![]() The result is a smoother, more efficient and quicker drive than an normal automatic. 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. This rev range is typically either the one that delivers the most efficiency, most power, or most torque. What this means for the driver is that the engine can go directly to the best rev range for a given situation. There’s a few different ways of achieving variation in pulley sizes, but the fundamental principle is always the same. ![]() In effect, you’re changing gear all the time. But as the car gathers speed, the engine’s pulley is smoothly reduced in size, exactly as the other pulley increases in size. So to start off with, the engine’s pulley will be small and the other one large, just like first gear. The magic of CVT is that the size of the pulleys can change from small to large, and infinite sizes in between. The engine turns one pulley, and the other one is connected to the rest of transmission to the wheels. Instead of two gears, there’s two pulleys connected by a belt. This design does away with gears entirely. Why is CVT better?Įnter CVT, or continuously variable transmission. There’s all sorts of technologies to help engines work over rev ranges such as variable valve timing, but nevertheless the fundamental problem remains. 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. The wider rev range the engine needs to work over, the more compromises the designers have to make. The reason this is less than ideal is because engines are best optimised to run only in a narrow rev range. So the standard automatic is somewhat compromised because the engine has to pick from only a few gear ratios. And automakers also go to a lot of trouble to smooth out the gearchanges as well as make them faster. But it’s always a compromise, even with nine speeds. ![]() 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. 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. 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. 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. 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.Īt higher speed it’s the opposite. It’s like using a low gear on your pushbike. For every revolution of the small gear, the bigger gear only turns a fraction. At low speed, the engine turns a small gear which is connected to a larger gear that turns the wheels via other transmission components. How an automatic gearbox worksĪ normal automatic gearbox uses gears to match engine speed to road speed. This article 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. The most obvious application for a CVT is when you’re trying to ensure maximum fuel efficiency, hence why Subaru has paired Boxer engines and all-wheel drive (all-wheel drive adds weight and can affect fuel consumption driving up the relative fuel consumption of a Subaru when compared with a non-AWD SUV in the same segment) with a CVT. ![]() Rather, the CVT allows the vehicle’s engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. 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).
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