Let's understand the engine operating cycles. Presentation on the topic "Piston internal combustion engines with the Atkinson-Miller cycle" So what is the difference

The Miller cycle is a thermodynamic cycle used in four-stroke internal combustion engines. The Miller cycle was proposed in 1947 by American engineer Ralph Miller as a way of combining 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 at the expense of the intake stroke, keeping the piston's up and down motion the same. speed (as in the classic Otto engine).

To do this, Miller proposed two different approaches: either close the intake valve significantly earlier than the end of the intake stroke (or open later than the beginning of this stroke), or close it significantly later than the end of this stroke. The first approach among engine experts is conventionally called “shortened intake”, and the second - “short compression”. Ultimately, both of these approaches give the same thing: a reduction in the actual compression ratio of the working mixture relative to the geometric one, while maintaining a constant expansion ratio (that is, the power stroke remains the same as in the Otto engine, and the compression stroke seems to be shortened - like in Atkinson, only it is reduced not by time, but by the degree of compression of the mixture). Let's take a closer look at Miller's second approach.- since it is somewhat more advantageous in terms of compression losses, and therefore it is 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 car for quite a long time, and recently The latest “aspirated” I4 engine of this type with a volume of 1.3 liters was received by the Mazda-2 model).

In such an engine, 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 on the intake stroke fuel-air mixture the entire volume of the cylinder has been filled, part of the mixture is forced back into intake manifold through the open intake valve as 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 locked into the cylinder. Thus, the mixture in the Miller engine is compressed less than it would be compressed in an Otto engine of the same mechanical geometry. This allows you to increase geometric degree compression (and, accordingly, the degree of expansion!) above the limits due to the detonation properties of the fuel - bringing the actual compression to acceptable values due to the above-described “shortening of the compression cycle”. In other words, for the same actual compression ratio (limited by the fuel), the Miller engine has a significantly higher expansion ratio than the Otto engine. This makes it possible to more fully utilize the energy of the gases expanding in the cylinder, which, in fact, increases the thermal efficiency of the motor, ensures high engine efficiency, and so on.

Of course, reverse charge displacement means a drop in engine power performance, and for atmospheric engines operation on such a cycle makes sense only in a relatively narrow part-load mode. In the case of constant valve timing, only the use of supercharging can compensate for this throughout the entire dynamic range. On hybrid models, the lack of traction in unfavorable conditions is compensated by the traction of the electric motor.

The benefit of 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 reduced cylinder filling. Since to obtain the same power output a Miller engine would be required bigger size than an Otto engine, the gain from increasing the thermal efficiency of the cycle will be partially spent on mechanical losses (friction, vibration, etc.) that increase with the size of the engine. That's why Mazda engineers built their first serial motor with the Miller cycle not atmospheric. When they attached a Lysholm-type supercharger to the engine, they were able to restore the high power density without losing much of the efficiency provided by the Miller cycle. It was this decision that determined the attractiveness Mazda engine V6 "Miller Cycle" installed on the Mazda Xedos-9 (Millenia or Eunos-800). After all, with a working volume of 2.3 liters, it produces a power of 213 hp. and torque of 290 Nm, which is equivalent to the characteristics of conventional 3-liter naturally aspirated engines, and at the same time, fuel consumption for such powerful motor on big car very low - on the highway 6.3 l/100 km, in the city - 11.8 l/100 km, which corresponds to the performance of much less powerful 1.8-liter engines. Further development of technology allowed Mazda engineers to build a Miller Cycle engine with acceptable specific power characteristics without the use of superchargers - new system sequentially changing the valve opening time Sequential Valve Timing System, dynamically controlling the intake and exhaust phases, allows you to partially compensate for the drop in maximum power inherent in the Miller cycle. The new engine will be produced in-line 4-cylinder, 1.3 liter, in two versions: 74 horsepower (118 Nm of torque) and 83 horsepower (121 Nm). At the same time, the fuel consumption of these engines has decreased by 20 percent compared to a conventional engine of the same power - to just over four liters per hundred kilometers. In addition, the toxicity of a Miller cycle engine is 75 percent lower than modern environmental requirements. Implementation In classic Toyota engines 90s with fixed phases, operating on the Otto cycle, the intake valve closes at 35-45° after BDC (according to the rotation angle crankshaft), the compression ratio is 9.5-10.0. In more modern engines with VVT possible closing range intake valve expanded to 5-70° after BDC, the compression ratio increased to 10.0-11.0. In engines of hybrid models operating only on the Miller cycle, the closing range of the intake valve is 80-120° ... 60-100° after BDC. Geometric compression ratio - 13.0-13.5. By the mid-2010s, new engines appeared with wide range variable valve timing (VVT-iW), which can operate both in the conventional 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 it is 10-100° and 10.0, respectively.

