If you’re anything like me, you plunk the fuel nozzle into the filler neck, squeeze the lever and top up your gas tank without ever thinking about what you’re putting into your motorcycle.
But how do you know what grade of fuel you should be putting into your tank? For example, are you planning on a sporty ride today? If so, then you might want premium, right? Well maybe, maybe not.
The grade of gas you put into your tank is dependent entirely on the type of engine you have in your motorcycle, and if you don’t have a high-compression motor, then adding premium gas is only putting your money into the pockets of the gas companies, despite what they may imply about any supposed performance gains.
With gas prices about to jump, CMG thought it would be a good time to take a look at what factors define what grade of gas you need to use so that you can do the most to keep your money in your pocket.
Okay, for starters lets take a look at what is happening inside your engine that determines the need for a certain grade of fuel.
If you studied physics at school then you may remember that when a gas is compressed it will (more or less) generate heat proportional to the amount of compression that it is subjected to. That is, the more you compress it, the more heat it generates.
Okay, so if you flunked physics, then in laymen’s terms, if you’ve ever used a hand pump to pump up a tire you’ll have noticed that the end gets warm as you pump, which is the same effect at work.
This also occurs when a fuel mixture is compressed inside your engine by the rising piston. Only this compression process moves a relatively large volume of air into a very small space prior to ignition (the smaller the relative space, the higher the engine’s compression ratio).
The more the effective compression, the more complete the burn (the gasoline and air molecules are literally squeezed closer together to enable a quicker and more complete burn) and thus the more energy is released from a given quantity of fuel.
This in turn creates more mechanical energy and a greater power output to boot, which is why high-performance engines are synonymous with high compression ratios.
All good so far? Good, then let’s get to the problem.
The problem with this is that — as with the bicycle pump — the compression of the fuel mixture generates a lot of heat, which (combined with the ambient heat produced by the engine) can actually be sufficient to ignite the fuel/air mixture without a spark.
In practice, a spark plug fires, ignites the mixture and produces a flame front that works its way through the combustion chamber – ignition isn’t an explosion, but rather a controlled burn.
But because the hot mixture has been compressed, it can also self-ignite, thus producing two flame fronts, which have the effect of amplifying the pressure waves that result.
It is these pressure waves pounding against the inside of the combustion chamber that cause the condition known as engine knock or detonation. You’ve probably heard the unpleasant sound of detonation, a metallic pinging noise that can be likened to rattling a couple of steel balls in a steel can.
Typically a spark plug ignites the fuel/air mixture 10 to 40 degrees before the piston reaches the top of the cylinder. This may seem counter-intuitive, but this ignition before Top Dead Centre (TDC) allows for the flame started by the spark-plug to spread through the fuel, ensuring that it generates peak pressure just after TDC and so maximizes efficiency of the burn.
However, detonation will result in peak pressure being generated before TDC – which then tries to push the piston back the way it came, before it reaches the top of the cylinder. It’s only the momentum of the crankshaft that keeps it from doing so, but the result is power loss and potential engine damage over the long run.
NOT JUST COMPRESSION RATIOS
Now, you’d think that the compression ratio alone would determine an engine’s likelihood to knock, but there are other factors to be considered. One of them is cylinder bore.
We spoke to Canadian Kawasaki’s technical guru Gary Comer, and he said the reason a ZX-6R can get away with a 13.3:1 compression ratio, while a Vulcan 1700 engine must run no more that 9.5:1 is because of the different bore sizes and their relationship to the time required for the flame front to travel across the cylinder.
A larger bore requires more time for the flame front to burn throughout the combustion chamber, thus allowing more time for the fuel mixture to self-ignite. This is why large-bore, air-cooled carburetted engines are so prone to engine knock — large bore, high heat and an erratic mixture; all the elements are there.
Thankfully, current advances in combustion chamber shape and a move to liquid cooling, combined with EFI, have reduced the likelihood of big twins to knock to almost nil.
Another factor is cam timing. Aggressive cam timing, like that used in Kawasaki’s KX450F motocrosser, reduces the effective compression ratio — the longer the intake valves stay open, the later the piston begins compressing the mixture on its way back up the cylinder.
This is why the KX with a 96 mm bore (8 mm larger than the bore of a Vulcan 900) can get away with a 12.5:1 compression ratio — well, that and use of modern EFI.
And yet another factor is elevation. The higher up into the atmosphere you live, the less dense the air is, therefore the less air there is to compress; less heat is generated and the risk of detonation is reduced.
|A backward burnout… strange but true.
BTW, detonation is not to be confused with pre-ignition, which is the result of high-heat areas in the combustion chamber—like heated carbon deposits—causing the fuel to ignite before the spark plug fires.
We’ve been discussing mostly four-strokes, but two-stroke fans may be familiar with an adverse, if somewhat comical effect of severe pre-ignition.
