Miller cycle is considered as an effective means to meet the regulation on Tier Ⅱ and to reduce CO2 emission. For this cycle, the amount of intake air supplied should be enough increased. Therefore, the intake system with minimized resistance for air flow is under consideration. In this study, the flow coefficients of intake valves were measured in order to obtain the basic data for the cycle simulation and intake port design. The flow coefficients were measured using the steady-flow test rig. As a test result for the poppet valve used the marine engine with medium speed, the flow coefficients are increased to about 0.62 with the valve lift. In addition it is confirmed that the flow coefficients have the characteristic value irrelevant to the S/B ratio.
The Atkinson cycle, where expansion ratio is higher than the compression ratio, is one of the methods used to improve thermal efficiency of engines. Miller improved the Atkinson cycle by controlling the intake- or exhaust-valve closing timing, a technique which is called the Miller cycle. The Otto–Miller cycle can improve thermal efficiency and reduce NOx emission by reducing compression work; however, it must compensate for the compression pressure and maintain the intake air mass through an effective compression ratio or turbocharge. Hence, we performed thermodynamic cycle analysis with changes in the intake-valve closing timing for the Otto–Miller cycle and evaluated the engine performance and Miller timing through the resulting problems and solutions. When only the compression ratio was compensated, the theoretical thermal efficiency of the Otto–Miller cycle improved by approximately 18.8% compared to that of the Otto cycle. In terms of thermal efficiency, it is more advantageous to compensate only the compression ratio; however, when considering the output of the engine, it is advantageous to also compensate the boost pressure to maintain the intake air mass flow rate.
In recent years, marine engine manufacturers have become increasingly interested in gas engines as an alternative to diesel engines to address rising crude oil prices and environmental regulations. In this study, a 1.6 MW dedicated gas engine was developed based on a diesel engine with bore 220, stroke 300. The developed gas engine had a precombustion chamber and exhibited excellent performance; the brake mean effective pressure was 2.1 MPa at 1000 rpm and NOx emissions were 50 ppm under 15% O2. In particular, it demonstrated excellent fuel economy with a thermal efficiency of 45%, and its carbon dioxide emissions were ~75% of the conventional diesel engines, thus demonstrating greenhouse gas reduction. These results indicate that suitably developed gas engines can provide a low-cost and energy-efficient alternative to diesel engines.
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