03 Combustion in compression ignition Diesel engine

Pagina 1

Combustion in compression-ignition (Diesel) engines In Diesel engines, a compression-ignition of a heterogeneous charge occurs, because the liquid fuel is injected (atomized into small droplets) into the cylinder, towards the end of the compression stroke. Here the air is at a temperature and pressure level enough to ignite the first elements of fuel already vaporized and mixed with air. Combustion Phases Compression ignition engines have high compression ratio (14-20), able to increase by 500-600°C the temperature of the cylinder air towards the end of the compression stroke. In this hot and compressed air, the liquid fuel is injected as a soft spray, at an optimized time advance relative to TDC. Then some of the smallest fuel droplets vaporize and mix with air, forming a local premixed charge. Because of their high temperature and pressure, some elements of this mixture auto-ignite, after a delay about one millisecond. The consequent increase of temperature and pressure of the cylinder charge speeds up the oxidation reactions in the rest of the premixed charge, that burns rapidly, reducing the combustion time of the remaining fuel. The injection continues until the required amount of fuel has entered the combustion chamber, mixing with the air and burning. Furthermore the rest of the cylinder air continues to mix with burning and already burned gases, during the combustion and the expansion processes. •The first phase (AB, ignition delay) starts when the fuel begins to enter the combustion chamber and ends at the combustion start, with the first energy release; •The second phase (BC, rapid combustion or premixed combustion) is characterized by a rapid combustion of all the air-fuel charge premixed during the ignition delay period, producing very high energy release…