Some of you older guys might remember an article Gordon Jennings wrote many years ago debunking various motorcycle myths. It was a fantastic article full of interesting facts and lots of good humor. I'm certainly no Gordon Jennings but I'd like to revive that idea with an article of my own but I'd like some thoughts from you guys on myths that you would like to see addressed.
Here's my starting point for the article with a couple of pet peeve myths of mine :
Myth # 1 – Toluene is a magic fuel additive that adds power by just dumping it in gas tank
Reference grade pure toluene vaporizes in the 240-250F range with a basically flat curve, but only lab guys with big checkbooks get that stuff. The commercially available stuff (Home Depot, paint store etc. ) has lots of distillation byproducts in it so just like pump fuel it has a high temperature distillation curve that varies from load to load, so you'll see a curve with anything in the 280- +400F range in commercial grade Toluene.
Anything in this temperature range isn't likely to vaporize above 7000rpm in current MX engines so all the additive accomplishes is tweaking the distillation curve and the overall fuel curve i.e the jetting.
There is no extra energy release value to Toluene either as the gross combustion value for it is on par with everything else normally used in fuels.
Toluene is a decent additive if all you want to do is protect the engine from fragging itself due to detonation. It has reasonably high octane and it is relatively cheap. The downside is because additives like these are fairly reluctant to vaporize they have proven to make for sloppy throttle response, poor carb circuit transitions and poor high rpm running in engines like ours that have short intake tracts and run for sustained periods above 7000 rpm. The greater the percentage of Toluene the more these issues are likely to surface. This will tend to be true for Toluene based octane boosters as well.
I’ve seen a number of instances of Internet automotive sites taking little chunks of info and using them out of context in an attempt to prove the efficacy of Toluene in producing additional power by virtue of “heat of combustion” numbers. In doing this they are are ignoring the fact that the maximum combustion heat produced by any fuel can be expressed as a function of its stoichiometric ratio and net heat of combustion. While fuels may have varying heats of combustion and stoichiometric ratios, when compared by their heat of combustion per pound of air, most aromatic hydrocarbons toluene included, and standard pump fuels are all pretty similar.
For those who haven't dozed off, here's how it works:
Reference grade toluene has a net heat of combustion = 17424 (btu/lb), and it's chemically correct (stoichiometric) A/F ratio is about 13.8:1 . This comes out to a heat of combustion per pound of air value of 1262. Average pump premium has net heat of combustion in the neighborhood of 19,000 (btu/lb) andthe A/F ratio is about 15:1 which comes out to a heat of combustion per pound of air value of 1266 just a touch better but nothing we would notice. A good oxygenated race fuel like Firepower 324)has net heat of combustion of 18,747 (btu/lb) and the A/F ratio is 13.84 which comes out to a heat of combustion per pound of air value of 1355, an increase of about 9% over standard pump fuel.
The point of all this you ask? Usable energy values only really change when you go to specialty race fuels that contain oxygen bearing components or other voodoo components like isoprene. If you have an engine that is detonating and dump a load of any aromatic hydrocarbon in your fuel and you feel a difference it's likely you just modified the fuel curve in a positive way, which is something you could have done just as easily with brass aka JETTING.
A little more about Toluene and combustion chemistry.
The willingness of a hydrocarbon based fuel to ignite is essentially a function of it's ability to vaporize. Once it's vaporized they ALL light off pretty much the same. Pump fuel is adjusted throughout the year in an attempt to ensure that the fuel will vaporize quick enough for the lowest temperatures encountered but not so easily that you run into a vapor lock condition in warm temps.
If you pick a high octane race that has a vaporization curve that is poorly suited to your riding conditions you can effect it's starting behavior. Fuels that vaporize easily will tend to richen the jetting while fuels that take large amounts of heat to vaporize will in effect lean the mixture and force you to compensate with richer jetting.
Myth # 2 Octane boosters, and combustion speed
It’s common practice for people to summon the gods of combustion speed to try and explain the magic powers of toluene, acetone and any number of Home Depot speed secrets they are trying to foist off on an unknowing audience. They’ll tell you that these paint store solvents raise the octane of the fuel and as a result slow down combustion and fend off detonation as a result. It’s just more pseudo-science BS from people who would have done well to pay more attention in Chemistry 101 in high school. Real combustion events follow the real laws of chemistry, and while it’s a pretty complex set of interactions there are a few things that are easy to explain and understand.
