The anti-knock quality of the gasoline fuel used in spark ignition internal combustion engine can be enhanced by the addition of lead alkyls . But it causes formation and emission of toxic lead compounds . These lead alkyls can be replaced by high octane , oxygen containing organic compounds called oxygenates. The physical and chemical properties of oxygenates influences the performance and combustion products of gasoline-oxygenate blends. This paper offers a comparision between the oxygenated and leaded fuels in terms of engine performance.
The anti-knock quality of gasoline fuel used in spark ignition internal combustion engine can be enhanced by the addition of lead alkyls .But this results in the formation and emission of toxic lead compounds .a recent practice is to enhance the anti-knock property of the fuel by using certain high octane oxygen containing compounds called oxygenates The use of oxygenates to replace the lead additives in gasoline is considered now as an alternative. The most commonly used oxygenates are MTBE (methyl tertiary butyl ether ), methanol and ethanol .MTBE is manufactured from isobutene and methanol , while methanol is manufactured from natural gas or synthesized from a variety of materials such as coal, municipal wastes and biomass. Ethanol is derived from the direct fermentation of sugars, fermentation of starches and cellulose after chemical or enzymatic pretreatment or made from petroleum sources. These three oxygenates have different chemical and physical properties when compared to gasoline
These differences are expected to influence the performance and combustion products of gasoline-oxygenates blends. The study offers a comparison between the oxygenated and leaded fuel in terms of engine performance.
The experimental were conducted using a six cylinder engine. It has a swept volume of 2960cm3 . It has a bore of 88.5mm, stroke of 80.2mm, a compression ratio of 9.2 and maximum power of 132kw at 5700rpm. The engine is equipped with ke-jetronic continuous fuel injection system. The engine has an electronic ignition system with an electronic spark timing adjustment. The temperature of the cooling water and lubrication oil are controlled by two fitted heat exchangers. The engine is coupled to an eddy current dynamometer. The eddy current dynamometer is electronically controlled and water cooled. It has a maximum power of 257kw, maximum torque of 1400nm and a maximum speed of 8000rpm.
A base fuel was prepared by mixing 20% of naphtha with 80% of reformate on volumetric basis .A leaded fuel was prepared by adding tetra ethyl lead (TEL) to the base fuel. The addition of TEL brought the lead concentration in the fuel to 0.4gpb/litre. The tested oxygenates are MTBE, methanol and ethanol. Each one of this oxygenates are blended with base fuel in the ratio 10, 15, 20%. The MTBE/base blends were designated
MTBE10 (10 vol% MTBE+ 90 vol% base), MTBE15 and MTBE20. The methanol/base
blends were designated METH10, METH15, and METH20. The ethanol/base blends were designated ETH10, ETH15, and ETH20. The purity of MTBE was 98.71 wt%, and the purity of methanol was 99.99 wt%. The ethanol had a purity of 91 wt% and contains 7.8 wt% of water.
All the performance tests were carried out with spark timing being manually adjusted to the maximum brake torque (MBT) timing and the engine operating with a stoichiometric mixture. The temperature of the cooling water and the lubrication oil were controlled by two fitted heat exchangers. In all the tests the temperature of the cooling water was kept at 80+_5c. The temperature of the lubrication oil was kept at 80+_2c during the exhaust emission test and ranged from 80c to 95c during the performance test. The test room temperature was kept at 25+_2c during the exhaust emission test and ranged from 20c to 30c during the performance
3.RESULTS AND DISCUSSION
The performance was evaluated in terms of the maximum output and the brake thermal efficiency of the engine .The maximum output was measured in the wide open throttle variable speed test. In the same test the corresponding values for the brake thermal efficiency was evaluated. The brake thermal efficiency was also evaluated at a constant speed, constant load test as a function of equivalence ratio. The MBT timing values and the exhaust gas temperatures were closely examined in order to help in understanding the performance and explaining the variations between the fuels.
