Week-4 : Basic Calibration of Single cylinder CI-Engine

                                                                         Introduction to CI engines

Compression - ignition or diesel engine combustion process can be summarized as follows:

Fuel is injected by the fuel injection system into the engine cylinder toward the end of the compression stroke, just before the desired start of combustion. The liquid fuel, usually injected at high velocity as one or more jets through small orifices or nozzles in the injection tip, atomizes into small drops and penetrates into the combustion chamber. The fuel vaporizes and mixes with the high-temperature high-pressure cylinder air. Since the compressed air temperature and pressure is more than the self-ignition temperature of the diesel fuel, spontaneous ignition takes place after fuel mixes with air with a delay period of a few crank angle degrees. 

                                                                     Difference between SI and CI engines

 Cetane Number (CN)

CN of fuel = (Percentage of n - cetane) + 0.15 * (Percentage of Heptamethylonane)

Lager cetane number indicates faster burning of diesel fuel

The smaller the hole size of the fuel injector higher will be the atomization. The size of the piston in CI is not flat as compared to the SI engine piston. CI piston has a bowl shape as shown in the picture below which helps in better vaporization of the fuel and better mixing with the air in a small amount of time. 

 Chemical Equation : 

 `C_xH_y + aO_2 +a(3.76)N_2 tobCO_2 + cCO +dc(s) +eH_2O +a(3.76)N_2`

If it is complete combustion then the bi-products are `CO_2, H_2O, N_2` This is not possible in reality because of variations in air-fuel ratios under different operating conditions.

Rich mixtures will have CO, C has major pollutants

Lean mixtures will have `NO_x` as a major pollutant

Methods to reduce engine pollutants

`NO_x`:  The formation of `NO_x` is completely dependent on in-cylinder temperatures and pressures. Therefore it is necessary to reduce the temperature of the incoming air entering the cylinder.  The use of intercoolers, Exhaust gas recirculation methods will help to reduce `NO_x` emissions. One more easy method to reduce the `NO_x` is by controlling the rate of fuel burning. This can be achieved by using ECU controlled CRDI (common rail direct injection) or other fuel injection systems. 

SOOT: Soot is solid carbon formed due to rich air-fuel mixtures. This can be reduced by designing the shape of the intake port. For diesel engines swirl motion is preferred in order to properly mix air and fuel. Swirl motion can be achieved by using a Helical port. Another solution to reduce soot is to supply more amount of air, and this can be achieved by using turbochargers/superchargers. 

 Piston Bowl

 The fuel is injected inside the bowl. This helps in giving a sufficient amount of time for atomization, vaporization of the fuel. Later proper mixing of air and fuel takes place and thus helps in developing controlled combustion and better fuel efficiency. 

 Combustion in CI engines : 

6 - cylinder Diesel engine 

Modern diesel engines make use of electronically controlled fuel injectors taking into account the Airflow rate to maintain air-fuel ratio above 18 (in order to maintain lean mixtures to avoid soot), Engine Rpm and Power. Fuel is injected into the cylinder when the piston reaches before or after 0 to 2 degrees of crank angle rotation at TDC. Better atomization is possible only with a reduction in the size with an increased number of holes of the injector at higher injection pressures. 

 BSFC: BSFC is a measure of the fuel efficiency of any engine that burns fuel and produces rotational power output. The BSFC value indicates how efficiently the engine converts fuel supplied into useful work.

SI engines have higher BSFC compared to CI engines due to the lack of presence of air required to burn the fuel (lower compression ratios (CR)). The air-fuel ratios of gasoline and Diesel engines are completely different. Gasoline engines are rich mixtures whereas CI engines have lean mixtures. An increase in CR could lead to the formation of NOx in SI engines. Higher BSFC in Si engines is due to homogeneous mixtures. Lower Bsfc in CI engines is due to heterogeneous mixtures. This lead to the formation of soot in CI engines. Knocking is the cause for the reduction in CR in SI engines. 

Exhaust temperatures: Due to higher BSFC and complete combustion in SI engines the exhaust gas temperatures are very high in SI engines whereas in DIesel engines the temperatures are less due to lower bsfc. 

A/F ratio: Air-fuel ratios in CI engines are very high (lean burned) whereas SI engines have lower air-fuel ratios (rich burned). This is due to a reduction in the compression ratio in Si engines. 

 MFB50 : 

 MFB50: Fraction of fuel burned is changed from default 100% to 50%. Simulation results show that there is a decrease in in-cylinder maximum pressure from 117 bar to 112 bar.