Two stroke engines are engines that require two strokes of the piston to complete one cycle of operation. In other words, the entire cycle of the engine operation which involves intake, compression, ignition and exhaust take place due to the movement of the piston in two strokes,(ie upward and downward movement of the piston in two strokes completes one cycle of operation). All diesel engines are compression ignition, a process in which fuel is injected after air is compressed to a very high temperature and pressure in the combustion chamber thus causing the fuel to self ignite.
Two stroke engines are simpler mechanically than four stroke engines and are widely used to power large ocean going ships. In a two stroke engine the cycles of operation which are, intake, compression, combustion and exhaust take place in one revolution of the piston( is upwards and downwards).
During the intake process, the piston moves close to the bottom dead centre, which is the lower part of the cylinder. The ports located in the cylinder walls are opened and air rushes into the combustion chamber in the cylinder. All two stroke engines require artificial aspiration which is the process of supplying high pressure air to the cylinder using a mechanically driven blower or turbo compressor. This process is also used to force out residual air trapped in the cylinder from the previous combustion cycle. The process of forcing out this residual air from the previous combustion process is known as scavenging.
As the piston rises, the air in the combustion chamber is now compressed to a very high temperature and pressure. Fuel is then injected and is immediately ignited due to the high temperature and pressure of the air. Explosion occurs and huge amounts of energy are released which drive the piston downwards.
As the piston moves downwards it reaches bottom dead centre where the exhaust ports are opened. Exhaust gases are expelled to clear the combustion chamber of these waste gases to clear the combustion chamber for fresh air to continue the combustion cycle.
The maintenance procedure for a two stroke marine diesel engine should be strictly carried out in accordance with laid down procedures. These are very complex procedures and must be carried out by experienced and qualified personnel.
The cylinder liner must be checked periodically for damage due to the high combustion temperature in the cylinder. The liner is an integral part of the combustion chamber where the fuel is ignited at high temperature and huge amounts of energy released. The liner must therefore be checked from time to time according to the manufacturer’s specification. The entire cylinder must be checked periodically for any damage which might affect the efficiency of the engine. These include; inspection of the the scavenge space, routine liner ovality check, piston overhaul, problem involving liner.
Scavenge Space inspection
This inspection is carried out every time after the scavenge space is cleaned from sludge and other deposits such as soot. After the cleaning, the responsible engineer will enter the scavenge space to inspect the general condition of the liner. For inspection these methods are used;
.In order to inspect a large area of the cylinder liner and piston, enter the scavenge air receiver and make observations from the exhaust side.
. Dismount the small covers from the scavenge air boxes and clean the openings.
. When the piston has been turned below the level of the scavenge air ports, inspect the cylinder liner walls and the piston crown.
. A tiltable mirror fixed to a telescopic rod can be used. Use a powerful light source for inspection.
Routine liner ovality check
Readings are taken with specialized instruments at port and starboard position in different levels to calculate the change in ovality of the liner.
When the piston is overhauled as per planned maintenance or due to breakdown, the liner ovality must be checked and the liner surface must be examined for any defects.
Problems like leaking liner water, O ring
These problems and others like cracked liner, piston blow past and others can be checked immediately to determine if the piston should be changed immediately or if sufficient repairs can be carried out to extend the working life of the piston.