Variable torque is described as an engine's ability to direct power on a limited basis when needed. The engine senses a load and directs more power to assist in the movement of that load. Conversely, it is also signals the engine to remove power when too much power is detected. This is most often found in an automobile's drive line system. In an automobile, the power is directed to a wheel when the other wheel is slipping to prevent the vehicle from becoming stuck.
The automatic transmission in a typical family sedan is in essence a variable torque supplier. At lower speeds, the torque is amplified by the transmission's lower gear. As the vehicle speeds up, the amount of torque required to move the vehicle is reduced so the transmission shifts to a higher gear. Vehicle speed increases as torque decreases.
The amount of torque required to keep an object in motion is less than the amount of torque required to set the object in motion. A variable torque engine is able to adjust the amount of torque that is being applied to the object. With variable torque, the engine saves fuel and operates more efficiently.
Hybrid automobiles use a variable torque system to decide which power plant to use. When the engine senses a great deal of load, such as starting to move an object, the variable torque sensor switches the vehicle to gasoline power. Once the load sensors read a drop in required torque, the electric battery engine switches on. This method of variable torque engine saves gasoline as well as provides a cleaner emission from the tail pipe.
Variable torque drive systems have been utilized in snowmobiles and all-terrain vehicles (ATVs) for decades. The vehicle's torque converter drives a clutch mechanism by way of a belt. As the vehicle's speed increases, the belt moves up on the torque converter while it drops in the clutch. This effectively changes the gear ratio in which the motor is operating as well as varies the amount of torque it is producing.
Aircraft have the ability to vary torque by adjusting the pitch of the propeller blades. The blades are adjusted for maximum torque at take off and then feathered out for speed once the aircraft reaches its cruising altitude. Variable torque is also used for slowing or stopping a vehicle. The compression brake, or Jake Brake as it is known on a semi truck, uses the engine's compression and torque to slow the vehicle and save the brake shoes.