Regular gas versus premium gas
Here’s the truth about whether regular gas burns faster than premium gas
You’ll find lots of misinformation on the internet about the difference between regular and premium gas. Here are the most common myths and just plain wrong facts:
• Regular gas ignites at a lower temperature than premium gas — FALSE. Both ignite at the same temperature.
• Regular gas burns faster than premium gas — FALSE. They burn at exactly the same speed in normal combustion events (see detonation below).
• Premium gas has more detergent — FALSE. Detergent content may vary from brand to brand, but you can’t make the generalized comment that all premium gas has more detergent.
• Premium gas doesn’t contain ethanol — FALSE. Some brands sell a non-ethanol premium fuel. Others actually use ethanol to increase the octane rating on their premium gas. If you want to know if premium gas contains ethanol, read the labels on the pump. They must say if they use ethanol.
• Premium gas gives you more power —False. Generally speaking, premium gas has the same energy content as regular gas.
You’ll find false information and myths even from some “expert” sites like:
Meineke: “Lower octane gas burns quicker than higher octane, and so require less energy to ignite. However, this also means that lower octanes burn more quickly in high-pressure environments, and can have a greater tendency to knock. Knocking occurs when the gasoline is burning fast and is also under high-pressure environments.”
Motortrend: “Fuel with an 87 octane rating burns more quickly while higher-octane fuels burn more slowly.”
Why do some “experts” think regular gas burns faster than premium gas?
Any time you compress a gas or air/fuel mixture, you create heat. A high compression engine creates more heat during the compression stroke than a lower compression engine. Some people think that the extra heat in a high compression engine causes regular gas to ignite too early (before the spark plug fires). They also think that it burns faster than premium gas.
What these “experts” are describing is actually PRE-IGNITION; a condition where hot carbon deposits in the combustion chamber ignite the air/fuel mixture before the spark plug fires. Pre-ignition can happen in any engine with carbon buildup; it isn’t limited to high compression engines.
But you don’t get pre-ignition in a clean combustion chamber of a high compression engine using regular (87-octane) fuel. You get detonation.
DETONATION is a different animal and that’s where all the confusion happens
Let’s say you fuel up with regular (87) octane gas in a high compression engine that requires premium (91 or 93-octane) fuel. Here’s what happens; -i
The compression stroke creates heat in the air/fuel mixture, but not enough heat to ignite the 87-octane fuel. As the piston reaches the top of the compression stroke, the air/fuel mixture is ignited by the spark plug (NOT by the heat of the compression stroke).
Here’s the important part to understand about detonation
As the flame front expands away from the spark plug, it dramatically increases pressure in the combustion chamber. The air/fuel mixture was already hot from the compression stroke when it was ignited by the spark plug. But now that 87-octane fuel experiences a second and much larger rise in pressure which creates far more heat than could be created by the compression stroke alone.
It’s this dramatic and rapid pressure rise caused by combustion expansion (not the compression stroke) that raises the temperature of the uncombusted air/fuel mixture to the point where it self-ignites. The rapid rise in temperature due to the added pressure causes multiple flame fronts to ignite across the combustion chamber. When those multiple flame fronts collide, they create audible shock waves known as detonation or “knock”.
Here’s where the “faster burn” misconception comes from. It’s a semantics issue.
You inject X amount of 87-octane fuel into the combustion chamber, compress it, and ignite it, creating multiple flame fronts. With multiple flame fronts occurring at the same time, of course it’s going to burn UP all the fuel faster. But that doesn’t mean the fuel itself burns faster than 93-octane. If you were to take the same about of 93-octane fuel and cause the exact same number of multiple flame fronts, it would burn up at exactly the same rate as the 87-octane fuel.
Think of it this way
Take two piles of leaves and spread them on your driveway. Light one pile multiple places and the other pile in just one place. The pile with multiple fires will burn up faster than the pile with just one ignition point. What does that prove? Only that multiple ignition points will burn up the leaves faster than one ignition point.
In other words, multiple flame fronts will cause ANY fuel (87 or 93-octane) to burn faster than a single flame front that propagates across the cylinder in a controlled manner.
Detonation robs the engine of power, and if allowed to continue, can destroy the engine.
Octane is what prevents the high-pressure unburned air/fuel mixture from self-igniting. So it prevents detonation and knock. 91 or 93-octane fuel ignites at the same temperature as 87-octane (remember, it started ignition from the spark plug), but the higher octane content allows the fuel to resist self-ignition due to rapid pressure rise.
The lesson here is:
87 and 93-octane fuel ignite at the same temperature.
87 and 93-octane fuel burn at the same speed
What the engine computer does to reduce detonation
The knock that occurs during detonation is detected by the knock sensor and reported to the ECM. The ECM retards spark timing. In other words, it initiates the spark earlier in the compression stroke so the air/fuel mixture isn’t compressed as much; so it’s at a lower pressure when it ignites from the spark. Since the cylinder pressure is lower at ignition, the expanding flame front doesn’t create as dramatic a pressure rise as it would if the spark had occurred later in the compression stroke. The lower pressure during ignition reduces the 87-octane fuel’s ability to self-ignite, thereby eliminating the multiple flame fronts that cause detonation. Most ECM can only retard spark timing by up to 25%. If the knock continues beyond that point, the ECM will set a check engine light.
What is an octane?
A fuel’s octane rating is a numerical index of the fuel’s ability to resist detonation. It is not an indication of power. Generally speaking, a higher octane fuel doesn’t contain any more power or detergent.
A higher octane rating is only an indication of its ability to resist detonation. However, since fuel is a combination of up to 150 different ingredients and the ingredients used by the refiner vary by market prices, there may be some instances where a refiner blends in a less expensive component that could possibly add an insignificant amount of BTUs to premium fuel.
Fun combustion facts about detonation
When an engine fires the spark plug, the combustion begins, the flame front moves smoothly across the combustion chamber, traveling at 45 to 90-MPH.
Late-model engines with high-swirl combustion chamber designs may have faster airflow and thus faster flame spread. However, when you use regular-grade (87-octane) gas in a high compression engine that was designed for 91 or 93-octane, detonation occurs. The combustion gasses in that scenario can reach a speed of over 700-MPH!
That kind of rapid expansion increases combustion temperatures and can damage pistons, gaskets, valves, and cylinder heads. The metallic knock you hear is the force of the combustion slamming against the piston while the piston is still moving upwards on its compression stroke.
What is pre-ignition?
Preignition is caused by hot carbon deposits in the engine that ignite the air/fuel mixture before the spark plug fires. The result is the same as detonation–it causes multiple flame fronts; one from the hot carbon and one from the spark plug. That’s why it’s often confused with detonation.
©, 2021 Rick Muscoplat
Posted on by Rick Muscoplat