The engine maximum output and brake thermal efficiency were evaluated as a function of engine speed. During the test the spark timing was adjusted to maximum brake torque timing and the mixture was set to stoichiometric .The exhaust temperatures were measured near the outlet of the exhaust manifold. In general the highest exhaust temperature is observed with the base fuel and lowest with the leaded fuel. In addition the exhaust temperature decreases as the oxygenate ratio in the blend increases .These variations in exhaust temperatures can be attributed to the increase in thermal efficiency and the decrease in the combustion temperatures. The increase in thermal efficiency means that a larger portion of the combustion heat has been converted into work and therefore lower exhaust temperatures are expected. In addition the lower combustion temperatures characterizing the oxygenated blends are expected to result in lower exhaust temperatures.
MAXIMUM ENGINE OUTPUT
The engine maximum output was measured in terms of the maximum brake torque exerted by the engine at different speeds. The brake mean effective pressure (bmep) is usually used instead of the brake torque to represent the engine output. The maximum brake torque and bmep results verses engine speed for all tested fuels are shown in figures 1, 2 and 3 .The points indicate the measured values while the lines indicate the least squares polynomial fit. Consistent and persisting fluctuations in the maximum torque measurements were observed. To reduce the fluctuation error average of three readings of each test condition is recorded. A possible cause of this abnormal vibration is misalignment of the shaft connecting the engine with the dynamometer.
The base fuel produced the lowest brake torque among all the tested fuel. The leaded fuel exhibited a substantial increase in the brake torque with respect to the base fuel. This substantial increase is a result of the improved antiknock behavior due to addition of TEL, which raised the octane number from 84.7 of the base fuel to 92 for the leaded fuel. The improved antiknock behavior allowed a more advanced MBT timing that results in higher combustion pressure and thus higher exerted torque.
The results of MTBE blends indicate an increase in brake torque with respect to the base fuel .The significance of this increase varied with engine speed and MTBE ratio in the blend. At lower speeds, increasing MTBE ratio in the blend resulted in gradually slight increase in brake torque. At higher speeds, however a considerable increase in the brake torque was obtained with the blend containing 10vol% of MTBE (MTBE10), but further addition of MTBE eventually led to the decline of brake torque. The gain in brake torque obtained with MTBE blends can be attributed to the improvement in anti-knock behavior which allows more advanced MBT timing and thus higher output .As the MTBE ratio in the blend increases the variation in the instantaneous oxygen/fuel equivalence ratio due to the change of fuel oxygen content in the combustion chamber and decreasing heating value of the fuel tend to affect the combustion flame temperature which in turn offset the improvement in performance.
The results for the methanol indicate an increasingly improving brake torque with the increasing methanol ratio in the blend .The improvement in brae torque persists over the whole range of the tested engine speed. This gain in brake torque obtained with the methanol blends can be attributed to the better anti-knock behavior of these blends and the improvement in engine volumetric efficiency. The research octane
numbers for the methanol blends are significantly higher than that of base fuel. The improved anti-knock behavior allows a more advanced MBT timing and thus a higher engine output. The improvement in engine volumetric efficiency is a result of the higher latent heat of vaporization characterizing the methanol blends. Most of this latent heat is provided by the air accompanying the fuel in its way to the engine cylinder. The absorption of heat from the air cools it and make it denser .This allows more air mass to be admitted into the cylinder during the induction process and thus increasing the volumetric efficiency. In increasing the alcohol ratio in a gasoline â€œalcohol blend will increase the latent heat of vaporization of the blend and thus improve the engine volumetric efficiency. Alcohols have lower heating values when compared to gasoline, which means a lower energy release during combustion and a lower work transfer during expansion process. However the gain in brake torque due to the improvement in anti-knock behavior and volumetric efficiency seems to overweigh the loses due to lower heating.
