Diesel layout, starting and fuel supply systems

A diesel power plant uses a compression-ignition (CI) engine coupled to an alternator. Air compressed in the cylinder becomes hot enough to ignite the injected fuel; the engine's shaft power is converted to electrical power by the alternator. Plants are usually 2–50 MW and are used for standby, peak load and isolated supply.

General layout

 Bulk tank -> Filter -> Day tank -> Inj. pump
                                       |
                                       v
 Air filter -> Supercharger -> +--------+
                               | DIESEL |==> ALTERNATOR
 Comp. -> Air bottle --------> | ENGINE |
                               +--------+
                                |  |  |
      Lube tank, filter, cooler-+  |  +-> Silencer->Stack
                                   v
      Cooling tower <- Heat exch <- Jacket pump

Main components: engine, alternator, air intake system (filter, supercharger), fuel system, exhaust system (silencer, stack), cooling system, lubricating system, starting system and governing system.

Starting system

A diesel engine cannot start by itself because the high compression needs an external torque to turn it to a speed at which compression temperature ignites the fuel. Methods:

1. Compressed-air starting (most common for large plants): an air compressor charges air bottles to about 17–25 bar. To start, a starting valve admits this air to some cylinders in firing order; the air pushes the pistons and rotates the crankshaft. When the engine reaches firing speed, fuel is injected and air supply is cut off.
2. Electric starting: a battery-driven DC motor turns the engine through a gear on the flywheel; used for small engines.
3. Auxiliary petrol engine: a small engine cranks the main engine through a clutch; rarely used now.

Fuel supply system

1. Fuel oil arrives by tanker and is pumped through a strainer into the bulk storage tank.
2. A transfer pump sends it through filters to a day tank (holding about one day's need), placed higher so oil flows by gravity.
3. From the day tank oil passes through fine filters to the fuel injection pump, which raises the pressure (about 100–200 bar or more) and meters the correct quantity according to the governor setting.
4. The injector (atomiser) sprays fine droplets into the hot compressed air at the right instant, where it ignites.
5. Excess oil from the injectors returns to the day tank. Heaters are used for heavy oils to reduce viscosity.

The fuel system may be a common-rail, individual pump or distributor type injection system.