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If oxygen %age of atmosphere was raised to i) 25% . ii) 30% . iii) 35%
How much would it affect the power output/fuel efficiency of current engines?

Engines being 1) Petrol Engines . 2) Diesel Engines . 3) Turbine Engines
and what sort of adjustment could these engine use to make better use this oxygen richer atmosphere?

Biosphere and humans are already adapted to this higher oxygen, so don't worry about that

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2 Answers 2

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Air-Fuel Ratio

As told here

For proper burning/combustion of fuel(petrol) engine require oxygen, and this oxygen is supplied to engine by atmospheric air. The ratio of mass of air sucked by engine to mass of fuel injected is air-fuel ratio. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as the Stoichiometric mixture or Stoichiometric air-fuel ratio. Stoichiometric air-fuel ratio for petrol(SI engine)is 14.5:1 . A/F ratio less than 14.5 means rich mixture and A/F ratio greater than 14.5 means lean mixture.

  • For cold starting (simply when your engine parts are not warmed)- 9:1 (rich of fuel)

  • For Idling(when you stop at signal without turning off engine)- 12:1 (rich mixture)

  • for cruising (when you run engine at constant medium/economy speed)- 16:1(lean mixture)

  • When you accelerate bike/car - 13:1 (Rich mixture but require less fuel than idling)

Higher oxygen content

At present, oxygen in atmosphere is 21%. If you increase or decrease the %age of oxygen in atmoshere to x%, then you will multiply the ratios above with 21/x.

Example:

Stoichiometric air-fuel ratio (at 21%) for petrol(SI engine)is 14.5:1.

If oxygen is 30%, then Stoichiometric air-fuel ratio will be 14.5*(21/30) = 10.15:1.

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  • $\begingroup$ Just to point out - modern cars are on much leaner mixture, even petrol-powered. Right now modern, Fuel-injected SI engines can near 20:1 ratio (which is higher than early Diesel engines). But that's just a small point, nothing to invalidate the answer, because if the oxygen levels in atmo would to increase, the modern engines would adjust automatically. Though I don't know how well. $\endgroup$
    – AcePL
    Commented Jul 1, 2022 at 11:00
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With a higher oxygen-percentage, you can either get more power from the same size engine, or you can build smaller engines for the same power output.

As imtaar explained in detail, the higher oxygen percentage means you can burn more fuel for any given volume of air. The size of any of these engines is determined by the size of the combustion chamber, which means that for the same amount of power, you can get by with smaller combustion chambers, and thus lighter engines. Since engines themselves are a substantial part of the weight of every vehicle, especially every airborne vehicle, you get a lower overall weight and in return an even smaller power demand, again reducing the size (and thus weight) of your engine.
Another substantial part of the weight of the vehicle is the fuel. A higher oxygen percentage does not reduce the amount of fuel you need to carry. for every given engine, the power output is directly linked to the amount of carbon atoms you feed to the engine, so baring efficiency gains that might or might not result from the size reduction, your fuel consumption would stay the same.

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