Method and system for providing fuel injection time scheduling for internal combustion engines using engine speed prediction

A system and process for determining fuel injection time scheduling in an internal combustion engine. The system and method uses a prediction of engine speed to compensate for the errors due to rapid speed change in fuel injection during crank/start and engine speed transition like engine tip-in and tip-out.

TECHNICAL FIELD

This invention relates to internal combustion engines, and more particularly to fuel injection system used in such internal combustion engines.

BACKGROUND AND SUMMARY OF THE INVENTION

As is know in the art, fuel injection systems are used to inject an amount of fuel into cylinders of an internal combustion engine. The fuel injection process is one of the most critical events in the preparation of air/fuel mixing in an internal combustion engine. It effects combustion quality and engine-out emissions. Electronic control units determine the amount of fuel to be injected into such cylinders and the time of such fuel injection and command the fuel injection system to provide such amount at such time. The amount of fuel to be injected into such cylinder is calculated by an engine control unit (ECU) as a function of the air quantity passing to the cylinder as measured by an mass air flow (MAF) sensor or indirectly measured by manifold absolute pressure (MAP) sensor. In one such system, the ECU determines the pulse width to turn on the injector to provide the desired amount of fuel to that cylinder; such systems are commonly called pulse width modulated (PWM). The constant amplitude pulse has a time duration, or fuel pulse width (FPW), related to the calculated fuel quantity. The ECU also determines the time at which such fuel is to be injected into the cylinder. The time of injection is such that the injection occurs at a proper engine crank angle associated with one of the cycles of a four-stroke engine, for example. That is, the time of injection, or targeting injection position, is crank angle based, while the quantity of fuel is time-based. There is a conversion provided by the ECU between crank angle based parameters and time-based parameters. The conversion is calculated based on engine speed. The ending location of fuel injection may be at either closed-valve or open-valve. More particularly, in port injection, closed-valve injection (CVI) strategy is targeting the end of injection (EOI) at closed-valve while the open valve injection (OVI) expects the end of injection (EOI) happens during intake valve open.

The fuel injection process is even more critical for the timing request for starting location of fuel injection in DISI stratified engine and camless engines. Optimized starting of fuel injection maximizes the formation of a fuel cloud around the spark plug in the time frame of spark release. The fuel injection pulse lasts from several milliseconds to dozens of milliseconds depending on design of injector, fuel rail pressure, cylinder volume, and engine conditions. In cold crank and start-up, it generally needs longer fuel pulses to get enough vaporized fuel into the port or cylinder to generate the proper fuel/air mixture for combustion.

As noted above, there is a conversion provided by the ECU between crank angle based parameters and time-based parameters. The conversion is calculated based on engine speed. We have recognized that there if there is any error in engine speed, such error introduces error into the delivered fuel injection position. The speed errors can be due to either measurement error or the speed variation. The invention provides an approach to compensate for the error due to rapid speed change in fuel injection during crank/start and engine speed transition like engine tip-in and tip-out in an internal combustion engine. The invention includes two methods of engine speed prediction. One is polynomial model based engine speed prediction

In accordance with the invention, a method is provided for determining fuel injection time scheduling in an internal combustion engine. The method includes calculating a fuel time schedule for the engine using a prediction of engine speed at the time of such fuel injection.

DETAILED DESCRIPTION

Referring now toFIG. 1, there is shown a schematic diagram of an internal combustion engine which incorporates the teachings of the present invention. The internal combustion engine10comprises a plurality of combustion chambers, or cylinders, one of which is shown in FIG.1. The engine10is controlled by an Electronic Control Unit (ECU)12having a Read Only Memory (ROM)11, a Central Processing Unit (CPU)13, a Random Access Memory (RAM)15, and a Keep Alive Memory (KAM)19, which retains information when the ignition key is turned off for use when the engine is subsequently restarted. The ECU12can be embodied by an electronically programmable microprocessor, a microcontroller, an application-specific integrated circuit, or a like device to provide the predetermined control logic. It is noted that while a port injection engine system is described the invention may be used with other engine systems such as direct injection engines. The present invention applies to engine with variable cam timing, variable cam lift, and variable compression ratio. The present invention further applies to engines used in hybrid electric vehicles.

