GB2218465A

GB2218465A - I.C. engine ignition system

Info

Publication number: GB2218465A
Application number: GB8811500A
Authority: GB
Inventors: Michael Wynn Dallaway; Peter Lloyd
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1988-05-14
Filing date: 1988-05-14
Publication date: 1989-11-15
Also published as: GB8811500D0

Abstract

An electronic ignition system has a load input to the timing control, e.g. a micro-processor, provided by a throttle valve position transducer, e.g. a potentiometer on a carburettor throttle valve spindle, and an engine speed sensor. The micro-processor may have additional inputs, e.g. engine temperature, charge temperature and barometric pressure, which determine co-ordinates accessed in a look-up table.

Description

IGNITION SYSTEM The present invention relates to an ignition system for a spark ignited internal combustion engine.

It is known to determine spark timing by the use of an on board computer which receives signals representing various operating parameters of the engine, such as engine speed, temperature and manifold pressure. The engine is calibrated on a test bed to establish optimum ignition settings under various operating conditions and these settings are stored in look-up tables in the computer. During use, by referring to the stored data, the computer can read off or calculate the optimum ignition setting for the prevailing operating conditions.

Hitherto, it has been assumed that the optimum setting of the ignition timing, for any engine temperature and atmospheric temperature, is related uniquely to the engine speed and the engine load. The load signal has been derived by monitoring the pressure in the intake manifold or in some systems by the use of air flow meters. Such measurement of the engine load is undesirable for reasons which will now be discussed.

Air flow meters are commonly used with fuel injection systems where it is necessary to measure the air charge in order to set the fuel quantity. These are however expensive and in the case of a carburetted engine they would not be viable. The more common transducer therefore used in the prior art has been a pressure sensor in the intake manifold. Such a sensor requires machining of the manifold, is itself costly to produce and its accuracy and reliability are below those of the remainder of the control system. Furthermore, in a wet manifold engine, the pressure sensor can itself be impaired in its operation by the fuel in the manifold.

With a view to mitigating the foregoing disadvantages, the present invention provides an electronic ignition system for a spark ignited internal combustion engine for determining the spark timing as a predetermined function of sensed engine operating parameters related to at least the engine speed and the engine load, wherein the engine load related input signal to the ignition system is derived from a transducer sensing the position of a throttle valve controlling the air supply to the engine cylinders.

Conveniently, the electronic ignition system comprises a micro-controller which includes a micro-processor and a look-up table in which there is stored data indicative of optimum settings of the ignition timing under different operating parameters.

The invention is predicated upon the realisation that while the manifold pressure and the throttle position may not be uniquely related to one another under all operating conditions and throughout the life of the engine, the throttle position is as useful an input parameter for determining the optimum ignition setting as the manifold pressure. So long as the calibration tables in the look up memory of the micro-controller are derived by using the throttle position rather than manifold pressure as an indication of engine load it is of no consequence if during operation the prevailing throttle position can correspond to more than one manifold pressure, for example due to a change in the prevailing atmospheric pressure, or due to a change in engine load as caused by depressing the clutch on overrun.

The look-up table requires two co-ordinates for each position in the spark advance map, the first being determined from engine speed alone and the second being a load related parameter derived from the throttle position. This second parameter is preferably computed as a function not only of the throttle position but also the engine speed. Optionally the second parameter may also vary with charge temperature and/or atmospheric pressure being itself derived, if necessary from a lookup table.

Though not exclusively restricted to carburetted engines, the invention finds most application in such engines as it can considerably reduce system complexity.

The position transducer can in this case be mounted on the spindle of the butterfly valve or the primary butterfly valve in the case of a twin-choke carburettor.

Advantageously, the throttle position transducer is a potentiometer. Recently reliable potentiometers have been designed which are well suited to this application.

In preferred embodiment of the invention, a potentiometer is mounted on the side of a carburettor to provide an analogue electrical output signal indicative of the throttle position. The signal is supplied to a micro-controller which includes a processor, a memory containing calibration data and various input-output circuits which, inter alia, carry out the necessary A/D conversions.

The position signal is converted in the micro-controller into a load indicative signal by the use of a first table of which the co-ordinates are the position of the throttle and the engine speed. This yields a first parameter, which can if desired be modified by such other factors as engine temperature, air charge temperature and atmospheric pressure to yield the parameter used to represent load in the main ignition map.

The optimum ignition setting is either directly read off the main calibration table or computed from this data using predetermined algorithms. These may be designed to effect compensation for such parameters as engine temperature, air charge temperature and fuel octane rating. It is possible for more than one calibration table to be stored and for selection of the appropriate table to be effected under the control of such a further parameter.

While the invention has been described above by reference to a micro-processor implementation, it will be clear that a look-up table merely provides a convenient method of defining a function which can be computed rapidly in real time. In principle, the invention is not restricted to any particular form of electronic ignition system and can alternatively be embodied in a system in which the ignition timing is computed by analogue or digital techniques as function of various operating parameters, the throttle position being used as the indication of the engine load.

It is believed that from the foregoing description, the person skilled in the art will be able to put the invention into practice without further explanation.

The apparatus of the invention is essentially analogous to prior art systems employing pressure sensors to indicate the engine load the primary difference being that the signal indicative of engine load is derived from the throttle position transducer rather than from the manifold pressure sensor. Of course, the calibration tables used in the invention will also be different from such prior art systems since the second co-ordinate used in the look-up table is not the same as in the prior art systems.

Claims

1. An electronic ignition system for a spark ignited internal combustion engine for determining the spark timing as a predetermined function of sensed engine operating parameters related to at least the engine speed and the engine load, wherein the engine load related input signal to the ignition system is derived from a transducer sensing the position of a throttle valve controlling the air supply to the engine cylinders.

2. An ignition system as claimed in claim 1, comprising a micro-contLcller which includes a microprocessor and a look-up table in which there is stored data indicative of optimum settings of the ignition timing under different operating parameters.

3. An ignition system as claimed in claim 2, wherein the look up table is accessed using two coordinates, the first being engine speed dependent and the second being load dependent, the second co-ordinate being itself derived from the throttle position and at least the engine speed.

4. An ignition system as claimed in claim 3, wherein the second parameter also varies as a function of charge temperature and/or atmospheric pressure.

5. An ignition system as claimed in any preceding claim, wherein the engine is a carburetted engine and the position transducer is mounted on the spindle of a butterfly throttle of the carburettor.

6. An ignition system as claimed in any preceding claim, wherein the throttle position transducer is a potentiometer.