It's a commonly held misconception that higher Octane fuel slows down the flame speed, which keeps the engine from knocking. What a load of crap.
A fuel's laminar Flame speed (basically fuel burning without the influence of combustion swirl or tumble) is a function of fuel chemistry (specifically Hydrogen/Carbon ratio), not the Octane rating.
The Hydrogen/Carbon make up of the fuel will determine it’s flame speed and the energy release whether it's a high-octane fuel or not. You can have fast and slow burning high octane fuels, but in most cases the differences are so slight as to not be an issue.
The heat of combustion at a simple level is basically a function of combustion efficiency and the calorific (energy) value of a fuel. There isn't a lot of energy difference between most conventional fuels, but combustion efficiency can sometimes be improved with race fuels so a bit more heat release is possible. Overall it's pretty much a wash in most cases though.
In other words, you aren't going to change the energy release or the speed of combustion in any significant way by switching from 87 to 110 octane gasoline, or anything in between . If you lose or gain power by switching fuels there are other things at work.
The way octane influences knock has to do with the way fuel is burned (well it’s really REACTED in the chamber) in the combustion chamber. The components in higher-octane fuels are less likely to have weak molecular bonds that can break easily and form active radicals. Higher-octane fuel is just made up of components that are (for lack of a better term) stouter under high temperatures and high cylinder pressures. Things like our long lost friend lead and metallic additives like MMT (common in octane boosters) tend to work as an anti-catalyst and block the formation of the active radicals that lead to auto-ignition of the fuel and ultimately detonation. The longer it takes to react all the fuel the greater the chances are that heat will accelerate the reactions beyond what the fuel components can bear. The idea that purposely SLOWING things down in the chamber is somehow beneficial in light of the reactions taking place is pretty funny really
Heat , pressure, and time tend to be the driving force in all of this. The more heat and pressure you subject the fuel to and the longer you keep the temps elevated the better and more stable the fuel you need . That's why engines that run at high rpm and have small bores can run very low octane fuels even with high compression ratios.
It's just simple chemistry really. If you look at the chemistry of explosives you can see a lot of similar reactions taking place, and it can give you some real insights into what's happening inside the combustion chamber when things go wrong. It's a fine line between engine tuner and explosives engineer.
... to be continued
Here's my starting point for the article with a couple of pet peeve myths of mine :
Myth # 1 – Toluene is a magic fuel additive that adds power by just dumping it in gas tank
Reference grade pure toluene vaporizes in the 240-250F range with a basically flat curve, but only lab guys with big checkbooks get that stuff. The commercially available stuff (Home Depot, paint store etc. ) has lots of distillation byproducts in it so just like pump fuel it has a high temperature distillation curve that varies from load to load, so you'll see a curve with anything in the 280- +400F range in commercial grade Toluene.
Anything in this temperature range isn't likely to vaporize above 7000rpm in current MX engines so all the additive accomplishes is tweaking the distillation curve and the overall fuel curve i.e the jetting.
There is no extra energy release value to Toluene either as the gross combustion value for it is on par with everything else normally used in fuels.
Toluene is a decent additive if all you want to do is protect the engine from fragging itself due to detonation. It has reasonably high octane and it is relatively cheap. The downside is because additives like these are fairly reluctant to vaporize they have proven to make for sloppy throttle response, poor carb circuit transitions and poor high rpm running in engines like ours that have short intake tracts and run for sustained periods above 7000 rpm. The greater the percentage of Toluene the more these issues are likely to surface. This will tend to be true for Toluene based octane boosters as well.
I’ve seen a number of instances of Internet automotive sites taking little chunks of info and using them out of context in an attempt to prove the efficacy of Toluene in producing additional power by virtue of “heat of combustion” numbers. In doing this they are are ignoring the fact that the maximum combustion heat produced by any fuel can be expressed as a function of its stoichiometric ratio and net heat of combustion. While fuels may have varying heats of combustion and stoichiometric ratios, when compared by their heat of combustion per pound of air, most aromatic hydrocarbons toluene included, and standard pump fuels are all pretty similar.
