficiency.

When fuel is injected under high pressure into the combustion chamber the air in front of the injector nozzle will be pushed forward.

This movement will be small at low load but will rapidly intensify as the pressure load increases.

The amount of air swirl in the cylinder of the compression ignition engine varies with the cylinder size and inlet valve arrangement.

When the bowl in the cylinder head is off-set too much, air swirl will be affected by uneven piston displacement. This causes the air movement to be changed to general turbulence.

It is the object of this invention to control the injected fuel velocity so that a more efficient combustion can be achieved. To improve the low load performance in the high swirl engine the injector is vertically angled so that the fuel will impinge and deflect off the hot exhaust valve. This reduces the injected fuel velocity and dispersion as weil as pre-heating the mixture so that ignition and vaporization is accelerated.

The invention provides an internal combustion engine having a piston reciprocabie in a cylinder, the cylinder head, having an insula- tion-lined combustion chamber formed therein.

The invention also provides an internal combustion engine having a cylinder, a piston reciprocable within the cylinder, and a cylinder head, the cylinder head having formed therein a combustion chamber which is lined with an insulating material and the piston being formed with a protrusion which enters and partially fills the combustion chamber at and near top dead centre, an injector being provided to discharge fuel into the combustion chamber.

In the present invention, to improve engine efficiency, the combustion chamber which may be part-spherical is provided as an insulated pocket formed in the cylinder head. Ideally this would be situated in the centre of the cylinder, round the exhaust valve in a valved engine. An injector nozzle can be positioned just off-centre of the radius of the sphere, so that when injection begins, a whirlpool effect will be created. By controlling the rate and timing of fuel injected, the air swirl ratio can be matched to the engine speed and load requirements. Although it is possible to use the standard injection sequence used in current diesel practice, a more ideal method of fuel injection is achieved by reversing the sequence of operation and spreading the weight of fuel injected over 60 of crank angle duration.

With this reversed injection sequence and extended period of injection duration, a more desirable method of fuel-air utilization is attainable. By delaying injection at tick-over, the noise now associated with the compression ignition engine will be greatly reduced.

This invention will be described further, by way of example, with reference to the accompanying drawings, wherein: Figure 1 is a plan view of part of a cylinder head of a preferred internal combustion engine of the invention; Figure 2 is a cross-sectional elevation through parts of a piston and cylinder head assembly of the preferred engine of the invention; Figure 3 is a view similar to that of Fig. 2, but showing a modification; Figure 4 shows a pair of diagrams illustrating the mode of operation of a conventional diesel engine; and Figure 5 shows a pair of diagrams illustrating a preferred mode of operation of the engine of the invention.

In a preferred engine of the invention a pocket 10 is formed in a cylinder head 11, surrounding an exhaust valve 12. The wall of the pocket 10 is lined with an insert 13 of ceramic or other material having a low coefficient of thermal conductivity. The internal surface 14 of the insert 13 is shaped to be partspherical to impart such a shape to the space inside the pocket which constitutes a combustion chamber 15. A conventional injector 16 projects tangentially into the chamber 15.

To increase turbulent mixing of fuel and air in the chamber 15, one or both of two additional modifications can be made. One of these is the provision, on an otherwise flat piston head 17, of a protrusion 18 which may be part-spherical and which enters chamber 15 shortly before T.D.C. The other modification is the provision of a pip 19 on the inside face of the exhaust valve 12. The pip 19 is shown in dotted lines in Figs. 1 and 2 and can be generally hemispherical in shape, its base merging smoothly with the otherwise flat inner face of the exhaust valve. Pip 19 can lie centrally of the exhaust valve, or can be offset if desired.

Fig. 3 shows a variation wherein the injector 16 is angled to cause injected fuel to impinge directly on the exhaust valve. Such impingement can be achieved by angling the whole injector 16 as shown, or simply by appropriate disposition of the jet nozzle or nozzles formed in the innermost lip of the injector.

The engine of the invention can operate in accordance with the usual operating conditions of a conventional diesel engine, but is preferably operated as described later.

