ELI5: If car engines have combustion problems due to lower oxygen in high altitudes, how come airplanes work well literally in the sky?
ELI5: If car engines have combustion problems due to lower oxygen in high altitudes, how come airplanes work well literally in the sky?
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Airplane engines have giant compressors to force air into the turbines, which alleviates the oxygen issue. Your typical car engine is naturally aspirated, which means it only uses the air flowing over it, though it’s become popular to use turbos to increase the compression ratio (basically, shovel more air into the engine to get more power out of the same amount of fuel).
The air up high is less dense, but if you smush it together it’s more or less the same at sea level.
Turbochargers on cars work in a similar way in negating the effect of altitude on power output.
High flying piston engines are turbocharged or supercharged to cram more air in. Jet engines have an air compressor built into the fundamentals of the design.
They don’t, unless they have superchargers or turbochargers, their engine performance will drop significantly at a few km altitude.
For example, in WW2 the P-51 and P-47 were high altitude escort planes operating at 5+km, while others planes, in particular the Soviet fighters lacked such engines and typically fought up to 3-4km only.
We have mixture controls in normally aspirated piston-engined airplanes, and we lose horsepower as we gain altitude.
Aircraft engines are literally designed to operate at those altitudes. The people designing them know how much oxygen is up there, and take that into account when making them.
If you went up to a car manufacturer and said “make an engine that can run 10km in the sky” they could do it as well. Though it’d be much more expensive, complex, probably bigger and heavier, and, most importantly, absolutely pointless to do.
Small airplanes suffer from the same problems. They have a service ceiling.
Other aircraft, including jets, have ways of compressing the air that is needed for combustion.
Airplanes make up for lack of air at higher altitudes by using compressors. Devices that suck in air and blow it into the combustion chamber of the engine.
On piston engines these are called super chargers or turbo chargers.
Jet engines are in a sense a modified turbo charger, they suck in air with a compressor, burn fuel with it, and eject it out the back to create thrust.
If you look at any small aircraft they have two leavers per engine. One is throttle and the other is fuel mixture. The pilots needs to turn the mixture down when climbing so the engine does not burn too rich. This is the problem with most cars as they go over mountain passes. Even with the right fuel mixture the engine will be performing worse at altitude though. Planes that go higher therefore needs compressors and huge intakes. These can take the low pressure air at altitude and compress it so the engine gets enough air. Big ram intakes does not work at lower altitudes because the airplane is not going fast enough. The compressor will help a bit at low altitude but it is tuned for high altitudes so it does not help as much. Jet engines takes this further by having huge ram air intakes and a giant compressor.