Study on Formaldehyde Emission in a DME-Fueled Direct-Injection Diesel Engine

Although dimethyl ether (DME) is known as a clean fuel, formaldehyde (HCHO) as a hazardous air pollutant and a product of incomplete combustion emitted from DME engines should be investigated. This paper deals with combustion and exhaust gas emission characteristics in a small direct injection diesel engine fueled with neat dimethyl ether(DME). In the quantitative analysis of DME combustion and its exhaust emissions, both experimental and numerical studies were carried out to see clearly the origin and mechanism of formaldehyde emissions. In the experimental work the sample acquisition system and analysis method were constructed. Fourier Transform Infrared spectroscopy (FTIR) method was used to quantitatively investigate the characteristics of formaldehyde emissions from the tested DME engine and compared with those from diesel fuel. The comparison results show that DME fueled engine gives a higher formaldehyde emission than fueled with diesel fuel under working conditions of medium and high loads only at the engine speed of 1200r/min, and the effects of engine speed and load on formaldehyde are obvious for the test conditions. The effects of operating parameters of DME fueled test engine have been clarified on HCHO emissions from the experiments. Experimental results indicated that the concentration of HCHO emission is increased in HCCI engine combustion under no-load condition.

In order to have an insight into HCHO formation, theoretical investigations concerning DME/Air mixture combustion were set forth with reference to HCHO formation mechanism and in-cylinder phenomena. Influence of turbulence on combustion was taken into account by using KIVA code coupled with detailed chemical kinetic models of pollutants formation. Based on the analyses of the variations in temperature, in-cylinder air motion and species concentration correspondent to the crank angle, the calculated results show that HCHO is relative stable in low and medium temperature conditions, and then its oxidation process is accelerated with increased in-cylinder gas temperature. A significant source of HCHO emission originates from local poor oxygen caused by DME combustion, lower gas temperature due to expansionary cooling of the power stroke and thermal nonuniformity in the burned gas as well as head-wall-quench-layer close to piston crevice owing to the existence of heat loss and cooled wall.

The validation experiments were undertaken in this paper and good agreement existed between the calculated results and experimental data.