Study on port matching characteristic of the OP2S dual-fuel engine
The global climate change is becoming increasingly severe, and carbon dioxide emissions are gradually increasing in fluctuations.The world energy is gradually developing in a low-carbon and clean direction.For the main force of global industrial power, the internal combustion engine, improving efficiency and reducing emissions are still the main theme of its adaptation to energy development and environmental protection.At the 21st United Nations Climate Change Conference, countries reached a consensus to limit the increase in global average temperature to less than 2℃ and achieve the global carbon neutrality goal from 2050 to 2100.This has brought great challenges to the internal combustion engine industry, requiring it to make breakthroughs in many aspects such as basic working principles and fuel types.The opposed piston two stroke (OP2S) diesel engine is a cylinder headless and valve-less engine, which has attracted attention for its simple structure, high power density, high thermal efficiency and low heat transfer loss.Combined with dual-fuel combustion technology, the OP2S diesel engine exhibits strong operating and cost advantages, making it ideal for future high-performance engine needs.The OP2S diesel engine usually adopts a valve-port uniflow scavenging.This scavenging mode will bring the high residual exhaust gas during the working process, which is not conducive to the clean combustion of the engine.Oxygenated methanol can reduce the air demand of OP2S diesel engines and reduce the residual exhaust gas.The full utilization of methanol's advantages depends on the design of the scavenging system, especially the design of the intake and exhaust ports.Therefore, under the dual-fuel strategy, the primary attention is to the matching characteristics of the intake and exhaust ports.So this work applies the GT-POWER to simulate the working process.As there is no OP2S cylinder module in GT-POWER, we develop an equivalent method through the two-stroke diesel engine model.The equivalent model complies with these principles: ① the main structural parameters of an equivalent model are the same as those of an OP2 S model;② the variation of the working volume remains constant; ③ the piston movement and port timing remain unchanged.In the equivalent model, the inner dead center (IDC) is the position at the minimum volume, and the outer dead center (ODC) is the position at the maximum volume.Through the equivalent model, this work studies the effects of the intake and exhaust port height stroke ratio under different methanol content on the power performance, fuel economy, scavenging performance and emission performance of the OP2S diesel engine.Our results show that there are two trade-off relationships for scavenging performance and emission performance.An increase in methanol content promotes an increase in the fuel economy and a decrease in the power performance, and promotes an increase in the scavenging efficiency and a decrease in the trapping efficiency.The matching design of the intake and exhaust ports is one of the keys to improving the performance of the OP2S diesel engine.The intake port actively promotes combustion by adding fresh air, while the exhaust port passively improves scavenging performance by reducing energy and fresh charge losses.As the port height of increases, the performance curve of the OP2S diesel engine has a double transition point characteristic.It divides the performance curve into 3 stages: low performance, high-performance, and sub-high performance.The high-performance stage is the optimal performance zone with the height to stroke ratios of intake ports of 0.056～0.122 and the height to stroke ratios of exhaust ports of 0.078～0.100.
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