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A Review of Ultra-lean and Stratified Charged Combustion in Natural Gas Spark Ignition Engines

Journal Article
ISSN: 1946-3936, e-ISSN: 1946-3944
Published March 15, 2023 by SAE International in United States
A Review of Ultra-lean and Stratified Charged Combustion in Natural
                    Gas Spark Ignition Engines
Citation: Ziyaei, S., Mazlan, S., and Lappas, P., "A Review of Ultra-lean and Stratified Charged Combustion in Natural Gas Spark Ignition Engines," SAE Int. J. Engines 16(7):2023,
Language: English


Natural gas (NG) can be compressed to a high pressure of around 200 bar for use in engines and other applications. Compressed natural gas (CNG) contains 87–92% methane (CH4) and has a low carbon-to-hydrogen ratio compared to other hydrocarbon (HC) fuels. Due to this, it can potentially reduce carbon dioxide (CO2) emissions by more than 20% compared to conventional fuels like diesel or gasoline. This makes CNG one of the most environmentally friendly fuels for internal combustion engines (ICEs). To improve the thermal efficiency of ICEs, higher compression ratios (CRs) and leaner combustion are essential. Since CNG is a gaseous fuel, it has several advantages over liquid fuels due to its favorable physical and chemical properties. A few of these advantages are minimal fuel evaporation issues, a low-carbon content in the fuel composition and a high-octane number. The CNG high-octane number allows for a high CR, resulting in higher thermal efficiency and lower emissions.
It should be noted that gaseous fuels, while offering some advantages, also present some disadvantages, such as a reduction in the volumetric efficiency of engines. During the fueling process, when gaseous fuel is introduced to the cylinder through the intake manifold (e.g., through port fuel injection [PFI]), intake air is displaced by the fuel, which reduces the volumetric efficiency of the engine. Through direct injection (DI) technology, spark ignition (SI) engines can achieve greater volumetric efficiency by introducing fuel directly into a combustion chamber. Furthermore, DI fueling reduces the need for throttling to control the engine output power in ultra-lean conditions in addition to stratified charges, which results in improved fuel consumption. A reduction in throttling during engine operation will result in a reduction in pumping losses. During the design and optimization process of an SI engine utilizing DI technology with CNG fuel, the spray formation process, the ignition probability, and the combustion propagation of CNG-DI need to be studied.
An in-depth review of CNG fueling strategies for SI engines is presented with a focus on ultra-lean combustion. In this context, the problems associated with ultra-lean combustion of CNG, and their possible solutions will be discussed. This article will be followed by a review on lean combustion of CNG in ICEs using turbulent jet ignition (TJI) as a potential method to solve the problem of lean-burn combustion of CNG with high-energy ignition systems, including TJI.