I live close to the Rocky Mountains and drive into them quite often. I have a small naturally-aspirated four-banger that tends to lose a very noticeable amount of power getting into the higher altitudes and it doesn't help that much of the drive will be uphill with two adults and luggage.
I understand that this is due to the density of air being lower in altitude than at sea level. Less air being sucked into the engine implies less fuel to maintain the stoichiometric ratio, so you lose power to maintain the efficiency of the burn. In fact, you can roughly calculate how much horsepower you are losing using this formula:
HP Loss = (elevation in feet * 0.03 * horsepower @ sea level)/1000
The solution to this is simple: More air! Turbos compress air and shove it in along with the atmospheric pressure to create a higher density of oxygen, which is detected by the vehicle and more fuel is added, giving a boost in power. The result is that turbocharged vehicles have hardly any noticeable change in power in altitude. Since diesels often are equipped with a turbo, they rarely suffer any power loss in altitude for the same reason.
This website states that diesels have a compression ratio of 20:1 versus 8:1 for gasoline. I know that the compression ratio is unrelated to actual air intake; it is compressing the air that comes in to a specified ratio, regardless of how much air is involved. However, I imagine this might still help non-turbo diesel engines perform in higher altitudes since they tend to run lean and might change the fuel/air ratio less strictly. I think I understand why I'm off base in believing this, but I assume I am not taking everything into consideration.
My question is this: Do non-turbo diesel engines lose power comparably to non-turbo gasoline engines in higher altitude?
Considering Zaid's answer below, I threw together a quick chart to see the difference between the formula I gave above and the more accurate one Zaid so kindly provided for the percent power reduction.
Altitude (ft) 3,000 5,000 10,152 14,000 Actual Reduction 5.91% 13.46% 30.18% 40.52% Alternate 9.00% 15.00% 30.46% 42.00%
So you can see that the simpler formula works best close to 10,000 feet and breaks down in lower altitudes. This was comparing the formulas for a gasoline engine.