Method of fluid injection

The present invention relates to a method of operating an internal combustion engine. With reference to FIGS. (1), (2) and (5), fluid is supplied to charge air using an injector (116, 3116a, 3116b) which in each operation delivers a set amount of fluid. The amount of fluid supplied to the charge air in each engine cycle is controlled by how many times the injector (116, 3116a, 3116b) operates in each cycle. A desired fluid demand is calculated as a number of operations of the injector per cycle, calculated to at least one decimal place. The desired fluid demand is always rounded down or always rounded up to a near integer to provide an output fluid demand for the injector as a number of operations of the injector for the next operating cycle in varying operating conditions of the engine. The rounding difference is aggregated and when the aggregate passes an integer then the fluid demand is adjusted.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under all applicable rules and statutes to International Application No. PCT/GB2009/001984, filed Aug. 13, 2009, and entitled A METHOD OF FLUID INJECTION, which claims priority to GB 0814982.5, filed Aug. 15, 2008, incorporated herein by reference in their entireties.

This invention relates to a method of operating an internal combustion engine.

Most internal combustion engines in automobiles currently use fuel injection systems to supply fuel to the combustion chambers of the engine. Fuel injection systems have replaced the earlier technology of carburettors because they give better control of the delivery of fuel and enable the engine to meet emission legislation targets as well as improving overall efficiency.

It is important that the fuel injection system delivers an appropriate amount of fuel at an appropriate time. Inappropriate delivery of the fuel may lead to a reduction in the output power of the engine, an increase in emissions and a wastage of fuel.

Whilst the sophisticated and highly developed fuel injection systems currently available are ideal for use in internal combustion engines in automobiles, there are many other applications for internal combustion engines where such a level of sophistication is not appropriate and too costly. For instance, small single cylinder engines as used for a variety of engine powered gardening devices (such as lawn mowers, hedge trimmers, chain saws, rotovators, lawn aerators, scarifiers and shredders), small generators, mopeds, scooters, etc. are built to very tight cost targets and therefore cannot afford the cost of a sophisticated fuel injection system. To date, such small engines have used traditional cheaper carburettor technology. However, small engines of this type will soon face the same kind of exhaust gas emission legislation as automobile engines and so must be modified to meet the emission targets. Therefore, a cheap and simple system of fuel injection is required for such small engines.

In GB 2425188 the applicant described a fuel injection unit suitable for a small engine. The injector described injects in each operation a set amount of fuel into the charge air; the controller of the unit decided in each engine cycle how much fuel was needed and then operated the injector a number of times to come closest to the ideal amount of fuel. Since the amount of fuel can only be controlled in steps equivalent to the volume dispensed by the injector, the control was quite coarse. The engine could be over-fuelled or under-fuelled, where fine control of the fuelling is required.

According to a first aspect of the present invention, there is provided a method of operating an internal combustion engine according to claim1.

According to a second aspect of the present invention, there is provided a method of operating an internal combustion engine according to claim6.

Without increasing the complexity or cost of the injection apparatus itself the applicant has devised a way to achieve finer control of the amount of fuel delivered to a combustion chamber in each cycle to improve the efficiency of the engine, its, fuel consumption and its emissions.

Internal combustion engines that make use of embodiments of the invention can do away with complicated, heavy and expensive fuel injection timing systems. Instead, they may make use of a cheaper and simpler system.

Further respective aspects and features of the invention are defined in the appended claims.

FIG. 1illustrates an internal combustion engine100comprising a cylinder102in which reciprocates a piston104with the piston104and the cylinder102defining between them a combustion chamber106. The piston104is connected by a connecting rod108to a crankshaft110. The crankshaft110drives a camshaft (not shown) which in turn drives an inlet valve112and an exhaust valve114. The inlet valve112and the exhaust valve114are driven in timed relationship to the movement of the piston104in the cylinder102, with return springs (not shown) biasing the valves112,114back into their valve seats.

The fuel injection system of the engine100comprises a fuel injector116arranged to deliver fuel118into an inlet passage120downstream of the inlet valve112. A throttle valve122is placed in the inlet passage120to control the flow of charge air into the inlet passage120and the combustion chamber106.

An engine control unit124controls the time at which the fuel118is injected into the charge air present in the inlet passage120and also controls the quantity of fuel118that is injected. The engine control unit124receives a signal from the throttle valve122via a control line126, the signal indicating the rotational position of the throttle valve122and hence the engine load. Additionally, the engine control unit124receives a timing signal from a crankshaft sensor128(which could be replaced by a camshaft sensor) via a control line130. The crankshaft sensor128is responsive to teeth132on the crankshaft110and to a gap134in the teeth132. The engine control unit124can determine, from the timing signal received from the crankshaft sensor128, the speed of the engine100and the position of the piston104within the cylinder102, this being used to determine the timing of opening and closing of the inlet valve112. Having regard to the timing signal produced by the crankshaft sensor128and the load signal produced by the sensor attached to the throttle valve122, the engine control unit124generates a control signal which is relayed to the injector116via a line136and controls the operation of the injector116.

