We have learnt about various components of a marine diesel engine till date including bedplate, crankshaft and learnt few tasks like taking crankshaft deflections and analyzing the same apart from ways to maintain the crankshaft. Now we move on to another important component of the engine namely the piston.
Broadly speaking a piston is a component which helps to convey the energy generated inside the combustion chamber of the engine to the external parts through the crankshaft out of the engine where it can be harnessed for any useful purpose which could be either moving the propeller inside the sea water in case of main propulsion plant or running an alternator to generate electricity in case of smaller auxiliary engines on board a ship. There are lots of aspects to be studied in context of a piston including their construction, design, cooling, cleaning, classification and so forth and we will take up these matters in the next few articles to follow.
The Marine Piston
Most of us own cars and even though we may never have repaired an engine ourselves, we know and have seen what a piston looks like. The marine pistons are not much different in their basic principles of operation but as they say – size does matter – and when the piston being talked about belongs to the main propulsion engine or main engine of a ship, the size is really huge. Whereas the piston of your automobile or car could fit into the palm of your hand (or both palms combined together) you need to be a monster to lift the marine piston of the bigger engines.
To have a rough idea about the dimensions we are talking about, just take a look at the image of a marine diesel engine piston shown below and you will know what I am talking about. You will get an idea about the relative size of the piston by considering the dimensions of the wall and ceiling shown in the background.
Of course this particular piston is really a huge one, possibly one of the largest in the world but even otherwise the size of a normal sized marine main diesel engine is comparable one.
What the Heck is it made up of?
Because of the huge size and the amount of stresses that a piston has to go through, it requires considerable effort to design and long performing piston. The various types of forces that a piston gets subjected to during its operation is as follows.
Thermal stresses are induced due to proximity to the place where the combustion takes place inside the cylinder. Infact the part which is at the top of the piston and in direct contact with the combustion chamber is known as the piston crown and we will learn more about it later.
There are dynamic forces due to up and down motion of the piston such as compressive forces and inertial forces.
The exact material to be used in a specific engine depends on factors such as size and speed on the engine. For example in case of larger slow speed engines used for main propulsion the weight of the piston does not matter so much unlike smaller higher speed engines where weight is an important parameter due to the relatively smaller size. While the former is normally made out of low alloy Steel along with Nickel, Chromium and Molybdenum, the latter is made out of light weight Aluminium alloys.
In the next article we will take up the working conditions of a piston and construction of various types of pistons commonly used in the marine industry.