GB2112859A - Spark ignition direct injection i.c. engine - Google Patents

Abstract

The start of fuel injection from a nozzle 13 between a pair of spark electrodes 15, 16 is advanced with increasing injection quantity, e.g. from 10 DEG before at low speed to 60 DEG before t.d.c. at full load, and injection terminates at a fixed point within 5 DEG of t.d.c., e.g. t.d.c. ignition occurs during injection, e.g. at 8 DEG before t.d.c. at idling and advances with speed increase. <IMAGE>

Description

SPECIFICATION Internal combustion engine There are basically two types of internal combustion engine, the spark ignition engine (petrol) and the compression ignition engine (diesel). In the spark ignition engine, air and fuel are drawn into the combustion chamber and compressed to form an explosive mixture. The mixture is then spark ignited and burnt at a constant volume. the pressure generated during a very short time whilst the combustion chamber volume changes little, pushes down the piston.

This cycle is known as the constant volume premixed cycle.

In the compression ignition engine, air only is drawn into the combustion chamber. It is then highly compressed by the piston. This raises the temperature of the air well above the spontaneous combustion temperature of the fuel to be used. The fuel is forced into the combustion chamber and when the correct balance of temperature is reached, it spontaneously ignites.

Fuel feed is continued for a considerable period after ignition and the rate of feed is adjusted so that the pressure generated by the burning fuel remains nearly constant throughout the combustion process. This is known as the constant pressure cycle.

The power of the spark ignition engine is controlled by varying the amount of fuel and air entering the cylinder but keeping the air to fuel ratio chemically correct. Unless this balance is maintained the mixture is difficult to ignite.

In the compression ignition engine, control of power output is governed by varying the quantity of fuel only. The air supply is left at a constant value.

The conventional spark ignition (petrol) engine is un-economical because of the following reasons: It is subject to many cold starts which require excess fuel.

The throttle is constantly opened and closed, causing excess fuel to be used.

When the air intake is partially closed (throttle) this has the disadvantage of reducing the effective compression ratio.

In the compression ignition engine (diesel), the compression rate remains constant. The fuel is metered to power requirements and although it is more difficult to start, it has the advantage of being able to be driven off at full load. This gives it a 30% fuel advantage over the petrol engine on cold starts and intermittent driving conditions.

The disadvantages of the compression ignition engine are it's heavy weight and low power output, when it is compared with a petrol engine of the same size. It is also noisy at low speed and it has a limited speed range. This is caused by the delay that takes place in all compression ignition engine, until the fuel absorbs enough heat to create combustion.

In trying to overcome this big disadvantage, high temperatures are generated by increasing the compression ratio beyond the point where any increase in thermal efficiency is cancelled by increased friction. Because of the high compression ratios used in high speed diesels, the constant volume cycle which has a higher specific output and thermal efficiency, cannot be used because of the excessive pressures that would be generated.

Over the last sixty years there has been extensive work carried out by the world's leading engine makers, to develop an engine that would bridge the gap between the pre-mixed spark ignition and the compression ignition (diesel) engine cycle.

This has led to the development of the stratified charge theory in the spark-ignited engine. The theory of the stratified charge engine is to divide the charge in the cylinder into a rich and lean zone. The rich mixture is first ignited to cause a hot flame front to raise the temperature in the cylinder to such an extent that the lean fuel/air mixture will burn.

To achieve a lean and rich zone a precombustion chamber is set in the cylinder head or a pre-combustion chamber is machined into the top of the piston crown. All are similar in concept to the engine shown in Fig. 1, 2 and 3.

The direct injected stratified charge piston cavity engines are divided into late injection and early injection engines. The late injection engines are based on diesel technology and the fuel is injected directly into the piston cavity, late on in the compression stroke. The fuel is ignited as soon as, or just before, injection is completed.

The early injected engines are those in which the fuel is injected early on in the piston stroke, so that only part of the fuel is contained in the cavity and the remainder is dispersed throughout the cylinder.

There are also injected cycles that are given a degree of centrifugal advance, with increased engine speed. So far none of these injection combinations have been wholly successful.

