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Gasoline Direct Injection

Old 02-04-2009, 02:05 PM
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Default Gasoline Direct Injection

Does the octane rating for gasoline matter in a direct injection engine? I would think not because detonation should not be an issue when the fuel is injected directly into the cylinder as is diesel but dealer ship is telling me it needs premium.
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Old 02-04-2009, 02:07 PM
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Default Re: Gasoline Direct Injection

what engine?
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Old 02-04-2009, 02:10 PM
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Default Re: Gasoline Direct Injection

Sportfisher236 - 2/4/2009 1:07 PM

what engine?
I was/am looking at cars but would think that this would be standard across the board, outboard/car or honda/yamaha/bmw. Why would this be engine specific?
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Old 02-04-2009, 02:38 PM
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Default Re: Gasoline Direct Injection

I was told by lexus and BMW that you did not have to run premium. It won't hurt anything to run regular. they both said it may hurt performance (mpg) but not the engine.
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Old 02-04-2009, 03:40 PM
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Default Re: Gasoline Direct Injection

If the engine has a knock sensor that detects preignition/detonation, the ECM will bump the timing back to accommodate lower octane fuels. At part throttle cruise you will notice no difference. Fuel can spontaneously combust with DI..this is what detonation is.
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Old 02-04-2009, 04:53 PM
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Default Re: Gasoline Direct Injection

Mazda/Ford 2.3 MZR turbo very unhappy with regular
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Old 02-04-2009, 06:09 PM
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Default Re: Gasoline Direct Injection

Compression will have more to do with octane requirements. High compression motors need high octane to keep from detonating. The old school rule was 10 times the octane to compression. 9.0 compression needed 90+ octane. I don't know if that still applies. My car was forced to use 87 octane during our gas shortage over the summer. It did not detonate but it clearly did not like it.
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Old 02-05-2009, 02:14 AM
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Default Re: Gasoline Direct Injection

Octane "matters" in all gasoline engines. If you're asking if GDI engines have special requirements, the answer would be "no". The static compression is typically higher than a similar port-injected engine, but spontaneous combustion / auto-ignition isn't an issue because fuel isn't present in the combustion chamber until just prior to ignition.

The GM 3.6 liter (3564 cc) V-6 LLT, for example, uses 87 octane (regular) gasoline.

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Old 02-05-2009, 03:26 PM
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Default Re: Gasoline Direct Injection

sport/bill/hevy/reel/pep, that is what I understand, the knock sensor detects it and then retards the timing as needed. The reason for the question is because the GDI injects the fuel on the compression stroke unlike a EFI which injects the fuel on the intake stroke so no fuel is present until the piston is moving up so even at extreme compression ratios there would not be fuel to detonate until it is injected. This is why I question wether fuel grade matters. It would seem as though if it started detonating then the injection timing could be adapted to stop it.
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Old 02-05-2009, 03:32 PM
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Default Re: Gasoline Direct Injection

Usually spontaneous combustion-detonation- only occurs when the heat of compression ignites the mixture like a diesel. Pre-ignition is lighting off the fuel burn early, but the fuel still burns at a finite rate. Detonation can occur independent of ignition.
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Old 02-05-2009, 05:05 PM
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Default Re: Gasoline Direct Injection

The key point is that there is no fuel in the combustion chamber to spontaneously ignite. The fuel is injected just prior to the the ignition event, and the highly atomized fuel significantly reduces combustion chamber air temperature, adding to knock resistance. This doesn't mean that GDI engines are "knock-proof" and that octane is irrelevant, as it is still undesirable for uncontrolled combustion to occur. These engines are designed and calibrated around normal commercially-available fuels, and thus have rather conventional octane requirements - and these are not "less than regular" requirements.

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Old 02-05-2009, 06:42 PM
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Default Re: Gasoline Direct Injection

Interesting, pep. All this new technology..I've still gotta screen under my model A carburetor to atomize fuel.......... Spraying the fuel at the ultra high pressures required must atomize it immediately. At those pressures alcohol, water, gasoline everything atomizes. It's amazing they can control the injection with the short event duration that exists at high rpms.


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