The results for the ethanol blends shows that there is a significant improvement in brake torque with 10vol% of ethanol blend when compared to the base fuel . At low engine speeds, further increase of ethanol ratio had no effect on the brae.
torque .At high speeds ETH15 performed slightly better but further addition of ethanol in a decline in brake torque. This attributes the improvement in the in anti-knock behavior and volumetric efficiency. As the ethanol ratio in the increases the variation in the instantaneous oxygen/fuel equivalence ratio due to the change of fuel oxygen content in the combustion chamber and decreasing heating value of the fuel tends to affect the combustion flame temperature which in turn offset the improvement in performance. In general the three ethanol blends resulted in higher brake torque than the base fuel
The results for the best performing blends in terms of the maximum brake torque compared to the base and leaded fuel are shown in the fig 4. The performance of METH20 and ETH15 is comparable with that of leaded fuel and represents a gain of about 5% in the brake torque when compared to the base fuel. The best performing MTBE blend, which is the MTBE15, shows a gain of 2%.
BRAKE THERMAL EFFICIIENCY
Brake thermal efficiency is defined as the ratio between the engine power and the rate of fuel energy input. Fig 5,6 and 7 show the brake thermal efficiency results for the variable speed test at wide â€œopen throttle , stoichiometric mixture and the MBT timing. In general the brake thermal efficiency improves with increasing speed up to about 2500 rpm where it becomes maximum then starts to decline as the speed increase. Among all the tested fuels, the base fuel show a significant improvement in the brake thermal efficiency at lower speeds with respect to the base fuel.
The results for the MTBE blends indicate a significant improvement in the brake thermal efficiency. As the MTBE ratio in the blend increases, the brake thermal efficiency continues to improve achieving a maximum gain of about 12.5% with respect to the base \fuel at lower speeds.
The improvement in the brake thermal efficiency with the three MTBE blends persists over the whole range of test speed. This improvement can be attributed to the more advanced MBT timing allowed by the improved anti-knock behavior and the lower heat loses due to the lower combustion temperatures. The MTBE blends have lower
heating values than that of the base fuel and therefore their combustion temperatures are expected to be also lower.
The results for the methanol blends show a continuous improvement in brake thermal efficiency as the methanol ratio in the blend increases .The improvement in brake thermal efficiency associated with the methanol blend is also due to the more advanced MBT timing and the lower heat losses. The heating values for the methanol blends are noticeable less than that of the base fuel due to the extremely low heating value of methanol which is less than 50% of that of typical gasoline.
The result for the three ethanol blends also show an improvement in the brake thermal efficiency .a maximum gain of about 9% is achieved at midrange speeds with the ETH20 blend .The improvement in brake thermal efficiency noticed here can be also explained by the improvement in anti-knock behavior that allows a more advanced MBT timing ,and the expected lower heat losses due the lower combustion temperature .Reported an improvement in the thermal efficiency with blends containing up to 10 vol.% hydrated ethanol . This contradicts the results in the current study which indicate a noticeable in thermal efficiency in the case of 20vol. % ethanol blend.
Figure 8 shows the results of brake thermal efficiency for the best performing blends compared to the base and leaded fuels. The highest efficiency was obtained with METH20 blend followed by MTBE20 then ETH20. At low speeds, the improvement associated with the three blends comparable to that of leaded fuel. As the speed increases, however, the brake thermal efficiency continues to imprint the case the three blends while declining in the case of leaded fuel.
3b.VARIABLE EQUIVALENCE RATIO TEST
The effect of equivalence ratio on the brake thermal efficiency was evaluated at a constant speed (2000rpm), constant load (680kpa) and MBT timing. MBT timing advances as the oxygenate ratio in the blend increases .This is probably due to the improvement in anti-knock behavior of the blends due to the addition of oxygenates.
The exhaust gas temperatures peak at about stoichiometric and sharply drop with lean and rich mixtures. This indicates that the flame temperature during the combustion and the prevailing gas temperature at the end of the expansion process and higher in the case of stoichiometric than in lean or rich mixtures .For stoichiometric at the same exhaust flow rate, the heat loss is maximum. In general, the exhaust temperature for the oxygenated blends was comparable to those of the base fuel in the case of rich mixtures. The leaded fuel over the entire test range. The decreased exhaust temperature is a result of lower combustion temperature and/or improved thermal efficiency.
