GB2327980A

GB2327980A - Gasoline i.c. engine with spark-ignition and auto-ignition in the same cylinder

Info

Publication number: GB2327980A
Application number: GB9716157A
Authority: GB
Inventors: Thomas Tsoi Hei Ma
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 1997-08-01
Filing date: 1997-08-01
Publication date: 1999-02-10
Also published as: GB9716157D0

Abstract

A method of operating a gasoline internal combustion spark ignition engine is disclosed, which comprises first separating the gasoline into a lighter vapour fraction and a heavier liquid fraction. The two fuel fractions are supplied separately to the combustion chambers of the engine in such a manner as to achieve stratification of the charge in each cylinder at the instant of spark when the engine is operating at part load. The stratified charge includes a first ignitable mixture cloud lying in the vicinity of spark plug at the instant of the spark and containing the heavier liquid fraction of the gasoline, and a second mixture cloud lying at a distance from the spark plug at the instant of the spark and containing the lighter vapour fraction of the gasoline. The fuel-to-air ratio and the composition of the fuel in the second cloud are such that the second cloud undergoes auto-ignition subsequent to the instant of spark ignition of the first cloud, the auto-ignition being brought about by the increased temperature and pressure resulting from the burning of the fuel in the first cloud.

Description

GASOLINE INTERNAL COMBUSTION ENGINE Field of the invention The present invention relates to a gasoline internal combustion engine in which the combustion intentionally takes place at least in part as a result of auto-ignition.

The term auto-ignition that is used herein refers to spontaneous ignition of the fuel that occurs under high pressure and temperature conditions without relying on propagation of a flame. Such ignition is the same as occurs in diesel engines and is also variously referred to as compression ignition and self-ignition.

Background of the invention Usually, steps are taken to avoid auto-ignition in a gasoline engine when it is operating under high loads because it causes knock and can damage the engine. However, under part load operating conditions, there are advantages in intentionally causing auto-ignition because such combustion produces very low NOx emissions and allows complete combustion of very lean mixtures.

US-5,535,716 discloses a compression ignition gasoline engine where gasoline fuel is injected into the intake port during the period when the intake valve is closed to allow the fuel to be fully vaporised by extracting heat from the port before being drawn into the combustion chamber. The heated fuel, mixed with intake air, is compressed and heated in the combustion chamber in the usual manner but the compression ratio of the engine is so high that the mixture auto-ignites without the need for a spark to ignite the mixture. To achieve auto-ignition, the compression ratio needs to be in the range from 14:1 to 20:1. The patent specification claims that stable operation by auto-ignition is obtained with the engine running unthrottled with an air-to-fuel ratio equal to or less than 57:1. The NOX emissions of this engine are stated to be one thirtieth of that produced by a diesel engine and one sixtieth of that produced by a direct injection stratified charge spark ignition engine under similar load conditions.

Such an engine however suffers from the disadvantage that the timing of auto-ignition cannot be directly controlled and the instant at which auto-ignition will occur will vary with the chemical and thermal state of the compressed mixture charge. This can result in poor thermodynamic efficiency and erratic operation.

Another disadvantage of such an engine is the limited range of low load and lean air-to-fuel ratio in which the engine can operate, because at higher loads and richer air-to-fuel ratios auto-ignition can cause severe knock, with resultant damage to the engine. The high fixed compression ratio of the engine precludes it from being used to produce a sufficiently wide range of operating conditions to be suitable for vehicle applications.

Object of the invention Accordingly, the present invention seeks to provide a gasoline engine in which the timing of auto-ignition can be controlled and that can operate over a wide range of loads.