Although the low swirl engine will run quietly at low speed its efficiency will be impaired when the speed increases. To overcome this high speed disadvantage the injector nozzle is positioned just off centre of the radius of the sphere, so that when injection begins the air in front of the injector nozzle will be pushed forward causing the air to rotate. This movement will be small at low load, but will rapidly intensify as the weight of fuel injected increases. By controlling the amount of fuel injected the air swirl ratio can be matched to the engine speed and load require ments.

Another novel feature of the invention is to re-direct piston air displacement to give maximum air turbulence after ignition commences.

As the piston approaches T.D.C. air is squeezed between the cylinder head and the piston crown (squish action). This movement of air is controlled and directed to produce intense air-movement. To achieve this a domed shaped protrusion is formed on the piston crown. This domed shaped protrusion enters the bottom of the spherical combustion chamber when the piston reaches T.D.C. Because the tolerance between the cylinder head and the piston crown is small the air trap between these opposing surfaces is squeezed sideways at high velocity and is deflected by the dome shaped protrusion to create high toridal turbulence and vigorous fuel-air mixing.

Most hydro-carbon fuels, especially diesel, are subject to pre-flame reaction (cool flame) and will produce products of partial combustion that are very unstable. This reaction will take place in the diesel engine when the fuel is injected at too low a temperature or it is impinged on a combustion chamber surface that is below a minimum critical temperature.

With early injection these products of low temperature combustion can lead to knock.

By forming a spherical combustion chamber in an insulated pocket round the exhaust valve, the heat from the combustion will be retained by the combustion chamber wall. This will be further enhanced by the passage of the hot exhaust gas. Because the combustion chamber wall will be over 300"C, the temperature for pre-flame reaction will be exceeded.

This makes the reversing of the injection sequence a practical proposition. By retaining the heat in the combustion chamber the final compression temperature will be higher. Because of this the delay period will be reduced allowing the injection at tick-over to be retarded and noise levels reduced.

By injecting forward to increase speed and load the injection will more readily match the increasing temperature that occurs with speed and will also be self-compensating for the increased delay time. Because the problems associated with low temperature fuel impingement are removed, advantage can be taken of the longer time available at full load for the air and fuel to mix so that a more complete combustion can be achieved.

This will cause the peak cylinder temperature to rise. With increased temperature the propogation of flammable combustion will be extended and more power is available at the crank shaft.

By this method of operation combustion will be more complete and the overall engine efficiency is greatly improved.

I will now describe the reversed injection sequence in more detail. A compression ignition engine will require one-third the weight of fuel needed at full load in order to maintain tick-over. Injection will cease at T.D.C. or thereabouts and will commence at a position about 15 (12 to 18 ) before T.D.C. This will give a 15 injection duration to maintain tickover. To increase speed and load, initiation of injection will be progressively advanced to from 25 to 35 , preferably to 30 before T.D.C. so that two-thirds of the weight of fuel required at full load will be spread over about 30 of crank.

Because the cylinder temperature from 60 to 30 before T.D.C. is below the fuel's critical spontaneous combustion temperature and the rate of injection per degree is low, it will be vapourised and diluted by air motion to below the limit of flammability. This early premixed fuel will only be ignited when the weight of fuel increases from 30 to T.D.C.

By using the injected fuel's velocity to enhance the combustion chamber movement, a more ideal method of fuel-air admixture is achieved. At low speed and load, with minimum fuel injection, the air speed will be low.

With little air movement the engine will idle quietly. With maximum injection air velocity will rapidly increase. This is further accelerated by expansion and combustion. By this method of injection the rate of air swirl will match the rate at which the fuel burns.

The aforementioned description relates to a four-stroke direct-injected engine. It will be appreciated, however, that the invention can be applied to a direct injection two-stroke engine.

If the engine has one or more exhaust valves the pocket can surround such valve(s). If the engine does not have exhaust valving, the pocket can be a closed pocket lined with insuation material and there being an injector arranged to direct fuel into the pocket. The above described injection sequence can be used in such a two-stroke engine.

