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Ethanol: The ins and outs of alcohol as a motor vehicle fuel.

Ethanol is the other fuel that can run in gasoline engines. Ethanol is however very different than gasoline in a number of ways. First and foremost ethanol tends to be much more expensive than gasoline, at least in large quantities. And it does take a lot of alcohol, at least 40% more or it than gasoline to do the same job.

What is Ethanol?
Jetting for Ethanol
Running Ethanol
Compression Ratios for Ethanol
Ethanol as a Motor Vehicle Fuel

What is Ethanol?

Ethanol is an ethane molecule with a hydroxide group in place of one of the end most hydrogen atoms. Ethanol is a small molecule consisting of only two carbon atoms. Ethanol has a substantial density of 6.6 pounds per gallon, but it's energy density is low. Just over one third of the weight of ethanol is that oxygen atom. At first this is a bit hard to grasp. One third of the mass of an ethanol molecule in just that one oxygen atom? Oxygen is big and heavy, especially compared to hydrogen. Even compared to carbon oxygen is big and heavy, 16g/mol for oxygen versus just 12g/mol for carbon. What this means is that although ethanol weighs 6.6 pounds per gallon there is actually only 4.3 pounds of combustion fuel in each gallon of ethanol.

Not only is ethanol a low energy density fuel, but it is also a rather weak fuel. Shorter hydrocarbon chain fuels generally have somewhat higher energy densities per pound than longer hydrocarbon chain fuels, but shorter hydrocarbon chain fuels also don't burn as hot as longer hydrocarbon chain fuels. The lower temperature of combustion potential means that ethanol can't support as high mean piston speeds as hotter burning gasoline. Ethanol would tend to require a much shorter stroke length engine to run as well as gasoline. In long three and a half and four inch stroke length automotive gasoline engines ethanol does not run as well as gasoline.

Jetting for Ethanol

Because ethanol has such a low energy density a much larger volume of it is required. To run a gasoline engine on ethanol the fuel flow rate needs to be at least 40% higher. On a carbureted engine that means giant jet sizes that would not work on gasoline. Jetting for ethanol is however complicated and tricky. Because it is oxygen that is reducing the energy density of ethanol there is more oxygen available in the combustion chamber. This is however a less significant fact than might at first be expected.

Combustion of one ethane molecule results in three molecules of water and two molecules of carbon dioxide which requires seven molecules of oxygen. Combustion of one molecule of ethanol requires only six molecules of oxygen because one of those seven molecules of oxygen is already present in the ethanol molecule. That's 14% of the oxygen required for combustion which is already present in the ethanol.

That 14% of the oxygen already being present in the ethanol would seem to imply that a further 17% higher fuel flow rate could be used with ethanol to make more power. This is sort of true, but again it is more complicated than it might at first seem. The thing about that atom of oxygen in each molecule of ethanol is that it is already single bonded to a carbon atom and a hydrogen atom. One of the carbon dioxide molecules is already partially formed, so there is only a reduced amount of combustion energy available from that part of the ethanol molecule. Essentially there is a bunch of water that gets in the way when running ethanol. Ethanol requires huge jet sizes to make power, but the power output still tends to be somewhat weak because that large volume of ethanol flowing through the intake valves displaces a rather substantial amount of the intake air.

In terms of fuel mileage and medium load cruising ethanol fuel flow rates are a bit more than 40% higher than for gasoline. For making maximum power though the ethanol fuel flow rates may be substantially higher than this. Ignoring the fact that ethanol displaces more of the intake air it might be expected that peak power on ethanol would come with fuel flow rates 1.6 times as high as for gasoline. Jetting for ethanol really is very tricky though. The higher volume of ethanol does displace more intake air, but then there is also the fact that more ethanol does also deliver more oxygen. The result is that ethanol makes more and more power with richer jetting up to the point where rather large quantities of partially burned fuel are being blown out the exhaust. Ethanol burns rather clean when the jetting is on the lean side for maximum efficiency and maximum fuel mileage, when the jetting is for absolute peak power production though an alcohol burning engine spews large quantities of unburned fuel.


Methanol would tend to be substantially more expensive than ethanol, it is more difficult to produce and requires larger quantities of biomass. The reason that methanol is often available for rather low prices is simply that natural gas is so cheap and methanol is fairly easy to make from methane. In sanctioned racing alcohol classes it is nearly always methanol that is used as a fuel. Why methanol instead of ethanol? That is hard to figure out. They say methanol has more power production potential than ethanol, but that is hard to verify. Methanol is a smaller molecule than ethanol, and is even more different from gasoline than ethanol is.

Methanol is a methane molecule with a hydroxide group in place of one of the hydrogen atoms. With only one carbon atom per molecule methane is the smallest of the alcohols.

Because methanol has only one carbon atom per molecule it would be expected to be a weaker colder burning fuel more like methane or hydrogen. From this perspective it would be expected that methanol would have less power production potential than ethanol for racing purposes. What methanol does have though is an even higher oxygen content than ethanol. Ethanol is just over one third oxygen by mass, where methanol 50% oxygen by mass.

