Air Compressor for Marine Engine

Hi,
I know that an Air Compressor is used for starting Marine Engine. However, is it required for all types of diesel Marine Engine? Is it mandatory? How to choose the specification (capacity) of an air compressor required for a vessel? Thank you.

 

Air starting is used mainly for big engines, say power levels 500 kw and upwards, and for operating conditions where battery charging from a grid is not available. Normally the engine is delivered with the compressor integrated in the package. Starting air is stored in air tanks, which also can be charged from a separate compressor driven by a smaller auxiliary engine. The pressure level is often somewhat higher than normal "shop air" in order to give enough momentum for starting.

 

Actually, air is a good starting medium for any "need to run" engine. When I was at college. Webb Institute, the emergency power, IIRC, was a 4-71 with air start. On loss of power, the ABT would shift and the air valve would open, air would automatically start the engine, then the power generated would close the air valve and the flywheel mounted compressor would recharge the air bank. As a student, you had weekly maintenance duty, which tested the start system: very noisy.

Edit to add: ABT means Automatic Bus Transfer...My mind knew exactly what I was talking about.

 

My experience is with air start c 32 cats and a kta 38. They worked quite well and were fast on the rotation. Made for crisp starts, especially compared to my time with a 24v 3508. Both worked but the reserve tank and the bendix style compressor was much more trim than a large 24v starter and alternator. All the components were sourced and sized from cat and cummins respectively.

 

There are two air start systems, direct injection and geared pneumatic motors.

The first is used on big engines and injects compressed air into the cylinders in sequence, starting with the piston at TDC. The cylinder heads have dedicated injection ports, and there is an automatic distributor. The air is stored at ~30 bar, and SOLAS mandates sufficient capacity for minimum 12 starts without starting the compressors. This is a system that's designed together with the engine by the manufacturer, there is nothing you can or have to do.
There are other methods for starting such big engines, but direct air injection is preferred.
Big engines are not the only ones using direct air injection, for example the Vedeneyev M14P has one, but displaces only 10.16l (620cu in).

The second is used on small engines and is a retrofit, you can mount it on any engine. There are ready made starters for different size ranges (defined by either displacement or HP), with mounting adapters for different positions. The starter motor comes with an air consumption figure from wich you derive the needed storage and compressor size.
The reasons for using an air starter is explosion protection (usually in mines), black start capability (no electricity involved), or the presence of other pneumatic equipment.
Pneumatic starters stand in direct competition with spring and hydraulic starters, wich have the additional bonus of beeing capable of hand starting an engine without decompression levers. While in theory it's possible to pressurize the air reservoir by hand, in practice it's extremely rare and reserve bottles are used.