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(b) Petrol production and the volatility of hydrocarbons related to octane rating

(c) The fuel octane rating of petrol and individual hydrocarbons  (octane number of fuels)

(d) A more technical and quantitative survey of selected fuels

(a) Important links associated with this page for essential reading

BASIC NOTES that go with the advanced level notes below

(which I expect you to read in conjunction with the EXTRA NOTES below - I'm not repeating here most of the basic organic chemistry notes from links below)

Ethanol, manufacture - use as bioethanol

Introduction to biofuels & alternative fuels including hydrogen, biogas, bioethanol and biodiesel

This page includes a relatively non-technical survey of the characteristics of selected fuels.

A more technical survey of selected fuels is included on this page for advanced level students.

Greenhouse effect, global warming, climate change, carbon footprint from fossil fuel burning

and in particular CRACKING - a problem of supply and demand, other products

Other notes for advanced level

The chemistry of cracking, isomerisation and reforming (on a separate advanced level page)

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(b) Petrol production and the volatility of hydrocarbons related to octane rating

As we have seen, cracking, isomerisation and reforming produces lots of superior fuel molecules compared to those originally in crude oil - or rather, there were insufficient suitable fuels molecules in the crude oil.

Petrol producers have to mix a variety of hydrocarbons with the right properties to produce a suitable fuel for road vehicles.

There are two important properties to take into consideration:

(i) The octane rating of the fuel mixture

The octane rating is the fuels ability to resist auto-ignition, that causes 'knocking'.

Octane number is explained in the next section and the higher the octane number the better!

The branched alkanes and cyclic compounds (alicyclic or aromatic) have the highest octane numbers, compared straight chain unbranched alkanes.

See detailed discussion in the next section (c)

(ii) The volatility of the hydrocarbons

Every liquid exerts a vapour pressure in the atmosphere above its surface. The maximum vapour pressure depends on the temperature of the liquid and rises exponentially with increase in temperature.

The diagram on the  right shows typical saturated vapour pressure curves of maximum p_vap (mmHg) versus temperature - in this case for tetrachloromethane CCl₄, ethanol C₂H₅OH, benzene C₆H₆, water H₂O and ethanoic acid CH₃COOH.

These are relatively volatile compounds, and this vapour pressure behaviour is relevant to hydrocarbons which can exert similar vapour pressures at room temperature.

The vapour pressure exerted by a liquid surface depends on the relative strength of the intermolecular forces - in the case of hydrocarbons this is almost entirely due to instantaneous dipole – induced dipole intermolecular forces.

See boiling points of alkanes

The vapour pressure a liquid exerts is a measure of how volatile a liquid is and this is relevant to a discussion about the design of petrol mixture which must take into account a wide range of ambient temperatures.

Branched alkanes have lower boiling points than longer unbranched (linear) alkanes of the same molecular formula - the more compact the molecule, the weaker instantaneous dipole – induced dipole intermolecular forces. .

This means the branched hydrocarbons are more volatile and vaporize more easily.

For example, the highly branched 2,2,4-trimethylpentane ('iso-octane') (CH₃)₃CCH₂CH(CH₃)₂ has boiling point of  99^oC, whereas for the same molecular formula (C₈H₁₈), the linear non-branched isomer octane ('n-octane') CH₃(CH₂)₆CH₃  has a much higher boiling point of 126^oC.

See isomerism for Explanation why the intermolecular forces are weaker in the branched isomer

Petrol must contain the appropriate mixture that gives the appropriate volatility required.

The exact mixture ('blend') differs around the world and also varies with the seasons.

The petrol blend must maintain, as far as is practical, a reasonably constant volatility.

This means winter blends, at a lower temperature, must be more volatile than summer blends - which would not be volatile enough in winter, making the car engine more difficult to start.

Conversely, a winter blend would vapourise to easily in warmer summer temperatures causing vapour lock.

