By Rich Rohrich
It's getting so you can't read the newsgroups or walk through the pits at a racetrack without bumping into a group of racers discussing fuel, and the problems associated with it. It's obvious to everyone that the fuel we use is a vital link in the performance chain, yet it is surrounded by more than the usual share of myth and misinformation. Given the sweeping changes on pump fuels mandated by the government, and the vast number of race gas offerings the subject is a hot topic. Through this series of articles I hope to shed a little light and hopefully squish a myth or two along the way.
What does the Octane Number at the pump mean?
The octane rating of a fuel is what most people are familiar with, but there seems to be a lot of confusion surrounding it. In simple terms the octane number you see at the pump is the average of two octane numbers; the Research Octane Number (RON) and the Motor Octane Number (MON) or (RON + MON) / 2. This final octane number is sometimes referred to as the Anti Knock Index or AKI. This pump octane number is a measure of the anti- knock characteristics of a given fuel.
MON and RON are determined by standardized ASTM laboratory tests. The details of the tests are not as important as what they mean in terms of performance. Low to medium-speed knock characteristics are determined by the Research (RON) method, while high-speed and partial throttle heavy load knock characteristics are determined by the Motor (MON) method. MON testing is conducted under more stringent conditions with the timing on the test engine advanced and run with a higher inlet air temperature, so the MON number tends to be lower but also more valid for high-performance applications. There are a number of more valid tests that have been developed to determine the anti-knock characteristics of fuels used in high performance engines, but the aren't in general use at this point so we are stuck with the old reliable pump octane number.
So what's that knocking sound coming from my engine?
The Knocking sound you hear when your engine is in trouble are the result of abnormal combustion. The most common combustion problems are detonation and pre-ignition. In simple terms detonation is the uncontrolled burning of the fuel in the combustion chamber, while pre-ignition can be defined as the starting of the burning process by any source other than the spark plug usually before the plug has fired.
To truly understand what detonation is, its important to understand that if you raise the temperature of any combustible mixture high enough, it will ignite on its own. This is sometimes called the "spontaneous combustion point" or the "auto ignition temperature". Detonation is a rapid uncontrolled rise in cylinder pressure caused by all or part of the fuel mixture reaching this auto-ignition temperature.
Following the ignition process through a cycle should help complete the explanation. As the piston rises and compresses the trapped mixture the pressure and temperature begins to rise. The spark plug fires somewhere before the piston reaches Top Dead Center (TDC) and starts burning the compressed mixture in the cylinder and as a consequence raises the combustion chamber temperature. While this burning is taking place, the piston is still rising and still compressing the air/fuel mixture which raises the cylinder pressure, and combustion chamber temperature even higher.
At this point, the pressure rise in the cylinder is very rapid, but it generally proceeds at a fairly even controlled rate. The remaining unburned mixture and the end gases at the edges of the combustion chamber are being raised to extremely high temperatures as the advancing flame front compresses and heats up the mixture directly in front of it. This activity before the flame front reaches the end gases at the edge of the chamber are sometimes called pre-flame reactions. The longer it takes for the complete burning to take place the greater the chances that these pre-flame reactions will force the end gases to reach the auto ignition point and cause a rapid uncontrolled pressure rise, along with a huge increase in cylinder temperature. If brought to the auto ignition point the end gases of the combustion chamber can cause a pressure and frequency rise that is high enough to be audible. That's the KNOCK or PING that you hear. Ideally, the burning of the mixture will be completed before any of these end gases have an opportunity to reach the point of auto ignition. If the ignition timing is set correctly this should happen around 15-20 degrees After Top Dead Center (ATDC).
It's hard to visualize the immense pressures we are talking about in the combustion chamber. In a normal combustion cycle the pressures can easily reach 100 times the trapped compression ratio, that's 800-1300 psi banging away at the piston crown and cylinder head and bearings. Once an engine starts to detonate the pressures can reach 3 to 4 times that high. The pressure rise during detonation can be almost instantaneous, so it's easy to see why the edges of the piston can be broken away during these cycles. It's like having a small bomb go off in the engine.
