Gasoline faq

Just about everything you wanted to know about 

 

 

 

For those who are interested in knowing a bit more about what gasoline is really about, well here it is. As mentioned earlier, it would take a complete website with mega megs to give all the info. However, here are some common questions without getting too technical. I trust that some info here will dispel some of the myths. Keep in mind that the info here is very US related.  However, for European readers the info is just as valid. 

 

 

 

 

Where does crude oil come from?. The generally-accepted origin of crude oil is from plant life up to 3 billion years ago, but predominantly from 100 to 600 million years ago. "Dead vegetarian dino dinner" is more correct than "dead dinos". The molecular structure of the hydrocarbons and other compounds present in fossil fuels can be linked to the leaf waxes and other plant molecules of marine and terrestrial plants believed to exist during that era. There are various biogenic marker chemicals such as isoprenoids from terpenes, porphyrins and aromatics from natural pigments, pristane and phytane from the hydrolysis of chlorophyll, and normal alkanes from waxes, whose size and shape can not be explained by known geological processes. The presence of optical activity and the carbon isotopic ratios also indicate a biological origin. There is another hypothesis that suggests crude oil is derived from methane from the earth's interior. The current main proponent of this abiotic theory is Thomas Gold, however abiotic and extraterrestrial origins for fossil fuels were also considered at the turn of the century, and were discarded then.

What is the history of gasoline?

In the late 19th Century the most suitable fuels for the automobile were coal tar distillates and the lighter fractions from the distillation of crude oil. During the early 20th Century the oil companies were producing gasoline as a simple distillate from petroleum, but the automotive engines were rapidly being improved and required a more suitable fuel. During the 1910s, laws prohibited the storage of gasolines on residential properties, so Charles F. Kettering ( yes - he of ignition system fame ) modified an IC engine to run on kerosine. However the kerosine-fuelled engine would "knock" and crack the cylinder head and pistons. He assigned Thomas Midgley Jr. to confirm that the cause was from the kerosine droplets vaporising on combustion as they presumed . Midgley demonstrated that the knock was caused by a rapid rise in pressure after ignition, not during preignition as believed. This then lead to the long search for anti-knock agents, culminating in tetra ethyl lead . Typical mid-1920s gasolines were 40 - 60 Octane.

Because sulfur in gasoline inhibited the octane-enhancing effect of the alkyl lead, the sulfur content of the thermally-cracked refinery streams for gasolines was restricted. By the 1930s, the petroleum industry had determined that the larger hydrocarbon molecules (kerosine) had major adverse effects on the octane of gasoline, and were developing consistent specifications for desired properties. By the 1940s catalytic cracking was introduced, and gasoline compositions became fairly consistent between brands during the various seasons.

The 1950s saw the start of the increase of the compression ratio, requiring higher octane fuels. Lead levels were increased, and some new refining processes ( such as hydrocracking ), specifically designed to provide hydrocarbons components with good lead response and octane, were introduced. Minor improvements were made to gasoline formulations to improve yields and octane until the 1970s - when unleaded fuels were introduced to protect the exhaust catalysts that were also being introduced for environmental reasons. From 1970 until 1990 gasolines were slowly changed as lead was phased out. In 1990 the Clean Air Act started forcing major compositional changes on gasoline, and these changes will continue into the 21st Century because gasoline is a major pollution source.

Why were alkyl lead compounds added?

The efficiency of a spark-ignited gasoline engine can be related to the compression ratio up to at least compression ratio 17:1 . However any "knock" caused by the fuel will rapidly mechanically destroy an engine, and General Motors was having major problems trying to improve engines without inducing knock. The problem was to identify economic additives that could be added to gasoline or kerosine to prevent knock, as it was apparent that engine development was being hindered. The kerosine for home fuels soon became a secondary issue, as the magnitude of the automotive knock problem increased throughout the 1910s, and so more resources were poured into the quest for an effective "anti-knock". A higher octane aviation gasoline was required urgently once the US entered WWI, and almost every possible chemical ( including melted butter ) was tested for anti-knock ability.

