November 1974
San Dimas Equipment Development Center

Guido C Kaminski
University of California at Riverside
Report of work conducted by author at Pacific
Southwest Forest and Range Experiment Station's
Forest Fire Laboratory in Riverside, California, under
sponsorship and technical direction of the Forest
Service Equipment Development Center, San Dimis,


The chain saw is an important tool of the logging industry. Unfortunately, this otherwise useful device has started fires responsible for large losses in human and natural resources. Previous research has determined that most chain saw fires were caused by escaping incandescent exhaust particles larger than 0.023-in in major diameter. To eliminate this problem the multi-position screen-type spark arrester was developed. Because of the small screen openings (not over 0.023 in) the screen arrester tends to raise the operating temperature of the chain saw. This is especially true of retrofit screens which are often undersized to fit an existing muffler resulting in an exhaust flow restriction.

A fire hazard problem presented by a spark-arrester-equipped chain saw is from high skin and high exhaust temperatures. If the saw, under such conditions, or its exhaust comes in contact with forest fuels, ignition of the fuels can result. Further, the exhaust gases are often accompanied with exhaust flames which increase the danger of fire.

As an aid to understanding some of the parameters that influence the start of chain saw fires, a stock chain saw muffler having a screen-type spark arrester was tested under controlled conditions. Of primary interest was determination of ignition temperature and contact times for forest fuels associated with chain saw fire starts.

A muffler with a screen-type spark arrester mounted on it, designed for use on a Stihl portable chain saw, was bisected into a back- and a front-half, and the front-half was instrumented for use in our experiments, figure 1. Nichrome wire was coiled into a heating-coil array and mounted in the hollow, rearward-facing cavity of the muffler front-half. Also, a chromalalumal thermocouple was attached at the front, in contact with the screen. The heating-coil array was subjected to varying voltages by means of variac in order to obtain the heat flux desired for various phases of the experiment. Temperatures were measured using the thermo-couple which, by virtue of its position when the muffler/spark-arrester simulator lay downward on a fuel bed, recorded the temperature of the fuel-muffler/arrester interface.
Observations of a test in progress were made either through a plate glass window at the front of the environmental chamber (in which case the chamber door remained closed) or, when tests were conducted with an air stream, through the side of the partially open chamber door, figure 2. In the course of running the tests, fuel moisture in the chamber was determined by means of fuel-moisture sticks and was found to be approximately 6 percent over the period of the experiments. Chamber temperatures were set to 35.6° C (96° F) dry-bulb and 200° C (68° F) wet-bulb throughout the tests.

Fuel material for the tests was of four types:

  • "Punky wood" consisting of decomposed tree wood and bark obtained from Oregon which was ground to a homogeneous dust having a particle size of 0.08 in (2 mm) and was spread over a large, flat piece of bark of the same origin and was then placed in a pan the same size as the bark; the punky wood varied in depth from 1/4 to 1/2 in

  • Cheat grass, figure 3, from a roadside near Banning, California, was used as it had grown, attached to a soil base; the stems of the plants varying up to 3-in long and lying flat

  • Sawdust from mahogany wood obtained from a local sawmill and having a particle size of 0.10-in was spread over soil to a depth of 1/2-in

  • Tree moss from Oregon which was ground to 0.08-in in size.

These fuels were cycled at chamber conditions for I hour before each test run; the first three fuels were tested concurrently; moss was tested separately in test runs conducted later. Data sheets were used to list the thermocouple temperature for each test situation to +/-1O( C ( +/- 18( F), the material tested, and the time to which either a result was observed or arbitrary cut-off times of 5 minutes and I0minutes were reached. Then the test would be terminated and the heater moved to observe the fuel beneath it for a reaction. Test results are summarized in tables 1 and 2. Tests were conducted either with the chamber door closed and heater and fan off, which meant no net air flow, or with the heater and fan operating-which is noted as "with air stream" in table 1.

In addition to the main part of the tests, covering contact of a heater with a fuel bed, two sets of related tests were run. The first set used all the apparatus and methodology of the arrester tests and added what is referred to as a "pilot flame" to simulate the exhaust flame in the exhaust gases. This pilot flame was produced by burning only acetelene using a miniature oxyacetelene welding torch, figure 4. This provided a cold yellow flame approximately spherical in shape and 1/8-in in diameter, registering 500° C (932° F) on a thermocouple. The procedure for the pilot flame tests was to heat the fuel bed to a steady temperature as if for the muffler/arrester alone tests, and to introduce this flame at the fuel-muffler/arrester juncture, making observations from the time of flame contact.

The second set of related tests was conducted at room temperature under ambient conditions- 26.7° C (80 °F), 40% relative humidity-outside the environmental chamber, with no net air flow. These tests were confined to the punky wood and consisted of placing 1-in cube blocks of this wood onto a laboratory hot plate with a chromal-alumal thermocouple attached to the heating surface. The reaction of the punky wood to a steady heating was then observed, see table 2.

Identical tests on tree moss (which were conducted subsequent to running experiments on the punky wood, cheat grass, and sawdust) are just being completed. Preliminary indications are that moss exhibits about the same ignition characteristics as the punky wood.


