Octane Rating and VW Engines


Someone posted to the RAMVA Newsgroup - Everything thing else being equal, 92 octane gas will lower cylinder head temperatures (marginally). You do not need "higher compression" in order to burn high octane gas. However, in a un-modified modern water cooled engine, it's seldom of any benefit. Then tell me, why do they put spacers in between the cylinders and the case to lower compression and burn low octane gas in vw engines. This is common practice to keep from pinging and frying a engine.

Rob responded - The octane number of a gasoline is NOT a measure of it's hottness or coolness in the burning process, and it is NOT a measure of how 'powerful' it is. The octane number is simply a measure of how good the gasoline is at resisting detonation (knocking/pinging).

The internal combustion engine is, in simple terms, a gas pump (that's "gas" as in vapour, not "gas" as in gasoline). The higher the gas pressure inside the cylinder, the more 'push' there is on the pistons, and this means the higher the power output will be. We create this pressure by heating a cylinder full of air; and we do THIS by adding a small amount of gasoline to the air and igniting it with a spark. The engineers aim to get the highest possible cylinder pressure without creating uncontrolled burning of the gasoline.

Detonation (pinging/knocking) occurs after the fuel is ignited by the spark plug, but before the flame front has finished racing across the cylinder to burn all the fuel/air mixture. Don't confuse it with pre-ignition, which occurs when the fuel is ignited before the spark occurs.

So why does detonation occur? It relates to the nature of gasoline. Gasoline is a mixture of different hydrocarbon molecules, and some of these molecules decompose more easily than others when heated under pressure.

So when we ignite the fuel/air mixture with a spark, the flame front starts moving across the cylinder, burning the mixture of air and gasoline vapour as it goes. This increases the temperature (and therefore the pressure) of the remaining fuel/air mixture rapidly. The remaining fuel starts to decompose before the flame front reaches it. If this decomposition produces 'auto-ignition' compounds (those which will start burning without a spark), you end up with an uncontrolled over-rapid burning of the remaining fuel/air mixture, which sets up an opposing pressure wave in the cylinder. This uncontrolled burning and the opposing pressure wave produces the characteristic clicking/pinging sound of detonation, and results in the piston getting a 'hammer blow' instead of a steady push.

(You can test for detonation/pinging quite easily. Get the engine up to normal running temperature and the in 4th gear at 30 mph floor the throttle (high load/low rpm). If you hear a harsh uneven clicking sound from the engine - that's detonation. You need to either adjust the timing and/or use a higher octane fuel). These hammer blows can quickly destroy the engine.

Higher octane fuels are better at controlling the decomposition into auto-ignition compounds than lower octane fuels. They do this in several ways - by interferring with, and reducing the actual decomposition of the fuel, or by chemically reacting with the decomposing gasoline so less auto-ignition compounds are formed.

There are three main sources of heat inside the cylinder which contribute to the decomposition of the fuel:

  1. The residual heat in the heads, cylinders and pistons. The VW engine is air-cooled and runs hotter than it's water cooled cousins, so more residual heat is present, compared to a water cooled engine.
  2. The heat produced by the ignition of the fuel itself. This depends on the nature of the fuel, and on the fuel/air mixture - rich mixtures are "pre cooled" from the evapouration of the extra fuel, so burn a little cooler, lean mixtures burn hotter.
  3. The heat of compression before the spark. Compression of a gas raises the temperature of the gas. We want the temperature and pressure to rise, because the higher the compression, the higher the pressure rise after the fuel is burned -- giving us more power.

The first two items are largely fixed - not easily adjusted. Altering #1 would need extensive alteration to the engine design. Altering #2 would require a change in the fuel formulation, which is not even in the hands of the engine designer.

#3 - the heat of compression (compression ratio), is easy to adjust in the design of an engine, so this is the one used to match an engine with the fuel it will be using.

It's all a balancing act, and because the air-cooled engine runs hotter than a water cooled engine (more residual heat), you need to limit the amount of additional heat produced in the cylinders prior to ignition (lower compression ratio). Air-cooled VW engines therefore use a slightly lower compression ratio than water cooled cars using the same octane rated gasoline.