The density of methanol is somewhat suspiciously listed as nearly identical to the density of ethanol, within two tenths of one percent at room temperature. Assuming that the density of methanol is similar to the density of ethanol then methanol has about 3.3 pounds of combustion fuel per gallon. A very low energy density. That would mean that in terms of fuel mileage and medium to medium heavy load cruising methanol would require a fuel flow rate more than 1.8 times what is required for gasoline.

In terms of maximum power production methanol flow rates would be even higher. Burning one molecule of methane produces one molecule of carbon dioxide and two molecules of water which requires four molecules of oxygen. Since methanol already has one molecule of oxygen it is only three molecules of oxygen that is required to burn a molecule of methanol. That would seem to suggest that a 33% higher methanol fuel flow rate could be used to make even more power. Again though the larger volume of methanol is going to displace a substantial amount of intake air. Ignoring the very substantial additional displacement of intake air it might be suspected that a fuel flow rate 2.4 times as high as for gasoline would produce peak power when running methanol.

The reality is that race engines running methanol do use extremely large jet sizes to dump in tons of fuel. Not all of that methanol is actually being burned though, much of it is blown out the exhaust partially burned. Hence the large blue flames shooting out the header pipes of alcohol cars at a drag strip.

Running Ethanol

Aside from the higher volumes required and the necessity for shorter stroke lengths ethanol runs fine in gasoline engines. Ethanol can be a bit harder starting than gasoline, but gasoline engines do start on 100% ethanol. Typically it just requires a somewhat higher cranking speed to get a gasoline engine to fire up on ethanol. Priming with a little squirt of gasoline is a sure fire way to get a gasoline engine to start with 100% ethanol in the tank.

Ethanol tends to require rather high compression ratios to work well, but this is a tricky subject. When running reasonably lean for efficiency ethanol works fine with moderate compression ratios. When alcohol is run extremely rich in big high output engines though higher compression ratios are required. When large amounts of the intake air are displaced by very rich mixtures a higher compression ratio is required to compensate. What it comes down to is that an extremely overly rich mixture requires running very close to the absolute maximum compression ratio for the fuel being used. Running a more reasonable mixture ratio allows a somewhat lower compression ratio to be used for ease of tuning and best possible light load performance.

The most important thing to keep in mind about ethanol is that it won't run in an engine jetted for gasoline. To run 100% ethanol much richer jetting is required. At a bare minimum about a 30% higher fuel flow rate than what works for gasoline, and it really is necessary to go all the way up to a 40% higher fuel flow rate because there is simply 40% more combustion fuel in a gallon of gasoline than in a gallon of ethanol.

Mixing ethanol and gasoline can work fine, but it doesn't really accomplish anything either. Mixing small quantities of gasoline with ethanol can improve starting performance, and small quantities of ethanol can be used as an additive in gasoline to modify the combustion properties. The reality though is that mixtures of gasoline and ethanol are worse than either gasoline or ethanol used alone. Especially mixtures of close to half gasoline and half ethanol are spectacularly worse than either fuel alone. It might be said that either E85 or E10 are reasonable fuels that don't accomplish much that is beneficial but that don't cause much in the way of problems either. Ten percent ethanol mixed in with gasoline does not really do anything beneficial, but it does slightly modify the combustion properties of the fuel. Mostly what 10% ethanol does is reduce the energy density of the gasoline so that a 3% to 4% higher fuel flow rate is required. Ten percent ethanol mixed in with regular gasoline though does provide some slight advantage because the overall flame front travel speed increases just a bit. Ethanol may be a very weak low energy densit and low temperatuer of combustion potential fuel, but the flame front travel speed of ethanol is rather high.

Especially when engines need to be jetted to run on either premium or regular gasoline mixing in 10% ethanol with the regular gasoline can be useful. Mostly what 10% ethanol mixed in with regular gasoline does is just prevent excessively rich mixtures on the slightly fatter jetting for premium gasoline. The problem of course is that just mixing 10% ethanol in with the regular does not solve the problem of regular requiring a higher compression ratio than premium. Ethanol is not a pressure lowering additive for most normal types of gasoline. Mixing in 20% premium and 10% ethanol with 70% regular does however allow an engine with the same compression ratio and the same jetting to run on either this regular blend or 100% premium gasoline. The regular blend does run worse and requires more spark advance, but advancing the spark timing is typically much easier than increasing the compression ratio and leaning out the jetting. If however 10% ethanol is mixed into both the regular and the premium gasoline then it does not accomplish anything other than to deliver slighlty worse performance and slightly lower overall efficiency.

Computerized EFI gasoline engines can rather easily be setup to compensate for fuels of different energy densities by adjusting the air/fuel mixture. These more sophisticated engines really don't have any need at all for ethanol as an additive. What the "Flex Fuel" vehicles that automatically compensate for fuels of different energy densities do however do is make it possible to run anything from 100% gasoline to E85 and everything in between. So, somewhat ironically, a Flex Fuel vehicle has absolutely no need for 10% ethanol in the gasoline, but it does not suffer so greatly if there happens to be 40% ethanol in one batch of "bad" gasoline either.

