Understanding Diesel Runaway: Causes, Solutions, and Prevention

A diesel engine is a type of internal combustion engine that operates on the principle of compression ignition. Unlike gasoline engines, which use spark plugs to ignite the air-fuel mixture, diesel engines rely on the heat generated by compressing air in the combustion chamber to ignite the fuel. This combustion process produces high pressure and drives the piston, which in turn powers the vehicle or machinery. Diesel engines are commonly used in trucks, buses, heavy machinery, and some passenger vehicles due to their efficiency and torque characteristics.

What is diesel runaway?

Diesel runaway, also known as engine runaway, is a potentially dangerous condition that can occur in a diesel engine. It happens when the engine starts to run uncontrollably and at an increasing speed, even when the ignition is turned off or the fuel supply is cut off. This can be caused by the engine ingesting an external fuel source, such as oil or hydraulic fluid, which acts as a fuel for the engine.

When a diesel engine runs on an external fuel source, it can continue to operate without any control or regulation. This is because diesel engines do not rely on a spark plug to ignite the fuel, but rather on the heat generated by compressing air in the combustion chamber. As long as there is a continuous source of fuel, the engine can keep running.

Diesel runaway can be extremely dangerous as the engine can reach very high speeds, leading to potential damage to the engine and other components. It can also pose a significant safety risk if the vehicle or equipment is not under control.

Diesel runaway can be caused by a few different factors. The most common cause is oil in the intake, which occurs when there is a leak that allows oil to mix with the air and enter the engine. Contaminated air, such as air mixed with natural gas or propane, can also cause a runaway. Another cause is a stuck accelerator pedal, where the pedal gets stuck in the open position and continuously supplies fuel to the engine. Older diesel engines with manual fuel pumps can experience a runaway if the pump fails and continuously supplies the engine with fuel. Overall, these causes result in the engine receiving fuel and air even when it is turned off, leading to uncontrollable RPMs and potential engine damage or accidents.

What is it look like and what to do ?

Diesel runaway can manifest in several ways, but some of the sign are:

  1. Uncontrolled Acceleration: The engine starts to rev up rapidly, even when the throttle is not being applied. The RPM (revolutions per minute) may increase to dangerous levels.
  2. Difficulty in Shutting Down: When attempting to turn off the engine by turning the ignition key or using the emergency shut-off, the engine continues to run at high speed, unaffected by these actions.
  3. Thick Smoke: The exhaust may emit thick, black smoke due to the excessive fuel being burned in the engine.
  4. Loss of Control: The vehicle or equipment may become difficult to control due to the sudden increase in engine speed. Braking and steering may be compromised.
  5. Overheating: The engine may start to overheat due to the increased workload and lack of control over the fuel supply.

To stop a diesel runaway, you have a few options:

  1. Cut off fuel and air: The fastest way to stop a diesel runaway is to cut off the fuel and air supply. Find the source of the fuel leak and stop it as safely as possible. If you cannot stop the fuel and air intake, you can let it burn in a safe location and extinguish the flames with a CO2 extinguisher.
  2. Get off the road: If the runaway occurs while driving, pull over as soon as possible. Slowly remove your foot from the accelerator and apply the brakes. After stopping, get out of the vehicle and contact your local fire department or others who can assist.
  3. Stop the engine: There are various ways to stop the engine. You can spray a fire extinguisher on the intake or throughout the engine to replace the oxygen with carbon dioxide. Another option is to physically block the intake with a rag or other object to stop the airflow. If your vehicle has a manual transmission, you can hold down the brakes, put the vehicle into high gear, and let go of the clutch to cut the engine off.

Remember, preventing diesel runaway is also important. Regular maintenance and monitoring of the intake hose and turbo can help prevent this dangerous problem.  Another way to prevent diesel engine runaway is by installing devices on the engine’s air intake hose that can sense overspeed and close off the air supply. These devices can shut down the engine in a safe and rapid manner. One such device is the Pacbrake’s PH2 PowerHalt Shut-Off Valve. This device is installed on the engine’s air intake hose and senses overspeed. When activated at a prescribed RPM, the valve plate closes, cutting off airflow and choking the engine, effectively shutting it down in a safe and rapid manner. Installing a shut-off valve like this is especially recommended for older rigs with high mileage, engines with excessive blow-by, or trucks with high horsepower used for competitions. It not only prevents catastrophic engine failure but also protects the occupants from potential harm caused by an engine explosion.

 

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