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An interest in measuring ion current in Homogeneous Charge Compression Ignition (HCCI) engines arises when one wants to use a cheaper probe for feedback of the combustion timing than expensive piezo electric pressure transducers. However the location of the ion current probe, in this case a spark plug, is of importance for both signal strength and the crank angle position where the signal is obtained. Different fuels will probably affect the ion current in both signal strength and timing and this is the main interest of this investigation. The measurements were performed on a Scania D12 engine in single cylinder operation and ion current was measured at 7 locations simultaneously. By arranging this setup there was a possibility to investigate if the ion current signals from the different spark plug locations would correlate with the fact that, for this particular engine, the combustion starts at the walls and propagates towards the centre of the combustion chamber.

The potential of a Homogeneous Charge Compression Ignition (HCCI) engine with variable compression ratio has been experimentally investigated. The experiments were carried out in a single cylinder engine, equipped with a modified cylinder head. Altering the position of a secondary piston in the cylinder head enabled a change of the compression ratio. The secondary piston was controlled by a hydraulic system, which was operated from the control room. Dual port injection systems were used, which made it possible to change the ratio of two different fuels with the engine running. By mixing iso-octane with octane number 100 and normal heptane with octane number 0, it was possible to obtain any octane rating between 0 and 100. By using an electrical heater for the inlet air, it was possible to adjust the inlet air temperature to a selected value.