The octane number came about as a result of research carried out in the 1920s and 30s by Sir Harry Ricardo ("The Internal Combustion Engine" 1925, 1935, and other books) and Charles Kettering (he also developed the distributor and coil ignition system). Harry Ricardo developed the concept of a test engine, in which the compression ratio, valve timing and other factors could be altered whilst the engine was running. Kettering assigned Thomas Midgley to investigate the problem of knocking which was destroying his test engines. Midgely conducted a long search of additives which would help a fuel to resist knocking.

Amongst the chemicals tried were Iodine, Aniline, Selenium Oxychloride, Methylclopentadienyl Manganese Tricarbonyl (MMT - currently used in Australia for Lead Replacement Petrol - known as LRP), and other Phosphorus, Sodium and Potassium compounds. Midgley even tried melted butter! Some compounds worked better than others as an anti-knock agent, but many had serious defects (Selenium oxychloride corroded the metals in the engine); and Midgley gradually focussed on organo-lead compounds, and eventually developed a combination of Tetra-ethyl Lead (TEL) with Ethylene Dibromide and Ethylene Dichoride acting as scavengers to prevent an excessive build up of lead oxides inside the engine.

During these tests, it was discovered that Iso-Octane had a very high natural knock resistance, but Heptane had a very poor knock resistance. Because these two compounds are very similar in other respects (similar boiling point and molecular weight), they made a useful comparison point for gasoline. So the octane number for any gasoline is a comparison with a mixture of Iso-Octane and Heptane. 91 Octane has the knock resistance equivalent of mixing 91% Iso-Octane with 9% Heptane.

The discovery in the late 1920s that organo-lead products enhanced the anti-detonating characteristics was a revolution in fuel design, as engines could be designed to operate at higher compressions for better efficiency. So gasolines became 'doped' with tetra-ethyl lead (and the associated bromide scavengers) to enhance their octane numbers.

Another useful feature of lead in gasoline is that the burned lead products (mainly lead oxide) coated the hot exhaust valve seating area, and prevented a problem called Valve Seat Recession (VSR) which results in the exhaust valve 'eating' it's way into the head. With the 'soft' cast iron heads of the day, this was a real bonus. Many older engines in use today which have cast iron heads will suffer VSR on unleaded fuels, and so additives or Lead Replacement Petrol (LRP) is often recommended for this type of engine.

VSR is not a problem with VW engines, as they have hardened valve seats inserts in their aluminium heads. The VW engine can therefore run on unleaded gasoline quite happily, provided the octane number is high enough.

Lead is being removed from fuels because it pollutes the atmosphere, and when ingested by animals, it builds up and causes health problems.

An additional feature of lead additives was that they provided a small but useful amount of lubrication to the valve stems. This was important in engines which had cast iron heads with the valve guides cut directly into the head metal. Most cars also required valve seals to prevent excess oil from dripping down the valve stems and causing a smokey exhaust, and so they had little engine oil available to lubricate the valve stems. TEL in the fuel provided a useful additional amount of lubrication - partially replacing the "missing" oil based lubrication.

The VW engine has bronze alloy valve guides which do not require any lead based lubrication, and the design of the valves does not need valves seals, so they are lubricated very well from the splashed oil on the spring end of the valves. The VW engine does not require leaded fuels for valve stem lubrication purposes.

Gasoline which is high in aromatic compounds like Benzene has a high 'natural' octane rating and so needs less additives to increase the octane rating. Unfortunately, these aromatic compounds are also those most responsible for atmospheric pollution, so these compounds are being reduced in gasoline in many countries. This creates another dilemma - how to increase the octane rating without lead additives, and with reduced aromatic compounds in the fuel.

A number of other chemical compounds called Oxygenates have been developed to enhance the natural octane number of gasolines. The most common one used is Methyl Tertiary Butyl Ether (MTBE). Other compounds include TAME, ETBE, Methyl Alcohol (Methanol) and Ethyl Alcohol (called Gasohol when mixed with gasoline). But MTBE and the other oxygenates contain 'used' oxygen, so cars using oxygenates fuels burn MORE fuel (because there is less 'fuel' in the fuel) and this increases pollution anyway (Source - "Cleaner Burning Gasoline" California EPA).

And there is a second effect here too - carburetor cars like most VWs cannot adjust the fuel/air mixture 'on the run' like computer equiped fuel injected cars can, so they run lean when run on oxygenated fuels. This is because carburettors meter out a volume of fuel into the intake air; they have no regard for the chemical content of the fuel, as as noted above, oxygenated fuels have less "fuel" in the fuel. The resulting lean burning creates more heat in the cylinders, and this 'excess' heat raises the octane number needed, or require adjustments to the carburettor as outlined below.

