After the article I wrote on Tuesday (Ride Without Spark Arrester, Get Bill for $10 million), I got email from a few readers who expressed doubt that a motorcycle without a spark arrester could start a fire. Others assumed that the sparks being ‘arrested’ came from backfires, and that only four-strokes or improperly-jetted two-strokes would create this discharge.
In a quest for further information, I came across the Spark Arrester Guide (Published by the National Wildfire Coordinating Group, and sponsored by the US Departments of Agriculture and the Interior). Here is an excerpt from the section entitled “Basic Definitions and Test Standards”:
Exhaust Particles – All internal combustion engines produce exhaust particles which are predominately carbon with contaminates. These particles originate from deposits formed on the internal surfaces of the engine or exhaust system and, depending on their exact origin, may be expelled at temperatures in excess of 3,000 degress F. Depending on the nature of the contaminates, these particles are capable of glowing or sometimes flaming combustion. When expelled through the exhaust system into the atmosphere, the combustion process may continue or even be accelerated during flight. Such particles, if larger than 0.023-inch in diameter and at temperatures of 1,200 degrees F are capable of igniting cellulose materials on contact.
A Spark Arrester is a device which traps or pulverizes exhaust carbon particles to a size below 0.023-inch in diamter, as they are expelled from an exhaust system. Trap type spark arresters must have a method for removal of accumulated carbon particles. Most spark arresters generally perform in the high 90 percent spark arresting effectiveness range.
As anyone who has ever rebuilt a two-stroke top end can tell you, the piston and parts of the cylinder and cylinder head often have carbon and other gunk built up on them. Four-stroke engines can contain, and expel, similar deposits. Like the report says, small particles of this gunk may detach and be expelled through the exhaust system. Having been in contact with very hot parts of the engine, these particles can themselves be at a very high temperature when expelled. So a spark arrester is important insurance for any bike that will be ridden near trees or brush.
