Method for controlling fuel supply to an engine

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This invention relates to a method of determining the mass of air induced per cycle to an internal combustion engine for the purposes of controlling the air/fuel ratio as part of the engine management system.

It is known to use various types of mass air flow sensors in the air induction system of an engine to determine the mass rate of air induced into the engine over the full range of operating conditions of the engine. Other means for determining the air flow have also been used, such as providing a calibration in the memory of an ECU (electronic calculating unit) of air flow in relation to engine speed and throttle position.

Although these known techniques for determining the mass of induced air are effective, they have disadvantages either from the point of view of the nature of the equipment required, including the cost and effective life thereof, and/or the quantity of memory capacity required to store relevant information.

It is therefore the object of the present invention to provide a method of determining the mass of air introduced to an internal combustion engine under operating conditions which is effective, and requires less hardware and/or memory storage capacity to provide an effective control of the air/fuel ratio of the engine under all operating conditions.

With this object in view, there is provided according to the present invention a method of determining the mass of air introduced per cylinder per cycle (IACC) of an internal combustion engine comprising the steps of:

calculating the IACC at wide open throttle (IACC.sub.WOT) for the existing engine speed and operating conditions,

selecting from predetermined coefficients indicating the relationship between IACC.sub.WOT and IACC at preselected part-load the coefficient relating to the current load and speed; and

applying said selected coefficient to said IACC.sub.WOT to determine the current IACC (IACC.sub.LD).

More specifically, there is provided a method of determining the mass of air introduced per cylinder per cycle (IACC) of an internal combustion engine comprising:

programming a processor with an algorithm to determine the IACC for the engine at wide open throttle (WOT) (IACC.sub.WOT) over a selected engine speed operating range,

storing in memory coefficients relating the IACC.sub.WOT to the IACC at selected load demands below WOT over said selected engine speed range,

sensing while the engine is operating the engine speed and load demand and selecting the respective coefficients for the sensed engine speed and load demand,

inputting to the programmed algorithm the IACC coefficient relating to the sensed engine load demand at the sensed engine speed

determining from said inputs the IACC for the existing engine operating conditions (IACC.sub.CALC), and

determining from said IACC.sub.CALC and sensed engine speed and load demand the required mass of fuel per cylinder per cycle (FPC).

On the basis of this determined FPC, a signal is issued to a fuel metering means to activate same to deliver to the engine FPC amount of fuel in timed relation to the engine cycle.

Conveniently the processor is programmed so the algorithm adjusts the IACC.sub.WOT in response to variations in selected engine operating conditions such as intake air temperature or pressure, or exhaust pressure. The selected engine operating conditions may be related to respective datum values, the datum values preferably are the values of the respective engine operating condition existing at calibration of the IACC coefficients stored in the memory.

The processor may be programmed so that if one or more of the engine operating conditions is sensed to be fluctuating regularly within a relatively short time interval, the effects of the fluctuations on the air mass calculation will be limited. The limiting of the effect of the fluctuations is preferably carried out within a select range of load demand and/or engine speed, preferably in the lower range. Alternatively, if it is known that the intended use of the engine can give rise to such fluctuation at certain operating conditions, then the processor program can be adapted to limit the effect of such fluctuation whenever it is operating at those certain operating conditions, irrespective of whether such fluctuation is or is not occurring. By way of example a marine engine operating at low speed such as while trolling may pass through a series of waves which will cause a near cyclic variation in exhaust pressure. This in turn may cause the engine to "hunt" for a stable operating condition. By reducing the effect of exhaust pressure the "hunting" can be reduced or eliminated.