Vapour lock happens when the 'too volatile' fuel boils in your carburetor or your fuel line.

The vaporized fuel creates back pressure in your fuel system and prevents gas from getting to your engine, which would stall.

 

So, to be an efficient fuel. a petrol mixture must have the appropriate volatility and high octane number - explained in detail in the next section (c), but, it is worth mentioning in advance the importance of processing oil fraction products into higher quality fuels e.g.

The importance of reforming, isomerisation and cracking and their environmental-industrial context - its important to relate these petrochemical processes to 'What makes a good fuel' including having molecules of high octane number in the fuel.

For details of these processes see The chemistry of cracking, isomerisation and reforming

Catalytic reforming (makes branched alkanes + arenes ==> increases octane number).

Cracking (produces shorter carbon chains ==> higher octane number molecules).

Isomerisation (straight ==> branched molecules of higher octane number).

and so:

higher‑octane number fuels allow higher compression ratios ==> more efficient engines.

Removes the historic past use of poisonous lead additives (e.g. the now banned Pb(C₂H₅)₄, tetraethyl lead to raise the octane number of the petrol fuel.

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(c) The fuel octane rating of petrol and individual hydrocarbons  (octane number of fuels)

Notes on knocking effects in petrol engines and octane values of alcohol fuels

When petrol is injected into the cylinders of a combustion engine, it should not ignite until a spark is produced with precise timing appropriate to the rotary motion of the engine cycle.

If the fuel ignites prematurely, you can hear a characteristic knocking sound, which is indicative of an inefficient under performing engine. The knocking can also physically damage the engine due to the extra vibration produced.

This effect is called auto-ignition and is caused by the high temperature compression of the petrol vapour in the engine cylinders.

The octane rating of a fuel molecule (or petrol mixture) is a measure of how likely it is to auto-ignite.

The higher the octane rating, the less likely is the fuel to auto-ignite and cause knocking.

The equipment: the CFR test engine

The official method uses a Cooperative Fuel Research (CFR) engine, a single-cylinder, variable-compression engine developed in the 1930s and still the global standard.

• Compression ratio can be adjusted while running.
• A knockmeter measures knock intensity in “knock units”.
• The operator adjusts compression until the test fuel produces a defined knock level, then compares it with reference blends.

This engine is used in refineries, regulatory labs, and accredited testing facilities and it is not a DIY method.

Molecular structure factors affecting octane number for hydrocarbons

1. The longer the carbon chain, the lower the octane number

You can see this in the steady trend in the  RON table below.

The longer the carbon chain, the lower the activation energy of C-C bond scission allowing the rapid, potentially uncontrolled, lots of branching free radical reactions, so less energy (and precise spark timing) to detonate. PLEASE note data sources vary in quoted RON values.

 

2. Branched alkanes have higher octane numbers than straight‑chain alkanes for the same molecular formula.

The more branching raises octane number compared to   straight chain alkanes with lower octane numbers (comparing isomers).

e.g. for isomers of C₈H₁₈, 2,4-trimethyl pentane is 100, straight chain octane is -25 (yes, minus 25).

You can see this in the RON table of data above.

Combustion is a series of rapid free radical reaction. The radicals formed by straight-chain (linear) alkanes are highly reactive, which leads to rapid, uncontrolled explosions from branched free radical chain reactions that cause premature ignition.  In contrast, highly branched alkanes such as isooctane, form more stable free radicals.  This stability slows the reaction rate and ensures the fuel burns more smoothly.

Also, long, unbranched chains have low activation energy for auto-ignition and form reactive peroxides more readily under compression - these readily split homolytically to give two free radicals.

 

3. Arenes (aromatic hydrocarbons) have very high octane numbers

e.g benzene C₆H₆ is ~104 (hexane is 25) and C₆H₅CH₃ methylbenzene ~117 (heptane is 0)

Aromatic hydrocarbons (like benzene and methylbenzene) have high octane numbers because their highly stable, aromatic ring molecular structures strongly resist premature self-ignition under high compression allowing for smooth, controlled burning.