Figures 9,10 and 11 show the break thermal efficiency at different equivalence ratios for all the test fuels. The results indicate equivalence ratios for the test fuels. The results indicate that the break thermal efficiency is significantly influence by the equivalence ratio .The efficiency drastically deteriorates with increasing richness of the mixture. On the other hand the efficiency improves as the mixture is leaned out up to the about =0.9 after which the improvement slows down.
Among all the tested fuels, the base fuel attained the lowest brake thermal efficiency values in the whole test range. In general additional of oxygenates resulted in a noticeable improvement in the brake thermal efficiency. The results here indicate however, less significant differences between the tested fuels at part load than in the case of wide open throttle tests.
The results for the MTBE blendes shown in figure indicate continuous improvements in the efficiency as the MTBE ratio increases. This improvement is sustained over the entire tested range of equivalence ratio .The maximum game in efficiency with respect to the base fuel is observed at the rich side and is about 6%.The gain in efficiency decreases as the mixture is leaned out reaching about 3.8% at =0.8.In the case of methanol blends, the efficiency also continuous to improve as the menthol ratio increases in the blind at all equivalence ratio .However the gain in the efficiency in this case is almost constant at all equivalence ratio and is about 6%.Different from the MTBE and methanol blends, the ethanol blends at rich equilvance ratios shows a slight gain in efficiency with respect to the base fuel. Further more the slight gain in efficiency is not affected by the increase of ethanol ratio in the blend. As the mixture is leaned out, however, the gain inefficiency increases and the variation between the three ethanol blends become more noticeable. A maximum gain in efficiency of about 6% is observed for ETH20 at
Similar to the previous test, the improvement in efficiency noticed in this test can be attributed to the more advanced MBT timing allowed by the improved anti-knock behavior, and the lower heat losses due to the lower combustion temperature.
The brake thermal efficiency values as a function of equivalence ratio for the best performing blends compared to the base and leaded fuels in fig 12. Again the METH20 is the best performer at the whole test range. At the rich side the result for the MTBE blend are comparable to those of METH20. At the lean side, the METH20 outperforms all other test fuels and is approached only by the leaded fuel at rich equivalence ratios is comparable to the best performing blend, but fall behind at the lean side.
The results from the variable speed test wide throttle test show that the leaded and oxygenated fuels performed better than the base fuel in terms of brake mean efficiency. The improvement in performance persists along the entire speed range .For leaded fuel; the average increase of maximum bmep with respect to base fuel is about 4%. The reduced knock property of oxygenate fuel is significant in the engine performance over the base fuel. For lower ratios of oxygenates, the ethanol blends perform better than methanol and MTBE blends. For higher ratios, methanol is the best oxygenate in terms of maximum bmep of the engine.
The results show that the oxygenated fuels resulted in higher brake thermal efficiency than the base fuel and than leaded fuel particularly at higher engine speeds. Although less than the oxygenated fuels, thermal efficiency improves with leaded fuel but this improvement declines as engine speed increases until eventually vanishes at high speed of 3500rpm.
At mid range speed of 2500rpm, the methanol blends are the best performers followed by MTBE, then ethanol blends . Methanol blends results in maximum increase in thermal efficiency of about 12.3% with respect to the base fuel . The maximum increase in thermal efficiency is about 8.8% in the case of MTBE blends and about 7.9% in the case of ethanol blends . Overall the methanol blends are the best performers in terms of brake thermal efficiency at wide open throttle condition .
The results from the variable-equivalence ratio tests show that the difference between fuels is less significant at part load than in wide open throttle condition. With all the tested fuels the brake thermal efficiency improves significantly as the equivalent ratio is decreased.
At stoichiometric mixture, constant load of 680kPa and constant speed of 2000rpm, the best oxygenate in terms of brake thermal efficiency is ethanol for low oxygenate ratio and methanol for higher ratio . The maximum increase in thermal efficiency with respect to the base fuel is about 4.7% in the case of methanol , about 4% in case of MTBE , about 2.3% in the case of ethanol and about 3.3% in the case of leaded fuel.
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