21-08-2014, 10:45 AM
In diesel engines air is drawn into the cylinder during the suction stroke and compressed to a very high pressure, thus raising the air temperature to a value required to ignite the fuel injected into the cylinder. The usual compression ratios for diesel engines vary from 12:1 to 20:1, the corresponding pressures and temperatures at the end of compression stroke being 28 bar to 70 bar and 520 ºC to 720 ºC, respectively.
Fuel is injected into the cylinder at the end of the compression stroke, thus requiring a high injection pressure. During the process of injection the fuel is broken into a fine spray of very fine droplets. These droplets take heat from the hot compressed air. The surfaces of these droplets form vapors, which in turn mixes with air to form a fuel-mixture. When due to continued heat transfer from hot air to fuel the temperature of this mixture rises to a value greater than its self-ignition temperature, it ignites. The period between the start of the injection and the start of ignition, called theignition delay, is about 0.001 second for high speed engines and 0.002 for low speed engines. After the ignition the temperature rises rapidly, so also the pressure.
The injection period covers about 25 degrees of crank rotation and, thus, the process of vaporization and mixing continues at an increasing rate after the delay period is over. However, the amount of oxygen available for mixing and burning reduces as the combustion advances, therefore, the heat release is affected.