Originally, iodine was the best anti-knock available, but was not a practical gasoline additive, and was used as the benchmark. In 1919 aniline was found to have superior antiknock ability to iodine, but also was not a practical additive, however aniline became the benchmark anti-knock, and various compounds were compared to it. The discovery of tetra ethyl lead, and the scavengers required to remove it from the engine were made by teams lead by Thomas Midgley Jr. in 1922. They tried selenium oxychloride which was an excellent antiknock, however it reacted with iron and "dissolved" the engine. Midgley was able to predict that other organometallics would work, and slowly focused on organoleads. They then had to remove the lead, which would otherwise accumulate and coat the engine and exhaust system with lead. They discovered and developed the halogenated lead scavengers that are still used in leaded fuels. The scavengers, ( ethylene dibromide and ethylene dichloride ), function by providing halogen atoms that react with the lead to form volatile lead halide salts that can escape out the exhaust. The quantity of scavengers added to the alkyl lead concentrate is calculated according to the amount of lead present. If sufficient scavenger is added to theoretically react with all the lead present, the amount is called one "theory". Typically, 1.0 to 1.5 theories are used, but aviation gasolines must only use one theory. This ensures there is no excess bromine that could react with the engine. The alkyl leads rapidly became the most cost-effective method of enhancing octane.

The development of the alkyl leads ( tetra methyl lead, tetra ethyl lead ) and the toxic halogenated scavengers meant that petroleum refiners could then configure refineries to produce hydrocarbon streams that would increase octane with small quantities of alkyl lead. If you keep adding alkyl lead compounds, the lead response of the gasoline decreases, and so there are economic limits to how much lead should be added.

Up until the late 1960s, alkyl leads were added to gasolines in increasing concentrations to obtain octane. The limit was 1.14g Pb/l, which is well above the diminishing returns part of the lead response curve for most refinery streams, thus it is unlikely that much fuel was ever made at that level. I believe 1.05 was about the maximum, and articles suggest that 1970 100 RON premiums were about 0.7-0.8 g Pb/l and 94 RON regulars 0.6-0.7 g Pb/l, which matches published lead response data eg. 

 

For Catalytic Reformate           Straight Run Naphtha.



Lead g/l        RON



   0                    96                           72
  0.1                  98                           79




  0.2                  99                           83




  0.3                 100                           85




  0.4                 101                           87




  0.5                 101.5                         88




  0.6                 102                           89




  0.7                 102.5                         89.5




  0.8                 102.75                        90

The alkyl lead anti-knocks work in a different stage of the pre-combustion reaction to oxygenates. In contrast to oxygenates, the alkyl lead interferes with hydrocarbon chain branching in the intermediate temperature range where HO2 is the most important radical species. Lead oxide, either as solid particles, or in the gas phase, reacts with HO2 and removes it from the available radical pool, thereby deactivating the major chain branching reaction sequence that results in undesirable, easily-autoignitable hydrocarbons.

Why not use other organometallic compounds?

First and foremost, the progressive reduction of lead, as it is commonly known, in the US gasoline was due primarily to extend the life of the catalytic converter. Leaded gasoline dramatically shorted the life of the early palladium catalysts, simply because after a period of use (less than the 50,000 miles required by EPA) the palladium pellets became coated by lead, hence the catalyst would eventually cease to do it's job.  

Certainly lead's toxicity became a concern, but apparently more political than scientific. Regardless of the reasons, alternatives were considered. The most famous of these is methylcyclopentadienyl manganese tricarbonyl (MMT), which was used in the USA until banned by the EPA from 27 Oct 1978 , but is approved for use in Canada and Australia. It is more expensive than alkyl leads and has been reported to increase unburned hydrocarbon emissions and block exhaust catalysts. Other compounds that enhance octane have been suggested, but usually have significant problems such as toxicity, cost, increased engine wear etc.. Examples include dicyclopentadienyl iron and nickel carbonyl.

 

Specified fuel properties

Volatility

This affects evaporative emissions and driveability, it is the property that must change with location and season. Fuel for mid-summer Arizona would be difficult to use in mid-winter Alaska. The US is divided into zones, according to altitude and seasonal temperatures, and the fuel volatility is adjusted accordingly. Incorrect fuel may result in difficult starting in cold weather, carburetter icing, vapour lock in hot weather, and crankcase oil dilution. Volatility is controlled by distillation and vapour pressure specifications. The higher boiling fractions of the gasoline have significant effects on the emission levels of undesirable hydrocarbons and aldehydes, and a reduction of 40C in the final boiling point will reduce the levels of benzene, butadiene, formaldehyde and acetaldehyde by 25%, and will reduce HC emissions by 20% .

