Diesel engine runaway is a rare condition affecting

diesel engine The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a so-call ...

s, in which the engine draws extra fuel from an unintended source and overspeeds at higher and higher

RPM Revolutions per minute (abbreviated rpm, RPM, rev/min, r/min, or with the notation min−1) is a unit of rotational speed or rotational frequency for rotating machines. Standards ISO 80000-3:2019 defines a unit of rotation as the dimensionl ...

, producing up to ten times the engine's rated output until destroyed by

mechanical failure Structural integrity and failure is an aspect of engineering that deals with the ability of a structure to support a designed structural load (weight, force, etc.) without breaking and includes the study of past structural failures in order to ...

or bearing seizure due to a lack of lubrication.

Hot-bulb engine The hot-bulb engine is a type of internal combustion engine in which fuel ignites by coming in contact with a red-hot metal surface inside a bulb, followed by the introduction of air (oxygen) compressed into the hot-bulb chamber by the rising p ...

s and

jet engines A jet engine is a type of reaction engine discharging a fast-moving jet of heated gas (usually air) that generates thrust by jet propulsion. While this broad definition can include rocket, water jet, and hybrid propulsion, the term typicall ...

can also run away via the same process.

Causes

In a diesel engine, the torque and the rotational speed are controlled by means of ''quality torque manipulation''. This means that, with each intake stroke, the engine draws in air which is not mixed with fuel; the fuel is injected into the cylinder ''after'' its contents have been compressed during the compression stroke. The high air temperature near the end of the compression stroke causes spontaneous combustion of the mixture as the fuel is injected. The output torque is controlled by adjusting the mass of injected fuel; the more fuel injected, the higher the torque produced. Adjusting the amount of fuel received per stroke alters the quality of the air-fuel-mixture, and adjusting the amount of the mixture itself is not required which is why diesel engines do not have throttle valves. Diesel engines can combust a large variety of fuels, including many sorts of oil, petrol, and combustible gasses. This means that if there is any type of leak or malfunction that increases the amount of oil or fuel unintentionally entering the combustion chamber, the quality of the air-fuel-mixture will increase, causing torque and rotational speed to increase. Fuel and oil leaks causing engine runaways can have both internal and external causes. Broken seals or a broken turbocharger may cause large amounts of oil mist to enter the inlet manifold, whereas defective injection pumps may cause an unintentionally large amount of fuel to be injected directly into the combustion chamber. If a diesel engine is operated in an environment where combustible gases are used, a gas leak may result in an engine runaway if the gas can enter the engine's inlet manifold.

Stopping a runaway engine

Several ways to stop a runaway diesel engine are to block off the air intake, either physically using a cover or plug, or alternatively by directing a

fire extinguisher A fire extinguisher is a handheld active fire protection device usually filled with a dry or wet chemical used to extinguish or control small fires, often in emergencies. It is not intended for use on an out-of-control fire, such as one which ha ...

into the air intake to smother the engine. Engines fitted with a decompressor can also be stopped by operating the decompressor, and in a vehicle with a manual transmission it is sometimes possible to stop the engine by engaging a high gear (i.e. 4th, 5th, 6th etc.), with foot brake and parking brake fully applied, and quickly letting out the clutch to slow the engine RPM to a stop, without moving the vehicle. This should be the last option because it can result in catastrophic damage to the whole transmission, mainly the gearbox, but this operation can save the engine.

Notable incidents involving diesel engine runaway

In the

Texas City Refinery explosion The Texas City Refinery explosion occurred on March 23, 2005, when a vapor cloud of natural gas and petroleum ignited and violently exploded at the isomerization (ISOM) process unit at the BP Texas City refinery in Texas City, Texas, killing 15 ...

, an instance of diesel engine runaway is thought to have provided the ignition source that triggered the massive explosion. After the refinery's

blowdown stack A blowdown stack is an elevated vent or vertical stack that is used to vent the pressure of components of a chemical, refinery or other plant if there is a process problem or emergency. A blowdown stack can be used to complement a flare stack or a ...

failed and started releasing

raffinate In chemical separation terminology, the raffinate (from French ''raffiner'', to refine) is a product which has had a component or components removed. The product having the removed materials is referred to as the extract. For example, in solvent ext ...

into the air, a pickup truck that had been parked near the

with its engine idling was engulfed by the vapor cloud released and the engine began to race. As staff at the refinery attempted to stop the truck's now-overheating engine, it

backfired "Backfired" is the debut solo single from American singer and Blondie vocalist Debbie Harry. Released in 1981, it was taken from her debut solo studio album, ''KooKoo''. Song information "Backfired" peaked at number 32 in the UK, and number 43 ...

, igniting the vapor cloud and triggering the disaster.U.S. Chemical Safety and Hazard Investigation Board. Investigation Report - Refinery Fire and Explosion and Fire. BP Texas City March 23, 2005, para 2.5.13 Ignition Source, p66

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