Two of your colleagues recently sounded off on the issue of CVT versus automatic transmissions, noting that they preferred automatic gear boxes - especially VW two-shaft systems - both for driving pleasure and gas mileage. Can you explain why this is their preference, as that wasn't clear in their columns. - Aubrey in Ottawa
Add me to that list. CVTs (continuously variable transmissions) are commonly used in motor scooters, snowmobiles, lawn tractors, etc.
Enthusiasts don't like them for two reasons. The primary one is the sound of the engine as it is constantly held at its power peak under acceleration - very high in the rev range - while a belt and pulley system continuously alters gear ratios. It sounds like the clutch in a manual transmission is slipping. This issue is hardly noticed under part throttle where the CVT is programmed to keep the engine within a very narrow and much lower rev range where fuel mileage, rather than power is optimized.
Reason number two is the lack of gear changes, once again more significant to someone who enjoys changing gears. To address these complaints, some companies provide a "sport" mode that programs the CVTs to simulate shifts.
While enthusiasts hate them, CVTs are incredibly efficient in terms of fuel mileage because they keep the engine within its most efficient operating range. A conventional automatic transmission is much less efficient because it uses a fluid coupling called a torque converter in place of a clutch to transfer power from the engine, which is always turning, to the gears.
This prevents the engine from stalling when the vehicle is stopped when in gear, but creates an inefficient connection between engine and drive wheels. This is addressed in the higher gears by "locking up" the converter and passing power directly from engine to drive system.
The DSG (Dual Shaft Gearbox) used by VW and many other manufacturers takes advantage of modern electronics. It has a pair of shafts, each containing different gear sets.
For example: one shaft may have first, third and fifth gears and the other second, fourth and sixth. Each shaft has its own clutch. When first gear is engaged second gear is "preselected" and when the electronic "shift" signal arrives, the clutch on one shaft disengages while the other engages. Third gear is then "preselected" on the first shaft in preparation for the next shift. These shifts are lightning-quick and the DSG commonly provides both improved fuel economy and performance. With its "real" shifts, it allows enthusiasts to change gears at will - or leave it in fully automatic mode.
The DSG is also lighter and less expensive to produce. Like eight-track versus cassette or Apple versus PC, there are two schools of thought.
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