PERFORMANCE PARAMETERS

I C ENGINES CLASS NOTES

 

Need for IC engines in automobiles and power

generation is increasing day by day. Therefore

design an efficient and economical engines.

Judge the performance of the engine from the

various parameters listed below:

1. Indicated Horse Power, IHP

2. Torque

3. Brake Horse Power

4. Mechanical Efficiency

5. Frictional Horse Power

6. Fuel Air-Ratio

7. Relative fuel Air Ratio

8. Thermal Efficiency

9. Brake Specific Fuel Consumption

10. Engine exhaust

11. Mean Effective Pressure

12. Volumetric Efficiency

13. Heat Balance

14. Indicated thermal efficiency

 

1. INDICATED HORSE POWER

An IC engine produces power by the combustion of fuel.

Power is rate of doing work. Power is the product of torque

and angular velocity. Measure the torque by a

Dynamometer and RPM by a Tachometer.  Engine produced

total power is Indicated horse power (IHP). The NET output

useful power available from the engine is the brake horse

power (BHP).

IHP =n p_m L A N” / 60

Where

n is the number of cylinders

p_m = mean effective pressure on the piston, N/m²

L= length of stroke, m

A= area of the piston, m²

N=RPM

But in 2 Revolutions, one cycle is completed

N” = N/2

IHP is indicated horse power in Watts

(ii) TORQUE, T

T = Tangential force x radius

(iii) BRAKE HORSE POWER

It is useful power available from engine shaft.

BHP = 2 πN T/60

Where

T is torque in N.m

N is RPM

BHP is brake horse power in Watts

(iv) MECHANICAL EFFICIENCY, η_mech

η_mech = BHP / IHP

(v)  FRICTIONAL POWER, FP

 Power lost in friction.

FP =  IHP – BHP

(vi) FUEL AIR RATIO

Fuel-Air Ratio = mass of fuel/mass of air

Fuel air ratio is of three types:

1. Rich fuel air ratio—Fuel is more and air is less than required for complete combustion

2. Normal fuel air ratio (Stoichiometric fuel air ratio)

         Fuel and air ratio required for complete combustion

       3. Lean fuel air ratio—fuel is less and air is much more than required for complete combustion

(vii) RELATIVE FUEL AIR RATIO

Used relative fuel air ratio in the analysis.

Relative fuel air ratio = Actual fuel air ratio/ Stoichiometric fuel air ratio

PERFORMANCE PARAMETERS-I C ENGINES-2

Thermal efficiency

η_th = BHP in heat units / heat produced from the fuel combustion in heat units

Mechanical efficiency = BHP/IHP

Fuel Consumption efficiency = It is combustion efficiency

                                               = % of the fuel completely burnt

 (viii) BRAKE SPECIFIC FUEL CONSUMPTION, BSFC

Firstly BSFC = rate of mass of fuel burnt / BHP produced =kg/s/ BHP) =kg/kWh

Secondly BSFC is fuel the engine consumes at a certain RPM to produce one brake horse power.

Thirdly  BSFC is different at different speeds for the same power produced.

Therefore there will be a range of BSFC for an engine.

Lower BSFC is good as it consumes less fuel per unit time for unit power produced.

Really it is fuel consumption efficiency.

BSFC is less for a bigger piston displacement.

At high speeds, BSFC increases due to increased friction.

At lower speeds also, BSFC increases. It is due to increased time for heat losses from the gas to the cylinder and piston wall.

Hence there has to be an optimum speed for lower value of BSFC for a certain engine.

Increase of compression ratio decreases the fuel consumption and hence decreases BSFC and is desirable.

In actual practice, its value varies between 0.4 and 0.6.

BSFC compares the performance of different engines.

(ix) ENGINE EXHAUST

Engine exhaust is wastage of heat to the atmosphere. It contains gases like carbon monoxide, carbon dioxide, Sulphur dioxide, hydrocarbons, lead, nitrogen oxides and particulate matter. It causes air pollution as well Global Warming. Smoke in exhaust is an indication of incomplete combustion of fuel. It adds more of carbon monoxide and drastically decreases the thermal efficiency.

(x) MEAN EFFECTIVE PRESSURE

1. p_mep = IHP x 60 / L A N

2. It is mean pressure acting on the piston during the entire power stroke.  Calculate it from IHP or BHP.

(xi) VOLUMETRIC EFFICIENCY, η_cv

There has to be clearance between the piston and cylinder head otherwise piston will strike the cylinder head. This will decrease the life of the cylinder. This clearance volume will contain high pressure and high temperature exhaust gases. These gases on expansion during the suction stroke will occupy significant volume of the cylinder. As a result of this, volume sucked will decrease. This efficiency varies between 70 to 80 %.

1. η_cv  =  actual volume sucked / stroke volume = 1 + C–C (PR) _1/n

Where C is % clearance i.e. if %  clearance is 2 %, then C = 0.02

PR = Pressure ratio = Discharge pressure / Suction pressure

n is index of expansion

(xii) HEAT BALANCE SHEET

Statement of heat input vs heat output.

Ideally heat input = heat output

Heat input/h = mass of fuel burned/h x calorific value

Heat output/h  consists of the followings:

(i)  Heat equivalent of work obtained / h 

(ii) heat lost in exhaust / h

(iii) heat lost in radiator / h

(iv) heat lost to atmosphere from various hot portions of the engine / h

(v) heat lost by friction / h