cr85racer12
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- Jun 15, 2006
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Intrinsic and Effective Octane, there is a difference
The Octane Rating of a fuel is defined by its ability to resist pre-ignition in a spark ignition engine. Octane rating has no effect on how much energy is contained in the fuel, but rather how efficiently the fuel can be controlled. High octane fuels are commonly seen in high performance engines because they are able to withstand high static compression ratios and high amounts of boost (in a forced induction engine, supercharged or turbocharged).
The octane number is taken by comparing the given fuel in a test engine with another fuel (a mixture of iso-octane and heptane). If the fuel has similar anti-knock properties to a mixture of 90% iso-octane and 10% heptane, then the fuel is given an octane rating of 90. Since iso-octane does not have the highest resistance to pre-ignition of any fuel, it is possible for fuels such as aviation gas, methanol, LPG and others to have an octane rating higher than 100.
There are 2 basic tests where octane ratings are tested. These are the Research Octane Number (RON) where fuel is put into a test engine at 600 RPM with a variable compression ratio, and there is the Motor Octane Number (MON) where fuel is put into a test engine at 900RPM with variable ignition timing to further stress the engine. In Canada and United States (and many other parts of the world) we use the average of these two numbers. This is denoted by (R+M)/2, we see this on the pump’s at the gas station.
Now this is where things get interesting. How is it that the new Mazda 6 has a gasoline engine that runs at 14:1 compression ratio, using pump gas? Considering that is about the same compression ratio as Mitch Payton’s Pro Circuit Race bikes which are running full race fuel, there is a clearly a difference between intrinsic octane rating (chemical) and effective octane rating.
The effective octane rating takes all conditions into consideration, and is a direct measure of what’s going on inside the engine. There are factors such as the latent heat of vaporization, the intake air temperature, laminar and turbulent flow in the ports, tumble and swirl of the mixture in the combustion chamber, and atomization of the fuel. All of these will play a role in the effective octane rating of a fuel.
One of the main reasons that the Mazda SkyActiv G engine is able to get away with such a high compression ratio is because it uses a high pressure direct injection fuel system injecting fuel into the combustion chamber at 3000 pounds per square inch!!! This allows the fuel to disperse with the air so well that it nearly completely vaporizes in the mixture, turning from a liquid into a gas. As the liquid turns into a gas, it absorbs heat due to the latent heat of vaporization, which in turn lowers the temperature of the given air/fuel mixture. The mixture becomes exceptionally atomized and the resulting lower temperature and stratified charge allows the fuel to further its resistance to knock.
Now you might ask, how does this play any role in the world of dirt bikes? Well take a look at the new twin-injection setup on the fuel injected Kawasaki motocross bikes. There is an initial injector just inside the airbox that delivers fuel into the intake stream so that by the time it reaches the intake valve it is completely vaporized, cooling the intake charge and optimizing the atomization of the mixture. Then a second injector much closer to the engine injects fuel before the intake valve increasing the fuel density of the mixture. This system allows for the benefits of a fuel mixture where the fuel molecules are extremely well dispersed with the air molecules, and the vaporized fuel has absorbed heat going into the intake manifold. This setup allows for higher compression ratios due to its high effective octane from the manner in which the fuel is injected.
There are other ways to increase the effective octane of an engine. A very popular method that has been used for decades is injecting a fine mist of water into the intake stream (aka Water Injection). Because water has among the highest latent heats of vaporization, its ability to absorb heat in the intake manifold is extremely effective, and don’t worry about corrosion, water is already a product of combustion anyways.
Mod Edit -- Dial-A-Jet SPAM removed- I'll leave it to the other members to decide if they want to address the technical aspects of the rest of your post.
Running 110 octane in your stock engine will yield little to no benefits. High octane fuel is only necessary when combating against high compression. Delivering the fuel in a cold, dense, stratified charge will mimic the effects of a higher octane rating gasoline, and this is due to the fuel’s effective octane. Intrinsic octane is merely the octane’s number, effective octane is how the fuel is implemented in the engine and is not so easily given a numerical value.
