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SAE J335: Multiposition Small Engine Exhaust System Fire Ignition Suppression Testing
SAE J335 is a recommended practice that establishes test methods for evaluating the performance of spark arrester exhaust systems on multiposition small engines (MSEs) used in portable applications. These engines, found in chainsaws, weed trimmers, brush cutters, and backpack blowers, present a unique fire ignition risk due to their mobility and operating positions. This standard provides a consistent method to assess whether an exhaust system can prevent the emission of sparks and hot particles that could ignite vegetative fuels.
While compliance with J335 indicates adequate performance under severe fire hazard conditions, the standard itself notes that during periods of extreme fire danger, even compliant systems may not offer absolute protection—additional operational restrictions may be necessary. Thus, J335 is a critical tool for manufacturers, certifying agencies, and users to minimize fire risk.
Key Test Procedures
The standard outlines several essential tests to verify spark arrester performance. The most fundamental is the Screen Test, which evaluates the integrity of the spark arrester screen and its housing.
Screen Test
The screen test applies to MSEs with removable (serviceable) screen-type spark arresters. The test uses a wire plug gage of 0.610 mm (0.024 in) diameter—slightly larger than the permitted screen opening of 0.023 inches (0.58 mm). The test involves three steps:
- Screen Housing Seal (with screen installed): Probe the entire periphery of the screen mounting structure. Any penetration with the gage, using force not exceeding 2 oz (0.56 N), fails the test.
- Test With Screen Removed: Remove the screen and randomly probe it at least 20 times, paying attention to bends, molding, or edges. Again, any penetration with ≤2 oz force fails the test.
- Reassembly Check: Reinstall the screen and repeat step 1 to confirm the housing seal is maintained.
Additionally, the screen material must be verified as heat and corrosion resistant metal alloy, typically through supplier certifications or metallurgical analysis.
⚠️ Critical Note: The wire plug gage test must be performed with a force not exceeding 2 oz (0.56 N). Excessive force can cause false failures or damage the screen.
Contact Plane Determination and Temperature Testing
To evaluate the potential of exhaust system surfaces to ignite surrounding vegetation, J335 defines a contact plane—an imaginary flat surface determined by the extremities of the MSE power unit. The standard provides detailed procedures for establishing three contact planes by rotating the power unit on a horizontal plane. The plane closest to the exhaust flow direction is used for exhaust gas temperature measurement; if any test plane contacts the exhaust system surface, surface temperature is measured at that point.
Temperature testing uses J or K type thermocouples with grounded junction and sheath diameter ≤3.2 mm. For chainsaws, a calibrated loading device is mandatory; for other MSEs, it is optional.
Engineering Design Insights
From the standard’s requirements, several design considerations emerge for manufacturers:
- Screen opening: Must be 0.023 inches (0.58 mm) or less. This ensures that particles large enough to ignite fuel are trapped.
- Housing seal: The screen mounting structure must be designed to prevent any gap that could allow unfiltered exhaust to bypass the screen. The wire plug gage test simulates a worst-case.
- Material selection: The screen must be heat and corrosion resistant metal alloy. Typical materials include stainless steel or high-temperature alloys.
- Non-serviceable mufflers: For sealed mufflers that do not allow screen access, the standard requires endurance testing per Appendix A instead of the screen test.
- Contact plane considerations: The points of contact (usually three) define the plane. For chainsaws, the uppermost bumper spike root is used as a reference. Designers should consider where the exhaust system might contact surfaces when the engine is placed in various positions.
| Test | Purpose | Key Criteria |
|---|---|---|
| Screen Housing Seal | Ensure no gaps around screen periphery | No penetration with 0.610 mm wire gage at ≤2 oz force |
| Screen Material Verification | Confirm heat/corrosion resistance | Supplier certification or metallurgical analysis |
| Contact Plane Determination | Identify points for temperature measurement | Three planes established from power unit extremities |
| Exhaust Gas Temperature Test | Measure temperature at impingement points | Thermocouples J/K, grounded, ≤3.2 mm sheath |
| Endurance Test (sealed mufflers) | Ensure long-term performance | Per Appendix A |
🛠️ Design Insight: The standard emphasizes that the screen housing seal must be re-verified after reassembly. This implies that the design should facilitate consistent sealing even after maintenance—a key reliability consideration.
Frequently Asked Questions
How is the contact plane determined for a multiposition small engine?
Place the power unit on a horizontal plane in the position that puts the exhaust system closest to the plane. Identify three contact points. Rotate the unit while keeping two points fixed to find a third, establishing the second plane. Repeat for a third plane. The plane closest to the exhaust flow is used for temperature measurement.
What are the pass/fail criteria for the screen test?
With or without the screen installed, the wire plug gage must not penetrate the screen or any gap in the housing seal when applied with a force not exceeding 2 oz (0.56 N). Any penetration constitutes a failure.
What loads and speeds are used for temperature testing of chainsaws?
Chainsaw temperature testing requires a calibrated loading device. The standard references ISO 7293 for engine performance; typically, testing is done at the speed corresponding to maximum torque or rated power (best power).
Can this standard be applied to vehicles or stationary equipment?
No. SAE J335 explicitly applies only to multiposition small engines (<19 kW) used in portable applications such as hand-held, hand-guided, and backpack-mounted devices. It is not applicable to spark arresters used in vehicles or stationary equipment.
For further details, refer to the full SAE J335 document and Appendix A for performance criteria.
