Frank Acampora
Supreme Mariner
- Joined
- Jan 19, 2007
- Messages
- 12,004
CAUTION! HISTORY AND SCIENCE CONTENT!
Periodically, the controversy over what octane fuel to use arises. Here is the answer. Use this infomation as you desire.
When air is compressed in a closed cylinder it heats up. The compression ratio can be high enough to auto-ignite the fuel. Indeed, this is the principal upon which a Diesel engine works: Air is compressed past the point of fuel ignition, then fuel is injected and it burns. Because it is injected at one point, it burns evenly and no harm is caused. A Diesel engine always injests full amount of air--there is no throttle-- and power is regulated by the amount of fuel injected. Enough about Diesels.
A gasoline engine is fundamentally different because of the fuel properties. Gasoline engines are regulated by controlling with a throttle the amount of air injested, and the proper amount of fuel is mixed with this air then ignited by a spark.
Because gasoline is much more volatile than diesel fuel and because it ignites at a lower temperature, gasoline engines have a lower compression ratio than Diesel. Now we get to the heart of the matter.
Back in the early days when automobiles were first being built and sold, the SAE ( Society of Automobile Engineers) arbitrarily set the octane rating of NORMAL OCTANE ( a liquid component of gasoline) to 100. All gasolines are measured against this standard and for example, aviation fuel is much higher than 100 octane rating. 87 octane pump gasoline has 87% of the knock resistance of normal octane--that's all the number indicates.
The octane rating is a measure of "knock" resistance. Knock is the auto-ignition of the fuel/air mixture due to compression temperature and can sometimes be heard as "pinging." High throttle settings and advanced timing will result in a heavier knocking sound.
In those days, poor quality fuel was raised in octane by adding a very cheap compound, tetraethyl lead. This was one lead molecule to which four ethyl molecules were attached. Lead was used because it is one of the few elements that can have four groups attached. Heat of combustion detached them, liberating ethane compounds which burn cooler and have a higher octane rating than gasoline. All gas had it but high test had more and for this reason was sometimes referred to as "Ethyl." This compound was used up until around the 1970 when the federal government mandated that lead be outlawed from fuel; lead levels in the environment were rising and we all know that lead is poisonous. This lead also collected on valves and exhaust systems. In two cycle outboards, it tended to plug the exhaust ports. So, losing it was a good thing. BTW: in those days Amoco was the only lead free gasoline and was a good bit more expensive. It was refined from more expensive, higher grade base stock, crude oil. Today, almost all gasoline contains some measure of ethanol. However, while it does raise knock resistance, it is done primarily for economic reasons, rather than raising the octane rating.
SO: in a low compression engine, it is sufficient to burn low (87) octane fuel because the heat of compression is not high enough to ignite it spontaneously. However, as compression is raised, higher octane ratings are necessary to prevent this auto-ignition. Since higher octane gasoline burns at about the same temperature as low octane, it generates the same amount of power. The reason a high compression, high octane engine develops more power is because the higher compression raises the BMEP--Base Mean Effective Pressure: More pressure = more horsepower. Timing is usually advanced a bit also for a better power curve and the engines are usually adjusted to injest more air/fuel mixture. They also usually produce this power at a higher RPM than a low compression engine. THUS: There is NO advantage to using high octane fuel in an engine that requires low octane, and at best, it is a waste of money. Indeed, some high octane fuels can contain compounds that could be harmful to low compression engines.
However, using low octane rated fuel in a high compression engine is harmful. Because the gasoline auto-ignites at a different point than the sparkplug and usually earlier, there are two flame fronts burning. When the flame fronts collide, excess temperature and pressure are generated. We may hear it as a knocking noise also known as "detonation." This melts the aluminum of the piston while at the same time hammering it. Total destruction is the result.
Conclusion: Use fuel with the octane rating recommended by the manufacturer.
Lean running conditions are a separate scenario not related to octane rating. Lean fuel mixtures also auto-ignite more easily and burn hotter than proper mixtures of gasoline and air regardless of octane rating. The result, however is the same: Total destruction of the piston.
