What Causes Diesel Engine Runaway?

Internal combustion engines are incredibly complicated machines that many of us interact with on a daily basis, but don’t completely understand. You get in your car, turn the key or push the start button, and it starts up. Then you drive to your destination, and turn the key back or push the stop button and it turns off. If one of those events doesn’t occur, you call the shop to fix it. 

Under rare circumstances, diesel engines, especially turbocharged diesel engines, can experience a phenomenon called runaway, where turning the key or hitting the stop button doesn’t turn the engine off. In fact, even after the key has been removed from the ignition, runaway can continue to worsen. This causes the engine to uncontrollably exceed its normal operating speeds until the engine fails or its fuel source is blocked. These incidents are rare, and usually occur in old, high-mileage diesel engines, but they are still terrifying and dramatic spectacles. After seeing a video that cited a diesel engine experiencing runaway as the source of ignition for the BP Texas City oil refinery explosion in 2005, I decided to do a deep dive into the phenomenon. 

Unlike gasoline-burning engines, diesel engines don’t require a spark to burn fuel and operate. Most automotive diesel engines don’t have any spark plugs; instead, diesel engines rely on compression to ignite the fuel. The combustion chamber inside each cylinder in a diesel engine facilitates combustion by housing air at extremely high pressures, which when compressed by the piston and combined with diesel fuel, heats up and ignites the fuel. This is why diesel engine blocks are almost always made of heavy cast iron, since cast iron has superior compressive strength and the ability to withstand intense thermal cycling.

Most diesel engines have glow plugs that function somewhat similarly to spark plugs, but don’t produce a spark. These glow plugs are important tools that allow diesels to start right up, even in extremely cold temperatures. Low temperatures and a cold engine block inherently make it difficult for a diesel engine to fire since the air inside the cylinder needs to be very hot to ignite the diesel fuel. Modern diesel engines are designed to house glow plugs inside the cylinders right next to the intake and exhaust valves and use electricity from a vehicle’s battery to heat up and glow hence the name glow plug. They reach optimal start-up temperatures in under four seconds, and computers control them to create the perfect temperature to induce combustion of the diesel fuel. Depending on operating conditions, some advanced diesel engine glow plugs can remain active while the engine is running to optimize the combustion process.

Diesel runaway occurs when fuel is unintentionally introduced to the combustion chamber from a source other than the vehicle’s dedicated fueling system. It causes the engine speed to uncontrollably increase in a self-perpetuating combustion cycle, since diesels only need air, fuel, and compression to fire. When a turbodiesel engine experiences runaway, the turbocharger feeds more and more air into the engine as it’s continuously and unintentionally receiving fuel, creating the runaway reaction. Since no spark is needed for the engine to fire, turning the vehicle off, even removing the key, has no effect on the uncontrolled reaction.

Turbocharged engines, whether fueled by diesel or gasoline, can experience a phenomenon called blow-by. Blow-by occurs when compressed air and fuel in the cylinder bore is greater than the pressure in the oil pan, and fuel leaks past piston rings and down into the crankcase. Some blow-by is normal, but excessive blow-by is a sign of serious engine, more specifically piston ring wear or damage, especially on high-mileage engines. Since virtually all diesel engines are turbocharged, blow-by is virtually an ever-present possibility, and it also happens to be one potential cause of diesel runaway.

Diesel runaway, though rare, most commonly occurs under a few different circumstances that are primarily classified as fueling failures. It can be caused by a fuel source other than the vehicle’s fueling system unintentionally and uncontrollably entering a turbodiesel engine. The phenomenon of blow-by can unintentionally allow unburned engine oil to reach the combustion chamber, leading to an uncontrolled continuous fuel flow, causing the positive feedback loop that creates runaway.

Most turbochargers are internally lubricated by engine oil, and a blown seal inside a turbodiesel engine’s turbocharger can cause motor oil to be unintentionally and uncontrollably drawn into the turbocharger intake. Once inside the turbo intake, the oil can enter the combustion chamber where it causes runaway. Failed diesel fuel-injection systems can also lead to the dangerous phenomenon.

I was recently introduced to the enthralling animated deep dives into chemical and work safety disasters published by the U.S. Chemical Safety and Hazard Investigation Board on YouTube. The CSB’s investigation into the devastating 2005 explosion at a BP oil refinery in Texas City, Texas which killed 15 workers and injured 180 others, cites a diesel pickup truck experiencing runaway as the source that ignited the explosion. In this freak accident, the diesel pickup truck was parked on the property when a disaster at the plant caused massive quantities of hot gasoline to spill. When the highly flammable vapor cloud that formed from the hot gasoline spill reached the idling diesel pickup truck’s air intake, it caused diesel runaway that eventually ignited the vapor cloud causing the deadly explosion.

Diesel engine runaway is a rare occurrence, especially among the advanced modern turbodiesel engines that are now produced and sold on both the private and commercial market. Modern diesels often mitigate the likelihood of dangerous blow-by by using an oil separator that prevents volatile derelict oil from entering the combustion chamber.

Regular maintenance and inspection of key components such as turbochargers, fuel injectors, and air intake systems can preemptively catch a failing component that could lead to runaway. Always using the correct engine oil and fuel will create optimal running conditions for as long as possible.

Older, high-mileage diesel engines are at higher risk for engine runaway, but can be fortified against it by using a couple different techniques. There are aftermarket inline devices that shut off the engine’s air supply at excessive engine speeds, thus halting the positive feedback loop that causes runaway. Some older diesel engines have a tube to ventilate extreme crankcase pressure which helps prevent flammable oils from leaving the crankcase and entering the combustion chamber. There are also some off-the-shelf oil treatment and stabilizer additives that can help to lubricate piston rings in diesel engines. This can improve the seal between the piston rings and the cylinder walls to prevent oil from entering the combustion chamber.

The only way to stop diesel engines experiencing runaway is by either completely cutting off the fuel source, or the air source. Turning off the ignition and/ or removing the key will have no effect on the runaway reaction. Sometimes engines experiencing runaway can suffer catastrophic damage like seized bearings or broken rods that may stop the reaction without human intervention. Stopping a diesel engine experiencing runaway is a simple yet daunting task that can rapidly become impeded by fire, heat, or by fear of the terrifying reaction.

Stopping a diesel engine experiencing runaway is a dangerous task, and should be done at your own risk. The easiest way to stop a runaway is usually by choking it out, or finding a way to cut off the air supply. Blocking the air intake using objects like rags or t-shirts can slow down the engine speeds and choke out the engine before things get too bad. Some videos also show people using flat objects to block the flow of air into the turbocharger that’s uncontrollably feeding the reaction. Fire extinguishers can sometimes be enough to eliminate the oxygen entering the engine, but this isn’t always sufficient to stop the reaction.

When in doubt, remember that vehicles are replaceable, and humans are not. If initial attempts to inhibit the runaway reaction are not successful, get yourself and any other people as far away from the vehicle as possible and allow the reaction to run its course. Engine parts can dislodge or explode during a runaway reaction, so treat the situation seriously and call for help as soon as the risk of human injury due to the reaction is mitigated.