GB2291119A - I.c.engine air intake and fuel atomising system - Google Patents

Abstract

An intake manifold plenum chamber 24 having intake tracts 34 leading to the intake ports of the respective cylinders has an ejector 10 for introducing atomised fuel. Gas is supplied under pressure to the ejector nozzle 12 and a heated fuel conduit 18 acts to heat and recirculate fuel from the lowest point in the chamber 24 to the entrainment section 14 of the ejector 10. A fuel metering unit 20 supplies the ejector 10 through an orifice 17 or the inlet of the conduit 18 (Fig. 2). Exhaust gas or engine coolant is passed through the chamber 40 surrounding the conduit 18 and an electric heater (42, 44, Fig. 2) may be used at starting. The nozzle gas supply may be air from a compressor or gas from the engine combustion chambers supplied through non-return valves at peak combustion pressure to a supply chamber. The chamber 24 may have a PTFE internal coating. <IMAGE>

Description

FUEL PREPARATION SYSTEM Field of the invention The present invention relates to a fuel preparation system for an internal combustion spark ignition engine.

Background of the invention Manifold systems have been proposed, for example as described in copending British Patent Application No.

9409400.0 that achieve a divided charge in the combustion chamber. One fraction of the charge in the combustion chamber contains a homogeneous fuel and air mixture while the remainder contains only exhaust gas recirculation (EGR) gases, the two fractions being, as far as possible, spatially separated from one another in the combustion chamber. Such an engine is desirable because it allows high overall dilution without disturbing the stoichiometry.

It is important in such an engine to avoid any fuel from finding its way into the EGR fraction of the charge as such fuel will not be able to burn in the combustion chamber and will cause high hydrocarbon emissions. If the EGR gases flow at some stage through the same manifold branches as the fuel and air mixture, then it is imperative for these branches to be totally dry, that is to say the walls of the branches should not be lined with liquid fuel at any time.

For this reason, atomisation of the fuel in the air and fuel mixture should be extremely fine, with all the droplets preferably having a diameter of no more than 10pom, so that the fuel being completely transported by the air flow and not deposited onto the walls of the manifold tracts.

Various proposals for charge preparation to avoid fuel deposition on the manifold walls have been suggested in the past, amongst them heating of the manifold and using pressurised air to improve atomisation. Heating the manifold to evaporate any fuel that has landed on it necessarily assumes that fuel will at times land on the manifold walls and because such fuel always takes a finite time to evaporate, a tract that is sequentially exposed to air and EGR gases cannot avoid fuel being carried away by the EGR gases.

Pressurised air improves atomisation but there will inevitably be a range of droplet sizes and while more fuel in fine droplets will be carried away by the air stream, the larger droplets will result in wall wetting in the intake manifold. For this reason, manifold tracts are normally sloped downwards towards the intake ports of the cylinders, so that even the deposited fuel will eventually find its way under gravity to the combustion chambers.

Systems have been proposed to improve charge preparation when the fuel is injected at the intake ports by causing recirculation of air using an ejector driven by a small pressurised air supply. The ejector results in a larger air circulation inside the intake port to assist in vaporising more of the fuel collecting in the intake port but this still does produce a dry intake port.

There are also known vaporisers where fuel is sprayed onto a heated surface and is evaporated off the surface. This however does not produce atomisation of fine droplets. The heating energy required is high in order to evaporate the fuel completely and not to allow any liquid fuel to slide off the surface. The heated surface also tends to heat up the air as well as evaporate the fuel which is not desirable because the air charge density is then reduced.

It is also known to heat the fuel supply before it is metered and passed through an atomiser. This is by far the most effective method of improving the fuel atomisation because of the reduced viscosity of the heated fuel allowing the fuel to break up into fine droplets, and because of the hotter droplets evaporating more quickly and reducing in size to even finer droplets. This requires less heating energy than that for fully evaporating the fuel and the air is not heated in the process. A fuel/air mixture consisting of fine fuel droplets suspended in cold air is desirable in providing a dry and uniform mixture with a high air charge density. Such a system is however difficult to implement because there is a risk of vapour pockets being created within the fuel supply that would interfere with the fuel metering.To avoid this potential problem very accurate control of the heating of the fuel is required.