Summary of the invention In accordance with a first aspect of the invention, there is provided a method of operating a gasoline internal combustion spark ignition engine, which comprises separating the gasoline fuel into a lighter vapour fraction and a heavier liquid fraction, and separately supplying the two fuel fractions to the combustion chambers of the engine in such a manner as to achieve stratification of the charge in each cylinder at the instant of spark when the engine is operating at part load, the stratified charge including a first ignitable mixture cloud lying in the vicinity of spark plug at the instant of the spark and containing the heavier liquid fraction of the gasoline, and a second mixture cloud lying at a distance from the spark plug at the instant of the spark and containing the lighter vapour fraction of the gasoline, the fuel-to-air ratio and the composition of the fuel in the second cloud being such that the second cloud undergoes auto-ignition subsequent to the instant of spark ignition of the first cloud, the auto-ignition being brought about by the increased temperature and pressure resulting - from the burning of the fuel in the first cloud.

In accordance with a second aspect of the invention, there is provided a gasoline internal combustion spark ignition engine having a continuous fuel vapour extraction system for separating the gasoline fuel into a lighter vapour fraction and a heavier liquid fraction, and means for introducing the two fuel fractions separately into the intake air charge at least during part load conditions, in such a manner as to produce a stratified charge within the combustion chamber at the instant of spark, wherein the stratified charge includes a first mixture cloud containing the heavier fraction of the fuel, which first cloud lies in the vicinity of the spark plug and is ignited by the spark, and a second mixture cloud lying at a distance from the spark plug and containing the lighter fraction of the fuel, the fuel-to-air ratio and the composition of the fuel in the second cloud being such that the second cloud undergoes auto-ignition subsequent to the instant of spark ignition of the first cloud, the auto-ignition being brought about by the increased temperature and pressure resulting from the burning of the fuel in the first cloud.

The invention takes advantage of the fact that the vapour fraction of the gasoline fuel blend has a lower octane number than the average octane rating for the whole blend and therefore auto-ignites more readily at the compression ratio of the engine designed for safe operation for the whole blend. By the same token, the heavier fraction has a higher octane number and is less prone to auto-ignition.

In the invention, steps are taken to ensure the fuel-to-air ratio and the composition of the vapour fraction of the fuel in the second cloud are such that the mixture is approaching auto-ignition during the compression stroke, yet does not auto-ignite until after the instant of spark ignition of the first cloud.

One such step is to set a predetermined ratio for the quantities of the liquid and the vapour fractions of the fuel supplied to the engine which are introduced into the first and the second clouds respectively. For a given total quantity of the fuel supplied to the engine, if the liquid fraction is increased while the vapour fraction is decreased, the first cloud will be richer and will contain more of the heavier fraction of the fuel while the second cloud will be leaner and will contain more of the lighter fraction of the fuel. Conversely, if the liquid fraction is decreased while the vapour fraction is increased, the first cloud will be leaner and will contain less of the heavier fraction of the fuel while the second cloud will be richer and will contain more of the heavier fraction of the fuel.

In each of the two cases, the overall fuel-to-air ratio supplied to the engine is the same but the propensity of the second cloud to auto-ignite is very different. It is therefore possible to adjust the ratio of the quantities of the liquid and the vapour fractions of the fuel precisely to achieve the optimum auto-ignition characteristics for the second cloud, the instant of auto-ignition being always controlled indirectly by the spark timing.

Another step is to pre-condition the vapour fraction to increase its reactivity before introducing it into the second cloud. Such pre-conditioning may including heating the vapour fraction in a heat exchanger or mixing the vapour fraction with hot exhaust gases recirculated from the exhaust system to the intake system of the engine.

As a further possibility, the compression ratio of the engine may be increased by a discrete amount, but not to such an extent as to cause auto-ignition without being first triggered by the spark ignition.

As will be appreciated from the foregoing discussiorr, the invention relies upon the availability of a system for continuously separating the gasoline fuel into a vapour containing the lighter fraction of the fuel and a liquid containing the heavier fraction. Such a vapour extraction system is described and claimed in a copending application No. , (Agent's reference P/4569) filed on the same date as the present application and will be described in more detail below.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section through a combustion chamber showing the an example of charge stratification that is suitable in implementing the present invention, and Figure 2 is a block diagram similar to the single drawing of copending patent application No. , that shows a continuous fuel vapour extraction system.

Detailed description of the preferred embodiment The present invention relies on the availability of separate continuous supplies of vapour fuel and liquid fuel.

