I seem to get slightly better mileage when I travel to British Columbia, which has more places with lower elevations. The lower mainland (Vancouver area) is just above sea level, but here at home, the elevation is 1000m to 1200m ASL (above sea level). It could be only from it being mostly highway travel, and perhaps more slipstreaming behind other vehicles. There are a lot more slopes, but I assume the upslopes even out with the downslopes. A friend of mine got one of the first electronically fuel injected vehicles back in 1985, and his work took him frequently between Calgary and Vancouver. He said the engine ran rough when he traveled from one place to the other, it ran rough and had to be adjusted by the dealers in both places. I always assumed that it was due to the elevation change, but I thought that altitude was automatically compensated for in the system. Am I off-base in my thinking here?
There are two aspects here - engine efficiency and vehicle efficiency. Normally aspirated engines do see higher fuel economy (and lower peak power) at altitude. This is because the throttle has to be open wider to get the same power, and thus pumping losses are reduced. This is not true of diesels, though, since diesel power is determined by amount of fuel injected, and there's no throttle plate. Atkinson cycle engines already do a lot to reduce pumping losses; I don't know if that effect reduces or eliminates the efficiency benefits from operating at higher altitude. As for vehicle efficiency, at higher speeds vehicles are always more efficient in lower density air.
Yes, you are wrong. It is not the altitude that is compensated for. Ecological aspects control modern cars, which are embedded in the engine control unit. The stoichiometric number says that to burn a unit of fuel mass, 14.7 more air mass is needed. The proportion of 14.7:1 is observed so that the output is as few harmful substances as possible. As the altitude increases, the air pressure decreases and the air density decreases, the number of oxygen molecules for burning a given volume of fuel decreases at altitude. Less air (oxygen) enters the engine and the ECU, following environmental aspects, regulates the reduction in fuel supply. Engine power drops. To climb to a height at an altitude of 3,000 feet, the pedal must be pressed much harder so that the throttle valve opens at a greater angle, compensating for the decrease in oxygen in the air volume.
Am I on the right track: at higher altitudes each cylinder’s volume is unchanged, but the air is thinner, hence less gasoline is needed to achieve proper fuel/air ratio? so better mpg but less power?
Yep, lower pressure means less air per volume. It results in a faux displacement reduction that leads to less fuel burned but lower power. It also means lower effective compression ratio, which is why lower octane can be used. Except in engines with chargers(turbo, super). Those push enough air in to keep pressure in the cylinder the same no matter the elevation. The lower pressure also lowers air resistance. Hybrids could be a special case depending on driving style. Lower engine output can be compensated by increased motor output. Which leads to more gas burned to recharge the battery.
I've always found that gasoline engines run richer at higher altitudes. Carburetors used to have special jets for specific altitudes, and gasoline-powered airplanes had a mixture setting for different altitudes. It doesn't seem to be an issue with most EFI systems. I think it depends on how the airflow sensor is constructed.
Nowadays the oxygen sensor does that job. It constantly looks at the exhaust and adjusts the mixture so that combustion is complete, no matter what the altitude.
The oxygen sensor does do that, but it's kind of a correction after the fact: its readings lead to "fuel trim" adjustments that feed back into the normal injection computations. Those computations are based in real time on the intake airflow reading. Paul is onto something here: There have been volume airflow sensor designs, which could give a misleading estimate of the air charge as the altitude goes up and there is less air per unit volume. But in the Prius (and lots of other EFI cars) a mass airflow sensor is used. It directly reads the mass of air entering the intake, so as the altitude goes up and the air density goes down, the ECM still accurately knows the actual mass of the air going in.
It does. And, sure, an ICE burns less fuel and also makes less power in thinner air. But, if you are driving, say, over 40 mph, the lower air density at higher altitudes also makes it so that your car creates far less air resistance. And, since aerodynamic drag is by far the biggest impediment to your car's forward motion at higher speeds, less air resistance = better fuel economy. It's also why most airplanes fly as high as practical: they also consume less fuel per mile where the air is thinner.
Yep. In engineering terms the air mass measurement is feedforward; the oxygen sensor is feedback. The feedforward allows very rapid response to changing conditions (like throttle position) and the feedback allows precise control over mixture.
