HANSA 06-2019

Schiffstechnik | Ship Technology
Figure 1: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 150 bar
valve is opened. The fuel energy released
in this way is most efficiently
converted to mechanical energy, which
is delivered to the piston.
••
Late part of the fuel injection profile:
fuel is injected and burned after the piston
has left TDC. The cylinder volume
is larger at this time and the combustion
gasses resulting from burning this portion
of the fuel will not expand and cool
to the same degree as earlier. The energy
in this portion of the fuel is less efficiently
converted to mechanical energy.
We want to inject and burn the fuel
quickly to achieve the highest efficiency
and power output. This is limited by the
maximum cylinder pressure that the engine
can withstand. Thus, it is important
to have an engine that is designed for a
high maximum cylinder pressure as can
be seen in Figures 1 and 2. In Figure 1,
combustion – and thereby heat release
– continues until the cylinder volume
is 0.28 times the maximum cylinder
volume. The last portion of fuel that is
burned can therefore expand by a ratio
of 1.00/0.28 = 3.6. Compare this to the
200bar engine illustrated in Figure 2.
In Figure 2, the same amount of fuel has
been injected as in Figure 1, but fuel injection
and combustion can occur faster than
in Figure 1 without exceeding the maximum-allowable
engine cylinder pressure.
Combustion and heat release finish at a
cylinder volume of 0.22 times the maximum
cylinder volume, which results
in a higher expansion ratio of 1.00/0.22
= 4.5 for the last portion of fuel burned.
This means that more of the chemical energy
in the fuel is extracted as mechanito
achieve high engine efficiency and power, it is important to have an engine that is designed for a
high maximum cylinder pressure; see Figure 1 and Figure 2.
In Figure 1, combustion – and thereby heat release – continues until the cylinder volume is 0.28 times
the maximum cylinder volume. The last portion of fuel that is burned can therefore expand by a ratio
of 1.00/0.28 = 3.6. Compare this to the 200 bar engine illustrated in Figure 2.
© MAN
Pressure as a function of cylinder volume for an engine
Figure 1: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 150 bar
with a maximum cylinder pressure of 150 bar
In Figure 1, combustion – and thereby heat release – continues until the cylinder volume is 0.28 times
the maximum cylinder volume. The last portion of fuel that is burned can therefore expand by a ratio
of 1.00/0.28 = 3.6. Compare this to the 200 bar engine illustrated in Figure 2.
Pressure as a function of cylinder volume for an engine
with a maximum cylinder pressure of 200 bar
Figure 2: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 200 bar
However, determination of the overall
efficiency effects of changes of the engine
type and layout is not as straightforward
and is the focus of this article.
The combustion engine’s basic job is
to burn fuel in a small volume and subsequently
expand the resulting, hot,
high-pressure combustion gases to a
larger volume, while delivering power
to that part of the combustion chamber
wall that provides the expansion
– namely the piston. It should be understood
that a high expansion ratio is
important in order to achieve high engine
efficiency. That is, the greater the
extent that the hot gasses can be expanded
and cooled before the exhaust
valve is opened, accordingly the more
energy can be extracted from the gases
and the higher the engine efficiency
and the colder the exhaust gas will be.
Figure 2: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 200 bar
Derating
In principle, it would be ideal to burn the
fuel in a volume so small that the hot,
high-pressure combustion gasses could
expand until the cylinder pressure reached
atmospheric pressure before the exhaust
valve was opened. This is not feasible for
a number of reasons that are outside the
scope of this article. However, it is relevant
to mention that if such small combustion
chambers were used, it would reduce the
amount of oxygen in the chamber unless
the pressure was very high – higher than
possible from an engine-design point of
view.Accordingly, a small combustion
chamber means a reduced amount of oxygen,
which itself means that it is not possible
to burn as much fuel. The result is
a lower engine torque and thereby lower
power-output. Despite this, the chamber
volume is often reduced to some extent
– this is called derating. A derated, marine
two-stroke engine has a combustion
chamber that is smaller than a fully-rated
version of the same engine type and this
is done to increase the expansion ratio,
and thereby the efficiency of the engine,
at the expense of a lower power-output.
High maximum cylinder-pressure
In the previous explanation, it was assumed
that all the fuel is burned when
the cylinder volume is at its smallest, at
the position in the engine cycle called
TDC (Top Dead Centre). In terms of engine
efficiency, it would also be beneficial
to burn all the fuel at TDC.
In an engine in operation the fuel cannot
all be delivered and burned at once.
Even if possible, it would result in cylinder
pressures that were too high. The fuel
is therefore injected following a carefully
designed injection profile. This gives a
high engine efficiency and power output
without exceeding the physical limits of
the engine and other restraints such as
exhaust-emission requirements. In order
to better understand the positive effect on
fuel consumption of an engine that can
withstand a high maximum cylinder
pressure, let’s take a look at the fuel-injection
process:
••
Early part of the fuel-injection profile:
fuel is injected and burned near TDC
and will expand and cool to the greatest
possible extent before the exhaust
HANSA International Maritime Journal 06 | 2019
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