The ECU12receives a plurality of signals from the engine10via an Input/Output (I/O) port17, including, but not limited to, an Engine Coolant Temperature (ECT) signal14from an engine coolant temperature sensor16which is exposed to engine coolant circulating through coolant sleeve18, a crank shaft angle sensor signal20from a crank shaft angle (CPS) sensor30, a throttle position signal24generated by a throttle position sensor26indicating the position of a throttle plate (not shown) operated by a driver, a Heated Exhaust Gas Oxygen (HEGO) signal32from a HEGO sensor34, an air intake temperature signal36from an air temperature sensor38, an air charge, or flow, signal40from a mass air flow (MAF) sensor42.

The ECU12processes these signals and generates corresponding signals, such as a fuel injector pulse waveform signal transmitted to the fuel injector44on signal line46to control the amount of fuel delivered by the fuel injector44. ECU12also generates a combustion initiation signal (not shown) for receipt by a spark plug (not shown,) to initiate combustion of the air and fuel in the cylinder. Intake valve48operates to open and close intake port50to control the entry of the air/fuel mixture into combustion chamber52.

Referring toFIG. 2, a functional diagram shows the process used to project the Start of Injection (SOI) of the fuel as a function of crank angle (φ). As will be described, the calculation for SOI (φ) is made is a function of a projected engine speed, NP.

More particularly, a targeted End of fuel Injection (EOI (φ)|target) is determined from a measure of Mass Air Flow (MAF) to the cylinder and the measured engine speed Nm. (It is noted that manifold absolute pressure (MAP) may be used to compute mass air flow, in place of basing the computation on a signal from a MAF sensor). The engine speed, Nm, is here measured from information obtained by the crank position sensor30. The targeted End of fuel Injection (EOI (φ)|target) for a typical port injected engine is here, in this example, obtained from a look up table or calculated as a function of engine speed Nmand air mass flow (MAF), block50.

As described above, the timing may also be changed for particular operating modes such as crank/start-up and during acceleration/deceleration demands of the operator.

Nominally, the fuel pulse width (FPW) is calculated to provide a desired air/fuel ratio (A/F) in the cylinders. Given an air flow rate, MAF, typically mass per unit time e.g. pounds/minute, and measured via an air flow sensor, or estimated from manifold pressure/engine speed, throttle position/engine speed, for example, and a desired air/fuel ratio (A/F), the necessary Fuel_Flow_Rate (in mass per time units) is calculated in accordance with:
Fuel_Flow_Rate=AirFlow/AirFuelRatio=MAF/(A/F)

From the calculated Fuel_Flow_Rate, (block52) a quantity of fuel_per_injection is calculated (block54) from the engine speed, (here a projected engine speed NP, at the time, or crank angle, (block53, i.e., from equations (1) and (2), below, or a table) when fuel is actually injected into the port or cylinder) and the number of injections desired per engine revolution (for a 4 cylinder, 4 stroke, PFI engine this would be typically 2 injections per engine revolution—one for each cylinder intake), in accordance with:
Fuel_Per_Injection=FuelFlow/(NP/InjectionsPerRevolution)
Given this fuel mass (i.e., Fuel_Per_Injection), for each injection event, i (here cylinder combustion), the necessary fuel pulse width FPW in units of time (second) is determined from a table using the known characteristics of the injector (block56), the data in such tables being shown by a curve such as that in shown in FIG.2A. It should be noted that these calculations typically include some modifications to account for the effects of the intake manifold filling time and fuel those puddles in the intake manifold.

Given a target end of injection, EOI (φ)|target, (typically in degrees of crankshaft rotation, φ, from block50) and the desired fuel pulse width FPW (in time, typically seconds, from block56), the FPW (in time) is converted into a fuel pulse width in terms of crank angle, block58, i.e., FPW (φ), where FPW (φ)=FPW (in time)/NP.

