Filed under: MPG, Greenlings
Not so long ago, it was common for automatic transmissions to be referred to as slushboxes, since that's how they often behaved. Rather than use a mechanical clutch, traditional automatic transmissions use a fluid coupling between the engine and the gear-sets to transmit drive torque. This provides some benefits, but isn't a perfect system.
Unless a mechanical clutch is worn out and slipping, it transfers nearly 100 percent of the torque that goes in. Automatics use a torque converter that consists of three main components: the pump, turbine and stator all within a cavity filled with hydraulic fluid. The pump is connected to the engine and, at lower speeds, it spins within the fluid without driving the turbine. As the engine speeds up, the slippage within the fluid increases and the turbine begins to rotate. This, in turn, drive the gears and the wheels. The stator increases the turbulence between the other two components providing a torque multiplication effect. There is generally no direct connection between the pump and turbine other than the fluid, which is why the efficiency is anywhere between zero and about 80 percent. All this removes a pedal, but it used to mean automatics got much worse mileage than manual transmission vehicles. Read on after the jump to find out what engineers have done to overcome this discrepancy.
Greenlings: Why do automatic transmissions now get better fuel efficiency than manuals? originally appeared on AutoblogGreen on Wed, 18 Aug 2010 19:55:00 EST. Please see our terms for use of feeds.
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