The automatic transmission is a critical component in today's search for improved fuel efficiency. In the search for better fuel mileage, engineers are concentrating on keeping an engine as close as possible to the narrow confines where it is most efficient. This has led to the growth in the number of gears from three a few decades ago to the six to eight common today – and as many as 10 only months away.
It is perhaps easier to understand this desire to keep an engine within a narrow rev range if you have ever driven a bicycle, or even watched someone do so. As the rider pushes down on the pedals, that energy is transferred by chain to turn the rear wheel. On a bike with no gears, the rider starts with slow, powerful leg movement, usually standing up to put more weight on the pedals. As speed increases, the rider's legs have to move faster as the rear wheel is directly connected to the pedals, which are in turn moving much faster. The speed of the bike is limited to how fast the rider can move his or her legs.
Enter the first bicycles with gears – usually three speeds. The speed of the pedals is no longer directly related to that of the rear wheel.
The rider could select a low gear that magnifies his or her effort. That low gear means less effort is required to get under way – but leg speed grows quickly with the speed of the bike. As the rider shifts into second, leg speed slows in relation to rear wheel or bike speed. In this case, it is much like an old bike with no gears. But as the bike picks up speed, the rider can shift into a higher gear which allows slower leg speed in relation to rear-wheel speed. Instead of pumping away furiously to maintain a decent speed, the rider could assume a more relaxed and less-taxing leg speed.
A hill coming up? Shift down a gear to allow more multiplication of leg stroke and torque. Bikes up to 10 or 12 speeds allow riders to maintain a relatively steady leg speed – where they are most comfortable – and thus efficient.
That same system works in cars. In an old two-speed automatic transmission, the engine would rev to say 4,000 rpm under acceleration and drop to 1,000 rpm after the shift. Three speeds came along quickly and instead of a 3,000 rpm spread it dropped to perhaps 2,500. (These numbers are used for explanatory purposes only.)
The idea is that more gears lower that spread in engine speed, allowing it to be kept within more narrow confines where everything from fuel delivery and valve activation to ignition timing can be fine-tuned.
More gears have always been the way to more performance – or efficiency. When automatic transmissions had two speeds, manuals had three. That grew to a common four-speed automatic and five- or six-speed manual. But with the vast majority of new vehicles being purchased or leased being automatics – more than 90 per cent – and advances in electronics and controls, automatics now commonly have more gears than manuals.
Within these developments has been the push to make the automatic transmission more efficient – to the stage where some new automatic transmissions allow faster acceleration and better fuel mileage than their manual counterparts.
Continuously variable transmissions (CVTs) are one answer as they keep the engine in a very narrow operating range and continually adjust ratios to suit. While efficient, the result is an unpleasant driving experience under acceleration.
Dual shaft gearboxes (DSG) are another approach. These pre-select gears and effect changes incredibly quickly through dry clutches. There are some drivability issue at very slow speeds but these are rapidly being overcome.
Both of the above alternatives have gained favour because the conventional automatic transmission is inherently less efficient because of the use of a torque converter. The very device that made automatics so popular is proving an issue when striving for every little spec of improved fuel mileage.
A torque converter is basically a fluid coupling that allows rotating energy from one source – the engine – to be transferred to another, the drive wheels. These two are joined in a manual transmission by a clutch. When the vehicle is stopped, the clutch is disengaged so the engine can keep running while the wheels are stopped. The torque converter operates by using automatic transmission fluid (ATF) to "connect" the engine and drive shaft or wheels. When the vehicle is stopped and the brake applied, the torque converter permits a high degree of slippage so the engine can keep turning.
But, once some throttle is applied, the difference in rotational speeds between input from the engine and output to the drive wheels and centrifugal forces link the two and allow the key characteristic of a torque converter – the ability to multiply torque when there is a substantial difference between input and output speeds – to work. It is common to "lock" a torque converter in high gear to eliminate the very slippage that makes the unit work, but detracts from its efficiency.