Internal Combustion

Engine Efficiencies

Prof. (Dr.) M P Poonia Director

NITTTR, Chandigarh

What is Engine?

Engine is a Machine which Converts Heat Energy of Fuel into Mechanical Power

ENGINE TERMINOLOGY

Chemical Energy Heat Energy Mech. Work

Electrical and Chemical Energy is Concentrated in a Small Space. The Molecules or Particles that Store these Forms of Energy are Highly Ordered and Compact.

High-Grade Energy

Low-Grade Energy

Molecules, in which energy is stored more randomly than the molecules of carbon in a coal, classified as low-grade energy. Heat is low-grade energy.

Indicated Power

Engine Power

Indicated

Brake

Suction Stroke

TDC

BDCS

1 2

VC VS

CR=(VC+VS)/VC

Compression Stroke

TDC

BDCS

1

3

2

Heat Addition

TDC

BDCS

1

3

2

4

Power Stroke

TDC

BDCS

1

3

4

5

Exhaust Stroke

TDC

BDC

S

1

3

4

5

Indicated Power (QH-QC)

BDCTDC

ɳth= (QH-QC)/QH

Indicated Power

Indicated Power

S = C1

2

3

4

S = C

V

P

Thermal Efficiency vs CR Otto Cycle

ɳth

CR

Air-Standard Cycle Assumptions

• Working fluid is air as ideal gas. • No change in composition. • Heat addition from an External

Source.• No Suction and Exhaust Strokes • All the processes are internally

reversible• Specific heats are constant

P

V

pm

patm

Indicated Mean Effective Pressure (bar)

VS

Actual P-V Diagram

PMEP

• L-2\Brake mean effective pressure Meaning.mp4

Indicated Power (IP)

IP=

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

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

N = Engine RPM

1. Only 25% of the power produced by the burning fuel is useful . 2. Now, Computers control every aspect of Car. 3. Even most improving effect, the valve timing. 4. The cam is the brains of an engine. 5. It tells the engine when to fire its spark , when and how long to breathe in and out.

Engine Future

An engine is setup to run most efficiently at one speed. Every other speed the engine has no chance to run as efficiently . There are companies that are now trying to make engines with variable valve timing.

Engine Future

FAQ

• The ratio of the heat equivalent to one kW hour to the heat in fuel per B.P. hour is termed as brake thermal efficiency.a)True b) False

FAQ

• The ratio of the work obtained at the crankshaft in a given time to the energy supplied during the same time is calleda) Mechanical efficiencyb) Overall efficiencyc) Indicated thermal efficiencyd) Volumetric efficiency

FAQ

• The ratio of the brake power to the indicated power is called

a) Mechanical efficiencyb) Overall efficiencyc) Indicated thermal efficiencyd) Volumetric efficiency

FAQ

• The ratio of the indicated thermal efficiency to the air standard efficiency is known asa) Mechanical efficiencyb) Overall efficiencyc) Relative efficiencyd) Volumetric efficiency

FAQ

• The correct sequence of the decreasing order of brake thermal efficiency

a) Four stroke C.I. , four stroke S.I. , two stroke S.I. engineb) Four stroke S.I. , four stroke C.I. , two stroke S.I. enginec) Four stroke C.I. , two stroke S.I. , four stroke S.I. engine

FAQ

• The brake power of a diesel engine, keeping other parameters constant, can be increased by

a) Decreasing the density of intake airb) Increasing the temperature of intake airc) Increasing the pressure of intake aird) Decreasing the pressure of intake air

Thanks