Traditional Culture Encyclopedia - Traditional virtues - Interpretation of the engine (V) Atkinson and Miller cycle

Interpretation of the engine (V) Atkinson and Miller cycle

The form of operation of this type of engine How efficiently the energy produced by the fuel is utilized is the key to improving engine efficiency. This topic has been going on since the dawn of the engine. From the Atkinson cycle at the end of the 19th century to the Miller cycle in the 1940s, an "alternative" operating mechanism broke the original compression ratio constant.

Once again, the topic is "compression ratio". If you want to increase power, increase compression ratio is a means. If you want to increase power, increasing the compression ratio is one way to do that. But compression ratios cannot be increased indefinitely, and in the "ancient" days of engine history, this problem was even more difficult to solve. However, human ingenuity often finds another way. Since the compression ratio cannot be increased, the "expansion rate" should be increased.

● Atkinson cycle engine

In 1882, James Atkinson invented an engine. Unlike the Otto cycle engines of the time, this engine had a different piston displacement between the compression stroke and the power stroke. The Atkinson engine used complex connecting rods as the power output from the piston to the crankshaft, and the actual stroke of the piston is shown in the diagram below.

The piston strokes are represented by blue, yellow, red, and green color blocks for the four strokes of intake, compression, power, and exhaust

The design is ingenious. Different connecting rod mechanism **** the same role, so that each stroke range is different, not only effectively improve the intake and exhaust conditions, but also the biggest feature of the Atkinson engine is the expansion ratio is greater than the compression ratio. The longer expansion stroke allows for more efficient utilization of the high pressure still present in the exhaust gas after combustion, resulting in higher fuel efficiency than the Otto cycle.

The introduction of connecting rods not only affects the piston stroke, but also changes the torque acting on the crankshaft.

But the complex connecting rods are inferior to the Otto engine in size and failure conditions, so they have not yet been popularized in automobiles. But marine, power generation and other large diesel engines largely borrowed this feature of the Atkinson engine can be tolerated. As for the Atkinson engine exists intake valve late closing whether it means that the compression ratio is less than the expansion ratio of the problem, there is no evidence, but the real use of this technology is the back of the engine.

● Miller cycle engine

In 1940, Miller regained this unequal expansion/compression ratio engine, but abandoned the complex connecting rod structure and used valve timing to create this effect. The solution was to delay closing the valves at the end of the suction stroke, which would "spit out" some of the inhaled mixture, and then close the valves to start the compression stroke.

The top graph shows valve timing for a conventional Alto cycle engine, the bottom graph shows valve timing for a Miller cycle"

"In the Miller cycle, the intake valves are closed later, with some of the intake gases being expelled in the reverse direction, and the exhaust valves are opened later, allowing for longer operating times, compared to a conventional engine. "

Simply controlling the opening and closing times of the valves produces an expansion ratio greater than the compression ratio. The energy contained in the exhaust gases can be utilized compared to a conventional Otto cycle engine.

1-2-3-4 are PV diagrams for a conventional engine and 6-2-3-5 are PV diagrams for the Atkinson/Miller cycle. The shaded area can be interpreted as the Atkinson/Miller extra piston stroke and its energy utilization.

Practical applications of this type of engine

● Defects of this type of engine

Many readers will realize that with variable intake timing, this technology is very easy to implement, but why is this technology not widely used in engines? The reasons are as follows:

◆ 1. The unique intake pattern makes low-speed torque very poor. At low speeds, the already thin air-fuel mixture "backflows" and becomes less, making the low-speed torque performance of these engines very poor. Obviously, there is not enough power to get the car started. No one wants their car to lose at the starting line, and manufacturers don't want their products to lag behind.

◆ 2. Long piston strokes are not conducive to high speed operation. Longer piston strokes do make better use of fuel energy and improve economy, but they also limit the speed increase, make acceleration performance worse, and the performance index of "power increase" will be very low. However, the ratio of stroke to piston diameter is usually very low for racing engines that seek performance, especially at high speeds. In civilian vehicles, the stroke and bore figures are usually close to each other for balance.

This puts the Atkinson / Miller cycle engine in a very awkward position, can only be effective in the middle of the speed stage of the power, for every day road conditions in the form of complex urban traffic is very unfavorable to the car, so the ordinary car will not use this technology. But there are many special cars.

● Modern Atkinson / Miller cycle engines

In reality, there are very few Atkinson cycle engines on the market today. The Toyota Prius (Check Deal Price | Model Details) claims to use an Atkinson engine, but in terms of actual construction, it's still essentially a Miller cycle approach. That's because in 1993, Mazda reacquired the Miller cycle engine and equipped it in production cars. To avoid more trouble, Toyota had to say it was the Atkinson cycle.

However, the two companies took two different ideas. Mazda used the Miller cycle engine to reduce detonation and increase power by doing so, so it was equipped with a supercharger to further increase power. The Toyota Prius aims to save fuel and take full advantage of the substantial benefits of the Atkinson/Miller cycle engine.

Uses a mechanically supercharged Miller cycle Mazda 2 .3S engine"

"This engine is in the Mazda Millenia"

The Atkinson/Miller cycle engine is favored by a wide range of fuel-efficient hybrids for its full use of energy. vehicles for their energy-efficient use. They don't care about low-speed "shutdowns" and high-speed "shutdowns" because the motor powers the wheels during both periods and the engine generates electricity most of the time, so the engine can run at the speed that gets the best gas mileage. The high torque of the electric motor makes up for the power shortcomings, and the complementary powertrain gives hybrid cars outstanding performance in terms of power and economy.

However, it may not be appropriate to build a high-power engine through the Atkinson/Miller cycle. Mazda's engine was not developed even for mass production, and the power increase was essentially the effect of supercharging, not the original intent of the cycle. As a result, the Atkinson/Miller cycle is used more for hybrids, where fuel conservation is its job.

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