When fuel is injected into a combustion recess formed in the top of the piston, the amount of containment of the fuel is dependent on the position of the piston from T.D.C. When late injection is used, the walls of cavity will prevent the fuel from diffusing into the rest of the compression chamber. If at this stage, the temperature in the cylinder was above the critical limits of the fuel used, combustion would begin, but when the temperature in the cylinder is below the fuel's critical limits, as in the pre-mixed engine cycle, the concentration of fuel in the piston cavity must be within the limits of flammability of the fuel used (85% of the chemically correct mixture).

If a vigorous squish action was introduced at this stage, to give improved mixing at maximum load, it would have an adverse effect on the charge compactness at light load. Therefore squish action is not desirable in a late injection cycle.

The next alternative to be considered is the injection of fuel earilier on in the compression cycle. If injection commenced too early in the cycle, the fuel would be diffused throughout the cylinder and would only achieve combustion when the whole of the compressed volume reached 85% of the chemically correct ratio. It is therefore necessary to delay injection in order that a sufficient quantity of fuel will enter the piston cavity, so that the spark flammability ratio will be reached.

After considering the limits imposed by direct fuel injection, it is obvious that the correct mixture of air and fuel is necessary before the charge can be ignited.

Because of the reasons stated, the divided charge pre-mixed direct injection engine is unable to complete in its present form, with the part load efficiency of the compression ignition engine.

Because of the problems involved with the stratified charge method of operation, there is still no volume production engine that is capable of achieving compression ignition economy, with pre-mixed engine power through the wide range of speed and load required for automotive application.

By mixing air with fuel jn the induction period of an engine, the fuel is well prepared to absorb oxygen. Most fuels ignite by a two-stage low and high temperature process. When the mixture is subjected to compression, oxidation is greatly accelerated. This causes a chain reaction so that at a given temperature and pressure, the high and low temperature bands merge.

This ignition region is approximately the compression and temperature generated in the pre-mixed engine cycle. Excess oxygen at this stage is known to promote combustion.

In conventional engines, to try to overcome the limits set by compression and temperatures, anti-oxidants are added to the fuel (Tetra-ethyl lead etc.,) but these additives are a major source of pollution.

After reviewing the progress made in modern engine technology, it is necessary to reassess the progress made to achieve an engine that has the combined advantages of the compression ignition and the pre-mixed cycle. The advantage of the compression ignition engine is its ability to run with a lean mixture strength nearly down to zero.

It has the disadvantage of being unable to cope with a mixture richer than 85% of the chemically correct value. The pre-mixed spark ignition engine can develop extra power by operating with a fuel mixture well above,the chemically correct value. It has the big disadvantage of being unable to function correctly with a mixture strength less than 85% of the chemically correct value. This means in effect that one engine takes over in power where the other finishes.

If a pre-mixed engine could be ignited as easily as a compression ignition engine, the engine speed and power could be regulated by varying the fuel quantity. At idling, the fuel to air ratio could be 80 to 1 and at full power 15 to 1 (chemically correct). Between these two extremes the mixture ratio could be varied to suit all load and speed requirements. Under these conditions an engine's overall performance would be greatly improved.

When fuel is injected into an engine cylinder at tick-over, it is not the shortage of air that is the problem because the cylinder contains all the air necessary for combustion. The main problem is how to achieve the correct combination of air and fuel compatible with each cycle of operation. The pre-mixed stratified engine cycle requires chargecompactness so that the fuel can be more readily ignited. When speed increases more fuel must be vaporised and pre-mixed evenly throughout the cylinder so that it is well prepared for the flame to spread evenly across the charge. In the compression ignition cycle where temperature is the most important factor, injection is delayed as late as possible in the compression stroke so that the fuel will ignite more readily. There must always be a delay period after the start of injection, until the fuel has absorbed enough heat to ignite.Because of these limiting factors, the time taken for injection and ignition is very limited and normally lasts from 200--300 of crank angle.

