**1) Indicated mean effective pressure:-**

The indicated mean effective pressure of an engine is obtained from the indicator diagram drawn with the help of an engine indicator.

mathematically,

**M.E.P = Area of indicator card X Scale of indicator spring / Length of indicator card.**

It may be noted that the mean effective pressure calculated on the basis of the theoretical indicator diagram, is known as theoretical mean effective pressure. If it is based on the actual indicator diagram, then it is called an actual mean effective pressure.

**2) indicated power:-**

The indicated power( briefly written as I.P) star power actually developed by the engine cylinder.

I.P = 100 K p_{m} L A n / 60 kW

Where, K = Number of cylinders,

p_{m} = Actual mean effective pressure in bar(1bar=100kN/m^{2})

L = Length of stroke in m.

A = Area of the piston in m^{2}

n = No. of working stroke / min

= speed of the engine for two stroke cycle engine.

= Half speed of the engine for four stroke cycle engine.

Note- I.P a multi-cylinder of spark ignition engine is determined by morse test.

**Brake power:-**

The brake power( briefly written as B.P) is the power available at the crankshaft. The brake power of IC engine is usually measured by means of the brake mechanism( prony brake for rope brake).

In case of prony brake, brake power of the engine,

B.P = Torque in N-m x Angle turned in radius through 1 revolution / 60 in watt.

= T x 2 π N / 60 = Wl x 2 π N / 60 watt

Where, W = Break load in newtons,

l = Length in arm in metters,

N = Speed of the engine in r.pm

In case of rope break, break power of the engine,

B.P = (W – S) π D N / 60 in watt.

= (W – S) π (D + d) N in watt.

Where, W= Dead load in newtons

S = Spring blance reading in N.

D = Dia. Of the break drum.

d = Dia. Of the rope. And N = Speed of the engine in r.p.m.

Note- the brake power(B.P) of an engine is always less than the indicated power(I.P) of an engine, because some Power is lost in overcoming the engine friction( known as frictional power).

Mathematically,

Frictional power(F.P) = I.P – B.P .

**Efficiency:-**

the efficiency of an engine is defined as the ratio of work done to the energy supplied to the engine. The following efficiencies of an IC engine are important,,…

a) mechanical efficiency:- it is the ratio of brake power to the indicated power.

Mathematically,

դ_{m }= B.P / I.P

Since, B.P is always less than I.P, therefore, դ_{m }is always less then unity(i.e, 100%)

* b) overall efficiency:-* what is the ratio of the work of detained at the concept in a given time to the energy supplied by the fuel during the same time.

Mathematically,

դ_{0} = B.P x 3600 / m_{f } X C m_{f }= mass of fuel consumption in kg/hr.

C = Calorific value of fuel in kJ/kg of fuel.

**c) indicated thermal efficiency:-**

it is the ratio of the heat equivalent to kW hour to the heat in the fuel per I.P hour,

Mathematically,

դ_{i } = Heat equivalent to 1 kW hour / Heat in fuel per I.P hour. = I.P X 3600 / m_{f} X C

**d) brake thermal efficiency:-**

it is the ratio of the heat equivalent 1kw to heat in the fuel per B.P hour.

Mathematically,

դ_{i } = Heat equivalent to 1 kW hour / Heat in fuel per B.P hour. = B.P X 3600 / m_{f} X C

**e) Air standard efficiency:-**

the general expression of the air standard efficiency is given as,

**f) relative efficiency:-**

it is also known as efficiency ratio. The relative efficiency of an IC engine is the ratio of the indicated thermal efficiency to the air standard efficiency.

**g) volumetric efficiency:-**

it is the ratio of the actual volume of charge admitted during the suction stroke at N.T.P swept volume of the piston.