Manual, automatic and CVT fuel efficiency
You've probably noticed when reading car fuel efficiency stats that manuals are always a bit more economical than auto versions of the same car.The reason for this is a manual gearbox allows a direct mechanical link to be maintained between the engine and wheels through the use of cogs, a metal chain and a clutch. First gear is the largest cog and top gear is the smallest cog. The clutch on manual transmissions is able to uncouple the engine from the cogs allowing gear selection and stopping of the car. You can vary the amount of torque applied by the engine by varying the degree of clutch engagement. The mechanical coupling of the engine to the gears in this way means that manual transmission is very fuel efficient.
Automatics on the other hand use a fluid filled torque converter to provide power to the wheels. A torque converter is a fluid coupling which hydraulically connects the engine to the gearset. Auto's have a worse fuel efficiency than manuals mainly due to pumping losses in both the torque converter and the hydraulic actuators. In addition the hydraulic control system requires power to run which necessitates the engine to work a little harder. In a manual your leg provides this power when you press the clutch pedal! Some automatics have an economy mode which saves fuel by upshifting at lower engine revs and sometimes by changing the throttle mapping to be less responsive.
Continuously variable transmission (CVT), which is becoming more commonplace, allows a direct mechanical link between engine and wheels but requires no gear changing by the driver. It does this by using a single cone shaped gear. At low speeds the larger end of the cone is utilized by the engine and at high speeds the smaller end of the cone is used. With such transmission you have an unlimited number of gears and no requirement for a clutch. In fact with true CVT you don't have preset gears as such since the gearing ratio is constantly changing as you drive. Having said that many CVT cars produced today come with a number of defined gearing points which when used with gear selectors give the feel of a clutchless manual.
Since CVT allows a direct link between engine and wheels its fuel efficiency is much better than that of an auto. CVT goes further than this though because it also has an advantage over manual transmission which gives it better fuel efficiency than manual cars. With manuals the engine speed (revs) for a given gear is determined by how fast the car is traveling. Not so for cars using true CVT who can vary their gear ratio in order to keep the engine revs at their most efficient level.
Continuously variable transmission is not a new concept. It was first used in cars in the 1950's but due to the materials around at the time its use was limited to engines of 100 horsepower or less. CVT technology was largely left neglected until around 10 years ago when improved, stronger CVT systems became more widespread. New materials mean't they were able to reliably handle more power. Manufacturers using CVT systems today include Nissan, Honda, Audi and Subaru.
So of all the transmission systems in use today, CVT has the potential for greatest fuel efficiency, followed by manual gearboxes, and lastly auto's.
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