Page 61 - Piston Engine-Based Power Plants

Diesel Engines 53

Diesel engines can be built to larger sizes than spark ignition
engines, with high-speed machines available in sizes up to 4 MW and
slow-speed diesels up to 80 MW. Large slow-speed engines can have
enormous cylinders. For example, a nine-cylinder, 24 MW engine used
in a power station in Macau has cylinders with a diameter of 800 mm.

Diesel engines can burn a range of diesel fuels including both oil-
derived fuels and biofuels. Smaller, high-speed engines normally use
high-quality distillate but the large slow-speed engines can burn very
low-quality heavy fuel oils which require a much longer combustion
time to burn completely. These fuels tend to be dirty and plants burn-
ing them usually require additional emission mitigation measures.

ENGINE TIMING AND SPEED CONTROL

The combustion of fuel in a diesel engine takes place after the fuel
has been injected into the hot compressed air within the cylinder of
the engine. However combustion will only take place once the fuel
has vaporised. This takes time because vaporisation of the initial dro-
plets cools the compressed gas around it. In order to take account of
this and ensure that the combustion takes place at the correct position
of the piston during the cycle, fuel injection begins before the piston
3
has reached TDC and finishes at TDC or marginally afterwards.
Once ignition of the fuel has started, the additional heat from the
combustion process helps vaporise the remaining fuel and this speeds
up the process. As the piston continues towards BDC, the exhaust
valve will start to open around three-quarters of the way though the
stroke. This allows the combustion gases to start to escape, forced out
by the high pressure and temperature within the cylinder.

As already noted, the speed at which a diesel engine will run is
determined mainly by the amount of fuel available. The high compres-
sion ratio used in the engines means that there will always be sufficient
oxygen available within the cylinder to burn all the fuel, so this will
never be a limiting factor. Primary speed control takes place by con-
trolling the amount of fuel available to the cylinders. However speed is
likely to vary for any fixed rate of fuel feed as engine conditions

3 Timing is usually measured in terms of the rotational position of the crankshaft. Injection of fuel
will start around 28 before the piston reaches TDC and ends at 3 after TDC.