This has lead to the development of the jerk injector pump, which will deliver a high pressure concentrated charge of fuel in the short time available.

Although there is adequate air in the cylinder, the highly concentrated fuel is difficult to mix, therefore the air in the cylinder has been given a high velocity directional swirl to rip the fuel apart., so that it is oxygenated as soon as possible. This quick break up of the fuel greatly increases the speed of combustion and therefore over rapid build-up in the cylinder pressure.

This rapid pressure rise at tick-over is the main cause of diesel engine knock. All stratified charge spark ignited engines, so far developed, have a part of the fuel pre-mixed before it can be ignited.

They all use premixing bowls set in the piston.

Directional air-flow is also created by a modified inlet valve or port. Although this directional airflow pattern is ideal for the modern jerk fuel injection pump used on the present compression ignition (diesel) engines, it is opposite to the requirements of the pre-mixed charge engine, which requires general turbulence throughout the cylinder. This directional movement of air is the main reason for the lack of success in the present stratified charge engines.

It is the universal adoption of the jerk fuel pump principle that has created most of the problems that are associated with the modern compression ignition engine. By adopting compression ignition technology, these problems have been passed on to the spark ignited stratified charge engine.

Therefore, in considering new engine development, .he problems associated with the jerk fuel pump must be resolved.

When the performance of the jerk fuel injector pump is studied in detaii, it becomes obvious that there are two main problems to be overcome. The first is caused by the highly concentrated fuel having to be discharged over a very limited crank angle and the second problem is created by using high velocity air to break up this highly concentrated fuel, resulting in rapid build-up of pressure. This makes the extension of injection beyond T.D.C. necessary, otherwise excess pressures would be generated when more fuel is required when the engine load increases.

Because of this extended injection sequence, it is not practical to incorporate the power advantage of the constant volume cycle. When fuel is injected during the time of maximum pressure build-up, air movement becomes unstable, making correct distribution of air and fuel very difficult to achieve. It is impossible, therefore, to seek more power from the engine by giving it a richer mixture than normal. Excess fuel cannot be properly burnt even though it is well above its spontaneous combustion temperature.

The next factor to be considered is the compression temperature and pressure. This will vary with the density of the air in the cylinder, the position of the piston and the speed of compression. When the final compression temperature is known, the critical temperature of the fuel can be matched to this temperature. The fuel to be selected must have a critical temperature above the maximum temperature in the cylinder so that the onset of spontaneous combustion will be delayed. When the compression temperature is measured in relation to piston movement, half the total temperature rise takes place in the final 15% of piston stroke and 10% of the total heat is generated in the last 2% of compression. This makes the starting point of injection, in relationship to piston movement, critical.

The spontaneous ignition time of hydro-carbon fuels varies with mixture strength, temperature and pressure. It is possible, by selecting the correct combination of fuel injection timing, compression and temperature, to achieve a more efficient engine. Before selecting the fuel to be used there are two important temperatures to be taken into account. These are the closed flash point and the critical temperature. The closed flash-point is the temperature at which the fuel can be spark ignited. The critical temperature is the point when the fuel will self-ignite instantaneously. When petrol or diesel is injected into an engine cylinder, the maximum compression temperature must be below the fuel's critical temperature. Although diesel fuel (Cetane No. 50) spontaneously ignites at a temperature below 2500C, the delay may amount to seconds.This delay rapidly reduces as the pressure and temperature increases.

Because of the limited time available for combustion the minimum cylinder temperature required must be over 5200C so that combustion will take place almost instantaneously. By injecting diesel fuel into the engine below this temperature, the timing of combustion can be controlled by spark ignition.

When petrol (which is a collective term to denote hydrocarbon fuels of various chemical structures) is mixed with air in the induction period of an engine, the fuel will be more readily oxidised. With early oxidation by compression and temperature, acetaldhyde and peroxides are generated. These are known promoters of combustion. When pressure and temperature increase, oxidation rapidly accelerates so that at a critical temperature and pressure combustion takes place. Because of this rapid oxidation the spontaneous and critical temperatures merge.

