Gasoline FAQ


  7.12) What is the effect of humidity?

An increase of absolute humidity of 1.0 g water/kg of dry air lowers the octane requirement of an engine by 0.25 - 0.32 MON [27,28,38].

  7.13) What does water injection achieve?

Water injection, as a separate liquid or emulsion with gasoline, or as a vapour, has been thoroughly researched. If engines can calibrated to operate with small amounts of water, knock can be suppressed, hydrocarbon emissions will slightly increase, NOx emissions will decrease, CO does not change significantly, and fuel and energy consumption are increased [113].

Water injection was used in WWII aviation engine to provide a large increase in available power for very short periods. The injection of water does decrease the dew point of the exhaust gases. This has potential corrosion problems. The very high specific heat and heat of vaporisation of water means that the combustion temperature will decrease. It has been shown that a 10% water addition to methanol reduces the power and efficiency by about 3%, and doubles the unburnt fuel emissions, but does reduce NOx by 25% [114]. A decrease in combustion temperature will reduce the theoretical maximum possible efficiency of an otto cycle engine that is operating correctly, but may improve efficiency in engines that are experiencing abnormal combustion on existing fuels.

Some aviation SI engines still use boost fluids. The water-methanol mixtures are used to provide increased power for short periods, up to 40% more - assuming adequate mechanical strength of the engine. The 40/60 or 45/55 water-methanol mixtures are used as boost fluids for aviation engines because water would freeze. Methanol is just "preburnt" methane, consequently it only has about half the energy content of gasoline, but it does have a higher heat of vaporisation, which has a significant cooling effect on the charge. Water-methanol blends are more cost-effective than gasoline for combustion cooling. The high Sensitivity of alcohol fuels has to be considered in the engine design and settings.

Boost fluids are used because they are far more economical than using the fuel. When a supercharged engine has to be operated at high boost, the mixture has to be enriched to keep the engine operating without knock. The extra fuel cools the cylinder walls and the charge, thus delaying the onset of knock which would otherwise occur at the associated higher temperatures.

The overall effect of boost fluid injection is to permit a considerable increase in knock-free engine power for the same combustion chamber temperature. The power increase is obtained from the higher allowable boost. In practice, the fuel mixture is usually weakened when using boost fluid injection, and the ratio of the two fuel fluids is approximately 100 parts of avgas to 25 parts of boost fluid. With that ratio, the resulting performance corresponds to an effective uprating of the fuel of about 25%, irrespective of its original value. Trying to increase power boosting above 40% is difficult, as the engine can drown because of excessive liquid [110].

Note that for water injection to provide useful power gains, the engine management and fuel systems must be able to monitor the knock and adjust both stoichiometry and ignition to obtain significant benefits. Aviation engines are designed to accommodate water injection, most automobile engines are not. Returns on investment are usually harder to achieve on engines that do not normal extend their performance envelope into those regions. Water injection has been used by some engine manufacturers - usually as an expedient way to maintain acceptable power after regulatory emissions baggage was added to the engine, but usually the manufacturer quickly produces a modified engine that does not require water injection.

Chapter 8) How can I identify and cure other fuel-related problems?

  8.1) What causes an empty fuel tank?

  8.2) Is knock the only abnormal combustion problem?

No. Many of the abnormal combustion problems are induced by the same conditions, and so one can lead to another.

Preignition occurs when the air-fuel mixture is ignited prematurely by glowing deposits or hot surfaces - such as exhaust valves and spark plugs. If it continues, it can increase in severity and become Run-away Surface Ignition (RSI) which prevents the combustion heat being converted into mechanical energy, thus rapidly melting pistons. The Ricardo method uses an electrically-heated wire in the engine to measure preignition tendency. The scale uses iso-octane as 100 and cyclohexane as 0.

Some common fuel components:

     paraffins       50-100
     benzene           26  
     toluene           93
     xylene          >100
     cyclopentane      70
     di-isobutylene    64
     hexene-2         -26
There is no direct correlation between antiknock ability and preignition tendency, however high combustion chamber temperatures favour both, and so one may lead to the other. An engine knocking during high-speed operation will increase in temperature and that can induce preignition, and conversely any preignition will result in higher temperatures than may induce knock.

Misfire is commonly caused by either a failure in the ignition system, or fouling of the spark plug by deposits. The most common cause of deposits was the alkyl lead additives in gasoline, and the yellow glaze of various lead salts was used by mechanics to assess engine tune. From the upper recess to the tip, the composition changed, but typical compounds ( going from cold to hot ) were PbClBr; 2PbO.PbClBr; PbO.PbSO4; 3Pb3(PO4)2.PbClBr.

