Fuel For Thought

Petrol/Gasoline, Ethanol and Methanol – they’re all just fuels right? RONs, MONs, octane ratings, additives, combustion points and alcohols – in this article we are going to cover the different types of fuel we use in our internal combustion engines, the effects these fuels have on power, and how we might adjust an engine’s tune-up based on the fuel being used.

Octane Rating

Before we get too deep into the weeds here, we need to get some terminology squared away, probably the most common piece of fuel information that most people have heard of is octane rating. There are a couple of different ways of measuring the octane of fuel. Whether it is the RON (which stands for Research Octane Number) or the MON (which stands for the Motor Octane Number) both are a measure of fuel’s resistance to detonation.  The higher the octane rating, the less likely the fuel is to detonate at any given temperature and pressure.

Alcohol

The two most common types of alcohol fuels are Ethanol and Methanol. While these 2 different fuels have similar names, chemically they are quite different. Both are alcohol fuels but Ethanol has twice as many carbon atoms as methanol in its chemical makeup.

Gasoline/Petrol

Petroleum-based fuels are the most common fuels we find at pumps and are made up of a complex mixture of hydrocarbon compounds. They are typically made by refining oil that has been pulled out of the ground and are available at just about every fuel pump across the globe.

These fuels come in both standard and a high octane blend, with the high octane varieties costing a few cents a liter or gallon more than their lower octane counterparts. It is these refined oil-based fuels that we are going to use as our base to compare all other fuels against today.

More Octane – More Better?

So what is the actual difference between lower octane and higher octane fuels at the pump, and, more importantly, which should you use in your engine?

The main difference is on the label – the Octane rating gives us an indication of the fuel’s combustion point but there’s more to it than that. It’s important to know how a fuel arrives at this increased octane rating, and that’s dependent on a lot of factors like the base oil stock, the refining process, the additives used in the process, and even the location of the end product. If you want to dig deeper on this topic check out this excellent article Where Does Gasoline Come From.

The important thing to remember is that Octane rating is simply a rating of a fuel’s ability to resist detonation at a given temperature and pressure. Why is this information important to us? When we are building an engine, the higher the compression ratio the engine is, the more pressure is built up in the combustion chamber on the compression stroke.

If we increase the compression ratio enough – we will build up so much cylinder pressure the air and fuel reach their auto-ignition temperature and pressure either just prior to or just after the spark plug firing (detonation).

The important detail to know is cheaper (or should we say “less expensive” given current fuel prices), lower octane fuels reach their detonation point at a lower temperature and pressure than the higher octane fuels. What does all this mean in the real world? It means the higher compression ratio your engine runs, the higher octane fuel you will need to use. 

It’s the same rule with turbo or supercharging because turbos and superchargers increase the air pressure in the intake manifold and therefore the combustion chamber, so the more boost you are running the higher octane fuel you need to run to prevent detonation.

What about low compression engines?

If you have a relatively low compression ratio, naturally aspirated engine, or a stock daily, is there any benefit to running high octane fuel? Honestly, no, there isn’t. If an 87 octane fuel is not reaching its detonation point in any of the operating conditions of your engine, the only performance gain that will be made by running a higher octane pump fuel in that engine – is the performance of the fuel company’s stock price. 

When you book a trip to the dyno to have your car tuned, make sure you fill the tank with whatever fuel you are going to run consistently.

There is no use going to the dyno with your own custom blend of 98, moonshine and rocket fuel octane boosters, if you can’t get the fuel again. The guy tuning your engine is going to tune it to the fuel that’s in the tank – if you turn around the next day and put whatever low dollar sludge you can find into the tank – chances are the engine is going to ping iteself to death.

Changing Fuels

What happens if your car is tuned on 98 or 95 but when you’re out on a road trip you simply can’t get it. Don’t worry, a tank or two of lower octane fuel isn’t going to cause the conrods to escape out the side of the block. If you can’t get high octane fuel, just fill the tank with what you can get and drive accordingly, so no limiter banging, no hitting boost cut, and no track days if you can’t get the quality of fuel that your engine is tuned for.

Another option, if you’re using a Haltech ECU, is setting up dual maps or an ignition trim that will switch between ignition timing or boost maps on low and high octane fuels.

Beautiful Blends

Let’s move on to petroleum-based fuels that have ethanol mixed with them. Fuels like E10 are inexpensive, but seem to have a slightly higher octane rating than the other fuels. The reason for that is the Ethanol content (that’s what the 10 stands for) 10% of the fuel is actually Ethanol.

Ethanol as an alcohol-based fuel for the most part has likely been manufactured using sugar from either cane or corn. The great thing about these alcohol-based fuels is they have really high octane ratings in excess of 110 Octane for Ethanol. So mixing just 10% of ethanol into a petroleum fuel will often bump the octane rating up a 2 or 3 points.

What about E85 or fuels that are 85% Ethanol and only 15% Petroleum – are these fuels really over 110 Octane? In short, yes. But that’s not the only benefit of running E85. Because E85 has a different stoichiometric air to fuel ratio to petrol or gasoline, we need to provide the engine with roughly 30% more E85 to meet the same stoichiometric air to fuel ratio that we would have on gasoline.

That’s important for two reasons, first – we can’t just fill up any vehicle with E85 and expect the engine to run correctly. If you want to run E85, the ECU will need to be programmed to suit – fortunately, that’s really easy with all modern Haltech ECUs and with the help of a Flex Fuel sensor it’s almost completely seamless. 

Secondly, that extra fuel that we add to the intake actually starts to have a significant cooling effect on the incoming air stream. What this means for turbocharged or supercharged engines is not only does E85 have a high resistance to knock – but it also cools the incoming air. It’s like having a mini intercooler!

Methanol

No discussion on alcohol fuels would be complete without talking about the big daddy of alcohol race fuels – Methanol. The most important thing about Methanol is its octane rating – it’s almost too high to measure, which is exactly why when you want to run 120psi of boost and 11,000RPM on your fire breathing drag car – the only fuel you even attempt to use is Methanol.

Methanol fuels have a stoichiometric air-fuel ratio around the 6.5:1 mark, which in round terms means we need to run twice as much methanol than we would gasoline to maintain the same chemical relationship of air and fuel in an engine. All that extra fuel means more cooling of the intake charge and cooler air is denser, and dense air equals horsepower.

Horses for courses

So why aren’t we all just running methanol in our daily drivers? Well, technically we could – but while Methanol is a great race fuel, it’s also a great solvent and reacts with many organic compounds so components like rubbers, seals and alloys of aluminium can all be damaged by methanol.

Methanol racers know this and they take great care to flush out the fuel lines, clean their fuel injectors, replace their fuel filters, and the like on a regular basis. So while Methanol is a fantastic fuel for high power, big boost engines, it requires a lot of upkeep and maintenance.