To overcome this big disadvantage when petrol is used in the pre-mixed engine cycle, combustion inhibiting catalysts (tetra-ethyl lead etc.,) have hereinbefore been added to delay the onset of spontaneous combustion so that higher compression ratios can be used. Although unleaded petrol has a low spontaneous combustion temperature, it is unsuitable for the compression ignition (diesel) cycle, because it has a low Cetane number. This is because petrol has two critical temperatures. The first critical spontaneous ignition (cool flame) takes place within the temperature pressure range of approximately five atmospheres and 2500C- 4000C.

Beyond this pressure and temperature the fuel is non-ignitable other than by spark-ignition until a higher critical temperature is reached 5000C to well over 7500C.

The present invention provides an internal combustion engine having one or more cylinders in the or each of which a piston is reciprocable, fuel injection means for the or each cylinder and ignition means for the or each cylinder, and a throttle arrangement controlling injection to the or all the cylinders, wherein injection always ceases at a fired position at or close to top dead centre and commences at a position which can vary up to a position considerably before the top dead centre, the engine having constant pressure characteristics at low throttle openings and, at high throttle openings characteristics which are at least partially constant volume characteristics.

In the engine of this invention air is compressed in the cylinder, as in the compression ignition cycle. A compression ratio is selected to give a pressure over five atmospheres and a temperature over 2500C when the piston is 600 before T.D.C. By keeping the fuel and air (oxygen) separated until the first critical temperature pressure stage (cool flame) is exceeded. Fuel can then be injected into the cylinder without the risk of spontaneous combustion until a higher critical temperature is reached. By selecting a final compression temperature that is lower than the fuel's second stage critical temperature, combustion can be controlled by spark ignition.

By adopting this method (high temperature cracking of fuel), lead free petrol can be used at a higher compression ratio than is now possible without the risk of pre-ignition.

After selecting the correct combination of fuel and compression ratio, the next factor is to select the injection timing necessary to achieve the injection sequence required. The injection pump should be able to supply the engine with a constant rate of discharge per degree of crank angle and the duration of injection must be able to be advanced and retarded at will. When the weight of fuel injected per degree of crank movement remains constant, there is always a point in the fuel-air stream where the mixture can be spark ignited. This is irrespective of the mixture ratio in the rest of the cylinder. This condition is unlike the pre-mixed stratified charge cycle where the conditions for flammability are constantly changing. It is also necessary to control the air movement leading up to the point of combustion and during the subsequent pressure build-up.

The ignition arrangement can be electrodes or a spark plug arranged to ignite an inflowing stream to cause a torch effect, to give a constant pressure or diesel type power generation from ignition to the end of injection.

The engine of the invention allows a constant pressure part of the cycle to take place especially at high speed so that excessive pressures are not generated. The injection of fuel at an early stage cools the cylinder contents to ensure the spontaneous combustion due to compression heating will be delayed beyond the point of useful work on the piston. At low engine revolutions, the engine of the invention has characteristics of power output similar to those of the constant pressure cycle (diesel) engine.

When the accelerator is fully opened to increase speed, the instant advance of injection (600 before T.D.C.) causes a constant volume reaction to give maximum power as in the premixed petrol engine.

With the advance of ignition at high speed, the characteristics of the engine change to a constant volume reaction when the accumulated fuel in the cylinder is ignited. This is extended by a constant pressure cycle until injection stops. When the accelerator is released the engine returns to a constant pressure cycle.

The invention will be described further, by way of example, with reference to the accompanying drawings, wherein Figs. 1, 2 and 3 are cross-sectional views showing the pistons of known engines and their operation in known engines; Figs. 4 and 5 illustrate, schematically, operation of a preferred embodiment of engine of the invention; Fig. 6 is a block diagram showing cetane numbers of various fuels; Fig. 7 illustrates spark generation at an atomising nozzle; Fig. 8 illustrates operation of various engines including the engine of the invention; Fig. 9 illustates a known fuel injection pump; and Fig. 10 illustrates a modified fuel injection pump.

