DIESEL ENGINE SPEED,DOE-HDBK-1018/1-93Diesel Engine FundamentalsFUEL CONTROLS, AND PROTECTIONStartingCircuitsDiesel engines have as many different types of starting circuits as there are types, sizes, andmanufacturers 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, ora complex auto-start circuit. But in almost all cases the following events must occur for thestarting 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 engineat a high enough rpm to allow the engine's compression to ignite the fuel and startthe engine running.3.The engine will then accelerate to idle speed. When the starter motor is overdrivenby the running motor it will disengage the flywheel.Because a diesel engine relies on compression heat to ignite the fuel, a cold engine can robenough heat from the gasses that the compressed air falls below the ignition temperature of thefuel. 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 useelectricity to heat up the electrode at the top of the glowplug. The heat added by the glowplugis sufficient to help ignite the fuel in the cold engine. Once the engine is running, the glowplugsare turned off and the heat of combustion is sufficient to heat the block and keep the enginerunning.Larger engines usually heat the block and/or have powerful starting motors that are able to spinthe engine long enough to allow the compression heat to fire the engine. Some large engines useair start manifolds that inject compressed air into the cylinders which rotates the engine duringthe start sequence.EngineProtectionA diesel engine is designed with protection systems to alert the operators of abnormal conditionsand to prevent the engine from destroying itself.Overspeed device -Because a diesel is not self-speed-limiting, a failure in the governor,injection system, or sudden loss of load could cause the diesel tooverspeed. An overspeed condition is extremely dangerous becauseengine failure is usually catastrophic and can possibly cause the engine tofly apart. ME-01Rev. 0Page 38
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