Engine management system - MEMS (Ignition Timing): Description and operation

During one engine revolution, 2 of the 4 cylinders draw in air. The ECM uses the CKP sensor signal to determine the potential air intake volume in the cylinders.

The oxygen content of the air contained in the cylinders can be calculated by the ECM using information from the MAP sensor and the IAT sensor. The pressure of the air in the intake manifold will vary according to the following factors: l The position of the throttle valve (driver input). l The atmospheric pressure (altitude and weather conditions). l The mechanical condition of the engine (volumetric efficiency).

The pressure in the intake manifold, downstream of the throttle valve, indicates how much air has flowed into the cylinders. This will decrease at higher altitudes as the air becomes 'thinner' or less dense. This will also mean that there will be less oxygen contained in the air which will be available for combustion of fuel.

The temperature of the air will also affect the oxygen content. Air which is cool has molecules packed closer together than hot air, therefore; cooler air contains more oxygen for any given volume than hotter air.

From the above information, the ECM can calculate how much air has been induced into the cylinders. By comparing these values to a fuelling map stored in the ECM memory, the amount of oxygen induced into the cylinders can be calculated. The values obtained from the ECT sensor, engine oil temperature sensor and TP sensor provide 'fine tuning' to the calculations.

To deliver the fuel the ECM completes an earth path to the injector coil, opening the injector for the precise amount of time required for the quantity determined. The correct cylinder order is determined by referencing the CMP sensor during start up to synchronise the CMP sensor signal to the CKP sensor signal. The fuel is injected into the inlet ports of the intake manifold and is drawn into the cylinder as an air and fuel mixture.

The ECM ensures that the amount of fuel injected is not affected by the variations in inlet manifold pressure. The ECM corrects the injector duration time, using MAP sensor information.

The ECM references battery voltage to adjust opening times to suit the state of battery charge. This is required because low battery voltage will mean slower response from the injectors, and could give a leaner AFR than intended.

Ignition Timing

The ignition timing is an important part of the ECM adaptive strategy. The ignition system consists of two double ended coils, mounted on the cam cover directly over the spark plugs, which operate using the wasted spark principle. Each coil is connected to a pair of spark plugs, 1 and 4, 2 and 3.

The spark plugs are connected in series with the secondary winding of the coil so a spark occurs in both cylinders at the same time. When a spark occurs in the two cylinders which are on the compression stroke the air/fuel charge is ignited. The spark has no effect on the two cylinders at the end of the exhaust stroke, hence the term 'wasted spark'.

The major advantage of this system is that a distributor cap and rotor arm are eliminated thereby improving performance and reliability. The timing of the spark will affect the quality of combustion and the power produced.

The ECM will reference all relevant sensors to achieve the optimum timing for any given condition. This electronically increases the primary coil charging time (dwell angle) as engine speed increases to maintain coil HT voltage at high engine speeds.

The ECM calculates ignition timing using inputs from the following: l CKP sensor. l TP sensor. l ECT sensor. l IAT sensor.

The ECM calculates dwell angle using inputs from the following: l CKP sensor. l Battery voltage.

At start up the ECM sets ignition timing by referencing the ECT sensor. After start up, the ignition timing will be controlled according to maps stored in the memory and modified according to additional sensor inputs.

The choice of ignition point is critical in maintaining engine power output with low emissions. Advancing the ignition may increase power output under certain conditions, but it also increases the amount of oxides of nitrogen (NOx) and carbon monoxide (CO) produced in the combustion chamber.