Can cars run on alcohol? Can fuel be replaced by alcohol?

Well, much has been discussed about this on CE and everywhere else. The low Calorific Values and the great anti-knocking properties are the basic considerations on the use of ethanol as a blend with gasoline or purely (100%, approved only in Brazil as of today) as fuel in engines. Wikipedia gives a wonderful description of this. From the documented material on Wikipedia, it appears that the corrosive properties of ethanol and the cold-start problems it poses are the only limiting factors on its use in engines. Otherwise, it can be run at par-sometimes even yielding better results with design changes-than gasoline engines.

Source: Wikipedia

Ethanol-based engines[#-Link-Snipped-#]
Ethanol is most commonly used to power automobiles, though it may be used to power other vehicles, such as <a href="https://en.wikipedia.org/wiki/Farm_tractor" target="_blank" rel="nofollow noopener noreferrer">Farm Tractor</a>, boats and <a href="https://en.wikipedia.org/wiki/Airplanes" target="_blank" rel="nofollow noopener noreferrer">Airplanes</a>. Ethanol (E100) consumption in an engine is approximately 51% higher than for gasoline since the energy per unit volume of ethanol is 34% lower than for gasoline.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-EEREFAQ-26" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Eerefaq 26</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-EIAATTF-27" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Eiaattf 27</a> The higher <a href="https://en.wikipedia.org/wiki/Compression_ratio" target="_blank" rel="nofollow noopener noreferrer">Compression Ratio</a> in an ethanol-only engine allow for increased <a href="https://en.wikipedia.org/wiki/Engine_power" target="_blank" rel="nofollow noopener noreferrer">Engine Power</a> output and better fuel economy than could be obtained with lower compression ratios.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-autogenerated6-28" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Autogenerated6 28</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-29" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 29</a> In general, ethanol-only engines are tuned to give slightly better power and <a href="https://en.wikipedia.org/wiki/Torque#Machine_torque" target="_blank" rel="nofollow noopener noreferrer">Torque Machine Torque</a> output than gasoline-powered engines. In <a href="https://en.wikipedia.org/wiki/Flexible_fuel_vehicle" target="_blank" rel="nofollow noopener noreferrer">Flexible Fuel Vehicle</a>, the lower compression ratio requires tunings that give the same output when using either gasoline or hydrated ethanol. For maximum use of ethanol's benefits, a much higher compression ratio should be used.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-30" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 30</a> Current high compression neat ethanol engine designs are approximately 20 to 30% more fuel efficient than their gasoline-only counterparts.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-31" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 31</a>
Ethanol contains soluble and insoluble contaminants.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-32" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 32</a> These soluble contaminants, halide ions such as chloride ions, have a large effect on the <a href="https://en.wikipedia.org/wiki/Corrosivity" target="_blank" rel="nofollow noopener noreferrer">Corrosivity</a> of alcohol fuels. <a href="https://en.wikipedia.org/wiki/Halide" target="_blank" rel="nofollow noopener noreferrer">Halide</a> increase corrosion in two ways; they chemically attack passivating oxide films on several metals causing pitting corrosion, and they increase the conductivity of the fuel. Increased electrical conductivity promotes electric, galvanic, and ordinary corrosion in the fuel system. Soluble contaminants, such as <a href="https://en.wikipedia.org/wiki/Aluminum_hydroxide" target="_blank" rel="nofollow noopener noreferrer">Aluminum Hydroxide</a>, itself a product of corrosion by halide ions, clog the fuel system over time.
Ethanol is <a href="https://en.wikipedia.org/wiki/Hygroscopic" target="_blank" rel="nofollow noopener noreferrer">Hygroscopic</a>, meaning it absorbs water vapor directly from the atmosphere. Because absorbed water dilutes the fuel value of the ethanol (although it suppresses engine knock) and may cause phase separation of ethanol-gasoline blends, containers of ethanol fuels must be kept tightly sealed. This high <a href="https://en.wikipedia.org/wiki/Miscibility" target="_blank" rel="nofollow noopener noreferrer">Miscibility</a> with water means that ethanol cannot be efficiently shipped through modern <a href="https://en.wikipedia.org/wiki/Pipeline_transport" target="_blank" rel="nofollow noopener noreferrer">Pipeline Transport</a>, like liquid hydrocarbons, over long distances.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-33" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 33</a> Mechanics also have seen increased cases of damage to small engines, in particular, the carburetor, attributable to the increased water retention by ethanol in fuel.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-34" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 34</a>
A 2004 <a href="https://en.wikipedia.org/wiki/Massachusetts_Institute_of_Technology" target="_blank" rel="nofollow noopener noreferrer">Massachusetts Institute Of Technology</a> study<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-35" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 35</a> and an earlier paper published by the Society of Automotive Engineers<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-36" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 36</a> identify a method to exploit the characteristics of fuel ethanol substantially more efficiently than mixing it with gasoline. The method presents the possibility of leveraging the use of alcohol to achieve definite improvement over the cost-effectiveness of hybrid electric. The improvement consists of using dual-fuel direct-injection of pure alcohol (or the azeotrope or E85) and gasoline, in any ratio up to 100% of either, in a turbocharged, high compression-ratio, small-displacement engine having performance similar to an engine having twice the displacement. Each fuel is carried separately, with a much smaller tank for alcohol. The high-compression (for higher efficiency) engine runs on ordinary gasoline under low-power cruise conditions. Alcohol is directly injected into the cylinders (and the gasoline injection simultaneously reduced) only when necessary to suppress ‘knock’ such as when significantly accelerating. Direct cylinder injection raises the already high octane rating of ethanol up to an effective 130. The calculated over-all reduction of gasoline use and CO2 emission is 30%. The consumer cost payback time shows a 4:1 improvement over turbo-diesel and a 5:1 improvement over hybrid. The problems of water absorption into pre-mixed gasoline (causing phase separation), supply issues of multiple mix ratios and cold-weather starting are also avoided.
Ethanol's higher <a href="https://en.wikipedia.org/wiki/Octane_rating" target="_blank" rel="nofollow noopener noreferrer">Octane Rating</a> allows an increase of an engine's compression ratio for increased <a href="https://en.wikipedia.org/wiki/Thermal_efficiency" target="_blank" rel="nofollow noopener noreferrer">Thermal Efficiency</a>.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-autogenerated6-28" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Autogenerated6 28</a> In one study, complex engine controls and increased exhaust gas recirculation allowed a compression ratio of 19.5 with fuels ranging from neat ethanol to E50. Thermal efficiency up to approximately that for a diesel was achieved.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-37" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 37</a> This would result in the <a href="https://en.wikipedia.org/wiki/Fuel_economy_in_automobiles" target="_blank" rel="nofollow noopener noreferrer">Fuel Economy In Automobiles</a> of a neat ethanol vehicle to be about the same as one burning gasoline.
Since 1989 there have also been ethanol engines based on the diesel principle operating in Sweden.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-Scania1-38" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Scania1 38</a> They are used primarily in city buses, but also in distribution trucks and waste collectors. The engines, made by <a href="https://en.wikipedia.org/wiki/Scania" target="_blank" rel="nofollow noopener noreferrer">Scania</a>, have a modified compression ratio, and the fuel (known as ED95) used is a mix of 93.6% ethanol and 3.6% ignition improver, and 2.8% <a href="https://en.wikipedia.org/wiki/Denaturation_(food)" target="_blank" rel="nofollow noopener noreferrer">Denaturation (Food)</a>.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-39" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 39</a> The ignition improver makes it possible for the fuel to ignite in the diesel combustion cycle. It is then also possible to use the energy efficiency of the diesel principle with ethanol. These engines have been used in the United Kingdom by <a href="https://en.wikipedia.org/wiki/Reading_Transport#Biofuel_usage" target="_blank" rel="nofollow noopener noreferrer">Reading Transport Biofuel Usage</a> but the use of bioethanol fuel is now being phased out.

