ABSTRACT
Dimethyl carbonate (DMC) is miscible with diesel fuel and the blended fuel can significantly reduce the smoke level of the exhaust gas. In the present study, experiments were carried out on a single cylinder diesel engine and a lightduty vehicle with a 4-cylinder diesel engine, with ultra low sulphur diesel oil blended with different proportions of DMC. The %DMC affects the fuel properties, the injection properties and hence the combustion process and the emissions. An analysis of the in-cylinder pressure obtained from the single cylinder diesel engine indicates, with an increase in %DMC, there is a corresponding increase in ignition delay, peak rate of pressure rise, peak heat release rate and the proportion of fuel burned in the premixed mode. Tests carried out on the single cylinder diesel engine indicate an increase in fuel consumption at fixed engine power, while for the diesel vehicle, there is a reduction in power output at fixed fuel pump setting, due to the lower calorific value of the blended fuel. Tests carried out on the single cylinder diesel engine indicate a significant decrease in submicron particles in the exhaust with an increase in %DMC. Coupled with this, there is an increase in HC and a reduction in NOx. On the diesel vehicle, with an increase in %DMC, there is a significant reduction in smoke opacity and CO, but an increase in both HC and NOx, per unit of the measured maximum power.
Keyword: Dimethyl carbonate, Diesel engine, Pollutant emissions
1.0 INTRODUCTION
In recent years, the reduction of soot and particulate emitted by diesel engines and vehicles has been an active area of research owing to increasingly stringent emission regulations [1,2]. Some of these efforts were directed to the application of oxygenated diesel [3,4]. Amongst all oxygenates, dimethyl carbonate (DMC) has been identified as a suitable fuel blend [5,6,7]. In these investigations, all found that, with the addition of DMC, there was a significant reduction of smoke and some reduction of CO, but a slight increase in NOx. As far as hydrocarbon was concerned, there were cases of increase and cases of decrease. In all these investigations, the experiments were conducted on engine test beds and the effect of DMC on particle concentration and size distribution was not investigated. In the present study, in order to understand better the effect of the DMC blended diesel in reducing pollutant emissions, experiments were carried out on a single cylinder diesel engine on an engine test bed, and on a light-duty diesel vehicle on a chassis dynamometer. Data of gaseous emissions, smoke opacity and particle concentration and size distribution were obtained. In addition, the physical properties, injection properties and engine performance for using DMC blended diesel were also investigated and related to the change in certain emissions.
2.0 PROPERTIES OF DMC AND BLENDED FUEL
2.1 Properties of DMC
DMC is a chemical comprising of carbon, hydrogen and oxygen. It is in a liquid state under atmospheric temperature and pressure, colorless, non-toxic and non-corrosive. There are only C-H and C-O bonds in its molecular structure. DMC contains 53% by weight of oxygen. The high oxygen content helps to reduce smoke and particulate in the engine exhaust. The main chemical-physical properties of DMC and diesel fuel are compared in Table 1. DMC can be blended with diesel fuel in any proportion.
2.2 Properties of Blended fuel
The properties of DMC are significantly different from those of diesel. Hence its blending in diesel will modify the chemical-physical properties and the injection properties of the original fuel. The effect of DMC on the properties of the blended fuel is shown in Table 2. When the proportion of DMC is increased., essentially, there is increase in density and surface tension, and decrease in viscosity, cetane number, lower calorific value and the stoichiometric air fuel ratio
3.0 TEST ENGINGS AND EXPERIMENTAL METHOD
Experiments were carried out on a single cylinder diesel engine and a light duty diesel vehicle. Specifications of the single cylinder diesel engine and those of the diesel engine on the diesel vehicle are shown in Table 3 and Table 4 The single cylinder diesel engine was operated at different steady states on an engine test bed for measuring the particle size distribution and the gaseous pollutants. Moreover, in-cylinder gas pressure was measured with a piezoelectric sensor for analyzing the combustion performance.Lug-down tests were carried out on the light duty diesel vehicle. Smoke opacity and gaseous pollutants were measured, with diesel fuel containing 0%, 5%, 10%, 15% and 20%DMC. The tests were carried out on a chassis dynamometer.In these experiments, the size distribution of particulate was measured with a scanning mobility particle sizer in the range of 0.02 micron to 0.7 micron, the smoke opacity was measured with a Dieseltune Smokemeter, while the NOx, HC and CO concentrations were measured with a heated chemiluminiscent analyzer, a heated flame ionized detector and an infra-red analyzer respectively. The fuel used was ultra-low sulphur diesel conforming to the following specifications: maximum sulphur content 50 ppm by weight, minimum cetane number 51, viscosity 2.0 – 4.5 mm2/s at 40oC and maximum density 0.835 kg/liter at 15oC. It is the current legislated fuel for all diesel vehicles in Hong Kong. The DMC used was purchased from Tangshan Chaoyang Chemical General Factory in Tangshan, Heibei, China.
