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Re: [ST] for Brett - fuel economy



>From: Rik Relph
>At altitude there are less oxygen molecules per cubic foot of air, therefore
>to get the correct mixture you lean the mixture.
>When riding, the bike will always be more efficient on a cold day during
>high pressure for the same reason, more oxygen will be drawn into the
>cylinders, making the burn more complete and producing more power. More
>power means less throttle is used for the same speed.

If you have a plane or bike with injection, that leaning will be done
automatically. Of course the engine gets less air at altitude, but so
does the fuselage / fairing / rider trying to plow through it. In order
to get the same air drag, the plane / bike at altitude can fly/ride
faster. Less drag means less power needed for the same speed or higher
speed for the same energy. If you would like to have the benefits of
less air drag at altitude without the disadvantage of running with lower
burn energy to reach the same ideal mixture, get a turbo :-).

But then you'd be talking about altitudes above 10.000 feet before the
turbo pays itself in terms of efficiency.

Going back to bikes, a cold high pressure day might bring more acceleration,
but that same air density should also restrict your top speed more (so more
efficiency only at lower (air)speeds)... I think. A proper injection system
should (in theory at least) help to keep the optimum mixture, independent
upon air density (autolean), for better fuel efficiency even at altitude.

Emile
www.piloot.com

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