Summarv of the invention With a view to mitigating the foregoing problems, the present invention provides an air-driven fuel atomising and vaporising system for an internal combustion spark ignition engine, comprising an intake manifold having a plenum chamber common to a plurality of engine cylinders and intake tracts leading from the plenum chamber to the intake ports of the respective cylinders, at least one ejector assembly for introducing atomised fuel into the plenum chamber, the ejector assembly including a gas nozzle, an entrainment section and a discharge section, means for supplying gas under pressure to the gas nozzle, means for introducing metered quantities of fuel directly or indirectly into the entrainment section of the ejector and a heated fuel conduit connecting the lowest point in the plenum chamber to the entrainment section of the ejector.

In the preferred embodiment of the invention, metered fuel is atomised by the ejector, which is itself a good atomiser.

A major proportion of the fuel will be atomised into droplets sufficiently small to be transported by the air streams leading to the intake ports of the individual cylinders from the common plenum chamber. However, some droplets will be too. large to be transported in this manner and will collect at the lowest point in the plenum chamber, which will then act as a drip tray. The entrainment section of the ejector will always be at a lower pressure than the plenum chamber by virtue of the entrainment caused by the air jet supplied through the nozzle under pressure and this reduced pressure is used to suck the fuel from the drip tray and introduce it a second time into the ejector for atomisation. This time, however, the fuel is heated on the way and will be vaporised as well as finely atomised.The heating of the fuel will not in this case be critical as only quantities of fuel that have already been metered are passed through the heated conduit connecting the ejector to the drip tray. It is therefore possible to apply as much heat as is necessary to promote evaporation or fine atomisation of the fuel when it is passed to the atomiser so that the full amount of the metered fuel will be transported to the combustion chambers without leaving any build up of fuel in the drip tray during continuous operation.

It is desirable in the present invention to arrange for the manifold tracts leading from the plenum chamber to the intake ports of the individual cylinders to slope downwards towards the plenum chamber (rather than towards the intake ports). The intake ports will therefore only receive fuel that has been finely atomised and suspended in the air and any fuel on the walls will drip back into the plenum chamber for recycling.

Within the plenum chamber, there is a continuous recirculation of fuel from the discharge section of the ejector, through the plenum chamber, onto the drip tray, through the heated conduit and back to the entrainment section of the ejector. Metered fuel can be introduced at any convenient point in this loop. If it is arranged for the metered fuel to be fed indirectly into the entrainment section, then all the fuel must pass a first time-through the heated conduit leading from the drip tray to the entrainment section of the ejector. On the other hand, if fuel is fed directly to the entrainment section of the ejector from the metering unit, then only the larger droplets will need to be heated in the heated conduit, thereby reducing the total required heat transfer within the heater.

The heating of the conduit may be effected either electrically or by means of a heat exchanger extracting heat from the coolant circuit of the engine or from the exhaust gases.

The means for supplying gas under pressure to the gas nozzle of the ejector may be derived from a dedicated air compressor or may be drawn from the combustion chamber through one-way valves that allow gas to escape into a pressurised gas supply chamber only when the combustion chambers are at peak combustion pressure.

The diameter and length of the plenum chamber should preferably be larger than the visible cloud of droplets emitted from the ejector so that droplets are not directly deposited by the ejector onto the walls of the plenum chamber.

Advantageously, the ejector should be arranged to direct the fuel spray downwards towards the drip tray in the plenum chamber in order to minimise the quantity and the residence time of the larger droplets in flight within the plenum chamber.

