Tech feature: Fuels ain't fuels

I've been part of many conversations about what fuel to use in a bike. On a recent ride I had another one. It went like this:
 "What fuel do you use in your 450?"
 "One of the 98 unleaded lot. BP, Caltex or Mobil because they're close to my place; mainly Mobil 'cause it's the closest."
"Why do you use 98?"
"Because it burns cooler and has more mumbo."
 "How does it do that?"
 "I dunno, some bloke just told me."

So I then did what anyone in my situation does. I was the pain in the arse that rang BP and asked them.

Before I go on I should let you know that I'm a knowledge freak. I love knowing how things work and why. So I wanted to know more than just what fuel to bung in my 450. Not only did I want to know this, I wanted to know about storage, shelf life, additives and whether or not I should mix peanut butter and Vegemite on my toast.

I spoke to Dale Brown-Kenyon, a fuel technologist for BP, who was damn helpful. This is what I've gleaned from a true expert in the field…

RON VERSUS MON
Petrol is sold in three varieties based on a Research Octane Number (RON). Each of these is manufactured to meet various standards. The men in white coats couldn't stop at one octane rating. Just to confuse you, there is another octane measure, the Motor Octane Number (MON). These both measure how a fuel burns.

The RON measures the peak power of the fuel, whereas the MON is a measure of the fuel's staying power. Consequently, RON is the more important measure for engines operating/accelerating from low engine speeds, whereas MON is more important for high-speed cruising.

SPECIAL NEEDS
The relationship between octane rating and engine compression is rather interesting, and it's an important factor in the fuel equation.

We all know that if you bump up compression ratios and let the engine breath better it runs more efficiently; i.e. it produces more power. If we go too far though, the fuel can't burn fast enough and little unburnt pockets remain, which explode under compression. These explosions produce the noise commonly called "pinging". Pinging can be prevented by changing to a fuel with higher octane rating.

Just about all modern dirtbikes have high compression engines. Dale provided me with a table listing fuel properties for commonly available BP petrol grades, with a corresponding list of the highest compression ratios they'll each suit. The compression ratios listed were all lower than 10.5:1, which is lower than those run in most modern dirtbike engines.

The difficulty here is that motorcyclists are a small sector. Virtually all the information gathered and provided is for car engines, and this doesn't directly translate to motorcycle engine compression ratios.

Our engines have small cylinders with short flame paths and run at high engine rpm. Both of these factors reduce the octane requirement.

This aside, Dale's data also showed that the higher the compression ratio the more beneficial a high octane fuel is, and that premium unleaded fuels have a lower corresponding air/fuel ratio. This latter point means we can jam in more fuel for the same engine capacity, giving a cooler running engine and more horsepower.

BENEFITS OF THE BEST
You pay more per litre for the higher rated fuel, but you're paying for more than just extra octane. If petrol is allowed to stand for any length of time the fuel can oxidise. This has two effects on the fuel. Firstly, gums can deposit in the fuel system. Secondly, the octane rating of the fuel will decrease. Additives can be added to stop the oxidation and also to stop or clean away any deposits.

What you find is that the higher rating fuels have more or better additives. The fuel companies can also add additives that reduce carbon deposits (from combustion) forming.

These additives are in the same boat as the anti-oxidant additives - you get better formulations in the higher octane fuels. This is an important point that I'll come back to.

SAFE KEEPING
Storage of fuel is another issue we should take heed of. Fuel is a mixture of squillions of hydrocarbons - a term for any chemical based on a carbon backbone - and with lots of hydrogen atoms attached, generally they form chains. Some hydrocarbons are small, others are quite long/large. The small chains are the lightest and they evaporate quickly.

Over time the small chains will evaporate, increasing the density of the fuel, which richens the fuel mixture. This makes starting more difficult, drops the power output and can foul plugs. This will only happen if the evaporated petrol can escape, like out through the breather in your tank. Petrol stored like this can go off in as quickly as two weeks given hot ambient temperatures. Fuel stored in airtight containers, however, will keep its octane rating for a considerable time. BP lists the shelf life of its fuels as a year if stored unopened in an airtight container at 20 degrees.

In summary, the cooler you keep the petrol and the less it is exposed to air, the longer it will maintain its octane rating and the less it will oxidise. Just like a good curry.

Petrol in your tank is given optimum opportunity to evaporate and oxidise because of the breather.

There are two things you should do if you leave your bike standing for several weeks between rides. One, never leave it sitting with more than half a tank of fuel. Two, top it up with fresh fuel just before your next ride. This replenishes the lost volatile components of the fuel.

HI RON RULES
So the age-old question remains: what fuel should you run in your bike? From the information that the men in white coats have provided us, there is no doubt your bike will run better on a premium fuel, but it also has extra benefits imparted from the additives.

These benefits are more valid to us dirtbike riders than any other motorist because of our infrequent use.

Summing it all up, 98 RON premium is the best choice for a high performance dirtbike. Those with lower compression engines, such as DRs, TT-Rs etc, will be better served by using 95 RON premium over the 91 variant for all the same reasons.

Now I feel justified in my wanton use of 98 premium over the last eight years. I think it's time for a green beef curry…

FUEL CHOICE 101
98 octane rated fuel is the highest octane rated fuel on the market. BP calls its 98 fuel "Ultimate". It has a RON of 98 to 100, and has additives to reduce deposits on the valves and combustion chamber and reduce oxidation/gum deposits in the fuel system.

95 octane rated fuel is formulated upon the European standards for fuel and meets the Australian standards for premium fuel. It has a lower additive treatment than the 98 rated fuel, but offers better performance and economy than the regular unleaded. Most modern car engines are designed to run on this fuel.

91 octane rated fuel (regular unleaded) is the lowest octane fuel on the market and is formulated to meet Australian standards for regular unleaded petrol.

91 octane rated fuel with 10 percent ethanol is exactly that. Ethanol has a higher octane rating than petrol and this bumps up the actual octane rating, typically to 94. However, the power gain from this is offset by a reduction in economy, caused by the oxygen in ethanol. Caution should be exercised when using ethanol blends in older bikes, as the ethanol may degrade fuel lines and fibreglass tanks.