FIG. 2shows an embodiment of an injector116. It comprises a piston1000slideable in a housing1001. The piston1000is acted upon by a solenoid1002and by a biasing valve spring1003. The piston is moveable to draw fluid into and dispense fluid from a fluid chamber1004. A one-way inlet valve1005allows fluid to flow into the fluid chamber1004from a fluid inlet1006, while preventing flow of fluid out of the fluid chamber1004to the fluid inlet1006. A one-way sprung-loaded outlet valve1007allows fluid to be dispensed from the fluid chamber1004to a fluid outlet1008, but prevents fluid being drawn back into the fluid chamber1004from the fluid outlet1008.

In the operation of the fluid injector116the activation of the solenoid1002moves the piston1000against the biasing force of the spring1003to displace fluid from the fluid chamber1004via the outlet valve1007to the fluid outlet1008. Then, when the solenoid1002is de-energised the biasing spring1003forces the piston1000to move to draw fluid into the fluid chamber1004via the inlet valve1005. The piston1000has a defined piston stroke XpThis piston stroke is defined by setting the travel of the piston between two end stops. By setting a definite piston travel the volume of fluid dispensed in each dispensing operation of the fluid injector116can be set at a fixed value. Thus, whenever the solenoid1002is operated then the fluid injector116dispenses a set amount of fluid. This means that in each engine cycle the total amount of fluid dispensed by the fluid injector116can be controlled by controlling the number of times that the solenoid1002is activated during the engine cycle. Unlike pulse width modulated injectors, the amount of fluid delivered by the fluid injector is insensitive to pressure variations in the intake passage1006or outlet passage1008.

The embodiments described above a four-stroke internal combustion engine, but the fuel injection strategy is also applicable to a two-stroke internal combustion engine. Such an engine can have not only an injector of the type described above to deliver fuel, but also an additional injector of the same type to deliver two-stroke engine oil.

Previously it has been proposed for each engine cycle to take measured engine speed and load and then use a look-up table to determine how many times in the engine cycle a fuel injector116should be operated. This was determined separately for each engine cycle, independently of all other engine cycles. However, this gives only a coarse control of the amount of fuel going into the engine for combustion.

The applicant has realised that not all fuel dispensed by the injector116prior to a combustion cycle reaches the combustion chamber and is combusted. Instead a significant amount of fuel hangs on the walls of the intake passage120. This is usually considered undesirable and so the injector116is usually situated as near as possible to the back of the valve head of valve112to minimise the length of the passage120on whose walls fuel can hang.

The applicant has realised that the fact that fuel hangs on walls, normally felt undesirable, can be used to advantage in the use of an injector as described above with reference toFIG. 2. The applicant has designed a new control strategy to be used by the electronic controller124ofFIG. 1.

The strategy is illustrated by the flow chart ofFIG. 3, which shows steps of a first method of the invention used to calculate an optimal integer number of operations of a fluid injector for a single engine cycle.

FIG. 3illustrates a method of operation according to the present invention. As described in the previous patent GB 2425188, the controller124will use measured engine load and speed to address a look-up table at step2000. This will give an idealised amount of fluid to be delivered by the injector116for a single engine cycle. Preferably, this will take place at the beginning of the engine cycle. As an alternative to using a look-up table, this amount of fluid could be calculated each cycle according to a pre-programmed algorithm, e.g., using engine speed and engine load as two input variables.

The controller will receive a measure of the rate of revolution of the engine (rpm) and from the crankshaft position signal will recognise the beginning of each new engine cycle.

This controller can also take account of factors such as changing engine temperature, atmospheric pressure, etc, provided by sensors associated with the engine, although this is optional.

A desired amount of fuel to be delivered is determined as a number of operations of the fluid injector, such number be calculated to one or two decimal places. For instance, a desired fuel demand might be 3.6 operations of the fluid injector. Obviously the injector itself can only operate 3 times a cycle or 4 times a cycle and cannot itself operate 3.6 times a cycle.

At2001the fuel demand is rounded down to the nearest integer. For instance, a 3.6 fuel demand would be rounded to 3. This is an output as a demand D. The difference between the output demand D and the input demand calculated at2000(or2004) is determined, in this case +0.6. This difference is output to2002.

The output demand D is relayed to2003. At2003the final output to the injector is determined.

The difference calculated at2001will be accumulated at2002. Then, at2005it is determined whether the accumulated difference is greater than 1. If the accumulated difference is greater than 1, then 1 is added to the number D at box2003so that the output from2003is D+1, and 1 is subtracted from the accumulated difference stored at box2002.

The method inFIG. 3allows an averaging to take place over a number of engine cycles to give a total fluid delivery to the cylinder102,3102which is closer to that determined at the step2000than if no differences are accumulated in the process.