A film of liquid fuel on the walls of the plenum chamber can also be avoided by coating the plenum chamber and the manifold intake tracts with a non-stick coating, such as PTFE. Any fuel landing on the walls will collect in droplets and slip quickly under gravity into the drip tray.

During transients, there will be a time delay or hysteresis as the equilibrium quantity of recycled fuel in the plenum chamber will be altered, but this can be compensated by a transient fuel calibration strategy in the usual manner.

During cold starts, before the heated fuel conduit reaches its operating temperature, the fuel metering should be set rich in order that the fraction of fuel droplets that are transported to the combustion chambers should be sufficient to give an ignitable mixture. Under these conditions, there will be an increasing accumulation of liquid fuel in the plenum chamber until the heated fuel conduit becomes effective in heating the recycled fuel fed to the atomiser.

Thereafter, the fuel metering should be set lean so that, when combined with the atomised recycled fuel, the mixture of fuel droplets and air reaching the combustion chambers should be of the desired strength. After the accumulated fuel in the plenum chamber has been consumed, the fuel metering can then be set at the steady state calibration.

To reduce the time and the extent of the fuel metering compensation during cold starts, it is desirable to bring the heated fuel conduit to operating temperature as quickly as possible. This may be assisted with electrical heating during and after a cold start. Alternatively, preheating the fuel conduit may be used before starting the engine.

After the heated conduit has reached it operating temperature, the electrical heating may be switched off and hot exhaust gases may be circulated through the heat exchanger to maintain the heating temperature. This makes use of waste heat and because the heating does not have to be accurately controlled there is no need to provide any special control system for regulating the exhaust gas flow.

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 schematic section through an air-driven fuel atomising and vaporising system for an internal combustion spark ignition engine of a first embodiment of the invention, and Figure 2 is a similar view of an alternative embodiment of the invention.

Detailed description of the preferred embodiments In Figure 1, a plenum chamber 24 common to a bank of cylinders of an engine is connected to an air filter (not shown) through a single duct 30 containing a butterfly throttle 32 that regulates the air supply to the cylinders.

A plurality of upwardly sloping tracts 34 of the intake manifold connect the plenum chamber 24 to the intake ports of the engine cylinders. Fuel is introduced into the inducted air by an ejector 10 which acts as an air-driven atomiser. Air under pressure is supplied to a gas nozzle 12 of the ejector that opens into an entrainment section 14 of the ejector into which fuel is introduced by a metering unit 20 through an orifice 17 and by a heated fuel conduit 18.

The fuel carried by the pressurised air from the nozzle 12 is discharged through the discharge section 16 of the ejector 10 and is atomised in the process to form a cloud of droplets 22 the envelope of which is represented by dotted lines in the drawings. As can be seen from the drawings, the cloud is smaller than the dimensions of the plenum chamber 24 and therefore fuel droplets do not immediately deposit on the walls of the plenum chamber 24.

The cloud of droplets will contain a whole range of sizes, a large proportion being smaller than 10 pm in diameter so that they are carried by the air stream and are transported to the combustion chambers. The cloud 22 however also contains larger droplets which will not be transported by the air and will instead fall under gravity into a drip tray 26 at the bottom of the plenum chamber 24. The fuel conduit 18 that is heated by a heater 40 returns the droplets collecting in the drip tray 26 to the entrainment section 14 of the ejector 10 so they may be re-atomised. The heating of the fuel on its passage through the heated conduit 18 assists in the vaporisation and atomisation of the fuel on its return to the ejector 10.

It can be seen that the fuel is therefore recycled through a loop made up of the internal volume of the plenum chamber 24 and the fuel conduit 18 and metered fuel may be introduced at any point in this loop. In the case of Figure 2, the metered fuel is introduced directly into the drip tray 26 which means that all the fuel is heated before it is introduced into the entrainment section 14 of the ejector 10. This has the advantage of improved fuel preparation but more heat is needed because of the mass of fuel flowing through the fuel conduit 18. The embodiment of Figure 1 requires a lower degree of heat transfer because only fuel droplets above a certain size are heated.

