Power Plant Engineering

256 POWER PLANT ENGINEERING

The heat carried away by exhaust gases
= W 2 × KP × (T 4 – T 3 ) kg cal.
(D) Heat Unaccounted for (Heat Lost Due to Friction, Radiation etc.). The heat balance
sheet is drawn as follows:

Item Head units kcal Percent

Heat in fuel supplied

(a) Heat absorbed by I.H.P.

(b) Heat rejected to cooling water

(c) Heat carried away by exhaust gases

(d) Heat unaccounted for (by difference)

Total

A typical heat balance sheet at full load for Diesel cycle (compression ignition) is as follows:

(1) Useful work = 30%

(2) Heat rejected to cooling water = 30%

(3) Heat carried away by exhaust gases = 26%

(4) Heat unaccounted (Heat lost due to friction, radiation etc.) = 10%.

SOLVED EXAMPLES

Example 1. A diesel engine has a brake thermal efficiency of 30%. If the calorific value of fuel
used in 10000 kcal kg, calculate the brake specific fuel consumption.

Solution. ηb = Brake thermal efficiency = 0.3

I.H.B. hr = 632.5 kcal

ηb = H.P. hr equivalent/(w × C.V.)

where, w = Specific fuel consumption per hr.

C.V. = Calorific value of fuel = 10,000 kcal/kg.

0.3 =

632.5
(w×10,000)
w = 0.21 kg/H.P. hr.
Example 2. A four-stroke diesel engine has a piston diameter of 16.5 cm and a stroke of 27 cm.
The compression ratio is 14.3, the cut-off 4.23% of the stroke and the mean effective pressure 4.12 bar.
The engine speed is 264 rev/min and the fuel consumption is 1.076 kg of oil per hour, having a calorific
value of 39150 kJ/kg. Calculate the relative efficiency of the engine.

Solution. I.P. =

PLA

(60 1000 2)

m n

××

, for two stroke engine