Re: facts about high octane petrol - Saab General Bulletin Board - Saabnet.com
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Re: facts about high octane petrol
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Posted by Dean (more from Dean) on Sun, 6 May 2012 08:55:51 Share Post by Email
In Reply to: facts about high octane petrol, JohnA [Profile/Gallery] , Sat, 5 May 2012 16:41:14
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Combustion deposits get hotter than the piston, head and valves and can act as ignition sources under conditions where knocking becomes possible. When this happens, one needs higher octane fuel to avoid knocking. This is called acquired octane demand. To deal with this problem, high octane fuels contain cleaners to avoid this problem. Otherwise, the engine will us a less aggressive ignition timing, which can reduce power and perhaps MPGs as well.

Fuel system cleaners can remove combustion chamber deposits, which can improve power if there were deposits that were increasing octane demand.

High compression engines, non turbo, are quite vulnerable. Turbo charged engines run rich [aka fat] on boost and that increases the effective octane.

High compression engines are more fuel efficient than turbo charged engines as one cannot turbo charge a high compression engine as the amount of boost is limited with available octane fuels.

There are now engines that have a CR=13 or so. These are direct injection engines and fuel is delivered over a period of time which avoids having all of the AF charge detonating in an instant. You will not see turbocharging at these high compression ratios. However, turbocharged and direct injection may play very well together.

Saab was working on a direct injection engine with variable compression. That allowed for high MPG's off boost and lower CR for on boost high power. It never got off of the test stand and had major issues that did not appear to be solvable in a production unit.

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