Engine fuelling rate control

Details Engine fuelling rate control Engine fuelling rate control

1999-01-07

2003-06-24

Vo, Hieu T. (Department: 3747)

Internal-combustion engines

Charge forming device

Fuel injection system

C701S104000, C701S105000, C123S480000

Reexamination Certificate

active

06581572

ABSTRACT:

The present invention generally relates to the control of the fuelling rate of internal combustion engines, and in particular to engines in which fuelling level and air flow level may be controlled independently, for example where fuel is supplied via electronically controlled fuel injection. In this specification, reference will be made to fuel delivery per cycle (fpc) and air flow per cycle (apc). A reference to either apc or fpc may refer to the level of fuelling/air flow determined to be required for appropriate operation of the engine (the “demand” apc/fpc), or to the fuel/air actually delivered to the engine, or to any other measure of air flow or fuelling level as the context requires.
In many internal combustion engines, such as carburettor fuelled four stroke engines, the relationship between air flow rate and fuelling rate is substantially monotonic. In these engines, each air flow rate value corresponds to a single fuelling rate value. Engines having this characteristic are able to operate under what is known as air led control. In air led control, an air flow rate is set by driver demand, and fuelling level is subsequently determined as a function of the air flow rate to the engine.
It is however not-normally possible to use such control in internal combustion engines having an air flow/fuelling level characteristic which provides non-unique values of fuelling level for a given air flow. One example of an engine having such a characteristic is the applicant's fuel injected two stroke crankcase scavenged engine. In this engine, airflow to the engine actually decreases with initial increases in fuelling level (or rate) before rising, as fuelling level increases further, to above the initial air flow rate. It can be seen that it is possible to obtain non-unique values for fuelling rate for a single air flow rate. Many variations providing non-unique values are possible. For example, initial increases in fuelling may correspond to substantially no change in airflow. It is therefore not generally possible to use air led control of the fuelling rate at low engine loads in such engines.
The Applicant's Australian Patent Application No. 34862/93, describes a method for controlling the fuelling rate of an internal combustion engine, in particular a fuel injected two stroke engine, where a fuelling rate, or “Demand_FPC” is initially determined and the required air flow rate, or “Demand_APC” is subsequently determined on the basis of the Demand_FPC value. This method of controlling the fuelling rate is referred to as fuel led control. The Demand_FPC is determined as a function of operator demand as measured, for example, by sensing the throttle pedal position and the engine speed. The Demand_FPC can then be determined by means of a look-up map provided within the engine management system plotting the Demand_FPC against the coordinates of pedal position and engine speed. This look-up map is known as the “pedal” map because the driver initiated fuelling level is assessed by determining the operator pedal position. The Demand_APC for the above determined Demand_FPC is then determined using a look-up map plotting Demand_APC against the coordinates of Demand_FPC and engine speed. The determined Demand_APC is then compared with the measured air supply rate to the engine, or Measured_APC, as measured by an air mass sensor and, if possible, the air mass flow rate adjusted to compensate for any difference between the two. The resultant air/fuel ratio of Demand_FPC against Demand_APC can also be compared with a censor air/fuel ratio which is preset on the basis of the engine load demand and engine speed. The censor air/fuel ratios are stored on a further look-up map and set predetermined minimum limits to the air/fuel ratio that can be applied for the existing speed and load. These limits to the air/fuel ratio are set to prevent specific engine malfunctions such as engine misfire, and take into account catalyst and/or emission considerations. If it is determined that the air fuel ratio is too low (ie rich mixture), the fuel supply may be clipped to avoid delivery of such rich mixtures to the engine.
Fuel led operation may be disadvantageous in certain situations. In certain types of fuelling systems, such as those using fuel injectors, fuelling level can be altered quickly and accurately, whilst variation of the air flow rate is generally less accurate, slower and more difficult to control, particularly under transient conditions, making control of the air fuel ratio in the combustion chamber more difficult. Supplying air and fuel at an accurate air fuel ratio is important for controlling combustion emissions. As such, it is preferable to have the airflow being set by driver demand and then to control the fuelling level to give the required air fuel ratio, that is, air led control.
Another advantage of using air led control at higher load/speed occurs at or near wide open throttle (WOT) conditions where air led control can be used to achieve maximum power output from the engine. In fuel led operation, calculation of maximum fuelling for a given engine speed is based on experimental calibration of test engine(s). The calibrated maximum fuelling would normally be set at slightly lower than the test results indicated to provide a margin of safety to ensure that an overly rich mixture was not obtained. However, in actual operation, airflow to the engine may be higher than the experimental data indicated, particularly under transient conditions. This may result in the air fuel ratio in the combustion chamber being less than that for which maximum power can be obtained. At wide open throttle, for example, air flow is at its maximum, but maximum fuelling corresponding to the air flow may not be supplied due to the calibrated maximum fuelling rates, reducing the power output of the engine.
Whilst fuel led control is necessary for low engine loads/speeds, this may not be so for higher load/speed conditions. In certain engines, such as the applicant's two stroke direct injected crankcase scavenged engine, there is a substantially monotonically increasing relationship between the fuelling rate and the air flow rate at higher loads. Under these loads it is possible, and preferable as discussed above, to use air led control of the fuelling rate.
The major difficulty that arises with such an arrangement is that there can be a discontinuity at the point of transition between the two control methods. The fuelling rate determined under fuel led control could be significantly different to the fuelling rate determined under air led control at the point where the engine management system transfers between the two fuelling rate control methods. This can cause a step change in the determined fuelling rate resulting in a step change in torque. Such sudden changes may be detrimental to engine control and are undesirable as they may result in jolting through the drive train of the vehicle producing, for example, an uncomfortable ride for the occupants of the vehicle.
It is therefore an object of the present invention to provide an improved method of controlling the fuelling rate of an engine.
With this in mind, the present invention provides a method of controlling the fuelling rate for an internal combustion engine including:
(a) controlling the fuelling rate in a fuel led control mode whereby the fuelling rate is controlled as a function of the operator demand on the engine during at least a portion of low engine load operation;
(b) controlling the fuelling rate in an air led control mode whereby the fuelling rate is controlled as a function of the air flow rate to the engine during at least a portion of medium to high engine load operation;
(c) providing a point of transition between the two control modes whereat each control mode provides substantially the same predetermined fuelling rate.
As the point of transition between the two control modes occurs when the fuelling rate determined by either control mode reaches substantially the same predetermined threshold fuelling rate, there can the

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