Runaway Diesel From Own Tank
Location: Outside U.S.
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?
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
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Update: June 2012