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