A specially designed cylinder head, enabling unthrottled operation with a standard cam-phasing mechanism, was tested in an optical single-cylinder engine. The in-cylinder flow was measured with particle image velocimetry (PIV) and the results were compared with heat release and emission measurements. The article also discusses effects of residual gas and effective compression ratio on heat-release and emissions.
The special design of the cylinder head, with one inlet and one exhaust valve per camshaft, made it possible to operate the engine unthrottled at part load. Cam phasing led to late inlet valve closing, but also to increased valve overlap. The exhaust valve closing was late in the intake stroke, resulting in high amounts of residual gases. Two different camshafts were used with late inlet valve closing. One of the camshafts had shorter valve open duration on the phased exhaust cam lobe. This camshaft did not have a very large valve overlap when phased, and was found to result in comparable amounts of residual gases to throttled operation, using standard valve timing. Residual gas content was calculated with an engine simulation model.
The engine was operated down to a load of 4 bar IMEPnet with unthrottled operation. Below that load, the engine had too large variations in load from cycle to cycle. The reduction in NOx was about 4-5 g/kWh. The lowered effective compression ratio had the largest effect on NOx reduction, and the residual content only had a small effect.
The engine was not possible to operate at very low loads (below 4 bar IMEPnet). This was caused by the low effective compression ratio and the relatively low turbulence.