ENGINEMAN 1 & C
Effect of Unbalance
From the preceding discussion, it can be
readily seen that, in general, the result of un-
balance will be overheating of the engine. The
clearances established by the engine designer allow
for sufficient expansion of the moving parts when
the engine is operating at the designed
temperatures, but an engine operating at
temperatures in excess of those for which it was
designed is subject to many casualties. Excessive
expansion soon leads to seizure and burning of
the engine parts. Should the temperatures in the
crankcase rise above the flash point of the
lubricating oil vapors, an explosion may result.
High temperature may destroy the oil film be-
tween adjacent parts, and the resulting increased
friction will further increase the temperature.
4. Keep cylinder temperatures and pressures
as evenly distributed as possible.
5. Train yourself to recognize the symptoms
of serious engine conditions.
ENGINE EFFICIENCY
Engine efficiency is the amount of power
developed as compared to the energy input which
is measured by the heating value of the fuel con-
sumed. The term “efficiency” is used to designate
the relationship between the result obtained and
the effort expended to produce the result.
Since power is directly proportional to the mep
developed in a cylinder, any increase in mep will
cause a corresponding increase in power. If the
meps in the individual cylinders vary, power will
not be evenly distributed among the cylinders.
The term “compression ratio” is frequently
used in connection with engine performance.
From your study of the principles of internal com-
bustion, you will recall that compression ratio is
the ratio of the volume of air above the piston,
when the piston is at the BDC position, to the
volume of air above the piston when the piston
is at the TDC position.
EFFICIENCIES
The quality of combustion obtained depends
upon the heat content of the fuel. The amount
of heat available for power depends upon
temperature. Temperature varies directly as
pressure; therefore a decrease in pressure will
result in a decrease in temperature, and in poor
combustion. Poor combustion will cause lowered
thermal efficiency and reduced engine output.
The principal efficiencies which must be
considered in the internal combustion process are
cycle, thermal, mechanical, and volumetric.
Cycle Efficiency
Cylinder load balance is essential if the desired
efficiency and performance of an engine is to be
obtained. To avoid the harmful effects of
overloading and unbalancing of load, the load on
an engine should be properly distributed among
the working cylinders; and no cylinder, or the
engine itself, should ever be overloaded.
In general, load balance in an engine can be
maintained if the following procedures are
observed:
1. Maintain the engine in proper mechanical
condition.
2. Adjust the fuel system according to the
manufacturer’s instructions.
3. Operate the engine within the temperature
limits specified in appropriate instructions.
The efficiency of any cycle is equal to the out-
put divided by the input. The efficiency of the
diesel cycle is considerably higher than the Otto
or constant volume cycle because of higher com-
pression ratio and because combustion starts at
a higher temperature. In other words, the heat
input in a diesel engine is at a higher average
temperature. Theoretically, a gasoline engine
using the Otto cycle would be more efficient than
the diesel engine if equivalent compression ratios
could be used. However, engines operating on the
Otto cycle cannot use a compression ratio com-
parable to that of diesel engines because fuel and
air are drawn together into the cylinder and com-
pressed. If comparable compression ratios were
used, the fuel would fire or detonate before the
piston reached the correct firing position.
Since temperature and amount of heat con-
tent which is available for power are proportional
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