Fuel Additives


Additives discussed in this article include -


In the USA there is a lot of possible variations in fuel additives because lead has been replaced first with MTBE and later with ethanol (alcohol), but this is not true everywhere. Some history -


Tetraethyl Lead (TEL)

Tetraethyl lead (TEL), colorless, poisonous, oily liquid, Pb(C2H5)4, was added to gasoline for internal-combustion engines as an antiknock agent. TEL needed ethylene dibromine and ethylene dicloride scavengers mixed in to prevent an excess of lead-oxide build up in the engines, and although the story revolved around lead - it was as much for the scavengers that lead was removed. BUT - along comes the MTBE (methyl tertiary butyl ether) alternative, and the need for higher proportions of aromatics (benzene etc). MTBE itself tends to increase the NOx levels in the exhaust (although it reduces carbon monoxide and hydrocarbons). It also results in a slight increase in fuel consumption since 11% MTBE results in 2% used oxygen in the fuel, meaning that there is slightly less enegry in each litre/gallon of fuel.

In the final days of lead in the late 80s they were adding only 0.2 gm/litre (down from 1.05 gm/litre - the practical limit was 1.1 gm/litre), and this reduced the halogen scavengers too, so the last leaded fuels were not nearly as nasty as they had been 20 years previously.

LRP (lead replacement petrol), which contains MMT (methylcyclopentadienyl manganese tricarbonyl) has been used in aome countries, including Australia, but has proven to be a poor substitute - it causes spark plug fouling with resultant hard starting and rough running. It is not popular, and lots of folks with older cars which needed lead are using unleaded with additives instead (some of which contain MMT so go figure!)

According to the lengthy FAQ on fuel I have, there is some variation in the US brands due to the no-lead rules and the resulting substitutes for higher octane fuels. (I don't know if different 'cheaper' brands or "name" brands would be better.)

The FAQ I have indicates it's usually better to stay away from additives, apart from a bit of upper cylinder lubrication. Most of these are just kerosine-based anyway, so I wouldn't bother. The VW engine doesn't need extra upper cylinder lubrication because the valves don't have valve seals (they slope down at the spring end so excess oil runs away from the cylinders), and since they don't use valves seals they get plenty of lubrication from the oil on the tappet end -- it works it's way up the valve stems by capilliary action. The cylinders get plenty of oil too, so it's not needed for the rings etc.

In the United States lead additives have been completely banned, and compounds such as Methyl Tertiary Butyl Ether (MTBE), and more recently Ethanol, are added to raise octane rating and promote cleaner combustion.


Methyl Tertiary Butyl Ether (MTBE)

MTBE (Methyl Tertiary Butyl Ether) is an oxygenate that was introduced as a gasoline additive in the US in the 1980s to replace TEL while remaining harmless to catalytic converters. The US fuel laws allowed up to 2% total oxygen in the fuel -- that's 11% MTBE. This varies a bit from State-to-State, and summer-to-winter. I don't think all states actually used it, and there are definitely different amounts in some areas where it IS used. Much of California had up to 11% MTBE, and Denver too, but areas outside Denver had a different fuel formulation (lower MTBE or no MTBE)-- some Denver folks filled up outside the city when they could because the car runs better.

Since MTBE is oxygenated, an 11% concentration makes carburetted cars run 2% lean. The slight hesitation that many VW owners experience is almost certainly a lean burn condition for a number of reasons, one of which was MTBE. VWs like a fractionally rich mix, which is part of what killed them off in the US (higher emissions). It is very difficult to sufficiently control the emissions from an aircooled engine, because the engine temperature varies a lot with engine load, and they don't like lean burn conditions.

Cars with engine-management computers cope with MTBE by upping the fuel delivery for correct mixtures, but older carburetted cars can't adjust themselves on the move.

Hopefully a slight choke adjustment will help this, or you may increase the size of the main jet (say from X125 to X127.5 -- more than enough to compensate for 11% MTBE in the fuel). So if your car is stuttering on acceleration and running a little hot when cruising, increasing the main jet by one size might be the fix that is needed.

Just a change of the brand of fuel might help, too. Some companies use more aromatics (which have natural high octane ratings) and less MTBE to get the right octane number. The 60 page FAQ and California EPA documents I have on it says there is enough variation in fuels in the US to make it worth experimenting with brands. They also say you should be prepared to change brands winter/summer if you do find an improvement, because the formulas change winter/summer.

California is requiring reduced aromatics because they also result in more pollutants, but this is a double whammy. You then need more MTBE or Tertiary Amyl Ether (TAME) or one of the other oxygenates to increase the octane number, and then you burn more fuel in the computerised cars, which increases total pollution anyway. Fascinating.

MTBE doesn't do anything for the valve seats. With vintage and classic cars the loss of lead is not a huge problem -- they do only small mileage, and valves seat damage usually only occurs when the valves are running really hot (engine working hard), so going for a car club cruise in your 1930 MG once a month is not usually a problem.

But for many of our cars on the road, being used every day, and still having cast iron heads, it's a REAL problem. They really do need the lead coating (or a substitute) on the exhaust valves to prevent a problem called Valve Seat Recession (VSR).

MTBE doesn't help there -- it's only an octane booster. In some countries Lead Replacement Petrol (LRP) is available which has lead subsitutes (MMT mentioned above) to protect the valve seats from valve seat recession, but fortunately for VW owners the VW engine has hardened steel valve seats in the aluminium heads so valve seat recession is not a problem. The VW engine does NOT need lead or lead substitutes in fuel. In fact if you have LRP additives in your country -- DON'T use it in the VW engine. The VW engine runs hotter than it's water-cooled cousins, and burning LRP in them tends to bake the LRP residues onto the spark plugs, causing rough running and hard starting.

