If you have ever heard the term “diesel engine runaway” but do not know what it means or how it happens, this article will explain the underlying causes that can make a diesel engine’s speed “run away”. One such possible cause is fitting hugely oversized turbochargers to diesel engines that are used for purposes they were not designed for, an example of which is shown in the image above. In this example an inappropriately sized turbo charger is fitted to a modified diesel engine that is used for drag racing, which begs this question-
Although diesel runaway conditions are relatively rare on standard diesel engines running with appropriately sized turbochargers, the term refers to a condition in which the engine is powered by a source of fuel from an unintended and often uncontrollable source. In practice, the unintended source of fuel causes the engine to over speed, which causes more fuel from the unintended source to be drawn into the engine, increasing its speed and power output even more. This process eventually sets up a self-sustaining cycle that ultimately destroys the engine through mechanical failure, or bearing and/or piston seizure because when an engine is in a runaway condition the lubrication oil cannot sustain effective bearing and/or piston lubrication beyond a certain engine speed, which brings us to the-
Since diesel engines are not throttled like petrol engines are, a diesel engines’ speed is controlled by varying the volume of fuel that is injected into the cylinders. Moreover, since diesel engines always run with excess air, it is very difficult to enrich the air/fuel mixture the point where combustion becomes impossible to initiate and sustain (as it is possible to do on petrol engines), which if it could, would be an effective limiting factor in how far a runaway condition can progress.
Therefore, for as long as fuel from an unintended source is available to a diesel engine in a runaway condition, the condition will persist without the air/fuel mixture ever reaching a point of over saturation before the engine destroys itself.
Nonetheless, several causes could create the conditions described above, such as the following-
A fuel-rich environment
Diesel engines that are used in industrial environments, whether as part of the installation, or fitted to vehicles moving around in the installation, are particularly susceptible to runaway conditions when the atmosphere in the environment contains substances that such a diesel engine could conceivably use as fuel. In these cases, an engine could draw the fuel-laden air in through the inlet tract, one such example being the Texas City Refinery disaster that killed 15 people and injured more than 180 others in March of 2005.
In this case, a major malfunction in the refinery released a huge cloud of hydrocarbon-rich vapour near a truck whose engine was idling at the time. The engine inhaled the vapour, which created a runaway condition, which in turn, likely created a cloud of sparks when the truck’s turbocharger exploded. The sparks ignited the vapour cloud, and the resulting explosion and fire destroyed a large part of the refinery, apart from causing the fatalities and injuries mentioned above.
Excess oil in the intake tracts of worn engines
In worn diesel engines in which combustion is not complete, unburnt fuel can blow by the piston rings, thus creating a mist of oil and unburnt fuel that enters the engine via the crankcase ventilation system. Once this happens, the engine’s speed becomes uncontrollable but with the added problem that the more the engines’ speed increases, the more oil and unburned fuel is forced out of the crankcase, thereby creating a self-sustaining positive feedback loop that will continue until the engine self-destructs through over speeding.
In a case like this, shutting off the regular fuel supply is useless since oil contains many, if not most of the combustible constituents of diesel fuel.
Damaged turbochargers
Although diesel runaway is a relatively rare occurrence on standard engines, oil leaking past turbo charger seals is nevertheless a major cause of runaway conditions and especially on applications where hugely oversized turbo chargers are fitted to engines that are used in competitive environments such as for instance, sled/weight pulling.
One might think that the compressor/turbine wheels on large turbochargers are stable because of their comparatively high mass, and this is true in most cases. However, the fact is that on some applications, poorly designed inlet tracts/ducting causes severe turbulence in the intake air, both before and after the compressor wheel, which can cause significant fluctuations in the turbocharger’s rotational speed.
Over time, speed fluctuations produce imbalances of the turbine/compressor assembly that in turn, causes the seals in the turbocharger casing to start leaking oil into the inlet tract. On standard engines fitted with appropriately sized turbochargers this is usually not an issue, but on an application that produces extremely high boost pressures, the high boost pressure in combination with the high flow rate of the intake air converts liquid oil into a fine mist, which the engine then uses as fuel. If in addition, there is some blow-by present, the unburnt fuel that is forced past the piston rings enters the engine through the crankcase ventilation system, and under these conditions, a runaway condition is inevitable.
It should be noted that when a runaway condition starts on a diesel engine running with an over-sized turbocharger, the engine could develop as much as ten times its rated power output before it self-destructs, even with the regular fuel supply shut off. The driving force behind the runaway condition in these cases is the over-sized turbocharger that forces progressively more oil mist past the piston rings ( into the crankcase) as the rings become progressively more worn due to a lack of proper lubrication caused by the increasing engine speed, which continually increases the turbocharger’s speed.
The positive feedback loop on such an engine is almost impossible to break, and the safest course of action bystanders can take is to remain at a safe distance (the further the better) until the turbocharger, or the engine explodes.
This is sometimes possible, but only if action is taken within the first few seconds of a runaway condition, which is often not possible given the kinds of situations in which diesel runaway conditions occur most commonly.
However, if a CO2 fire extinguisher is available, discharging it into the air intake might smother the combustion process, but this is not guaranteed to work. In fact, discharging a fire extinguisher into an overspeeding turbocharger could very well cause rotating parts in the turbocharger to disintegrate with the power of a small explosive device, and since most turbochargers in these applications are generally not equipped with containment devices, parts of the turbo charger could begin to fly out of the casing with the speed of big-calibre rifle bullets.
A safer approach might be to stuff a rag or some kind of plug into the air intake to cut off the intake airflow, but only if the object cannot come into contact with the spinning compressor wheel. If it does, the turbocharger will almost certainly explode violently, which could have fatal consequences for persons that are within range of the flying pieces.
One other method might be to jam a manual transmission into a high gear, and to let out the clutch quickly while holding the brakes. However, this will almost certainly destroy the clutch before it stops an engine that is in a runaway condition, and is therefore developing several times the power the clutch is designed to cope with.
Diesel engine runaway conditions may be spectacular to watch, but they usually only last for a few seconds before the engine is destroyed. These events are also highly dangerous to those that are in close proximity to the event, so if ever you happen to witness such an event, play it safe and wait for the engine to self-destruct from a safe distance, as opposed to making some kind of effort to break the positive feedback loop. Chances are good that you won’t succeed, anyway.
