Slide 4

Atkinson cycle

Before the war, British engineer James Atkinson invented his own cycle, which is slightly different from Otto's cycle - his indicator chart is marked in green. What is the difference? Firstly, the volume of the combustion chamber of such a motor (with the same working volume) is less, and, accordingly, the compression ratio is higher. Therefore, the topmost point on the indicator chart is located to the left, in the area of ​​smaller overpiston volume. And the expansion ratio (the same as the compression ratio, only vice versa) is also larger - which means that we are more efficient, at a longer piston stroke we use the energy of the exhaust gases and have less exhaust losses (this is reflected by a smaller step on the right). Then everything is the same - there are exhaust and intake strokes.

Slide 5

Now, if everything happened in accordance with the Otto cycle and the intake valve closed at BDC, then the compression curve would go up, and the pressure at the end of the stroke would be excessive - because the compression ratio is higher here! The spark would not be followed by a flash of the mixture, but a detonation explosion - and the engine, without having worked even an hour, died with an explosion. But this was not the British engineer James Atkinson! He decided to extend the intake phase - the piston reaches BDC and goes up, and the intake valve, meanwhile, remains open until about half of the full piston stroke. Part of the fresh combustible mixture is pushed back into the intake manifold, which increases the pressure there - or rather, reduces the vacuum. This allows you to open the throttle valve more at low to medium loads. This is why the intake line in the Atkinson cycle diagram is higher and the pumping losses of the engine are lower than in the Otto cycle.

Slide 6

Cycle "Atkinson"

So the compression stroke when the intake valve closes starts at a lower above-piston volume, as illustrated by the green compression line starting at half the horizontal lower intake line. It would seem that what is easier: to increase the compression ratio, change the profile of the intake cams, and the trick is in the bag - the engine with the Atkinson cycle is ready! But the fact is that in order to achieve good dynamic performance in the entire operating range of engine revolutions, it is necessary to compensate for the pushing out of the combustible mixture during an extended intake cycle, using supercharging, in this case a mechanical supercharger. And its drive takes away from the motor the lion's share of the energy that it manages to win back on pumping and exhaust losses. The use of the Atkinson cycle on the naturally aspirated Toyota Prius hybrid engine was made possible by the fact that it operates in a light mode.

Slide 7

The Miller cycle

Miller's cycle is a thermodynamic cycle used in four-stroke internal combustion engines. The Miller cycle was proposed in 1947 by the American engineer Ralph Miller as a way to combine the advantages of the Antkinson engine with the simpler piston mechanism of the Otto engine.

Slide 8

Instead of making the compression stroke mechanically shorter than the power stroke (as in the classic Atkinson engine, where the piston moves up faster than down), Miller came up with the idea of ​​shortening the compression stroke by using the intake stroke, keeping the piston movement up and down the same in speed (as in the classic Otto engine).

Slide 9

To do this, Miller proposed two different approaches: to close the intake valve much earlier than the end of the intake stroke (or open later than the start of this stroke), and to close it much later than the end of this stroke.

Slide 10

The first approach for engines is conventionally called "shortened intake", and the second - "shortened compression". Both of these approaches give the same thing: a decrease in the actual compression ratio of the working mixture relative to the geometric one, while maintaining the same expansion ratio (that is, the stroke of the working stroke remains the same as in the Otto engine, and the compression stroke, as it were, is reduced - as in Atkinson, only is reduced not in time, but in the degree of compression of the mixture)

Slide 11

Miller's second approach

This approach is somewhat more beneficial from the point of view of compression losses, and therefore it is precisely this approach that is practically implemented in the serial Mazda MillerCycle automobile engines. In such a motor, the intake valve does not close at the end of the intake stroke, but remains open during the first part of the compression stroke. Although the entire volume of the cylinder was filled with the air / fuel mixture during the intake stroke, some of the mixture is forced back into the intake manifold through the open intake valve when the piston moves up on the compression stroke.

