Review
Natural-gas fueled spark-ignition (SI) and compression-ignition (CI) engine performance and emissions

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Abstract

Natural gas is a fossil fuel that has been used and investigated extensively for use in spark-ignition (SI) and compression-ignition (CI) engines. Compared with conventional gasoline engines, SI engines using natural gas can run at higher compression ratios, thus producing higher thermal efficiencies but also increased nitrogen oxide (NOx) emissions, while producing lower emissions of carbon dioxide (CO2), unburned hydrocarbons (HC) and carbon monoxide (CO). These engines also produce relatively less power than gasoline-fueled engines because of the convergence of one or more of three factors: a reduction in volumetric efficiency due to natural-gas injection in the intake manifold; the lower stoichiometric fuel/air ratio of natural gas compared to gasoline; and the lower equivalence ratio at which these engines may be run in order to reduce NOx emissions. High NOx emissions, especially at high loads, reduce with exhaust gas recirculation (EGR). However, EGR rates above a maximum value result in misfire and erratic engine operation. Hydrogen gas addition increases this EGR threshold significantly. In addition, hydrogen increases the flame speed of the natural gas–hydrogen mixture. Power levels can be increased with supercharging or turbocharging and intercooling. Natural gas is used to power CI engines via the dual-fuel mode, where a high-cetane fuel is injected along with the natural gas in order to provide a source of ignition for the charge. Thermal efficiency levels compared with normal diesel-fueled CI-engine operation are generally maintained with dual-fuel operation, and smoke levels are reduced significantly. At the same time, lower NOx and CO2 emissions, as well as higher HC and CO emissions compared with normal CI-engine operation at low and intermediate loads are recorded. These trends are caused by the low charge temperature and increased ignition delay, resulting in low combustion temperatures. Another factor is insufficient penetration and distribution of the pilot fuel in the charge, resulting in a lack of ignition centers. EGR admission at low and intermediate loads increases combustion temperatures, lowering unburned HC and CO emissions. Larger pilot fuel quantities at these load levels and hydrogen gas addition can also help increase combustion efficiency. Power output is lower at certain conditions than diesel-fueled engines, for reasons similar to those affecting power output of SI engines. In both cases the power output can be maintained with direct injection. Overall, natural gas can be used in both engine types; however further refinement and optimization of engines and fuel-injection systems is needed.

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Document layout

A detailed review of previous work investigating natural gas as a fuel in reciprocating piston engines, both spark-ignited (SI) and compression-ignited (CI), is presented. Previous reviews (such as reference [1]) only discussed SI engines, while this paper includes CI engines in dual-fueling. In addition, in reference [1] there is little discussion of exhaust emissions, while it is very specific about other topics, such as lean burn, spark timings and intake flow passages. We first discuss the

Natural gas in spark-ignition engines

The UK Department of Transport [38] reports that about 66% of new passenger cars sold in 2008, and the majority of personal vehicles in the USA, are fueled by gasoline, i.e. using SI engines. As a result, any alternative fuel should possess qualities that allow its use in these current engines. In other words, consumers should still be able to use their vehicles if and when a new alternative is phased in to replace gasoline. Natural gas is usually inducted or injected in the intake manifold,

Natural gas in compression-ignition engines

While SI engines are the dominant automotive powerplants [38], there is still a significant percentage of passenger vehicles in Europe that use diesel fuel, i.e. CI engine-powered (about 34% of the fleet). In addition, almost 100% of goods vehicles as well as buses use CI engines in Europe; and diesel-electric trains are extensively used in the USA. Natural gas as an alternative fuel should be usable in CI and SI engines. CI engines have inherently higher compression ratios than SI engines, and

Summary and conclusions

Natural gas is a practical fuel for SI engines, and for CI engines in the dual-fuel mode, with varying degrees of success. Natural-gas fueled SI engines can operate at higher compression ratios resulting in similar or slightly higher thermal efficiencies compared to gasoline-fueled engines. Natural gas injection or induction in the intake manifold adversely affects volumetric efficiency ηv. The 2.2% lower LHVf and 17.2% higher (F/A)st of natural gas compared to gasoline also affects power.

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