Combustion Characteristics

As gasolines contain mainly hydrocarbons, the only significant variable between different grades is the octane rating of the fuel, as most other properties are similar. Octane is discussed in detail in Section 6. There are only slight differences in combustion temperatures ( most are around 2000C in isobaric adiabatic combustion). Note that the actual temperature in the combustion chamber is also determined by other factors, such as load and engine design. The addition of oxygenates changes the pre-flame reaction pathways, and also reduces the energy content of the fuel. The levels of oxygen in the fuel is regulated according to regional air quality standards.

Stability

Motor gasolines may be stored up to six months, consequently they must not form gums which may precipitate. Gums are usually the result of copper-catalysed reactions of the unsaturated HCs, so antioxidants and metal deactivators are added. Existent Gum is used to measure the gum in the fuel at the time tested, whereas the Oxidation Stability measures the time it takes for the gasoline to break down at 100C with 100psi of oxygen. A 240 minutes test period has been found to be sufficient for most storage and distribution systems.

Corrosiveness

Sulfur in the fuel creates corrosion, and when combusted will form corrosive gases that attack the engine, exhaust and environment. Sulfur also adversely affects the alkyl lead octane response and may poison exhaust catalysts. The copper strip corrosion test and the sulfur specification are used to ensure fuel quality. The copper strip test measures active sulfur, whereas the sulfur content reports the total sulfur present.

Are brands different?

Yes. The above specifications are intended to ensure minimal quality standards are maintained, however as well as the fuel hydrocarbons, the manufacturers add their own special ingredients to provide additional benefits. A quality gasoline additive package would include:-

During the 1980s significant problems with deposits accumulating on intake valve surfaces occurred as new fuel injections systems were introduced. These intake valve deposits (IVD) were different to the injector deposits, in part because the valve can reach 300C. Engine design changes that prevent deposits usually consist of ensuring the valve is flushed with liquid gasoline, and provision of adequate valve rotation. Gasoline factors that cause deposits are the presence of alcohols or olefins. Gasoline manufacturers now routinely use additives that prevent IVD and also maintain the cleanliness of injectors. These usually include a surfactant and light oil to maintain the wetting of important surfaces. 

Texaco demonstrated that a well-formulated package could improve fuel economy, reduce NOx emissions, and restore engine performance because, as well as the traditional liquid-phase deposit removal, some additives can work in the vapour phase to remove existing engine deposits without adversely affecting performance ( as happens when water is poured into a running engine to remove carbon deposits:-) ). Most suppliers of quality gasolines will formulate similar additives into their products, and cheaper lines are less like to have such additives added. As different brands use different additives and oxygenates, it is probable that important parameters, such as octane distribution, are different, even though the pump octane ratings are the same.

So, if you know your car is well-tuned, and in good condition, but the driveability is pathetic on the correct octane, try another brand. Remember that the composition will change with the season, so if you lose driveability, try yet another brand. As various Clean Air Act changes are introduced over the next few years, gasoline will continue to change.

Who invented Octane Ratings?

Since 1912 the spark ignition internal combustion engine's compression ratio had been constrained by the unwanted "knock" that could rapidly destroy engines. "Knocking" is a very good description of the sound heard from an engine using fuel of too low octane. The engineers had blamed the "knock" on the battery ignition system that was added to cars along with the electric self-starter. The engine developers knew that they could improve power and efficiency if knock could be overcome.

Kettering assigned Thomas Midgley, Jr. to the task of finding the exact cause of knock. They used a Dobbie-McInnes manograph to demonstrate that the knock did not arise from preignition, as was commonly supposed, but arose from a violent pressure rise _after_ ignition. The manograph was not suitable for further research, so Midgley and Boyd developed a high-speed camera to see what was happening. They also developed a "bouncing pin" indicator that measured the amount of knock . Ricardo had developed an alternative concept of HUCF ( Highest Useful Compression Ratio ) using a variable-compression engine. His numbers were not absolute, as there were many variables, such as ignition timing, cleanliness, spark plug position, engine temperature. etc.

