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An ionization sensor has been used to study the combustion process in a six-cylinder lean burn, truck-sized engine fueled with natural gas and optimized for low emissions of nitric oxides. The final goal of the investigations is to study the prospects of using the ionization sensor for finding the optimal operating position with respect to low NOx emission and stable engine operation. The results indicate that unstable combustion can be detected by analyzing the coefficient of variation (CoV) of the detector current amplitude. Close relationships between this measure and the CoV of the indicated mean effective pressure have been found during an air-fuel ratio scan with fixed ignition advance.

Continuously Variable Cam Phasers (CVCP) controlling both the exhaust and intake camshaft phasing in combination with air assist direct fuel injection and a wide variable spark gap is utilized to reduce the fuel consumption for a 4-valve Turbo Charged (TC) Spark Ignition (SI) engine. The spark plug and the air-assisted direct fuel injector are integrated into one unit in order to facilitate packaging in a modern 4-valve combustion chamber. This integrated component is referred to as the Spark Plug Injector (SPI). CVCP's are used to reduce pumping work by diluting the charge with large amounts of residual exhaust gas. This strategy, along with stoichiometric homogeneous operation enables the use of a Three Way Catalyst (TWC), considered to be a prerequisite to meet the emission standards of Ultra Low Emission Vehicles (ULEV) II and beyond. The ignition quality deteriorates with increasing levels of residual exhaust gas dilution.