Factors Affecting the Engine Performance (Automobile)

4.7.

Factors Affecting the Engine Performance

The factors due to which the indicated power developed by actual engines differs from that
of ideal engines are as follows:
(i) The working media is not air but mixture of air and fuel in case of actual engine.
(ii) The chemical composition of working media changes during combustion.
(Hi) The process of combustion is never at constant volume or at constant pressure.
(iv) The process of compression and expansion are not adiabatic.
(v) The specific heats of gases of working media vary considerably with temperature.
(vi) The combustion may be incomplete.
(vii) The residual gases changes the composition, temperature and actual amount of fresh
charge.
(viii) The amount of fresh charge is decreased due to pumping losses.

Heat Transfer.

The heat is exchanged in both directions between the gases and engine cylinder walls and
the other parts of the engine coming in contact with the gases. During combustion, expansion,
exhaust and the later part of the compression, heat transfer takes place from the gases to the
walls and from the wall to the cooling water or ambient air. During suction and the earlier part
of the compression, heat transfer takes place from the walls to the gases. The heat lost to the
walls during latter part of compression is almost equal to<the heat received by the gases from
the walls during early part of compression. The amount of heat lost during exhaust stroke is
unavoidable and unavailable. The heat lost during combustion and expansion lowers the
thermal efficiency of the engine. The factors that affect the heat losses to the walls are as follows:
(i) Duration of combustion of the charge. This increases the heat loss.
(ii) Temperature of combustion. This is turn depends upon the fuel, compression ratio
and the load on the engine. The temperature increases with load and compression ratio. This
increases the thermal loss.
(Hi) Speed of the engine. The increase of the engine speed decreases the duration of
combustion hence decreases the heat loss.
(iv) Shape of the combustion space. The increase in ratio of combustion chamber surface
to volume decreases the heat loss. However, turbulence and flame propagation also effect the
heat transfer to combustion chamber wall.
(v) Size of the cylinder. The effect of cylinder size is rather complicated. An increase in
the cylinder size decreases the ratio of surface to volume but increases the frame travel. This
increases the combustion duration and hence engine speed is decreased.
(vi) Ignition timing in S.I. engines and fuel injection timing in C.I. engines. Proper
ignition and injection timings give rise in quicker combustion with less after burning and hence
less heat loss. The heat flow from the walls to fresh charge during suction stroke increases the
temperature of the charge and hence decreases the quantity of charge. This decreases the power
that the engine can develop.

Residual Gas

The residual gases left in the compression space from the previous cycle dilute the fresh
charge by increasing the amount of inert gases in it. This affects the ignition and combustion.
The residual gases also lower the volumetric efficiency of the suction stroke and raise the
temperature of the charge. Both these lower the amount of fresh charge induction.

Valve Resistance

In theoretical cycle of four-stroke engines, it is assumed that the exhaust and intake
pressure are equal to atmospheric. But the exhaust pressure is higher and the suction pressure
is lower than atmospheric pressure due to the resistances in exhaust and intake manifolds and
valves. The valve resistance affects the volumetric efficiency. The valve resistance causes the
pumping losses, which is the negative loop on the indicator diagram. The pumping losses
increase with an increase in speed. In two-stroke engines, the power consumption of scavenge
and charging pumps corresponds to the pumping losses in four-stroke engines.

Valve Timing.

In ideal cycle it is assumed that opening and closing of intake and exhaust valves take place
on dead centres. In actual case the exhaust valve closes and intake valve opens approximately
on TDC, but the opening of the exhaust valve and the closing of the intake valve vary
considerably from the BDC, depending principally on the desired speed. The net result due to
deviations of valve opening and closing other than at dead centres is that the indicator diagram
is rounded at the exhaust corner. This reduces the work output by 1 to 2%.

Combustion Time.

In ideal cycle it is assumed that the time of combustion is zero for constant-volume process
and combustion occurs at a rate necessary to maintain constant pressure during the constant
pressure process. Actually combustion process requires an appreciable amount of time, which
depends upon various factors. The increase in the combustion time decreases the ideal efficiency
by 2 to 3%.

Incomplete Combustion.

A volumetric analysis of the constituents of the products of combustion indicates an
incomplete combustion that amounts to about 2% of the heating value of the fuel. Mixture with
excess air tends to reduce this loss to zero; on the other hand rich mixtures result in considerable
unburnt fuel due to oxygen deficiency.

Atmospheric Conditions.

The temperature of air, humidity of air and barometric pressure affect the air charge. The
weight of the air charge found to be inversely proportional to the square root of the temperature,
especially in high-speed automobile engines.
For obtaining the performance at the standard conditions, the following corrections on
pressure, temperature and humidity are to be adopted.

Pressure. The standard pressure is taken as 760 mm of Hg. Adopting correction on observed
b.p.:
where p is the pressure in the test house, mm of Hg.
Temperature. The standard temperature is taken as 25°C

Humidity. The correction for water vapour pressure present in atmosphere is to be made
for getting accurate results. The vapour pressure can be obtained by knowing the wet-bulb and
dry-bulb temperatures and using psychometric chart.
If p_v is the vapour pressure in the test house in mm of Hg, then the corrected barometric
pressure of the test house IS p — Pv.

It can be seen that the effect of change of pressure is to increase or decrease the output
power as the level in the barometer rises or falls. The b.p. varies inversely as the absolute
temperature of the intake air.
Note: The units used in these expressions are the one actually used for the measurement
of the parameters.