What's behind the recent shift to CVT and automatic gearboxes?

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At one time, it boiled down to a choice between an automatic transmission and a manual gearbox. Of late, the type of transmission found in any given vehicle has started to, well, shift. Here’s a brief overview of each and how they differ.

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While the manual transmission has long been popular, it is rapidly becoming the modern-day Dodo bird. Its benefits are obvious — the driver is in complete control and, as such, it makes for a more engaging drive. The drawbacks, however, include a steep initial learning curve; the fact it is less fuel-efficient than other types of transmission; and how it can be a pain in stop-and-go traffic — the perpetual left-leg workout loses its shine very quickly.

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What's behind the recent shift to CVT and automatic gearboxes? Back to video

A popular replacement is the automated manual, or dual-clutch transmission (DCT) , like the one pictured at top. It is a manual gearbox with real gears, but it has no clutch pedal. An internal brain operates the two separate clutches — hence the “DCT” name — and the switching of the gears is automated. The result is a slick and refined unit that’s found favour in sports cars. However, it was not always this way. The single dry-clutch units used in the likes of the original Audi R8 and BMW M5 were dreadful. The uncoordinated shifts and stinky burnt clutch smell killed them.

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For a nanosecond, the Dodge Dart was offered with a dry-plate DCT. It used a fan to blow air into the clutch housing and down a pipe that exhausted at the back-end of the car. It was an unsuccessful attempt at hiding the smelly clutches! Today, the better units typically run the clutch packs in an oil-bath, so the burning issue has pretty much disappeared.

The reason for the efficiency is simple — the DCT has a purely mechanical link between the engine and drive wheels, and no power-sapping torque convertor. In a seven-speed DCT, one clutch controls reverse, first, third, fifth, and seventh gears; the other handles second, fourth, and sixth. This allows the transmission to pre-select the next gear, so when a gear change is initiated, it is instantaneous. The speed of the shift is so fast, only Lewis Hamilton might be able to switch gears faster.

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The set-up also lends itself particularly well to the use of paddle shifters to control the shifts. As the engine stays at full bore between shifts, the flow of power is uninterrupted. This maximizes performance without forsaking fuel economy. Per a joint U.S. Department of Transportation and National Highway Traffic Safety Administration report, the upside of a modern dual-clutch transmission is a fuel economy improvement of between 7.5 and 13.5 per cent, depending on the vehicle type.

The continuously variable transmission (CVT) became a popular choice a number of years ago because its operating strategy maximizes efficiency and fuel economy. Typically, these transmissions are found in smaller cars with lower-output engines. Rather than using gears, there are two pulleys linked by a steel belt or chain — one pulley is driven by the engine; the other drives the wheels. By changing the diameter of these two pulleys in unison, the effective “gear” ratio changes. Where other transmissions have a fixed number of ratios (six, seven, eight, nine, or 10) the number of ratios between what would be “first” and “top” gear in a CVT is infinite.

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The key operating difference is that unlike other transmissions, where the engine works for the transmission (the engine revs drop with each shift and then climb until the next shift and so on, so it is a stepped affair) the CVT holds the engine at its optimum operating point and changes the ratios to allow the vehicle to accelerate. In other words, it is working for the engine.

While the CVT, in its purest form, is the most efficient transmission, it is not without its drawbacks. First, the initial take-off is lethargic because the design can only deal with a limited amount of drive torque. Toyota has addressed this by using a real first gear for take-off before shifting to normal CVT operation. Second, there is little in the way of engine braking when the gas pedal is lifted.

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Finally, hard accelerations sees the CVT peg the engine in upper reaches of the rev-range and hold it there until the desired speed is attained or the driver lifts to kill the monotonous racket. This trait is generally referred to as “motorboating.”

The nuisance factor has led many manufacturers to introduce six or seven pre-set shift points that are designed to mimic the feel and sound of an automatic or dual-clutch transmission. While this works to quell the racket, it kills a big part of the reason it is so efficient — the engine is now working for the transmission in the same manner as any other type of transmission. The EPA says this limits the overall fuel economy advantage to three or four per cent when compared to other transmissions.

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In its infancy, the automatic transmission had two or three speeds. Now it’s often eight, nine, or 10. The key is the greater the number of gears (or speeds) means the engine’s rpm drop between each shift is significantly less, which brings it closer to the CVT in terms of operation. As such, the engine spends more of its operating life in its most fuel-efficient sweet spot. More importantly, increasing the number of gears brings a much wider spread between first and top gear. This means better initial acceleration, a stronger mid-range, and yet there are still enough gears left to have two or three highway overdrives.

In terms of the driving fun, the modern automatic is almost as good as a dual-clutch. The difference is there’s a slight reduction in engine power when the shift occurs, which makes the shift speed slightly slower. Again, the fuel economy improvements cited in the U.S. Department of Transportation and National Highway Traffic Safety Administration report says an eight-, nine-, or ten-speed transmission improves the fuel economy of a mid-sized car by 6.9 per cent; by 8.5 per cent for a small crossover; and by up to 11 per cent for a small car. In short, the more gears in a transmission the better, although at 10-speeds the point of diminishing returns is fast approaching!