Large marine diesel engines use high pressure compressed air to start them. The air flows into the cylinder when the piston is moving down the cylinder on the power stroke. To minimise the risk of an air start explosion, fuel is not injected into the cylinder whilst the air is being admitted.
Air start systems vary in their design and can be quite complex. There will be a means to start the engine locally as well as from a remote location (The Bridge or the engine control room). This system is not representative of one type of engine but is simplified to give a basic understanding.
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