Compressed natural gas (CNG) is a gaseous fuel stored under high pressure in a cylindrical tank. When the CNG is injected into the intake manifold, it expands in the manifold and displaces the air which reduces the volumetric efficiency of the engine. Thus, the corresponding overall energy inside the combustion chamber is also reduced, resulting in reduced power output. CNG injection timing with respect to intake valve opening and closing can be optimized to minimize the losses in volumetric efficiency loss and hence, the power output of the engine. In this study, the combustion instability of the CNG fueled engine has been studied under low throttle condition and compared with gasoline. A comparative investigation of the effects of the start of injection (SOI) timing on combustion stability was conducted on a single-cylinder spark-ignition (SI) engine fuelled with gasoline and CNG at 25% throttle opening condition. The ignition timing was kept fixed at 35°, and 40° CA bTDC for gasoline and CNG, respectively, and the SOI timing was varied from 240° CA bTDC to 100° CA aTDC of the intake stroke. The results indicated that SOI timing significantly affects in-cylinder air-fuel mixture formation and combustion stability for gasoline and CNG engine. SOI timing corresponds to the closed valve period, shows the higher fluctuation in combustion for gasoline compared to CNG. CNG shows lower fluctuation throughout SOI timing due to homogeneous air-fuel mixture formation. Early SOI timing forms fuel stratification in the intake port and forms a rich air-fuel mixture, which reduces the net power output of the CNG engine. CNG requires optimum SOI timing for improved combustion compared to gasoline. It reduced the combustion instability and fluctuation in indicated mean effective pressure. The results will benefit the development of a closed-loop engine control system for a CNG system.

中文翻译：

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喷射正时对双燃料压缩天然气 SI 发动机燃烧和 IMEP 变化的影响

压缩天然气 (CNG) 是一种在高压下储存在圆柱形罐中的气态燃料。当压缩天然气注入进气歧管时，它在歧管中膨胀并排出空气，从而降低发动机的容积效率。因此，燃烧室内相应的总能量也降低，导致功率输出降低。可以优化与进气门打开和关闭相关的 CNG 喷射正时，以最大限度地减少容积效率损失，从而最大限度地减少发动机的功率输出。在这项研究中，CNG 燃料发动机在低油门条件下的燃烧不稳定性已经过研究，并与汽油进行了比较。在 25% 节气门开度条件下，对以汽油和 CNG 为燃料的单缸火花点火 (SI) 发动机进行了喷射开始 (SOI) 正时对燃烧稳定性影响的比较研究。汽油和压缩天然气的点火正时分别固定在 35°和 40° CA bTDC，SOI 正时从 240° CA bTDC 变化到 100° CA aTDC 进气冲程。结果表明，SOI正时显着影响汽油和CNG发动机的缸内混合气形成和燃烧稳定性。SOI 正时对应于关闭阀门的时间，表明与 CNG 相比，汽油的燃烧波动更大。由于形成均匀的空气燃料混合物，CNG 在整个 SOI 时间显示出较低的波动。过早的 SOI 正时在进气口形成燃油分层，形成浓空燃混合物，降低了 CNG 发动机的净功率输出。与汽油相比，CNG 需要最佳 SOI 正时以改善燃烧。它减少了燃烧不稳定性和指示平均有效压力的波动。结果将有利于开发用于 CNG 系统的闭环发动机控制系统。