Slide 12

Compression of the mixture actually begins later when the intake valve finally closes and the mixture is trapped in the cylinder. Thus, the mixture in a Miller engine compresses less than it would have to compress in an Otto engine of the same mechanical geometry. This allows you to increase the geometric compression ratio (and, accordingly, the expansion ratio!) Above the limits determined by the knock properties of the fuel - bringing the actual compression to acceptable values ​​due to the above-described "shortening the compression cycle". Slide 15

Conclusion

If you look closely at the cycle - both Atkinson and Miller, you will notice that there is an additional fifth bar in both. It has its own characteristics and is, in fact, neither an intake stroke nor a compression stroke, but an intermediate independent stroke between them. Therefore, engines operating on the Atkinson or Miller principle are called five-stroke.

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Among other merits, the Mazda company (aka Toyo Cogyo Corp) is known as a great admirer of unconventional solutions. Having a fair amount of experience in the development and operation of the usual four-stroke piston engines, Mazda pays great attention to alternative solutions, and we are not talking about some purely experimental technologies, but about products installed in serial cars. The most famous are two developments: a piston engine with a Miller cycle and a rotary Wankel engine, in relation to which it is worth noting that the ideas underlying these motors were not born in Mazda laboratories, but it was this company that managed to bring original innovations to mind. It often happens that all the progressiveness of a technology is nullified by an expensive production process, inefficiency in the composition of the final product, or some other reason. In our case, the stars formed a successful combination, and Miller and Wankel got a start in life as Mazda units.

The combustion cycle of the air-fuel mixture in a four-stroke engine is called the Otto cycle. But few car enthusiasts know that there is an improved version of this cycle - the Miller cycle, and it was Mazda who managed to build a real working engine in accordance with the provisions of the Miller cycle - this engine was equipped in 1993 with the Xedos 9 cars, also known as Millenia and Eunos 800. This 2.3-liter V-6 was the world's first production Miller engine. Compared to conventional engines, it develops the torque of a three-liter engine with a fuel consumption of a two-liter one. The Miller cycle more efficiently uses the combustion energy of the air-fuel mixture, so a powerful motor is more compact and more efficient in terms of environmental requirements.

Mazda Miller has the following characteristics: power 220 liters. with. at 5500 rpm, a torque of 295 Nm at 5500 rpm - and this was achieved in 1993 with a volume of 2.3 liters. How was this achieved? Due to some disproportionality of the measures. Their duration is different, therefore, the compression ratio and expansion ratio, the main values ​​describing the operation of the internal combustion engine, are not the same. For comparison, in an Otto engine, the duration of all four strokes is the same: intake, compression of the mixture, working stroke of the piston, exhaust - and the compression ratio of the mixture is equal to the expansion ratio of the combustion gases.

Increasing the expansion ratio means that the piston is able to do more work - this significantly increases the efficiency of the engine. But, according to the logic of the Otto cycle, the compression ratio also increases, and here there is a certain limit, above which it is impossible to compress the mixture, its detonation occurs. An ideal variant suggests itself: increase the expansion ratio, reduce the compression ratio as much as possible, which is impossible in relation to the Otto cycle.

Mazda has managed to overcome this contradiction. In her Miller cycle engine, lowering the compression ratio is achieved by introducing a delay in the intake valve - it remains open, and part of the mixture is returned back to the intake manifold. In this case, the compression of the mixture begins not when the piston has passed the bottom dead center, but at the moment when it has already passed a fifth of the way to the top dead center. In addition, a preliminarily slightly compressed mixture is fed into the cylinder by a Lisholm compressor, a kind of analogue of a supercharger. This is how the paradox is easily overcome: the duration of the compression stroke is slightly shorter than the expansion stroke, and in addition, the engine temperature decreases and the combustion process becomes much cleaner.

Another successful Mazda idea is the development of a rotary piston engine based on ideas proposed almost fifty years ago by engineer Felix Wankel. Today's delightful sports cars RX-7 and RX-8 with the characteristic "alien" engine sound are hidden under the hoods of rotary engines, which are theoretically similar to conventional piston engines, but practically - completely out of this world. The use of Wankel rotary engines in the RX-8 allowed Mazda to supply its brainchild with 190 or even 230 horsepower with an engine displacement of only 1.3 liters.