As you may have guessed from the earlier discussion of octane numbers, high octane fuels have a considerably higher auto ignition temperature to keep these pre-flame reactions from causing sudden uncontrolled pressure rises. If the charge burns fast enough or the fuel is resistant enough to auto ignition (high octane) then all is well and the pressure rise isn't too extreme. Hopefully it should be fairly clear that if you can shorten the burn time (10% to 90% burned) enough then the octane requirement of the engine will be reduced. As a general rule, the first half of combustion 0-50% burned, speeds up in direct proportion to rpm, while the 50-100% burned time speeds up exponentially with rpm. So all other things equal, the faster you spin an engine, the faster the charge will burn and the more knock resistant the engine will be. Small bore, high rpm motors are by design, very knock resistant.
We defined pre-ignition previously as the starting of the burning process by a source other than the plug. This has the same effect as advancing the timing. This causes the engine to be subjected to huge amounts of heat, because the piston and cylinder walls are subjected to the burning process for a longer period of time, this in turn raises the combustion chamber pressure. It's possible for pre-ignition to melt the top of pistons because of the extreme temperatures that this advanced timing causes. Keep in mind that any time you raise the temperature in the cylinder you get a corresponding rise in pressure , or conversely raising the pressure also raises the temperature. So it's easy to see how pre-ignition and detonation are very closely linked.
So it pretty much boils down to this. If you can control the pressure and temperature in the combustion chamber , things will go along with out too many problems. But once you cross that temperature/pressure threshold a number of interrelated actions can take place that causes all hell to break lose. High octane fuel is one way to keep the carnage in check.
How much octane do we need?
Cylinder pressure is one of the key factors in determining the octane requirement of an engine. Intake valve closing time on four-stroke engines, and exhaust timing on two-strokes will have a major influence on the dynamic cylinder pressure . It's a commonly held misconception that higher Octane fuel slows down the flame speed which keeps the engine from knocking. Flame speed is a function of fuel chemistry, not the Octane rating. The component make up of the fuel will determine the flame speed whether it's a high octane fuel or not. . Racing fuels designed for high rpm applications tend to have higher flame speeds than normal to help reduce burn time. There isn't much time available to complete the combustion cycle at 10,000rpm, so choosing the right fuel can really make a difference. Choosing a faster burning fuel will allow you to run less ignition advance, and ultimately make more power at higher revs.
Every engine can have radically different requirements. Even two similarly modified engines can have requirements as different as 5 to 8 MON numbers in some cases. The factors affecting octane requirements should be of great interest to every racer. By changing these factors around you can raise or lower the octane requirement of your engine. Some of the more obvious factors are :
D E S I G N F A C T O R S O P E R A T I N G C O N D I T I O N S
Compression ratio Outside air temperature/ Intake air temperature
Trapped Compression Ratio Altitude
Ignition timing Humidity
Combustion chamber shape Barometric pressure
Charge Motion in the cylinder Premix ratio (two-strokes)
Air/fuel ratio Engine RPM
Cooling efficiency Engine load
Spark plug location
Spark plug heat range
In looking at this list, it should be apparent to you that a number of these factors/conditions are pretty much out of our control. We will concentrate on those that are more easily changed.
Is it better to raise fuel octane or lower the octane requirement of the engine? The answer to that question is yes. You want to lower the octane requirement of the engine as much as possible without lowering engine performance. You also want to use a fuel with an octane rating just high enough to keep your engine from ever detonating.
Engine timing is one of the factors on the list, but on most modern engines the timing is fixed or electronically controlled. You have more to lose than gain if you play with the timing of these ignitions, it's best to leave them alone.
