Studies on the Application of LP gas for Farm Tractor Drive (IV):On Suitable Value of Compression Ratio Necessary to the Improvement of Engine Performance
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In applying Liquefied Petroleum gas for small size watercooled kerosene farm engine, we should let many advantagious points which LP gas has in it work fully. Paying special attention to the greatest merit of LP gas that anti-knock is high, we changed an actual cylinder-head to three new ones, in order to rise thermal efficiency; compression ratio 5.2, 5.7, 6.2. We measured fuel consumption, air-fuel ratio, temperature of every part of engine and noise etc, thus we compared with engine performances and considerd economical relation of both fuels; i. e., LP gas and kerosene:(i) Relation between compression ratio and ignition timing: The following fact must be observed, that, under the same compression ratio, best ignition timing of LP gas became earlier about 5 degrees in crank angle than that of kerosene. As to the LP gas only, the higher the compression ratio rises, the slower the best ignition timing becomes. (Table. 3)(ii) Air-fuel ratio and Excess-air ratio: In case of kerosene, air-fuel ratio was about 17, excess-air ratio was about 1.2. As to LP gas, however, it was respectively, 22 and 1.4. It seems that the differences of volatility, maximum flame velocity of both fuels, and the burning characteristics of those give influence to those values. (Figure. 6. diagram above)(iii) Fuel consumption, Specific fuel consumption and Fuel consumption falling rate: Being based upon Figure. 6: fuel consumption falling rate, namely, falling rate of specific fuel consumption of LP gas to that of kerosene, showed a drop from 14 per cent to 18 per cent in compression ratio 5.2, 17 to 25 in 5.7, 23 to 30 in 6. 2. (Figure. 7). The reason must be that, in case of LP gas, the use of rarefied mixture-ratio is possible.(iv) Operation cost: On the actual stage, though specific fuel consumption of LP gas is better than keroseen's, from 20 per cent to 30 per cent, its operating cost became about 35 per cent more expensive than kerosene's, because the net price of LP gas is about two times more expensive than that of kerosene. (Figure. 9)(v) Temperature of every parts of engine: Combustion chamber temperature became higher by LP gas than by kerosene. As to LP gas only, as compression ratio rises, it became higher. Generally, exhaust-gas temperature used to fall down with rising of compression ratio. (Figure. 11). Concerning the relation between exhaust-gas temperature and ignition timing, as ignition timing is advanced, its temperature dropped down. (Figure. 12)(vi) Engine noise: There was not a remarkable difference between kerosene and LP gas, and seems not to be so harmful to the human being.As above mentioned, judging from such a method of rising of compression ratio taken in this experiment, if specific fuel consumption is 160g/PS-h, and thermal efficiency is above 35 per cent, too, LP gas will be more economic benefit than kerosene. So this shows that compression ratio must be above 8. It must be our concern of future research that taking into consideration a form of combustion chamber fitting to LP gas, we should try to rise compression ratio, and according to rising of compression ratio, we must try to rise charging efficiency so that mechanical efficiency may not drop down.
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