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AP Chemistry
Octane and Engine Performance
You may have heard the terms octane or seen numbers on the gas pump like this:
What is octane and why is it important? To
understand this we need to understand how an Otto cycle engine works. (Insert link
here). During the compression stroke, the fuel air mixture increases in temperature
because the piston does work on it. As the temperature of the mixture increases fuel
molecules can start to react. The reaction is exothermic and it increases the pressure in
the cylinder. If too much fuel reacts it causes downward force on the piston. This is
called knock or ping and it reduces engine life, power, and fuel efficiency. The
phenomenon of fuel burning too early is called predetonation. Engines that use the Otto
cycle (we call them gasoline or gas engines) need a fuel that resists predetonation. The
higher the compression in the engine the more power and efficiency it can deliver. On
the other hand the higher the compression, the higher the temperature before the spark
plug fires. Unfortunately the fuel provided by the refining of oil is liable to
predetonation. Chemists have worked on this problem for nearly 100 years and a number
of solutions have been found.
In order to understand the issue of predetonation and fuel we need a unit to
describe the phenomenon. We use the octane scale. Isooctane (2,2,4-trimethylpentane)
is assigned a value of 100 on the octane scale. A compound called n-Heptane is assigned
a value of 0. You may have seen 87 octane gas for sale at the gas station. 87 octane gas
has the same behavior with regard to predetonation as a mixture of 87% isooctane and
13% heptane. The gas for sale is actually a mixture of many compounds that give an
octane rating of 87.
Why does isooctane resist predetonation better than n-heptane? It has to do with
the stability of radicals. Radicals are chemical species with one or more unpaired
electrons. Most radicals are highly reactive. Otto cycle fuels will work better if they
burn when the spark plug fires rather than burning when the cylinder gets hot. Ideally the
radicals formed by a fuel should be less reactive. Alternatively, the fuel molecule itself
can be more stable and resist forming radicals in the first place. Molecules with
hydrogen bonded to sp2 carbons are less likely to form radicals while molecules that are
branched form more stable radicals. Note that n-heptane has no branching while
isooctane has much branching.
Unfortunately the gasoline distilled from oil has an octane rating of about 40. It is
expensive to change natural gasoline to more branched molecules. A cheaper way of
improving the quality of fuel is to add an inhibitor. This may seem counterintuitive but
know that once the spark plug fires the inhibitor is overcome. The most widely used
inhibitor for gasoline was tetraethyl lead.
This substance has been phased out because of its toxicity. It works because the bonds
between lead and carbon are weak. When it is heated these bonds undergo homolytic
cleavage and scavenge radicals formed from fuel, forming stable species that can wait
until the spark plug fires to react. To get the lead out of the engine ethyl chloride or ethyl
bromide was added to leaded gasoline to form lead halide that was volatile enough to
leave with the exhaust.
Let’s look at some fuel molecules:
Hexadecane:
n-octane:
2,2,3-trimethylbutane
Benzene:
2,3,3-Trimethylpentane
2,2,4-Trimethylpentane
2-Methylheptane
Heptene
2,2-Dimethylbutane
2,2-dimethylpropane:
Rank these from lowest to highest estimated octane rating and explain your ranking.
Answer:
The correct order, with octane rating shown is:
Hexadecane (-30), n-octane (-10), 2-Methylheptane (23), Heptene (60), 2,2dimethylpropane (80), 2,2-Dimethylbutane (93), 2,2,4-Trimethylpentane (100), Benzene
(101), 2,3,3-Trimethylpentane (106), 2,2,3-trimethylbutane (112)
This is a really hard problem. It is easy for us to understand some of the positions based
on structure and others are hard. It is easy to see that hexadecane is large and has lots of
opportunities for radicals to form prematurely. There is no branching at all. 2,2,3trimethyl butane is the most branched unless we look at 2,2-dimethyl propane. We can’t
really use our knowledge of chemistry so far to predict where benzene should go.
Overall you did well if you ranked from what looks the least branched to what looks the
most branched. Obviously there is more to octane rating than just branching.
Questions:
1. A problem with the World War Two era R2800 engine that ran on 150 octane fuel was
erosion of the steel valve seals due to hydrobromic acid. Where did the hydrobromic acid
come from? Predict whether hydrochloric or hydrobromic acid would erode the seals
more. You need to know that what makes acids corrosive is the formation of H+ ions
from acid molecules. H+ attacks many metals and corrodes them.
2. Why did ethyl fluid contain 1,2-dibromoethane and 1,2-dichloroethane in addition to
tetraethyl lead? Keep reading for a hint if you need it but stop now if you want to think it
out. Don’t keep reading unless you want the hint. The hint is in the next sentence so stop
now. (Without these halogenated additives the lead in tetraethyl lead formed lead metal
and/or lead oxide upon combustion.)