With a mass and dimensions two to three times less than that of a piston engine, a rotary engine is capable of developing a power approximately equal to that of a piston engine, twice that in volume. A kind of devil in a snuff-box, which deserves the utmost attention. In the entire history of the automotive industry, only two companies in the world have managed to create efficient and not too expensive rotors - this is Mazda and ... VAZ.

Mazda RX-7

The functions of a piston in a rotary piston engine are performed by a rotor with three peaks, with the help of which the pressure of the burnt gases is converted into a rotary motion of the shaft. The rotor, as it were, rolls around the shaft, forcing the latter to rotate, and the rotor moves along a complex curve called the "epitrochoid". For one revolution of the shaft, the rotor turns 120 degrees, and for a full revolution of the rotor in each of the chambers into which the rotor divides the stationary housing-stator, a complete four-stroke cycle "intake - compression - working stroke - exhaust" occurs.

Interestingly, this process does not require a gas distribution mechanism, there are only intake and exhaust ports that overlap with one of the three rotor tops. Another indisputable advantage of the Wankel engine is that the number of moving parts is much smaller compared to the usual piston engine, which significantly reduces vibration of both the engine and the car.

It must be admitted that the very effective nature of such an engine does not at all exclude many disadvantages. Firstly, these are very high-speed, and therefore highly loaded motors, which require additional lubrication and cooling. For example, the consumption of 500 to 1000 grams of special mineral oil for Wankel is quite common, because it has to be injected directly into the combustion chamber to reduce loads (synthetics are not suitable due to increased coking of individual engine components).

The design flaw is perhaps the only one: the high cost of production and repair, because the precision rotor and stator have a very complex shape, and therefore many Mazda dealers have serious warranty repair of such motors is extremely simple: replacement! The difficulty is also in the fact that the stator must successfully withstand thermal deformations: unlike a conventional motor, where a heat-loaded combustion chamber is partially cooled in the intake and compression phase with a fresh working mixture, here the combustion process always takes place in one part of the engine, and the intake - in another ...

The Miller cycle is a thermodynamic cycle used in four-stroke internal combustion engines. The Miller cycle was proposed in 1947 by the American engineer Ralph Miller as a way to combine the advantages of the Atkinson engine with the simpler piston mechanism of the Otto engine. Instead of making the compression stroke mechanically shorter than the power stroke (as in the classic Atkinson engine, where the piston moves up faster than down), Miller came up with the idea of ​​shortening the compression stroke by using the intake stroke, keeping the piston movement up and down the same in speed (as in the classic Otto engine).

To do this, Miller proposed two different approaches: either close the intake valve much earlier than the end of the intake stroke (or open later than the start of this stroke), or close it much later than the end of this stroke. The first approach among engine engineers is conventionally called "shortened intake", and the second - "shortened compression". Ultimately, both of these approaches give the same thing: a decrease in the actual compression ratio of the working mixture relative to the geometric one, while maintaining the same expansion ratio (that is, the stroke of the working stroke remains the same as in the Otto engine, and the compression stroke, as it were, is reduced - as in Atkinson, only it is reduced not in time, but in the degree of compression of the mixture). Let's take a closer look at Miller's second approach.- since it is somewhat more profitable in terms of compression losses, and therefore it is precisely it that is practically implemented in serial Mazda "Miller Cycle" automobile engines (such a 2.3-liter V6 engine with a mechanical supercharger has been installed on the Mazda Xedos-9 for a long time, and recently the newest "atmospheric" engine I4 of this type with a volume of 1.3 liters was received by the Mazda-2 model).

In such a motor, the intake valve does not close at the end of the intake stroke, but remains open during the first part of the compression stroke. Although the entire volume of the cylinder was filled with the air / fuel mixture during the intake stroke, some of the mixture is forced back into the intake manifold through the open intake valve when the piston moves up on the compression stroke. Compression of the mixture actually begins later when the intake valve finally closes and the mixture is trapped in the cylinder. Thus, the mixture in a Miller engine compresses less than it would have to compress in an Otto engine of the same mechanical geometry. This allows the geometric compression ratio (and, accordingly, the expansion ratio!) To be increased above the limits due to the knock properties of the fuel - bringing the actual compression to acceptable values ​​due to the above-described "shortening the compression cycle". In other words, at the same actual compression ratio (fuel limited), the Miller motor has a significantly higher expansion ratio than the Otto motor. This makes it possible to more fully use the energy of the gases expanding in the cylinder, which, in fact, increases the thermal efficiency of the engine, ensures high efficiency of the engine, and so on.

