Heat Balance Sheet

Heat Balance Sheet It is a chart in which complete record of heat interaction such as heat supply and heat rejection during a particular period of time is tabulated. The following items were found in the heat balance sheet. Total Heat Supply In an internal combustion engine, heat is supplied only by combustion of fuel. Accordingly heat supplied by the fuel

=

m  CV 3600

kW

m f

where

= total fuel consumption in kg/hr, CV = calorific value of the

fuel kJ/kg Total Heat Rejection Heat Absorbed in Useful Work It is nothing but the brake power developed by the engine in kW. Heat absorbed in useful work, Q1 = BP in kW. Heat Rejected in Cooling Water The amount of heat rejected to cooling water is given by Q2  m w C pw  t 2  t1 

where

m w

kW

= mass of cooling water circulated in kg/s

Cpw = specific heat of water = 4.187 kJ/kg oK t2 = exit temperature of cooling water t1 = inlet temperature of cooling water Heat Carried Away by Exhaust Gases

It is given as

Q3  m g C pg  t g  t a 

kW

m g

Where

= mass of exhaust gas which is equal to the sum of mass of air

and mass of fuel in kg/s Cpg = specific heat of gas in oC tg = temperature of exhaust gas in oC, and ta = atmospheric temperature in oC

Unaccounted Losses These are the losses such as radiation losses, loss due to friction, losses due to leakage etc. these losses cannot be measured by using direct methods. But its magnitude can be calculated as follows; Unaccounted losses = Heat supplied by the fuel – (Heat absorbed in useful work + Heat rejected to cooling water + Heat carried away by the exhaust gases) These losses are expressed as percentage in heat balance sheet with respect to heat supplied by the fuel. Finally all these things are listed in the form of a table as shown in table below; Sl. No.

Particulars

Heat rejection in %

Total Heat Supplied

kJ/sec %

1. 2. 3.

Heat absorbed in useful work done Heat rejected to cooling water Heat carried away by exhaust

4.

gases Unaccounted losses

RATING OF ENGINES It has become necessary to formulate certain standard methods of rating the performance of engines as the number of production designs of engines for commercial use is going up day by day. Sometimes engines are commercially rated at an output only half of that which could be obtained. As engines are subjected to excise duty in India a manufacturer sometimes try to save the duty by under-cutting his product. Sometimes engines are over-rated in order to attract the customers. All types of engines are rated in horse power; the measure of the rate at which they can do work. It has already been correlated that one horsepower equals to 75 m-kg/sec or 4500 m-kg/min. The engine power can be measured in two ways;  The power developed by expansion of gases in the cylinder, as by indicator cards, where indicated horse power is obtained.  The power which the engine actually delivers can be measured by some measuring instruments like prony brake or dynamometer- in which case brake horse power is obtained.

The horse power rating of an engine is always given at its maximum value and is stated in terms of engine speed at which it is developed. For example Fiat develops 43 B.H.P. at 5000 rpm, and Rajdoot motorcycle develops 9 BHP at 5000 r.p.m.

SAE Rating The Society for Automotive Engineers (SAE) recommended a method of obtaining the engine output power for the purpose of standardization. The engine is run without the generator, air cleaner, cooling fan, etc. However, the standard water circulating pump is fitted to the engine. The horse power to the engine is measured with a dynamometer, and the measured value is then corrected for standard temperature, pressure and humidity conditions. The horse power thus obtained is known as SAE horse power or SAE rating. RAC Rating The RAC (Royal Automobile Club of England) horse power rating of engines is used to compare engines on the basis of the number and diameter of cylinders. RAC horse power is given by;

RAC horse power =

D2  N 2.5

Where D = Diameter of the cylinder in inches and N = number of cylinders

This horse power rating is usually for tax purposes, and hence is called taxable horse power (T.H.P.). DIN Rating It is a German method of engine rating. In this method the engine is run with all the accessories such as generator, air cleaner, cooling fan etc. fitted on the engine. The horse power is measured with a dynamometer and the measured value is corrected for standard temperature, pressure and humidity. The horse power thus obtained is known as DIN horse power or DIN rating. D.E.M.A. Rating The trade association of builders of diesel engines known as the Diesel Engine Manufacturers Association (DEMA) has adopted standard ratings for low speed and medium speed stationary diesel engines. According to DEMA the sea level rating of an engine is the net brake horse power

that the engine will deliver continuously when in good operating

condition and located at an altitude of not over 457.2 meters, above sea level with atmospheric temperature not over 32.20C and barometric pressure not less than 71.7 cm of mercury. Engine manufacturers offer engines with sufficiently conservative sea level ratings so that the engines will be capable of delivering an output of 10 per cent in excess of full load ratings so that the engines.