Not relying on valves to supply a timed dose of fuel mixture, two-strokes can run backwards if enough carbon build-up is present to cause severe pre-ignition. Just try explaining that to your insurance company when you front-ended a parked car…
Sustained knocking and pre-ignition however, can do serious damage to an engine, most commonly damage of the piston crown.
So now the question is, how do you avoid it from occurring? Ah, that’s where we come to the fuel.
The first line of defence towards warding off detonation is the fuel. Fuel contains one or more additives (it used to be tetra-ethyl lead, but that has now been replaced with less toxic chemicals) that actually inhibit the ability of the fuel to ignite and thus allow for a higher temperature before ignition.
The rate at which a fuel can resist knocking is its octane rating. But as with many things, it’s not as simple as it could be and we actually have three different measurements for obtaining octane numbers for fuel.
The Research Octane Number (RON) is measured at light loads simulating city driving (in a car, of course). This method provides the highest octane number.
The Motor Octane Number (MON) is measured in extreme conditions and provides the lowest number. The Anti-Knock Index (AKI) is an average of the two measurements and gives a more realistic reading of a fuel’s knock resistance properties.
For example, a fuel that is rated at 91 RON and 83 MON would have a number at the fuel pump (AKI) that actually reads 87 (R+M)/2. Typical AKI grades that you’ll find at the pump are regular at 87 (R+M)/2, midgrade at 89 (R+M)/2, and premium at 91 (R+M)/2.
Of course, some fuel companies take advantage of consumers’ misunderstanding of octane ratings and in some areas may display the higher RON reading, so you should check the pump to see which method they use when filling up.
However, perhaps the most common misconception about octane is its relation to power output. Many people believe that the higher the octane, the more power the fuel will produce in an engine.
And it’s hard to fault the end user in believing so, as oil companies have been associating high-octane fuels with high horsepower for decades.
Technically they’re not exactly wrong, as you need a high-octane fuel in a high-compression motor to prevent knocking. But the fuel doesn’t have any more energy in it; it just resists higher temperatures before burning — it’s the high compression of the engine that produces the power.
It’s in the oil companies’ interest to do so, after all, because ‘premium’ high-octane fuel demands a premium price.
But interestingly, higher-octane fuels can actually reduce the amount of power from an engine too. Comer noted that some Kawasaki owners have actually complained that they saw a drop in performance when using the highest-octane fuel available (such as Petro Canada and Sunoco’s Ultra 94 fuel).
He attributed this drop in performance to the possibility that in order to boost octane numbers, that fuel may contain the maximum allowable percentage of ethanol (up to 10 per cent by volume), which increases the octane rating but reduces the fuel’s calorific value as well.
BUT THEN AGAIN
Having said that, there is some truth to the myth that higher octane produces more power in today’s electronically controlled engines, but it does so indirectly.
In the past, when ignition systems used rudimentary mapping to control ignition advance, ignition timing wasn’t precise enough to cope with varying engine conditions and running low octane fuel in high-performance engines would induce engine knock.
Modern fuel-injected engines use complex electronics to finely monitor ignition timing, as well as the fuel mixture, which burns much more efficiently and therefore cooler than with an erratic carburetor mixture.
These two factors alone reduce combustion chamber heat, and thus the likelihood of engine knock. But this has also allowed manufacturers to run optimum timing settings over much wider operating conditions. All of these factors combined allow modern engines to run ever-higher compression ratios, and therefore produce more power.
Some motorcycles today also use knock sensors to detect detonation and retard the ignition timing, which has the effect of running the engine cooler to counter the knocking, and it’s these bikes that are most likely to display a loss of power when using a lower-grade fuel than recommended.
The result with this setup is knock-free running, even when using low-octane fuel, however peak power also drops off as a result of the altered timing, though it must be stressed that this has nothing to do with the energy available in the fuel.
BMW’s 1200GS is a good example of this, which allows the use of lower grade fuels that you may find while travelling abroad.
Using the recommended high-octane fuel in these engines reduces the tendency for knock, allowing the timing run at maximum advance, thus restoring power output to where it was designed to be.
Engines that use ride-by-wire throttle control can even go so far as to manipulate the throttle to lighten acceleration loads when knocking is sensed, adding to the feeling that power is reduced.
So really, the main reason manufacturers recommend high-octane fuel in high-performance bikes isn’t because the fuel has more energy content, it’s to prevent knocking, pure and simple.
There’s simply no advantage to using a higher-octane fuel than that recommended by the manufacturer (it’s written in your owner’s manual – you read it, right?) in a well-tuned engine.
The exception is that if you ride an older bike, the recommended fuel may no longer be effective at preventing knock due to possible carbon build-up, so if you hear that characteristic pinging you should avoid lugging the engine (higher rpms reduce the likelihood of detonation) and then fill up with a higher grade at your next fuel stop.
Choose your fuel wisely and save your money — gas prices are on the way up, after all, and just in time for riding season.