1994-03-01

Measurement of the Wall-Wetting Dynamics of a Sequential Injection Spark Ignition Engine 940447

In this paper the fuel path of a sequentially injected gasoline engine is discussed. Since a fraction of the injected fuel suffers a delay due to the wall-wetting phenomenon, in transient phases a significant deviation of the air-to-fuel ratio from its setpoint can arise.
The amount of fuel on the manifold wall and its rate of evaporation cannot be measured directly. Therefore, the effects of the wall-wetting on exhaust lambda and engine torque have to be considered for the identification of the dynamics.
The dynamics of the exhaust-gas-oxygen (EGO) sensor is not negligible for the interpretation of the lambda measurement. Since both the dynamics and the statics of a ZrO2 Sensor are very nonlinear, a normal EGO-sensor is not suitable for these investigations. On the other hand, the engine torque is a good measure for the cylinder lambda when all other effects which lead to torque changes can be eliminated. The engine torque has to be calculated from the load torque and the resulting engine speed.
On a dynamic engine test bench step response and frequency response measurements are used for the identification of the wall-wetting dynamics. The measurements are executed on a 6-cylinder 3.4 liter BMW engine with sequential injection. In sequentially injected engines, a wall film can be located at each intake valve. These parallel dynamic systems are multiplexed in input and output as well. This effect can be shown clearly in the measurements. A mathematical model taking into account the multiplexing is given and the parameters are identified.

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