Variable Valve Actuation in a Spark Ignition Engine - A Flow Field and Heat Release Study
Sammanfattning: The objective of this thesis was to find ways of improving part-load efficiency for a spark ignition engine by studying the effects of variable-valve actuation on fluid flow and combustion. The in-cylinder flow was measured with laser doppler velocimetry, LDV, and particle image velocimetry, PIV. A piezo-electric pressure transducer was used to measure the in-cylinder pressure, which was used for calculations of efficiency and heat-release. Late intake cam phasing, combined with valve deactivation, generated very high turbulence and this made it possible to operate an engine very lean, l = 1.8, which will improve the part-load efficiency. Early and late inlet valve closing was tested for unthrottled operation. The generated turbulence varied between 0.9 and 2.8 m/s depending on the valve strategy. Stable unthrottled operation was possible at low loads (4 bar IMEPnet) and the efficiency was improved with about 13%. The same improvement was achieved with lean-burn operation, l = 1.5. In both cases, the improvement was caused by the reduction in pumping losses and heat losses. A newly developed cross-flow cylinder head was tested for unthrottled operation using a simple cam phasing mechanism. The efficiency was improved with about 9% at part-load, but the combustion stability deteriorated at low loads. This was caused by the (assumed) low turbulence generation. This was illustrated with stationary CFD simulations in the same geometry. The cylinder head would probably generate higher turbulence with horizontal intake ports instead of the existing vertical ports. Wavelet analysis was applied to in-cylinder flow measurements. The technique was compared to traditional moving window techniques and was found to be applicable. The technique was later used for examining the flow behavior in tumbling and swirling flows. The tumble breakdown, with energy transfer from large to small eddies, could clearly be seen. In a swirling flow, all turbulence frequencies peaked almost simultaneously at 5-15 CAD before compression TDC. A variable compression ratio engine (Alvar engine) was compared to a standard engine. The Alvar engine improved the part-load efficiency with about 14% compared to the standard engine. A stationary CFD analysis was performed in the cross-flow cylinder head. The calculations were compared to LDV measurements in the same geometry. The agreement between calculations and measurements were not very good, but were considered adequate for a qualitative examination of the in-cylinder flow. The calculations showed a double swirl, which decayed quickly.
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