Engine cold start during the winter[#-Link-Snipped-#]
High <a href="https://en.wikipedia.org/wiki/Common_ethanol_fuel_mixtures" target="_blank" rel="nofollow noopener noreferrer">Common Ethanol Fuel Mixtures</a> present a problem to achieve enough <a href="https://en.wikipedia.org/wiki/Vapor_pressure" target="_blank" rel="nofollow noopener noreferrer">Vapor Pressure</a> for the fuel to evaporate and spark the ignition during cold weather (since ethanol tends to increase fuel <a href="https://en.wikipedia.org/wiki/Enthalpy_of_vaporization" target="_blank" rel="nofollow noopener noreferrer">Enthalpy Of Vaporization</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-Balabin_2007-40" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Balabin 2007 40</a>). When vapor pressure is below 45 <a href="https://en.wikipedia.org/wiki/KPa" target="_blank" rel="nofollow noopener noreferrer">Kpa</a> starting a cold engine becomes difficult.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-41" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 41</a> To avoid this problem at temperatures below 11 <a href="https://en.wikipedia.org/wiki/Celsius" target="_blank" rel="nofollow noopener noreferrer">Celsius</a> (52 <a href="https://en.wikipedia.org/wiki/Fahrenheit" target="_blank" rel="nofollow noopener noreferrer">Fahrenheit</a>)), and to reduce ethanol higher emissions during cold weather, both the US and the European markets adopted E85 as the maximum blend to be used in their flexible fuel vehicles, and they are optimized to run at such a blend. At places with harsh cold weather, the ethanol blend in the US has a seasonal reduction to <a href="https://en.wikipedia.org/wiki/Common_ethanol_fuel_mixtures#E70.2C_E75" target="_blank" rel="nofollow noopener noreferrer">Common Ethanol Fuel Mixtures E70.2C E75</a> for these very cold regions, though it is still sold as E85.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-E70green-42" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note E70Green 42</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-E70-43" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note E70 43</a> At places where temperatures fall below −12 <a href="https://en.wikipedia.org/wiki/Celsius" target="_blank" rel="nofollow noopener noreferrer">Celsius</a> (10 <a href="https://en.wikipedia.org/wiki/Fahrenheit" target="_blank" rel="nofollow noopener noreferrer">Fahrenheit</a>) during the winter, it is recommended to install an engine heater system, both for gasoline and E85 vehicles. Sweden has a similar seasonal reduction, but the ethanol content in the blend is reduced to <a href="https://en.wikipedia.org/wiki/Common_ethanol_fuel_mixtures#E70.2C_E75" target="_blank" rel="nofollow noopener noreferrer">Common Ethanol Fuel Mixtures E70.2C E75</a> during the winter months.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-E70-43" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note E70 43</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-E75-44" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note E75 44</a>
Brazilian flex fuel vehicles can operate with ethanol mixtures up to <a href="https://en.wikipedia.org/wiki/Neat_alcohol_fuel" target="_blank" rel="nofollow noopener noreferrer">Neat Alcohol Fuel</a>, which is <a href="https://en.wikipedia.org/wiki/Hydrous" target="_blank" rel="nofollow noopener noreferrer">Hydrous</a> ethanol (with up to 4% water), which causes vapor pressure to drop faster as compared to E85 vehicles. As a result, Brazilian flex vehicles are built with a small secondary gasoline reservoir located near the engine. During a cold start pure gasoline is injected to avoid starting problems at low temperatures. This provision is particularly necessary for users of Brazil's southern and central regions, where temperatures normally drop below 15 <a href="https://en.wikipedia.org/wiki/Celsius" target="_blank" rel="nofollow noopener noreferrer">Celsius</a> (59 <a href="https://en.wikipedia.org/wiki/Fahrenheit" target="_blank" rel="nofollow noopener noreferrer">Fahrenheit</a>) during the winter. An improved flex engine generation was launched in 2009 that eliminates the need for the secondary gas storage tank.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-BioEnergia-45" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Bioenergia 45</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-Gazeta-46" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Gazeta 46</a> In March 2009 <a href="https://en.wikipedia.org/wiki/Volkswagen_do_Brasil" target="_blank" rel="nofollow noopener noreferrer">Volkswagen Do Brasil</a> launched the <a href="https://en.wikipedia.org/wiki/Volkswagen_Polo" target="_blank" rel="nofollow noopener noreferrer">Volkswagen Polo</a>, the first Brazilian flex fuel model without an auxiliary tank for cold start.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-FlexStart1-47" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Flexstart1 47</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-FlexStart2-48" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Flexstart2 48</a>

