4.8.
Energy Losses (Heat Balance)
Only a part of the energy supplied to the engine is transformed into useful work whereas
the rest is either wasted or utilized for heating purposes. The main part of the unutilized heat
goes to exhaust gases and to the cooling system. In order to draw a heat balance chart for an
engine, tests should be conducted to give the following information.
(i) Energy supplied to an engine which is known from the heating value of the fuel
consumed.
(ii) Heat converted to useful work.
{Hi) Heat carried away by cooling water.
(iv) Heat carried away by exhaust gases.
(v) Heat unaccounted for (radiation etc.)
It is expected that the heat balance results of CI engine must differ from that of petrol engine
due to much higher compression and expansion ratios in the former. The higher compression
ratio results in lower exhaust gas temperature and also lower flame temperature that in turn
causes lower heat loss to the cylinder walls in CI engines.
The utilization of the fuel’s heat energy is also higher in CI engines because of its higher
compression ratio.
Although the actual value of heat utilization is dependent upon a number of factors like
compression ratio, engine load, fuel injection quantity, timing etc. some average figures for heat
| Item | S.I. Engine | C.I. Engine |
| Heat converted to useful work (i.p.) | 25 to 32% | 36 to 45% |
| Heat carried away by cooling water | 33 to 30% | 30 to 28% |
| Heat carried away by exhaust gases | 35 to 28% | 29 to 20% |
| Heat unaccounted for | 7 to 10% | 5 to 7% |
| Total (= Energy supplied) | 100% | 100% |
balances for both the engines are given below :
If the shaft work (b.p.) is considered instead of useful work, the mechanical losses are to be
accounted for or are generally included in the cooling water heat.
Example 4.1. An eight-cylinder automobile engine of 85.7 mm bore and 82.5 mm stroke with
a compression ratio of 7 is tested at 4000 r.p.m. on a dynamometer which has a 0.5335 m arm.
During a 10 minutes test at a dynamometer scale beam reading of400 N, 4.55 kg of gasoline for
which the heating value is 46,000 kJ I kg are burnt, and air at 294Kand 10 x 104N/m2 is supplied
to the carburettor at the rate of 5.44 kg per min. Find (a) the b.p. delivered, (b) the b.m.e.p., (c)
the b.s.f.c, (d) the specific air consumption, (e) the brake thermal efficiency, (f) the volumetric
efficiency, (g) the air-fuel ratio.
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