Of course, the reverse displacement of the charge means a drop in the power parameters of the engine, and for atmospheric engines it makes sense to work on such a cycle only in a relatively narrow mode of partial loads. In the case of constant valve timing, this can only be compensated for in the entire dynamic range by using boost. On hybrid models, the lack of traction in unfavorable conditions is compensated for by the thrust of the electric motor.

The benefit of the increased thermal efficiency of the Miller cycle relative to the Otto cycle is accompanied by a loss of peak power output for a given engine size (and weight) due to degraded cylinder filling. Since a larger Miller motor would be required to achieve the same power output than an Otto motor, the gains from increased cycle thermal efficiency will be partly spent on increased mechanical losses (friction, vibration, etc.) with the size of the motor. That is why Mazda engineers built their first production engine with a non-atmospheric Miller cycle. When they attached a Lysholm supercharger to the engine, they were able to regain the high power density without losing much of the efficiency provided by the Miller cycle. It was this decision that made the Mazda V6 “Miller Cycle” engine attractive to the Mazda Xedos-9 (Millenia or Eunos-800). Indeed, with a working volume of 2.3 liters, it produces a power of 213 hp. and a torque of 290 Nm, which is equivalent to the characteristics of conventional 3-liter atmospheric engines, and at the same time, the fuel consumption for such a powerful engine in a large car is very low - on the highway 6.3 l / 100 km, in the city - 11.8 l / 100 km, which is in line with the much less powerful 1.8-liter engines. Further development of technology allowed Mazda engineers to build a Miller Cycle engine with acceptable power density characteristics already without using superchargers - the new Sequential Valve Timing System, dynamically controlling the intake and exhaust phases, allows to partially compensate for the drop in maximum power inherent in the Miller cycle. The new engine will be produced in an in-line 4-cylinder with a volume of 1.3 liters, in two versions: with a capacity of 74 horsepower (118 Nm of torque) and 83 horsepower (121 Nm). At the same time, the fuel consumption of these engines has decreased in comparison with a conventional engine of the same power by 20 percent - up to a little over four liters per hundred kilometers. In addition, the toxicity of a Miller cycle engine is 75 percent lower than current environmental requirements. Implementation In classic Toyota engines of the 90s with fixed phases operating on the Otto cycle, the intake valve closes 35-45 ° after the BDC (in terms of the crankshaft angle), the compression ratio is 9.5-10.0. In more modern engines with VVT, the possible closing range of the intake valve has expanded to 5-70 ° after BDC, the compression ratio has increased to 10.0-11.0. In engines of hybrid models operating only according to the Miller cycle, the closing range of the intake valve is 80-120 ° ... 60-100 ° after BDC. The geometric compression ratio is 13.0-13.5. By the mid-2010s, new engines with a wide range of variable valve timing (VVT-iW) appeared, which can operate both in the normal cycle and in the Miller cycle. For atmospheric versions, the intake valve closing range is 30-110 ° after BDC with a geometric compression ratio of 12.5-12.7, for turbo versions - 10-100 ° and 10.0, respectively.

The Miller cycle was proposed in 1947 by the American engineer Ralph Miller as a way to combine the advantages of the Atkinson engine with the simpler piston mechanism of the Otto engine. Instead of making the compression stroke mechanically shorter than the power stroke (as in the classic Atkinson engine, where the piston moves up faster than down), Miller came up with the idea of ​​shortening the compression stroke by using the intake stroke, keeping the piston movement up and down the same in speed (as in the classic Otto engine).

