In-cylinder mixture analysis

An optimal ignition and a complete combustion of the air/fuel-mixture in combustion engines is mainly influenced by the mixture generation. Fuel concentration, temperature and pressure of the mixture are the critical values during the initialization of the combustion. A fundamental knowledge of the dynamics of the mixture formation is necessary to optimize the internal combustion. A highly time-resolved data acquisition of the mentioned values, during many consecutive cycles, is preferable to observe mixture formation phenomena and their effects.

The only access to the combustion chamber for optical sensors, without complex modifications on the engine block, is the spark plug hole. We successfully integrated an optical sensor in a modified spark plug which is able to detect the fuel concentration and the lambda value at the ignition position time-resolved. There is no further optical access needed, what leads to unchanged mechanical and thermodynamic properties of the engine.

: Figure 1: Head of the measurement spark plug

Figure 1 shows such an ignitable measurement spark plug with integrated absorption path. A tungsten halide lamp is used as an infrared light source. The radiation is guided by an optical fiber to the absorption path and reflected by a small metal mirror to the second fiber, which leads to the detector. Fuel molecules within the absorption path absorb infrared radiation due to dependence of the fuel density in the absorption path. It is possible to calculate the crank angle resolved fuel concentration and the local lambda value, with a corresponding calibration, from the absorption data.

: Figure 2: Different fuel concentration curves: a) Misfire, b) Delayed combustion, c) Ideal combustion

The time-resolved fuel concentration measurement is possible during the whole engine cycle, with motored and fired engine. Figure 2 shows exemplary conspicuous concentration curves, caused by misfire and delayed combustion. The concentrations mainly differ after ignition, because fuel was not at all or not complete combusted. In this case the measurements were performed on a gasoline research engine with injection into the intake manifold and homogenous mixture formation.

Co-operation partners:

LaVision GmbH
Robert Bosch GmbH
Volkswagen AG

Further information:

A. Grosch, V. Beushausen, O. Thiele, R. Grzeszik, Crank Angle Resolved Determination of Fuel Concentration and Air/Fuel Ratio in a SI-Internal Combustion Engine Using a Modified Optical Spark Plug, SAE Paper 2007-01-0644

Grosch A., Beushausen V. , Thiele O., Crank Angle Resolved Determination of Fuel-Concentration and Air/Fuel Ratio in a SI-Production Engine by Using a Modified Optical Spark Plug, in Valldorf J., Gessner W (Hrsg): Advanced Microsystems for Automotive Applications 2008, S. 105, Springer-Verlag, ISBN: 978-3-540-77979-7