4.0 EXPERIMENTAL RESULTS OFTHE SINGLE CYLINDER DIESEL ENGINE
4.1 Injection Properties of Blended Fuel
The injection properties, such as droplet size and spray angle, of the blended fuel might be different from the original fuel because of changes in the physical properties. These changes will affect the combustion process and hence on the emissions. The effects of %DMC on droplet size of injection and spray angle were calculated and shown in Figure 1 and Figure 2 respectively. With an increasing amount of DMC in the blended fuel, there is a decrease in droplet size but an increase in spray angle.
4.2 Effect of Blended Fuel on Combustion Process
Figure 3 shows effect of %DMC on the gas pressure inside the cylinder of the single cylinder diesel engine. The corresponding rate of pressure rise and rate of heat release are shown in Figures 4 and 5. With an increase in %DMC, there was an increase in ignition delay, an increase in the peak value of the rate of pressure rise and an increase in the peak value of rate of heat release. Coupled with these, there was an increase in the fuel consumed in the premixed mode. Table 5 lists out these changes with %DMC for the single cylinder diesel engine operating at 1800r/min and 11kW. It can be concluded that the %DMC has significant effect on the proportion of premixed and diffusion combustion. The proportion of premixed combustion increases with the proportion of DMC in the blended fuel. It is generally accepted that soot is produced in the diffusion mode of combustion. Hence, addition of DMC in the fuel will lead to a corresponding reduction in soot formation.Fig 6 shows the fuel consumption at different blending ratios. When the % DMC was increased, the blended fuel
4.3 Effect of Blended Fuel on Particle Size Distribution
Figures 7 and 8 show the distribution of particle size measured on the single cylinder diesel engine. Figure 7 shows the case of the engine operating at 1800 r/min, unloaded. The peak particle concentration dropped from 6.59107 particles per cm3 to 2.90107 particles per cm3 with 5% DMC added, and further reduced to 1.36107 particles per cm3 with 10% DMC added. Figure 8 shows a loaded case, in which again, the particle concentration was significantly reduced with 10% DMC added. These tests demonstrated that the blended fuel could significantly reduce the submicron particles emitted by a diesel engine. Such reductions can be attributed to the reduced proportion of fuel burned in the diffusion mode.
4.4 Effect of Blended Fuel on Gaseous Emissions
Table 6 shows the HC and NOx emissions of the single cylinder diesel engine with different percentage of DMC blended. In the cases shown, HC emission increased and NOx emission reduced, when DMC was added.
5.0 EXPERIMENT RESULTS OF THE LIGHT DUTY DIESEL VEHICLE
The light duty diesel vehicle was arranged for lug-down tests on the chassis dynamometer with different %DMC The lug down test is the legislated test procedure in Hong Kong for smoke opacity test of diesel vehicles. During the lug down test, the vehicle with fixed fuel pump setting will be operated on the chassis dynamometer to find out its maximum horsepower and the corresponding engine speed. Test will then proceed with the vehicle operating at this maximum power and engine speed, subsequently at 90% of the engine speed at maximum power and finally at 80% of the engine speed at maximum power. The whole test period for each lug down is about 40 seconds. It represents the most severe operating condition the motor vehicle is facing. In our tests, the same lug down procedure as required by legislation was adopted. The gaseous pollutant concentrations were measured at the tailpipe. Appendix 1 shows the power, RPM and speed changes in one of the lug down tests. Figures 9 to12 show the measured results of smoke opacity, hydrocarbon, nitrogen oxides and carbon monoxide respectively during the lug down tests at different %DMC. The measured maximum power was 42kW with 0%DMC but dropped to 35kW with 20%DMC, which again is due to the lower calorific value of the blended fuel. Figures 9 – 12 show basically reduction in the measured values with an increase in %DMC. Such reductions are due mainly to the reduced power output. Table 7gives the average smoke and exhaust emissions per unit measured power. It indicates that with an increase in %DMC in the fuel, there was a significant decrease in smoke opacity and carbon monoxide, a significant increase in HC anda marginal increase in NOx.
6.0 CONCLUSIONS
From the present study, the following conclusions can be obtained.
1. For the DMC blended diesel fuel, when the percentage of DMC increases, the density and surface tension of thefuel increase, while the viscosity, cetane number and lower calorific value of the fuel decrease.
2. When the percentage of DMC increases, droplet size decreases and spray angle increases.
3. An analysis of the in-cylinder pressure obtained from the single cylinder diesel engine indicates, with an increase in %DMC, there is a corresponding increase in ignition delay, peak rate of pressure rise, peak heat release rate and the proportion of fuel burned in the premixed mode.
4. Tests carried out on the single cylinder diesel engine indicate an increase in fuel consumption at fixed engine power. Tests carried out on the diesel vehicle indicate a reduction in power output at fixed fuel pump setting. These are due to the lower calorific value of the blended fuel.
5. Tests carried out on the single cylinder diesel engine indicate a significant decrease in submicron particles in the exhaust with an increase in %DMC. Coupled with this, there is an increase in HC and a reduction in NOx.
6. Tests carried out on the diesel vehicle indicate a significant reduction in smoke opacity and CO, with an increase in both HC and NOx, per unit of the measured maximum power.
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Article By:
Jaswant Kuar Chunduru
Mechanical 4th year
Mechanical 4th year
Sphoorthy Engineering College
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