READ ALSO ON THE SITE

Honda NR500 8 valves per cylinder with two connecting rods per cylinder, a very rare, very interesting and quite expensive motorcycle in the world, the Honda people were smart and smart for racing))) About 300 pieces were produced and now the prices are...

In 1989, Toyota introduced a new family of engines to the market, the UZ series. Three engines appeared in the line, differing in cylinder displacement, 1UZ-FE, 2UZ-FE and 3UZ-FE. Structurally they are V-shaped eight from the department...

Miller cycle ( Miller Cycle) was proposed in 1947 by American engineer Ralph Miller as a way to combine the advantages of an Atkinson engine with the simpler piston mechanism of a Diesel or Otto engine.

The cycle was designed to reduce ( reduce) temperature and pressure of the fresh air charge ( charge air temperature) before compression ( compression) in a cylinder. As a result, the combustion temperature in the cylinder decreases due to adiabatic expansion ( adiabatic expansion) fresh air charge upon entering the cylinder.

The concept of the Miller cycle includes two options ( two variants):

a) choosing a premature closing time ( advanced closure timing) intake valve ( intake valve) or closing advance - before bottom dead center ( bottom dead center);

b) selection of the delayed closing time of the intake valve - after the bottom dead center (BDC).

The Miller cycle was originally used ( initially used) to increase the specific power of some diesel engines ( some engines). Reducing the temperature of the fresh air charge ( Reducing the temperature of the charge) in the engine cylinder led to an increase in power without any significant changes ( major changes) cylinder block ( cylinder unit). This was explained by the fact that the decrease in temperature at the beginning of the theoretical cycle ( at the beginning of the cycle) increases the air charge density ( air density) without changing pressure ( change in pressure) in a cylinder. While the mechanical strength limit of the engine ( mechanical limit of the engine) shifts to more high power (higher power), thermal load limit ( thermal load limit) shifts to lower average temperatures ( lower mean temperatures) cycle.

Subsequently, the Miller cycle aroused interest from the point of view of reducing NOx emissions. Intense release of harmful NOx emissions begins when the temperature in the engine cylinder exceeds 1500 °C - in this state, nitrogen atoms become chemically active as a result of the loss of one or more atoms. And when using the Miller cycle, when the temperature of the cycle decreases ( reduce the cycle temperatures) without changing power ( constant power) a 10% reduction in NOx emissions was achieved at full load and a 1% ( per cent) reduction of fuel consumption. Mainly ( mainly) this is explained by a decrease in heat losses ( heat losses) at the same pressure in the cylinder ( cylinder pressure level).

However, significantly higher boost pressure ( significantly higher boost pressure) at the same power and air to fuel ratio ( air/fuel ratio) made it difficult for the Miller cycle to become widespread. If the maximum achievable gas turbocharger pressure ( maximum achievable boost pressure) will be too low relative to the desired value of the mean effective pressure ( desired mean effective pressure), this will lead to a significant limitation in performance ( significant derating). Even if it's enough high pressure supercharging, the possibility of reducing fuel consumption will be partially neutralized ( partially neutralized) due to too fast ( too rapidly) reducing the efficiency of the compressor and turbine ( compressor and turbine) gas turbocharger at high compression ratios ( high compression ratios). Thus, the practical use of the Miller cycle required the use of a gas turbocharger with a very high pressure compression ratio ( very high compressor pressure ratios) And high efficiency at high compression ratios ( excellent efficiency at high pressure ratios).