The Start of Injection (SOI (φ)) is thus equal to the sum of EOI (φ) and −FPW (φ) block60, which sum is fed to the ECU. Thus, a determination is made as to the number of crankshaft degrees correspond to the injector fuel pulse width (SOI (φ)). It would involve one more conversion between time-based parameter and angle-based parameter if the SOI is required to be time-based.

As described above, instead of using a current estimate of engine speed Nmfor this calculation for SOI (φ), a projection of engine speed NPis used. Use of projected engine speed, NP, particularly during engine start-up, more accurately determines the start of injection SOI (φ) that will allow the injection to finish near the desired end of injection angle EOI (φ).

Some applications use hardware that provides frequent updates of the crankshaft (e.g. every 10 degrees) that allow the injection activated (with reasonable accuracy) at the specified angle. Other applications receive much less frequent updates. For example, one known 4-cylinder engine gets position information every 90-crankshaft degrees (via an interrupt called Profile Ignition Pickup (PIP) edges. The start of injection must then be scheduled by specifying a time delay following one of these updates. Again, using the current engine speed to calculate the delay will result in a late start of injection (SOI (φ) if the engine speed increases significantly such as it does during the start and run up. As noted above, in accordance with the present invention, the projected speed, NP, is used to allow the actual start of injection (SOI (φ)) to occur much closer to the desired timing.

Determination of Projected Engine Speed NP

The projected speed Np for determination of the End of Injection (EOI) or Start of Injection (SOI (φ)) relative to the proper crank angle position (φ) associated with a next one of the cylinders to have fuel injection quantity determined therefore is as follows:
Np=Nm+ΔNm|i(1)
where: Nmis measured engine speed; andΔNmis a projected change in engine speed.

In one embodiment, the projected change in engine speed ΔNmis here a polynomial model based:Δ⁢⁢Nmi=∑1n⁢ai⁢Nmn-i(2)
where i is the event index (an event here being cylinder combustion) and n is the order of the polynomial used to estimate ΔNm. The constants in the polynomial are determined experimentally. Equation 2 indicates a dependence only on engine speed. Further improvements on the model are made by having the projected change in engine speed depends on other engine parameters: engine coolant temperature, ambient temperature, accessory loads, as examples.

For an engine run-up example, let it be assumed that the event index i is 1 (i.e., the event is for the calculation of cylinder, C, and that current measured engine speed is Nm(1), or say 300 rpm. A calculation is made of the expected change in engine speed ΔNm(1) in accordance with:

Since, for a particular engine, the coefficients a0through anare known a priori as a result of testing the particular type engine at the factory and are functions of starting temperature for the start-up phase, to be described, and for tip-in/tip-out (i.e., acceleration/deceleration) are functions of the magnitude of the tip-in and tip-out, to be described. The calculated value for, ΔNm(1) is used to determine the projected engine speed NPfor next event—i.e., the 2ndevent. Let it be assumed in this example that the calculated result is ΔNm(1)=350 rpm, then the predicted engine speed, NPfor 1stevent is Np=Nm(1)+ΔNm(1)=300 rpm+350 rpm=650 rpm. For the second event, i=2, (i.e., for the next cylinder in the firing sequence), assume the measured engine speed is 655 rpm, or Nm(2)=655 rpm. The predicted engine speed increment ΔNm(2) is ΔNm(2)=a0+a1Nm(2)1+a2Nm(2)2+ . . . +anNm(2)n−1or ΔNm(2)=a0+a1(655)1+a2(655)2+ . . . +an(655)n−1. Thus, the predicted engine + . . . +an(655)n-1. Thus, the predicted engine speed for 2nd event is determined from the engine speed change ΔNm(2). More particularly, in this example, assume the result is for ΔNm(2) is 250 rpm. The final predicted engine speed for 2ndevent is Np=Nm(2)+ΔNm(2)=655+250=905 rpm. The process continues as described for successively fired cylinders Thus, assuming that there is no misfiring in the start-up. We would able to cover other transient, tip-in and tip-out based on the same idea.