These can be derived from separation of gasoline into a lighter and a heavier fraction in the manner that will now be described with reference to Figure 2, this being the subject of the above mentioned copending patent application No. .

An engine 10 has an intake manifold 16, a main throttle 14 and an intake passage containing a venturi 12. A fuel injection system for the engine comprising a fuel circulation pump 32 that supplies fuel under pressure into a fuel rail 34 from which fuel is dispensed to the individual cylinders of the engine by fuel injectors 18. The pressure in the fuel rail 34 is regulated by a relief valve 36 that derives a reference pressure from the intake manifold 16.

Surplus fuel is spilled by the relief valve 36 into a fuel return pipe 38.

While it is conventional for the pump 32 and the return pipe 38 to be directly connected to the main fuel storage tank, designated 20 in the drawing, they are connected instead to a volatising chamber 30 that contains a much smaller quantity of fuel. The volatising chamber 30 is connected to the main fuel tank 20 by a supply pipe 24 containing a fuel lifter pump 22 and the level of fuel within the chamber 30 is maintained constant by means of a float 28 and a valve 26.

An evaporator 40 is disposed in the vapour filled space of the chamber 30 above the liquid level and in the path of the fuel returned by way of the fuel return pipe 38. The return fuel is sprayed over the evaporator and the latter is designed to have a large surface area that is coated with a film of fuel. The large surface area may be achieved by using a matrix of capillaries or a porous or sintered block for the evaporator 40. Neither the evaporator 40 nor the fuel in the chamber 30 is heated and evaporation relies on the reduced pressure in the vapour space, the dispersion of the spray droplets, the large surface area of the evaporator 40 and such heat as the return fuel picks up during its circulation flow. The matrix of the evaporator 40 may be formed of a hydrocarbon storage material such as activated carbon to increase the quantity of vapour that can readily be extracted under dynamic conditions.

To maintain the vapour space in the volatising chamber30 below atmospheric pressure, a pipe 42 leading from it-is connected by way of a first pipe 46 and a regulating valve 56 to the venturi 12 and by way of a second pipe 44 and a regulating valve 54 to the intake manifold 16. The pipe 46 is also connected by way of a pipe 48 and a regulating valve 58 to a vapour canister 50 that is itself connected to the ullage space of the main fuel tank 20 by a pipe 52. Instead of the pipe 48 being connected to the pipe 46 to allow fuel vapour stored in the vapour canister 50 to be purged directly into the venturi 12, it is alternatively possible as represented by the pipe 48' shown in dotted lines to route the purge flow to the venturi 12 through the volatising chamber 30.

Under idling and low load conditions, a high vacuum will be present in the intake manifold 16 which will result in a high rate of evaporation of the fuel in the volatising chamber 30 and the bulk of the fuel requirement will be delivered to the engine in vapour form. A small quantity of liquid fuel corresponding to the unvaporised fraction of the fuel will be supplied by the fuel injection system so as to maintain the composition of the fuel consumed overall the same as that present in the fuel storage tank 20.

As the engine load is increased progressively, the pressure in the intake manifold 16 will rise towards atmospheric pressure while the venturi pressure will drop with increasing air flow. By suitable selection of the position of the regulating valves 54 and 56 the vacuum pressure in the volatising chamber 30 can be set to supply vapour at any desired rate while the balance of the fuel to make up the original composition of the fuel is injected by the fuel injectors. During this mode of operation the vacuum alone would not be sufficient to maintain the rate of vapour supply continuously but as a large proportion of the fuel is recirculated in the loop 32, 34, 36, 38 the cooling of the evaporator 40 will be compensated by heat picked up by the recirculating fuel and the evaporation rate will stabilise.

The rate of supply of fuel in vapour form to the engine depends upon the pressure and temperature prevailing in the volatising chamber 30 and the position of the regulating valves 54 and 56. The engine control system will first decide the total quantity of fuel to be burnt and the fractions to be supplied in vapour and liquid forms. Based upon these variables, as can be prior determined by conventional engine fuel calibration maps, the engine management system can set the positions of the regulating valves 54 and 56 to achieve the desired vapour flow rate and the pulse width of the fuel injectors 18 to achieve the desired liquid flow rate.

