Massachusetts Hoisting License Practice Test

How should you respond if a diesel engine is runaway?

1. Shut off fuel supply

2. Shut off air supply

3. Turn off the ignition system

4. Use the emergency brake

The correct answer is: Shut off air supply

When dealing with a runaway diesel engine, the best response is to shut off the air supply. A runaway condition occurs when the engine is unintentionally running at a speed higher than its normal operating range, often due to an uncontrolled supply of fuel or unwanted ignition sources. Diesel engines rely on a mixture of air and fuel for combustion, and they can continue to run even without a traditional fuel source if there is an excess of air or secondary fuel sources present. By cutting off the air supply, you are effectively depriving the engine of one of its critical components needed for combustion. This action is essential in stopping the runaway condition, as it eliminates the possibility of the engine having the air required to sustain the runaway state. It is a more effective method compared to attempting to cut the fuel supply directly, as fuel may still be present, or there could be other sources of ignition or fuel that you are unaware of during a runaway situation. In contrast, shutting off the fuel supply might not be effective if there is still air available, as the engine can often run on its own lubricating oil or any residual fuel in the system. Turning off the ignition system in a diesel engine typically does not stop the engine, as diesel engines do not operate on the