The motoring curve is a plot of the IMEP (Indicated Mean Effective Pressure) versus the engine speed. It is used to determine the pumping losses of an engine. The pumping losses are the work required to move the piston and other reciprocating components of the engine against the compression and expansion of the gases in the cylinder.
The motoring curve is obtained by motoring the engine, which means that the engine is run without fuel being injected. This is done by disabling the fuel injector or by closing the fuel supply valve. When the engine is motored, the only work that is done is the work required to move the reciprocating components.
The motoring curve can be used to calculate the pumping losses by subtracting the IMEP of the motoring curve from the IMEP of the firing curve. The firing curve is a plot of the IMEP versus the engine speed when the engine is running with fuel being injected.
The pumping losses are an important factor in determining the overall efficiency of an engine. The lower the pumping losses, the more efficient the engine will be.
Stages of SI Engine Combustion
The combustion process in a spark ignition (SI) engine can be divided into four stages:
1. Induction
2. Compression
3. Power
4. Exhaust
Induction
During the induction stroke, the piston moves down in the cylinder and draws in a mixture of air and fuel from the intake manifold. The intake valve is open during the induction stroke and the exhaust valve is closed.
Compression
During the compression stroke, the piston moves up in the cylinder and compresses the air-fuel mixture. The compression ratio is the ratio of the volume of the cylinder when the piston is at bottom dead center (BDC) to the volume of the cylinder when the piston is at top dead center (TDC). The higher the compression ratio, the more efficient the engine will be.
Power
During the power stroke, the spark plug ignites the air-fuel mixture and the resulting expansion of the gases drives the piston down in the cylinder. The power stroke is the only stroke that produces power.
Exhaust
During the exhaust stroke, the piston moves up in the cylinder and expels the exhaust gases from the cylinder. The exhaust valve is open during the exhaust stroke and the intake valve is closed.
The four strokes of the SI engine are repeated continuously to produce power.
