Page 63 - Piston Engine-Based Power Plants

Diesel Engines 55

One of the advantages of large two-stroke engines is their simplic-
ity. The engines have no valves and so requires fewer parts. Another
major advantage is the ability of these engines to burn heavy fuel oil
with high efficiency. This heavy or residual fuel oil is the fraction left
after all the more valuable parts of crude oil have been removed by
fractional distillation. It is a cheap fuel but extremely dirty so plants
that burn it need extensive emission control facilities. This can be cost-
effective in a large plant whereas it might not be in a small diesel
engine generating unit.

The size of these engines the largest can have power generating
capacities of up to 80 MW means that it is also cost-effective to add
a bottoming cycle. This involves using waste heat from the engine to
generate steam which is then exploited to drive a small steam turbine,
creating a combined cycle power plant. This can add a few percentage
points to the overall efficiency in a large installation. This is illustrated
in Fig. 5.3 which compares the thermal efficiencies of medium- and
low (slow)-speed diesel power plants. As the curves in the
figure indicate, slow-speed diesel engines are the most efficient with
potential efficiencies of up to 60% when operated in combined cycle
mode. However practical engine efficiencies are lower than this with
the best achieved around 52%.

Thermal efficiencies % Low speed diesel engine
65 in combined cycle

60
Low-speed diesel engine
55

50
45
Medium-speed
40
diesel engine
35
30

25
20
1 5 10 50 100 500
Unit capacity (MW)
Figure 5.3 Comparative efficiency of low-speed diesel engines. Source: MAN Diesel and Turbo.