In a preferred form, the method of determining the mass of induced air per cylinder per cycle (IACC) of a particular engine comprises:

programming a processor with an algorithm to determine the IACC for the engine speed operating range dependent upon atmospheric pressure (P.sub.AT), exhaust pressure (P.sub.EX), and manifold charge temperature (T.sub.CH),

determining in advance and storing in memory respective coefficients relating to P.sub.AT, P.sub.EX and T.sub.CH for selected engine speeds within the operating speed range,

determining and storing in memory coefficients relating the IACC.sub.WOT to the IACC at selected load demands below WOT at each said selected speed,

sensing while the engine is operating the P.sub.AT, P.sub.EX, T.sub.CH, engine speed and load demand and selecting the respective coefficients for each at the sensed load demand and engine speed,

detecting and inputting to the programmed algorithm respective signals indicating the existing P.sub.AT, P.sub.EX and T.sub.CH,

inputting to the programmed algorithm the IACC coefficient relating to the sensed engined load demand at the sensed engine speed,

determining from said inputs the IACC for the existing engine operating conditions (IACC.sub.LD),

determining from said IACC.sub.LD and sensed engine speed and load demand the required mass of fuel per cylinder per cycle (FPC).

It will be appreciated that the method of determining IACC as hereinbefore discussed requires no specific equipment to measure the IACC as this is determined by the inputs from simple temperature, pressure, speed and load demand sensors to an ECU suitably programmed and with the relevant coefficients previously determined and stored in memory.

The present method of determining the mass of induced air is based on the discovery that the air flow at a selected position of the throttle remains a substantially constant ratio to the air flow at wide open throttle for any given engine speed, and is basically independent of ambient conditions, provided the same ambient conditions exist at both the selected and the wide open throttle positions.

Accordingly, if the air flow at wide open throttle is known for a particular engine speed at specific temperature and pressure operating conditions, then the air flow for any throttle position at that speed can be readily determined. This is achieved by programming the ECU to determine the air flow at wide open throttle and a particular engine speed under the specific operating conditions, and by applying the appropriate coefficients, calculating the air flow at the same speed for a range of load conditions covering those normally encountered by the engine in normal operation. ##EQU1##

Thus, if the IACC.sub.WOT is calculated for a specific engine speed, atmospheric pressure, charge temperature, and exhaust pressure, using the above algorithm, the ECU can determine the IACC for all load demand as may be sensed, such as by the throttle position, at that selected engine speed, for which coefficients have been determined and stored in memory.

The actual IACC at any selected speed is determined by:

IACC.sub.LD =IACC.sub.WOT .times.K.sub.LD

IACC.sub.LD =induced mass air per cylinder per cycle at selected load demand

K.sub.LD =selected load demand coefficient.

It is thus seen that by updating the base IACC.sub.WOT values for the existing speed and atmospheric and engine conditions, the IACC for any combination of operating speeds and loads (throttle positions) can be calculated.

The algorithm may include provision to allow for trapping efficiency by reference to a trapping efficiency map provided in the ECU so that calculations can be on the basis of the actual mass of air trapped in the engine cylinder per cycle. This may be particularly desirable with respect to a two stroke cycle engine. Also as an alternative to the providing of a map, the algorithm may be modified to actually directly calculated trapped mass of air per cylinder per cycle.

Using the above discussed speed and load demand as look-up parameters there is determined the required fuel mass per cylinder per cycle based on the calculated air rate for the particular existing operating conditions, referred to as FPC.sub.CALC, for the existing P.sub.AT, P.sub.EX and T.sub.CH. This FPC.sub.CALC is determined as for a homogeneous charge as is desirable under WOT and other high fuelling rates. However, under stratified charge conditions, it may be advantageous to disassociate that fuelling level from the calculated air flow.

It is proposed that a weighting map, again utilising speed and throttle-position as look-ups, be used such that the actual fuel delivered (FPC.sub.DELV) is at a level between FPC.sub.CALIB and FPC.sub.CALC, FPC.sub.CALIB being the calibrated FPC based directly on engine load and speed alone.

ie: FPC.sub.DELV =FPC.sub.CALIB +Alpha* (FPC.sub.CALC -FPC.sub.CALIB)

By defining the alpha (weighting) term between zero and one, the calibration can be selected to provide the desired control path, or percentage of each control path. By way of example, it may be elected to maintain FPC.sub.DELV =FPC.sub.CALIB until homogeneous conditions were present and to then ramp the alpha term up to 1 as a function of throttle position. Under WOT conditions, the alpha value is always 1 to encompass the full correction for a change in the ambient conditions.