When aromatic molecules are exposed to high heat and pressure, they form highly stable resonance-stabilized free radicals which are less reactive than those radicals produced by straight-chain alkanes.

You can see this in with the RON table data above.

 

For relative octane number (RON) Comparison Table for linear hydrocarbon alkanes versus linear alcohols (primary alcohols 1-Alkanols from C1 to C10

See alcohol chemistry section 4. on comparing the octane numbers of alkanes and alcohols

Different hydrocarbons have different octane ratings.

Therefore, the different hydrocarbons are blended together to give a petrol mixture the appropriate octane rating - the mixture with the best antiknock performance.

Linear unbranched alkanes tend to have lower octane ratings, a higher tendency to auto-ignite, than isomeric branched alkanes of the same molecular formula, cyclic alkanes (alicyclic) and aromatic hydrocarbons like benzene.

  Linear heptane (C₇H₁₆), far to readily auto-ignites and has a defined octane rating of 0 - not good!

  Highly branched 2,2,4-trimethylpentane (C₈H₁₈) has a defined octane rating of 100 (very good).

These two alkanes are the arbitrarily defined octane rating numbers scale.

The octane rating of petrol in the UK is usually 95, but you can pay a bit more for higher octane 99 rated petrol.

 

If you take hydrocarbons you can do an interesting comparison of the octane rating of several molecules,

I've deliberately quoted octane ratings for structural isomers (carbon chain isomers) e.g.

(a)   pentane (C₅H₁₂) has an octane rating of 62

isomeric   2-methylbutane (C₅H₁₂) has an octane rating of 93. In (a) you can see introducing branching considerably increases the octane rating.

(b)   linear (unbranched) hexane (C₆H₁₄), octane rating 25

isomeric 2-methypentane (C₆H₁₄) has an octane rating of 73

and 3-methypentane (C₆H₁₄) has an octane rating of 75/86? (data sources differ)

As in (a), in (b) you can see introducing branching considerably increases the octane rating.

These branched isomers are produced in reforming processes.

(c) linear heptane (C₇H₁₆) has an octane number of 0.

isomeric 3-methylhexane has an octane rating of 65.

and 2,3-dimethylpentane (C₇H₁₆) has an octane rating of 91

Again, in (c) you can see introducing branching considerably increases the octane rating.

You can also see that the greater the branching of the isomer, the higher the octane number.

These branched isomers are produced in reforming processes.

(d)  cyclic alkane (alicyclic) cyclohexane (C₆H₁₂), octane rating 83

  aromatic benzene (C₆H₆), octane rating 106, much higher than cyclohexane.

These hydrocarbons are produced in reforming processes from the linear alkane hexane.

(e)   methylcyclohexane (C₇H₁₄) has an octane rating of 70

  methylbenzene (C₇H₈) has an octane number of 120, much higher than methylcyclohexane.

These hydrocarbons are produced in reforming processes from the linear alkane heptane.

From (c) (d) and (e) you can see that moving from a linear alkane to cyclic alkane and then to a cyclic aromatic hydrocarbon greatly increases the octane rating at each stage in the reforming process.

 

Branched and cyclic alkane and aromatic hydrocarbon compounds are important components in petrol mixtures designed to produce the cleanest most efficient burning, with good antiknock properties, particularly as lead tetraethyl is now banned.

You should now appreciate much more one important consequence of cracking crude oil fractions.

