Heat Balance Sheet of IC Engine

Heat Balance Sheet is an account of heat supplied and heat utilized in various ways in the IC engine. Heat balance sheet is used to get necessary information regarding the performance of IC Engine.

Heat balance sheet can be done on second basis or minute basis or hour basis.

Since heat balance sheet is account of heat supplied and heat utilized by engine. So we should keep list of heat supplied to engine and heat utilized by engine.

The heat supplied to engine is only by combustion of fuel and is equal to :-
Q_s = m_f X CV
Where,
m_f = mass of fuel used in kg/min
CV = Calorific value of fuel in KJ/kg

List of heat which are utlized by the engines are :-

1) Heat equivalent to brake power of engine.
2) Heat carried away by cooling water.
3) Heat exhausted with the exhaust gases
4) Unaccounted heat loss.

1) Heat equivalent to brake power of engine.
Q B.P = B.P X 60 = _ K J/Min

2) Heat carried away by cooling water.

Qw = M_w X CP_w X ( T_wo – T_wi ) in KJ/min
Where ,
M_w =Mass of cooling water ciculated in Kg/Min
CP_w = Specific heat of cooling water which is equal to 4.186 KJ/khK
T_wi = Temperature of cooling water at inlet in °C .
T_wo= Temperature of cooling water at outlet of the engine in °C

3) Heat carried away by the exhaust gases (Q_g)

Q_{g =} m_g CP_g (T_g– T_R)
Where ,
m_g = mass of exhaust gases in kg/min
m_a = mass of air consumed in kg/min
m_f = mass of guel consumed in kg/min
CP_g = Specific heat of exhaust gases
= 1.005 KJ/kgK
T_g = Temperature of exhaust gases in °C
T_R = Room temperature in °C

4) Unaccounted heat losses :-
A part of heat is lost by leakage of gases as well as by convection and radiation. Part of heat is also used to run other accessories like lubricating pump, cam shaft and water circulating pump. These part of heat cannot be measured accurately, so this is known as unaccounted heat. It is denoted by Q_un.

Q_un = Q_s – (Q_B.P + Q_w + Q_g) in KJ / min

Now, we have values of heat supplied and heat utilized by the engine.From these values, we have to prepare a balance sheet.

The result of the different values of heat supplied and heat utilized are tabulated in a table and this table is known Heat Balance Sheet.
This table also has percentage representation of heat supplied and heat utilized.
Heat supplied has only one value and that is heat supplied by fuel combustion which covers whole 100% of heat supplied.
Heat utilized column has four values heat in BP, heat carried away by cooling water, heat carried away by exhaust gases and the rest of unaccounted heat. This four together meets to form 100% of utilized heat.