Under the stratified charge conditions, such as at low loads, provided that the required airflow is not set sufficiently close to the rich misfire limit airflow, that is, enough allowance for changes in the ambient conditions is made, it is possible to utilise only FPC.sub.CALIB. An advantage of this is that the resulting fuelling level can be extremely stable without usage of system filtering that detracts from the transient performance.

The determination of the various constants and coefficients is achieved by a calibration process and will be individual to each particular engine family configuration. The principal characteristics of the engine configuration that will influence the constants and coefficients are the engine induction system and exhaust system, together with the inlet and exhaust porting. To determine these constants and coefficients, a representative model of the engine is run on a particular day with known ambient conditions and then induced variations in those conditions are created to determine the effect of these variations on the air flow.

Initially the engine is run with wide open throttle at the prevailing ambient conditions and the actual air per cylinder per cycle is measured at a number of selected speeds within the normal range of operation of the engine. Further sets of measurements are made of the induced air per cylinder per cycle with introduced variations in the ambient pressure, exhaust pressure and charge temperature at the same selected speeds within the normal operating speed range. On the basis of this information the coefficients can be determined relating to the individual influence of atmospheric pressure, exhaust pressure and charge temperature. Thereafter the above measurements are repeated for a range of partial open throttle positions and from these results the coefficient determining the relationship between airflow at wide open throttle and airflow at the respective partial throttle open positions are determined.

The coefficients determined as above indicated, can then apply to all engines of the same construction as that of the engine used for calibration and thus appropriate maps can be produced for storage in the memory of the ECU to be used in controlling the fuel injection system and the management of such engines.

As previously referred to the stated preferred algorithm enables calculation of the air flow through an engine at wide-open throttle and provides the basis of a simple method to determine the air flow through an engine without the need for a dedicated air flow sensor. This is possible by the important discovery that for the same operating conditions of P.sub.EX, P.sub.AT and T.sub.CH the ratio of the air flow at any particular throttle position is a constant proportion of the air flow at WOT for any given speed.

It is important to appreciate that the P.sub.AT, T.sub.CH and P.sub.EX conditions must be the same for both part-load and WOT conditions.

Intuitively P.sub.AT and T.sub.CH will remain approximately steady at normal part-load operation and at WOT. However, as the load is increased from part-load to WOT, P.sub.EX will increase. This is particularly so with two stroke cycle engines and thus to keep P.sub.EX constant is an artificial state which would not be expected in practice.

Thus, by running the engine at varying loads and speeds with the same P.sub.AT and T.sub.CH a map of K.sub.LD can be established that takes account of the changes that arise directly from the influence of load and speed on exhaust pressure P.sub.EX. The appropriate look-up map can then be incorporated into the ECU memory so that IACC.sub.LD is determined by IACC.sub.LD =IACC.sub.WOT .times.K.sub.LD.

The temperature constant T.sub.CM of the preferred algorithm is also variable with speed and load and by derivation from the algorithm it is shown ##EQU2##

Thus by conducting two tests

at ambient conditions

at elevated T.sub.CH whilst keeping all other conditions equal

and repeating these tests at a series of speed and load combinations, appropriate look-up maps can be developed and incorporated into the ECU memory so that T.sub.CM may be looked up for any combination of engine load and speed.

To determine the constants K.sub.1 and K.sub.2, it is known that at WOT conditions K.sub.LD =1 and thus it can be derived from the preferred algorithm that ##EQU3##

By conducting two tests on the engine, both at WOT and over a range of selected engine speeds:

(1) at ambient conditions

(2) at induced exhaust back pressure

and repeating these tests at a series of engine speeds, and taking T.sub.CM at WOT from the previously referred to maps, an appropriate look-up map for K.sub.1 at K.sub.2 and WOT can be developed.