Compression Ratios for Ethanol

Ethanol is not normally observed to work as a pressure lowering additive for normal types of gasoline, but it sort of seems like it should. Ethanol is a small, short carbon chain molecule and it would be expected to pop off at a somewhat lower temperature and pressure point than larger carbon chain molecules.

At atmospheric pressure ethanol is observed to be resistant to being lit by a flame. Instead of just jumping into combustion like gasoline or other solvents ethanol requires that a flame be held on it for a longer period of time before it will ignite. At first this might be assumed to be just a difference in volatility, but that does not seem to be true. Ethanol evaporates rather readily, disappearing from a warm surface in a short period of time. When ethanol is sprayed into the air as an aerosol it can be ignited by a flame, but it produces sort of a weak and unspectacular ball of fire. Even light oil will burst into a much more spectacular cloud of flame when sprayed as an aerosol and ignited with a flame.

This is interesting, but it does not necessarily provide much good information about what temperature and pressure point is required for ethanol to pop off on late compression ignition in a gasoline engine. Fuels behave considerably differently under elevated temperature and pressure conditions so properties at atmospheric pressure are not always significant. Ethanol behaves dramatically differently than oil or gasoline at atmospheric pressure, but in a gasoline engine ethanol burns much the same. It is just harder to start from cold and does not run as well at elevated mean piston speeds.

Both ethanol and methanol are usually run at a 14:1 compression ratio in race engines, which seems to indicate that ethanol is a fairly high pressure feul. When 10% ethanol is mixed with gasoline it always seems to require slightly more spark advance in 9:1 and 10:1 engines, which also indicates that ethanol is a fairly high pressure fuel. Even in 11:1 and 12:1 engines mixing in 10% ethanol is sometimes observed to require a slight increase in the spark advance to compensate for the reduced energy density of the E10 fuel. Would this always be true for all types of gasoline? No, absolutely not. The highest pressure race gas that runs with substantial amounts of full load spark advance in 13:1 and 14:1 engines clearly is a higher pressure fuel than ethanol. By most accounts that is however far from normal gasoline. Most normal types of gasoline tend to run with rather small amounts of full load spark advance in 12:1 engines; if they will run at that high of a compression ratio at all.

What it comes down to is that ethanol is not all that high pressure of a fuel and it would be expected to function as a pressure lowering additive to allow higher pressure types of gasoline to run in lower compression ratio engines. The observed reality though is that gasoline normally is already such a low pressure fuel that adding ethanol does not allow it to run in lower compression ratio engines. Clearly very high pressure race gas is a higher pressure fuel than ethanol, and there may be some types of slow flame front travel speed regular gasoline that also are higher pressure fuels than ethanol.

If ethanol really is a lower pressure fuel, why is it usually run in 14:1 gasoline engines? The answer of course is that the much higher fuel flow rates for ethanol displace considerable intake air and a higher compression ratio is required to compensate. Ethanol probably pops off on late compression ignition at similar temperature and pressure levels to gasoline that is normally run in 10:1 and 11:1 engines with moderate amounts of spark advance. Ethanol could be run in an 11:1 engine, but usually higher compression ratios are used to compensate for extremely rich mixture ratios that displace large amounts of the intake air.

Should ethanol really be run at the extremely high fuel flow rate that produces maximum power? No, not for most purposes. For practicality and efficiency ethanol should be run with a 40% higher fuel flow rate than gasoline, that is pretty lean for ethanol. With this rather lean mixture that delivers clean operation and peak efficiency the compression ratio for 100% ethanol might be around 11.5:1 or slightly higher. Ethanol does not burn as hot and does not make as much power as gasoline, so an engine running 100% ethanol would be able to run well with a slighlty higher compression ratio than an engine running 10% ethanol mixed into high pressure gasoline as a pressure lowering addative.

Ethanol as a Motor Vehicle Fuel

Both ethanol and methanol have been considered as alternatives to gasoline, but for very different reasons. Methanol can be considered a viable alternative to gasoline simply because natural gas is so abundant and cheap. It is far more efficient to burn the methane itself, but methanol is a considerably more convenient liquid fuel for small portable gasoline engines. Ethanol is significant as a bio fuel.

Waste corn husks, stalks and cobs as well as other agricultural waste can fairly easily be used to produce some substantial small amounts of ethanol. Much higher ethanol yields per acre are possible by using the corn itself. Ethanol is a convenient bio fuel, but large quantities require huge areas of arable land. Ethanol is only a viable alternative to gasoline when much smaller quantities are used. Arguably a much better biofuel is vegetable oil run in diesel engines. Yields per acre are higher and the energy density of the fuel is higher for longer range between fill ups in small vehicles. It still remains true though that biofuels are only a viable alternative to petroleum when much smaller quantities are used.

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