Octane, Compression Ratio, and Detonation.
#41
03-30-2007, 01:14 PM
RE: Octane, Compression Ratio, and Detonation.
I understand the argument...or a better word...reason...to use low octane fuel.
I also understand how an engine works...haven't repaired or rebuilt every single one made...but have some and understand...
Intake..the piston in a given cylinder is traveling down...creating a vaccum...thus INTAKING...the fuel...
Compression...piston is beginning to travel up...making the cylinder area smaller and smaller as the piston rises...and gets closer to both
the intake valves and the spark plug, which ignites the gas molecules, tiny droplets of both fuel and air, aka atomized mixture.
This gets all that wonderful gasoline and air mixture, which is what explodes/burns and forces the piston to turn the crankshaft,
into the most compact and efficient gaseous form possible.
Ignition...this starts just about when the piston is at it's highest point...just before top...dead...center...or TDC.
When the piston is at full TDC...ideally this is when the fuel and air mixture should be at it's most dense, causing the cleanest burn
(controlled explosion) possible.
(Before TDC is how you set the timing for a particular engine...but that's another topic.)
Exhaust...any unspent energy not efficient enough to turn the crank, which turns the sprockets, which drive the chain,
which turns the rear wheel, which propels you forward....wheeeeeeeee....
the waste products from the unburned fuel, fuel is not absolutely pristine and sterile stuff afterall...
...and to dis-allow a build up of so much pressure that the enitre engine would eventually explode...
The Exhaust valve opens...the gases run out the valves...into the headers...down the pipe(s)...and out the exhaust canister.
That is what is known in laymen's terms as Suck...Squeeze...Bang...Blow.
Having said all that....?
My 94 F2 run better on 93 octane fuel...whether it's supposed to or not.
The engine has more power...has less naturally occuring stumble...and makes a lovelier hum on 93.
On 86...it runs like ***.
Go figure.
I also understand how an engine works...haven't repaired or rebuilt every single one made...but have some and understand...
Intake..the piston in a given cylinder is traveling down...creating a vaccum...thus INTAKING...the fuel...
Compression...piston is beginning to travel up...making the cylinder area smaller and smaller as the piston rises...and gets closer to both
the intake valves and the spark plug, which ignites the gas molecules, tiny droplets of both fuel and air, aka atomized mixture.
This gets all that wonderful gasoline and air mixture, which is what explodes/burns and forces the piston to turn the crankshaft,
into the most compact and efficient gaseous form possible.
Ignition...this starts just about when the piston is at it's highest point...just before top...dead...center...or TDC.
When the piston is at full TDC...ideally this is when the fuel and air mixture should be at it's most dense, causing the cleanest burn
(controlled explosion) possible.
(Before TDC is how you set the timing for a particular engine...but that's another topic.)
Exhaust...any unspent energy not efficient enough to turn the crank, which turns the sprockets, which drive the chain,
which turns the rear wheel, which propels you forward....wheeeeeeeee....
the waste products from the unburned fuel, fuel is not absolutely pristine and sterile stuff afterall...
...and to dis-allow a build up of so much pressure that the enitre engine would eventually explode...
The Exhaust valve opens...the gases run out the valves...into the headers...down the pipe(s)...and out the exhaust canister.
That is what is known in laymen's terms as Suck...Squeeze...Bang...Blow.
Having said all that....?
My 94 F2 run better on 93 octane fuel...whether it's supposed to or not.
The engine has more power...has less naturally occuring stumble...and makes a lovelier hum on 93.
On 86...it runs like ***.
Go figure.
#42
03-30-2007, 04:13 PM
RE: Octane, Compression Ratio, and Detonation.
this topic needs some science...not just blather. I wrote most of this some time ago on another forum...for sleds in particular.
Tahoe mentioned the RON and (R+M)/2 versions of octane number..he's right on. The octane requirement of a straight RON number would be a higher number than an (R+M)/2....so check what the mfr calls for.
Octane is a measure of the energy it takes to ignite a fuel (resistance to detonation is a different perspective, but gets the same point across). It is NOT a measure of the energy in the fuel. Power improvements from utilizing higher octane are a result of improved effeciency in the combustion process. Energy in the cylinder has various forms and comes from the compression of the air/fuel, from the heat in the piston/head/cylinder walls/spark plug and the last energy to throw gas over the hump is from the spark itself. This energy input is necessary to create the downhill chemical process....fuel+O2+energy = water, CO2, and LOTS of energy. When you don't have enough energy to get it over the hump the combustion process is not completed in the intial event.
