How To Calculate Heat Balance Sheet For IC Engine
What is Heat Balance Sheet?
A heat balance sheet is an account of heat supplied and heat utilized in various ways in the system. Necessary information concerning the performance of the engine is obtained from the heat balance.
The performance of an engine is generally given by a heat balance sheet. To draw a heat balance sheet for the I.C. engine, it is run at constant load and constant speed. The Indicator diagram is drawn with the help of an indicator. The quantity of fuel used in a given time and its calorific value, the amount, inlet, and outlet temperature of cooling water and the mass of exhaust gases are recorded.
The heat balance sheet is generally done on a second basis or a minute basis or an hourly basis.
The engine should be equipped with a suitable loading arrangement to measure the brake power of the engine. Provisions are also made to measure the amount of air intake. amount of fuel consumed, the temperature of cooling water at inlet and outlet of the engine amount of cooling water circulated and temperature of exhaust gases.
Use Of Heat Balance sheet :
1. To know an account of heat supplied and heat distributed in various ways in the system.
2. To analyses the performance of the engine.
Heat Balance Sheet Calculation :
The heat balance is generally done on a second basis or a minute basis or an hourly basis.
The heat supplied to the engine is only in the form of fuel-heat and that is given by
Qs = mf X CV
Where mf is the mass of fuel supplied per minute or per sec. and CV is the lower calorific value of the fuel.
The various ways in which heat is used up in the system is given by
(a) Heat equivalent of BP = kW = kJ/sec. = kJ/min.
(b) Heat carried away by cooling water
= Cpw X mw (Two – Twi) kJ/min.
- mw is the mass of cooling water in kg/min or kg/sec circulated through the cooling jacket
- (Two – Twi) is the rise in temperature of the water passing through the cooling jacket of the engine
- Cpw is the specific heat of water in kJ/kg-K.
(c) Heat carried away by exhaust gases
= mg Cpg (Tge – Ta) (kJ/min.) or (kJ/sec)
- mg is the mass of exhaust gases in kg/min. or kg/sec and it is calculated by using one of the methods already explained.
- Tg= Temperature of burnt gases coming out of the engine.
- Ta = Ambient Temperature.
- Cpg = Sp. The heat of exhaust gases in (kJ/kg-K)
(d) A part of the heat is lost by convection and radiation as well as due to the leakage of gases. Part of the power developed inside the engine is also used to run the accessories as a lubricating pump, camshaft and water circulating pump. These cannot be measured precisely and so this is known as unaccounted ‘losses’.
The results of the above calculations are tabulated in a table and this table is known as “Heat Balance Sheet”. It is generally practice to represent the heat distribution as percentage of heat supplied. This is also tabulated in the same heat balance sheet.This unaccounted heat energy is calculated by the different between heat supplied Qs and the sum of (a) + (b) (c).
Heat Balance sheet Example :
Problem : An I.C. engine uses 6 kg fuel having calorific value 44000 kJ/kg. in one hour. The brake power developed is 18kW. The temperature of 11.5 kg of cooling water found to rise through 25 0C per minute. The temperature of 4.2 kg of exhaust gas with specific heat 1 kJ/kg K was found to rise though 220 0C. Draw heat balance sheet for the engine.
Given data :
Mass of Fuel = 6 Kg/hr = 6/60 = 0.1 Kg/min. BP = 18 KW CV = 44000 kJ/kg
Mass of cooling water Mw = 11.5 Kg/min Cpw = 4.187 kJ/kg K
Temp rise of cooling water Δtw= 25 0C
Mass of exhaust gas Meg = 4.2 Kg/hr =4.2/60 Kg/min = 0.7 Kg/min
Temp rise of gas Δtg= 220 0C Cpg = 1 kJ/kg K
i) Heat equivalent in Fuel (Hf ) = Mf x C V = 0.1 x 44000 = 4400 Kg/min
ii) Heat converted in B P (Hb ) = B P x 60 = 18 x 60 = 1080 Kg/min
iii) Heat carried by cooling water (Hw) = Mw x Cpw x Δtw
Hw = 11.5 x 4.187 x 25 = 1203.76 Kg/min
iv) Heat in Exhaust Gas (Hg )= Mg x Cpg x Δtg
Hg = 0.7 x 1 x 220 = 154 Kg/min
v) Heat lost as Unaccounted (Ha)= Hf- (Hb+Hw+Hg)
Ha = 4400 – (1080+1203.76+154) = 1962.24 Kg/min
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