Rice. 6. Two-stage turbocharging system

So in the high-speed 32FX engines of the company " Niigata Engineering» maximum pressure combustion P max and temperature in the combustion chamber ( combustion chamber) are maintained at a reduced normal level ( normal level). But at the same time, the average effective pressure is increased ( brake mean effective pressure) and reduced the level of harmful NOx emissions ( reduce NOx emissions).

Niigata's 6L32FX diesel engine chooses the first Miller cycle option: premature intake valve closing timing 10 degrees before BDC (BDC), instead of 35 degrees after BDC ( after BDC) like the 6L32CX engine. Since the filling time is reduced, at normal boost pressure ( normal boost pressure) a smaller volume of fresh air charge enters the cylinder ( air volume is reduced). Accordingly, the process of fuel combustion in the cylinder worsens and, as a result, the output power decreases and the temperature of the exhaust gases increases ( exhaust temperature rises).

To obtain the same specified output power ( targeted output) it is necessary to increase the volume of air with a reduced time of its entry into the cylinder. To do this, increase the boost pressure ( increase the boost pressure).

At the same time, a single-stage gas turbocharging system ( single-stage turbocharging) cannot provide higher boost pressure ( higher boost pressure).

Therefore, a two-stage system was developed ( two-stage system) gas turbocharging, in which low and high pressure turbochargers ( low pressure and high pressure turbochargers) are arranged sequentially ( connected in series) in sequence. After each turbocharger, two air intercoolers are installed ( intervening air coolers).

The introduction of the Miller cycle together with a two-stage gas turbocharging system made it possible to increase the power factor to 38.2 (average effective pressure - 3.09 MPa, average speed piston - 12.4 m/s) at 110% load ( maximum load-claimed). This is the best result achieved for engines with a piston diameter of 32 cm.

In addition, in parallel, a 20% reduction in NOx emissions was achieved ( NOx emission level) up to 5.8 g/kWh with the IMO requirements being 11.2 g/kWh. Fuel consumption ( Fuel consumption) was slightly increased when operating at low loads ( low loads) work. However, with average and high loads (higher loads) fuel consumption decreased by 75%.

Thus, Engine efficiency Atkinson is increased due to a mechanical decrease in time (the piston moves up faster than down) of the compression stroke relative to the power stroke (expansion stroke). In the Miller cycle compression stroke in relation to the working stroke reduced or increased by the intake process . At the same time, the speed of the piston moving up and down is kept the same (as in the classic Otto-Diesel engine).

At the same boost pressure, charging the cylinder with fresh air is reduced due to a decrease in time ( reduced by suitable timing) opening the intake valve ( inlet valve). Therefore, a fresh charge of air ( charge air) in the turbocharger is compressed ( compressed) to a higher boost pressure than required for the engine cycle ( engine cycle). Thus, by increasing the boost pressure with a reduced opening time of the intake valve, the same portion of fresh air enters the cylinder. In this case, a fresh air charge, passing through a relatively narrow inlet flow area, expands (throttle effect) in the cylinders ( cylinders) and is cooled accordingly ( consequent cooling).

The Miller cycle was proposed in 1947 by American engineer Ralph Miller as a way of combining 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 at the expense of the intake stroke, keeping the piston's up and down motion the same. speed (as in the classic Otto engine).

Thus, the mixture in the Miller engine is compressed less than it would be compressed 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 detonation properties of the fuel - bringing the actual compression to acceptable values due to the above-described “shortening of the compression cycle”. In other words, for the same actual compression ratio (limited by fuel), the Miller engine has a significantly higher expansion ratio than the Otto engine. This makes it possible to more fully utilize the energy of the gases expanding in the cylinder, which, in fact, increases the thermal efficiency of the motor, ensures high engine efficiency, and so on.

