Re: Only if you have a lead foot? - Saab 9-5 Bulletin Board - Saabnet.com
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Re: Only if you have a lead foot?
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Posted by jeff in MD (more from jeff in MD) on Sat, 26 Mar 2011 07:26:38 Share Post by Email
In Reply to: Re: Only if you have a lead foot?, Sam Stoney, Sat, 26 Mar 2011 05:20:04
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I think you may have it wrong. Turbocharging an engine improves the efficiency of the engine, ie more power for the same amount of fuel, by increasing the effective compression ratio with "free" energy from the exhaust gas. If you do something to reduce this effect, ie use regular fuel which causes a reduction in boost pressure, you are necessarily reducing the efficiency of the engine, therefore requiring more fuel to produce the same amount of power.

The other way to think about it is that a turbo car has a lower native compression ratio (and therefore less efficient) than otherwise would be optimal because it's designed to operate with boost. Reducing the available boost would seem to take away from the available benefit.

Consider the E85-optimized Saab Biopower engines you can get overseas. They are optimized for running on E85 with a heavy dose of extra boost for a significant power increase, on the order of 20%. But, when you normalize the fuel efficiency to account for the ethanol's lower energy content, the engine actually proves to be more efficient - 3% on the highway test and 7% on the city test. See the report here https://www.eecbg.energy.gov/biomass/pdfs/analysis_saab2007.pdf
So there's some evidence that a turbo engine can produce both more power and better fuel economy with a higher octane fuel.

jeff



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