Originally Posted by DJ
Nope.
Consider what happens when you are rolling along at a very low throttle setting, even at sea level. Do you have any idea how thin the air is inside the intake manifold, where the fuel is mixed with it? Put a manifold vacuum gauge on it and watch it. At full throttle, the two are pretty close to each other, but at lower throttle settings where you normally drive, that low vacuum gauge reading tells you the air in the manifold is at very much lower pressure than the outside air. It's really thin. This ought to tell you that problems of "vaporizing the fuel in the thinner air" is a red herring. The fuel is vaporized quite nicely because it is sprayed though the tiny orifice of the injector at very high pressure. Literally, it comes out of the injector as a "fog" of fuel that is nothing but vapor.
The "mass air flow sensor" measures just what its name implies -- the mass flow rate of air into the manifold. The engine computer injects fuel via the injectors at a corresponding mass flow rate of fuel such that the mixture is the same at all altitudes and all throttle settings. It uses the exhaust gas oxygen sensors to "close the loop" and, in effect, continuously calibrate the injectors.
This is remarkably different from the operation of carbureted engines, and the "rules of thumb" of how carbureted engines behave at altitude don't apply to it. A carburetor uses the difference in air pressures within the carburetor itself to force fuel to flow through the jets. The lower air pressure at altitude causes the carburetor to work with lower pressure differences, and so fuel is pushed at a lower rate than the air mass flow rate would call for, and the lower pressure at the jets causes the fuel to be atomized more poorly, resulting in a slightly leaner mixture that is less well mixed. An engine with a mass air flow sensor, O2 sensors, fuel injectors, and an engine computer doesn't suffer from these problems.
Oddly enough, I once had an ultralight aircraft, which I bought new. It had a very nice two-cylinder, two-cycle engine of the type normally used in snowmobiles. In setting it up, we chose a jet (from a table provided by the manufacturer) for the carburetor based on the local altitude and the season. The mixture was sensitive to air pressure and temperature. But, remember that this was a carbureted engine, not a fuel injected engine with a mass air flow sensor, O2 sensors, fuel injectors, and an engine computer.
Yup.
You have less maximum power available at higher altitude because the maximum mass air flow rate into the engine is less. The fuel that is injected matches that mass air flow rate, whatever it is, at all altitudes. But, until you open it up to full throttle, you can get the same power at higher altitude that you get at lower altitude -- you simply open the throttle more to get it.
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