The principle of operation of heat engines. Efficiency factor (COP) of heat engines. What does "coefficient of performance" mean?

Among the many characteristics of various mechanisms in the car, the decisive factor is Efficiency of an internal combustion engine. In order to find out the essence of this concept, you need to know exactly what a classic internal combustion engine is.

The efficiency of an internal combustion engine - what is it?

First of all, the motor converts the thermal energy that occurs during the combustion of fuel into a certain amount of mechanical work. Unlike steam engines, these engines are lighter and more compact. They are much more economical and consume strictly defined liquid and gaseous fuels. Thus, the efficiency of modern engines is calculated based on their technical characteristics and other indicators.

Efficiency (coefficient of performance) is the ratio of the power actually transmitted to the engine shaft to the power received by the piston due to the action of gases. If we compare the efficiency of engines of different power, we can establish that this value for each of them has its own characteristics.

Both engines, despite the similarity of design, have different types of mixture formation. Therefore, the pistons of a carburetor engine operate at higher temperatures that require high-quality cooling. Because of this, thermal energy that could turn into mechanical energy is dissipated to no avail, lowering the overall efficiency value.

However, in order to increase the efficiency of a gasoline engine, certain measures are taken. For example, two intake and exhaust valves can be installed per cylinder, instead of one intake and one exhaust valve. In addition, some engines have a separate ignition coil for each spark plug. Throttle control in many cases is carried out with the help of an electric drive, and not with an ordinary cable.

Diesel engine efficiency – noticeable efficiency

Diesel is one of the varieties of internal combustion engines, in which the ignition of the working mixture is carried out as a result of compression. Therefore, the air pressure in the cylinder is much higher than that of a gasoline engine. Comparing the efficiency of a diesel engine with the efficiency of other designs, one can note its highest efficiency.

In the presence of low speeds and a large displacement, the efficiency index can exceed 50%.

Attention should be paid to the relatively low consumption of diesel fuel and the low content of harmful substances in the exhaust gases. Thus, the value of the efficiency of an internal combustion engine depends entirely on its type and design. In many vehicles, low efficiency is offset by various improvements to improve overall performance.

Energy supplied to the mechanism in the form of work of driving forces A dv.s. and moments for a cycle of steady motion, is spent on useful work A p.s. , as well as to work A Ftr associated with overcoming the forces of friction in kinematic pairs and the forces of resistance of the medium.

Consider steady motion. The increment of kinetic energy is equal to zero, i.e.

In this case, the work of the forces of inertia and the forces of gravity are equal to zero A Ri = 0, And G = 0. Then, for a steady motion, the work of the driving forces is equal to

And dv.s. =A p.s. + A Ftr.

Consequently, for a full cycle of steady motion, the work of all driving forces is equal to the sum of the work of the forces of production resistance and non-production resistance (friction forces).

Mechanical efficiency η (efficiency)- the ratio of the work of the forces of production resistance to the work of all driving forces during the steady motion:

η = . (3.61)

As can be seen from formula (3.61), the efficiency shows what fraction of the mechanical energy brought to the machine is usefully spent on doing the work for which the machine was created.

The ratio of the work of the forces of non-productive resistance to the work of the driving forces is called loss factor :

ψ = . (3.62)

The mechanical loss factor shows what proportion of the mechanical energy supplied to the machine is ultimately converted into heat and wasted uselessly in the surrounding space.

From here we have a relationship between efficiency and loss factor

η =1- ψ.

From this formula it follows that in no mechanism the work of the forces of non-productive resistances can be equal to zero, therefore the efficiency is always less than one ( η <1 ). From the same formula it follows that the efficiency can be equal to zero if A dv.s \u003d A Ftr. The movement in which A dv.s \u003d A Ftr is called single . The efficiency cannot be less than zero, because for this it is necessary that A dv.s<А Fтр . The phenomenon in which the mechanism is at rest and at the same time the condition A dv.s is satisfied<А Fтр, называется the phenomenon of self-braking mechanism. The mechanism for which η = 1 is called perpetual motion machine .

Thus, the efficiency is in the range

0 £ η < 1 .

Consider the definition of efficiency for various ways of connecting mechanisms.

3.2.2.1. Determination of efficiency in series connection

Let there be n sequentially connected mechanisms (Figure 3.16).

