Engines

There are two main types of the engines: Otto engines and Diesel ones. The engine operating on heavy fuel is called Diesel engine or internal combustion engines. The heavy fuel is injected in atomized state into the cylinder at the end of the compression stroke and ignites spontaneously in the hot compressed air. Each engine consists of (has) moving and stationary parts. The stationary parts are: cylinder liners and cylinder heads, block of cylinders, a crankcase, a bed plate, a frame. The main moving parts are: a crankshaft, a connecting rod, a piston, bearings and valves.

There are two types of the ICE four-and two-stroke diesel engines. The working cycle of the four-stroke engine is completed during four strokes of the piston. During the first stroke the position of the piston is at top dead centre and the intake valve is open. The piston moves down and air enters the cylinder liner. This stroke is called suction stroke. During the second stroke the piston moves upward, all the valves are closed and air is compressed. As a result of the compression the temperature and pressure are increased. This stroke is a compression stroke. At the end of compression stroke the injector supplies some atomized fuel into the cylinder. The process of combustion starts. The gases expand and push the piston downward. This stroke is called a combustion stroke. The fourth stroke is exhaust stroke during which the piston goes up discharging all products of combustion out of the cylinder through the opened exhaust valve. At the same time the crankshaft makes two revolutions completing the cycle.

With two-stroke engines the working cycle is completed in two strokes of the piston and one revolution of the crankshaft. There are some differences in construction of the cylinder liners. With this construction the liners have scavenge and exhaust ports instead of valves.

The cycle starts when the piston is at its bottom dead centre and the ports are completely open. Here scavenging takes place that continues until the piston moves upward and closes the scavenging ports. Then the piston closes the exhaust ports and the air in the cylinder is compressed. When the piston is at the top dead position injection of the fuel takes place and process of combustion occurs. The products of combustion expand. The piston moves down opening the exhaust ports and the exhaust gases leave the cylinder. Then the piston opens the scavenge ports and fresh air rushes into the cylinder taking away all remaining products of combustion.

Theoretically, with the same size of the cylinder and an equal speed a two-stroke engine can develop a power two times that of a four-stroke engine.

Scavenging

Scavenging is a process of removing any products of combustion from the cylinder by supplying the fresh air into the liner. Scavenging is used only in two-stroke engines where the cylinder liner has scavenging and exhaust ports. There are three types of scavenging which are used in the diesel engines. They are: cross scavenging, loop scavenging and uniflow scavenging.

With cross scavenging the scavenge and exhaust ports are placed on the opposite sides of the liner, with loop scavenging they are placed on one side. With these types of scavenging the construction of the cylinder heads is simple because it doesn’t carry any valves. But the efficiency of scavenging is low.

The uniflow scavenging is more efficient due to the different design of the cylinder liner. The scavenge ports are placed at the bottom of the liner on circumference and there is an exhaust valve in the cylinder head. It makes the construction of the head more complicated but the efficiency increases. It reaches 100%.

scavenging	['skxvinNgiN]	продувка
remove	[ri'mHv]	удалять
supply	[sq'plai]	подавать
use	[jHz]	использовать
cross	[krOs]	поперечная
loop	[lHp]	петлевая
uniflow	['jHniflou]	прямоточная
place	[pleis]	размещать
opposite	['Opqzit]	противоположный
simple	[simpl]	простой
carry	[kxri]	нести
efficiency	[i'fiSqnsi]	эффективность, кпд
low	[lou]	низкий
circumference	[sq'kAmfqrqns]	окружность
complicate	['kOmplikeit]	усложнять