Here’s a primer on continuously variable transmissions

I read your column with interest each Saturday in a local newspaper and was wondering if you would be willing to comment on the pros and cons of continuously variable transmissions (CVT) and multi-geared transmissions.

Absolutely! So that everyone is on the same page, let's identify what a CVT transmission is. It's important to recognize that the purpose of any automotive transmission is twofold — transmit engine power to the drive wheels and to optimize vehicle performance at any and all speeds by keeping the engine in its most efficient RPM range as much as possible.

Multi-gear transmissions have historically been the transmission of choice for this purpose. Either manually — the driver selects each gear with a lever — or automatically selecting individual gear ratios based on road speed and load, these transmissions do a reasonably efficient job of keeping the engine in its "power band" at varying road speeds.

But, of course, road speed is infinitely variable, and these transmissions have only a fixed number of gear ratios. Remember the original GM Dynaflow and Powerglide transmissions? They had only two forward gears, coupled to a torque converter to multiply engine torque. Today's electronically controlled automatic transmissions feature up to eight forward speeds to help optimize efficiency and performance.

CVTs feature a drive belt riding between variable pulleys. The pulleys change diameter in proportion to speed and load by varying the distance between the inner and outer tapered "sheaves." When starting from a stop or under heavy load, the engine-driven pulley is smaller and the driveline pulley is proportionally larger so that engine RPM is higher, providing more torque to the drive pulley. As vehicle speed rises, the engine pulley gets larger in diameter while the driveline pulley gets proportionally smaller, meaning the engine is turning lower RPM as it drives the vehicle at a higher speed.

The fundamental advantage of CVTs is their ability to keep the engine in a very narrow, highly efficient RPM range to maximize performance, efficiency and fuel mileage.

Snowmobiles, ATVs and some motorcycles have had CVTs for decades, and they have been used in cars since at least the early 1960s. In fact, the very first vehicle I ever drove — in 1960 at age 11 on a long, looping private driveway — was a very small Dutch-built DAF with a belt-driven, variable-ratio CVT.

While engineers and carmakers have known the advantages of CVTs for decades, it wasn't until relatively recently that technology has provided a drive belt for them that could handle the much higher horsepower and torque of today's engines. The belts are no longer fabric and rubber; they are steel and provide the strength and durability necessary for modern automobiles.

Driving a CVT-equipped vehicle is a somewhat peculiar experience. Under acceleration, engine speed rapidly climbs into its optimum RPM range, which tends to create a sense of slippage although none is occurring, and the vehicle speed "catches up" to engine speed. When slowing, engine speed falls until re-accelerating when RPM jumps back up and vehicle speed starts "catching up" again. The engine, under load, is always operating in its most efficient RPM range, maximizing performance and fuel economy.

Paul Brand, author of "How to Repair Your Car," is an automotive troubleshooter, driving instructor and former race-car driver. Readers may write to him at: Star Tribune, 425 Portland Ave. S., Minneapolis, Minn., 55488 or via email at paulbrand@startribune.com. Please explain the problem in detail and include a daytime phone number. We cannot provide personal replies.

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