Thus, the method used to determine projected engine speed Npduring engine start-up and during engine acceleration or deceleration is shown in FIG.2. More particularly, as shown inFIG. 2, the mass airflow sensor provides a measure of the mass airflow (MAF) to the cylinder for which the fuel is being scheduled. Also the engine speed NPis measured from the crank position sensor. This measured MAF is combined with the measured engine speed, Nm, to determine a target EOI(φ)|target. The CID with CPS is used to identify the cylinder time scheduled to be fueled (i.e., the event, i). The polynomial in equation (2) predicts the change in speed ΔNm. This polynomial could be evaluated in real time by the PCM or used to generate a look up table stored in the PCM. The predicted engine speed NPis calculated in accordance with equation (1). The projected engine speed NPis used in the calculation of fuel_per_revolution and fuel pulse width FPW(φ).

The methods to determine the predicted speed described above, depend on a prediction of the expected amount of speed change due to the engine firing. As discussed above, this can determined from a model of the engine start up or a lookup table. In the most simple form, the lookup table or polynomial model are based on engine speed alone. Alternatively, the lookup table can be multi-dimensional to include additional engine parameters, which are known in the ECU and which are known to affect the predicted speed change during startup. In the case of a polynomial model, the model can also have factors relating to the additional engine variables, e.g., engine temperature. The methods described above are based on there being successful combustion in each cycle, that is, no misfire. Misfire is a rare event in a modern engine; thus, assuming no misfire is nearly accurate. In the unlikely event of a misfire, though, the engine turns more slowly than if the engine fired normally. According to the assumption of no misfire, the fuel injection, if a misfire occurs, is earlier than desired. Although not desirable, the injected fuel remains in the intake port until the intake valve opens and the fuel and air are drawn into the cylinder. This “error” in timing does not lead to a further misfire.

Referring to FIG3A the process used during engine start-up is described, in Step100, engine crank is sensed. In Step102, an event, i, counter is initialized. At Step104, a determination is made as to whether the engine is synchronized (i.e., whether the engine rotational position has been established). If the engine is synchronized, in Step104, the fuel injection calculations are started and die event counter is incremented (Step106a).

In Step108, a determination is made of a target END_OF_INJECTION (EOI), in crank angle, (i.e., EOI(φ)|target) and the fuel pulse width (FPW) in time. The projected engine speed NPis calculated (as described above in connection withFIG. 2) by determining the projected change in engine speed ΔNmfrom equation (2) in Step108aand adding such projected change is engine speed to the measured engine speed Nmin accordance with equation (1) above (Step108b).

In Step110, the determined fuel pulse width (FPW) is converted to angle-based or EOI(φ) is converted to time-based using the projected engine speed, NP=measured engine speed (Nm)+ΔNm, from equation (1) above. Using the projected engine speed, NP, a final injection start SOI (in time) is determined if the control requires SOI in time using either:SOI=(EOI/NP)−FPW,or determine SOI in staring position based on:SOI=EOI−(FPW*NP).

The final SOI, i.e., SOI_final with other compensations, and FPW are loaded into the hardware system (Step112). The process then returns to the next event (Step114).

A timing diagram of the process is shown in FIG.4.

Acceleration and Deceleration After Start-Up

Referring now toFIG. 3B, the process used during an acceleration or deceleration after engine start-up is described. In Step200, a large throttle change is detected indicating an acceleration or deceleration. In Step202, the process initializes the fuel injection time schedule with predicted engine speed Npas determined in accordance with Step108a(FIG.3A), it being understood that engine speed is being predicted during both start-up and subsequent engine operation in accordance with the process described above in equation (1) (as described above in connection withFIG. 2) by determining the projected change in engine speed ΔNmfrom equation (2).

The process thus continues in accordance with Steps106,108,106a,108a,108b,110,112and114, as described in FIG.3A. After step114, however, in Step116, a determination is made as to whether the actual engine speed is the described engine speed (i.e., whether an acceleration or deceleration is still being demanded). If no whether an acceleration or deceleration is being demanded, the process exits (Step218). If, on the other hand, the acceleration or deceleration is still being demanded, the process then returns to the next event (Step114).