Investigators say the Apple Fire was caused by a vehicle

Hot carbon particles from the exhaust created multiple ignition points

Diesel carbon particles
File photo of diesel carbon particles (Photo from Guide to Wildland Fire Origin and Cause Determination)

Investigators have concluded the Apple Fire in southern California near Cherry Valley was caused by hot carbon particles from the exhaust system of a diesel-powered truck, which is not an uncommon cause of vegetation fires along roadways. Witnesses corroborated the investigators findings. At least three ignition points were found which all merged into one fire.

All internal combustion engines emit carbon particles which is why spark arrestors are required on chain saws, for example. The smallest are invisible, but particles from diesel engines can be much larger than those from small engines or gasoline engines. The bigger the engine, the larger the particles. As a qualified Cause and Origin Investigator I have picked up along railroads particles that were two inches long.

(To see all articles on Wildfire Today about the Apple Fire, including the most recent, click here.)

When a diesel engine begins ejecting carbon particles it can occur over a distance along a road or railroad. Multiple fire ignition points can be created.

Volatile hydrocarbons contained within the particle may extend the time the particle is thermally active. Larger particles may auto-ignite upon ejection and contact with the air.

A Diesel engine is more likely to throw out large carbon particles if it has been idling, it has suddenly been throttled up (such as pulling a heavy load up a hill), or if the engine is not running properly.

A discarded lit cigarette usually will not ignite dry grass unless the relative humidity is less than 22 to 25 percent, but carbon particles have been known to start fires at up to 80 percent. Many fires along roads blindly blamed on “someone tossing a cigarette” are more likely caused by hot carbon particles.

Pieces of catalytic converters can also be discharged from exhaust pipes. Normally catalytic converters can reach up to 1,380°F. When they malfunction and overheat they can break apart at temperatures of 2,400 to 2,800°F. Hot ceramic particles discharge from the exhaust system either through the tail pipe or through failures in the outer shell of the converter itself.

Apple Fire
The Apple Fire, San Bernardino NF, August 2, 2020. Screenshot from ABC7 video.

To learn more about investigating the cause of vegetation fires, spend some time with the 337-page “Guide to Wildland Fire Origin and Cause Determination” (NWCG publication, PMS 412).