For those who haven't dozed off, here's how it works:
Reference grade toluene has a net heat of combustion = 17424 (btu/lb), and it's chemically correct (stoichiometric) A/F ratio is about 13.8:1 . This comes out to a heat of combustion per pound of air value of 1262. Average pump premium has net heat of combustion in the neighborhood of 19,000 (btu/lb) andthe A/F ratio is about 15:1 which comes out to a heat of combustion per pound of air value of 1266 just a touch better but nothing we would notice. A good oxygenated race fuel like Firepower 324)has net heat of combustion of 18,747 (btu/lb) and the A/F ratio is 13.84 which comes out to a heat of combustion per pound of air value of 1355, an increase of about 9% over standard pump fuel.
The point of all this you ask? Usable energy values only really change when you go to specialty race fuels that contain oxygen bearing components or other voodoo components like isoprene. If you have an engine that is detonating and dump a load of any aromatic hydrocarbon in your fuel and you feel a difference it's likely you just modified the fuel curve in a positive way, which is something you could have done just as easily with brass aka JETTING.
A little more about Toluene and combustion chemistry.
The willingness of a hydrocarbon based fuel to ignite is essentially a function of it's ability to vaporize. Once it's vaporized they ALL light off pretty much the same. Pump fuel is adjusted throughout the year in an attempt to ensure that the fuel will vaporize quick enough for the lowest temperatures encountered but not so easily that you run into a vapor lock condition in warm temps.
If you pick a high octane race that has a vaporization curve that is poorly suited to your riding conditions you can effect it's starting behavior. Fuels that vaporize easily will tend to richen the jetting while fuels that take large amounts of heat to vaporize will in effect lean the mixture and force you to compensate with richer jetting.
Myth # 2 Octane boosters, and combustion speed
It’s common practice for people to summon the gods of combustion speed to try and explain the magic powers of toluene, acetone and any number of Home Depot speed secrets they are trying to foist off on an unknowing audience. They’ll tell you that these paint store solvents raise the octane of the fuel and as a result slow down combustion and fend off detonation as a result. It’s just more pseudo-science BS from people who would have done well to pay more attention in Chemistry 101 in high school. Real combustion events follow the real laws of chemistry, and while it’s a pretty complex set of interactions there are a few things that are easy to explain and understand.
It's a commonly held misconception that higher Octane fuel slows down the flame speed, which keeps the engine from knocking. What a load of crap.
A fuel's laminar Flame speed (basically fuel burning without the influence of combustion swirl or tumble) is a function of fuel chemistry (specifically Hydrogen/Carbon ratio), not the Octane rating.
The Hydrogen/Carbon make up of the fuel will determine it’s flame speed and the energy release whether it's a high-octane fuel or not. You can have fast and slow burning high octane fuels, but in most cases the differences are so slight as to not be an issue.
The heat of combustion at a simple level is basically a function of combustion efficiency and the calorific (energy) value of a fuel. There isn't a lot of energy difference between most conventional fuels, but combustion efficiency can sometimes be improved with race fuels so a bit more heat release is possible. Overall it's pretty much a wash in most cases though.
In other words, you aren't going to change the energy release or the speed of combustion in any significant way by switching from 87 to 110 octane gasoline, or anything in between . If you lose or gain power by switching fuels there are other things at work.
The way octane influences knock has to do with the way fuel is burned (well it’s really REACTED in the chamber) in the combustion chamber. The components in higher-octane fuels are less likely to have weak molecular bonds that can break easily and form active radicals. Higher-octane fuel is just made up of components that are (for lack of a better term) stouter under high temperatures and high cylinder pressures. Things like our long lost friend lead and metallic additives like MMT (common in octane boosters) tend to work as an anti-catalyst and block the formation of the active radicals that lead to auto-ignition of the fuel and ultimately detonation. The longer it takes to react all the fuel the greater the chances are that heat will accelerate the reactions beyond what the fuel components can bear. The idea that purposely SLOWING things down in the chamber is somehow beneficial in light of the reactions taking place is pretty funny really
Heat , pressure, and time tend to be the driving force in all of this. The more heat and pressure you subject the fuel to and the longer you keep the temps elevated the better and more stable the fuel you need . That's why engines that run at high rpm and have small bores can run very low octane fuels even with high compression ratios.
It's just simple chemistry really. If you look at the chemistry of explosives you can see a lot of similar reactions taking place, and it can give you some real insights into what's happening inside the combustion chamber when things go wrong. It's a fine line between engine tuner and explosives engineer.
... to be continued