A preferred internal combustion engine of the invention has one or more cylinders 10 in the or each of which a piston 11 is reciprocable. The engine as a whole is generally of conventional construction and is not wholly illustrated. The engine has means for injecting fuel into each cylinder at desired positions in the operating cycle of the cylinder. Such means are well known and therefore not described in detail, although a particularly suitable such means is discussed later.

In a cylinder head 12 of the engine for the or each cylinder 10 is an injection nozzle 13 and a spark ignition means 14 which as illustrated takes the form of a pair of electrodes 1 5, 1 6 between which a spark is generated at a low determined by a conventional ignition circuit of the engine. The provision of the two electrodes 1 5, 1 6 takes advantage of the effect that the jet of incoming fuel at 17 can deflect the incipient spark tending to cross its path to give a longer spark with an efficient igniting action as illustrated in Fig. 7. It must be appreciated, however, that the electrodes 15, 1 6 could be replaced by one or more conventional spark plugs.Because in this preferred engine the rate of fuel supply by the injection nozzle 14 is constant, ie a fixed amount of fuel is introduced for each degree of crank angle, there is always a position at which the richness of the mixture is correct for ignition, ie within 85% of the stoichiometrically ideal fuel/oxygen ratio, so that reliable ignition can be ensured without the separation which is necessary in stratified charge engines. A torch effect is always generated which will ignite and burn any weak mixture in the cylinder 10 which has been accumulated during early injection.

The fuel supply means to the or each injector 14 is so connected to a throttle or accelerator that fuel injection to the or each cylinder always terminates a point within 5 of top dead centre (T.D.C.). Preferably this point is at T.D.C., but can be varied from 50 before T.D.C. to 50 after T.D.C.

without making a major departure from the ideal operating conditions of the engine. The rate of injection is constant, ie a fixed amount of fuel is injected per degree of crank angle and the commencement of injection is variable from a position shortly before termination, for example 5 before injection termination up to a position considerably before termination, such as 600 before T.D.C. Ignition is effected at a position which will vary from the start of injection, at low speeds, and will be centrifugally advanced to a position in advance thereof at higher speeds.

Thus at low speeds and low throttle openings injection and ignition will occur close together and the engine will operate on a cycle close to if not identical to a touch ignited constant pressure (diesel) cycle. Centrifugal ignition advance will ensure that this condition is stili loosely satisfied during increasing speed with slow throttle opening, centrifugal ignition advance keeping pace with or slightly behind the throttle as it causes advanced initiation of injection. When the throttle is depressed rapidly for high acceleration the initiation of injection advances rapidly whilst centrifugal ignition advance lags significantly.

Under these conditions the engine operates on a cycle which is to a great degree similar to the petrol cycle, that is a pre-mixed charge is built up in the cylinder(s) and eventually burnt at in a constant volume ignition phase.

When petrol is used in the engine the design parameters are chosen such that this early injection takes place above the lower combustion point of the fuel (conditions of temperature and pressure) and below the upper combustion point of the petrol allowing an additive-free petrol to be used.

Fig. 8 illustrates the injection/ignition conditions of conventional petrol and diesel engines (Figs. 8(a) and 8(b)) and of the engine of the present invention at ticksver low speeds (8(c)) and at a throttle-depressed high acceleration condition of the engine (8(d)).

The spark ignited engine pre-mixes the charge in the cylinder within the limits of the fuel's flammability. This mixture is then spark ignited.

The compression ignition engine is a part stratified charge pre-mix sequence, until ignition takes place. This is followed by an extended torch burning period that is terminated when injection stops.

The basic concept of my engine is to combine the best features of both engines. To achieve this the engine is run on the compression ignition cycle but with a lower final compression temperature, so that the onset of spontaneous combustion is delayed. A spark plug is fitted in the cylinder head so that the delay period can be controlled by spark ignition. The fuel injection can be advanced and retarded at will so that the engine can be varied from torch burning to a premixed or stratified charge sequence, In order to keep the crank pressure angle constant, the spark ignition timing is advanced in proportion to the speed.

