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Previous studies have shown that increase compression ratio (CR) is an effective way to improve thermal efficiency of gasoline engine without changing the mechanical structure and working cycle, however, it is limited by engine knock when increasing the intake boosting under high load operation. This study aimed to solve the knock problem of gasoline engine with higher CR by application of Miller cycle, which can be implemented by either early or late intake valve closing (EIVC or LIVC). Therefore, in this paper, based on the engine with CR of 13.5 and electromagnetic valves train (EMVT), a comparative study was carried out to investigate the effects of EIVC and LIVC on engine performance, by theoretical modeling and calculation. The results show that, at high load, EIVC strategy is more preferred than LIVC owing to its lower total power consumption, which can improve the indicated mean effective pressure (IMEP) by 0.0371 bar, while enhance turbulence intensity and improve combustion. And at part load, the advantage for EIVC declines gradually, nevertheless, it can still sensitively adjust the EGR rate and thus reduce NOx. This results of quantitative analysis about two Miller cycles can provide valuable reference for engine designers and researchers. [ABSTRACT FROM AUTHOR]

Highlights: [•] A new intake port for a turbocharged spark ignition engine was designed and tested. [•] The new port promotes turbulence by increasing the tumble motion at low valve lifts. [•] Target of enhancing poor turbulence levels typical of EIVC at part load was reached. [•] Full load performance targets were not reached due to the slower combustion. [•] Combustion characteristics were related to in cylinder flow characteristics. [ABSTRACT FROM AUTHOR]

This study provides an experimental evaluation of the effectiveness of Miller cycles with various combinations of lift and intake valve closing angle for a passenger car engine with premixed combustion in naturally aspirated operation. A fully variable electro-hydraulic valve train provided different valve lift profiles. Six load points, from 1.5 up to 5 bar brake mean effective pressure at a constant engine speed of 2000 min−1, were tested with 6 different intake valve lift/intake valve closing angle combinations. The intake valve closing angle was always set before bottom dead center to achieve the desired load with unthrottled operations. Experimental comparison with throttled operation outlines an indicated efficiency increase of up to 10% using high intake lift with early valve closing angle. Furthermore, this analysis outlines the influences that early intake valve closing angle has on fuel energy disposition. Longer combustion duration occurs using early intake valve closing angle because of turbulence dissipation effects, leading to slight reductions in the heat-to-work efficiency. However, overall pressure and temperature levels decrease and consequently heat losses and losses due to incomplete combustion decrease as well. Overall, we found that combustion deterioration is compensated/mitigated by the reduction of the heat losses so that reductions of pumping losses using early intake valve closing can be fully exploited to increase the engine's efficiency. [ABSTRACT FROM AUTHOR]