In this great article originally featured in PracticalMotoring.com, Robert Pepper uses plain English to explain how a CVT works, why manufacturers are using them and how driving a car with one takes a little getting used to:
In a nutshell, a continuously variable transmission, or CVT, doesn’t offer fixed gear ratios like a either a manual gearbox, automatic transmission or a DSG (Direct Sequential Gearbox, a newer type of auto). Rather, the CVT allows the vehicle’s engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds.
A normal automatic gearbox uses gears to match engine speed to road speed. At low speed, the engine turns a small gear which is connected to a larger gear that turns the wheels via other transmission components. For every revolution of the small gear, the bigger gear only turns a fraction. It’s like using a low gear on your pushbike. And when you drive a car in first (lowest) gear, you’ll know that you can rev the engine quite high and the car doesn’t go very fast – this gearing is the reason.
At higher speed it’s the opposite. Now the engine turns a large gear connected to a small gear, so for every rotation of the larger gear the smaller gear turns multiple times. You’ll know from driving that in top gear your engine only needs to do say 3000rpm for 100km/h – whereas 3000rpm in first gear would see you only at about 25km/h.
In between the first and top gears are other gear ratios, and these days there’s usually a total of six but some newer automatics have as many as nine. The combinations of gears are carefully chosen so the engine can remain in its best rev range for either power, torque (turning force) or efficiency. But it’s always a compromise, even with nine speeds. And automakers also go to a lot of trouble to smooth out the gearchanges as well as make them faster.
So the standard automatic is somewhat compromised because the engine has to pick from only a few gear ratios. The reason this is less than ideal is because engines are best optimised to run only in a narrow rev range. The wider rev range the engine needs to work over, the more compromises the designers have to make. It’s like anything else that’s multi-purpose; you buy a camera lens that can do 18mm to 300mm and it’ll never be as good as a two lenses that just do 18 and 300mm respectively. There’s all sorts of technologies to help engines work over rev ranges such as variable valve timing, but nevertheless the fundamental problem remains.
Enter CVT, or continuously variable transmission. This design does away with gears entirely. Instead of two gears, there’s two pulleys connected by a belt. The engine turns one pulley, and the other one is connected to the rest of transmission to the wheels. The magic of CVT is that the size of the pulleys can change from small to large, and infinite sizes in between.
So to start off with, the engine’s pulley will be small and the other one large, just like first gear. But as the car gathers speed, the engine’s pulley is smoothly reduced in size, exactly as the other pulley increases in size. In effect, you’re changing gear all the time. There’s a few different ways of achieving variation in pulley sizes, but the fundamental principle is always the same.
What this means for the driver is that the engine can go directly to the best rev range for a given situation. This rev range is typically either the one that delivers the most efficiency, most power, or most torque. Once in its happy rev range, the engine just holds its revs and the gears “change” as the relative sizes of the two pulleys change. The result is a smoother, more efficient and quicker drive than an normal automatic. There is a downside though, and that’s the energy losses due to friction are greater with a CVT – about 5 to 15% depending on the type, whereas with normal gears it’s more like 2-5%. As ever, development is closing the gap, and the fact the CVT allows the engine to remain in its optimum rev changes more than compensates for the extra efficiency loss.
So everybody should be happy, right?
CVTs sometimes feel awful; like a rubber band is stretching, and many drivers think there’s something wrong with the car. This is because when you drive off the engine will jump directly to a rev point and just stay there, while the car accelerates. This feels and sounds like the clutch is slipping in the car, or an automatic transmission gone wrong. It’s a bit like going out in a boat with a small outboard engine, you rev the outboard and the revs stay the same while the boat gathers speed. You also don’t get that immediacy of throttle control if you increase or decrease the revs, and that’s sometimes unpopular with drivers. In short, nobody likes the CVT experience, initially at least, because it jars horribly with everything they’re used to.
So manufacturers sidestep the problem by artificially creating “gears” in their CVTs. Simply, these are pre-set points where the designers decide that the two pulleys will be of certain relative sizes, just like normal gears. They then set the CVT to fix those pulley sizes, and when the engine runs out of revs it “changes gear” to the next set of relative pulley sizes.
Isn't this just destroying the point of a CVT though? Well, yes it is, but not entirely. Firstly, those “gears” are nothing more than software settings in a computer specifying the relative sizes of the two pulleys, not physically interlocking gears. So the “gear ratios” can be changed at any time, a bit easier to do than physically changing the lumps of metal called gears. Subaru have done this gear trick with the WRX, which in economy mode has six speeds, and in performance mode, eight speeds. It’s also how Toyota claim the CVT Corolla has seven speeds. It kind of does, but they could just as easily have put 16 speeds in there but nobody would take it seriously – it’s amazing, but CVTs are a good example of where marketing trumps good solid logical engineering.
Anyway, we need to examine why Subaru have chosen, apparently illogically, six speeds for efficiency and eight for performance. Why not eight for each? The answer lines within the heart of the CVT system. When in efficiency mode, the CVT in the WRX doesn’t precisely match an increase in revs to an increase in speed. You can see the vehicle change speed and the revs stay the same, within each of the six ratios. This is a small but important fuel efficiency measure. It’s not really noticeable unless you’re looking for it – because otherwise drivers would be upset – and of course it’d be better to have no gears and keep the engine in just one rev range, but we discussed why that’s not done.
In performance mode Subaru have locked the CVT out – any change in revs means a change in speed. The relative size of the pulleys changes only in the eight predefined ratios (“gears”). The effect is a more direct connection between throttle pedal and speed, and the sensation of revs rising exactly as speed rises – this is what performance and sports-minded drivers look for, although it’s ironic that the effect they desire is actually slower and less efficient.
CVT transmissions also cannot handle as much power as conventional transmissions, although this is slowly changing. This is one reason you don’t see them on high-performance cars or large trucks. CVTs are often found in small cars and SUVs. Small industrial vehicles such as light tractors often use them, and you see them on snowmobiles too.
A CVT car will be, on balance, more fuel efficient than a normal auto, particularly for stop/start traffic or when you’re constantly changing speed. It should be smoother and quieter too.