Run-on is the tendency of an engine to continue running after the ignition has been switched off. It is usually caused by the spontaneous ignition of the fuel-air mixture, rather than by surface ignition from hotspots or deposits, as commonly believed. The narrow range of conditions for spontaneous ignition of the fuel-air mixture ( engine speed, charge temperature, cylinder pressure ) may be created when the engine is switched off. The engine may refire, thus taking the conditions out of the critical range for a couple of cycles, and then refire again, until overall cooling of the engine drops it out of the critical region. The octane rating of the fuel is the appropriate parameter, and it is not rare for an engine to require a higher Octane fuel to prevent run-on than to avoid knock [27,28]. Obviously, engines with fuel injection systems do not have the problem, and idle speed is an important factor. Later model carburettors have an idle stop solenoid which partially closes the throttle blades when the ignition key was off, and (if set correctly) thus prevents run-on.

  8.3) Can I prevent carburetter icing?

Yes, carburettor icing is caused by the combination of highly volatile fuel, high humidity and low ambient temperature. The extent of cooling, caused by the latent heat of the vaporised gasoline in the carburettor, can be as much as 20C, perhaps dropping below the dew point of the charge. If this happens, water will condense on the cooler carburettor surfaces, and will freeze if the temperature is low enough. The fuel volatility can not always be reduced to eliminate icing, so anti-icing additives are used. In the US, anti-icing additives are seldom required because of the widespread use heated intake air and fuel injection [28].

Two types of additive are added to gasoline to inhibit icing:

If you have icing problems, the addition of 100-200mls of alcohols to a full tank of dry gasoline will prevent icing under moderately-cold conditions. If you believe there is a small amount of water in the fuel tank, add 500mls of isopropyl alcohol as the first treatment, and isopropyl alcohol is also preferred for more severe conditions. Oxygenated gasolines using alcohols can also be used.

  8.4) Should I store fuel to avoid the oxygenate season?

No. The fuel will be from a different season, and will have significantly different volatility properties that may induce driveability problems. You can tune your engine to perform on oxygenated gasoline as well as it did on traditional gasoline, however you will have increased fuel consumption due to the useless oxygen in the oxygenates. Some engines may not initially perform well on some oxygenated fuels, usually because of the slightly different volatility and combustion characteristics. A good mechanic should be able to recover any lost performance or driveability, providing the engine is in reasonable condition.

  8.5) Can I improve fuel economy by using quality gasolines?

Yes, several manufacturers have demonstrated that their new gasoline additive packages are more effective than traditional gasoline formulations. Texaco claimed their new vapour-phase fuel additive can reduce existing deposits by up to 30%, improve fuel economy, and reduce NOx tailpipe emissions by 15%, when compared to other advanced liquid phase additives [49]. The advertising claims have been successfully disputed in court by Chevron - who demonstrated that their existing fuel additive already offered similar benefits. Other reputable gasoline manufacturers will have similar additive packages in their premium quality gasolines [50]. Quality gasolines, of whatever octane ratings, will include a full range of gasoline additives designed to provide consistent fuel quality.

Note that oxygenated gasolines must decrease fuel economy for the same power. If your engine is initially well-tuned on hydrocarbon gasolines, the stoichiometry will move to lean, and maximum power is slightly rich, so either the management system ( if you have one ) or your mechanic has to increase the fuel flow. The minor improvements in combustion efficiency that oxygenates may provide, can not compensate for 2+% of oxygen in the fuel that will not provide energy.

  8.6) What is "stale" fuel, and should I use it?

"Stale" fuel is caused by improper storage, and usually smells sour. The gasoline has been allowed to get warm, thus catalysing olefin decomposition reactions, and perhaps also losing volatile material in unsealed containers. Such fuel will tend to rapidly form gums, and will usually have a significant reduction in octane rating. The fuel can be used by blending with twice the volume of new gasoline, but the blended fuel should be used immediately, otherwise teh old fuel will catalyse rapid decomposition of the new, resulting in even larger quantities of stale fuel. Some stale fuels can drop several octane numbers, so be generous with the dilution.

  8.7) How can I remove water in the fuel tank?

If you only have a small quantity of water, then the addition of 500mls of dry isopropanol (IPA) to a near-full 30-40 litre tank will absorb the water, and will not significantly affect combustion. Once you have mopped up the water with IPA, small, regular doses of any anhydrous alcohol will help keep the tank dry. This technique will not work if you have very large amounts of water, and the addition of greater amounts of IPA may result in poor driveability.

Water in fuel tanks can be minimised by keeping the fuel tank near full, and filling in the morning from a service station that allows storage tanks to stand for several hours after refilling before using the fuel. Note that oxygenated gasolines have greater water solubility, and should cope with small quantities of water.

  8.8) Can I used unleaded on older vehicles?