3. In aviation gasoline 1,2-dibromoethane is preferred over 1,2-dichloroethane. This is
to reduce corrosion of aluminum aircraft parts. Why might the chloride corrode
aluminum more than the bromide?
4. Vapor lock is a phenomenon where some fuel in the pipes supplying an engine
vaporizes and prevents fuel from moving through the system. More volatile fuels are
more prone to vapor lock. Rank these fuels from highest to lowest boiling point:
Methane, methanol, ethanol, benzene, octane, isooctane.
5. Butanol is a potential biofuel. Predict whether butanol is more or less soluble in water
than ethanol. Also predict whether butanol is more or less energetic than ethanol per
mole and per gram.
6. Water injection is a technique where water is added to the fuel-air mixture in an Otto
cycle engine. The water evaporates and absorbs heat. Describe how this increases the
power output of an Otto cycle engine.
7. Water for water injection must be contained in a tank separate from the gasoline.
Give two reasons for this.
8. Water freezes easily so it is often mixed with methanol for injection into engines. The
mixture of the two freezes at a lower temperature than water alone. You can learn why if
you look up “freezing point depression.” We’ll learn about that later. For now, tell why
water has a higher melting point than methanol.
9. Nitrous oxide is injected into engines to give more power. It works because the
nitrous oxide (N2O) decomposes to produce oxygen gas that can participate in
combustion. Give two reasons why nitrous injection can increase the power of an engine.
10. In order to improve performance of aircraft either water-methanol or nitrous injection
can be used. It turns out that water-methanol injection improves performance more at
low altitudes while nitrous injection improves performance more at high altitudes. In
fact, at too high an altitude water-methanol injection has no effect on engine
performance. Explain why this is true.
11. Gasohol is a mixture of gasoline and alcohol. It is important that gasohol never be
contaminated with water. Why might this be the case? For a hint, look at how soap and
emulsifiers work and compare their structure to the structure of alcohols.
Answers:
1. We predict that hydrobromic acid is worse than hydrochloric because the bond
between bromine and hydrogen is weaker than the bond between chlorine and hydrogen.
This allows the hydrobromic acid to break up more easily and make more H+ to attack the
steel valves.
2. Lead chloride and lead bromide have lower melting and boiling points than lead oxide
and lead metal. The halides are more volatile and are therefore more likely to be
removed with the exhaust. Despite this lead compounds built up in leaded gas engines.
3. After doing #1 this problem seems strange. We might expect the bromide to be worse
as in #1. However, it is not and we need to find another explanation. The best guess is
that aluminum has more affinity for chlorine than for bromine because of the greater
bond strength. This makes sense because the aluminum aircraft parts are being exposed
to lead chloride and lead bromide in the exhaust and not hydrochloric and hydrobromic
acid in the cylinder.
4. It’s easy to see that methane is the lowest. The rest are much harder but you need to
have methanol lower than ethanol for sure. You should put octane higher than isooctane
because straight chains have a greater surface area to volume ratio than branched
molecules. This means they have more area for dispersion forces to operate. Both
octanes are obviously higher than benzene. The next problem is deciding the relative
place of the alcohols and the octanes. It turns out methanol boils at 64C while the
octanes are over 100C. Ethanol is 78.5C and benzene is 80.1C. Once a molecule gets
big enough dispersion forces can be stronger than hydrogen bonding. It’s ok to miss the
relative values of octane alcohol at this point in your chemical career.
5. Butanol is less soluble in water than ethanol because it has one hydrogen bonding
group (OH) for 4 carbons while ethanol has 1 OH for two carbons. For the same reason
butanol delivers more energy per gram. Obviously butanol delivers more energy per
mole.
6. The water evaporates and allows more pressure without reaching temperatures that
cause predetonation.
7. One reason is that water and gasoline are not miscible. They separate with water
sinking to the bottom of the gasoline. The other reason is that if the water is held in a
separate tank it can be injected in controlled quantities as needed.
8. Water has one O with two lone pairs and two H’s. This means it has four places to
hydrogen bond. Methanol has one less H so it has three places. The fact that methanol
has a CH3 does not make up for the loss of hydrogen bonding since the dispersion forces
in the CH3 are so much weaker than the extra hydrogen bonding in water.
9. The extra oxygen means more fuel can be burnt. The decomposition of nitrous is
endothermic so it cools the fuel-air mixture (see #6).
10. This one is tough unless we think about the atmosphere. The atmosphere is most
dense at sea level and decreases in density as altitude increases. Eventually an altitude is
reached where the air cannot support combustion. Once we know this we can infer that
at high altitude the nitrous helps alleviate a shortage of oxygen. More difficult is
understanding how the methanol water helps more at low altitude. At low altitude there
is enough air to compress it beyond the temperature where predetonation occurs. Once
the altitude is reached where the engine can no longer compress the fuel air mixture
beyond the predetonation point the water-methanol injection loses most of its effect.
11. The water dissolves the alcohol and the two separate from the gasoline.