chapter5-diesel-engine-combustion-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,*,*,*,Combustion in compression ignition engines,Near the end of the compression stroke,liquid fuel is injected as one or more jets.,The injector receives fuel at very high pressures in order to produce rapid injection,with high velocity jet(s)of small cross-sectional area;in all but the largest engines,there is a single injector.,The fuel jets entrain air and break up into droplets;this provides rapid mixing which is essential if the combustion is to occur sufficiently fast.Sometimes the fuel jet is designed to impinge on to the combustion chamber wall;this can help to vaporise the fuel and break up the jet.There will be large,variations in fuel/air mixtures on both a large and small scale within the combustion chamber.,Figure 3.1 1 shows the pressure diagram for a compression ignition engine,(i),Ignition delay,AB.After injection there is initially no apparent deviation from the unfired cycle.During this period the fuel is breaking up into droplets being vaporised,and mixing with air.Chemical reactions will be starting,albeit slowly.,Combustion chamber,1.,Direct injection(DI)systems,Some typical direct injection combustion chambers given by Howarth(1 966).,Despite the variety of shapes,，,all the combustion chambers are claimed to give equally good performance in terms of fuel economy,，,power and emissions,，,when properly developed,The most important air motion in direct injection diesel engines is swirl,，,the ordered rotation of air about the cylinder axis,(a)hemispherical combustion chamber,(b)shallow bowl combustionchamber,(d)deep toroidal bowl combustion chamber(d/h 2),The methods by which the swirl is measured,，,how the swirl ratio is defined,，,and how the averaging is undertaken all vary widely,Heywood(1988)summarises some of the approaches,，,but a more comprehensive discussion is provided by Stone and,Ladommatos(1992b),For steadyflow tests,，,the angular velocity of the swirl will vary across the bore,，,but an averaged value is given directly by paddle wheel(vane)anemometer,（叶片风速仪）,Indirect injection(IDl)systems,Indirect injection systems,have a divided combustion chamber,with some form of pre-chamber in which the fuel is injected,and a main chamber with the piston and valves.The purpose of a divided combustion chamber is to speed up the combustion process,in order to increase the engine output by increasing engine speed.,By far the most successful combustion chamber for this size range is the Ricardo Comet combustion chamber.The Comet combustion chamber dates back to the 1930s and the Mk V version is shown in,figure 5.7.The volume of the pre-chamber is about half the total clearance volume.The two depressions in the piston induce two vortices of opposing rotation in the gas ejected from the pre-chamber.The insert or hot plug has to be made from a heat-resistant material,since temperatures in the throat can rise to 700,C.Heat transfer from the hot plug to the cylinder head is reduced by minimizing the contact area.The temperature of the plug should be sufficient to maintain combustion,otherwise products of partial combustion such as aldehydes would lead to odour in the exhaust.,A final type of divided combustion chamber is the air cell,of which the MAN air cell is an example,see figure 5.9.Fuel is injected into the main chamber and ignites.As combustion proceeds,fuel and air will be forced into the secondary chamber or air cell,so producing turbulence.As the expansion stroke continues the air,fuel and combustion products will flow out of the air cell,so generating further turbulence.In comparison with swirl chambers,starting will be easier since the spray is directed into the main chamber.As the combustion-generated turbulence and.swirl will be less,the speed range and performance will be more restricted than in swirl chambers;the air cell is not in common use.,