In one of our previous articles, we learnt about the compressed air system on board a ship for control purposes, and the uses to which compressed air is put on board various types of ships. In this article we will learn about one of the most important uses of compressed air on board a ship, namely to start the diesel engines; which could be either main engine or auxiliary engines.
Diesel Engine Starting Problems
If you live in a very cold climate and own a diesel automobile such as a car or an SUV, I am sure you know what it means to start the engine on a cold morning. In some vehicles such as tractors and other diesel engine machines, there is a cranking arrangement wherein the driver can turn the engine by putting a rod which meshes with the crankshaft and an arrangement to rotate the whole thing is given like the adjacent picture where the person is starting a farming machine by rotating the engine by foot.
Of course in most situations batteries are used to crank the engine during starting and this is exactly what happens when you insert your ignition key in the car and simply go zip zap zoom. If you car has run out of battery, you could ask a couple of people to give it a push till some distance and then you engage the gears. In other cases such as small home generators you have a rope to pull with full force.
The net effect of all the above methods is just to give an initial rotation to the engine so that it starts and once the ignition has taken place, there is no need for further external power; rather the engine starts to supply power to the external source through its shaft.
Marine Engine Starting
It is all very well to start an automobile by pushing it across the road or a small boat overboard engine by pulling its string but how would you think of starting an engine as massive as the main engine of even an average sized ship, or the huge generator engines which supply power to the entire ship. You certainly would need Herculean power to achieve these results; which unfortunately is not a practical possibility.
Even the though of using batteries is not a practical reality because of the immense power required to rotate such an engine, it might require an army of batteries to achieve the same.
It is here that the compressed air comes into the picture and the literally invisible air shows its immense strength. Before we learn the exact mechanism by which compressed air system works, let us see why a marine diesel engine should be difficult to start in the first place.
- We know that inertia is a property which resists change in state of motion which means that anything at rest would resist motion and the initial energy required to put that object in motion is quite more as compared to the force required when it is set into motion.
- There is lubricating oil between all parts having relative motion. When the engine is running the oil is in a hot state and very viscous but the same oil gets thickened and very sticky when in stopped position.
- In the automobile it is possible to de-clutch the engine from the tyres thus effectively reducing the load to zero, but in case of main diesel engine it is not possible to cut of the propeller from the engine hence the load is always present.
- The movement of the engine means that the entire piston has to move up and down inside the engine cylinder which also has cold lubricating oil and offers much resistance.
Because of the above mentioned reasons the power of the compressed air is used to turn the engine for a short duration until ignition takes place and the engine is in a self sustaining mode. A lot of care has to be taken in designing this air starting system since an engine should be capable of being started from any position and not a particular position otherwise it would defeat the whole purpose of the setup.
In our next articles we will learn more about the starting arrangement of marine engines in the form of master air valve, cylinder air valves and their construction and working.