Emissions of automobiles fueled with alternative fuels based on engine technology: A review

Diversification of alternative fuels for automobiles is not only an actual situation, but also a development trend. Whether the alternative fuels are clean is an important issue. Emissions of automobiles fueled with natural gas (NG), methanol, ethanol, biodiesel, dimethyl ether (DME) and polyoxymethylene dimethyl ethers (PODE) are investigated and reviewed based on engine technology and fuel properties. Compared to gasoline, NG/gasoline bi-fuel and NG automobiles have higher brake thermal efficiencies (BTE) and produce less HC, CO and PM emissions, while more NO emission. Compared to diesel, NG/diesel dual fuel automobiles have lower BTE and emit lower soot and NO emissions, but higher HC and CO emissions. Methanol and ethanol blending in gasoline can obviously reduce the HC, CO and PM emissions of spark ignition (SI) automobiles. Methanol or ethanol blending in diesel may prolong the ignition delay, shorten the combustion duration and improve the BTE, resulting in lower soot emissions. However, the HC, CO and NO emissions of methanol or ethanol diesel blend fuels are uncertain due to low cetane number, high latent heat of vaporization. Most biodiesel has higher viscosity, distillation temperature, cetane number and oxygen content than diesel. Soot emission of biodiesel is lower than that of diesel, while NO emission is higher. Both DME and PODE do not contain CC bonds and their blend with diesel can prohibit the formation of soot. PODE has high cetane number and low viscosity, resulting in better ignitability and spray quality respectively. PODE blending shortens both the ignition delay and the combustion duration, improves the BTE, and increases the temperature in the diffusion combustion phase, leading to a higher NO emission.