The method ofFIG. 3can provide substantially the optimum supply of fuel over a number of engine cycles. There will be no noticeable unevenness in the running of the engine because the effect of the fuel ‘hanging’ on the walls serves to average the fuel delivered to the combustion chamber in any event.

Although in the method shown inFIG. 3, the fuel demand is rounded down to the nearest whole number (integer) of operations of the fuel injector, it will be apparent to the skilled person that the disclosed method could be modified such that the fuel demand is rounded down to any near integer. The operation of box2002, which stores the accumulated difference would, of course, be modified accordingly.

For example, the fuel demand, as a number of operations of the fuel injector, could be rounded down to the nearest even integer. In which case, the accumulated difference stored at box2002would modify the number D at box2003by two, when the accumulated difference is greater than two.

In theFIG. 3method there is a consistent rounding down to a near integer. As an alternative there could be a consistent rounding up to a nearest integer (or near integer, e.g., nearest even integer). This is illustrated inFIG. 4. When the accumulated difference is less than −1 the count D is reduced by 1 and 1 is added to the accumulated difference.

The methods illustrated by the flowcharts ofFIGS. 3 and 4are operated continuously during operation of the engine (and not just in steady state conditions).

FIG. 5shows a two-stroke internal combustion engine3100in accordance with the present invention. The engine comprises a fuel injector3116aof the type shown inFIG. 2and also a two-stroke oil injector3116b, again of the type shown inFIG. 2. Either of the methods disclosed above with reference toFIGS. 3 and 4can be applied to deliver both fuel and two-stroke engine oil to the engine cylinder.

Engine3100is a crank case scavenged two-stroke internal combustion engine comprising a cylinder3102in which reciprocates a piston3104, with the cylinder3102and the piston3104defining between them a combustion chamber. The piston3104is connected by a connecting rod3108to a crankshaft3110.

The operation of engine3100is controlled by the electronic controller124ofFIG. 1.

The fuel injection system of the engine3100comprises the first injector3116aarranged to deliver fuel from a fuel chamber3115ainto an inlet passage3120downstream of a throttle valve3112and a second injector3116aarranged to deliver two-stroke engine oil from an oil chamber3115binto the inlet passage3120again downstream of the throttle valve3112. The delivered fuel and two-stroke oil are both drawn into a crank case3400and the mixture of fuel, oil and air in the crank case3400is pressurised by downward motion of the piston3104with the pressurised mixture then flowing into the combustion chamber via a transfer passage3401, with a reed valve3402preventing flow back up the inlet passage3120.

The engine3100is provided with a mixing chamber3300in which the fuel delivered by first injector3116aand the oil delivered by second injector3116bare mixed prior to entering the inlet passage3120. The engine3100is provided with air by-pass passage3310and air is drawn through the bypass passage3310and through the mixing chamber3300, entraining oil and fuel therein, into the inlet passage3120downstream of the throttle valve3112. A depression downstream of the throttle will draw air through the bypass passage3310.

In theFIG. 5engine, the control strategy ofFIG. 3orFIG. 4is implemented twice; once to calculate the amount of fuel to deliver and once to calculate the amount of oil to deliver. The method associated withFIG. 3orFIG. 4is applicable to the delivery of two-stroke oil as well as fuel, although in each engine cycle the total volume of two-stroke engine oil delivered will be smaller than the volume of fuel which is delivered and so the oil injector3116bwill be operated a smaller number of times per engine cycle than the first injector3116a. Typically the calculated number of operations per cycle of the two-stroke oil injector3116bwill be in the range 0.01 to 0.3 (with 0 operations for an oil free starting condition). Thus the strategy will typically result in one operation of the injector only in each of selected engine cycles (i.e. in some cycles no oil will be injected).

In theFIG. 5embodiment, electronic controller124will carry out the method depicted inFIG. 3orFIG. 4for each fluid, fuel and oil, separately. The method may be carried out for each fluid either in parallel or in series.

It will be apparent to the skilled person, that although in the engine3100ofFIG. 5the electronic controller124applies the method ofFIG. 3orFIG. 4to control the delivery of two fluids to the cylinder3102, the above disclosed methods of delivering fluid is applicable to any number of fluids. Whilst the above injection system has been described as delivering either fuel only or both fuel and engine oil, the present invention is also applicable to the delivery of any two fluids, or in fact, any number of fluids. For instance one injector could inject regular gasoline and the other ethanol biofuels (both could be delivered simultaneously in a single engine cycle or the engine operated selectively on either gasoline or biofuel, either in response to driver control or a preprogrammed control strategy).

Whilst theFIG. 2injector shows the spring1003used to move the piston1000to draw fluid into chamber1004and the solenoid1002used to dispense fluid from the chamber1004, an injector could be used with the operations reversed and a solenoid used to move a piston to draw fluid into a chamber and a spring then used to dispense fluid from the chamber.