9

ENGINE PERFORMANCE AND OPERATION
A. COMBUSTION, AND EFFICIENCY 9A1. Combustion. Engine efficiency is
a comparison of the amount of power
developed by an engine to the energy
input as measured by the heating value
of the fuel consumed. In order to
understand the various factors
responsible for differences in engine
efficiency, it is necessary to have some
knowledge of the combustion process
which takes place in the engine.
In the diesel engine, ignition of the fuel
is accomplished by the heat of
compression alone. To support
combustion, air is required.
Approximately 14 pounds of air are
required for the combustion of 1 pound
of fuel oil. However, to insure complete
combustion of the fuel, an excess
amount of air is always supplied to the
cylinders. The ratio of the amount of air
supplied to the quantity of fuel injected
during each power stroke is called the

relation to the position of the piston.
2. The fuel must enter the cylinder in a
fine mist or fog.
3. The fuel must mix thoroughly with the
air that supports its combustion.
4. Sufficient air must be present to assure
complete combustion.
5. The temperature of compression must
be sufficient to ignite the fuel.
Figure 9-1 is a reproduction of a
pressure-time diagram of a mechanical
injection engine. The lower curvy part of
which is a dotted line, is the curve of
compression and expansion when no fuel
is injected. At A the injection valve
opens, fuel enters the combustion
chamber and ignition occurs at B. The
pressure from A to B should fall slightly
below the compression curve without
fuel due to absorption of heat by the fuel
from the air. The period from A to B is
the ignition delay. From B the pressure
rises rapidly until it reaches a maximum
at C. This maximum, in some instances,
may occur at top dead center. At D the
injection valve closes, the fuel is cut off,
but burning of the fuel continues to some
undetermined point along the expansion
stroke.
The height of the diagram from B to C is

causing it A black-smoking cylinder
usually shows a higher exhaust
temperature which can be observed
from pyrometers installed in the
individual exhaust lines from the
cylinders. Opening the indicator cock
on each cylinder to observe the color of
the exhaust is another check. Still
another method is cutting off the fuel
supply to one cylinder at a time to see
what effect it has on the engine
exhaust. This latter should never be
done when the engine is operating at
full load as overloading of the other
cylinders will result if the engine is
governor controlled.
9A2. Engine losses. It is obvious that
not all of the heat content of a fuel can
be transferred into useful work during
the combustion process. The many
different losses that take place in the
transformation of heat energy into work
may be divided into two classes,
thermodynamic and mechanical. The

radiation and convection to the
surrounding air.
2. Heat rejected and lost to the
atmosphere in the exhaust.
3. Inefficient combustion or lack of

according to the part of the cycle in
which they occur. The 175
heat appearing in the jacket cooling
water is not a true measure of cooling
loss because this heat includes:
1. Heat losses to jackets during
compression, combustion, and
expansion phases of the working cycle.
2. Heat losses during the exhaust
stroke.
3. Heat losses absorbed by the walls of
the exhaust passages.
4. Heat generated by piston friction on
cylinder walls.
Heat losses to the atmosphere through
the exhaust are inevitable because the
engine cylinder must be cleared of the
still hot exhaust gases before another
fresh air charge can be introduced and
another power stroke begun. The heat
lost to the exhaust is determined by the
temperature within the cylinder when
exhaust begins. It depends upon the
amount of fuel injected and the weight
of air compressed within the cylinder.
Improper timing of the exhaust valves,
whether early or late, will result in

extensively in connection with engine
performance and various types of
efficiencies. It may be defined as the
ratio of the total volume of a cylinder to
the clearance volume of the cylinder. It
may be best explained by reference to the
designer. It is essential that the valve be
tight and properly timed in order to
maintain the loss to the exhaust at a
minimum. This is also true for air inlet
valve setting on 4-cycle type engines.
If an indicator card is taken of a diesel
engine cylinder, it is possible to
calculate the horsepower developed
within the cylinder. This calculation
does not take into account the power
loss resulting from mechanical or
friction losses, as will be discussed
later, but it reflects the actual work
produced within the cylinder.
Mechanical losses are of several kinds,
not all of them present in every engine.
The sum total of these mechanical
losses deducted from the indicated
horsepower developed in the cylinders
will give the brake horsepower finally
delivered as useful work by the engine.
These mechanical or friction losses
include bearing friction, piston and
piston ring friction, and

amount of heat input and the amount of
heat rejected. For example, in Figure 9-4,
the T-S diagram of a modified diesel
cycle, the heat input is represented by the
area FBDG and the heat rejected to the
exhaust by the area FAEG. The heat
represented in doing useful work is
represented by the difference between
these two, or area ABDE. The efficiency
of the cycle can then be expressed as
(H
1
-H
2
)/H
1
where H
1
is the heat input
along lines BC and CD (the lines
representing the constant volume and
constant pressure combustion), and H
2
is
the heat rejected along line EA (the line
representing the constant volume
exhaust). Since heat and temperature are
proportional to each other, the cycle
efficiency is actually computed from
measurements made of the temperature.

