chapter4-fuel-and-combustion-chemical-发动机原理英文.ppt

单击此处编辑母版标题样式,单击此处编辑母版文本样式,第二级,第三级,第四级,第五级,*,*,*,Fuels and additives,1.Characteristics of petrol,The two most important characteristics of petrol are its,volatility,and,octane number,(its resistance to self-ignition).,Volatility,is expressed in terms of the volume percentage that is distilled at or below fixed temperatures.If a petrol is too volatile,when it is used at high ambient temperatures the petrol is liable to vaporise in the fuel lines of carburetted engines and form,vapour locks,This problem is most pronounced in vehicles that are being restarted,since under these conditions the engine compartment is hottest.If the fuel is not sufficiently volatile the engine will be difficult to start,especially at low ambient temperatures.The volatility also influences the cold start fuel economy.,Spark ignition engines are started on very rich mixtures,and continue to operate on rich mixtures until they reach their normal operating temperature;this is to ensure adequate vaporisation of fuel.Increasing the volatility of the petrol at low temperatures will evidently improve the fuel economy during and after starting.,The,octane number,of a fuel is a measure of its anti-knock performance.A scale of 0-100 is devised by assigning a value of 0 to n-heptane(,庚烷,)(a fuel prone to knock),and a value of 100 to iso-octane(,异辛烷,).A 95 octane fuel has the performance equivalent to that of a mixture of,95 per cent iso-octane and 5 per cent n-heptane,by volume,.The octane requirement of an engine varies with compression ratio,geometrical and,mechanical considerations,and also its operating conditions.There are two commonly used octane scales,research octane number(RON),and,motor octane number(MON),The tests for determining octane number are performed using the ASTM-CFR(Cooperative Fuel Research)engine;this is a variable compression ratio engine similar to the Ricardo E6 engine.In a test the compression ratio of the engine is varied to obtain standard knock intensity.With the same compression ratio two reference fuel blends are found whose knock intensities bracket(归类)that of the sample.The octane rating of the sample can then be found by interpolation（插值）.,Table 3.4 shows that the motor octane number has more severe test conditions since the mixture temperature is greater and the ignition occurs earlier.There is not necessarily any correlation between MON and RON as the way fuel components of different volatility contribute to the octane rating will vary.Furthermore,when an engine has a transient increase in load,excess fuel is supplied.Under these conditions it is the octane rating of the more volatile components that determines whether or not knock occurs.,2.Characteristics of diesel fuel,The most important characteristic of diesel fuel is the,cetane number,as this indicates how readily the fuel self-ignites.,Viscosity,is also important,especially for the lower-grade fuels used in the larger engines;sometimes it is necessary to have,heated fuel lines.Another problem with diesel fuels is that,at low temperatures,the high molecular mass components can precipitate to form a waxy deposit.This is defined in terms of the,cold filter plugging point.,（冷滤塞点）,The,flashpoint,is the temperature to which the liquid has to be heated for the vapour to form a combustible mixture with air at atmospheric pressure.Since the flashpoint of diesel fuel is at least 55C,this makes it a safer fuel to store than either petrol or kerosene.The flashpoints of petrol and kerosene are about-40,C and 30,C respectively.,