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Fuel Basics
Cycle Data
Engine Data
Engine Numbers
Fuel Basics
Paint Chart
Port Data - Early
Port Data - Later
Torque Settings


THE Internal Combuston engine, as used in the car as well as the VeloSoleX S 3800, is a major cause of environmental pollution contributing as much as 50% to all Carbon Monoxide, Nitrogen Oxide and Hydro-Carbon emissions in the US (Source: US Department Of Energy, 1992).

The 3 Primary Emissions of a car are:

  1. Nitrogen N2 - Air is 78% Nitrogen gas and most of this passes straight through the car engine
  2. Carbon Dioxide CO2 - Carbon in the fuel bonds with oxygen in the air
  3. Water Vapour H2O -  Hydrogen in the fuel bonds with oxygen in the air

Apart from Carbon Dioxide, which is thought to contribute to global warming, 3 other Harmful Secondary Emissionsare also produced:

  1. Carbon Monoxide CO - A poisonous, colourless and odourless gas
  2. Hydro-Carbons HC and Volatile Organic Compounds VOC - Mainly produced from unburnt fuel that evaporates and react with Nitrogen Oxides to create ground-level Ozones O3 which a major component of smog
  3. Nitrogen Oxides NO and NO2 - Known to contribute to smog and acid rain and to cause irritation to human mucus membranes

These 3 last harmful emissions are what Catalytic Converters were primarily designed to reduce and the reason why they are sometimes called 3-way Catalytic Converters.

Catalytic Converters

CATALYTIC CONVERTERS typically have a 2-Stage Ceramic Honeycomb Structure (to provide a large surface area) coated with the expensive metals of Platinum, Rhodium and Palladium.

The 1st stage Platinum-Rhodium Reduction Catalyst holds on to the Nitrogen molecule of NO or NO2 freeing oxygen as O2. This Nitrogen molecule bonds with other Nitrogen molecules already present to produce Nitrogen N2 which is the major component of air.

The 2nd stage Platinum-Palladium Oxidation Catalyst works by burning (oxidizing) unburnt Hydro-Carbons and Carbon Monoxide using the Oxygen remaining in the exhaust gas to primarily produce Carbon Dioxide CO2.

An Oxygen (Lambda) Sensor upstream of the Catalytic Converter can measure the amount of Oxygen present and tell the engine computer to increase or decrease the amount of Oxygen in the exhaust gas by adjusting the air-fuel mixture ratio, while not changing it too much from the ideal stoichiometric air-fuel mixture ratio of 14.6 : 1.

Catalytic Converters are used on modern 4-Stroke and 2-Stroke mopeds. Catalytic Converters require a temperature in the range of 300C to 600C to work properly, so for the VeloSoleX S 3800 (which is often used for short trips), it must be located as close as possible to the engine manifold to enable a quick warm-up. This unfortunately can result in reduced life expectancy for the Catalytic Converter. Another cause of reduced life expectancy for the Catalytic Converter is unburnt fuel in the exhaust gas which unfortunately is how the VeloSoleX S 3800 is often tuned, since a slightly rich mixture helps to prevent the air-cooled engine from overheating while still giving good acceleration.

Modern Catalytic Converters use Variable-Conductance Vacuum Insulation (VCI) to keep the temperature of the catalysts above 300C for at least 24 hours after stopping the engine thus allowing them to work immediately the vehicle is started the next day.

As far as failures are concerned, when a Catalytic Converter fails the only way of knowing about it may be by a total lack of engine power or the engine overheating because of blocked exhaust gases. These days, shaking it and listening for loose or broken pieces may not be a reliable indicator anymore.

Leaded Fuel

SINCE the 1920's, fuel has been 'Leaded' (the Alkyl compound Tetra Ethyl Lead was added to fuel) to allow engines to use higher compression ratios and hence produce more power without the problem of engine detonation ("knocking", "pinging", "pinking"). Lead also had a secondary benefit of reducing Valve Seat wear on 4-Stroke engines. Typically, 100 RON Premium Leaded Fuel of the 1970's had about 0.7 - 0.8 grams per litre and 94 RON Regular Leaded Fuel about 0.6 - 0.7 grams per litre.

