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I am going to be moving from sea level to 7,000 feet and at the same time replacing my 02 Accord with something else so I am looking around at all sorts of options. I know that at altitude an internal combustion engine will lose power from lack of oxygen. From this question about diesels vs four strokes the OP says

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.

I generally understand this statement, but what I can't work through in my head is what this means in terms of fuel economy.

So given two engines rated at about the same power at sea level, one normally aspirated and one turbo charged, in roughly the same sized car and driven in a similar manner1 at the same altitude would the turbo engine have better or worse fuel economy?

On one hand I feel like injecting more fuel with the turbo would decrease mpg, but on the other hand I'm not sure if that is balanced out by pushing the non-turbo harder.

Just found this Canadian government site Learn the facts: Turbocharging and its impact on fuel consumption, that states:

Turbochargers force air into the engine’s cylinders – in contrast to a naturally aspirated engine that draws air in at atmospheric pressure. This feature enables a smaller displacement, turbocharged engine to produce the same power as a naturally aspirated engine that has a larger displacement. Using a smaller, turbocharged engine can reduce fuel consumption by 2 to 6% for equal vehicle performance, saving you money and reducing your impact on the environment.

This looks to be a statement along the same lines as Mohan's answer, but I still can't picture how this relates to being at altitude.

  1. Discounting any lead-foot guilty pleasure of pushing the turbo to the max.
Peter M
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1 Answers1

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If you think of fuel simply as a unit of energy, so a lite of fuel has x amount of calories*

The engine, by burning this converts the energy into (the bit you're interested in) kenitic energy. The tricky part is the efficiency with which the engine can do this.

The ideal ratio of petrol to air is 1:14.7 -https://www.google.com/search?q=stoichiometric+air+fuel+ratio

So at the ideal, less air just means less fuel. So at altitude, you will just have less power.

Sadly, when accelerating the mix runs rich, so you're using more fuel then the above ideal, and the more time you are in this state the worse the fuel economy.

In your case, a turbocharged engine will be closer to it's usual operating air pressure, therefore fuel use so you'll have little loss of power, and probably not have to accelerate as hard and as often and will probably be closer to the manufacturers MPG rating... However, in this instance and very much literally, your mileage may vary ;-)

This graph shows that the best torque (acceleration) is at 12:1, so to maintain that torque for longer will use more fuel

http://www.endtuning.com/images/airfuel.jpg

  • There are 9,241,059 calories in a litre of diesel and 8,325,818 in a litre of petrol.
RemarkLima
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