Which gearbox is the best?

Car fans have always had different opinions about this question: Which transmission is the best? Purists and nostalgics also swear by the “authenticity” of a “hand-made” car, because automatics are for lazy people who can’t drive. But more and more choose an automatic transmission when buying a car. Today you usually have the choice between a torque converter or a dual-clutch transmission. Why is automatic becoming more popular? What are the advantages and what are the differences between the types of transmission? We will clarify and you will find out everything you need to know about gearboxes.

The hand switch

Most people learn to drive in driving school with a manual transmission. Left foot on the clutch, right foot on the accelerator and brake, shifting by hand. Gear changes work by disengaging the gear you are in and engaging the new one while you have the clutch depressed and then releasing it slowly, especially when starting off. These instructions should still be familiar to all driver’s license holders. Many former learner drivers will look back and have already asked themselves back then: Isn’t there an easier way? The car manufacturers thought so too, which is why they developed the automatic transmission, which relieves the driver of changing gears. Nevertheless, manual transmissions still have some advantages over other types of transmissions:

Advantages

• Simple mechanical construction, therefore inexpensive repairs
• Very cheap due to large quantities

Disadvantages

• Demands a relatively large amount of work from the driver
• Engine may stall/over-rev
• Incorrect shifting can cause major damage to the clutch/motor
• Loss of traction during shifting

The first automatic transmissions

ASG: Automated manual transmission. That was the name of the transmission of the first cars that the driver no longer had to shift himself. The technical basis was manual transmissions, which were converted by control units in such a way that they themselves selected the correct gears. The driver only had to determine the “speed level” himself. Every automatic driver knows them to this day: P for parking, N for neutral, R for reverse and D for normal driving ahead – including gear changes. Many manufacturers came up with their own names for their own ASG. The most well-known are probably the SMG (BMW in M ​​models) and the ASG transmissions in particularly fuel-efficient small cars such as the Audi A2 Tdi 3L, which advertised a consumption of only three liters per 100 kilometers. These automatic transmissions had some advantages and a few disadvantages:

Advantages

• Compact and space-saving design
• Consumes energy only during switching operations
• No more stalling / switching / over-revving possible
• Transmission can be tuned to be particularly efficient / sporty / gentle on materials
• Design is based on manual switches, therefore similar structure, similar components and low costs

Disadvantages

• Loss of traction during shifting
• Relatively long switching time

The dual clutch transmission

Today there are actually only two types of transmission in widespread use: dual clutch and torque converter transmissions. The dual clutch transmission originally comes from rallying and was first developed by Porsche. However, Volkswagen succeeded much more quickly in developing it to the point where it was ready for large-scale production. Today, most manufacturers have their own designations for their dual-clutch transmissions: DSG (Volkswagen), PDK (Porsche), DKG (BMW), DCT (Mercedes-Benz) and so on. Audi calls its S tronic and it is considered robust and technically mature. It can be found, for example, in used Audi A3s, which are very popular because of the driving characteristics of the S tronic. The technical structure is easy to explain, the technical implementation is much more difficult. Two clutches are used, one of which is responsible for the “even” gears 2,4,6 and the other for 1,3,5 and – if available – 7. Reverse gear can usually be assigned to both. One gear is always engaged and the next higher “preselected”: This means that the gear is engaged but the clutch is not connected to the engine. If a shift command is received, the current gear is not taken out, but simply switched to the other clutch and thus changed to the higher gear. This change happens within fractions of a second. The transmission then pre-engages the next gear and the cycle begins again. Double-clutch transmissions also have their individual characteristics:

Advantages

• No interruption of tractive power when changing gears
• Very fast switching operations
• Suitable for racing due to the high switching speed
• Good efficiency
• Pleasant design for vehicles with engines installed transversely in the front of the vehicle
• Several common parts with manual transmissions, hence low cost
• The disadvantage compared to the ASG is the often permanent energy requirement to keep the clutch closed in the power branch
• No stalling / switching / overspeeding possible
• Transmission can be tuned to be particularly efficient / sporty / gentle on materials

Disadvantages

• No shift jumps possible, gears must be shifted through one after the other
• Permanent energy requirement to keep the clutch closed

The automatic converter

It is considered by many to be the epitome of carefree gliding: the automatic torque converter and its incredibly smooth gear changes. The way it works has nothing to do with a manual transmission. To put it simply, a converter has two wheels that do not touch each other in a housing. This housing is filled with a liquid (usually oil). One of the two wheels is connected to the engine, the other to the rest of the drive train. When the engine rotates, wheel 1 (pump wheel) also rotates and sets the oil in motion. This movement is picked up by wheel 2 (turbine wheel) and passed on to the drive train.

In modern cars, a frictional connection can be generated between the impeller and turbine wheel, which enormously increases the efficiency of the automatic converter. This is precisely the main problem: Due to the low mechanical load on the transmission parts, the automatic converter is considered to be very robust, but it has poor energy efficiency. In addition, it does not have such a spontaneous response as a dual clutch transmission, which makes it uninteresting for racing. The individual gears are then switched through via multi-plate clutches, brakes and freewheels. Here is an overview of the advantages and disadvantages of torque converter transmissions:

Advantages

• No interruption in traction
• Very smooth, stepless gear changes
• Reduced vibrations in the drive train
• Almost wear-free starting
• Compact design
• Very robust and material-friendly

Disadvantages

• Poor efficiency
• Cannot be towed with the drive axle rolling due to lack of lubrication in the gearbox
• High costs due to strict tolerances in production
• High weight, resulting in consumption disadvantages compared to DSG

That was an overview of all currently common types of transmission in comparison. Of course, there are other variants such as the continuously variable transmission or electric cars that no longer have a transmission. In addition, new developments are constantly being added, such as the two-speed transmission of the all-electric Porsche Taycan. As far as the automobile is concerned, we are in very exciting times! In any case, this comparison is a solid overview of the established state of the art.

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