Computer control of the valves allows you to change the degree of filling of the cylinder during operation. This makes it possible to squeeze out of the engine maximum power, when economic indicators deteriorate, or achieve better efficiency while reducing power.

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

An internal combustion engine is very far from ideal, at best it reaches 20 - 25%, a diesel engine 40 - 50% (that is, the rest of the fuel is burned almost empty). To increase efficiency (increase the coefficient accordingly useful action) the motor design needs to be improved. Many engineers are working on this, to this day, but the first were only a few engineers, such as Nikolaus August OTTO, James ATKINSON and Ralph Miller. Everyone made certain changes and tried to make the engines more economical and productive. Each proposed a specific cycle of work, which could differ radically from the opponent’s design. Today I will try in simple words, explain to you what the main differences are in the operation of internal combustion engines, and of course the video version at the end...

The article will be written for beginners, so if you are an experienced engineer, you don’t have to read it, it is written for a general understanding of loops internal combustion engine operation.

I would also like to note that variations various designs a lot, the most famous ones that we can still know are the DIESEL, STIRLING, CARNO, ERICSONN cycles, etc. If you count the designs, there can be about 15 of them. And not all internal combustion engines, but, for example, the STIRLING external one.

But the most famous, which are still used in cars today, are OTTO, ATKINSON and MILLER. That's what we'll talk about.

In fact, this is an ordinary internal combustion heat engine with forced ignition of the combustible mixture (through a spark plug), which is now used in 60 - 65% of cars. YES - yes, the one you have under the hood works according to the OTTO cycle.

However, if you dig into history, the first principle of such an internal combustion engine was proposed in 1862 by the French engineer Alphonse BEAU DE ROCHE. But this was a theoretical principle of operation. OTTO in 1878 (16 years later) embodied this engine in metal (in practice) and patented this technology

Essentially it is a four-stroke engine, which is characterized by:

Inlet . Supply of fresh air-fuel mixture. The inlet valve opens.
Compression . The piston goes up, compressing this mixture. Both valves are closed
Working stroke . The spark plug ignites the compressed mixture, the ignited gases push the piston down
Exhaust gas removal . The piston goes up, pushing out the burnt gases. Opens Exhaust valve

I would like to note that the intake and exhaust valves operate in strict sequence - THE SAME at high and at low revs. That is, there is no change in performance at different speeds.

In his engine, OTTO was the first to use compression of the working mixture to raise the maximum temperature of the cycle. Which was carried out adiabatically (in simple words, without heat exchange with the external environment).

After the mixture was compressed, it was ignited by a spark plug, after which the process of heat removal began, which proceeded almost along an isochore (that is, at a constant volume of the engine cylinder).

Since OTTO patented its technology, its industrial use was not possible. To get around the patents, James Atkinson decided to modify the OTTO cycle in 1886. And he proposed his own type of operation of an internal combustion engine.

He proposed changing the ratio of stroke times, due to which the power stroke was increased by complicating the crank structure. It should be noted that the test copy he built was single-cylinder, and did not receive widespread due to the complexity of the design.

If we describe in a nutshell the operating principle of this internal combustion engine, it turns out:

All 4 strokes (injection, compression, power stroke, exhaust) occurred in one rotation of the crankshaft (OTTO has two rotations). Thanks to a complex system of levers that were attached next to the “crankshaft”.

In this design, it was possible to implement certain ratios of lever lengths. To put it in simple words, the piston stroke on the intake and exhaust strokes is LONGER than the piston stroke on the compression and power strokes.

What does this give? YES, the fact that you can “play” with the compression ratio (changing it) due to the ratio of the lengths of the levers, and not due to “throttle” of the intake! From this we deduce the advantage of the ACTISON cycle in terms of pumping losses

Such engines turned out to be quite efficient with high efficiency and low fuel consumption.

