CI engine modelling of 2019 Chevrolet Silverado Duramax Diesel Engine Using GT Suite

                                                                         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

SI Engines	CI Engines
Fuel with a higher octane number and lower Cetane number	Fuel with a higher Cetane number and Lower Octane number
Uses Spark to start combustion	Ignition starts after fuel reach its self-ignition temperature
Higher RPM and lower torque is obtained	Lower Rpm and Higher torque is obtained
Used in Race cars, high-performance cars	Used in heavy-duty applications like Tractors, Trucks, Bulldozer etc
Fuels like petrol, CNG, Lpg are used	Diesel fuel is used
Emissions like CO, Nox are the main pollutants	Emissions like Particulate matter CO, NOx, HC are obtained after combustion
After treatment devices like 3-way catalytic converter are used	DOC, DPF, SCR used as after-treatment devices
Direct port tumble motion is desired	Swirl motion is desired so Helical ports are desired
Lower compression ratios	High compression ratios
Thermal, volumetric efficiencies are lower	Higher Thermal and volumetric efficiencies
Engine knocking is a common phenomenon in SI engines and it happens at the end of the combustion	Engine Detonation is a common phenomenon in CI engines and it happens at the start of the combustion

 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 of holes at higher injection pressures. 

 

 Turbocharging :

Turbocharging in SI engines is very difficult due to the higher knocking. Whereas turbocharging modelling in CI engines is very beneficial compared to normal diesel engines. Turbocharging helps in sending more air into the engine. Due to the presence of high air inside the cylinder, it is possible to burn more fuel which helps in developing required/more power. Limitations in developing turbocharged engines are in-cylinder pressures and exhaust temperatures. In a turbocharger, the turbine is connected to a compressor with a shaft and bearing arrangement. The exhaust gas is sent through the turbine and the kinetic energy of the exhaust gas helps in spinning the turbine, which in turn drives the compressor. This helps in reducing the overall fuel consumption (BSFC) of the vehicle with improved efficiency.  

For supercharged engines, the energy required to run the compressor is provided by the engine itself or an external energy source such as an e-motor. The work done by the supercharger is negative work and it should be subtracted from the engine work to determine the net-work done and net power generated by the engine. For diesel engines, turbochargers are more preferable compared to superchargers because turbocharged engines generate more torque than acceleration which is very much preferable for heavy-duty applications. The supercharger is used in place of a turbocharger is to avoid turbo lag. 

 

 

 

 The turbine inlet could have multiple entries (twin-scroll turbines) in order to avoid interference of pulses in the engines with more cylinders. The speed of the turbine usually varies from 100,000 rpm to 300,000 rpm. For smooth functioning of the shaft which is connecting the turbine and compressor, the assembly is equipped with bearings and oil is passed through one end and collected at the other end for effective heat transfer. Due to higher operating load conditions, turbochargers tend to fail or has durability issues. The shape of the impeller in the turbine and compressor decides the operating and working conditions of turbochargers. 

 

 

 

 

 

 

 

 Turbochargers have been evolved over a period of time starting from Euro 1 to the present Euro 6. It started with fixed geometry turbochargers then later evolved to Wastegate turbochargers. Later on to Variable gate turbochargers and then to 2 stage turbochargers for high power requirements. The latest development is the use of e- turbo where the exhaust gas was sent to run the turbine that is used to run the motor-generator. thereby running the compressor using electrical energy. This arrangement is extremely useful for hybrid engines in order to maintain the required pressure and temperature of dense inlet air entering the cylinder.  

 

 Compressor Map in a Turbocharger

 

 

 

Compressor maps give us the relation between pressure ratios and the mass flow rate of the compressor. The map also consists of constant speed lines and efficiency lines. The operating condition of the compressor should lie in between these lines. Engineers should make sure the operating condition should not fall in either surge zones, Excess speed zone or Choke zones. If the compressor is operating in the Surge zone then the compressor experiences fluctuations it is also called 'Barking of the compressor'. In the choke zone, there will be a rapid drop in the efficiency of the compressor. A drop in efficiency leads to an excessive increase in the temperature of the compressor outlet. To avoid mechanical failure they should not be operated at excess speeds. This happens when the engine is operated at higher altitudes. 

 

Engine Specifications :

 Built-in 6 cylinder turbocharged (twin-scroll turbine) CI engine

 

 

Compressor inlet temperature and pressure are 298k and 1 bar respectively. 

Initial run when the engine is operating at 3600 Rpm

 

 Compressor Map

 

Compressor Speed Monitor

Compressor Efficiency Monitor

The operating region of the compressor map is not feasible as the compressor is operating at higher RPM and therefore fine-tuning of various parameters is required in order to make it feasible. 

 

Diesel engines require higher Air-fuel ratios (lean mixtures) for effective combustion. An increase in the quantity of air is extremely important which can be achieved through turbochargers. At higher altitudes, it is very difficult for engines to operate due to a lack of air and lower atmospheric pressures. Therefore compressors put more work and run at higher Rpm's to supply air which is not feasible. This causes reliability issues. Modern diesel engines come with electronically controlled high-pressure fuel injectors to control fuel supply taking into account the available air for combustion. 

 case 1:

Engines running at 3600 constant rpm with variable altitudes and fuel injection rates

 

 Results:

 

 It can be observed that despite changes in the altitudes the air-fuel ratios are the same for all 3 cases. Also due to the decrease in airflow rate and fuel injection rate, the power output in these 3 cases is quite different and less. This engine setup uses a fixed geometry turbocharger which is prone to damage over a period of time due to the fixed kinetic energy of the exhaust running the turbines. Therefore wastegate turbochargers are used to control the pressure outlet of the compressor by bypassing the excess air entering the turbine. 

 Wastegate turbines use PID controllers to adjust the valve and thereby bypassing the excess air set by the user. The compressor outlet pressure is sensed by the diaphragm. It activates when the pressure outlet is more than required. Due to this, the compressor will not operate at higher speeds. 

 

 

 

 

 

 The wastegate controller controls the boost pressure being set at the target value. The wastegate controller gives output to the turbine to control the diameter of the wastegate to bypass exhaust gas. 

 In this case, the target boost pressure has been set to 2 bar. anything more than this will actuate the wastegate controller to bypass the excess exhaust gas by maintaining its diameter not less than 8 mm. This can be observed in the graph below

 

 

 

At different speeds, the diameter of the wastegate changes accordingly.

Variable Geometry Turbine:

 A variable geometry turbocharger controls engine exhaust flow through the turbine wheel using a row of vanes. These vanes were open and close to matching the engine’s exact boost requirements. zero position defines fully closed and one defines fully open. The position of vanes lines within this range. Electronic control units control the position of these vanes according to the required boost pressure demands. This makes VGT's more preferable over WGT's. 

At LOW SPEEDS, the vanes close, which:

• Restricts exhaust airflow through the turbine
• Increases turbine power
• Increases boost pressure

At HIGH SPEEDS, the vanes open, which:

• Maximizes exhaust gas flow
• Avoids turbo over-speed
• Maintains required boost pressure

 

 

 

 

 

 

 

 

 

 

 

 

 

Euro - 6 

• On-board diagnostic (ODB) requirements have been strengthened compared with previous regulations