It's a vicious circle, so If you can avoid using oxygenated fuels in your VW - do so. If you have to use oxygenated fuels, you may improve the car's performance by using a slightly larger main jet in the carburetor. Doing this brings the mixture back to the correct setting, which helps reduce the extra unwanted heat in the engine, and reduces the likelihood you'll need a higher than normal octane gasoline to compensate. And if your engine is due for a rebuild, and you have to use oxygenated fuels, consider using a slightly lower compression ratio.

Octane numbers are measured in two ways. The 'research' method of measuring the octane number uses a constant speed (1500 rpm) engine in laboratory conditions. This is the RON - Research Octane Number. The other method is the MON - Motor Octane Number, which uses the same test engine, but has a harsher test regime more closely related to road conditions. So the MON is usually lower than the RON for the same fuel.

Often you may see the octane rating quoted as (R+M)/2. This means an average of the two methods is used to give the fuel a number. This number method is often called 'pump octane' or AKI (anti-knock index) in the US.

The 1200 VW engine running around 6.6-7:1 compression ratio, and the 1300 with 7.3:1 compression ratio needs a minimum of 87RON octane (about 84AKI). The 1500/1600 engine running around 7.5:1 compression needs 91RON octane (87AKI) or higher. Using a higher octane gasoline in an engine designed for low octane WILL NOT increase it's performance - the octane number is a MINIMUM needed to eliminate detonation, and that's all it is.

I have a 1500 VW with unmodified 1500 heads running 1600 cylinders and pistons. This has raised my compression ratio from 7.5:1 to 8:1 (more fuel/air mix squeezed into the same head space). It was originally designed to run on 91 RON (about 87AKI) but now prefers at least 93 RON (89AKI). If I were to increase the capacity more without modifying the heads, I would need spacers under the cylinders to lower the compression back to a reasonable number (usually around 7.5:1), otherwise I would need to run on Unleaded Premium (95 RON in Australia where I live) to prevent detonation. I include this description to demonstrate the effect of compression ratios on the octane number required.

In conclusion, the octane rating is a measure of the fuel's ability to CONTROL the burning process (to prevent detonation); it is not a function of burning 'hotter' or 'colder'. And the higher the compression ratio (in the same engine), the higher the octane number needed.

Oxygenates are bad news for carburetted engines, and if you have to use them, expect to increase your main jet to compensate for the reduced amount of "fuel" in the fuel.

For Australian readers - in 2002-2003 some states started allowing Ethanol blend petrol (gasoline). As noted above, oxygenates like Ethanol cause a carburetted engine to run lean. 10% Ethanol causes a 3.9% lean mixture (Ethanol is 39% "used" oxygen). But some petrol stations are adding up to 20% Ethanol and not signposting the pumps - these stations are mostly in New South Wales. Southern Queensland and nothern NSW has many BP stations offering 10% ethanol blend (called E10) and these ARE signposted. In South Australia and Western Australia, it is illegal to sell ethanol blended fuels, and in Western Australia it is also illegal to sell MTBE blend fuels.

10% Ethanol blend (E10) requires a main jet 2 sizes larger than a straight hydrocarbon fuel (for example, from a 125 to a 130; or 127.5 to a 132.5), to get the mixture back in to balance (stoichiometry). 20% Ethanol blend requires a main jet 4 sizes larger, and will probably need a larger idle jet too. Fuel consumption WILL increase with these changes.

Up until about 2001, MTBE was not an issue in Australia, but Woolworths Petrol Plus oulets all over Australia (except Western Australia) have fuel with an average of 2-3% MTBE (most Petrol Plus fuel is imported from overseas - they don't get much fuel from Australian refineries). The Woolworths web site is a little confusing because they also mention up to 7% MTBE (so does the average 2-3% mean none in Western Australia and 7% on other states)???

Why the fuss? Well, 2-3% MTBE is not going to bother your VW engine to much, but with 7% MTBE, your engine will be running just over 1% lean, which might be noticable in some engines - especially those using the 34PICT/3 carburettor, which is generally set to run a little leaner than the smaller 30 and 28 series carburettors. So if you experience detonation - try changing brands and see if this makes a difference.

Hope this helps explain a complex issue.

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