LIQUEFIED PETROLEUM GAS FUEL FOR COMPRESSION IGNITION ENGINE

Abstract: A clean liquefied petroleum gas fuel usable in compression ignition engines as a fuel substituting for diesel oil such as light and heavy oil. The fuel is supplied to the combustion chamber of a compression ignition engine with a compression ratio set in the range of from 11 to 23, and comprises a liquefied petroleum gas and a radical generating agent which is not less than 0.1 % by volume of the fuel.

Description: DESCRIPTION LIQUEFIED PETROLEUM GAS FUEL FOR COMPRESSION IGNITION ENGINE TECHNICAL FIELD The present invention relates to a liquefied petroleum gas fuel capable of operating compression ignition engines presently used. BACKGROUND ART Because of their high thermal efficiency, compression ignition engines are generally used as power sources for industrial machines that require large power, or as driving sources for trucks and industrial vehicles. The compression ignition engines use diesel oil, e.g. light and heavy oil, as a fuel. On the other hand, liquefied petroleum gases consist essentially of propane and butane and suffer from a low cetane number and low ignition quality. Therefore, when a liquefied petroleum gas is used as a fuel for a compression ignition engine, it is necessary to increase the compression ratio to 26 or higher. With such a high compression ratio, however, it is impossible to obtain high thermal efficiency, which is the chief advantage of compression ignition engines , In addition, noise and vibration increase unfavorably. Accordingly, liquefied petroleum gases have heretofore been impractical fuels for compression ignition engines. Compression ignition engines that use conventional diesel oil, e.g. light and heavy oil, as a fuel cause the problem that they emit a large amount of black smoke. T. Inomata et al. "The Role of Additives as Sensitizers for the Spontaneous Ignition of Hydrocarbons" [Twenty-Third Symposium (International) on Combustion, The Combustion Institute, 1990] discloses that they studied the spontaneous ignition of normal butane mixed with small amounts of isopropyl nitrate and di-t-butyl peroxide, and found that ignition delays in gases compressed beyond 765 K (492°C) became extremely short when the peroxide or nitrate was present at less than 5% by volume of the mixture. As a result of a study of the above-described conventional technique and experimental results, the present invention provides a specific liquefied petroleum gas fuel for compression ignition engines. DISCLOSURE OF INVENTION An object of the present invention is to provide a technique whereby the liquefied petroleum gas , which is regarded as a clean fuel, is made usable as a fuel for compression ignition engines. To attain the above-described object, the present invention provides a liquefied petroleum gas fuel that is supplied to the combustion chamber of a compression ignition engine with a compression ratio set in the range of from 11 to 23, which is equal to the range of compression ratios for compression ignition engines that use diesel oil, e.g. light and heavy oil, at present. The liquefied petroleum gas fuel consists essentially of a liquefied petroleum gas and a radical generating agent (cetane number improver) which is not less than 0.1%, preferably from 0.1% to 15%, by volume of the fuel. The radical generating agent is e.g. an organic peroxide or a nitrate compound. According to the present invention, a liquefied petroleum gas fuel comprises a liquefied petroleum gas and a radical generating agent which is not less than 0.1% by volume of the fuel preferably from 0.1% to 15% by volume of the fuel. The liquefied petroleum gas fuel is supplied to the combustion chamber of a compression ignition engine with a compression ratio set in the range of from 11 to 23. The ignition delay in the combustion chamber of the compression ignition engine is not more than 3 milliseconds . The use of the liquefied petroleum gas fuel allows the power and thermal efficiency to be maintained at levels that are substantially equal to those in the case of using diesel oil such as light and heavy oil, and, at the same time, makes it possible to reduce the emission of black smoke to a considerable extent and also reduce the emission of carbon dioxide . Liquefied petroleum gases basically consist of carbon and hydrogen. Therefore, when a liquefied petroleum gas is used as a fuel for a compression ignition engine, it does not emit SO x and other air pollutants , which are unavoidably emitted during combustion of diesel oil. In addition, because there is substantially no emission of black smoke, as stated above, NO x and HC can be readily removed with a simple pollutant remover, e.g. a catalytic converter. The above and other objects, features and advantages of the present invention will become more apparent from the following description of the preferred embodiments thereof, taken in con unction with the accompanying drawings . BRIEF DESCRIPTION OF DRAWINGS Fig. 1 is a graph showing thermal efficiency. Fig. 2 is