When petrol or diesel is injected in very light quantities vertically into the cylinder of the high speed automotove engine, it is desirable for the air in the cylinder to be adjusted to the fuel's critical temperature. The critical temperature is the point beyond which the fuel will ignite instantaneously. Although diesel fuels (Cetane No.

50) spontaneously ignites at a temperature below 2500C., the delay may amount to seconds. This delay rapidly reduces as the temperature increases. Because of the limited time available for combustion, the minimum cylinder temperature required must be over 5200C so that combustion will take place almost instantaneously. By injecting diesel fuel into an engine below this temperature, the timing of combustion can be controlled by spark ignition.

The critical temperature of petrol (which is a collective term to denote hydro carbon fuels of various chemical structures) has a spontaneous ignition temperature below 2500C. Because petrol is more readily oxidised than diesel the spontaneous and critical temperatures coincide.

To overcome this disadvantage when it is used in the pre-mixed engine cycle, combustion inhibiting catalysts (tetra ethyl lead etc,) are added to delay the onset of spontaneous combustion, so that higher engine compression ratios can be used.

Although unleaded petrol has a low spontaneous combustion temperature, it is unsuitable for the compression ignition (diesel) cycle as it has a Cetane No. of 20.

(The Cetane Numbers of various fuels are set out in Fig. 6).

This is because petrol has two critical temperatures. The first critical spontaneous ignition takes place within the temperature range 2500C-4000C. Beyond this temperature the fuel is non-ignitable other than by spark-ignition until the higher critical temperature is reached, 5000C to well over 7500C. (See reference I to Veil).

When compression temperature is measured by piston movement, half the total temperature rise takes place in the last 600 of crank movement.

When petrol is injected into an engine with the temperature in the cylinder at 600 before T.D.C., higher than the first critical temperature of the fuel (2500--4000C) the injected fuel will pass through this temperature region so quickly that spontaneous ignition will not take place until the second critical temperature region (5000 C- 7500 plus) is reached.

Because fuel temperature is the most important factor, this makes the starting point of injection in relationship to piston movement critical. When fuel is injected vertically into the engine cylinder, the pressure of the spray will create a partial vacuum behind the injector. This will be filled by air rushing in to fill the void. (See reference IX Ford Combustion Process). This tends to create radial air movement as the air mixes with the advancing fuel stream.

If a means of spark-ignition is super-imposed above this air stream in the area of relative vacuum, any spark created will be carried forward with the advancing fuel-air mixture. A spark can arc from one electrode to the other only when a sufficiently high voltage is present. The amount of voltage discharge required is governed by gas pressure and turbulence at the instant of firing. The higher the pressure the higher the voltage required to overcome this resistance. If the fuel-air mixture is very turbulent because of high speed compression, the spark can split up and will require more voltage to over-ride this extra suppression.

When pressure and turbulence have a suppressive effect on the spark discharge at the instant of firing, the opposite will occur by the sudden release of pressure at the moment of discharge. When fuel is injected under pressure into the air stream in a pressure jet burner, a partial vacuum is created behind the injector.

Electrodes are positioned above the injector nozzle (see Fig. 7), and when the spark is created between the electrodes at 'A' the spark is stretched forward and deflected into the air stream (position 'B') so that the fuel-air mixture will easily ignite.

The ability to stretch and deflect a spark is the main reason for ease of ignition in the pressure jet burner. When a spark can be extended and deflected by air movement this factor should be taken into consideration when the spark plug position is decided in a direct injection engine.

When a spark can be stretched forward and deflected into the advancing fuel-air mixture, it will automatically ingite the mixture as soon as the limits of flammability are reached. By this method of igniting the incoming fuel the premixing bowl that is necessary in the current premixed stratified charge engines is eliminated, because no mixing of the charge is required.

The principle of igniting the incoming fuel is based on the theory that there will always be the correct fuel-air ratio in the injected fuel-air stream. This will even apply when the minimum quantity of fuel is injected to maintain the tickover.