Another way to fend off detonation is to make sure the engine's cooling system works as efficiently as possible. That means clean radiators at all times, and unrestricted airflow across the cooling fins on air cooled engines. It's important to keep the water temperature between 50 and 70 degrees Centigrade. This range produces the most power while keeping the octane requirement low.
Most two-cycle engines have a squish band machined at the outer edge of the cylinder head, this band serves two purposes . The close proximity of the piston to the cylinder head helps to cool (or quench) the end gases that were heated by the pre-flame reactions, but more importantly the squish band helps to add motion to the burning charge, which helps speed up the burning and limits the amount of time available for pre-flame reactions to heat the end gases. A fast burning chamber will tend to be very resistant to detonation . This is part of the reason you've seen manufacturers switching from domed pistons, to flat top pistons and back again. They are always looking for that magic combination of scavenging efficiency and charge motion. Unfortunately, on most stock engines that squish band doesn't serve much purpose. In theory squish bands work very well, but production line tolerances leave squish clearances so great that the only thing being squished is horsepower. Cutting the cylinder head to bring the squish clearance into spec while still retaining the original cylinder head shape and volume is not the easiest thing to do, though it will definitely pay dividends .
Four-stroke engines usually have a quench area designed into the cylinder head to aid in lowering the temperature of the end gases, while charge motion is often generated as a combination of intake port velocity, port angles, piston shape and combustion chamber shape. We'll go into the specific interactions in a future part of this series. Large bore engines tend to have a higher octane requirement then smaller bore engines given the same compression ratio, because of the longer distance the flame front has to travel and the additional time available for pre-flame reactions to take place. It is possible to use two spark plugs per cylinder to lower your engine's octane needs in large bore engines. Adding a second spark plug shortens the fuel burn time and decreases the distance the flame front needs to travel.
One of the single most important things you can do to lower the octane requirement and save your engine some self destruction via the detonation express is to LEARN TO JET! Running too lean causes excessive heat build up that can pave the way for Detonation and Pre-ignition. Check out the Technical Articles section for a good jetting tutorial.
This all brings us back to the octane question : How much do you need. We'll go into more detail in Part II of the series but here are a few suggestions to get you started:
Start simple and work your way up. Try a good grade of premium gas that doesn't contain ANY ALCOHOL. In most states they will have a sign on the side of the pump warning you about the percentage of alcohol (ethanol or methanol) in the fuel. Most well modified normally aspirated engines can run on 95 -100 octane gasoline. Good porting with flow matched transfer ports can significantly lower octane requirement on two cycle engines. If your engine detonates try one (or all) of the measures to lower the octane requirement of the engine. If all these measures fail, try mixing pump gas 50/50 with a good of good quality race gas (Phillips 66, Power Mist, UNION 76, Sunoco, VP, ELF, etc..) with your gasoline. Make sure you use race gas specifically designed for you your type of application. The fuel manufacturers can make recommendations based on your engines rpm range, bore size, and the type of riding you'll be doing. It's best to stay away from AvGas for your bike. We'll go into the specifics of AvGas in the next installment. Keep in mind that octane requirement is lower at high altitude and high humidity. An engine that ran fine at 10,000 feet could very easily detonate at sea-level, or a sudden drop in humidity on a hot day can cause knocking that never appeared before.
References and further reading :
Harold H. Schobert - The Chemistry of Hydrocarbon Fuels - Butterworth-Heinemann Ltd.
Keith Owen, Trevor Coley - Automotive Fuels Reference Book - SAE - R151
H.P. Lenz - Mixture Formation in Spark-Ignition Engines - Springer-Verlag
Jeff Hartman - Fuel Injection - Motorbooks International
Germane, Wood, Hess - Lean Combustion in Spark-Ignited Internal Combustion Engines - A Review - SAE paper 831694
Z. Warhaft - An Introduction to Thermal Fluid Engineering - Cambridge University Press
Copyright © 2002 Richard Rohrich
All Rights Reserved