Although it’s not an everyday occurrence, engine overspeed can have disastrous results. Here’s a simple solution to the problem.

A petrochemical storage site is in flames. Firefighters train master streams on the fire in a defensive mode of operations. Clouds of chemical vapors and smoke fill the immediate area.

The engineer at pumper number 3 carefully monitors discharge pressure and engine RPMs. Without warning the engine begins to race. He throttles back to reduce speed. Still the RPMs increase. He shuts off the engine fuel valve but the engine races even faster— 500 horsepower out of control. The tachometer needle is buried in the red zone. The engineer issues a warning cry and begins to run just as the pumper explodes into flames. The force of the blast lifts him off his feet and throws him to the ground.

The engine literally has torn itself apart. It’s no longer a firefighting tool; rather, it becomes another fire hazard that must be dealt with.

Engine overspeed—or runaway—is a real problem. It doesn’t happen every day but the potential is there. It can occur when airborne combustible vapors or dust is introduced into the diesel engine system as an uncontrolled fuel source not subject to normal engine shut-down procedures. The vapors and dusts that can cause such a reaction are not exotic— they run the gamut from widely encountered petrochemicals and other hydrocarbons to soot from burning house shingles.

I’ve spoken with firefighters who have experienced engine overspeed. A central Florida firefighter tells about drastic increases in engine RPMs from a broken gas line. A New England pumper engineer tells of a near runaway situation caused by vapors from a paint warehouse fire. A West Coast firefighter witnessed an engine tearing itself apart from overspeed caused by leaking oil from an oil bath-type air filter.


Diesel engines operate on a specific air-to-liquid fuel ratio. Fuel flow is regulated to control engine speed. Under normal circumstances the more fuel available, the faster the engine speed. This usually is not a problem because the fuel source is regulated either by the engine governor, a component that automatically monitors engine RPMs and increases or decreases fuel flow for constant engine speed, or manually by the engineer.

If a new, uncontrolled fuel source is added to the picture —that is, if an atmosphere of combustible vapors enters the air intake—engine speed will increase. As more fuel is added from this new source the engine will continue to run faster and faster. The engine governor will attempt to slow the engine by restricting the normal fuel supply. The engineer may even reduce the throttle to idle. However, neither the engineer nor the engine governor has any control over this new fuel source. It continues to accelerate the engine.

If enough uncontrolled fuel is available the engine may reach overspeed, running out of control. If RPMs increase high enough, the engine will self-destruct. Overspeeding engines tear themselves apart. Connecting rods blow through the sides of the engine block as pistons and bearings overheat. Metals distort, break down, and flake apart. At best the engine is severely damaged or destroyed. At worst the engine parts become flying shrapnel or the entire engine explodes into fire, presenting a lifethreatening danger to personnel.


The most common causes of engine overspeed are combustible vapors or gases produced by industrial liquids. Gasoline, liquefied petroleum gas, propane, benzene, cleaning solvents, and paints are just a few of them. Escaping natural gas from broken lines also can fuel an engine into overspeed. The burning of common combustibles at a fire scene produces combustible gases and particulates that can be an uncontrolled fuel source; likewise for grain dust, which has explosive properties under certain conditions.

Even if your pumper is located downwind from the fire, a sudden wind shift can bring these new fuel sources upon you. Engines respond amazingly quickly to combustible vapors. RPMs can jump from normal to overspeed so fast that the engineer has very little time to consider his options and react.

Airborne vapors and dusts are not the only causes of overspeed. A defective engine governor, an overfilled oil bath-type air filter, and leaking oil or other fluids from an overturned or severely tilted engine also could cause runaway.


One of the first reactions when RPMs begin to climb is to reduce or shut off the diesel fuel supply. Obviously, this is ineffective when external combustibles fuel the engine. Another reaction, though certainly an uneducated one, might be to shut off the ignition system, which has no effect on the diesel engine since it is not spark-ignited. The operator also might try to disengage the load from the engine, without success: Load is one of the factors of engine speed; remove the load when an uncontrolled fuel source is present and the engine simply accelerates even faster.

However, engine overspeed need not be an uncontrolled hazard. Shutdown equipment is available from many fire apparatus manufacturers and used by fire departments across the country. The simplest and most effective solution to an uncontrolled fuel supply is to shut off the intake air supply —no air, no combustion. To that end, the air intake shut-off valve has proven successful.

The National Fire Protection Association’s NFPA 1901, Standard on Automotive Fire Apparatus, recommends (in Section 2- that the emergency engine shut-off mechanism “be of the type that will shut off either the air supply or the exhaust gas flow of the engine,” and the 1991 edition of NFPA 1901, 1902, 1903, and 1904 (Section A-3-20401) lists an air intake shut-off valve as a desirable additional feature.

The air intake shut-off valve, either manually or automatically operated, is situated between the engine’s air filter and turbo charger. Quite simply, it prevents the flow of air and vapors through the air intake piping to the engine where it forms an ignitable mixture with the fuel. There’s no mystery to the device, but remember two important considerations for effective operation:

  • Use a tight-sealing device for a positive seal. If even a small amount of air leaks through, the engine can continue to operate.
  • Locate the shut-off valve as close to the air intake as possible. A shut-off valve located at the end of 20 inches of air intake piping can make for 20 inches of potential problems.

If you never have to use the emergency air intake shut-off valve, so much the better. As is true in other aspects of the fire service, prevention is a desirable alternative to suppression.

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