Electronic Ignition Based on In-Cylinder Pressure from a Pressure Sensor Glow Plug

The use of piston location to time the spark ignition in a free piston engine is problematic since piston travel is not defined and constrained by the crank and connecting rod but is a function of the energy available to the piston and the in-cylinder pressure. The recent availability of diesel engine glow plugs/heaters with integrated in-cylinder pressure sensing provided the opportunity to manage spark timing based on in-cylinder pressure. Therefore the combustion initiation can be better controlled. These modern glow plugs are now being utilized on production cars and hence their price is reasonable. This paper describes the calibrations performed on these glow plug sensors at low and also high pressures as expected in an engine. It details the drop in output voltage of the sensors, which, in a way, is similar to that from a piezoelectric type sensor. Therefore the calibration was not as straightforward as expected. The paper details the circuit designed to filter, amplify and threshold the in-cylinder pressure signal and as a result issue the spark plug trigger. The paper outlines the successful implementation of these glow plugs on the free piston engine which resulted in better combustion events than in previous location-based spark triggers.