In-cylinder pressure of an internal combustion engine is considered to be a major source of information about combustion process. It is a generally accepted method to obtain an in-cylinder pressure signal using a pressure sensor (transducer). A different method of approach is presented in this study. The information about the in-cylinder pressure can be obtained by measuring breakdown voltage across the spark-plug gap.
The density of gas inside the combustion chamber effect on the breakdown voltage of the spark plug, which is derived by the application of a high bias voltage (30kV) to the sparkplug gap continuously. The correlation between maximum breakdown voltage position and peak pressure position is established by this principle. So it is possible to detect the peak pressure position by measuring the breakdown voltage of the spark plug. The analyzing method of the breakdown voltage signal is also presented.