Periodically, the controversy over what octane fuel to use arises. Here is the answer. Use this infomation as you desire.
When air is compressed in a closed cylinder it heats up. The compression ratio can be high enough to auto-ignite the fuel. Indeed, this is the principal upon which a Diesel engine works: Air is compressed past the point of fuel ignition, then fuel is injected and it burns. Because it is injected at one point, it burns evenly and no harm is caused. A Diesel engine always injests full amount of air--there is no throttle-- and power is regulated by the amount of fuel injected. Enough about Diesels.
A gasoline engine is fundamentally different because of the fuel properties. Gasoline engines are regulated by controlling with a throttle the amount of air injested, and the proper amount of fuel is mixed with this air then ignited by a spark.
Because gasoline is much more volatile than diesel fuel and because it ignites at a lower temperature, gasoline engines have a lower compression ratio than Diesel. Now we get to the heart of the matter.
Back in the early days when automobiles were first being built and sold, the SAE ( Society of Automobile Engineers) arbitrarily set the octane rating of NORMAL OCTANE ( a liquid component of gasoline) to 100. All gasolines are measured against this standard and for example, aviation fuel is much higher than 100 octane rating. 87 octane pump gasoline has 87% of the knock resistance of normal octane--that's all the number indicates.
The octane rating is a measure of "knock" resistance. Knock is the auto-ignition of the fuel/air mixture due to compression temperature and can sometimes be heard as "pinging." High throttle settings and advanced timing will result in a heavier knocking sound.
In those days, poor quality fuel was raised in octane by adding a very cheap compound, tetraethyl lead. This was one lead molecule to which four ethyl molecules were attached. Lead was used because it is one of the few elements that can have four groups attached. Heat of combustion detached them, liberating ethane compounds which burn cooler and have a higher octane rating than gasoline. All gas had it but high test had more and for this reason was sometimes referred to as "Ethyl." This compound was used up until around the 1970 when the federal government mandated that lead be outlawed from fuel; lead levels in the environment were rising and we all know that lead is poisonous. This lead also collected on valves and exhaust systems. In two cycle outboards, it tended to plug the exhaust ports. So, losing it was a good thing. BTW: in those days Amoco was the only lead free gasoline and was a good bit more expensive. It was refined from more expensive, higher grade base stock, crude oil. Today, almost all gasoline contains some measure of ethanol. However, while it does raise knock resistance, it is done primarily for economic reasons, rather than raising the octane rating.
SO: in a low compression engine, it is sufficient to burn low (87) octane fuel because the heat of compression is not high enough to ignite it spontaneously. However, as compression is raised, higher octane ratings are necessary to prevent this auto-ignition. Since higher octane gasoline burns at about the same temperature as low octane, it generates the same amount of power. The reason a high compression, high octane engine develops more power is because the higher compression raises the BMEP--Base Mean Effective Pressure: More pressure = more horsepower. Timing is usually advanced a bit also for a better power curve and the engines are usually adjusted to injest more air/fuel mixture. They also usually produce this power at a higher RPM than a low compression engine. THUS: There is NO advantage to using high octane fuel in an engine that requires low octane, and at best, it is a waste of money. Indeed, some high octane fuels can contain compounds that could be harmful to low compression engines.
However, using low octane rated fuel in a high compression engine is harmful. Because the gasoline auto-ignites at a different point than the sparkplug and usually earlier, there are two flame fronts burning. When the flame fronts collide, excess temperature and pressure are generated. We may hear it as a knocking noise also known as "detonation." This melts the aluminum of the piston while at the same time hammering it. Total destruction is the result.
Conclusion: Use fuel with the octane rating recommended by the manufacturer.
Lean running conditions are a separate scenario not related to octane rating. Lean fuel mixtures also auto-ignite more easily and burn hotter than proper mixtures of gasoline and air regardless of octane rating. The result, however is the same: Total destruction of the piston.
