AUTOMOBILE ENGINE COOLANT RELATED FIRES
Charles C. Roberts, Jr., Ph. D., P.E.
Automobile engine coolant related fires may result from engine
coolant leakage, an increase in the concentration of the glycol in
the water/glycol mixture, the nature of the vapor/particle
distribution, and contact with an ignition source in the engine
compartment. Ethylene glycol, a common coolant, is a flammable
liquid with an ignition temperature near 800F. In recent years,
propylene glycol is being used because of environmental reasons.
Propylene glycol is also a flammable liquid with an ignition
temperature near 700F. In an automotive application, the glycol is
mixed with water at about a 50/50 ratio. Ignition of this
concentration of coolant is difficult because of the water. When
released at high temperatures into the atmosphere where the water
evaporates, the glycol vapor/liquid droplets can reach the state of
an ignitable mixture. Typical ignition sources in the engine
compartment include hot surfaces (exhaust manifold, exhaust
system) and electrical components (relays, distributor, spark plug
wires). Automobile accidents, resulting in hot vapor expulsion
from the coolant system, are also known to cause fires.
Figure 1 is a view of a late model sedan that was being driven
home from a radiator shop where the radiator had been repaired.
The repair entailed draining and replacing the coolant. After
driving several miles, the owner of the vehicle noticed smoke
under the hood. The vehicle was stopped and a fire ensued in the
engine compartment. Burn patterns in Figure 1 confirmed the
engine compartment origin of the fire.
Figure 2 is a closer view of the engine compartment with the hood
raised. Inspection revealed an open engine coolant system cap as
indicated by the arrow in Figure 2 and shown in detail in Figure 3.
Apparently, the radiator coolant system cap was not found secured to
the flange assembly.
Figure 4 shows an exemplar cap constructed of steel. The cap has a
locking feature, which keeps it in place and reduces the chance of
accidental removal when the coolant is hot. Because of the steel
construction, such a cap would not be consumed in a fire of this
magnitude and should have remained in place.
The absence of the cap suggests that the radiator repair shop did
not properly secure it to the cooling system flange assembly. This
results in heating of the coolant with no pressure in the system.
Water will boil first at about 212 F, while the glycol will not boil
off until about 370 F. Once the water had boiled away and the
glycol coolant was expelled from the open flange, it was ignited by
several possible ignition sources in the engine compartment and
resulted in the coolant related fire.
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