Ideally the fuel/air mix is ignited by the spark before TDC and the flame front travels out radially from that point....and by the time the process createspressure, the piston is on the way down (we are talking a couple degrees after TDC). This event is obviously VERY fast...but so is the piston speed at 7000rpm.
In lean mixture, the flame front has trouble propagating. In a rich mixture, the extra fuel robs heat from the reaction, or prevents the spark from jumping the gap in the plug.
If you use a higher octane than called for, not all the fuel ignites from the initial energy put into the chamber by the spark (remember the lower octane RON number portion of the fuel)...for the rest ofthe fuelto ignite, it takes the added energy of heat and pressure caused by theflame front to ignite it. This can create other smaller flame fronts in remote area's that impeed progress of the initial front in the worst cases. Regardless, by the time full combustion has happened its too late for it to contribute to the power making process....well, not very efficiently anyway....you are building pressure, but the piston is already moving away.
Higher octane is not running your engine lean in the sense of mixture, but a similar effect on power....a more comparable description would be retarding your timing. You wouldn't change jetting to make the best use of higher octane, you would change your ignition timing or compression. Newer cars with knock sensors do the prior. You bike doesn't have a knock sensor, and the ecu has no data logging or avergaing capbilities...there'sno reseting or whatever.
Ethanol (alcohols) actually has a higher octane rating than gasoline. Many gas station sell 87 octane with 10% ethanol added to sell you89 octane. However, ethanolhas a lower energy content and is hydrophillic (absorbs water). Yeah, thats bad. It creates a lot of difficulties for the gas companies in terms of mixing and transporting. Ethanol mixtures will run lean in carbureted engines...bikes, sleds, weed eaters...you name it. It can be compensated for in jetting.
Compression ratio alone doesn't dictate octane...though they go hand in hand. 2 strokes will always be lower compression than a 4 stroke on the same octane because the cylinder is warmer....it doesn't get a whole pumping cycle to cool off.
Static compression is the number everyone throws around, and is a ratioof the swept cylinder volume to compressed volume. And truely, running compression ratio's are mostly anything but that number....cylinder filling efficiecies are 75-80% at low rpms (and WOT)but theyexceed 100% at high rpms (and Wide Open Throttle)...how is by acc
Tahoe mentioned the RON and (R+M)/2 versions of octane number..he's right on. The octane requirement of a straight RON number would be a higher number than an (R+M)/2....so check what the mfr calls for.
Octane is a measure of the energy it takes to ignite a fuel (resistance to detonation is a different perspective, but gets the same point across). It is NOT a measure of the energy in the fuel. Power improvements from utilizing higher octane are a result of improved effeciency in the combustion process. Energy in the cylinder has various forms and comes from the compression of the air/fuel, from the heat in the piston/head/cylinder walls/spark plug and the last energy to throw gas over the hump is from the spark itself. This energy input is necessary to create the downhill chemical process....fuel+O2+energy = water, CO2, and LOTS of energy. When you don't have enough energy to get it over the hump the combustion process is not completed in the intial event.
Ideally the fuel/air mix is ignited by the spark before TDC and the flame front travels out radially from that point....and by the time the process createspressure, the piston is on the way down (we are talking a couple degrees after TDC). This event is obviously VERY fast...but so is the piston speed at 7000rpm.
In lean mixture, the flame front has trouble propagating. In a rich mixture, the extra fuel robs heat from the reaction, or prevents the spark from jumping the gap in the plug.
If you use a higher octane than called for, not all the fuel ignites from the initial energy put into the chamber by the spark (remember the lower octane RON number portion of the fuel)...for the rest ofthe fuelto ignite, it takes the added energy of heat and pressure caused by theflame front to ignite it. This can create other smaller flame fronts in remote area's that impeed progress of the initial front in the worst cases. Regardless, by the time full combustion has happened its too late for it to contribute to the power making process....well, not very efficiently anyway....you are building pressure, but the piston is already moving away.
Higher octane is not running your engine lean in the sense of mixture, but a similar effect on power....a more comparable description would be retarding your timing. You wouldn't change jetting to make the best use of higher octane, you would change your ignition timing or compression. Newer cars with knock sensors do the prior. You bike doesn't have a knock sensor, and the ecu has no data logging or avergaing capbilities...there'sno reseting or whatever.
Ethanol (alcohols) actually has a higher octane rating than gasoline. Many gas station sell 87 octane with 10% ethanol added to sell you89 octane. However, ethanolhas a lower energy content and is hydrophillic (absorbs water). Yeah, thats bad. It creates a lot of difficulties for the gas companies in terms of mixing and transporting. Ethanol mixtures will run lean in carbureted engines...bikes, sleds, weed eaters...you name it. It can be compensated for in jetting.