Claims

1. An internal combustion engine including a piston reciprocable in a cylinder and a cylinder head, a combustion chamber being formed in the cylinder head and a lining of low thermal conductivity material being provided to surround the combustion chamber, a fuel injector being arranged to be capable of injecting fuel into the combustion chamber.

2. An engine as claimed in claim 1 and being a four-stroke engine including an exhaust valve, the combustion chamber being formed in register with the exhaust valve.

3. An engine as claimed in claim 1 or 2, wherein the combustion chamber is circular in cross-section in a plane transverse to the axis of piston movement.

4. An engine as claimed in claim 2 or 3, wherein the combustion chamber is partspherical in shape.

5. An engine as claimed in claim 2, 3 or 4 wherein the injector is positioned to direct in jected fuel along a path intermediate directions radial and tangential to the combustion chamber.

6. An engine as claimed in any preceding claim wherein the injector is positioned to direct fuel to impinge on the exhaust valve.

7. An engine as claimed in any preceding claim wherein the combustion chamber is lined with ceramic material.

8. An engine as claimed in any preceding claim wherein the combustion chamber is coaxial with the exhaust valve.

9. An engine as claimed in any of claims 1 to 7, and wherein two or more grouped exhaust valves are provided, the combustion chamber being provided in register with the group.

10. An engine as claimed in any preceding claim wherein the piston has a protrusion which enters the combustion chamber near to T.D.C.

11. An internal combustion engine substantially as hereinbefore described with reference to the accompanying drawings.

12. An internal combustion engine having a cylinder, a piston reciprocable within the cylinder, and a cylinder head, the cylinder head having formed therein a combustion chamber which is lined with an insulating material and the piston being formed with a protrusion which enters and partially fills the combustion chamber at and near top dead centre, an injector being provided to discharge fuel into the combustion chamber.

13. An engine as claimed in claim 12, wherein the protrusion on the piston is wholly or substantially wholly within the combustion chamber at T.D.C.

14. An engine as claimed in claim 12 or 13, wherein the protrusion is circular in crosssection about an axis parallel to the cylinder axis.

15. An engine as claimed in claim 12, 13 or 14, wherein the protrusion is part circular in cross-section on a plane transverse to an axis parallel to the cylinder axis.

16. An engine as claimed in claim 13, 14 or 15 and being a four-stroke-cycle compression ignition engine, the combustion chamber being in register with an exhaust valve or a group of exhaust valves of the engine.

17. An engine as claimed in any of claims 13, 14 and 15 and being a direct-injected two stroke engine.

18. An engine as claimed in claim 17 and having one or a group of exhaust valves with which the combustion chamber is in register.

19. An engine as claimed in claim 17 and having no valves, exhaust of spent gases occuring via an exhaust port in walling of the cylinder.

20. A method of operating an internal combustion engine as claimed in any preceding claim wherein injection is caused to cease at a point at or near T.D.C. and injection is initiated at a point whose angular spacing from T.D.C. progresively increases from a minimum at tickover to a maximum at full power.

21. A method as claimed in claim 20, wherein the rate of fuel injection is constant over a second part of the angle range adjacent T.D.C.

22. A method as claimed in claim 21, wherein a lower rate of fuel injection is over a first part of the angle range remote from T.D.C.

23. A method as claimed in claim 20, 21 or 22, wherein during at least a fraction of said first part of the range the fuel injected does not ignite immediately, but forms a premixture with air in the cylinder which is burnt at ignition at the initiation of said second part.

24. A method as claimed in any of claims 20 to 23 wherein the second part of said range commences at from 12" to 18 before T.D.C.

25. A method as claimed in claim 24, wherein the second part of said range commences at 15 before T.D.C.

26. A method as claimed in any of claims 20 to 25, wherein the first part commences at a point some 50 to 70" before T.D.C., preferably about 60".

27. A method of operating an internal combustion engine substantially as hereinbefore described.