MTBE is falling out of favor in the US -- it's being found everywhere in the environment, with what effect I don't know, as a result of leakage from service station sites and tanker spills, etc. It mixes readily with water, so it easily contaminates ground water and so on. And of course since it contains "used" oxygen there's less "fuel" in the fuel, so fuel consumption INCREASES, which increases the total exhaust emissions and so on. California has moved to ban the addition of MTBE by 2002.

Until recently we hadn't gone down the MTBE path in Australia, and I was in hopes that we wouldn't. However now (March 2003) Woolworths (a large supermarket chain in Australia which has an associated fuel outlet) began selling a fuel called Woolworths Petrol Plus. Their web site indicates that this petrol (imported into Australia, not refined in here) contains up to 7% MTBE, and an average of between 2 and 3% MTBE -- but not in Western Australia or South Australia, where oxygenates are banned.

2-3% MTBE causes no problems for the VW engine, but 7% MTBE will run the engine about 1% lean, which might be noticable in some cars (hesitation, running hot). So if your Australian bug has a tank full of Petrol Plus and the engine feels a little sick, try switching to one of the other brands and see if that helps. If it does, then you'll now know WHY it helps, as no other brand in Australia uses MTBE laced petrol.

Folks in Australia are getting REALLY confused!

With MTBE or gone and the aromatics like methyl benzine (toluene) being reduced too (because it's carcenogenic), it's going to be hard to get the octane number high enough.



Ethyl alcohol (ethanol) is added to fuel in many places to reduce costs and to extend the life of our oil reserves. It also encourages clean burning of the gasoline, and has a high natural octane number. Ethanol is soluble in gasoline, and in some countries it's common enough to see 10% ethanol blends. In Australia for example, a common mix is E10 (10% ethanol), and E85 (85% Ethanol) is also available.

There has been lots of discussion about how the addition of ethanol helps the combustion process and how it will help farmers. In the USA the main source of Ethanol is from corn. In Australia it's often made from sugar grown in Queensland and northern New South Wales, where they make it from otherwise excess molasses.

However, no one has picked up the subject of older carburetted cars. The addition of ethanol to gasoline causes big problems in a VW engine, as ethanol makes the engine run 4% leaner. To compensate for 10% ethanol in gasoline (E10), you must increase the main jet by one or two steps (e.g., from X125 to X127.5 or 130) and possibly the idle jet by one step (from 55 to 60), although sometimes just adjusting the Volume Screw on the carburettor will be enough to adjust the idle mixture.

Another consideration -- production of pure anhydrous ethanol is difficult and expensive, as it forms an azeotrope with water on distillation -- resulting in about 96% ethanol. This 4% or so of water will require an increase the jet sizes just a little to cope with the water (main jet maybe 185, and idle maybe 80-82 for an ethanol-only fuel).

Ethanol attracts water of course, so storage in a car's fuel tank in the open weather means it will absorb condensation, and you won't know it's happening until the engine starts to sputter.

Some VW enthusiasts wonder if they can run their 1500cc or 1600cc engines on ethanol. They envision the ability to produce ethanol on their own from substances that don't release hydrocarbons when burned (such as corn and other biomass). It's all part of a hope to have a bio-Bug, a totally enviro-friendly Bug.

It would be easy enough to run a VW engine on ethanol. The main thing is that using ethanol would require fuel jets approximately 200% bigger in cross section than gasoline needs, because ethanol contains a lot of "used" oxygen which is no longer available as fuel.

So if you have a 34PICT/3 carburetor with size 55 idle and 127.5 main jets, you'd need to increase those jet sizes to about 78 idle, and the main jet to about 180.

For information, the sizes indicate the hole size in mm, so 127.5 jet has a hole size of 1.275mm. So a hole size of 1.8mm has about twice the cross section of a hole size of 1.275mm - hope that makes sense.

I really don't know what size you'd need the air correction jet though - whether larger (for leaner at high speed) or smaller (for richer at high speeds) I don't know. I'd GUESS smaller (for richer, since you need MORE fuel with the ethanol), but that's only a guess. So it if the air correction is normally about an 80, you could try a 60 or something like that.

I think the engine itself would cope okay, so long as you weren't asking for bags more power - just what the standard cam etc gives you.

The engine would tend to run much cooler than running on gasoline, since it has a much higher latent heat of vapourisation (it cools a lot as it evapourates). So a cylinder head temperature guage might be useful (before the change too, to get an idea of normal temperatures) so you could perhaps partly block the fan intake (a circular piece of metal with a hole in the middle would be the way to do it) to keep the operating temperatures near normal - otherwise the wear rate on your cylinders and pistons would accelerate.

Ethanol is about 115 octane (using the US AKI measurement) so you could increase the compression ratio to about 10:1 to make use of it. That's a lower compression than burning ethanol in a water-cooled car because the VW runs hotter and so must use a lower compression for the same octane rating. For that sort of compression you'd have to make sure your engine had case savers, and it would be wise to use the 10mm head studs with 14mm case savers, rather than the 8mm studs and 12mm case savers. The larger case savers will give more protection from studs pulling out with the higher combustion pressures (if you increase the compression).

If you did increase the compression, you should normally reduce your amount of ignition advance a little, but the higher octane number of ethanol would indicate more advance, so you may have to play about with that one.

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