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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.