In 1926 Graham Edgar suggested using two hydrocarbons that could be produced in sufficient purity and quantity. These were "normal heptane", that was already obtainable in sufficient purity from the distillation of Jeffrey pine oil, and " an octane, named 2,4,4-trimethyl pentane " that he first synthesized. Today we call it " iso-octane " or 2,2,4-trimethyl pentane. The octane had a high anti-knock value, and he suggested using the ratio of the two as a reference fuel number. He demonstrated that all the commercially- available gasolines could be bracketed between 60:40 and 40:60 parts by volume heptane:iso-octane.

The reason for using normal heptane and iso-octane was because they both have similar volatility properties, specifically boiling point, thus the varying ratios 0:100 to 100:0 should not exhibit large differences in volatility that could affect the rating test. 

                                                           Heat of




                  Melting Point     Boiling Point       Density    Vaporisation




                              C              C          g/ml         MJ/kg 




normal heptane    -90.7            98.4       0.684          0.365 @ 25C
iso octane       -107.45             99.3       0.6919         0.308 @ 25C

Having decided on standard reference fuels, a whole range of engines and test conditions appeared, but today the most common are the Research Octane Number ( RON ), and the Motor Octane Number ( MON ).

Why do we need Octane Ratings?

To obtain the maximum energy from the gasoline, the compressed fuel/air mixture inside the combustion chamber needs to burn evenly, propagating out from the spark plug until all the fuel is consumed. This would deliver an optimum power stroke. In real life, a series of pre-flame reactions will occur in the unburnt "end gases" in the combustion chamber before the flame front arrives. If these reactions form molecules or species that can autoignite before the flame front arrives, knock will occur .

Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used. If autoignition occurs, it results in an extremely rapid pressure rise, as both the desired spark-initiated flame front, and the undesired autoignited end gas flames are expanding. The combined pressure peak arrives slightly ahead of the normal operating pressure peak, leading to a loss of power and eventual overheating. The end gas pressure waves are superimposed on the main pressure wave, leading to a sawtooth pattern of pressure oscillations that create the "knocking" sound.

The combination of intense pressure waves and overheating can induce piston failure in a few minutes. Knock and preignition are both favoured by high temperatures, so one may lead to the other. Under high-speed conditions knock can lead to preignition, which then accelerates engine destruction .

What does the Motor Octane rating measure?

The conditions of the Motor method represent severe, sustained high speed, high load driving. For most hydrocarbon fuels, including those with either lead or oxygenates, the motor octane number (MON) will be lower than the research octane number (RON). 

 

Test Engine conditions                Motor Octane




Test Method                         ASTM D2700-92 [66]




Engine                       Cooperative Fuels Research ( CFR )




Engine RPM                               900 RPM




Intake air temperature                    38 C




Intake air humidity           3.56 - 7.12 g H2O / kg dry air




Intake mixture temperature               149 C




Coolant temperature                      100 C




Oil Temperature                           57 C




Ignition Advance - variable     Varies with compression ratio




                                 ( eg 14 - 26 degrees BTDC )




Carburettor Venturi                       14.3 mm

What does the Research Octane rating measure?

The Research method settings represent typical mild driving, without consistent heavy loads on the engine. 

 




Test Engine conditions               Research Octane




Test Method                         ASTM D2699-92 [67]




Engine                       Cooperative Fuels Research ( CFR )




Engine RPM                               600 RPM




Intake air temperature       Varies with barometric pressure




                           ( eg 88kPa = 19.4C, 101.6kPa = 52.2C )
Intake air humidity           3.56 - 7.12 g H2O / kg dry air




Intake mixture temperature            Not specified




Coolant temperature                      100 C




Oil Temperature                           57 C




Ignition Advance - fixed            13 degrees BTDC




Carburettor Venturi           Set according to engine altitude




                           ( eg 0-500m=14.3mm, 500-1000m=15.1mm )

How is the Octane rating determined?