The Octane Rating of a fuel is defined by its ability to resist pre-ignition in a spark ignition engine. Octane rating has no effect on how much energy is contained in the fuel, but rather how efficiently the fuel can be controlled. High octane fuels are commonly seen in high performance engines because they are able to withstand high static compression ratios and high amounts of boost (in a forced induction engine, supercharged or turbocharged).
The octane number is taken by comparing the given fuel in a test engine with another fuel (a mixture of iso-octane and heptane). If the fuel has similar anti-knock properties to a mixture of 90% iso-octane and 10% heptane, then the fuel is given an octane rating of 90. Since iso-octane does not have the highest resistance to pre-ignition of any fuel, it is possible for fuels such as aviation gas, methanol, LPG and others to have an octane rating higher than 100.
There are 2 basic tests where octane ratings are tested. These are the Research Octane Number (RON) where fuel is put into a test engine at 600 RPM with a variable compression ratio, and there is the Motor Octane Number (MON) where fuel is put into a test engine at 900RPM with variable ignition timing to further stress the engine. In Canada and United States (and many other parts of the world) we use the average of these two numbers. This is denoted by (R+M)/2, we see this on the pump’s at the gas station.
Now this is where things get interesting. How is it that the new Mazda 6 has a gasoline engine that runs at 14:1 compression ratio, using pump gas? Considering that is about the same compression ratio as Mitch Payton’s Pro Circuit Race bikes which are running full race fuel, there is a clearly a difference between intrinsic octane rating (chemical) and effective octane rating.
The effective octane rating takes all conditions into consideration, and is a direct measure of what’s going on inside the engine. There are factors such as the latent heat of vaporization, the intake air temperature, laminar and turbulent flow in the ports, tumble and swirl of the mixture in the combustion chamber, and atomization of the fuel. All of these will play a role in the effective octane rating of a fuel.
One of the main reasons that the Mazda SkyActiv G engine is able to get away with such a high compression ratio is because it uses a high pressure direct injection fuel system injecting fuel into the combustion chamber at 3000 pounds per square inch!!! This allows the fuel to disperse with the air so well that it nearly completely vaporizes in the mixture, turning from a liquid into a gas. As the liquid turns into a gas, it absorbs heat due to the latent heat of vaporization, which in turn lowers the temperature of the given air/fuel mixture. The mixture becomes exceptionally atomized and the resulting lower temperature and stratified charge allows the fuel to further its resistance to knock.
Now you might ask, how does this play any role in the world of dirt bikes? Well take a look at the new twin-injection setup on the fuel injected Kawasaki motocross bikes. There is an initial injector just inside the airbox that delivers fuel into the intake stream so that by the time it reaches the intake valve it is completely vaporized, cooling the intake charge and optimizing the atomization of the mixture. Then a second injector much closer to the engine injects fuel before the intake valve increasing the fuel density of the mixture. This system allows for the benefits of a fuel mixture where the fuel molecules are extremely well dispersed with the air molecules, and the vaporized fuel has absorbed heat going into the intake manifold. This setup allows for higher compression ratios due to its high effective octane from the manner in which the fuel is injected.
There are other ways to increase the effective octane of an engine. A very popular method that has been used for decades is injecting a fine mist of water into the intake stream (aka Water Injection). Because water has among the highest latent heats of vaporization, its ability to absorb heat in the intake manifold is extremely effective, and don’t worry about corrosion, water is already a product of combustion anyways.
Mod Edit -- Dial-A-Jet SPAM removed- I'll leave it to the other members to decide if they want to address the technical aspects of the rest of your post.
Running 110 octane in your stock engine will yield little to no benefits. High octane fuel is only necessary when combating against high compression. Delivering the fuel in a cold, dense, stratified charge will mimic the effects of a higher octane rating gasoline, and this is due to the fuel’s effective octane. Intrinsic octane is merely the octane’s number, effective octane is how the fuel is implemented in the engine and is not so easily given a numerical value.
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