Let us assume that a jet of fuel entering the cylinder will mix with air and can be ignited just after leaving the injector nozzle, so that a blow torch will be created. The torch will continue to burn until all the air in the cylinder is consumed or the injection of fuel is cut off.

When the injection timing is advanced, a stream of fuel-air mixture will be discharged into the cylinder. When ignition takes place before this fuel-air stream has time to spread, the flame will rapidly advance to consume this highly combustable mixture. This will cause a rapid rise in the cylinder temperature. The cylinder temperature will continue to rise until injection ceases. Because of the rise in cylinder temperature the speed of the engine will increase.

If the ignition timing is centrifugally advanced, the spark will be moved forward. By this method of operation the torch burning duration can be extended by slowly advancing the injection timing.

When injection timing is advanced faster than the speed of ignition advance, part of the fuel-air stream diffuses into the cylinder. This will change the engine from a torch burning to a torch ignited stratified charge sequence. When the injection timing is advanced to a maximum of 600 before T.D.C. at tick-over with the spark ignition at 80 before T.D.C., the fuel will be diffused throughout the cylinder. This gives the engine a torch ignited pre-mixed sequence.

To achieve the injection sequence required, the present diesel fuel injection jerk pump must be reversed and the length of stroke increased.

(reference X). The present diesel jerk pump has an injection time of approx. 250 duration.

Injection timing starts from a fixed point 100 or 1 50 before T.D.C. and stops at T.D.C. To increase speed, injection timing is progressively extended, so that at maximum load injection stops 100 after T.D.C.

The modified jerk pump will have an injection timing of 600 duration and the sequence of operation is reversed. Injection will have a fixed stopping point at T.D.C. Injection is advanced to 100 before T.D.C. to maintain tick-over. To increase speed the injection is progressively advanced to 600 before T.D.C. When fuel is injected vertically into the cylinder of an engine, as soon as it begins to burn, there is a rapid increase in expansion and convection. This greatly accelerates velocity. This increased velocity will cause the burning gas to be deflected off the rising piston, so that a free-dimensional radial swirl is created (Fig. 5).

This will re-cycle the combustion gases back through the burning zone. As a result of this recycled air motion, a more complete combustion will occur. When the fuel injected vertically in light concentration has the ability to be spark ignited at will, the fuel injection and ignition timing can be selected to give the most advantageous crank pressure angle, under all conditions of power and load. The modified pump will have a timing that stops injection at T.D.C.

but retains the capacity to extend injection duration forward from T.D.C. to 600 before T.D.C.

If we assume that an engine with an injected fuel duration of 600 of crank angle will need a minimum requirement of 100 of fuel injection duration, to maintain a stable tick-over, the tickover injection timing will begin at 100 before T.D.C. and stop at T.D.C. The spark ignition will begin at 80 before T.D.C. to give a torch burning duration of 80 (constant pressure cycle).

When the accelerator (pump advance link) is gradually depressed, the injection timing will slowly advance. This will increase the torch burning time because the extra fuel will be consumed by the advancing flame. This will raise the cylinder temperature thus causing the engine speed to increase. This will slowly advance ignition (which is centrifugally controlled by engine-speed). This will again extend torch burning duration in the cylinder. Because the injection advance timing is independent of the speed of the engine, the ignition timing (which is controlled by engine speed) will tend to lag behind injection so that a degree of pre-mixing will occur as the load increases. This will cause the engine gradually to change its cylinder burning sequence to a part-premixed and part torch accelerated burning process.

Under maximum acceleration when the accelerator (pump advance link) is fully depressed, injection will advance to 600 before T.D.C. By this means, injection will continue from 600 before T.D.C. to T.D.C. When the engine is idling and the accelerator is kicked down, injection is advanced to 600 before T.D.C. This will give the engine the characteristics of the premixed constant volume cycle, because there will be 520 of pre-mixed fuel injection which will be ignited by 80 of torch combustion. As the engine approaches maximum speed, ignition timing will have centrifugally advanced so that the pre-mixed and torch burning duration will have reached equilibrium. (Constant volume cycle followed by constant pressure cycle).