Yes, providing the octane is appropriate. There are some older engines that cut the valve seats directly into the cylinder head ( eg BMC minis ). The absence of lead, which lubricated the valve seat, causes the very hard oxidation products of the exhaust valve to wear down the seat. This valve seat recession is usually corrected by installing seat inserts, hardening the seats, or use of specific valve seat recession protection additives ( such as Valvemaster ). Most other problems arise because the fuels have different volatility, or the reduction of combustion chamber deposits. These can usually be cured by reference to the vehicle manufacturer, who will probably have a publication with the changes. Some vehicles will perform as well on unleaded with a slightly lower octane than recommended leaded fuel, due to the significant reduction in deposits from modern unleaded gasolines.

  8.9) How serious is valve seat recession on older vehicles?

The amount of exhaust valve seat recession is very dependent on the load on the engine. There have been several major studies on valve seat recession, and they conclude that most damage occurs under high-speed, high-power conditions. Engine load is not a primary factor in valve seat wear for moderate operating conditions, and low to medium speed engines under moderate loads do not suffer rapid recession, as has been demonstrated on fuels such as CNG and LPG. Under severe conditions, damage occurs rapidly, however there are significant cylinder-to-cylinder variations on the same engine. A 1970 engine operated at 70 mph conditions exhibited an average 1.5mm of seat recession in 12,000km. The difference between cylinders has been attributed to different rates of valve rotation, and experiments have confirmed that more rotation does increase the recession rate [29]. The mechanism of valve seat wear is a mixture of two major mechanisms. Iron oxide from the combustion chamber surfaces adheres to the valve face and becomes embedded. These hard particles then allow the valve act as a grinding wheel and cut into the valve seat [115]. The significance of valve seat recession is that should it occur to the extent that the valve does not seat, serious engine damage can result from the localised hot spot.

There are a range of additives, usually based on potassium, sodium or phosphorus that can be added to the gasoline to combat valve seat recession. As phosphorus has adverse effects on exhaust catalysts, it is seldom used. The best long term solution is to induction harden the seats or install inserts, usually when the head is removed for other work, however additives are routinely and successfully used during transition periods.

Chapter 9) Alternative Fuels and Additives

  9.1) Do fuel additives work?

Most aftermarket fuel additives are not cost-effective. These include the octane-enhancer solutions discussed in section 6.18. There are various other pills, tablets, magnets, filters, etc. that all claim to improve either fuel economy or performance. Some of these have perfectly sound scientific mechanisms, unfortunately they are not cost-effective. Some do not even have sound scientific mechanisms. Because the same model production vehicles can vary significantly, it's expensive to unambiguously demonstrate these additives are not cost-effective. If you wish to try them, remember the biggest gain is likely to be caused by the lower mass of your wallet/purse.

There is one aftermarket additive that may be cost-effective, the lubricity additive used with unleaded gasolines to combat exhaust valve seat recession on engines that do not have seat inserts. This additive may be routinely added during the first few years of unleaded by the gasoline producers, but in the US this could not occur because they did not have EPA waivers, and also may be incompatible with 2-stroke engine oil additives and form a gel that blocks filters. The amount of recession is very dependent on the engine design and driving style. The long-term solution is to install inserts, or have the seats hardened, at the next top overhaul.

Some other fuel additives work, especially those that are carefully formulated into the gasoline by the manufacturer at the refinery, and have often been subjected to decades-long evaluation and use [12,13].

A typical gasoline may contain [16,27,32,38,111]:

Oil-soluble Dye
Initially added to leaded gasoline at about 10 ppm to prevent its misuse as an industrial solvent

Typically phenylene diamines or hindered phenols, are added to prevent oxidation of unsaturated hydrocarbons.

Metal Deactivators
Typically about 10ppm of chelating agent such as N,N'-disalicylidene-1,2-propanediamine is added to inhibit copper, which can rapidly catalyze oxidation of unsaturated hydrocarbons.

Corrosion Inhibitors
About 5ppm of oil-soluble surfactants are added to prevent corrosion caused either by water condensing from cooling, water-saturated gasoline, or from condensation from air onto the walls of almost-empty gasoline tanks that drop below the dew point. If your gasoline travels along a pipeline, it's possible the pipeline owner will add additional corrosion inhibitor to the fuel.

Anti-icing Additives
Used mainly with carburetted cars, and usually either a surfactant, alcohol or glycol.

Anti-wear Additives
These are used to control wear in the upper cylinder and piston ring area that the gasoline contacts, and are usually very light hydrocarbon oils. Phosphorus additives can also be used on engines without exhaust catalyst systems.

