1. Combustibility of diesel fuel
Increasing the fuel temperature, causes the fuel to ignite spontaneously, even it is not exposed to a flame. The minimum temperature that this occurs is called the autogenous ignition point (self-ignition temperature). The fuel is injected into the combustion chamber and heated by the air of high temperature and pressure. Then, the fuel self-ignites and burns. In a diesel engine, the fuel ignitability is improved because as the compression ratio increases, the temperature increases quickly. Also, ignition performance is improved when using fuel with a high cetane number.
Cetane number
The cetane number of the diesel fuel corresponds to the octane number of gasoline and represents ignitability of the fuel.
The higher the number is, the lower the ignition point and the better the fuel.
For diesel engine fuel, a cetane number of at least 4- 45 is usually required.
Generally a cetane of 53 – 55 is used.A high cetane number corresponds to the following effects.
Good startability
Clean exhaust gas
Large output
Improvement of fuel consumption
Engine operates smoothly and there is less noise.
2. Relationship between compression ratio and compression
pressure or temperature
The diesel engine compresses the air inside the cylinder and raises the temperature for combustion. The graph on the left shows the relationship between the compression ratio and compression pressure or temperature. It is assumed that no leakage of air and heat loss between the piston and the cylinder occurs. When the compression ratio is 16, for example, the graph shows that the compression pressure and temperature can go as high as approx. 5 MPa (5kgf/ cm2) and 56C (1,04F) respectively. However, in an actual engine, the compression pressure and air temperature values are usually somewhat lower than the theoretical values shown in the graph because the heat is released.
3. Diesel engine combustion process
For the combustion process that occurs in the diesel engine, there is a relationship between the pressure inside the combustion chamber and the crank angle as shown on the left. This combustion process can be divided into the following four stages.
(1) Ignition delay (A – B) In preparation for combustion fine particles of the injected fuel evaporate and mix with the air in the cylinder to form an ignitable mixture.
(2) Flame propagation (B – C) In this stage, ignition starts from the areas within the air-fuel gas that have reached the proper ratio, and then continues to burn outward. From point B to C, the pressure rises sharply. The rise of pressure is affected by the volume of fuel injected at ignition delay, fuel spraying condition and airfuel mixture, etc.
(3) Direct combustion (C – D) In this stage, the fuel is burned with the flame in the combustion chamber immediately after the injection. The pressure from combustion rises more gradually because the fuel burns immediately after injection. The pressure at this time can be adjusted to a certain extent by adjusting the fuel injection volume.
(4) After burning (D – E) The fuel injection into the combustion chamber ends at point D. However, the remaining fuel, which could not burn, burns during this period. As the after burning period gets longer, the exhaust temperature rises and the heat efficiency*1 lowers. *1With heat engines, the heat efficiency means the ratio of heat energy converted into the workload and the heat energy of the supplied fuel.
Combustion process (A – E)