FAA Powerplant Written Practice Test

When operating at high altitudes, why does a turbocharged engine's exhaust system face potential failure?

1. Increased engine cooling

2. Higher fuel mixture pressure

3. Lower external pressure

4. Expanded exhaust flow

The correct answer is: Lower external pressure

The reasoning behind the answer is rooted in the principles of pressure differentials and their impact on the exhaust system of a turbocharged engine. At high altitudes, the ambient air pressure decreases significantly. This lower external pressure affects the exhaust system in several ways. In turbocharged engines, the exhaust gases play a crucial role in driving the turbocharger, which forces additional air into the combustion chamber to maintain performance despite the thinner air at high altitudes. However, as external pressure decreases, the ability of the exhaust system to properly expel those gases can be compromised. The decreased pressure can lead to insufficient exhaust flow, which may not be optimal for turbocharger operation, potentially resulting in back pressure. If back pressure becomes excessive, it can result in overheating and damage to exhaust components, including the turbocharger itself, leading to potential exhaust system failure. This is contrasted with the other options, where increased engine cooling does not directly contribute to exhaust system failure, higher fuel mixture pressure is not typically a concern with turbocharged engines at altitude since they compensate for air density loss, and expanded exhaust flow generally refers to performance improvement rather than failure risk. Thus, the interaction of lower external pressure with the exhaust system is critical in understanding the potential for failure in these conditions.