Deposit-modifying Additives
Usually surfactants.
  1. Carburettor Deposits, additives to prevent these were required when crankcase blow-by (PCV) and exhaust gas recirculation (EGR) controls were introduced. Some fuel components reacted with these gas streams to form deposits on the throat and throttle plate of carburettors.

  2. Fuel Injector tips operate about 100C, and deposits form in the annulus during hot soak, mainly from the oxidation and polymerisation of the larger unsaturated hydrocarbons. The additives that prevent and unclog these tips are usually polybutene succinimides or polyether amines.

  3. Intake Valve Deposits caused major problems in the mid-1980s when some engines had reduced driveability when fully warmed, even though the amount of deposit was below previously acceptable limits. It is believed that the new fuels and engine designs were producing a more absorbent deposit that grabbed some passing fuel vapour, causing lean hesitation. Intake valves operate about 300C, and if the valve is is kept wet, deposits tend not to form, thus intermittent injectors tend to promote deposits. Oil leaking through the valve guides can be either harmful or beneficial, depending on the type and quantity. Gasoline factors implicated in these deposits include unsaturates and alcohols. Additives to prevent these deposits contain a detergent and/or dispersant in a higher molecular weight solvent or light oil whose low volatility keeps the valve surface wetted [46,47,48].

  4. Combustion Chamber Deposits have been targeted in the 1990s, as they are responsible for significant increases in emissions. Recent detergent-dispersant additives have the ability to function in both the liquid and vapour phases to remove existing deposits that have resulted from the use of other additives, and prevent deposit formation. Note that these additives can not remove all deposits, just those resulting from the use of additives.
Octane Enhancers
These are usually formulated blends of alkyl lead or MMT compounds in a solvent such as toluene, and added at the 100-1000 ppm levels. They have been replaced by hydrocarbons with higher octanes such as aromatics and olefins. These hydrocarbons are now being replaced by a mixture of saturated hydrocarbons and and oxygenates.

If you wish to play with different fuels and additives, be aware that some parts of your engine management systems, such as the oxygen sensor, can be confused by different exhaust gas compositions. An example is increased quantities of hydrogen from methanol combustion.

  9.2) Can a quality fuel help a sick engine?

It depends on the ailment. Nothing can compensate for poor tuning and wear. If the problem is caused by deposits or combustion quality, then modern premium quality gasolines have been shown to improve engine performance significantly. The new generation of additive packages for gasolines include components that will dissolve existing carbon deposits, and have been shown to improve fuel economy, NOx emissions, and driveability [49,50,111]. While there may be some disputes amongst the various producers about relative merits, it is quite clear that premium quality fuels do have superior additive packages that help to maintain engine condition [16,28,111]

  9.3) What are the advantages of alcohols and ethers?

This section discusses only the use of high ( >80% ) alcohol or ether fuels.

Alcohol fuels can be made from sources other than imported crude oil, and the nations that have researched/used alcohol fuels have mainly based their choice on import substitution. Alcohol fuels can burn more efficiently, and can reduce photochemically-active emissions. Most vehicle manufacturers favoured the use of liquid fuels over compressed or liquified gases. The alcohol fuels have high research octane ratings, but also high sensitivity and high latent heats [8,27,80,116].

                                Methanol       Ethanol     Unleaded Gasoline
                                ========       =======     =================
RON                               106            107           92 - 98
MON                                92             89           80 - 90
Heat of Vaporisation    (MJ/kg)     1.154          0.913        0.3044
Nett Heating Value      (MJ/kg)    19.95          26.68        42 - 44
Vapour Pressure @ 38C    (kPa)     31.9           16.0         48 - 108
Flame Temperature        ( C )   1870           1920          2030 
Stoich. Flame Speed.    ( m/s )     0.43           -             0.34
Minimum Ignition Energy ( mJ )      0.14           -             0.29
Lower Flammable Limit   ( vol% )    6.7            3.3           1.3           
Upper Flammable Limit   ( vol% )   36.0           19.0           7.1
Autoignition Temperature ( C )    460            360          260 - 460     
Flash Point              ( C )     11             13          -43 - -39
The major advantages are gained when pure fuels ( M100, and E100 ) are used, as the addition of hydrocarbons to overcome the cold start problems also significantly reduces, if not totally eliminates, any emission benefits. Methanol will produce significant amounts of formaldehyde, a suspected human carcinogen, until the exhaust catalyst reaches operating temperature. Ethanol produces acetaldehyde. The cold-start problems have been addressed, and alcohol fuels are technically viable, however with crude oil at <$30/bbl they are not economically viable, especially as the demand for then as precursors for gasoline oxygenates has elevated the world prices. Methanol almost doubled in price during 1994. There have also been trials of pure MTBE as a fuel, however there are no unique or significant advantages that would outweigh the poor economic viability [15].

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