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Modern Gasoline motors have an average efficiency of about 25% when used to power an automobile. In other words, of the total energy of gasoline, 75% is consumed by the motor itself and dissipated in the form of heat and only 25% of energy moves the vehicle. At idle and slow speed the efficiency is much lower than average and improves considerably at open road speeds. Diesel motors are more efficient. The most efficient type, Direct Injection Diesels , are able to reach an efficiency of about 40% in the engine speed range of idle to about 1,800 RPM . Beyond this speed, efficiency begins to decline due to air pumping losses within the engine. The efficiency depends on several factors, one of them is the value. Diesel engines have a compression ratio between 14:1 to 25:1. In this case the general rule does not apply because Diesels with compression ratios over 20:1 are Indirect Injection Diesels . these use a prechamber to make possible high RPM operation as is required in automobiles and light trucks. The thermal and gas dynamic losses from the prechamber result in direct injection Diesels despite their lower compression ratio being more efficient. An engine has many parts that produce Friction and this friction increases at high RPM. A motor is more efficient at low RPM than at high RPM. The loss of efficiency as RPM rise becomes proportionately greater due to air pumping losses which increase much faster than friction losses. A gasoline motor burns a mix of gasoline and air, consisting of about one part of fuel to thirty-five parts of air. The combustion is a reaction which takes the Oxygen from air and makes chemical changes with hydrocarbons of fuel, and produces heat, energy, water, carbon monoxide, carbon dioxide, unburned hydrocarbons, and carbon. This mixture is what we see going out from the exhaust. The air has approximately 20% of oxygen; if there is not enough oxygen for combustion, the fuel will not burn and will not produce energy, it is senseless to introduce more fuel to the engine in order to make a richer mix (more fuel) of fuel and air, this will not produce more power, instead it will cause an increase of pollutants from the engine. There are a few exceptions where introducing fuel upstream of the combustion chamber can cool down the incoming air through evaporative cooling. The extra fuel that isn't burned in the combustion chamber cools down the intake air resulting in more power. With direct injection this effect isn't as dramatic but it can cool down the combustion chamber enough to reduce certain pollutants, nitrous oxides to be exact while raising others like hydro carbons. The air enters the engine because the vacuum produced by the motion of the pistons. But there are several engines that have a Turbine that forces air to enter the engine; thus, the combustion process has more oxigen available, and it is possible to increase the amount of fuel, the result, is a more powerful engine without increasing it size, this kind of motor is known as Turbocharged . There are other methods to increase the amount of oxygen available inside the engine; one of them, is to inject nitrous oxide, ( Nitrous )to the mixture, and some especial engines use nitromethane,it is a fuel that provides the oxygen itself it needs to burn, because of that, the mixture could be 1 part of fuel and 3 parts of air; thus, it is possible to burn huge quantities of fuel inside the engine, and get enormous quantities of power. Piston steam engines are very inefficient which is why there are no longer any steam locomotives in commercial use. Large output steam turbines equal or exceed the efficiency of the Diesel, which is why they are used for electric utility generating plants. The Stirling engine has the highest efficiency of any thermal engine but it is very expensive to make and is not competitive with other types for normal commercial use. The gas turbine is most efficient at maximum power output. Efficiency declines steadily with reduced power output and is very poor in the low power range. This is one reason, among several, why the gas turbine is not used for automobiles and trucks where much of the operating cycle is at idle and low to intermediate speeds. Detroit at one time tried to make a gas turbine for an automobile and gave up. This is also why gas turbines can be used for peak power electric plants. In this application they are only run at full power where they are efficient or shut down when not needed. SEE ALSO |
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