Diesel engines have as many different types of starting circuits as there are types, sizes, and
manufacturers of diesel engines. Commonly, they can be started by air motors, electric motors,
hydraulic motors, and manually. The start circuit can be a simple manual start pushbutton, or
a complex auto-start circuit. But in almost all cases the following events must occur for the
starting engine to start.
1. The start signal is sent to the starting motor. The air, electric, or hydraulic motor,
will engage the engine's flywheel.
2. The starting motor will crank the engine. The starting motor will spin the engine
at a high enough rpm to allow the engine's compression to ignite the fuel and start
the engine running.
3. The engine will then accelerate to idle speed. When the starter motor is overdriven
by the running motor it will disengage the flywheel.
Because a diesel engine relies on compression heat to ignite the fuel, a cold engine can rob
enough heat from the gasses that the compressed air falls below the ignition temperature of the
fuel. To help overcome this condition, some engines (usually small to medium sized engines)
have glowplugs. Glowplugs are located in the cylinder head of the combustion chamber and use
electricity to heat up the electrode at the top of the glowplug. The heat added by the glowplug
is sufficient to help ignite the fuel in the cold engine. Once the engine is running, the glowplugs
are turned off and the heat of combustion is sufficient to heat the block and keep the engine
Larger engines usually heat the block and/or have powerful starting motors that are able to spin
the engine long enough to allow the compression heat to fire the engine. Some large engines use
air start manifolds that inject compressed air into the cylinders which rotates the engine during
the start sequence.