Simulation of petrol and gas engines with spark ignition
The program DIESEL - 2/4t based on a
multizone model simulates phenomena in spark ignition engines including prechamber ones. Both a
benzine and a gas can be used as a fuel.
It is assumed that there are 2 zones in the volume cylinder: 1) a cold fresh charge; 2) hot products of combustion. The zones are divided by a moving thin layer of flame front and interact intensively. For each zone, heat exchange is calculated, besides, dissociation and heat - mass interchanging between zones are taken into account.
The velocity of spherical flame front is defined either directly by solution of the equations of turbulent flame velocity or by solution of the system of balance conservation equations for both zones with usage the Vibe heat release function. This calculation method allows to define the temperature of a fresh charge zone during entire compression - combustion process that is very important for calculation of a knock limit.
Curves of pressure and temperatures in the zones during combustion
(SI petrol engine)
Calculation of gas engines
The program DIESEL - 2/4t id developed to calculate and optimize parameters of spark ignition engines which use a natural or petroleum gas as the fuel. The engine can be equipped with a usual combustion chamber or a pre-combustion chamber which is charged with a rich mixture on the intake stroke. The spark ignition system ignites this rich charge, which creates a "flame jet" down into the lean mixture, igniting it. In this case, parameters of gas are calculated separately for the main chamber and prechamber. The equation of non-steady motion is used for calculation of gas velocity in the connective channel
where: W is the gas velocity in the channel; P is the gas pressure; r is the gas density; õ is the length of the channel, t is the time.
The heat exchange factor for prechamber is calculated by the Woschni formula in which coefficient differs from original one.
The heat release in the prechamber is calculated by theVibe formula. The combustion in the main volume of cylinder begins after 15 % - 20 % of the gas flow out from the prechamber. The heat release in the main chamber is calculated by the Vibe formula too. The calculation of gas parameters (pressure, temperature, composition) in the prechamber is carried out during entire duty cycle.
Calculation of a gas engine with prechamber (diesel MTU 396 converted for operation with a natural gas). Comparison between calculated and experimental curves of pressure in the cylinder. The trials were conducted without supercharging.
|Prechamber volume is 35 cm3.
Connect channel bore is 8 mm.
|Compression ratio is 13.5.
Ignition lead is 40 deg.
Curves of pressure in the cylinder and prechamber.
Calculated curves of temperature in the cylinder and prechamber.