LO 6-5 Identify and describe the hazards associated with engines as ignition sources and outline control practices

LO 6-05: Identify and describe the hazards associated with engines as ignition sources and outline control practices.

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.

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