The heating in both embodiments is effected by a heat exchanger 40 through which exhaust gases flow. The embodiment of Figure 2 also has an electric heating element 42, 44 to allow the heater to work during cold starts of the engine. It is possible to use exhaust gases because the heating of the fuel is not critical and no harm can result from overheating, as the fuel has already been metered.

The effect of a dry manifold wall is enhanced by the vertical attitude of the plenum chamber 24 and the fact that the fuel is sprayed downwards into the drip tray 26. The sloping of the intake tracts 34 into the plenum chamber 24 also ensures that fuel will drip back into the drip tray 26 instead of flowing into the combustion chambers to disturb the fuel calibration. If the walls of the intake tracts 34 and plenum chamber 24 are coated with a non-stick coating such as PTFE, then the wall wetting will be reduced further because the fuel will tend to form droplets that slide back under gravity into the drip tray 26.

The air for the gas nozzle 12 can be derived from a compressor but it is preferred to provide a supply chamber connected by conduits containing one-way valves to the engine combustion chambers. Such an arrangement will tap off a part of the combustion charge at the peak combustion pressure and store it in the supply chamber. As long as the mass of the pressurised charge tapped off from the combustion chambers is small, the performance of the engine will not be adversely affected. Furthermore, because the pressurised gas is drawn from the combustion chambers rather than from a compressed ambient air source, this gas is merely circulated in a closed loop positioned downstream of the engine throttle and it will not therefore affect the engine idling speed.

The system described is a "dry" central fuel injection system (CFI) which has little or no liquid fuel deposited on the walls of the intake manifold leading to the engine.

While the invention fully meets the requirements of a divided charge engine described earlier, its application is not restricted to such an engine as the improved fuel preparation is advantageous to other central fuel injection engines.

The invention also finds application with fuels that do not evaporate easily, for example, methanol, and it is advantageous for cold starts and transient fuel control of such a methanol-fuelled engine or other gasoline-fuelled engines.

Claims (10)

1. An air-driven fuel atomising and vaporising system for an internal combustion spark ignition engine, comprising an intake manifold having a plenum chamber common to a plurality of engine cylinders and intake tracts leading from the plenum chamber to the intake ports of the respective cylinders, at least one ejector assembly for introducing atomised fuel into the plenum chamber, the ejector assembly including a gas nozzle, an entrainment section and a discharge section, means for supplying gas under pressure to the gas nozzle, means for introducing metered quantities of fuel directly or indirectly into the entrainment section of the ejector and a heated fuel conduit connecting the lowest point in the plenum chamber to the entrainment section of the ejector.

2. A system as claimed in claim 1, wherein the fuel conduit is heated by a heat exchanger for transferring hec:í.

to the fuel from the exhaust gases of the engine.

3. A system as claimed in claim 1 or 2, wherein the fuel conduit is heated by means of an electric heating element.

4. A system as claimed in any preceding claim, wherein the gas nozzle of the ejector is connected to a source of pressurised air that comprises a compressor.

5. A system as claimed in any of claims 1 to 3, wherein the gas nozzle of the ejector is connected to a source of pressurised gas comprising a pressurised chamber connected by conduits containing one-way valves to the combustion chambers of the engine.

6. A system as claimed in any preceding claim, wherein the internal dimensions of the plenum chamber exceeds those of the fuel and air spray emitted by the ejector.

7. A system as claimed in any preceding claim, wherein the ejector is positioned to direct a spray of fuel and air downwards into the plenum chamber.

8. A system as claimed in any preceding claim, wherein the intake tracts connecting the plenum chamber to the combustion chambers slope downwards into the plenum chamber.

9. A system as claimed in any preceding claim, wherein the internal walls of the plenum chamber and of the intake tracts leading to the combustion chambers are coated with a non-stick coating.

10. An air-driven fuel atomising and vaporising system for an internal combustion spark ignition engine, constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.