LO 6-5 Identify and describe the hazards associated with engines as ignition sources and outline control practices
| Site: | CDX Learning Systems |
| Course: | DE Section 2-1: Engine Blocks and Cylinder Heads |
| Book: | LO 6-5 Identify and describe the hazards associated with engines as ignition sources and outline control practices |
| Printed by: | Guest user |
| Date: | Thursday, 5 November 2026, 7:44 PM |
1. Engines as Ignition Sources
LO 6-05: Identify and describe the hazards associated with engines as ignition sources and outline control practices. |
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Engines as Ignition Sources Internal combustion engines, whether fueled by gasoline, propane, natural gas, or diesel, are themselves sources of ignition. The most common example of an engine as an ignition source occurs when intake air is blended with another fuel, which enriches the air-fuel mixture. Increased cylinder fueling can cause the engine to overspeed or run away. Over-speeding can not only damage the engine but also cause engine parts to scatter and result in lethal injuries to anyone nearby. Flammable vapors or gases near a running vehicle is one source of supplemental cylinder fueling. Operating engines around fuel refineries or near gas wells that emit natural gas can allow combustible gases to mix with fresh intake air. Another common source of supplemental fueling is lubricating oil that has collected in a pocket or reservoir in the intake air system (most frequently in the charge air cooler). If a turbocharger seal has leaked, or a closed crankcase ventilation system has supplied oil that has accumulated in the intake air system, a sudden change of intake air velocity can begin to draw oil into a diesel engine’s combustion chambers. The engine speed will rapidly increase and draw in even more oil, which acts as a supplemental fuel. Because a diesel does not depend on a spark to ignite fuel, shutting off the ignition switch does not shut off the engine. Cutting the regular supply of diesel fuel is no help either. In these situations, the supply of intake air needs to be quickly choked off. Devices that mechanically shut off intake airflow, called positive intake shutoff valves, are mandatory in areas where the potential for supplemental fueling from flammable gases is possible Figure 27. |
FIGURE 06-27: A positive intake air shutoff valve will close the intake air system in seconds and prevent an engine from running away. |
©2026 - CDX |
1.1. Engines as Ignition Sources (continued)
LO 6-05: Identify and describe the hazards associated with engines as ignition sources and outline control practices. |
Aftertreatment systems performing a regeneration are another way a diesel engine can become an ignition source and fire hazard. During exhaust regeneration events, the exhaust system temperatures rise to over 1000° F (538° C), becoming hot enough to ignite dry grass, paper, and any debris in contact with the exhaust pipe. Landfills and the interior of factories are two common places where this hazard can start a fire Figure 28. Similarly, spark arresters are used in off-road applications when hot carbon particles can exit the exhaust and ignite forest fires or grass fires. The spark arresters generally consist of screens at the exhaust pipe tip or layers of metal discs that the spark must pass across at the tip, which cools and quenches sparks.
FIGURE 06-28: The exhaust tailpipe outlets are double walled to allow the circulation of cooler air around the tip to reduce its temperature. |
When working on diesel engines, whether it is in a shop, broken down on a job site, or outdoors, controls for ignition hazards include:
- Establishing acceptable areas, boundaries, and entry routes for vehicles and machines’ combustion to avoid potential flammable gases
- Marking off and enforcing acceptable traffic routes through hazardous locations
- Installing automatic overspeed shutdown devices on engines
- Installing exhaust spark and flame arrestors
- Inhibiting the operation of the aftertreatment system anytime it is indoors or around potentially flammable waste or products
- Flushing out charge air coolers and intake air systems, if the intake air system has oil accumulations, subsequent to a repair but before starting an engine
| Technicians Tip | ||
| When a turbocharger has failed or the charge air cooler is potentially filled with lubricating oil, always remove and flush the cooler. Oil in the cooler will fuel the engine and cause it to overspeed. |
©2026 - CDX |