Internal Combustion

Fundamentals

Prof. (Dr.) M P Poonia Director

NITTTR, Chandigarh

AIR STANDARD CYCLE

Pressure‐volume graphs for thermodynamic processes: (a) isobaric, (b) isothermal, (c) adiabatic, (d) isochoric.

AIR STANDARD CYCLEAir standard cycles are idealized cycles based onthe following approximations:- the working fluid is air (ideal gas)- all the processes are internally reversible- the combustion process is replaced by heat input from external source- heat rejection is used to restore fluid to initial state

Gas at Constant Pressure

Gas

Movable Piston

Heat

Specific Heat

EFFECT OF SPECIFIC HEAT

1-2-3-4 Constant Specific Heat

1-2”-3”-4” Variable Specific Heat

P

V

A: F Ratio

Excess Fuel Excess Air

A: F Ratio

EFFECT OF DISSOCIATION

P

V

Tmax

Actual Cycles

POOR UTILISATION OF HEATTypical heat balance of an engine

Useful Output (Brake Power) 34%Cooling Loss 30%Exhaust Loss 26%Friction, Radiation, etc. 10%

----------Total Heat Input 100%

Flame front Fuel spray flame

Premixed charge (gasoline)

Non-premixed charge (Diesel)

Spark plug Fuel injector

Fuel + air mixture Air only

PREMIXED vs NON-PREMIXED

CHARGE ENGINES

IDEAL HEAT ADDITION

THE IDEAL AIR STANDARD

OTTO CYCLE

ACTUAL HEAT ADDITION

LOSSES IN ACTUAL CYCLES Time Loss Heat Loss Blow down Loss Blow by Loss Rubbing Friction Loss Pumping Loss

TIME LOSS

Heat addition is not instantaneous, and spread over a period (30 to 40 degrees of crankshaft revolution). Therefore, Pmax is not at TDC, but just after TDC.Time loss depends upon flame velocity which, in turn, again depends on type of fuel used, A/F ratio, and shape of combustion chamber.

HEAT LOSS

This is due to the transfer of heat through water jackets & cooling fins. Also, same heat is being transferred during compression and expansion processes. Due to heat loss, temperature (Tmax) decreases, and specific heat gets reduced. This decreases the efficiency.

HEAT LOSS

BLOW DOWN LOSSBlow down loss is due to the early opening of exhaust valves. This results in drop in pressure, and a loss of work output during expansion stroke. Too early opening results in loss of expansion work. Best compromise is between 40 – 60 bBDC.

BLOW DOWN LOSS

BLOW BYE LOSSES

RUBBING FRICTION LOSS Rubbing friction loss is due to friction between the piston and chamber walls, friction in various bearings and also includes the energy spent in operating various auxiliary equipments such as cooling fans water pumps etc.The piston ring friction increases rapidly with engine speed. It also increases to a small extent with increase in mean effective pressure. The bearing friction and the auxiliary friction also increase with engine speed.

PUMPING LOSS

Blow Down Loss

Heat Transfer Loss

Combustion Loss

Pumping Loss V

P

LOSSES DURING CYCLE

MAGNITUDE OF LOSSES

Engine Load

MAGNITUDE OF LOSSES

The actual efficiency of a good engine is about ……… of the estimated air fuel cycle efficiency. a) 100% b) 85% c) 50% d) 25

FAQ

Fuel air efficiency is less than air standard efficiency by an amount equal to a) pumping loss b) friction loss c) loss due to specific heat and chemical equilibrium d) exhaust blow down loss

FAQ

Fuel air ratio affects maximum power output of the engine due to a) higher specific heats b) chemical equilibrium losses c) all of the mentioned d) none of the mentioned

FAQ

As compared to air cycle, in actual working, the effect of variation in specific heats is to: a) increase maximum pressure and maximum temperature b) reduce maximum pressure and maximum temperature c) increase maximum pressure and decrease maximum temperature d) none of the mentioned

FAQ

Thanks