Under high load conditions, there will be hardly any vacuum in the intake manifold 14 but a high vacuum at the venturi 12. However under such high load it is not desirable to supply fuel vapour as it would reduce the volumetric efficiency and maximum power output of the engine, for which reason the valve 56 can be closed so that all the fuel requirement is met by the injected liquid fuel.

The fact that fuel vapour is used efficiently in running the engine allows proper use of such vapour as is stored in the vapour canister 50. Whereas normally fuel purged from the canister 50 is merely dumped into the intake system in an uncontrolled fashion to regenerate the canister 50, by routing the purge flow through the volatising chamber 30, such vapour flow is taken into consideration in determining the total amount of fuel vapour to be metered to the engine.

The vapour extraction system of Figure 2 copes well with a steady demand for fuel vapour as the operating pressure and temperature will move automatically to match the demand cope with sudden changes in the vapour demand, there is a need for a vapour store to act as a buffer. Such a vapour store is already present in the form of the canister 50 the content of which may be used by opening the valve 58 whenever a sudden surge occurs in the demand for fuel vapour. A second vapour store can be formed by using a storage material, such as activated carbon, in the evaporator 40 which will be replenished more rapidly than the vapour canister 50.

Referring to Figure 1, the engine is run under part load operating conditions with a stratified charge made up of two or more mixture clouds, shown schematically in a section through a combustion chamber of the engine. In the figure, a piston reciprocates in an engine block 100 and compresses the charge as it moves upwards towards a cylinder head 104. The intake and exhaust valves are not shown in the drawing but a spark plug 106 is shown located centrally in the combustion chamber.

A first mixture cloud 108 of the stratified charge contains the heavier liquid fraction of the fuel introduced by the fuel injectors 18, and lies next to the spark plug 106 when a spark occurs and is ignited by the spark. The lighter vapour fraction of the fuel is drawn from the pipes 42 and 46 via the regulating valve 56 to mix with the intake air at the venturi 12, and forms a second mixture cloud 110 that lies further away from the spark plug 106 and is not directly ignited by the spark.

The ratio of the quantities of the liquid and the vapour fractions of the fuel to be introduced into the first and the second clouds respectively is set by an engine management system and the respective fractions are drawn accordingly from the fuel vapour extraction system. This ratio should be such that, in the absence of a spark, neither cloud would auto-ignite during any part of the compression stroke, and in the presence of a spark, only the first cloud is ignited and burns by flame propagation in the same manner as in all spark ignition engines. This combustion raises the temperature and pressure in the combustion chamber to such an extent that the fuel in the second cloud is brought into auto-ignition and undergoes spontaneous combustion, initiated from multiple ignition sites occurring from within the second cloud.

The two parameters that affect the ability of the second cloud to auto-ignite are the fuel-to-air ratio and the composition of the vapour fraction of the fuel in the second cloud. As explained earlier, these parameters can be adjusted across a wide range, by varying the ratio of the liquid and the vapour fractions drawn from the fuel vapour extraction system while keeping the total fuel quantity supplied to the engine the same and the overall fuel-to-air ratio of the stratified charge unchanged.

For a given engine load, corresponding to a predetermined overall fuel-to-air ratio, the relative mixture strength and the relative fuel composition contained within the two clouds can be optimised to achieve the desired auto-ignition characteristics for the second cloud, resulting in consistent initiation of the auto-ignition always triggered by the spark ignition. By applying the calibration process across a range of engine loads, it is possible to maintain auto-ignition combustion covering a substantial area of the engine speed and load operating map used most frequently during city driving conditions.

The engine operation over its full range of loads may therefore be divided into three distinct modes.

At the lowest engine load condition, i.e., near idling, all the fuel must be included in the first cloud so that thestratified charge may be ignitable reliably by the spark Thus in this mode, 100% liquid fuel is drawn from the fuel vapour extraction system while the vapour supply is shut off.

