Editing Air-start system (section)

===Direct starting===
Compared to a [[gasoline]] (petrol) engine, a [[diesel engine]] has a very high [[compression ratio]], an essential design feature, as it is the heat of compression that ignites the fuel. An electric starter with sufficient power to turn a large diesel engine would itself be so large as to be impractical so there is a need for an alternative system.

An air start system has three main components along with various safety components, namely the air start injector, the distributor and the air receivers. When the system is initiated, starting air from the receivers is distributed by the distributor unit to each respective air start valve according to the firing order of the engine’s cylinders. Safety components include flame traps, to prevent air line explosions, a turning gear interlock which ensures that air-start cannot be initiated when the turning gear is engaged, relief valves and isolation valves, as well as drain valves at certain points along the system. Control components include the control valves and remote air-start valves.<ref>{{cite web| url=http://machineryspaces.com/starting-air-system.html|title=Starting air system for Marine diesel engine|publisher=MachinerySpaces.com|access-date=6 March 2023|archive-date=15 December 2022|archive-url=https://web.archive.org/web/20221215161933/http://www.machineryspaces.com/starting-air-system.html}}</ref> A direct start system as used on a marine slow-speed diesel is required to have up to 12 starts on a non-reversing engine or 6 starts on a reversible, or geared, engine.

When starting the engine, [[compressed air]] is admitted to whichever cylinder has a [[piston]] just over [[four stroke cycle|top dead center]], forcing it downward.<ref>{{cite web |url=http://www.marinediesels.info/2_stroke_engine_parts/Other_info/air_start.htm |title=The air start system at marinediesels.co.uk How a marine diesel engine starts using compressed air |publisher=Marinediesels.info |access-date=2013-04-15 |archive-date=2013-04-28 |archive-url=https://web.archive.org/web/20130428153825/http://www.marinediesels.info/2_stroke_engine_parts/Other_info/air_start.htm |url-status=dead }}</ref> As the engine starts to turn, the air-start valve on the next cylinder in line opens to continue the rotation. After several rotations, fuel is injected into the cylinders, the engine starts running and the air is cut off.

To further complicate matters, a large engine is usually "blown over" first with zero fuel settings and the indicator cocks open, to prove that the engine is clear of any water build up and that everything is free to turn. After a successful blow ahead and a blow astern, the indicator cocks are closed on all the cylinders, and then the engine can be started on fuel. Significant complexity is added to the engine by using an air-start system, as the [[cylinder head]] must have an extra valve in each cylinder to admit the air in for starting, plus the required control systems. This added complexity and cost limits the use of air-starters to very large and expensive reciprocating engines.
[[File:VideoGaliAir.webm|thumb|Air starter on Caterpillar Diesel Engine]]