Fuel economy
In theory, all fuel-driven vehicles have a <a href="https://en.wikipedia.org/wiki/Fuel_economy_in_automobiles" target="_blank" rel="nofollow noopener noreferrer">Fuel Economy In Automobiles</a> (measured as miles per US gallon, or liters per 100 km) that is directly proportional to the fuel's energy content.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-55" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 55</a> In reality, there are many other variables that come into play that affect the performance of a particular fuel in a particular engine. Ethanol contains approx. 34% less energy per unit volume than gasoline, and therefore in theory, burning pure ethanol in a vehicle reduces miles per US gallon 34%, given the same fuel economy, compared to burning pure gasoline. Since ethanol has a higher octane rating, the engine can be made more efficient by raising its compression ratio. In fact, using a variable <a href="https://en.wikipedia.org/wiki/Turbocharger" target="_blank" rel="nofollow noopener noreferrer">Turbocharger</a>, the compression ratio can be optimized for the fuel, making fuel economy almost constant for any blend.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-EEREFAQ-26" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Eerefaq 26</a><a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-EIAATTF-27" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Eiaattf 27</a> For E10 (10% ethanol and 90% gasoline), the effect is small (~3%) when compared to conventional gasoline,<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-56" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 56</a> and even smaller (1–2%) when compared to oxygenated and reformulated blends.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-57" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 57</a> For E85 (85% ethanol), the effect becomes significant. E85 produces lower mileage than gasoline, and requires more frequent refueling. Actual performance may vary depending on the vehicle. Based on EPA tests for all 2006 E85 models, the average fuel economy for E85 vehicles resulted 25.56% lower than unleaded gasoline.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-Ethanol-58" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Ethanol 58</a> The EPA-rated mileage of current United States flex-fuel vehicles<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-59" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 59</a> should be considered when making price comparisons, but E85 is a high performance fuel, with an octane rating of about 94–96, and should be compared to premium.<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-60" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note 60</a> In one estimate<a href="https://en.wikipedia.org/wiki/Ethanol_fuel#cite_note-bourne-61" target="_blank" rel="nofollow noopener noreferrer">Ethanol Fuel Cite Note Bourne 61</a> the US <a href="https://en.wikipedia.org/wiki/Retail_price" target="_blank" rel="nofollow noopener noreferrer">Retail Price</a> for E85 ethanol is 2.62 <a href="https://en.wikipedia.org/wiki/US_dollar" target="_blank" rel="nofollow noopener noreferrer">Us Dollar</a> per <a href="https://en.wikipedia.org/wiki/Gallon" target="_blank" rel="nofollow noopener noreferrer">Gallon</a> or 3.71-dollar corrected for energy equivalency compared to a gallon of gasoline priced at 3.03-dollar. Brazilian cane ethanol (100%) is priced at 3.88-dollar against 4.91-dollar for E25 (as July 2007).