Compression ratio alone doesn't dictate octane...though they go hand in hand. 2 strokes will always be lower compression than a 4 stroke on the same octane because the cylinder is warmer....it doesn't get a whole pumping cycle to cool off.
Static compression is the number everyone throws around, and is a ratioof the swept cylinder volume to compressed volume. And truely, running compression ratio's are mostly anything but that number....cylinder filling efficiecies are 75-80% at low rpms (and WOT)but theyexceed 100% at high rpms (and Wide Open Throttle)...how is by acc
#43
03-30-2007, 05:29 PM
RE: Octane, Compression Ratio, and Detonation.
First of all...excellent post...though some may find it a bit...wordy...
I liked it...
Ethanol ruined the fuel system in my old...220k+ Mits Eclipse.
Also, it costs more to make a gallon of ethanol gas than it is sold for...so yes...politics.
Where do you get this U4 race gas...and what jetting kit are we talking?
I liked it...
Ethanol (alcohols) actually has a higher octane rating than gasoline. Many gas station sell 87 octane with 10% ethanol added to sell you89 octane. However, ethanolhas a lower energy content and is hydrophillic (absorbs water). Yeah, thats bad. It creates a lot of difficulties for the gas companies in terms of mixing and transporting. Ethanol mixtures will run lean in carbureted engines...
Also, it costs more to make a gallon of ethanol gas than it is sold for...so yes...politics.
Where do you get this U4 race gas...and what jetting kit are we talking?
#44
03-30-2007, 06:56 PM
RE: Octane, Compression Ratio, and Detonation.
ORIGINAL: chainstretcher
Generally the super contains extra detergents -- so if you get carbon buildup on your piston crown from incomplete burn then hopefully the detergents will clean it up
Generally the super contains extra detergents -- so if you get carbon buildup on your piston crown from incomplete burn then hopefully the detergents will clean it up
Can get it from Hein Gericke and probably other places too.
#45
03-30-2007, 07:24 PM
RE: Octane, Compression Ratio, and Detonation.
now why would a car designed to run on 87 require higher octane?
it's possible that due to age, there is enough build up of carbon deposits, etc. within the cylinders to increase operating temps dramatically, pressure, etc. lower octane fuels in this environment would combust even before TDC due to the high operating temps...so you stick in higher octane and it makes the engine run much better.
i'm not saying this is what is up with BMG's bike, but just some possible answers.
anyone remember dad's old a$$ olds or buick that always pinged and ran like crap...and if you turned off the engine, crazy car would still sit there idling for a bit?
put high octane in there and it ran great...and we stood around and said...WTF? hahahahahhah
it's possible that due to age, there is enough build up of carbon deposits, etc. within the cylinders to increase operating temps dramatically, pressure, etc. lower octane fuels in this environment would combust even before TDC due to the high operating temps...so you stick in higher octane and it makes the engine run much better.
i'm not saying this is what is up with BMG's bike, but just some possible answers.
anyone remember dad's old a$$ olds or buick that always pinged and ran like crap...and if you turned off the engine, crazy car would still sit there idling for a bit?
put high octane in there and it ran great...and we stood around and said...WTF? hahahahahhah
#46
03-30-2007, 08:11 PM
RE: Octane, Compression Ratio, and Detonation.
ORIGINAL: Tahoe SC
now why would a car designed to run on 87 require higher octane?
it's possible that due to age, there is enough build up of carbon deposits, etc. within the cylinders to increase operating temps dramatically, pressure, etc. lower octane fuels in this environment would combust even before TDC due to the high operating temps...so you stick in higher octane and it makes the engine run much better.
i'm not saying this is what is up with BMG's bike, but just some possible answers.
anyone remember dad's old a$$ olds or buick that always pinged and ran like crap...and if you turned off the engine, crazy car would still sit there idling for a bit?
put high octane in there and it ran great...and we stood around and said...WTF? hahahahahhah
now why would a car designed to run on 87 require higher octane?
it's possible that due to age, there is enough build up of carbon deposits, etc. within the cylinders to increase operating temps dramatically, pressure, etc. lower octane fuels in this environment would combust even before TDC due to the high operating temps...so you stick in higher octane and it makes the engine run much better.
i'm not saying this is what is up with BMG's bike, but just some possible answers.
anyone remember dad's old a$$ olds or buick that always pinged and ran like crap...and if you turned off the engine, crazy car would still sit there idling for a bit?
put high octane in there and it ran great...and we stood around and said...WTF? hahahahahhah
and then letting off and blipping the throttle, or downshifting...and it's smooth as silk.
