Estimation of in-cylinder pressure using spark plug discharge current measurements

In-cylinder pressure of spark ignition engines is correlated with several variables of the compression, injection, and ignition processes. Therefore, by monitoring the pressure of each cylinder we can improve the electronic engine supervision and control in terms of fast response and accuracy, thus enabling online diagnosis and overall efficiency improvement. However, the pressure measurement methods proposed until now have not been generally accepted at the production level due to their cost and/or complexity. In this paper we describe a novel approach that derives the pressure information from the measurement of the ordinary spark plug discharge current. In this manner, the mechanical part of the engine does not require substantial modification. To demonstrate the viability of the proposed technique, a prototype measurement system was set up and tested. Preliminary experimental data show that pressure in stationary conditions can be estimated with an error lower than 5% for pressure above 3 atmospheres.