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This paper presents a start algorithm that is able to control the air/fuel ratio (AFR) during the cranking phase and immediately hereafter, where the ordinary λ-control is not yet enabled. The control is based on the ion sense principle, which means that a current through the spark plug is measured directly after the spark has disappeared. This current is a measure for the temperature and therefore of the combustion in the cylinder. This is an excellent way to start a CNG (Compressed Natural Gas) engine with unknown gas qualities. A typical example of application is when the vehicle is almost out of fuel and is refueled at a motel stop. The small amount of old fuel that is left in the system will mix with the new fuel resulting in an unknown fuel quality. The control system shall then be able to start the engine directly or after an accommodation over night. During the last condition, the oxygen sensor is still cold and thus not able to correct for fuel quality changes.

In this paper we use the idea of parameterizing the ionization current using the sum of two Gaussian functions in an indirect scheme to estimate the AFR. In the first step of the scheme, the Gaussian functions are fitted to the ion signal using a standard least-squares fit. Then, as a second step, the AFR is estimated using the six parameters of the Gaussian functions plus the ignition angle and measurements of the engine speed. The experimental tests show that it is possible to estimate the AFR with good accuracy, using this approach. The best results were obtained using a neural network approach and it is shown in the paper that the AFR can be estimated from the ionization current down to approximately 0.1% in mean square error.