Cylinder pressure sensors for smart combustion control
Different sensor concepts for time-resolved cylinder pressure monitoring of combustion engines are realized and evaluated in this paper. We distinguish a non-intrusive form of measurement outside the cylinder, performed by means of a force compression rod from intrusive, real in-cylinder measurement by means of pressure membrane sensors being exposed to the hot combustion process. The force compression rod has the shape of a sine wave with thinner zones equipped with highly sensitive foil strain gauges that experience a relatively moderate temperature level of 120 ∘C. The sensor rod delivers a relative pressure value that may be influenced by neighbour cylinders due to mechanical coupling. For the intrusive sensor type, two different materials for the membrane-type sensor element were simulated and tested, one based on the ceramic zirconia and the other based on stainless steel. Due to the higher thermal conductivity of steel, the element experiences only 200 ∘C while the zirconia element reaches 300 ∘C. Metallic chromium thin films with high strain sensitivity (gauge factor of 15) and high-temperature capability were deposited on the membranes and subsequently structured to a Wheatstone bridge. The pressure evolution can be measured with both types in full detail, comparable to the signals of test bench cylinder pressure sensors. For the preferential steel-based sensor type, a reliable laser-welded electrical connection between the thin films on the membrane and a copper wire was developed. The in-cylinder pressure sensors were tested both on a diesel test bench and on a gas-fired engine. On the latter, an endurance test with 20 million cycles was passed. Reliable cylinder pressure sensors with a minimum of internal components are thus provided. The signals will be processed inside the sensor housing to provide analysis and aggregated data, i.e. mass fraction burned (MFB50) and other parameters as an output to allow for smart combustion control.