Just runs better on 93 octane.
Also runs best on Chevron gas as opposed to say Shell or a local Mobile station.
Part of it may be my imagination...part of it may be the gas...and part of it may be the never ending debate over this topic.
I've read it both ways...
A low octane fuel may burn all at once (detonate) or even be
ignited by hot spots or carbon chunks in the cylender and ignite too early.
Preignition and detonation cause severe wear and damage to the lower end of
the engine such as pistons, con rods and bearings. The high octane fuel
burns slowly and evenly and should be finished by the time the valve opens.
The higher octane also has a higher combustion temp. than the low octane and
will not preignite.
On the other hand, using too high of an octane can burn valves. Because
of the slower burning properties of high octane fuels, the low octane
engines will open the exhaust valve before the fuel/air mixture has finished
burning and will elevate the valve and exhaust manifold temps causing
premature valve wear or failiure and may burn holes in the exhaust.
So, the best thing to do is use the octane rating in the owner's manual
ignited by hot spots or carbon chunks in the cylender and ignite too early.
Preignition and detonation cause severe wear and damage to the lower end of
the engine such as pistons, con rods and bearings. The high octane fuel
burns slowly and evenly and should be finished by the time the valve opens.
The higher octane also has a higher combustion temp. than the low octane and
will not preignite.
On the other hand, using too high of an octane can burn valves. Because
of the slower burning properties of high octane fuels, the low octane
engines will open the exhaust valve before the fuel/air mixture has finished
burning and will elevate the valve and exhaust manifold temps causing
premature valve wear or failiure and may burn holes in the exhaust.
So, the best thing to do is use the octane rating in the owner's manual
Then this guy...
There have been a lot of ideas thrown out there about this, so here's
another one.
Think of higher octane gasoline as a slower burning fuel. All things
being equal, it takes longer to ignite and longer to burn. Thus, high
octane fuel is desireable in situations where the tendency to knock
is high, such as high compression ratios and forced induction. In
essence (no pun intended), higher octane fuel needs more ambient heat
in order to start the reaction.
In order to make use of the higher octane fuel in an engine which is
designed for low octane fuel (while keeping the same spark plugs),
one must advance the timing--so that the spark plug fires a little
prematurely. In this configuration, higher octane fuel will generally
make a low octane engine feel smoother.
However with everything setup stock, you will gain nothing from
higher octane fuel. If you hear pinging and you cannot eliminate it,
then you should try a slightly higher octane--but only enough to
eliminate it.
another one.
Think of higher octane gasoline as a slower burning fuel. All things
being equal, it takes longer to ignite and longer to burn. Thus, high
octane fuel is desireable in situations where the tendency to knock
is high, such as high compression ratios and forced induction. In
essence (no pun intended), higher octane fuel needs more ambient heat
in order to start the reaction.
In order to make use of the higher octane fuel in an engine which is
designed for low octane fuel (while keeping the same spark plugs),
one must advance the timing--so that the spark plug fires a little
prematurely. In this configuration, higher octane fuel will generally
make a low octane engine feel smoother.
However with everything setup stock, you will gain nothing from
higher octane fuel. If you hear pinging and you cannot eliminate it,
then you should try a slightly higher octane--but only enough to
eliminate it.
[quote]
Effects of octane rating Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause knocking. (Note that it is the absolute pressure (compression) in the combustion chamber which is important - not the compression ratio. The compression rati
#47
03-30-2007, 08:16 PM
RE: Octane, Compression Ratio, and Detonation.
It might seem odd that fuels with higher octane ratings explode less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist compression detonation as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.
#48
04-01-2007, 03:16 PM
RE: Octane, Compression Ratio, and Detonation.
I'm in agreement with the Wikipedia definition. Activation energy is perfect wording.
If your bike runs better on higher octane than called forit could be either the carbon build up issues, or your jetting.....remember, a high octane has an effect of leaning out the mixture....those fuel molecules with high "activation energy" don't combust in the initial event and hence use none of the oxygen.
If your bike runs better on higher octane than called forit could be either the carbon build up issues, or your jetting.....remember, a high octane has an effect of leaning out the mixture....those fuel molecules with high "activation energy" don't combust in the initial event and hence use none of the oxygen.
Thread
Thread Starter
Forum
Replies
Last Post
