ENGINE EFFICIENCIES

Prof. (Dr.) M P Poonia Director

NITTTR, Chandigarh

Revision

1. Indicated and Brake Mean Effective Pressure2. Indicated Power, IP3. Brake Power, BP4. Mechanical Efficiency5. Friction Power6. Indicated Thermal Efficiency7 Brake Thermal Efficiency

Performance Parameters

8. Specific Fuel Consumption9.10 Heat Balance11. Fuel Air-Ratio12. Equivalence Ratio

Performance Parameters

Volumetric Efficiency

IDEAL P-V Diagram

Average IMEP 4-12 bar

Actual P-V Diagram

PMEP

= IMEP (g)AREA 1

AREA2

IMEP (n)= AREA1-AREA2

BMEP=IMEP (n)-FMEP

Metric Horse Power1 hp= 735.5 W

The indicated power is the power actually developed by the engine cylinder.

INDICATED POWER (IP)

The indicated power is the power actually developed by the engine cylinder.

IP=

Where IP= Indicated Power, WPm= Mean Effective Pressure N/m2

L = Stroke Length , mA = Piston Top Area , m2

N = Engine RPM

BRAKE POWER :The brake power is the power available at the crankshaft of the engine.

Indicated Power

BP

BRAKE POWER (BP)

ɳm=

BP= ɳm× IP =

=

BP=Where:T=Torque in N-mN= Rotational Speed in MinutesBP=Brake Power in watt

BRAKE POWER (BP)

Mechanical Efficiency

Mechanical Efficiency(i)

Mechanical Efficiency

Friction Losses1.Crank Shaft Bearing

Friction Losses2. Connecting Rod Bearing

Friction Losses3. Cylinder Piston Assembly

Friction Losses4. Piston Rings

Friction Losses5. Valve Train

Friction Losses5. Oil Pump

Friction Losses6. Water Pump

Friction Losses7. Generator and Starter Motor

S I Engine Friction

Friction Power

Thermal Efficiency

Typically Gasoline Engines will have an sfc of about 0.3kg/kW-h

Specific Fuel Consumption(SFC)

Volumetric Efficiency• Volumetric efficiency is

a comparison of the actual volume of air–fuel mixture drawn into an engine to the theoretical maximum volume that could be drawn in.

• Volumetric efficiency is expressed as a percentage

Volumetric Efficiency

Heat Balance

Heat Balance

Air Fuel Ratio

Air Fuel Ratio

Air Fuel RatioRich mixture

- more fuel than necessary(AF) mixture < (AF)stoich

Lean mixture- more air than necessary

(AF) mixture > (AF)stoich

Air Fuel Ratio

It shows the deviation of an actual mixture from stoichiometric conditions.

Equivalence Ratio (ɸ)

actual

stoich

stoich

actual

AFAF

FAFA

)()(

)()(

The ratio of the volume of charge admitted at N.T.P. to the swept volume of the piston is calleda) mechanical efficiencyb) overall efficiencyc) relative efficiencyd) volumetric efficiency

Answer: d

FAQ

FAQExplanation: Volumetric efficiency is the ratio of the volume of charge admitted at N.T.P. to the swept volume of the piston while mechanical efficiency is the ratio of the brake power to the indicated power and relative efficiency is the ratio of the indicated thermal efficiency to the air standard efficiency whereas overall efficiency is the ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time.

Which one of the following event would reduce volumetric efficiency of an engine?a) Inlet valve closing after bdcb) inlet valve closing before bdcc) inlet valve opening before tdcd) exhaust valve closing after tdcAnswer: bExplanation: Volumetric efficiency of an engine is reduced by closing inlet valve before bdc.

FAQ

Thanks