When you get behind the wheel and shift gears, have you ever wondered what's happening beneath the surface of your vehicle? Transmissions are a pivotal component in automotive engineering, allowing your car to shift smoothly and efficiently through various speeds. In this blog, we'll delve into the inner workings of transmissions, demystifying the mechanical magic that enables your vehicle to glide down the road. For cars with “Standardantrieb”, BMW uses 6 and 8-Speed automatic gearboxes from ZF (Zahnradfabrik Friedrichshafen) and the 7-Speed DCT transmission built by Getrag. “Standardantrieb” refers to the classic BMW layout with a longitudinally, front-mounted engine, paired with Rear-Wheel-Drive or All-Wheel-Drive. For their FWD platforms (Mini and alike) BMW also uses other suppliers like Aisin.
The Purpose of a Transmission
At its core, the transmission is a torque converter. Through its gears, it transforms a given input torque from the engine to a higher or lower level. A typical conversion ratio in 1st gear (short gear) for the standard BMW 8-Speed Auto is 4.7:1 which means the input torque gets multiplied by 4.7 before it reaches the output flange. This happens at the expense of speed, as the engine’s redline will be already reached at around 35 mph in 1st Gear. In 8th Gear (long gear), the ratio is much lower, so your vehicle can reach much higher speeds but at the expense of a lower output torque. Without a transmission, your car would either accelerate quickly or reach a high top speed, but not both.
Manual vs. Automatic: Two Different Worlds
Manual Transmission
In manual transmissions, drivers are actively engaged in the gear-shifting process. When the clutch pedal is depressed, it disengages the engine from the transmission. The driver then selects the appropriate gear using a gear lever and releases the clutch pedal, re-engaging the engine. This mechanical connection lets the driver control the gear changes and the power delivered to the wheels.
Automatic Transmission
Automatic transmissions, on the other hand, manage gear shifts without driver intervention. The transmission's intricate network of gears, clutches, and electro-hydraulic systems automatically selects the right gear for any driving condition or lets the driver change gears by himself with the pull of a paddle. Automatic Transmissions have come a long way and nowadays are far faster and responsive, compared to what any human would be able to achieve with a manual transmission. From a technological standpoint, there is no reason to use manual transmissions anymore, with the only real downside of auto transmissions being the cost of development and manufacturing. Even the weight nowadays comes close to manuals..
How Gear Shifting Works
Whether manual or automatic, gear shifting involves the manipulation of gear ratios; different gears are used for various driving conditions. Lower gears (1st and 2nd) provide more power but lower speed, making them suitable for accelerating from a standstill. Higher gears (3rd, 4th, etc.) allow for higher speed and fuel efficiency but lower power output.
In an automatic transmission shifting always happens by opening a certain clutch, while at the same time closing another. This is true for standard automatics, as well as for Dual-Clutch-Transmissions (DCT).
As the name suggests, DCT’s have 2 clutches, but each has a separate part-transmission behind them. One clutch can engage even gears (2-4-6) while the other engages the odd gears (1-3-5-7). For carrying out a certain shift, the transmission just opens one clutch, while closing the other. This automatically engages the pre-selected gear in one of the two part-transmissions.
In a standard auto, there are multiple planetary gear sets with usually five clutches to connect or disconnect a certain gear set. The big advantage of the standard auto is the flexibility in gear selection during normal road driving. The ZF 8-Speed can directly jump from 8th to 2nd Gear, as easily and quick as it can shift from 8th to 7th gear. It’s the same operation, just a different pair of clutches gets opened/closed. A DCT at this point, would first need to pre-select the gear in the corresponding part-transmission and afterwards carry out the opening/closing of its clutches to actually carry out the shift. Depending on the current and the target gear, this may even involve shifting through intermediate gears to reach the final gear. This is the reason why DCT transmissions sometimes can feel a bit sluggish in response, when quickly flooring the throttle during cruising.
Torque Converter Transmission (AT)
Torque Converter Autos are the most common type of automatic transmission and have existed since the early 1940s. Although the operating principle has remained unchanged since then, torque converter autos have come a long way and nowadays combine many of the advantages of other transmission types while overcoming the downsides.
A classic AT uses a hydraulic fluid coupling (the “Torque Converter”) as a take-off element, together with multiple planetary gear sets and clutches. Gears are shifted by opening and closing the clutches, so different combinations of gear sets are used. This creates the gear ratios, converting the Engine's RPM to various speeds at the wheels. In the past, such transmissions were often associated with slow gear shifts and a very sluggish driving experience.
However, with continuous improvements by manufacturers and the use of modern technology in the TCU (Transmission Control Unit), they are now the preferred choice of BMW and many other brands for their complete vehicle line-up. From an economy-focused 520d to the most recent M3 Touring, all BMW now use classic torque converter autos, which are just adjusted by software for those very different vehicles.
Dual Clutch Transmission (DCT)
A DCT is basically an automated manual transmission. It utilizes two separate clutches, one for even gears and another for odd gears. While driving, one clutch is engaged in the current gear while the other pre-selects the next gear. This setup allows for lightning-fast and smooth gear changes.
- Driving Experience: DCTs are known for providing a sportier and more engaging driving experience. They offer rapid gear changes, often with paddle shifters. Because of their quick and precise shifts while maintaining a relatively low weight, they are often preferred in high-performance and sports cars. DCT’s are also easier to adapt to high-power applications, due to the simpler power-flow inside the transmission.
- Fuel Efficiency: DCTs are typically more efficient than traditional automatic transmissions because they deliver power more directly. However, with the advancement of standard AT transmissions, this difference has become negligible.
- Application: DCTs are commonly used in high-performance and sports cars where quick gear changes and a sporty driving feel are desired. BMW used them in their M-Cars for some time (March 2008 - 2019) but switched over to classic ATs for their whole lineup afterward.
The Heart of Your Vehicle
Transmissions are your vehicle's unsung heroes, working tirelessly to ensure that power is delivered precisely when needed. Whether you prefer the tactile engagement of a manual transmission or the effortless convenience of an automatic, the transmission is an integral part of the automotive experience. Understanding how transmissions work allows you to appreciate the intricate mechanics as you cruise down the road, making your journey a true mechanical marvel.
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