In a runaway diesel situation, what should you do to stop the engine?

To stop a runaway diesel engine, shutting off the air supply is the most effective method. A runaway condition occurs when the engine is running uncontrollably, often due to an external fuel source igniting the fuel-air mixture without proper control. Diesel engines operate on the principle of compression ignition, so they need both fuel and air to continue running. By cutting off the air supply, you deprive the engine of the oxygen it requires to maintain combustion, resulting in a shutdown of the engine.

While shutting off the fuel supply could also help in some scenarios, if the engine is already running away, it may still continue to operate on residual fuel or from other sources of unintended fuel—even if the primary fuel supply is cut off. Therefore, immediately stopping the airflow is a more reliable and swift solution to regain control of the situation.

Engaging the throttle would only exacerbate the issue by adding more fuel into the combustion chamber, while activating the emergency shutdown might not effectively address the engine's need for air. Hence, influencing the air supply directly is the key action to take in such a runaway situation.