Analysis of energy release and NOx emissions of a CI engine fuelled on diesel and DME

https://doi.org/10.1016/j.applthermaleng.2006.11.016Get rights and content

Abstract

Combustion and NOx emissions from a dimethyl ether (DME) fuelled compression ignition engine were investigated. The test engine used consisted of an unmodified two-cylinder direct injection four-stroke air cooled type. The injection timing and injector opening pressure were left unaltered from their diesel fuelling settings. Analysis of the fuel line pressure shows that due to the compressibility of DME the rate of pressure rise was lower, resulting in injection occurring later when compared to diesel injection. The maximum combustion chamber pressure was found to be higher in the case of diesel fuelling. In terms of energy release it was found that with DME this occurs later than in the case of diesel fuelling with the larger proportion occurring just after top dead centre. A comparison of NOx emissions revealed that, at all loads tested, these were higher in the case of DME fuelling and decreased steadily with increasing speed. At the higher speeds however, the levels of NOx monitored were noted to be less than those of their diesel counterpart. Some of the factors influencing the promotion of NOx emissions with DME fuelling are discussed and analysed.

Introduction

When an alternative fuel is used in a compression ignition engine, invariably modifications are required to ensure proper functioning. The ideal situation would be to ensure ability to switch from one fuel to the other without having to make changes to any of the settings.

One of the primary requirements for a fuel to be considered for use in compression ignition engines is that it must possess a cetane number sufficiently high for self-ignition, as well as lubricating qualities. The alternative fuel must also produce emission levels that are below those of diesel fuel when used in comparable situations. One such fuel is dimethyl ether (DME). This meets most of the above requirements, except that of lubricity.

DME was initially used as an ignition promoter for methanol [1], [2] and later for ethanol in compression ignition engines [3]. In both instances when using the alcohol as the main fuel it was shown that reduced emissions with no smoke were attained. It has also been shown that DME can be used as a single fuel in an unmodified compression ignition engine [4], [5], [6]. The power produced by the engine was similar to that achieved by diesel fuelling, with the added advantage of a better overall efficiency. Generally reduced emissions were reported and no smoke was visible. One of the reasons for no smoke being present when DME was used either as an ignition promoter or as a fuel is due to the presence of oxygen in the fuel. Furthermore, the lack of carbon–carbon bonds in the molecule and the low boiling point of DME also contribute to smokeless emissions.

The main thrust of the present work is particularly to take a closer look at the NOx emissions from DME fuelling of a compression ignition engine under varying conditions. Initially tests using DME as a fuel were performed at constant speed with torque increasing [4]. The injector opening pressure was reduced from the recommended 21 MPa for diesel fuelling to 10 MPa. Briefly, the results obtained showed that the maximum power attained with DME and diesel fuelling were quite similar. In terms of emissions, considerable improvements were observed with regard to THC and NOx. THC emissions showed a very slow increase with load but remained some four times less than those of diesel fuelling, throughout the load range. NOx emissions were found to be similar in magnitude with both fuels at low load. However, as the load and therefore, the DME fuel flow rate was increased, the concentrations of NOx increased only slightly, while those achieved by diesel fuelling showed a steady increase throughout the load range. At maximum load NOx emissions with DME were some five times less than those attained with diesel fuelling. Concentrations of CO were similar with both fuelling methods up to mid load, after which those of DME surpassed their diesel counterparts.

The results reported in the initial tests above were performed at constant speed and with a reduced injector pressure, as mentioned. The next stage was to run the engine on DME at the same conditions and settings used for diesel fuelling. This was the basis of the work reported in [5]. In this research, the injector opening pressure was therefore increased in stages. For each increase the engine was run for a period of time and all relevant measurements were taken and analysed. This was repeated until the injector opening pressure was the same as that recommended for diesel fuelling, namely 21 MPa. The engine was found to perform very well producing similar brake power to that achieved with diesel fuel. The advantage was that the thermal efficiency was found to be equal to, and in certain conditions, better than that attained with diesel fuelling. These tests were performed at constant torque with the engine speed ranging from 1100 to 1900 rpm. These results indicated that it was possible to fuel the engine on either fuel, without alteration of the settings on the engine being necessary.

Section snippets

Objectives

The present work is an extension of the research reported above, where the effects of changing engine conditions on some combustion characteristics and NOx emissions are analysed. These include injection line pressure, combustion chamber pressure and cumulative energy release. Thus tests were performed on diesel and then on DME fuelling at four loading conditions with increasing speed. The tests were performed with all fuel injection settings maintained as recommended for diesel fuelling.

Nitrous oxide emissions

Emissions from compression ignition engines comprise of a number of species. Some of them are not considered as serious, for example unburned hydrocarbons and CO, except at light loads. Soot and nitrous oxides are viewed as more harmful emissions. However, the concern in the present work is with NOx emissions only, thus only this specie will be discussed.

NOx is formed in the combustion chamber as a result of the reaction of atomic oxygen and nitrogen. The reactions forming NOx are very

Equipment and instrumentation

Engine: The engine used for the experimental work consisted of a naturally aspirated, four-stroke, direct injection, air-cooled of the compression ignition type. The engine was instrumented to measure combustion chamber pressure and fuel line pressure. An optical crank angle marker, which included the top dead centre marker, was coupled to the front shaft of the engine. The engine specifications are given in Table 1.

Fuel system: The standard fuel system of the engine consisted of a fuel pump

Energy release analysis

The energy release model developed here was based on the first law of thermodynamics. This model also uses some of the assumptions as proposed in [15]. It was assumed that at each instant:

  • (i)

    the entire cylinder contains a homogeneous mixture of air and combustion products,

  • (ii)

    temperature gradients, pressure waves, non-equilibrium mixture, fuel vaporization and mixing were not considered,

  • (iii)

    burning takes place incrementally as a homogeneous combustion, thus acting as a uniform heat source.

The first law of

Test results and discussion

The discussion comprises two sections. The first one relates to aspects that ultimately affect the combustion process as well as an analysis of some combustion characteristics. This is followed by a discussion of NOx emissions. Before delving into the analysis of the results, it is important to again emphasize the basis upon which these tests were performed. Initially tests were performed on diesel fuelling at constant torque while the speed was increased from a minimum of 1100 to 1800 rpm. The

Conclusions

The aim of this work was to study the effect of using the same injection pressure for both diesel and DME fuel on some combustion aspects and on NOx in particular. A number of differences were observed in the injector line pressure, the combustion chamber pressure and the cumulative energy release of the two fuels. With regard to emissions, it was found that specific concentrations of NOx with DME fuelling were higher than those of diesel fuelling at lower speeds and decreased to similar levels

Acknowledgements

The author wishes to express his thanks to the University of the Witwatersrand and to the National Research Foundation for their financial assistance.

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