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A diesel engine protection systems and alarms

A diesel engine is designed with protection systems to alert the operators of abnormal conditions
and 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 to
overspeed. An overspeed condition is extremely dangerous because
engine failure is usually catastrophic and can possibly cause the engine to
fly apart.
An overspeed device, usually some type of mechanical flyweight, will act
to cut off fuel to the engine and alarm at a certain preset rpm. This is
usually accomplished by isolating the governor from its oil supply, causing
it to travel to the no-fuel position, or it can override the governor and
directly trip the fuel rack to the no-fuel position.

Water jacket -
Water-cooled engines can overheat if the cooling water system fails to
remove waste heat. Removal of the waste heat prevents the engine from
seizing due to excessive expansion of the components under a high
temperature condition. The cooling water jacket is commonly where the
sensor for the cooling water system is located.
The water jacket temperature sensors provide early warning of abnormal
engine temperature, usually an alarm function only. The setpoint is set
such that if the condition is corrected in a timely manner, significant
engine damage will be avoided. But continued engine operation at the
alarm temperature or higher temperatures will lead to engine damage.

Exhaust temperatures -
In a diesel engine, exhaust temperatures are very important and can
 provide a vast amount of information regarding the operation of the
engine. High exhaust temperature can indicate an overloading of the
engine or possible poor performance due to inadequate scavenging (the
cooling effect) in the engine. Extended operation with high exhaust
temperatures can result in damage to the exhaust valves, piston, and
cylinders. The exhaust temperature usually provides only an alarm

Low lube oil pressure -
Low oil pressure or loss of oil pressure can destroy an engine in short
order. Therefore, most medium to larger engines will stop upon low or
loss of oil pressure. Loss of oil pressure can result in the engine seizing
due to lack of lubrication. Engines with mechanical-hydraulic governors
will also stop due to the lack of oil to the governor.
The oil pressure sensor usually stops the engine. The oil pressure sensors
on larger engines usually have two low pressure setpoints. One setpoint
provides early warning of abnormal oil pressure, an alarm function only.
The second setpoint can be set to shutdown the engine before permanent
damage is done.

High crankcase pressure - 
High crankcase pressure is usually caused by excessive blow-by (gas
pressure in the cylinder blowing by the piston rings and into the
crankcase). The high pressure condition indicates the engine is in poor
condition. The high crankcase pressure is usually used only as an alarm

Most mid-sized to large diesel engines have (as a minimum) the following
protective alarms and trips.

Engine overspeed alarm/trip
High water jacket temperature alarm
High exhaust temperature alarm
Low lube oil pressure (alarm and/or trip)
High crankcase pressure alarm
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