Spark ignition natural gas engines—A review
Introduction
In recent years, air quality has become a particularly severe problem in many countries. Growing concern with exhaust emissions from internal combustion engines has resulted in the implementation of strict emission regulations in many industrial areas such as the United States and Europe. In the meantime, the Kyoto protocol calls for a reduction in greenhouse gas emissions between 2008 and 2012 to the levels that are 5.2% below 1990 levels in 38 industrialized countries. Therefore, how to reduce hazardous emissions and greenhouse gases from engines has now become a research focus. If driven according to the certifying cycle, modern spark ignition (SI) engines with three way catalyst emit very low amounts of hazardous emissions, along with large amounts of water and carbon dioxide (CO2) emissions.
CO2 is a greenhouse gas in the exhaust gases from SI engines. Improving fuel economy, using a fuel with higher hydrogen to carbon ratio (H/C) or using a renewable fuel can all reduce CO2 emissions from engines. The fuel economy of SI engines can be improved by operating the engine with diluted mixtures through extra air or exhaust gas recirculation (EGR) due to low temperature combustion, low heat transfer losses and low pumping losses at part loads. Direct injection SI engines have reduced pumping losses and heat transfer losses and, hence, have low fuel consumption. Homogenous charge compression ignition (HCCI) gasoline engines using diluted mixtures can also improve their fuel economy.
Natural gas (NG), which is primarily composed of methane, is regarded as one of the most promising alternative fuels due to its interesting chemical properties with high H/C ratio and high research octane number (about 130). When changing the fuel from Diesel to natural gas, its H/C ratio is approximately changed from 1.8 to 3.7 to 4.0. Also, natural gas has relatively wide flammability limits. The lower peak combustion temperatures under ultra lean conditions in comparison to stoichiometric conditions [1] lead to a lower knock tendency of natural gas engines, allowing a higher power for the same engine displacement by increasing the boost pressure level [2]. Accordingly, NG engines using high compression ratio, lean burn mixture or high exhaust gas recirculation would be expected to outperform gasoline engines in torque, power [3], [4] and can allow a remarkable reduction in pollutant emissions and an improvement in thermal efficiency [4]. In the meantime, natural gas engines can achieve CO2 levels below those of Diesel engines at the same air–fuel ratio, while keeping almost the same thermal efficiency under very lean conditions [5], [6]. CO2 emissions of natural gas engines can be reduced by more than 20% compared with gasoline engines at equal power [7].
In addition, particulate matter (PM) and nitrogen oxides (NOx) emissions have serious health and environmental implications when present in high enough concentrations, causing and exacerbating human respiratory illnesses such as asthma [8]. NG engines produce lower PM than Diesel engines do [9], since natural gas does not contain aromatic compounds such as benzene and contains less dissolved impurities (e.g., sulphur compounds) than petroleum fuels do [5]. A relatively low flame speed and low temperature combustion of NG engines [10] help to mitigate engine out NOx emissions when operating with high compression ratio or when the engine is supercharged [11]. Very low levels of NOx and carbon monoxide (CO) emissions can be achieved at lean equivalence ratios [12]. Moreover, engine out unburned hydrocarbons (HC) emissions can also be reduced below the corresponding levels for gasoline engines, since the gaseous state of compressed natural gas (CNG) avoids wall wetting effects on intake manifold and cylinder liner, especially at cold start conditions, which improves cold startability of CNG engines [13] and induces fuel consumption savings. The smaller percentage of HC emissions from oil film adsorption–desorption phenomena also contributes to the reduction of engine out HC emissions compared to those of gasoline base line engines [7], [14]. Low density and high dispersal rates also are advantages when safety is considered, since an explosive mixture is unlikely to form in the event of a leak [11].
Even if natural gas cannot prove itself as an intrinsically better fuel than gasoline and Diesel fuels in terms of engine emissions, natural gas vehicles that operate on CNG fuel are expected to find widespread use because the sources of natural gas are far bigger than those of oil, and natural gas will be available at a competitive cost for a long time [15]. Consequently, various research projects have been undertaken all over the world to convert light duty vehicles, passenger cars, heavy duty trucks and buses, as well as locomotive engines to use natural gas.
Section snippets
The operating envelope of lean burn engines
Low emission spark ignition CNG engines can be achieved by the lean burn engine and the stoichiometric engine with a three way catalyst. In a lean burn natural gas engine, air–fuel ratio is extremely critical. As the mixture is leaned out beyond a critical point to suppress NOx emissions, the burning rate in the lean conditions is reduced compared to that under stoichiometric conditions, which results in an increase in the overall combustion duration and, in turn, leads to increased heat
Lean burn natural gas engines
Although, the fuel economy of a lean burn natural gas engine is not as high as a Diesel engine, it is higher than that of a stoichiometric engine due to the increase of specific heat ratio [16], [17], [18], [19], [20]. The thermal efficiency of natural gas engines is dependent largely on lambda, compression ratio, burning rate and NOx emission levels. The optimum value of lambda for maximizing the trade-off between specific fuel consumption and specific NOx emissions strongly depends on the
Stoichiometric natural gas engines
Compared to SI gasoline engines, engine out NOx emissions for stoichiometric natural gas engines with early spark timing are lower due to lower combustion velocity while engine out THC emissions stay low at full loads [14]. However, the thermal efficiency of stoichiometric natural gas engines is lower than that at lean conditions. One way to get better fuel economy than pure stoichiometric operation is to use EGR.
Usually, the flame conditions during the early flame development are nearly
Problems needed to be solved
There are several major problems when using lean burn natural gas engines. First, the set point for the best compromise between emissions and fuel economy is not clear, although wide range exhaust gas oxygen sensors have recently become available. Second, even if this set point is known for a given fuel and operating condition, the optimum air–fuel ratio changes with both operating conditions and fuel properties [22]. Third, the exhaust temperatures of natural gas engines operating in lean burn
Conclusions
- 1.
Lean burn is an effective way to improve fuel efficiency and reduce NOx emissions. Lean burn limits are dependent on combustion chamber geometry, ignition timings, ignition energy and turbulence. Cycle-to-cycle variation in indicated mean effective pressure should be controlled to operate natural gas engines under lean burn conditions. To enhance the power density of natural gas engines, turbocharging technology should be used. To meet stringent emission regulations, lean burn engines need a
Acknowledgements
This work was supported by the Korea Research Foundation Grant (KRF-2005-212-102028).
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