Increasing urbanization and industrialization have led to a phenomenal growth in transportation demand worldwide, coupled with a concentration of vehicles in metropolitan cities. With regard to increasingly stringent emission legislation natural gas is gaining interest as a transportation fuel with worldwide over 19 million natural gas vehicles in operation. This paper presents the worldwide background, prospects and challenges of natural gas fuel and natural gas fueled vehicles along with environmental and economic aspects of compressed natural gas as a transformation fuel. Technical aspects of compressed natural gas properties, storage, safety problems and its effect on engine performance, efficiency, emissions and barriers to natural gas vehicles adaptation are discussed in detail. The main indicators selected for the comparative assessment of natural gas as vehicular fuel are: economic, emission performance and safety aspect. The results showed that CNG has several advantages over both diesel and gasoline fuel, including considerable emission and cost reductions.

Technical overview of compressed natural gas (CNG) as a transportation fuel

Lean burn natural gas fuelled spark-ignition engines are particularly attractive regarding environmental performance. Nevertheless, few data exist concerning lean combustion in gas engines in terms of ignition delay, combustion duration and combustion rate. Such data are necessary when using thermodynamic models to predict energy and environmental performance. The present study proposes a contribution to such combustion characterization as a function of spark timing, air excess and engine load. This work is based on experimental cylinder pressure measurements in an open chamber engine on the one hand, and on a one-zone thermodynamic model used as a heat release analysis tool on the other hand. Results obtained, such as mass fraction burned, ignition delay and combustion duration, allow identification of the parameters used in the Wiebe function, a semi-empirical law that is frequently used to describe the combustion in predictive models.

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Combustion characterization of natural gas in a lean burn spark-ignition engine

The effect of fuel injection timing relative to ignition timing on natural gas direct-injection combustion was studied by using a rapid compression machine (RCM). The ignition timing was fixed at 80 ms after the compression start. When the injection timing was relatively early (injection start at 60 ms), the heat release pattern showed a slower burn in the initial stage and a faster burn in the late stage, which is similar to that of flame propagation of a premixed gas. In contrast to this, when the injection timing was relatively late (injection start at 75 ms), the heat release rate showed a faster burn in the initial stage and a slower burn in the late stage, which is similar to that of diesel combustion. The shortest duration was realised at the injection end timing of 80 ms (the same timing as the ignition timing) over a wide range of equivalence ratio. The degree of charge stratification and the intensity of turbulence generated by the fuel jet are considered to cause this behavior. Early injection leads to longer duration of the initial combustion, whereas late injection leads to a longer duration of the late combustion. Early injection showed relatively lower CO concentration in the combustion products while late injection gave relatively lower NO x . It was suggested that early injection leads to combustion with weaker stratification, and late injection leads to combustion with stronger stratification. Combustion efficiency was kept at a high value over a wider range of equivalence ratio.

Effect of Fuel Injection Timing Relative to Ignition Timing on the Natural-Gas Direct-Injection Combustion

Performance and Emissions of a Natural Gas-Fueled 16 Valve DOHC Four-Cylinder Engine

The characteristics of natural-gas direct-injection combustion under various fuel injection timings were studied by using a rapid compression machine. Results show that natural-gas direct injection can result in combustion that is much faster than homogeneous combustion while shortening the time interval between injection timing and ignition timing can markedly decrease the combustion duration. Unburned hydrocarbon would increase over a wide range of equivalence ratios, shortening the time interval between injection timing and ignition timing can decrease the value to that of homogeneous-mixture combustion. The NOx level is high but the CO level is low over a wide range of equivalence ratios and is little affected by fuel injection timing. High values of pressure rise due to combustion can be realized and it is insensitive to the variation in fuel injection timing. High combustion efficiency can be achieved, which is also independent of injection timing.

Combustion characteristics of natural-gas direct-injection combustion under various fuel injection timings:

A series of tests was conducted on a Toyota, four-cylinder, spark ignition engine which was modified to run on either gasoline or natural gas. The aim of the experiment was to investigate the performance and combustion behavior of natural gas, with particular emphasis on its low burning velocity. A pressure transducer installed in the cylinder head was used to obtain pressure versus crank angle curves from which mass burn rates and burning velocities were calculated, using a heat release analysis program. Results indicate that the low laminar burning velocity of natural gas extends its ignition delay period (time to 1 percent burned) by up to 100 percent compared with gasoline. This contrasts with the remainder of the combustion period which is dominated by turbulence effects that produce very similar burning velocities for the two fuels.

A. L. Jones

Papers

Comparison of Burning Rates in a Natural-Gas-Fueled Spark Ignition Engine