However negative points there were also many:

Complexity and cumbersome design
Low at low rpm
Poor throttle control, whether ()

There are persistent rumors that the ATKINSON principle was used in hybrid cars, in particular the TOYOTA company. However, this is a little untrue, only his principle was used there, but the design was used by another engineer, namely Miller. In their pure form, ATKINSON motors were more likely to be isolated rather than widespread.

Ralph Miller also decided to play with the compression ratio in 1947. That is, he will, as it were, continue the work of ATKINSON, but he did not take him complex engine(with levers), and a regular internal combustion engine is OTTO.

What did he suggest . He did not make the compression stroke mechanically shorter than the power stroke (as Atkinson suggested, his piston moves faster up than down). He came up with the idea of shortening the compression stroke at the expense of the intake stroke, keeping the up and down motion of the pistons the same (classic OTTO engine).

There were two ways to go:

Close the intake valves before the end of the intake stroke - this principle is called “Short intake”
Or close the intake valves later than the intake stroke - this option is called “Shortened compression”

Ultimately, both principles give the same thing - a decrease in the compression ratio of the working mixture relative to the geometric one! However, the degree of expansion is maintained, that is, the power stroke is maintained (as in the OTTO internal combustion engine), and the compression stroke seems to be shortened (as in the Atkinson internal combustion engine).

In simple words — the air-fuel mixture in MILLER is compressed much less than it should have been compressed in the same engine in OTTO. This allows you to increase the geometric degree of compression, and accordingly the physical degree of expansion. Much greater than is due to the detonation properties of the fuel (that is, gasoline cannot be compressed indefinitely, detonation will begin)! Thus, when the fuel ignites at TDC (or rather dead center), it has a much greater degree of expansion than the OTTO design. This makes it possible to use the energy of the gases expanding in the cylinder much more, which increases the thermal efficiency of the structure, which leads to high savings, elasticity, etc.

It is also worth considering that pumping losses are reduced during the compression stroke, that is, it is easier to compress fuel with MILLER and requires less energy.

Negative sides – this is a reduction in peak output power (especially at high speed) due to worse filling of the cylinders. To produce the same power as the OTTO (at high speeds), the engine had to be built larger (larger cylinders) and more massive.

On modern engines

So what's the difference?

The article turned out to be more complicated than I expected, but to summarize. THEN it turns out:

OTTO - this is the standard principle of a conventional engine that is now installed on most modern cars

ATKINSON - offered a more efficient internal combustion engine, by changing the compression ratio using a complex structure of levers that were connected to the crankshaft.

PROS - fuel economy, more flexible engine, less noise.

CONS – bulky and complex design, low torque at low speeds, poor throttle control

In its pure form it is now practically not used.

MILLER - suggested using a lower compression ratio in the cylinder, using late closing of the intake valve. The difference with ATKINSON is huge, because he did not use his design, but OTTO, but not in its pure form, but with a modified timing system.

It is assumed that the piston (on the compression stroke) goes with less resistance (pumping losses), and better geometrically compresses the air-fuel mixture (excluding its detonation), however, the degree of expansion (when ignited by a spark plug) remains almost the same as in the OTTO cycle .

PROS - fuel economy (especially at low speeds), elasticity of operation, low noise.

DISADVANTAGES – reduction in power at high speeds (due to worse cylinder filling).

It is worth noting that the MILLER principle is now used on some cars at low speeds. Allows you to adjust the intake and exhaust phases (expanding or narrowing them using

Before talking about the features of the Mazda Miller engine, I will note that it is not a five-stroke, but a four-stroke, like the Otto engine. The Miller motor is nothing more than an improved classic engine internal combustion. Structurally, these motors are almost identical. The difference lies in the valve timing. What distinguishes them is that the classic engine operates according to the cycle of the German engineer Nicholas Otto, and the Mazda Miller engine operates according to the cycle of the British engineer James Atkinson, although for some reason it is named after the American engineer Ralph Miller. The latter also created its own internal combustion engine operating cycle, but in terms of its efficiency it is inferior to the Atkinson cycle.

