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Name: Gary
Status: Other
Grade: Other
Location: Outside U.S.
Country: Canada
Date: Summer 2009

Diesel engines can "runaway" on hydrocarbon vapours contained in the air they injest. Therefore, could a diesel engine "runaway" on diesel fumes from its own fuel tank?

Hi Gary,

In fact, a diesel engine cannot runaway on the nearly insignificant amount of unburned hydrocarbons (HC) present in air pollution. The concentration of HC is in the low parts-per-million range. Thus, the amount of HC in even very polluted air is still far to low to provide enough energy to keep a diesel motor running. Air with sufficiently high levels of HC to actually cause runaway, would be unbreathable.

This sounds like an urban legend that started from the fact that worn (and therefore loose) valve guides and valve seals can in extreme cases, draw enough motor oil down the valve guides from the oil galleries in the cylinder head, to cause a motor to continue to run and even run away. For this to happen, the valve guides and valve stem seals need to be very seriously worn.

The answer to your question is, no, a diesel cannot run away on the fumes from its own tank, because there is no way for the fumes to get into the motor. A diesel motor's fuel injection pump can only pump liquid fuel. It cannot pump "fumes" from the tank to the motor. That is why diesel motors always stall and stop running when they run out of fuel.


Bob Wilson

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