Single Cylinder CI Engine modelling using GT Power
Objective
To Compare SI and CI engine and study the performance parameters of CI Engine
CI Engine Model
BSFC (Brake specific fuel consumption)
The brake specific fuel consumption(BSFC) is the ratio of the mass flow rate of fuel to the brake power.
The BSFC for diesel engines is lower compared to SI engines for the same power output as the thermal efficiency of diesel engines is higher due to their high compression ratio. Since Diesel engines are operated at low engine speed conditions they have low pumping and friction losses as well.
The BSFC for Diesel engines is around 200 g/kw-hr.
Exhaust Temperature
The temperature of the exhaust gases at the outlet is called exhaust temperature.
The exhaust gases of petrol engines are hotter than Diesel engine as Diesel engines employ higher compression ratio and as the gas expands more it gets cooler.
The Diesel engines also employ turbochargers in which the exhaust is allowed to further expand which cools them.
The Diesel engines also operate at higher air-fuel ratio(diluted) as there are no throttling losses and hence lower exhaust temperature.
Air-Fuel ratio
The air-fuel ratio is the ratio of the mass of air to the mass of fuel. Diesel engines employ higher air-fuel ratios as there are no throttling losses, hence they operate at leaner air-fuel ratio throughout the engine operating conditions.
Typical operating ranges of diesel engines spread between an air/fuel ratio of 18 and up to 70, depending on the operation point.
Effect of MFB 50 on Engine performance
The MFB 50 is the 50% mass fraction burned(MFB).
The below table is in the order of MB50= 25CAD, 30CAD, 35CAD, 40CAD. CAD(Crank angle degree ATDC)
When MFB 50 is low then more fuel is burned in a short duration which will increase the BMEP as the engine will travel at higher average pressure. It is always desirable to have peak pressure at the beginning of power stroke. The maximum in-cylinder pressure is also higher for lower MFB50.
The exhaust temperature is comparatively low for low mfb50 as there is uniform expansion of gases for a comparatively longer duration which reflects on the higher thermal efficiency of the engine.
It can be seen from the above tables that lower MFB50 compartively produces more engine power, torque and MEP.
However for low MFB50 the engine should be designed to handle high pressure which increases the inertia of the moving parts and decreases the power to weight ratio.