It is necessary to also obtain K.sub.1 and K.sub.2 at pan-load operation as the sensitivity of the engine to exhaust pressure varies with load (throttle position). Accordingly, the two tests, previously referred to in relation to K.sub.1 and K.sub.2 at WOT, are repeated for each speed and load point.

Using the data from these tests, and the previously developed data regarding T.sub.CM and K.sub.LD, K.sub.1 and K.sub.2 at part-load and over the normal speed range is determined by the following formula: ##EQU4##

By combining the K.sub.1 and K.sub.2 data for both WOT and throughout the load and speed operating ranges respective look-up maps for K.sub.1 and K.sub.2 can be developed and incorporated into the memory of the ECU so that in operation the relevant coefficients can be used in the algorithm for the prevailing engine operating conditions in the determination of IAACC.sub.WOT.

D.sub.CM is a constant related to geometry and other physical characteristics of the engine. This constant is determined experimentally and is specifically related to the engine cylinder volume at top dead centre.

The accompanying drawing depicts a logic diagram of one practical manner of operation of the method of the present invention.

The logic diagram as depicted relates to the use of the preferred algorithm as previously identified and to the use of the various maps and equations previously discussed. The procedure as represented in the logic diagram is carried out on a periodic basis whilst the engine is operating. The frequency of readings may be related to the cycle period of the engine, however, it is preferably time-based independent of engine speed.

Step 1 is to read the signal from sensors indicating respectively the engine load, engine speed, manifold charge air temperature, ambient pressure and exhaust pressure.

Step 2 is to look up on the respective maps the values of K.sub.1, K.sub.2 and T.sub.CM for the sensed engine load and speed and feed the look up values to the algorithm. Also inputs relating to the sensed P.sub.AT, T.sub.CH and P.sub.EX are fed to the algorithm.

Step 3 is to calculate IACC.sub.WOT based on the inputs of Step 2 to the algorithm.

Step 4 is to look up the K.sub.LD value for the sensed engine load and speed and to calculate IACC.sub.TP from the K.sub.LD value and the IACC.sub.WOT. At this stage, the calculation of the currently existing air flow to the engine has been determined and that may be used in a number of different ways to subsequently determine the required fuel per cycle of the engine to achieve the required air fuel ratio in the engine combustion chamber.

One convenient way of proceeding to determine the FPC required by the engine is:

Step 5: look up on an appropriate air fuel ratio map the required air fuel ratio for the existing load and speed of the engine and apply this to the calculated IACC.sub.TP to calculated FPC.sub.CALC.

As previously discussed in the specification, for a stratified charge engine, at low loads and hence high air fuel ratios, there is an oversupply of air available to ensure combustion of all of the fuel and thus a fuelling rate in accordance with FPC.sub.CALC is acceptable and desirable. However, in conditions where the air fuel mixture is substantially homogeneous, such as at WOT, it is desirable to change the fuelling rate APC.sub.CALIB such as in accordance with the formula previously referred to, namely, FPC.sub.DELV =FPC.sub.CALIB +Alpha (FPC.sub.CALC -FPC.sub.CALIB).

For the purpose of effecting this adjustment to the FPC respective look up maps for FPC.sub.CALIB and Alpha each related to engine load and speed are looked up at Step 6 to effect a variation to FPC.sub.CALC based on the above referred to formula to provide FPC.sub.DELV.

On the basis of the newly calculated FPC.sub.DELV, at Step 7 the appropriate signal is given to the fuel injector to effect delivery for the required amount of fuel to the respective cylinders of the engine.

In carrying out the invention conventional sensors as commonly used in engine management systems provide inputs to the ECU in respect of atmospheric pressure and temperature, exhaust pressure and engine load demand, the latter conveniently being a throttle position indicator. Components for these purposes are well known and are readily available, accordingly no specific description thereof is provided.

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