And dv.s. 1 A 1 2 A 2 3 A 3 A n-1 n A n

Figure 3.16 - Scheme of series-connected mechanisms

The first mechanism is set in motion by driving forces that do work A dv.s. Since the useful work of each previous mechanism spent on production resistances is the work of the driving forces for each subsequent mechanism, the efficiency of the first mechanism will be equal to:

η 1 \u003d A 1 /A dv.s ..

For the second mechanism, the efficiency is:

η 2 \u003d A 2 /A 1 .

And, finally, for the nth mechanism, the efficiency will look like:

η n \u003d A n /A n-1

The overall efficiency is:

η 1 n \u003d A n /And dv.s.

The value of the overall efficiency can be obtained by multiplying the efficiency of each individual mechanism, namely:

η 1 n = η 1 η 2 η 3 …η n= .

Consequently, general mechanical efficiency in series connected mechanisms equals work mechanical efficiency of individual mechanisms that make up one common system:

η 1 n = η 1 η 2 η 3 …η n .(3.63)

3.2.2.2 Determining the efficiency in a mixed connection

In practice, the connection of mechanisms turns out to be more complicated. More often series connection is combined with parallel. Such a connection is called mixed. Consider an example of a complex connection (Figure 3.17).

The flow of energy from mechanism 2 is distributed in two directions. In turn, from the mechanism 3 ¢¢ the energy flow is also distributed in two directions. The total work of the forces of production resistance is equal to:

And p.s. = A ¢ n + A ¢ ¢ n + A ¢ ¢¢ n.

The overall efficiency of the entire system will be equal to:

η \u003d A p.s /A dv.s =(A ¢ n + A ¢ ¢ n + A ¢ ¢¢ n)/A dv.s . (3.64)

To determine the overall efficiency, it is necessary to isolate the energy flows in which the mechanisms are connected in series, and calculate the efficiency of each flow. Figure 3.17 shows the solid line I-I, the dashed line II-II and the dash-dotted line III-III three energy flows from a common source.

And dv.s. A 1 A ¢ 2 A ¢ 3 ... A ¢ n-1 A ¢ n

II A ¢¢ 2 II

A ¢¢ 3 4 ¢¢ A ¢¢ 4 A ¢¢ n-1 n ¢¢ A ¢¢ n

Suppose we are relaxing in the country, and we need to bring water from the well. We lower a bucket into it, scoop up water and begin to raise it. Have you forgotten what our goal is? That's right: get some water. But look: we are lifting not only water, but also the bucket itself, as well as the heavy chain on which it hangs. This is symbolized by a two-color arrow: the weight of the load we lift is the sum of the weight of the water and the weight of the bucket and chain.

Considering the situation qualitatively, we say: along with the useful work of raising water, we also perform other work - lifting a bucket and a chain. Of course, without a chain and a bucket, we would not be able to draw water, however, from the point of view of the final goal, their weight "harms" us. If this weight were less, then complete perfect work would also be less (with the same useful).

Now let's move on to quantitative study these works and introduce a physical quantity called efficiency.

A task. Apples selected for processing, the loader pours out of the baskets into the truck. The mass of an empty basket is 2 kg, and the apples in it are 18 kg. What is the share of the useful work of the loader from his total work?

Solution. The full job is moving apples in baskets. This work consists of lifting apples and lifting baskets. Important: lifting apples is useful work, but lifting baskets is “useless”, because the purpose of the loader's work is to move only apples.

Let's introduce the notation: Fя is the force with which the hands lift only apples, and Fк is the force with which the hands lift only the basket. Each of these forces is equal to the corresponding force of gravity: F=mg.

Using the formula A = ±(F||  l) , we “write out” the work of these two forces:

Auseful \u003d + Fya lya \u003d mya g h and Auseless \u003d + Fk lk \u003d mk g h

The full work consists of two works, that is, it is equal to their sum:

Afull \u003d Auseful + Auseless \u003d mi g h + mk g h \u003d (mi + mk) g h

In the problem, we are asked to calculate the share of the loader's useful work from his total work. We do this by dividing the useful work by the total:

In physics, such shares are usually expressed as a percentage and denoted by the Greek letter "η" (read: "this"). As a result, we get:

η \u003d 0.9 or η \u003d 0.9 100% \u003d 90%, which is the same.

This number shows that out of 100% of the full work of the loader, the share of his useful work is 90%. Problem solved.

A physical quantity equal to the ratio useful work to complete perfect work, in physics it has its own name - efficiency - efficiency:

After calculating the efficiency using this formula, it is customary to multiply it by 100%. And vice versa: to substitute the efficiency in this formula, its value must be converted from a percentage to a decimal fraction, dividing by 100%.