As the engine load is increased progressively, more and more vapour fraction of the fuel is drawn from the fuel vapour extraction system and introduced into the second cloud, increasing within the second cloud the fuel-to-air ratio and the relative concentration of the heavier fuel fraction.

While the increasing fuel-to-air ratio would increase the fuel density and the tendency to auto-ignite, this is counteracted by the increasing octane number of the heavier fuel fraction which increases the resistance to autoignition. The net effect will be that for a substantial range of engine loads, the propensity of the second cloud to auto-ignite is maintained substantially constant, approaching auto-ignition during the compression stroke, yet does not auto-ignite until after the instant of spark ignition of the first cloud.

At the upper load range, because of the high risk of engine damage from auto-ignition, the engine is not run with a stratified charge and instead the fuel and air are homogeneous mixed, thus bringing into play the normal octane rating of the complete gasoline fuel distributed evenly throughout the entire charge.

Various methods of creating a stratified charge may be used in the engine of the present invention. For example, the vapour fuel fraction may be introduced into the intake system as a homogeneous mixture and the liquid fuel fraction injected directly into the combustion chamber to form a stratified charge inside the homogeneous mixture.

Alternatively, the vapour fuel fraction and the liquid fuel fraction may be guided along separate passages in the intake system and drawn into the combustion chamber in parallel streams with minimum mixing between the streams so as to retain the charge stratification.

Claims

1. A gasoline internal combustion spark ignition engine having a continuous fuel vapour extraction system for separating the gasoline fuel into a lighter vapour fraction and a heavier liquid fraction, and means for introducing the two fuel fractions separately into the intake air charge at least during part load conditions, in such a manner as to produce a stratified charge within the combustion chamber at the instant of spark, wherein the stratified charge includes a first mixture cloud containing the heavier fraction of the fuel, which first cloud lies in the vicinity of the spark plug and is ignited by the spark, and a second mixture cloud lying at a distance from the spark plug and containing the lighter fraction of the fuel, the fuel-to-air ratio and the composition of the fuel in the second cloud being such that the second cloud undergoes auto-ignition subsequent to the instant of spark ignition of the first cloud, the autoignition being brought about by the increased temperature and pressure resul-ting from the burning of the fuel in the first cloud.

2. An engine as claimed in claim 1, wherein means are provided to pre-condition the vapour fraction to increase its reactivity before introducing it into the second cloud.

3. An engine as claimed in claim 2, wherein the preconditioning means include means for heating the vapour fraction in a heat exchanger.

4. An engine as claimed in claim 2, wherein the preconditioning means include means for mixing the vapour fraction with hot exhaust gases recirculated from the exhaust system to the intake system of the engine.

5. A method of operating a gasoline internal combustion spark ignition engine, which comprises separating the gasoline fuel into a lighter vapour fraction and a heavier liquid fraction, and separately supplying the two fuel fractions to the combustion chambers of the engine in such a manner as to achieve stratification of the charge in each cylinder at the instant of spark when the engine is operating at part load, the stratified charge including a first ignitable mixture cloud lying in the vicinity of spark plug at the instant of the spark and containing the heavier liquid fraction of the gasoline, and a second mixture cloud lying at a distance from the spark plug at the instant of the spark and containing the lighter vapour fraction of the gasoline, the fuel-to-air ratio and the composition of the fuel in the second cloud being such that the second cloud undergoes auto-ignition subsequent to the instant of spark ignition of the first cloud, the auto-ignition being brought about by the increased temperature and pressure resulting from the burning of the fuel in the first cloud.

6. A method as claimed in claim 5, wherein the proportions of the gasoline contained in the vapour and the liquid fractions supplied to the engine are varied in dependence upon the load/speed operating conditions of the engine.

7. A method of operating a gasoline internal combustion spark ignition engine substantially as herein described with reference to and as illustrated in the accompanying drawings.

8. A gasoline internal combustion spark ignition engine having a continuous fuel vapour extraction system for separating the gasoline fuel into a lighter vapour fraction and a heavier liquid fraction, substantially as herein described with reference to and as illustrated in the accompanying drawings.