When the accelerator is suddenly released at maximum speed, injection returns to the tick-over position 100 before T.D.C. to T.D.C. (Torch burning constant pressure cycle). Because the ignition timing is centrifugally advanced, there will be a time lag until ignition and injection balance.

To overcome this, the ignition arc of duration must be extended.

Let us now consider the commercial compression ignition engine, with a modified jerk injector pump that can spread the rate of fuel injection over 600 of crank angle. If we assume that 100 of fuel injection duration will be the minimum weight of fuel required to maintain tickover, injection would end at 100 after T.D.C. and start at to T.D.C. The start of the ignition at T.D.C.

would be when the temperature in the cylinder had reached its maximum. By injecting fuel at 1/3rd the present concentration per degree of crank angle, vertically into the cylinder, when the air in the cylinder is well above the fuel's critical temperature, the injected fuel will rapidly increase its velocity because of the pressure of injection and the very high temperature expanding each droplet of fuel into vapour.

This injected rapidly expanding fuel creates a partial vacuum behind the injector which will be filled by air rushing in to fill the void. This will create radial air movement as the air mixes with the expanding vapour. Velocity is further increased by the start of spontaneous combustion. This causes the rapidly accelerating burning mixture to be deflected off the piston so that a three dimensional radial swirl is created.

This will recycle the combustion gases back through the burning zone. The radial swirl can be greatly increased by a squish action created by the piston when it reaches T.D.C. This will accelerate the speed of combustion thus causing a more complete combustion to occur. Because combustion takes place after T.D.C. at tick-over, the knock associated with high compression ignition engines at tick-over will be eliminated.

Injection is progressively advanced to increase power and speed. By advancing injection timing to match engine speed, more time is available in the cylinder for the less concentrated fuel to mix evenly with the air. Because of this, high velocity air is not required to rip the fuel apart. This makes the rate of pressure rise more controllable so that it will allow the compression ignition cycle to gain the power advantage of the constant volume cycle.

When a high compression ignition engine has an injection timing of 100 after T.D.C., advancing to T.D.C. to maintain tick-over, injection will be progressively extended forward by 600 duration, so that maximum speed will be reached by extending injection from 100 after T.D.C to 500 before T.D.C. By this method of pump operation the compression ignition engine will have an extended speed and power range that will be comparable in performance to the pre-mixed petrol engine.

Claims (11)

Claims

1. An internal combustion engine having one or more cylinders in the or each of which a piston is reciprocable, fuel injection means for the or each cylinder and ignition means for the or each cylinder, and a throttle arrangement controlling injection to the or all the cylinders, wherein injection always ceases at a fixed position at or close to top dead centre and commences at a position which can vary up to a position considerably before top dead centre, the engine having constant pressure charactersitics at low throttle openings and, at higher throttle openings characteristics which are at least partially constant volume characteristics.

2. An engine as claimed in claim 1, wherein the position of termination of injection is within 50 of T.D.C.

3. An engine as claimed in claim 2, wherein the position of termination of injection is at T.D.C.

4. An engine as claimed in claim 1, 2 and 3, wherein ignition occurs, at tick-over at a position 50 before termination of injection.

5. An engine as claimed in any preceding claim wherein injection commences at up to 600 before T.D.C.

6. An engine as claimed in any preceding claim and operating with petrol as fuel, the temperature and pressure within the or each cylinder being chosen so that at all times they are above the low level combustion conditions of the petrol to enable the petrol to be used without any anti oxidant additives.

7. An engine as claimed in any of claims 1 to 5 and operating with diesel fuel on the compression ignition cycle.

8. An engine as claimed in claim 6, wherein the cylinder temperature is above 2500C.

9. An engine as claimed in claim 6, wherein the cylinder temperature is above 4000 C.

1 0. An engine as claimed in claim 6, wherein the pressure ia above five atmospheres.

11. An engine as claimed in claim 6, wherein the pressure is above six atmospheres.

1 2. An internal combustion engine substantially as hereinbefore described with reference to and as illustrated in Figs. 4 and 5 of the accompanying drawings.