Internal Combustion Engines

Internal-combustion engines are machines that convert heat into mechanical energy. In internal-combustion engines, burning of the fuel inside a tightly closed cylinder results in expansion of gases. The pressure created on top of a piston makes it move. The back-and-forth motion of a piston is known as the reciprocating motion (straight-line motion). This motion must be changed to rotating (turning) motion to perform a useful function, such as propelling a ship or driving a generator to produce electricity.

All internal-combustion engines rely on three things: fuel, air and ignition. Fuel contains energy for engine operation, air contains oxygen necessary for combustion, and ignition starts the process of combustion.

All internal-combustion engines consist of one or more cylinders that are closed off at one end and have a piston driving up the other end. Cylinders may be arranged either in a straight line (in-line) or in a V shape. When a piston slides downward as a consequence of the pressure of expanding gases inside a cylinder, the upper end of the connecting rod moves downward together with the piston. The lower end of the connecting rod moves down in a circular motion. This makes the crankshaft rotate.

There are different kinds of internal-combustion engines. The most commonly used nowadays are diesel and petrol engines. Diesel engines are extensively used in ship propulsion.

FOUR-STROKE CYCLE

The four-stroke engine was first introduced by Nikolaus Otto at the end of 19th century and since then it has also been known as the Otto cycle. The commonly used term, however, is “four-stroke”. It takes its name from the four strokes of the piston needed to complete the processes of converting fuel energy into work. The four strokes of the piston are known as the suction (intake or induction) stroke, the compression stroke, the power stroke and the exhaust stroke.

(a) SUCTION. During this stroke, the crankshaft rotates clockwise and the piston moves down the cylinder. The inlet valve is open and a fresh air charge is drawn into the cylinder.

(b) COMPRESSION. The inlet valve closes and the air charge is compressed by the piston moving up. Its pressure and temperature increase. By the time the piston approaches the cylinder top, known as Top Dead Centre (TDC), the pressure is over 100 bar.

(c) POWER. Just before TDC, fuel is injected into the cylinder by the fuel injector. The fuel is atomised into tiny droplets. They are very small so they heat up very quickly and then start to burn. The expanding gases force the piston down the cylinder, thus turning the crankshaft. During this stroke work is put into engine.

(d) EXHAUST. When the piston approaches the bottom of the cylinder, known as Bottom Dead Centre (BDC), the exhaust valve starts to open and the hot gases are expelled from the cylinder.

Adapted from: MARENG Software