Definition [ | ]

Efficiency

Mathematically, the definition of efficiency can be written as:

where BUT- useful work (energy), and Q- wasted energy.

If the efficiency is expressed as a percentage, then it is calculated by the formula:

where Q X (\displaystyle Q_(\mathrm (X) ))- heat taken from the cold end (refrigeration capacity in refrigeration machines); A (\displaystyle A)

For heat pumps use the term transformation ratio

where Q Γ (\displaystyle Q_(\Gamma ))- condensation heat transferred to the coolant; A (\displaystyle A)- the work (or electricity) spent on this process.

In the perfect car Q Γ = Q X + A (\displaystyle Q_(\Gamma )=Q_(\mathrm (X) )+A), hence for the ideal machine ε Γ = ε X + 1 (\displaystyle \varepsilon _(\Gamma )=\varepsilon _(\mathrm (X) )+1)

The coefficient of performance (COP) is a value that expresses in percentage terms the efficiency of a particular mechanism (engine, system) regarding the conversion of the received energy into useful work.

Read in this article

Why diesel efficiency is higher

The efficiency index for different engines can vary greatly and depends on a number of factors. have a relatively low efficiency due to the large number of mechanical and thermal losses that occur during the operation of a power unit of this type.

The second factor is the friction that occurs during the interaction of mating parts. Most of the useful energy consumption is the driving of the engine pistons, as well as the rotation of the parts inside the motor, which are structurally fixed on the bearings. About 60% of the combustion energy of gasoline is spent only to ensure the operation of these units.

Additional losses are caused by the operation of other mechanisms, systems and attachments. It also takes into account the percentage of losses due to resistance at the time of the next charge of fuel and air, and then the release of exhaust gases from the internal combustion engine cylinder.

If we compare a diesel plant and a gasoline engine, a diesel engine has a noticeably higher efficiency compared to a gasoline unit. Gasoline power units have an efficiency of about 25-30% of the total amount of energy received.

In other words, out of 10 liters of gasoline spent on the engine, only 3 liters are spent on useful work. The rest of the energy from the combustion of fuel went to waste.

With the same displacement indicator, the power of an atmospheric gasoline engine is higher, but is achieved at higher speeds. The engine needs to be “turned”, losses increase, fuel consumption increases. It is also necessary to mention the torque, which literally means the force that is transmitted from the motor to the wheels and drives the car. Gasoline ICEs reach their maximum torque at higher RPMs.

A similar naturally aspirated diesel achieves peak torque at low rpm, while using less diesel to do useful work, which means higher efficiency and fuel economy.

Diesel fuel generates more heat compared to gasoline, the combustion temperature of diesel fuel is higher, and the knock resistance index is higher. It turns out that a diesel internal combustion engine has more useful work done on a certain amount of fuel.

Energy value of diesel fuel and gasoline

Diesel fuel is made up of heavier hydrocarbons than gasoline. The lower efficiency of a gasoline plant compared to a diesel engine also lies in the energy component of gasoline and the features of its combustion. Complete combustion of an equal amount of diesel fuel and gasoline will give more heat in the first case. Heat in a diesel engine is more fully converted into useful mechanical energy. It turns out that when burning the same amount of fuel per unit of time, it is the diesel engine that will do more work.

It is also worth considering the features of injection and the creation of appropriate conditions for the full combustion of the mixture. In a diesel engine, fuel is supplied separately from air, it is not injected into the intake manifold, but directly into the cylinder at the very end of the compression stroke. The result is a higher temperature and the most complete combustion of a portion of the working fuel-air mixture.

Results

Designers are constantly striving to improve the efficiency of both diesel and gasoline engines. An increase in the number of intake and exhaust valves per cylinder, active use, electronic control of fuel injection, throttle valve and other solutions can significantly increase efficiency. To a greater extent this applies to the diesel engine.

Thanks to these features, a modern diesel engine is able to completely burn a portion of diesel fuel saturated with hydrocarbons in the cylinder and produce a large amount of torque at low revs. Low RPMs mean less friction loss and the resulting drag. For this reason, a diesel engine is today one of the most productive and economical types of internal combustion engines, the efficiency of which often exceeds 50%.

Read also

Why it's better to warm up the engine before driving: lubrication, fuel, wear of cold parts. How to warm up a diesel engine in winter.