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The cyclic changes of the cylinder pressure are mainly influenced by the primary inflammation phase, which in turn depends on the local air/fuel ratio and the residual-gas fraction at the spark plug. The ion-current measurement technique is based on the conductivity of the mixture during the internal combustion. It is therefore possible to use the signal for combustion diagnostics when using the spark plug as a sensor. This article demonstrates the potential of ion sensing at the spark plug and in the combustion chamber to detect sources of interference which prevent an optimal combustion process. Comparing the ion signals of consecutive combustion cycles delivers explanations of phenomena that could not yet be sufficiently characterized by cylinder-pressure indication. The results allow new fundamental approaches to the optimization of the combustion process.

When developing effective exhaust emission reduction measures, a better understanding of the complex working cycle in crankcase scavenged two-stroke gasoline engines. However, in a two-stroke gasoline engine detailed measurement and analysis of combustion data requires significantly more effort, when compared to a lower speed four-stroke engine. Particularly demanding are the requirements regarding the high speed (>10,000 rpm) which inevitably goes along with heavy vibrations and high temperatures of the air cooled cylinders. Another major challenge to the measuring equipment is the increased cleaning demand of the optical sensor surface due to the two-stroke gasoline mixture. In addition, the measuring equipment has to be adapted to the small size engines. Therefore, only a fiber optical approach can deliver insight into the cylinder for analyzing the combustion performance.