From the 1970's, because of increasing smog levels in cities, laws were passed so that Catalytic Converters had to be fitted to cars. At the same time, action groups tried to get Tetra Ethyl Lead removed from fuel believing that the Lead was ending up in people's blood and affecting their health. The average Catalytic Converter was very quickly damaged by Lead in fuel and so Leaded Fuel was conveniently targeted as the main culprit and was phased out as Unleaded Fuel was phased in.

Since then, there have been a number of reports suggesting that the removal of Lead from fuel, which was so eagerly pounced on, has notquite resulted in the reduction of Lead levels in blood expected.

Since the Lead scavengers Ethylene Dibromide and Ethylene Dichloride, also required in Leaded Fuel, could react with Hydro-Carbons to produce highly toxic Organo-Halogen emissions such as Dioxin, even if catalysts were removed or Lead-tolerant catalysts discovered, Alkyl Lead compounds would still remain banned because of their toxicity and toxic emissions.

Sulphur inhibits the octane-enhancing effect of Lead and can damage Catalytic Converters, so from the beginning it was advantageous for motorists to have Sulphur reduced to an absolute minimum particularly from the 1970's with the introduction of Catalytic Converters.

Since Sulphur Dioxide is widely associated with pollution, many fuel distributors capitalize on this and proudly boast of Low-Sulphur Content to attract environmentally-concious motorists while not mentioning too much about the undesirable effects of the other fuel components which may have been increased.

Unleaded Fuel

WHEN Tetra Ethyl Lead was reduced from about 0.6 - 0.7 grams per litre in Regular Leaded Fuel to about 0.015 grams per litre in Premium and Super Unleaded Fuels, the percentage of Fuel Aromatics increased because they were used to replace Lead as the Octane Enhancer and anti-knock additive.

The typical percentage of the some important Fuel Aromatics in Leaded Fuel is:

  • 10 % Toluene
  • 5% Benzene
  • 5% Xylene
  • 1% Ethyl Benzene

There has been mounting evidence to suggest that Fuel Aromatics have the opportunity to escape at filling stations and tanker spills. Benzene as well as 1,3 Butadeine combustion by-products are known to be highly carcinogenic. So to reduce the escape of these Fuel Aromatics, many countries have had special fuel pumps installed to prevent the escape of fuel vapours at filling stations.

The carcinogenic effects of the other 30 or so components of fuel are still being investigated. Using Unleaded Fuel without a Catalytic Converter or using a defective Catalytic Converter is widely believed to introduce many new pollutants into the atmosphere than when Leaded Fuel was used.

Unleaded Fuel is seasonally adjusted to compensate for changing ambient temperatures and fuel volatility. Consequently fuel sold in summer does not have the same composition as fuel sold in winter.

On the topic of pollution, it should be noted that 10% of all Carbon Monoxide polluters produce over 50% of the pollution. Hence, it may make more sense to focus on these polluters than to spend large amounts of money on researching how to further improve the emissions of the modern car.

Fuel Aromatics versus Oxygenates

HIGH Octane Fuel Aromatics can produce disproportionate amounts of Carbon Dioxide CO2 and Hydro-Carbons HC in  exhaust emissions. In the US, the Clean Air Acts forced manufacturers to produce Re-Formulated Gas (RFG) from 1995 onwards to reduce pollution. Re-Formulated Gas contains less Benzene, evaporates slower and contains more Oxygenates. High blending octane Oxygenates began to be used increasingly as the levels of the high octane Fuel Aromatics were reduced. There was some early evidence to suggest that Oxygenates might reduce the smog-forming tendencies of exhaust fumes if the Hydro-Carbon fraction is carefully modified.

Oxygenates include the alcohols and ethers such as Methyl Tertiary Butyl Ether (MTBE), Tertiary Amyl Methyl Ether (TAME) or Ethyl Tertiary Butyl Ether (ETBE). Fuel typically contains as much as 15% MTBE. The major concern with Oxygenates at present is not with atmospheric pollution but with their carcinogenic properties, water solubility and very slow biodegradability in groundwater. There has been some pressure since the 1990's to replace MTBE with the alcohol Ethanol as it may be less carcinogenic (although more expensive).


