Medium speed engines. Starting air
There are several methods of starting a diesel engine, including manual, electrical and mechanical devices. The techniques used on a particular engine depend largely on its size, design and service requirements. Small diesels, such as those employed in lifeboats etc., may well be hand, or perhaps electrically, started. In main propulsion engines, or even diesel generators, such methods are unable to supply the substantial torque required to overcome the inertia of the large masses involved. These engines usually employ a system using the energy stored in compressed air.
Where the main engine is of the direct drive reversible type, it is essential that it is capable of starting in either direction from any position of rest. To achieve this, it is necessary for each cylinder to be fitted with a starting air valve, the opening of which is dictated by a ‘distributor’. This distributor ensures that air is introduced into the relevant cylinder at the correct time to achieve staring in the desired direction from any position of rest. There will be an overlap period during which two cylinders, at the extremities of their air injection periods, will both receive air. This ensures positive staring in the correct direction (the starting sequence is the same as the firing order for the engine). The amount of overlap is dependent upon the number of cylinders, the timing of the exhaust opening and so on.
Modern practice is to introduce air into the cylinder slightly before TDC (the alignment of piston rod with con rod at this point is such that little, if any turning moment is developed). This allows the air to accumulate in the clearance volume ready to force down the piston once it is over TDC. At the same time, another cylinder will be receiving air (because of the overlap). This unit will be one in which the crank is well past TDC so that it generates an adequate turning moment to any the above unit over TDC. The first unit, already pressurised, will be able to accelerate the engine up to the ‘fuel initiation’ speed. The useful expansion of the starting air will cease at the opening of the exhaust. To continue air injection any further would be wasteful and futile. This limit is normal to 3-cylinder engines but is unnecessarily long in engines with more than three units.
A starting air pressure well below the compression pressure of an engine will be able to turn the engine over against the compression because the compression pressure is only reached towards the end of the stroke, whereas starting air is introduced for a much longer period of the stroke. The starting air ‘indicator’ diagram shows that there is a far greater energy release below the starting air curve than that required to achieve the compression. Areas below the curves represent, to scale, the energy involved in the relevant operation.
The momentum built up in the rotating elements of the crankshaft will help pin smooth starting once the initial inertia has been overcome.
Reversibility can be achieved by introducing air into a cylinder where the piston is approaching TDC, in the direction of rotation in which it was stopped. Exactly the same concepts as discussed above then apply, but in the reverse firing order. Control can be achieved through the distributor or by varying the position of the starting air cams (sliding cam shaft, usually independent of the fuel pump cam shaft). Lost motion clutches had some bearing on the distributor on some engines but the advent of constant pressure turbocharging has led to radical simplification in the design of lost motion clutches.
Where the starting air system is concerned, the following features are usually considered desirable.
To ensure that the cylinder valves open in the correct sequence, a distributor is required. The distributor provides the air start timing with correct overlap whether going ahead or astern. Distributors may be cylindrical or circular discs both suitably ported, or radially distributed spool type valves around a central cam, or perhaps similar spool type valves aligned above a laterally sliding independent cam shaft.