【作者】

G.H.J. Van Dijk，S. Gersen

【摘要】

论文已在中国上海举行的第27届CIMAC大会上发表。论文的版权归CIMAC所有。In this paper we present a method for characterizing the knock resistance of gaseous fuels based on the physical and chemical properties of the fuel, and their effects on in-cylinder processes. As a result of the globalization of the energy market and the drive towards sustainability, natural gases with significantly different composition are being traded and distributed. Natural gases that contain substantially larger fractions of higher hydrocarbons than the traditional pipeline gas are being introduced into grids; and the introduction of hydrogen-containing gases is being discussed in many countries. In addition, fuel gases derived from local sources, such as industrial or well-head gases are becoming more popular as engine fuel. The compositions of these gases, possibly containing H2, CO, unsaturated hydrocarbons and inerts, can vary greatly as compared to pipeline natural gas. Given the impact of non-methane components on engine knock, the correct characterization of the effects of fuel composition on engine knock has reemerged as a critical issue. In addition, a correct and accepted method for characterizing knock resistance is an essential enabler for the success of the emerging market for liquefied natural gas (LNG) as a transport fuel. Rather than rely on the empirical methods using gas mixtures and ’standard’ engines traditionally employed for this purpose, we derived a method based on the combustion properties of the fuel mixtures, and have tested its predictions in our engine.Engine knock is characterized by spontaneous ignition (autoignition) of the unburned fuel mixture, the so-called end-gas, ahead of the propagating flame in the engine cylinder. Obviously, engine knock should be avoided since it can physically damage the engine and increase pollutant emissions. As a result, engine knock imposes limits to the (variation in) fuel gas composition. The core of the method described in the paper is the computation of the autoignition process during the burn period. The detailed chemical mechanism used in the simulations has been tested against experimentally determined autoignition delay times of the alkanes up to pentane (including the isomers of butane and pentane), H2, CO and CO2, measured in our Rapid Compression Machine (RCM). In addition to the effects on autoignition itself, fuel composition has other effects on in-cylinder processes that exercise a direct influence on autoignition, which have also been observed in experiments in our engine. Since autoignition of the end gas is critically sensitive to the pressure and temperature during the burn period, changes in the heat capacity of the fuel-air mixture, variations in initial pressure arising from changes in heating value and changes in the ’phasing’ of the combustion process with varying fuel composition can all affect the occurrence of autoignition during the cycle. We consider and identify the magnitudes of these effects, and their impact on autoignition and engine knock; these aspects are all incorporated in our method for characterizing knock. The predicted ’ranking’ of different gas compositions determined using the method are seen to agree very well with the measured ranking using knock-limited spark timing in our engine. The results thus show that the effect of higher hydrocarbons in engine knock is predominantly caused by the (chemical) autoignition behavior of the hydrocarbons themselves, while the impact of hydrogen is seen to arise from substantial changes in the ’phasing’ of the combustion process. These and other observations based on the method will be discussed in the paper.In addition to being valuable as a physically correct and unambiguous basis for agreeing on fuel specifications,the possibility of coupling the combustion cycle of a given engine to the determination of autoignition and the occurrence of knock will provide an excellent tool for engine manufacturers to define knock-free gas engine per

【会议名称】

第27届CIMAC会议

【会议地点】

上海

【下载次数】

1

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