To help it get from A to B a car’s engine sucks in air and some fuel, combusting the two to generate rotating motion, which is then transferred to the wheels via a transmission.
Not that long ago choosing between transmissions was easy: manual or automatic. Those who prioritised value, fuel economy, performance or driving pleasure went for manuals, while everyone else chose automatics. Simple.
Now, though, there are Lineartronics, DSGs, X-Tronics and tonnes of other marketing names to choose between. What are they? Are they any different to the automatics and manuals of yore? And what are their benefits?
Internal combustion engines can’t be directly connected to a car's wheels as engines typically rotate a lot faster. For instance, petrol motors generally operate between 600 and 6,500rpm, while a car’s wheels usually rotate between zero and 1,500 times per minute.
By introducing a gear ratio, the engine’s output speed can be reduced to match that of the wheels. Mind you, having just one gear ratio is rather limiting. Imagine a single gear petrol car that's been engineered for a top speed of 120km/h. In effect its solitary gear ratio would be the equivalent of third gear in most of today's cars.
If you’ve ever tried accelerating from standstill in third gear you’ll know what a torturously slow and painful exercise it is. Not only that, but when the car finally hits 110km/h, the engine will be screaming at around 5,500rpm. Neither is great on one’s patience, ear drums nor the longevity of the car.
Multiple gear ratio transmissions, then, allow cars to strike an acceptable balance between acceleration, top speed and fuel economy.
This is the simplest and lightest type of transmission available, as gear selection and gear changing is solely the domain of the driver.
The gears contained inside the transmission's metal housing — typically five or six forward gears, although the Porsche 911 and Chevrolet Corvette both sport seven — are accessed via a shifter in the cabin.
To change gears, the driver depresses the clutch pedal, which detaches the clutch from the engine. This allows the engine to spin at one speed, and the transmission and wheels at another. After selecting a new gear ratio via the stick shift, the driver connects the engine, transmission and wheels together again by releasing the clutch pedal.
Two pieces of tech have made it easier to shift gears over the years: synchromeshed gears and hill start assistance.
Synchromesh gears have not only removed much of the gear grinding that used to plague manuals, but has also eliminated the need to double declutch, where drivers had to declutch once to enter neutral and declutch again to select the next gear. Hill start assist is a more recent development that tries to prevent manual cars from rolling backwards downhill during standing starts. It does this by keeping the brakes active for a few seconds after the brake pedal is released, giving the driver a bit of grace to operate the clutch and accelerator.
For the greater part of automotive history, manual transmissions have rewarded drivers' efforts with more control, more gears, faster acceleration, higher top speeds, lower fuel consumption and, if driven well, smoother gear shifts. But with the computer-led evolution of automotive transmissions, some cars are reporting better performance and fuel economy figures with automated transmissions.
This is good news, especially in Australia and the USA, where the vast majority of cars are sold without a clutch pedal.
While enthusiasts revel in mastering the art of perfectly executed shifts and heel-toe manoeuvres, for many the act of declutching, selecting a gear and clutching is a burdensome chore. So, why not stick some hydraulics or servos onto a manual gearbox to operate the clutch for us?
Numerous car makers have dabbled with clutch-pedal-free manual transmissions over the years but, despite the simplicity of the concept, it's an idea has still yet to be executed well in road-going cars.
Automated manuals go back as far as the iconic Citroen DS, and more recent attempts have come from Alfa Romeo (Selespeed), Audi (R-Tronic), BMW (Sequential Manual Gearbox), Citroen (Servotronic), Ferrari, Lexus, Maserati, Peugeot and Smart (pictured above).
Jerky gear changes are the main problem with automated manual transmissions. In semi-automatic mode, where the driver changes gears via a +/- gate or paddles behind the steering wheel, this can be partially alleviated by lifting off the throttle when changing gears. As the driver is unable to finesse the clutch, it usually only serves to minimise the amount of lurching.
Many systems also offer a fully automatic mode, but these are plagued with either brutally aggressive or overly slurred gear shifts. Again lifting off the throttle during gear changes helps, but it's often less taxing to shift gears yourself rather than playing clairvoyant to the transmission's electronic brain.
Given the failings we've noted above with automated manual transmissions, it's easy to see why traditional automatics differ quite markedly from their manual brethren.
By having a torque converter, which permanently connects the engine and the transmission, automatics avoid having to disengage and re-engage a clutch whenever gears are changed or the car comes to a stop.
Whereas a clutch physically connects engine and transmission, a torque converter makes an indirect link through a bath of transmission fluid. Whenever the engine's running it spins this oil-like hydraulic liquid, and if the car isn't stopped with the brakes engaged, the fluid then spins the transmission's impeller that in time turns the wheels.