TYPICALLY, Premium Unleaded Fuel contains some or all of the following additives:

  1. Anti-Oxidants to inhibit gum formation and improve fuel stability
  2. Corrosion Inhibitors to prevent fuel from corroding storage tanks
  3. Deposit Modifiers to reduce deposits, spark-plug fouling and pre-ignition
  4. Dyes to colour fuel for safety or regulatory purposes
  5. Freezing-Point Depressants to prevent icing
  6. Metal De-Activators to inhibit gum formation and improve fuel stability
  7. Octane-Enhancing Additives to improve the octane rating
  8. Surfactants to prevent icing, improve vapourisation, inhibit deposits and reduce Nitrogen Oxide emissions

Octane Ratings

THE two main Octane Ratings are currently RON and MON:

  1. RON (Research Octane Number) is determined by typically using the Cooperative Fuels Research (CFR) ASTM D2700-92 612cc [cm] Single Cylinder Test Engine using the fuel to be tested. The engine is run at 900 RPM, the inlet air temperatrure is set to 38C and the ignition timing is varied from 14 to 26 BTDC as the Compression Ratio is varied from 4 : 1 to 18 : 1 (by moving the cylinder head) until detonation (measured by a Magneto-Restrictive Detonation Sensor in the Combustion Chamber) reaches 50 on a scale of 0 to 100 of a Detonation Meter. Next, a reference fuel mixture of Heptane and Iso-Octane is used in the engine and their ratios altered until 50 is reached once again on the Detonation Meter. Heptane and Iso-Octane were chosen as reference fuels because they have similar boiling points and therefore less likely to introduce spurious effects. Heptane ignites easily when compressed; Iso-Octane ignites with great difficulty. The percentage of the Iso-Octane used gives the Research Octane Number (Octane Rating) of the fuel being measured (eg: 95% Iso-Octane used means 95 RON).
  2. MON (Motor Octane Number) is determined by typically using the Cooperative Fuels Research (CFR) ASTM D2699-92 Test Engine (actually a similar test engine as that used to measure RON but set up differently to better represent an engine in the real-world). Here, the engine is run at 600 RPM, the inlet air temperatrure is set between 20C and 52C (depending on barometric pressure) and the ignition timing is fixed at 13 BTDC. These conditions tend to give lower Iso-Octane numbers (eg: 85% Octane used means 85 MON).

The difference between the RON and MON numbers is known as the Fuel Sensitivity. If the Octane Ratings of a test fuel varies a lot with changes of inlet air temperature or ignition timing (which is typical for highly cracked fuels), it is said to be more sensitive.

The US AKI (Anti-Knock Index) is the Average of the RON and MON numbers and is the figure shown on US fuel pumps while RON is shown on European fuel pumps.

So, if one uses the examples above (RON = 95, MON = 85) the AKI would be:

Generally, the closer the AKI Number rating to the fuel's RON number, the more reliable the AKI rating.

Fuel Recommendation

OXYGENATES have been known toswell or shrink Elastomers depending on the level of Fuel Aromatics (Arenes) and Olefins (Alkenes) also in the fuel. Elastomers are used for the SPI Lip Seal on the VeloSoleX S 3800 crankshaft and also sometimes as a seal on one or both sides of the Crankshaft Roller Bearing depending on the type.

Oxygenates may also lead to increased Spark Plug fouling in 2-Stroke engines that were designed a long time ago.

Since reliable information on the levels of Oxygenates, Fuel Aromatics and Olefins is not readily available for the various fuels in use around the world it is NOT RECOMMENDED to use the higher octane Premium and Super Unleaded Fuels in the VeloSoleX S 3800 until more information becomes available on the subject.

The recommendation at the moment is to use only Regular Unleaded Fuel (eg: UK 95 RON or US Pump 89 AKI).


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Copyright 2008 BRIAN'S VELOSOLEX. All rights reserved. This is a personal website containing information for and by VeloSoleX enthusiasts. It is not intended to replace any official information and whilst every effort is made to ensure the accuracy of all published information no guarantee is offered nor liability accepted for any loss or damage that may arise from any errors or omissions.