To do this, Miller proposed two different approaches: either close the intake valve much earlier than the end of the intake stroke (or open later than the start of this stroke), or close it much later than the end of this stroke. The first approach among engine engineers is conventionally called "shortened intake", and the second - "shortened compression". Ultimately, both of these approaches give the same thing: reducing actual the compression ratio of the working mixture relative to the geometric, while maintaining the same expansion ratio (that is, the stroke of the working stroke remains the same as in the Otto engine, and the compression stroke seems to be reduced - like that of Atkinson, only it is reduced not in time, but in the compression ratio of the mixture) ...

Thus, the mixture in a Miller engine compresses less than it would have to compress in an Otto engine of the same mechanical geometry. This makes it possible to increase the geometric compression ratio (and, accordingly, the expansion ratio!) Above the limits determined by the knock properties of the fuel - bringing the actual compression to acceptable values ​​due to the above-described "shortening the compression cycle". In other words, for the same actual compression ratio (fuel limited), the Miller motor has a significantly higher expansion ratio than the Otto motor. This makes it possible to more fully use the energy of the gases expanding in the cylinder, which, in fact, increases the thermal efficiency of the engine, ensures high efficiency of the engine, and so on.

The benefit of the increased thermal efficiency of the Miller cycle relative to the Otto cycle is accompanied by a loss of peak power output for a given engine size (and weight) due to degraded cylinder filling. Since a larger Miller motor would be required to achieve the same power output than an Otto motor, the gains from increased cycle thermal efficiency will be partly spent on increased mechanical losses (friction, vibration, etc.) with the size of the motor.

Computer control of the valves allows you to change the degree of cylinder filling during operation. This makes it possible to squeeze out the maximum power from the motor, with a deterioration in economic performance, or achieve better efficiency with a decrease in power.

A similar problem is solved by a five-stroke engine, in which additional expansion is performed in a separate cylinder.

Atkinson, Miller, Otto and others in our little technical tour.

First, let's figure out what an engine cycle is. An internal combustion engine is an object that converts the pressure from the combustion of fuel into mechanical energy, and since it works with heat, it is a heat engine. So, a cycle for a heat engine is a circular process in which the initial and final parameters coincide, which determine the state of the working fluid (in our case, it is a cylinder with a piston). These parameters are pressure, volume, temperature and entropy.

It is these parameters and their change that determine how the engine will work, in other words, what its cycle will be. Therefore, if you have the desire and knowledge of thermodynamics, you can create your own cycle of operation of a heat engine. The main thing then is to make your engine work in order to prove the right to exist.

Otto cycle

We will start with the most important cycle of work, which is used by almost all internal combustion engines in our time. It is named after Nikolaus August Otto, a German inventor. Initially, Otto used the work of the Belgian Jean Lenoir. A little understanding of the original design will give this model of the Lenoir engine.

Since Lenoir and Otto were not familiar with electrical engineering, the ignition in their prototypes was created by an open flame, which ignited the mixture inside the cylinder through a tube. The main difference between the Otto engine and the Lenoir engine was in the vertical placement of the cylinder, which prompted Otto to use the energy of the exhaust gases to raise the piston after the working stroke. The downward working stroke of the piston was initiated by atmospheric pressure. And after the pressure in the cylinder reached atmospheric, the exhaust valve opened, and the piston displaced the exhaust gases with its mass. It was the completeness of energy use that made it possible to raise the efficiency to a mind-blowing 15% at that time, which exceeded the efficiency of even steam engines. In addition, such a design made it possible to use five times less fuel, which then led to the total dominance of such a design on the market.

But the main merit of Otto is the invention of the four-stroke process of the internal combustion engine. This invention was made in 1877 and was patented at the same time. But French industrialists dug into their archives and found that the idea of ​​a four-stroke operation several years before Otto's patent had been described by the Frenchman Beau de Roche. This made it possible to reduce patent payments and start developing their own motors. But thanks to experience, Otto's engines were head and shoulders above the competition. And by 1897 42 thousand of them were made.

But what exactly is the Otto cycle? These are the four ICE strokes familiar to us from school - intake, compression, working stroke and exhaust. All these processes take an equal amount of time, and the thermal characteristics of the motor are shown in the following graph:

Where 1-2 is compression, 2-3 is a working stroke, 3-4 is an outlet, 4-1 is an inlet. The efficiency of such an engine depends on the compression ratio and the adiabatic index:

, where n is the compression ratio, k is the adiabatic index, or the ratio of the heat capacity of the gas at constant pressure to the heat capacity of the gas at constant volume.

