What Gives gasoline it's Kick?

[DLazok]

In the old days of gas engine and gasoline technology, more power was teased out of gasoline by advancing engine timing, and using premium gas (with Tetra Ethyl Lead) as a flame-front moderator, so the charge burned through more of the down stroke - instead of exploding at the top of the cylinder [as I was led to believe].
Now that the lead is gone, what gives the unleaded fuel the extra kick, so that my variable valve timing (etc.) Lexus gets great mileage and power availability on premium, or almost the same performance (but much less fuel economy) by using regular unleaded gas (RUG)?
Do the refiners work with different molecule lengths for different grades? How does my car feel the difference in octane?
I'm not trying to concoct a fuel booster ... I just want to know. I'm also trying to justify filling with midgrade or premium. Is there such thing as an additive anymore (like 104 or some such, or is it all snake oil?)
Dave

 

[Tree Hugger]

Higher octane gas simply burns slower than low octane gas.
There is no difference in the energy contained in them, or release by burning them.
What is happening in your engine is that when you use low octane gas, the computer retards the ignition to prevent detonation and so the main burning takes place later in the stroke, not at the most effective place as the engine was designed for using high octane gas.
You are getting the same energy out of the gasoline in both cases, but you're wasting more of it as heat with low octane gas in your engine, instead of turning as much of that heat into useful work as the engine was designed to do.

 

[punisher]

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Higher octane gas simply burns slower than low octane gas.

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Gotta pick on that one. Burn rate and octane rating are independant properties. The octane rating only applies to the antiknock qualities of the gasoline.

 

[another Todd]

I agree with the Tree Hugger (did I just say that?). The slower burn prevents the knocking.

 

[Steve S]

The engine works the same way except a computer controls the ignition advance ,fuel mixture etc by using inputs from sensors that tell the computer what is going on. The gas works the same way but with out lead.

 

[Dominic]

And there is a very slight increase in BTU content in premium fuels. I can look it up if I have to, because I once did. It's something like 126,000 BTU for non EtOH laden gasoline and 128,400 for premium.

 

[Shannow]

Nope, the flame speeds of both high and low octane petrols are all but identical.

This "burns slower" is a myth that needs squashing everywhere it appears.

 

[Merkava_4]

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What is happening in your engine is that when you use low octane gas, the computer retards the ignition to prevent detonation and so the main burning takes place later in the stroke, not at the most effective place as the engine was designed for using high octane gas. You are getting the same energy out of the gasoline in both cases, but you're wasting more of it as heat with low octane gas in your engine, instead of turning as much of that heat into useful work as the engine was designed to do.

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DEAD ON!

 

[moribundman]

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Nope, the flame speeds of both high and low octane petrols are all but identical.

This "burns slower" is a myth that needs squashing everywhere it appears.

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I suppose "ignites later" is a more concise description that still maintains the temporal element.

 

[Shannow]

nope, the delay times are still very similar.

It's a "resistance to auto-ignition".

 

[Autodarken]

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What is happening in your engine is that when you use low octane gas, the computer retards the ignition to prevent detonation and so the main burning takes place later in the stroke, not at the most effective place as the engine was designed for using high octane gas. You are getting the same energy out of the gasoline in both cases, but you're wasting more of it as heat with low octane gas in your engine, instead of turning as much of that heat into useful work as the engine was designed to do.

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DEAD ON!

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No, actually there's one word that makes the statement false. "Prevent". The ECM does NOT "prevent" knocking or preignition, the ECM "reacts" to it, and THEN retards timing and or other parameters just enough to stop the knock. All this happens in split seconds and you may never even realize it.

Computers are not "proactive" is really what I am saying, they are "reactive" per code to change parameters based only the inputs recieved, also called "closed loop". The only exception is when not at operating temp, and running mainly on a set program - the term is "open loop".

And thanks to most of you squashing the "slow burn" myth of 93 octane pump gas. As said, its only an added resistance to preignition and knock.

The actual compounds that control the octane level are easily found on the web. (ethanol can be one of them for those forced to run E10)

 

[Zaedock]

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nope, the delay times are still very similar.

It's a "resistance to auto-ignition".

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This is correct. The octane rating of gas indicates how much the fuel can be compressed before it spontaneously ignites. Higher HP engines typically have higher compression and require higher octane fuel to prevent detonation.

 

[TimVipond]

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Do the refiners work with different molecule lengths for different grades? How does my car feel the difference in octane?

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It's been a few years since I spent some time at a refinery, but I think they always try to make a high octane premium gasoline. It is then tested in an engine to determine the motor octane number. That test determines if it can be sold as Premium or Regular gas. The mid range is usually just a blend of the two grades at the pump. The octane number on the pump is a minimum value. The true octane number can be higher than posted, but not lower. Sometimes too much Premium is made and has to be put in the Regular tank. So sometimes you pay for Regular and get Premium.

 

[mechtech2]

Higher octane fuel is more resistant to detonation and pre-ignition. Once burning, it may be a few nanoseconds slower, but that is not the advantage of it.
You can use more ignition advance with it, which usually helps power and economy.
With today's cars, there only so much advance in the computer map, and it will only take away ignition advance, primarily when the knock sensor tells it to.
So if you never activate the knock sensor on mid grade, premium is a waste.
Knock sensors are pretty crude microphones. They are set off by all sorts of engine noise, and when the computer pulls timing, it PULLS timing - a lot.
I have disconnected mine, and have a smoother and better powerband. I am careful to use the proper fuel, though.
Many enthusiast forums talk about the 'knock sensor mod', and it can sure be a benefit.

 

[CaspianM]

If I recall correctly from school, higher octane is an index indicating how uniform in combustion properties a sameple of fuel is. What it means is that the combustion curve (start time~finish time & duration)is more predicatble.
Think of it like a subwoofer with boomy response and wrong delay vs a tight bass with accurate timing.
There is also more to it but memory is fuzzy.

 

[Max_Wander]

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I have disconnected mine, and have a smoother and better powerband. I am careful to use the proper fuel, though.
Many enthusiast forums talk about the 'knock sensor mod', and it can sure be a benefit.

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Howd u pull that off, which car? When i pull mine, i get a check engine light and the ECU refers to a special "no knock sensor present" ignition table .. which is very conservative down low.. I used to have a spare knock sensor that I would plug in and ground somewhere in a quiet area. performed real good with that mod

 

[ARB1977]

FP60, hehe.

 

[Black Bart]

Here is a site that deals with the subject of detonation.
Some who have posted in this thread should read this.
http://zhome.com/ZCMnL/PICS/detonation/detonation.html

 

[Tree Hugger]

direct quote from that site...
8. Run the best unleaded fuel you can. If the compression ratio of the engine is up near 11.0:1, it will love 100-octane unleaded. Remember, the higher the octane, the slower the fuel burns.

 

[mechtech2]

Max - I have a Ford Zetec in a ZX2. Honda guys also do this regularly.
Like you noticed, sometimes insulating the sensor while leaving it plugged in is necessary.
But isn't it nice?