For non-turbocharged engines, rising density altitude causes engine power to:

• A. Increase
• B. Remain the same
• C. Become unpredictable
• D. Decrease

Answer: (D)

Rising density altitude means the air is thinner, with fewer air molecules and less oxygen in each cubic meter. In a normally aspirated (non-turbocharged) engine, there’s no mechanism to push more air into the cylinders to compensate for that loss, so the mass of air entering the engine decreases as altitude and temperature rise. Since fuel is burned with the available oxygen to create power, less oxygen means less fuel can burn effectively, leading to a drop in peak combustion pressure and, consequently, lower indicated horsepower. In short, power falls because the engine can ingest less air and produce less energy per cycle as density altitude increases. Turbocharged engines counter this effect by compressing intake air, which is why the drop in power is much less pronounced for them.