In other words, it is the amount of energy that needs to be spent to return the gas inside the cylinder to its previous state.

Atkinson cycle

It was invented in 1882 by James Atkinson, a British engineer. The Atkinson cycle increases the efficiency of the Otto cycle, but decreases the power output. The main difference is the different execution time of different strokes of the motor.

The special design of the levers of the Atkinson engine allows all four piston strokes to be made in just one turn of the crankshaft. Also, this design makes the piston strokes of different lengths: the piston stroke during intake and exhaust is longer than during compression and expansion.

Another feature of the engine is that the cams of the valve timing (valve opening and closing) are located directly on the crankshaft. This eliminates the need for a separate camshaft installation. In addition, there is no need to install a gearbox, since the crankshaft rotates at half the speed. In the 19th century, the engine did not receive distribution due to its complex mechanics, but at the end of the 20th century it became more popular, as it began to be used on hybrids.

So, are there such strange units in expensive Lexus? By no means, no one was going to implement the Atkinson cycle in its pure form, but it is quite possible to modify ordinary motors for it. Therefore, we will not long rant about Atkinson and move on to the cycle that brought him to reality.

Miller cycle

The Miller cycle was proposed in 1947 by the American engineer Ralph Miller as a way to combine the advantages of the Atkinson engine with the simpler Otto engine. Rather than mechanically making the compression stroke shorter than the power stroke (as in the classic Atkinson engine, where the piston moves up faster than down), Miller came up with the idea of ​​reducing the compression stroke by using the intake stroke, keeping the piston movement up and down the same in speed (as in the classic Otto engine).

To do this, Miller proposed two different approaches: either close the intake valve much earlier than the end of the intake stroke, or close it much later than the end of this stroke. The first approach among the minders is conventionally called "shortened intake", and the second - "shortened compression". Ultimately, both of these approaches give the same thing: a decrease in the actual compression ratio of the working mixture relative to the geometric one, while maintaining the same expansion ratio (that is, the stroke of the working stroke remains the same as in the Otto engine, and the compression stroke, as it were, is reduced - as in Atkinson, only decreases not in time, but in the degree of compression of the mixture).

Thus, the mixture in a Miller engine compresses less than it would have to compress in an Otto engine of the same mechanical geometry. This makes it possible to increase the geometric compression ratio (and, accordingly, the expansion ratio!) Above the limits determined by the knock properties of the fuel - bringing the actual compression to acceptable values ​​due to the above-described "shortening the compression cycle". In other words, at the same actual compression ratio (fuel limited), the Miller motor has a significantly higher expansion ratio than the Otto motor. This makes it possible to more fully use the energy of the gases expanding in the cylinder, which, in fact, increases the thermal efficiency of the engine, ensures high efficiency of the engine, and so on. Also one of the advantages of the Miller cycle is the possibility of a wider variation in ignition timing without the risk of detonation, which provides more opportunities for engineers.

The benefit of the increased thermal efficiency of the Miller cycle relative to the Otto cycle is accompanied by a loss of peak power output for a given engine size (and weight) due to degraded cylinder filling. Since a larger Miller motor would be required to achieve the same power output than an Otto motor, the gains from increased cycle thermal efficiency will be partly spent on increased mechanical losses (friction, vibration, etc.) along with the size of the motor.

Diesel cycle

And finally, it is worth at least briefly recalling the Diesel cycle. Rudolph Diesel initially wanted to create an engine that would be as close as possible to the Carnot cycle, in which the efficiency is determined only by the difference in temperatures of the working fluid. But since cooling the engine to absolute zero is not cool, Diesel went the other way. He increased the maximum temperature, for which he began to compress the fuel to values ​​that were beyond the limit at that time. His motor turned out with a really high efficiency, but initially it worked on kerosene. Rudolph built the first prototypes in 1893, and only by the beginning of the twentieth century switched to other types of fuel, including diesel.

• , 17 Jul 2015