Cylinder-to-cylinder and cycle-to-cycle variations at HCCI operation with trapped residuals

A naturally aspirated in-line six-cylinder 2.9-litre Volvo engine is operated in Homogeneous Charge Compression Ignition (HCCI) mode, using camshafts with low lift and short duration generating negative valve overlap. Standard port fuel injection is used and pistons and cylinder head are unchanged from the automotive application. HCCI through negative valve overlap is recognized as one of the possible implementation strategies of HCCI closest to production. It is important to gain knowledge of the constraints and limits on the possible operating region. In this work, the emphasis is on investigating how cycle-to-cycle and cylinder-to-cylinder deviations limit the operating region, how these effects change in different parts of the operating region and how they can be controlled. At low load the cycle-to-cycle phenomena cause periodic behavior in combustion timing; together with cylinder deviations this is found responsible for decreasing the operating regime. Increasing the operating region by spark assistance is demonstrated and also the possibility to phase combustion timing by advance of the spark timing. The combustion is monitored in real time using cylinder pressure sensors. Emissions are measured individually for each cylinder. Results are discussed in terms of Coefficient Of Variation (COV), auto correlation coefficient (R), Crank Angle 50% burned (CA50) and emissions in terms of Indicated Specific Carbon Monoxides (ISCO), Hydro Carbons (ISHC) and Nitrogen Oxides (ISNOx).