To rate a fuel, the engine is set to an appropriate compression ratio that will produce a knock of about 50 on the knockmeter for the sample when the air/fuel ratio is adjusted on the carburettor bowl to obtain maximum knock. Normal heptane and iso-octane are known as primary reference fuels. Two blends of these are made, one that is one octane number above the expected rating, and another that is one octane number below the expected rating. These are placed in different bowls, and are also rated with each air/fuel ratio being adjusted for maximum knock. The higher octane reference fuel should produce a reading around 30-40, and the lower reference fuel should produce a reading of 60-70. The sample is again tested, and if it does not fit between the reference fuels, further reference fuels are prepared, and the engine readjusted to obtain the required knock. The actual fuel rating is interpolated from the knockmeter readings .

Other affects on octane

Several other properties affect knock. The most significant determinant of octane is the chemical structure of the hydrocarbons and their response to the addition of octane enhancing additives. Other factors include:-

Can higher octane fuels give me more power?

Not if you are already using the proper octane fuel. The engine will be already operating at optimum settings, and a higher octane should have no effect on the management system. Your driveability and fuel economy will remain the same. The higher octane fuel costs more, so you are just throwing money away. If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy. You may be able to change octanes between seasons ( reduce octane in winter ) to obtain the most cost-effective fuel without loss of driveability.

Once you have identified the fuel that keeps the engine at optimum settings, there is no advantage in moving to an even higher octane fuel. The manufacturer's recommendation is conservative, so you may be able to carefully reduce the fuel octane. The penalty for getting it badly wrong, and not realising that you have, could be expensive engine damage.

Does low octane fuel increase engine wear?

Not if you are meeting the octane requirement of the engine. If you are not meeting the octane requirement, the engine will rapidly suffer major damage due to knock. You must not use fuels that produce sustained audible knock, engine damage will occur. If the octane is just sufficient, the engine management system will move settings to a less optimal position, and the only major penalty will be increased costs due to poor fuel economy. Whenever possible, engines should be operated at the optimum position for long-term reliability. Engine wear is mainly related to design, manufacturing, maintenance and lubrication factors. Once the octane and run-on requirements of the engine are satisfied, increased octane will have no beneficial effect on the engine. The quality of gasoline, and the additive package used, would be more likely to affect the rate of engine wear, rather than the octane rating.

Can I mix different octane fuel grades?

Yes, however attempts to blend in your fuel tank should be carefully planned. You should not allow the tank to become empty, and then add 50% of lower octane, followed by 50% of higher octane. The fuels may not completely mix immediately, especially if there is a density difference. You may get a slug of low octane that causes severe knock. You should refill when your tank is half full. In general the octane response will be linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.

Attempts to mix leaded high octane to unleaded high octane to obtain higher octane are useless. The lead response of the unleaded fuel does not overcome the dilution effect, thus 50:50 of 96 leaded and 91 unleaded will give 94. Some blends of oxygenated fuels with ordinary gasoline can result in undesirable increases in volatility due to volatile azeotropes, and that some oxygenates can have negative lead responses. Also note that the octane requirement of some engines is determined by the need to avoid run-on, not to avoid knock.

What happens if I use the wrong octane fuel?

If you use a fuel with an octane rating below the requirement of the engine, the management system may move the engine settings into an area of less efficient combustion, resulting in reduced power and reduced fuel economy. You will be losing both money and driveability. If you use a fuel with an octane rating higher than what the engine can use, you are just wasting money by paying for octane that you can not utilise. Forget the stories about higher octanes having superior additive packages - they do not. If your vehicle does not have a knock sensor, then using an octane significantly below the requirement means that the little men with hammers will gleefully pummel your engine to pieces.

You should initially be guided by the vehicle manufacturer's recommendations, however you can experiment, as the variations in vehicle tolerances can mean that Octane Number Requirement for a given vehicle model can range over 6 Octane Numbers. Caution should be used, and remember to compensate if the conditions change, such as carrying more people or driving in different ambient conditions. You can often reduce the octane of the fuel you use in winter because the temperature decrease and possible humidity changes may significantly reduce the octane requirement of the engine.

Use the octane that provides cost-effective driveability and performance, using anything more is waste of money, and anything less could result in an unscheduled, expensive visit to your mechanic.

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 anti-knock 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 .