Surely many wonder what is that of the thermal performance or thermal efficiency, how it can be calculated, or how much your vehicle's engine has. All these doubts are understandable, since the efficiency of an engine will depend on it. That is, the ability to transform the energy provided by the fuel into real mechanical performance.
On the other hand, not to be confused with volumetric efficiency of an engine, which is the ratio between the air sucked into the cylinder and the volume it could contain according to the displacement of the same. This is another factor that directly intervenes in power, which is why naturally aspirated engines, with equal capacity, achieve less power than supercharged (turbo) engines.
What is the thermal efficiency of an internal combustion engine?
La thermal efficiency of an engine is the ability of an engine to transform the energy supplied by the fuel during combustion into mechanical performance. All designers hope to create an engine with 100% efficiency, as that would be ideal. However, in practice this is not possible, since the efficiency is hampered by the friction of the parts, the loss in the form of heat, etc.
In street cars, gasoline engines have a thermal performance or efficiency of 30%. This means that 30% of the energy provided by gasoline is actually used to generate power, the rest is wasted as heat.
In the case of Diesel engine, the efficiency is somewhat better, with 40%, since they have a higher level of compression. Therefore, they are somewhat more efficient than gasoline, although it is not too high a figure either.
Currently, they have achieved efficiency improvements considerable thanks to hybrid systems. For example, the thermal efficiency in Formula 1 it is much higher, thanks to the arrival of the V6 Turbo engines with MGU-K and MGU-H. Specifically, it has risen from 30% of conventional gasoline engines, to about 50%. That means that half of the energy provided by the fuel is used.
This particular case is a consequence of the inclusion of systems such as the MGU-K, or regenerative brake, capable of obtaining energy from braking, and from the MGU-H, which also harnesses the energy of the exhaust gases.
Ideal, effective and maximum thermal performance
In 1824, the physicist French Sadi Carnot, studied the thermal efficiency of an ideal heat engine as a function of the temperature between the hot and cold sources. This is currently applied to calculate the efficiency or thermal performance of any heat engine, whether it is a heat pump, a combustion engine, or a cooling system. Although in this article we will focus on combustion engines, given the theme of AM.
In addition, there are three types of thermal performance When you talk about an Otto or Diesel cycle engine:
- Ideal: is defined as the ratio between the amount of energy transformed into useful work and the amount of energy supplied. According to the second law of thermodynamics, no engine can convert all of its energy into mechanical work. Therefore, in practice there is no motor with an ideal thermal efficiency.
- Cash: is the actual thermal performance of a motor.
- Maximum: is the maximum performance of a type of engine, for example the 30% and 40% (approx.) mentioned above for gasoline and diesel. In other words, the architectural or limiting limitations of a type of thermal machine.
Diesel thermal efficiency vs. gasoline
The thermal efficiency in a motor Otto cycle (gasoline) is not the same as that of a motor diesel cycle. They behave differently at a thermodynamic level, and this causes them to have differences in terms of efficiency. In addition, an alternative 4-stroke engine is not the same as a Wankel engine, etc.
Otto cycle
in an engine of Otto cycle, which corresponds to thermodynamic cycles for gasoline, ethanol, or gas engines, whether they are 2-stroke (1 crankshaft turn) or 4-stroke (2 crankshaft turns). In these engines there are a series of stages such as intake, compression, combustion or ignition, expansion and escape.
In these motors, the efficiency or thermal performance depends on the compression ratio, that is, between the ratio between the maximum and minimum volume of the combustion chamber. The higher the ratio, the better the efficiency, although high octane fuels would also be needed to avoid a phenomenon known as detonation (self-ignition of the fuel before the spark plug occurs).
In short, the average thermal performance of a good 4-stroke Otto engine is between 25 and 30%, depending on the type of engine and the manufacturer.
Diesel Cycle
This diesel cycle it also establishes the thermal behavior diagram for a 4-stroke or 2-stroke diesel engine. In this case, there are some particularities, to which must be added its higher compression ratio compared to gasoline engines. Therefore, the thermal efficiency in this case ranges from 30 to 45%.
As a curiosity, the most efficient diesel engine in the world at the moment it measures 5 meters high and 9 meters long, with 13.142 horsepower. Its about Wärtsilä 31, manufactured in Finland and intended for naval use. This engine consumes about 38.8 tons of fuel per day, something that seems outrageous, but it is not so much when talking about this type of mass for boats...
Is a 100% efficient heat engine possible?
This question is very recurrent, and the truth is that it is not possible to get a motor or machine with a 100% yield. that's just theoretical. Achieving a perfect machine would be quite an achievement and would reduce energy consumption, but that would mean creating an engine so efficient that it can transform all the energy of the fuel into movement.
But this is not the case, in real engines, since there is friction between parts, the heat that escapes, etc. With improved lubricants, new engine architectures, injection technologies, Etc., efficiency is being improved, but it is impossible to reach that 100%.
And, of course, it is also not possible to achieve a motor with a thermal efficiency over 100%, since that would be like saying that new energy is obtained, something that directly violates the first law of thermodynamics.
Calculation of thermal efficiency
FOR calculate the thermal efficiency or thermal performance of a motor either, the Carnot formula must be applied:
Where Th is the temperature of the hot source of the machine, in this case the internal combustion engine, and Tc is the temperature of the cold source. Therefore, it follows that to achieve greater efficiency, the temperature between the hot and cold fluid must be as disparate as possible.
This is obviously very generic, and if you want to apply to a Internal combustion engine like that of cars, then the formula should be like this:
In this case, W is the work done, Qc is the hot source or source, Qf is the cold source to which the engine gives off heat. In addition, so that the law of conservation of energy is not violated, it must be fulfilled that the hot focus is equal to the work added to the heat of the cold focus. And it will always be true that 0<η<1. On the other hand, it should also be clear that the work (W) will be equal to the heat input minus the heat output.
Another way to calculate the efficiency of a motor is knowing the useful power and the power consumed in kW. This is the key engine compression ratio, since the higher it is, the better the efficiency.
By example, if you have 200 J of thermal energy as heat input, and the motor can do 80 J of work, then 80/200 = 0.4 (0.4 x 100 = 40% efficiency). The same thing would happen if the heat of the engine exhaust is measured, for example, if the energy put into the engine is 200 J by the fuel and an exhaust output of 120 J is observed, then the work done is 80 J ( 200-120) and the efficiency is 40%, since if you divide 120/200=0.6, which is the energy that is wasted and therefore is not transformed into work...