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In this study, a homogeneous mixture of natural-gas and air was used in a compression ignition engine to reduce NOx emissions and improve thermal efficiency. In order to control ignition timing and combustion, a small amount of DME was mixed with the natural-gas. Engine performance and the exhaust characteristics were investigated experimentally. Results show the following: the engine can run over quite a large load range if a certain amount of DME is added into natural-gas. By optimizing the proportion of DME to natural-gas, NOx emissions can be lowered to near zero levels if the mixture is lean enough. Thermal efficiency is higher than that obtained with normal diesel fuel operation.

In this study, the NO emission characteristics of a dimethyl ether fueled compression ignition (CI) engine were studied, and a suitable combustion control concept was developed. A three-zone thermo-chemical model was used to understand the basic NO formation characteristics with dimethyl ether. The experimental study was carried out using a small direct-injection diesel engine. Comparison of the experimental and calculated results showed that the dimethyl ether / air mixing process was relatively slow compared with diesel fuel, which is the main reason for the relatively high NO emissions with dimethyl ether operation, in spite of its lower adiabatic flame temperature. To reduce the high temperature period, turbulence was introduced into the combustion chamber by a high-turbulence combustion system, which reduced NO emissions. It became clear that acceleration of the mixing process is an important factor for NO reduction with dimethyl ether spray combustion.

Dimethyl Ether (DME) is a promising new alternative fuel for compression ignition DI engines. However, some problems arise from the poor lubricity of DME. Breakdown of the film bearing between needle and sleeve of the injector can lead to mechanical wear and leakage, a problem that is not mitigated easily. For example, the application of returning the leakage to fuel tank could raise a back pressure on the injection needle. This pressure can affect injection rate and consequently engine performance. In this study, fuels based on various DME to gas oil (diesel fuel) ratios were investigated, in part. Physical and chemical properties of DME and gas oil are shown to lead to mutual solubility at any ratio. Blended fuels have a higher lubricity compared with pure’ DME and a better injection spray compared with pure gas oil.