gas.
c. Volumetric efficiency. The volumetric
efficiency of an engine is the ratio of the
volume that would be occupied by the air
charge at atmospheric temperature and
pressure to the cylinder displacement (the
product of the

Figure 9-4. Temperature-entropy
diagram of modified diesel cycle. 177
area of the bore times the stroke of the
piston). The volumetric efficiency
determines the amount of air available
for combustion of the fuel, and hence
influences the maximum power output
of the engine.
Volumetric efficiency is actually the
completeness of filling of the cylinder
with fresh air at atmospheric pressure.
The volumetric efficiency of an engine
may be increased by enlarging the areas
of intake and exhaust valves or ports,
and by having all valves properly timed
so that as much air as possible will
enter the cylinders. Since any burned
gases will reduce the charge of fresh
air, the supercharging effect gained by

d. Thermal efficiency. Thermal
efficiency may be regarded as a
measure of the efficiency and
completeness of combustion of the
injected fuel. Thermal efficiencies are
generally considered as being of two
kinds, indicated thermal efficiency and
over-all thermal efficiency.
If all the potential heat in the fuel were
delivered as work, the thermal
efficiency would be 100 percent. This
is not possible in practice, of course. To
determine the values of the above
efficiencies the amount of fuel injected
is known, and from its heating value, or
Btu per pound, the total heat content of
the injected fuel can be found. From the
mechanical equivalent of heat (778
foot-pounds are equal to 1 Btu), the
number of foot-pounds of work
contained in the fuel can be computed.
If the amount of fuel injected is
measured over a period of time, the rate
at which the heat is put into the engine
can be converted into potential power.
Then, if the indicated horsepower
developed by the engine is
is equal to the indicated horsepower
minus the mechanical losses. The ratio of
brake horsepower to indicated


9B1. Engine performance. a. General.
Many factors affect the engine
performance of an engine. Some of
these factors are inherent in the engine
design; others can be controlled by the
operator. The following list of variable
conditions affecting the performance of
a diesel engine is not complete, but
contains all the important factors that
should be familiar to operating
personnel.
b. Fuel characteristics. The cetane
number of the fuel has an important
effect on engine performance. Fuels
with low cetane rating have high
ignition lag. A considerable amount of
fuel collects in the combustion space
before ignition occurs, with the result

which the engine will operate with a
smoky exhaust.
f. Injection rate. The rate of injection is
important because it determines the rate
of combustion and influences engine
efficiency. Injection should start slowly
so that a limited amount of fuel will
accumulate in the cylinder during the
initial ignition lag before combustion
begins. It should proceed at such a rate

e. Injection timing. The injection timing
has a pronounced effect on engine
performance. For many engines, the
optimum is between 5 degrees to 10
degrees before top dead center, but it
varies with engine design. Early
injection tends toward the development
of high cylinder pressures, because the
fuel is injected during a part of the
cycle when the piston is moving slowly
and combustion is therefore at nearly
constant volume. Extreme injection
advance will cause knocking. Late
injection tends "to decrease the mean
indicated pressure (mip) of the engine
and to lower the power output.
Extremely late injection tends toward
incomplete combustion, as a result of
of the fuel particles affects the ignition
lag and influences the completeness of
combustion. Small-sized particles are
desirable because-they burn more
rapidly. Opposed to this requirement is
the fact that small particles have a low
penetration, and there is therefore a
tendency toward incomplete mixing of
the fuel and the combustion air, which
leads to incomplete combustion.
h. Combustion chamber design. The
amount of turbulence present in the
179
indicated pressure is that developed in
the cylinder and can be measured. The
other is bmep or brake mean effective
pressure and is computed from the bhp
delivered by the engine.
NOTE. Maximum pressure developed

single-acting, 2-stroke cycle engine, there
is a power stroke for each revolution.
Having defined the factors influencing
the power capable of being developed,
the general formula for calculating