a graph showing the emission of black smoke. Fig. 3 is a graph showing the amount of carbon dioxide in exhaust gas. Fig. 4 is a graph showing the relationship between the radical generating agent concentration and the ignition delay. BEST MODE FOR CARRYING OUT THE INVENTION The present invention enables a compression ignition engine using diesel oil as a fuel and exhibiting high thermal efficiency to operate on a liquefied petroleum gas fuel which comprises a liquefied petroleum gas and a radical generating agent . The concentration of the radical generating agent is from 0.1% to 15% by volume of the fuel. The radical generating agent is at least one selected from among a nitrate, a nitrite, an organic peroxide, and an azo compound, which have heretofore been known as radical generating agents. According to the present invention, the compression ratio of the compression ignition engine is set at from 11 to 23. The reason for this is as follows. The higher the compression ratio, the smaller the required amount of radical generating agent added to the liquefied petroleum gas. However, in an engine with a high compression ratio, vibration and noise increase. In addition, the frictional resistance between the cylinder and the piston and in the valve operating system increases, and thereby reducing the power efficiency. On the other hand, if the compression ratio is excessively low, the amount of heat generated during compression becomes insufficient. Therefore, the compression ratio of the compression ignition engine is set at from 11 to 23 so that the liquefied petroleum gas fuel can be used in the range of compression ratios of compression ignition engines widely used at present. The reason why the amount of radical generating agent added is set in the range of from 0.1% to 15% by volume of the fuel is as follows . If the amount of radical generating agent added is below 0.1% when the compression ratio is within the above-described range, the ignition delay increases. That is, the time lag between the injection of the liquefied petroleum gas fuel into the combustion chamber under compression and combustion increases. This deteriorates the engine running condition. When the amount of radical generating agent added exceeds 15%, the ignition delay remains approximately constant even if the amount of radical generating agent added is increased. Moreover, in Japan, the fuel cost rises 20% or more with respect to the fuel cost when light oil is used. Thus, the liquefied petroleum gas fuel becomes unfavorable for use as a fuel substituting for diesel oil such as light and heavy oil. As an example, normal butane mixed with 3% by volume of di-t-butyl peroxide (an organic peroxide) as a radical generating agent of the mixture was used as a fuel in a compression ignition engine having a piston displacement of 1,800 cc, a compression ratio of 17, an engine speed of 1,800 rpm, a plunger diameter of 13 mm, and fuel injection timing set at 26 degrees before the top dead center, and the thermal efficiency, the emission of black smoke and the emission of carbon dioxide were measured. As a comparative example, light oil was used as a fuel, and the thermal efficiency, the emission of black smoke and the emission of carbon dioxide were measured. The results of the measurement are shown in Figs . 1 to 3. Fig. 1 is a graph showing the thermal efficiency. It will be understood from Fig. 1 that the use of the liquefied petroleum gas mixed with the radical generating agent exhibits thermal efficiency that is approximately equal to that in the case of light oil. Fig. 2 is a graph showing the emission of black smoke. As will be clear from Fig. 2, when light oil was used as a fuel, black smoke was emitted over the entire range of brake mean effective pressures, and the emission of black smoke increased rapidly when the brake mean effective pressure exceeded 0.5 MPa. On the other hand, when the liquefied petroleum gas mixed with the radical generating agent was used as a fuel, substantially no black smoke was detected over the entire brake means effective pressure range. Fig. 3 is a graph showing the amount of carbon dioxide in the exhaust gas. It will be understood from Fig. 3 that the use of the liquefied petroleum gas mixed with the radical generating agent allows the emission of carbon dioxide to reduce about 13% with respect to that in the case of light oil. Although in the above-described example normal butane is used as a fuel substituting for diesel oil such as light and heavy oil, it should be noted that the fuel substitute may be a mixed gas of liquefied butane and liquefied propane, or liquefied propane. In the case of a mixed gas of liquefied butane and liquefied propane, it is desirable for the mixed gas to be a liquefied gas consisting mainly of butane, which contains 50% or more of liquefied butane, from the viewpoint of the cetane number relationship. Examples of substances usable as a radical generating agent added to the liquefied petroleum gas include, in addition to the above-described di-t-butyl