The attractiveness of the V-shaped “six” installed on the Xedos 9 model (Millenia or Eunos 800) is that with a displacement of 2.3 liters it produces 213 hp. and torque of 290 Nm, which is equivalent to the characteristics of 3-liter engines. At the same time, the fuel consumption of such a powerful engine is very low - on the highway 6.3 (!) l/100 km, in the city - 11.8 l/100 km, which corresponds to the performance of 1.8-2-liter engines. Not bad.

To understand the secret of the Miller motor, you should remember the operating principle of the familiar Otto four-stroke motor. The first stroke is the intake stroke. It begins after the intake valve opens when the piston is near top dead points (TDC). Moving down, the piston creates a vacuum in the cylinder, which helps suck air and fuel into them. At the same time, in low and medium engine speed modes, when the throttle valve is partially open, so-called pumping losses appear. Their essence is that due to the large vacuum in the intake manifold, the pistons have to work in pump mode, which consumes part of the engine power. In addition, this deteriorates the filling of the cylinders with fresh charge and, accordingly, increases fuel consumption and emissions harmful substances in atmosphere. When the piston reaches the bottom dead center(BDC), the intake valve closes. After this, the piston, moving upward, compresses the combustible mixture - a compression stroke occurs. Near TDC, the mixture is ignited, the pressure in the combustion chamber increases, the piston moves down - the power stroke. At BDC the exhaust valve opens. When the piston moves upward - the exhaust stroke - the exhaust gases remaining in the cylinders are pushed into the exhaust system.

It is worth noting that when the exhaust valve opens, the gases in the cylinders are still under pressure, so the release of this unused energy is called exhaust losses. The function of reducing noise was assigned to the exhaust system muffler.

To reduce the negative phenomena that arise when an engine operates with a classic valve timing scheme, in the Mazda Miller engine the valve timing was changed in accordance with the Atkinson cycle. The intake valve does not close near bottom dead center, but much later - when the crankshaft rotates 700 from BDC (in Ralph Miller's engine the valve closes the other way around - much earlier than the piston passes BDC). The Atkinson cycle gives whole line benefits. Firstly, pumping losses are reduced, since part of the mixture, when the piston moves upward, is pushed into the intake manifold, reducing the vacuum in it.

Secondly, the compression ratio changes. Theoretically, it remains the same, since the piston stroke and the volume of the combustion chamber do not change, but in fact, due to the delayed closing of the intake valve, it decreases from 10 to 8. And this already reduces the probability of occurrence detonation combustion fuel, which means there is no need to increase engine speed by switching to a lower gear when the load increases. The likelihood of detonation combustion is also reduced by the fact that the combustible mixture, pushed out of the cylinders when the piston moves upward until the valve closes, carries with it into the intake manifold some of the heat taken from the walls of the combustion chamber.

Thirdly, the relationship between the degrees of compression and expansion was disrupted, since due to the later closing of the intake valve, the duration of the compression stroke in relation to the duration of the expansion stroke, when the exhaust valve is open, was significantly reduced. The engine operates on the so-called high expansion ratio cycle, in which the energy of the exhaust gases is used over a longer period, i.e. with a reduction in output losses. This makes it possible to more fully utilize the energy of exhaust gases, which, in fact, ensures high engine efficiency.

To obtain high power and torque, which are necessary for the elite Mazda model, the Miller engine uses mechanical compressor Lysholm, installed in the camber of the cylinder block.

In addition to the 2.3-liter engine of the Xedos 9 car, the Atkinson cycle began to be used in lightly loaded engines hybrid installation car Toyota Prius. It differs from the Mazda one in that it does not have an air blower, and the compression ratio is high - 13.5.

READ ALSO ON THE SITE

So what's the difference?

Read also