Maximizing Power Output in an Automotive Scale Multi-Cylinder Homogeneous Charge Compression Ignition (HCCI) Engine

Experimental investigations were conducted on a multi-cylinder automotive scale HCCI engine in determining a strategy that yields high power output, sufficient for passenger vehicles. A 1.9L Volkswagen TDI, modified for HCCI operation, is used with a compression ratio of 17:1 and boost pressures between 1.0 and 2.0 bar absolute. Various equivalence ratios and combustion times are explored at 1800 RPM with commercial-grade gasoline. The effects of exhaust backpressure that would be caused by a turbocharger in production engines are also explored.

The results reveal that the highest power output can be achieved with high boost pressures and high equivalence ratios, and highly delayed combustion timing for controlling ringing. The optimal power output conditions exist near the boundaries of ringing, peak in-cylinder pressure, misfire and controllability. The results of the highest power output condition are displayed for a single cylinder; however, similar trends were seen across all four cylinders of the HCCI engine. The maximum power output identified in this study exceeded 9 bar gross IMEP, and high indicated efficiency points (exceeding 40%) were also found. NO_x emissions were very low for all test points, well below US2010 standards. For different equivalence ratios and boost pressures, detailed trends were explored for the effects of the various controllable parameters upon power output, ringing, efficiency, NO_x, hydrocarbon and carbon monoxide emissions. For multi-cylinder HCCI, the importance of individual cylinder control is emphasized by showing that cylinders require different intake temperatures for maintaining the same combustion timing. The effects of coupled flow dynamics between cylinders is also explored, particularly because at high power output conditions disturbance propagations between cylinders can cause disturbances from one cylinder to drive neighboring cylinders into unstable combustion modes. Finally, the effects of exhaust backpressure are discussed, and it is found that increasing exhaust backpressures cause lower intake temperature requirements, however other engine characteristics are largely unaffected by increasing exhaust backpressure.