Because the engine spins the driveshaft indirectly, a certain amount of power (generally less than 10 percent) is lost. Modern autos compensate for this with a lock-up clutch that engages at cruising speeds and effectively transforms the entire torque converter assembly into one giant clutch.
While a manual gearbox has a physical gear for each of its gear ratios, automatics have a much more complex series of interconnected planetary gearsets. Each gearset is comprised sun, planet and ring gears, and any of these smaller gears can be held still, spun by the engine or left to rotate freely. The transmission's gear ratios are produced by altering the parts in each gearset that are fixed, powered or free to spin.
The elements of each planetary gearset are manipulated via a intricate chain of hydraulics and small clutches. In the past the logic behind when to change and hold gears was controlled by another hydraulic system. Improvements in computing technology have allowed modern autos to move to electronic control, allowing for smoother and more logical gear shifts.
Although automatics are still heavier and more expensive to purchase or repair, nowadays they give up little ground to manual transmissions in terms of performance, efficiency and gear count — five- and six-speed transmissions are the norm, and seven-, eight- and nine-speed models are becoming increasingly common.
In the late 1990s “Tiptronic transmissions" became all the rage. They supposedly combined the best of both automatic (look ma, no clutch pedal!) and manual transmissions (driver selectable gears). In reality, Tiptronics weren't really a different type of transmission, but rather an automatic gearbox that allowed for easy gear selection via a +/- gate and, sometimes, paddles or buttons around the steering wheel.
Once relatively rare, Tiptronic features are now available in most automatic vehicles.
Although dual clutch transmissions (DCTs) are generally employed as replacements for traditional automatic gearboxes, their mechanical workings actually bear more in common with the humble manual transmission. In fact it's probably easiest to think of a DCT as housing two manual transmissions — one for even numbered gears and the other for odd numbered ones — each with their own clutch (hence the name).
Say, for example, you’re accelerating along in third gear. In this situation the even numbered gearbox will have fourth gear pre-selected and primed to go. When the transmission’s computer or the driver thinks the time is right to change up, the clutch for the odd numbered gears is disengaged and the even clutch engaged.
Changing from one clutch to another takes anywhere between eight and 200 milliseconds; that's considerably faster than the half second or more required by most manual drivers to change gears. By wasting less time between gears, DCTs are often able to outsprint their manual equivalents.
Some transmissions, particularly those from the Volkswagen Group (VW, Audi, Skoda, et al), have lightning quick gear changes, which are a delight to see in action. The downside with fast-shifting DCTs is that at lower speeds they tend to jerk and lurch around. In tight parking spaces, it can be a little frightening to suddenly lunge forward, even if it's just a few centimetres, when you're feathering the throttle with the greatest of care.
There are two types of dual-clutch transmission on the market: dry and wet clutch. Wet clutch models are so called because the clutch is bathed in a sea of oil and this type is often found in high-power cars. Dry clutch versions are more efficient, but restricted in the amount of power and torque they can handle.
As with many new technologies, most car makers have decided to market DCTs under their own trademarked brand names: DSG (Volkswagen, Skoda, Seat), EcoShift (Hyundai), PDK (Porsche), PowerShift (Ford, Volvo), S-Tronic (Audi), SpeedShift (Mercedes-Benz), and TC-SST (Mitsubishi). For some companies, most notably Volkswagen and Ford, DCTs are available on mainstream vehicles. For others, like Nissan, BMW and Mitsubishi, usage is restricted to high-performance models.
Continuously variable transmission
Internal combustion engines deliver their maximum power and torque over a narrow rev range. For example, non-turbocharged petrol motors typically deliver peak power around 5500rpm and maximum torque at 4000rpm.
With the transmission systems we've detailed above, the engine is often operating outside of its sweet spot for either power, torque or efficiency. A continuously variable transmission (CVT) seeks to overcome this by offering an infinite number of gear ratios between a transmission's upper and lower ratio limit.
Most CVTs feature two pulleys connected via a V-shaped belt: one pulley is driven by the engine, and the other is connected to the wheels. To change gear ratios the transmission manipulates the ride height of the belt across the pulleys.
Unlike cars with other transmissions, giving a CVT car a little bit more gas doesn't necessarily cause engine revs to rise unless you're really flooring it.
For undemanding drivers CVTs are both elegantly simple and efficient as the engine is almost always operating at maximum efficiency. For drivers who harbour dreams of becoming a Senna-like deity, CVTs aren't really much fun. Instead of the (hopefully) pleasant sound of an engine surging up and down through the rev range, CVTs holds the engine at high revs, commonly resulting in a dull drone when you're pressing on.
All CVTs have a driver selectable low ratio or engine braking mode for steep hills, while some go a step further in offering a number of artificial "gears" or fixed gear ratios for drivers to cycle through.