Design features of automatic transmissions

The number of cars with automatic transmissions is steadily increasing. The reason is in their constant improvement, which causes changes in the minds of car owners. At first, automatic transmissions were perceived as exotic, then they were treated with irony, considering that such a car is intended only for those who are not “able to drive”. Nowadays, to have a car with automatic transmission is prestige, in fact, they are almost as good as the “mechanics” in operational characteristics, and surpass it in comfort and convenience. The modern automatic transmission is no stranger to a sporty driving style, thanks to the emergence of sequential manual shift mode. Currently, there are several varieties of automatic transmissions: the classical automatic transmission with a torque converter, a robotized manual transmission and a continuously variable variator.

The design of the “classics” is based on a planetary gear consisting of three elements: a sun gear, a crown gear, and a driver with satellites. They can rotate freely relative to each other. During operation, one of the elements is stationary, the second is the master and the third is the slave. This changes the direction of rotation and the ratio of the planetary series. By varying the number of planetary rows and the way they are connected, you get a different number of stages. The torque converter acts as the clutch. The shifts are “managed” by a hydraulic control unit. The first serial automatic machine appeared on the American cars as early as 1939. For more than seventy years, the design has been significantly improved, bringing the maximum number of ranges up to eight. The automatic transmission has its own electronic control unit and torque converter lockup, allowing a rigid connection between the engine and the drive wheels. The rigid connection provides more efficient acceleration and the ability to brake the engine. Manual range-shifting is now available. Compared to the three-range automatic transmissions of 20-30 years ago, today’s eight-range are 20% more fuel efficient and are virtually the same as manual transmissions in terms of fuel consumption.

Another thing is that such automatic transmissions are still relatively expensive, and on budget models car manufacturers usually put four-band devices. In the city such an automatic transmission is still convenient, but on the highway it is not economical and dynamic enough. “The golden mean” – six-band transmissions, representing the best combination of price and performance. Drivers, who have switched to automatic transmissions for the first time, will have to re-learn some driving techniques. There are only two pedals, so the left foot always rests. During parking the lever of automatic transmission must be in position “P” (parking), so the car will not spontaneously roll. The selector can lead to very unpleasant consequences by accidentally catching the selector while the car is moving. Some designs do not require the brake pedal to shift from “neutral” to “drive”, so the car will start moving with the engine running. So, the advantages of automatic transmission include: high smoothness of work (read – comfort for driver and passengers); possibility of transmission of big torque; economy on a freeway, like on an automatic transmission (except for four-range automatic transmissions). And the disadvantages? The torque converter is quite voracious in city mode – consumption can increase from 15 to 30%. In service and especially in repair, a modern automatic transmission is quite expensive, even with minor malfunctions.

The next type of automatic transmission is the dual-clutch robot box. It came to mass production from motor racing. The idea is to combine two independent, but coordinated, transmissions in one body. Each has its own clutch. One engages the odd-numbered gears and the other engages the even-numbered gears. When the car accelerates, for example in third gear, the gears of the fourth are already in mesh, but the “even” clutch is open. At the right moment, the “odd” clutch opens and the “black” clutch closes. The shifting process takes milliseconds. Similarly, the “down” shifting takes place. For the first time such a miracle-box was used on Porsche cars in 1986. After a couple of decades, thanks to advances in electronics, the gearbox with two clutches appeared on the road cars. Volkswagen became the “pioneer”. The rest started following it, having estimated the advantages of the novelty. Volkswagen is producing two variants of such clutches – with dry single-disc clutches and multi-disc clutches in oil bath. The first option is seven-speed. It’s more economical, easier to service and cheaper. The “wet” clutch variant has six clutches. It is more comfortable and capable of transmitting more torque. The two-clutch box is the most successful compromise. It is more economical than the hydraulic automatic, more comfortable than the mechanics, and can transmit more torque than the variators. However, in terms of consumption it loses to a manual transmission, and in terms of comfort and transmitted torque it loses to an automatic. It is something in between, which has disadvantages of both types as well as advantages!

The third type of automatic transmission is a variator. From the point of theoretical mechanics it is an ideal type of transmission. It has no steps and the gear ratio changes smoothly and always exactly matches the load on the engine. The design of the variator is almost primitive – the torque is transmitted by a chain or metal belt sandwiched between halves of the pulleys, driving and driven. The force from the engine to the drive pulley is transmitted through a torque converter or multi-disc clutch. The reverse is engaged via an additional planetary gear. But putting the idea into practice encountered some technical problems. The chain has to be clamped with a very high force to avoid slippage, which requires extra engine power and affects fuel consumption. Variator oil needs to be special and not cheap in order to ensure reliable operation. In order to make the chains and belts last long enough, expensive technology had to be developed. In short, the initial enthusiasm gradually faded, and now only the Japanese remain ardent adherents of this type of transmission, as their quiet driving style corresponds perfectly to the variator. Dynamic driving reduces life of chains and belts. Besides during sharp acceleration the engine immediately reaches high revolutions, creating unpleasant “noise” effect.