ENGINE OPERATION 3 fuel metering

Fuel metering With “ignition on, engine off”, the fuel pump runs for one second to build up the system pressure and is then deactivated for safety reasons. The fuel metering is determined via open or closed-loop control. The open control loop differs from the closed control loop in that the lambda control is deactivated. The PCM switches back from closed to open-loop control when the HO2S cools down or fails, as well as during during acceleration, coasting and Wide Open Throttle operation. The calculation of fuel metering by the PCM depends on the operating state and comprises: Control of the fuel pump Calculation of the fuel quantity during starting (cranking) of the engine Calculation of the desired air/fuel ratio Air mass calculation Calculation of fuel quantity for the different operating states and corresponding fuel adjustment measures. Open loop control Open loop control is used primarily for fuel injection, as long as the signals of the HO2S are not involved in the calculation of the PCM. The two most important reasons that make it absolutely essential to run the engine without lambda control (open-loop control) are the following operating conditions: Cold engine (starting, warm-up phase) Full-load operation (WOT (wide open throttle)) Under these operating conditions the engine needs a rich air/fuel mixture with lambda values below λ = 1 in order to achieve optimum running or optimum performance. Closed-loop control Closed loop control ensures strict control of exhaust emissions in conjunction with the TWC and economical fuel consumption. With closed loop control, the signals from the HO2S are analyzed by the PCM and the engine always runs in the optimum range of λ = 1. In addition to the normal HO2S, the signal from the monitoring sensor for the catalytic converter is also included in the control. The lambda control is optimized on the basis of this data. Ignition angle calculation Since propagation of the flame front in the air/fuel mixture always takes the same amount of time, the ignition of the air/fuel mixture has to take place earlier or later depending on the engine speed. The higher the speed, the earlier ignition must occur. This ensures that maximum combustion pressure is achieved immediately after Top Dead Centre. The following provide the basis for calculating the ignition angle: Engine speed Engine load. Engine coolant temperature KS signal The ignition angle has a major impact on engine operation. It influences: engine performance exhaust emissions and fuel consumption The higher the engine load, i.e. the torque demand, the richer the air/fuel mixture, the longer the combustion period and the earlier the ignition. The engine load is determined by the PCM on the basis of the MAF signal and the throttle valve position. The ignition timing is calculated taking both of these variables and the engine speed into account. It is based on ignition maps stored in the PCM. The ignition timing is adjusted according to the operating condition of the engine, for cold starting for example. Ignition map ItemDescription 1--Engine load 2--Engine speed 3--Ignition angle The data in the ignition maps is obtained from test series. Particular attention is paid to the emission behaviour, power and fuel consumption of the engine. The ignition map is stored in the data memory of the PCM. By adjusting the ignition timing it is also possible to influence the engine speed to some extent without having to change the throttle valve position. This has advantages for idling stabilization, as the engine speed and hence the engine torque respond far more quickly to a change in the ignition timing than to a change in the throttle valve position. Furthermore, the ignition timing can be changed much more quickly. To keep the ignition point as close as possible to the knock limit and so optimize the efficiency of the engine, two KS are installed in the engine, which pick up the mechanical vibrations of the engine and convert them into an electrical signal for the PCM.

ItemDescription ANormal combustion BKnocking combustion 1-Pressure characteristics in cylinder 2-Output signal from KS Knocking combustion takes place when flame speeds near the speed of sound occur. This can happen towards the end of combustion in particular, when unburnt gaseous mixture on the combustion chamber walls self-ignites due to the increase in pressure following initiation of regular combustion. The resulting pressure peaks damage the pistons, cylinder head gasket and cylinder head. If knocking combustion occurs, the ignition point is adjusted in the late direction only for the cylinder concerned, until knocking combustion ceases. The ignition point is then slowly adjusted back in the early direction, until the ignition point specified by the PCM is reached again.