The effect of intake temperature on HCCI operation using negative valve overlap

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. This implementation requires only minor modifications of the standard SI engine and allows SI operation outside the operating range of HCCI. Standard port fuel injection is used and pistons and cylinder head are unchanged from the automotive application. A heat exchanger is utilized to heat or cool the intake air, not as a means of combustion control but in order to simulate realistic variations in ambient temperature. The combustion is monitored in real time using cylinder pressure sensors. HCCI through negative valve overlap is recognized as one of the possible implementation strategies of HCCI closest to production. However, for a practical application the intake temperature will vary both geographically and from time to time. It is therefore very important to gain knowledge of how this variation affects the combustion process. The operating range in HCCI mode for this specific engine stretches from 650 rpm to 4000 rpm and from 1 bar IMEP to 4 bar IMEP. The emissions of NO_x are approximately two orders of magnitude lower in HCCI mode compared to SI mode. The emissions of CO are slightly lower while the emissions of HC are slightly higher for HCCI operation. When run in HCCI mode the engine's efficiency is improved throughout the operating range by as much as 10 percentage units compared to SI operation. The effect of inlet air temperature variation on HCCI combustion through negative valve overlap is insignificant when the engine runs well inside of the HCCI operating range. The effect is however prominent when the engine is operated close to the border of the operating range.