peroxide, organic peroxides, such as methyl isobutyl ketone peroxide, tris-t- butyl peroxy triazine, 2, 5-dimethyl-2, 5-di-t-butyl peroxy hexane, 1 , 1-di-t-butyl peroxy cyclohexane, and 2, 2-di-butyl peroxy butane, nitrates such as isooctyl nitrate, isoamyl nitrate, normal amyl nitrate, and isopropyl nitrate, nitrites such as normal propyl nitrite, and normal butyl nitrite, and azo compounds. Fig. 4 is a graph showing changes in the ignition delay (milliseconds; plotted along the ordinate axis) when a liquefied petroleum gas (butane) mixed with a radical generating agent (di-t-butyl peroxide) was used as a fuel for a compression ignition engine with a compression ratio set at a certain value (at 17) in the range of from 11 to 23 and the concentration of the radical generating agent (% by volume of the fuel; plotted along the abscissa axis) was changed. The ignition delay in compression ignition engines is preferably not longer than 3 milliseconds under normal operating conditions . In the graph of Fig. 4, therefore, the lower limit of the radical generating agent concentration is shown by L . The upper-limit is represented by the concentration H, beyond which the ignition delay becomes approximately constant independently of the radical generating agent concentration. In the graph of Fig. 4, the compression ratio is smaller than 23, and hence L is greater than 0.1% by volume. The lower-limit concentration L 0 when the compression ratio is 23 is determined to be 0.1% by volume by correction based on experimental values. The upper-limit concentration H 0 when the compression ratio is 11 is 15% by volume which is considered not to relate directly to the compression ratio. According to the present invention, a liquefied petroleum gas fuel formed by mixing a liquefied petroleum gas with from 0.1% to 15% by volume of a radical generating agent is supplied to the combustion chamber of a compression ignition engine with a compression ratio set in the range of from 11 to 23, thereby enabling the power and thermal efficiency to be maintained at levels that are substantially equal to those in the case of using diesel oil such as light and heavy oil and, at the same time, making it possible to reduce the emission of black smoke to a considerable extent and also reduce the emission of carbon dioxide. Furthermore, when a liquefied petroleum gas is used as a fuel for a compression ignition engine, because the liquefied petroleum gas basically consists of carbon and hydrogen, it does not emit S0 X and other air pollutants, which are unavoidably emitted during combustion of diesel oil. In addition, because there is substantially no emission of black smoke, as stated above, NO x and HC can be readily removed with a simple pollutant remover, e.g. a catalytic converter. For the reasons stated above, when the fuel according to the present invention is applied to a compression ignition engine, environmental pollution is favorably reduced. At the same time, it is possible to maintain the power and thermal efficiency at high levels that are substantially equal to those in the case of conventional diesel fuel. It should be noted that the present invention is not necessarily limited to the above-described embodiment, and that various changes and modifications may be imparted thereto without departing from the gist of the present invention.

Claims: CLAIMS 1. A liquefied petroleum gas fuel supplied to a combustion chamber of a compression ignition engine with a compression ratio set in a range of from 11 to 23, said liquefied petroleum gas fuel comprising a liquefied petroleum gas and a radical generating agent which is not less than 0.1% by volume of the fuel. 2. A liquefied petroleum gas fuel according to claim 1 , wherein the volume of said radical generating agent is from 0.1% to 15% of the fuel. 3. A liquefied petroleum gas fuel according to claim 1 , wherein said radical generating agent is one selected from the group consisting of a nitrate, a nitrite, an organic peroxide, and an azo compound. 4. A liquefied petroleum gas fuel supplied to a combustion chamber of a compression ignition engine with a compression ratio set in a range of from 11 to 23, said liquefied petroleum gas fuel comprising a liquefied petroleum gas consisting essentially of propane and butane, and a radical generating agent which is from 0.1% to 15% by volume of the fuel, said radical generating agent being selected from the group consisting of a nitrate, a nitrite, an organic peroxide, and an azo compound, wherein an ignition delay in the combustion chamber of said compression ignition engine is not more than 3 milliseconds .

Inventors: GOTO SHINICHI, MAMEDA TAKESHI, TAMURA MASAMITSU, WAKAO YOSHITAKA, GOTO, SHINICHI, MAMEDA, TAKESHI, TAMURA, MASAMITSU, WAKAO, YOSHITAKA
Applicants: GOTO SHINICHI, IWATANI INT CORP, MAMEDA TAKESHI, TAMURA MASAMITSU, WAKAO YOSHITAKA, GOTO, SHINICHI, IWATANI INTERNATIONAL CORPORATION, MAMEDA, TAKESHI, TAMURA, MASAMITSU, WAKAO, YOSHITAKA
Application year: 1999
Main classification: C10L3
IPC codes: C10L3