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(d) A more technical and quantitative survey of selected fuels

Data comments

1. All of these are used as a fuel

2. Name and physical state at 298K (25^oC) and 101.3 kPa (1 atm. 'normal pressure')

3. Relative molecular mass M_r of the fuel in g/mol  (relative atomic masses C = 12,  H = 1  and  O = 16)

4. ΔH^θ_comb is the standard enthalpy of combustion kJ/mol at 298K and 101,3 kPa

5. A measure of fuel density: (ΔH^θ_comb) x 1000 / M_r  in kJ/kg of fuel

6. Mass of CO₂ (M_r = 44) released per unit mass of fuel burned in kg CO₂/kg fuel: (C_n x 44) / M_r (n = C atoms in molecule)

7. kg CO₂ formed per kJ of energy released on fuel combustion (calculated from 6. / 5.)

8. kJ energy released per kg CO₂ formed on fuel combustion (calculated from 5. / 6.)

Comments on the information

Hydrogen stands out as the best fuel to combat the greenhouse effect.

It has a very high energy density in terms of kJ/kg BUT it is not as convenient to source and supply compared to hydrocarbon fuels e.g. hydrogen is difficult to liquefy, very explosive and requires high pressure technology to reduce the storage volume.

2H₂(g)  +  O₂(g)  ==> 2H₂O(l/g)

There is another potential drawback if used in a flame combustion situation - the flame temperature is higher than a hydrocarbon combustion flame and nitrogen(II) oxide is formed as a pollutant. NO rapidly forms the acidic and lung irritating gas nitrogen (IV) oxide. (old names nitric oxide and nitrogen dioxide

N₂(g)  + O₂(g)  ===>  2NO  then  2NO +  O₂(g)  ==> 2NO₂(g)

You need a lot of renewable electrical energy to produce it from the electrolysis of water and at the moment it is manufactured from the fossil fuel gas methane, therefore its production via the reaction

CH₄(g)  +  H₂O(g)  === high temp/catalyst  ==> CO(g)  +  3H₂(g)

leaves a big carbon footprint, even if burning the hydrogen only produces water!

The best fossil fuel is methane (if you can call it that!), it produces significantly less carbon dioxide per unit of energy released (it has the lowest kJ/kg fuel ratio, BUT it is NOT a renewable fuel and has a big carbon footprint.

Higher alkane hydrocarbons have a very high energy density, but a bigger carbon footprint - as the carbon chain length increases you produce more CO₂ per unit of energy released.

Oxygenated fuels like methanol and ethanol are cleaner burning than hydrocarbons, so less polluting, but their energy density (kJ/kg) is much less than hydrocarbon fuels, so larger volumes are required e.g. for the same car journey.

Methanol is synthesised from carbon monoxide and hydrogen, so all manufacturing is, at the moment, dependant on the oil industry:

CO(g)  +  H₂(g)  == high temp/catalyst ==>  CH₃OH(l)

Methanol is a good fuel, less polluting a low kg CO₂/kg fuel, but the kg CO₂/kJ energy is still high and it is toxic and expensive to make.

BUT, at the moment, the hydrogen is made from methane, so methanol has a big carbon footprint!

Bioethanol, ethanol from a renewable energy sources (e.g. fermentation of carbohydrates - sugar cane, cereal crops), is also a good fuel, cleaner burning than hydrocarbon fuels so less polluting and can be blended with petrol.

Being from a renewable source, bioethanol should create a much smaller carbon footprint in terms of the 'life-cycle' of the product, BUT, it is NOT, as often claimed, a carbon neutral product.

See discussion section (5) is bioethanol a carbon neutral fuel? on another page

Biodiesel has a moderately high energy density and is cleaner burning the purely hydrocarbon diesel, never-the-less, kg CO₂/Kg fuel burned is similar to hydrocarbons and the kg CO₂/kJ is higher than for hydrocarbon fuels.

However, the important point here is that biodiesel is obtained from renewable sources, the net carbon footprint is much less than for hydrocarbon fuels.

BUT, it is NOT carbon neutral, see the link above about bioethanol, as many of the points apply.

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For less technical comparisons of fuels but including many VERY important points see my

Comparison of biofuels and other alternative fuels including hydrogen notes

most of which I am NOT REPEATING here!