The main thrust of the experiment was concerned with fuel-muffler/arrester simulator contact. Consistently, fuel material would adhere to the heater unit when the unit was removed from the fuel bed and smoke would issue from the adhering material. The movement through the air as the removal took place would cause glowing points or sparks. In the case of the punky wood, this varied in size up to 1/4 in; some of the glowing points would fall in transit and glow outside the test area for 5 to 30 see with some smoke. In the case of the sawdust, adhering material would char and smoke without glowing, even though there would be no evidence of smoke on the fuel bed proper for that test run. This effect is not apparent with the cheat grass because it had no tendency to detach from the soil substrate.

On the basis of these observations it becomes conceivable that an operator of a chain saw could lay a saw down while it is running or has just been turned off; material could adhere to the muffler/arrester case and either burn or remain inert until the next time the saw is used-whereupon the combination of heat and motion through the air of the operating saw could kindle glowing and/or flaming combustion in the adhering material. This material may either fall, or be shaken loose by engine vibration, and upon contact with fuel on the ground start a fire-even though no burn or smoke could be seen at the place where the operator originally rested the chain saw.

A similar deviation from the results of the main series of the contact tests would occur with the addition of a minimum of air movement when the chamber door was opened while testing was in progress. On two runs, when the punk was heated to 300° C (572° F), names and smoke were sustained for IO to 1 5 sec on removal of the heater unit; the flames were less than a 1/4-in high and they extinguished of their own accord. The heater had been in place for 5 minutes.

Bearing in mind these observations, it is obvious on the basis of the fuel-muffler/arrester contact tests that, within the limitations of the test, smoke and glowing occurs with the most sensitive of the four fuels-punky wood at 270° C (518° F); while the less reactive materials-cheat grass and sawdust react with only smoke at this temperature, and the cheat grass glowed at 330° C (626° F) or higher and then, but briefly, under these conditions.

The tests using the pilot flame in addition to a heating body provided additional data supporting the basic temperature sensitivity of the fuels-but with a different fire behavior for the four types. At below threshold temperatures, the contact of the flame would scorch the fuel bed, but no sustained flame would appear. A threshold appeared rather dramatically in the case of the cheat grass at approximately 270° C(518° F). At this temperature the flame of the pilot jumped across the entire fuel bed, consuming it before the test operator could contain it. The threshold in the case of the sawdust was at 260° C (500° F); at this temperature a slowly moving yellow flame about 3/4-in high spread from the pilot flame, markedly slowing when leaving the surroundings of the muffler/arrester simulator.

The threshold for the punky wood was also at 260° C (5OO° F) and was a weak, short flame that did not extend beyond the base of the muffler/arrester as it lay in contact with the fuel. It soon reduced itself to a steady glow that persisted, with smoke, in excess of 5 min. The tests of the 1-in cube punky wood sample on the laboratory hot plate with steady heating (see table 2) was found smoking at 230° C (446F° ) and was totally consumed by glowing combustion within 7 minutes after the initial combustion on exposure to 300° C (572° F) for 40 sec.


It appears that safe chain saw operation would result if the muffler with a screen-type spark arrester on the chain saw had a shell temperature that did not exceed 260C° (500° F) and a gaseous exhaust temperature that did not exceed 232° C (450° F). This is based on the data, plotted in figure 5. Alternately, repositioning the muffler/spark arrester to prevent any contact of the muffler/arrester or the exhaust stream with forest fuel under any circumstance during operation is worthy of consideration.

Table 1. Chamber Test Data
200 punky wood 5 No ignition, no effect
200 punky wood 10 No ignition, no effect
200 sawdust 5 No ignition, no effect
200 sawdust 10 No ignition, no effect
200 cheat grass 5 No ignition, no effect
200 cheat grass 10 No ignition, no effect
230 punky wood 5 Slight browning of contact areas
230 punky wood 10 Slight browning of contact areas
230 cheat grass 5 Slight browning of contact areas
230 cheat grass 10 Slight browning of contact areas
230 sawdust 5 No ignition, no effect
230 sawdust 10 Slight browning of contact areas
250 punky wood 5 Browning and slight smoking
250 punky wood 10 Browning and smoke, no spread
250 cheat grass 5 Browning, no spread
250 cheat grass 10 Browning, no spread
250 sawdust 5 Browning, no spread
250 sawdust 10 Browning, no spread
270 punky wood 4-4 1/2 With air stream: much smoke,glowing combustion
270 punky wood 5 Smoke, glowing combustion
270 punky wood 10 Smoke, glowing combustion, no spread
270 cheat grass 5 Browning, substantial
270 cheat grass 5-6 With air stream: very little smoke, no glowing
270 cheat grass 10 Browning, out to perimeter of arrester
270 sawdust 5 Smoke and browning (slight)
270 sawdust 10 Smoke and browning, no spread
270 sawdust 10 With air stream: slight browning
300 punky wood 2 Smoke, glowing, combustion
300 punky wood 10 Smoke, glowing combustion, no spread
300 cheat grass 3 With air stream: smoke, no glow

The Forest Service, U.S. Department of Agriculture has developed this information
for the guidance of its employees, its contractors, and its cooperating Federal and
State agencies, and is not responsible for the interpretation or use of this information
by anyone except its own employees. The use of trade, firm, or corporation names
in this publication is for the information and convenience of the reader and does not
constitute an endorsement by the US. Department of Agriculture of any product or
service to the exclusion of others that may be suitable.