![]() ![]() As long as this mechanical lockup remains fully engaged, there is no slip, which improves fuel economy (typically by around 4% at cruising speeds) and provides throttle response and engine braking comparable to a vehicle with a manual gearbox. All engine power flows through the mechanical clutch directly to the input shaft. When that connection is fully engaged, the fluid clutch’s torus members are effectively locked together - they must turn together at engine speed - and transmit no torque. There are several ways to arrange such a clutch, but the common object is to create a mechanical connection between the engine and the transmission input shaft (which otherwise is driven by the fluid clutch’s turbine(s)). One way to minimize coupling slip is to supplement the fluid clutch with a plate clutch that engages when the vehicle reaches cruising speed. This is why lifting off the throttle in a car with a fluid coupling or torque converter can feel almost like shifting into neutral. When coasting, the turbine overruns the impeller, which causes slippage, but relatively little braking effect. ![]() That isn’t true of a fluid clutch, whose torus members are free to rotate at different speeds. With a plate clutch, that effect is often quite pronounced, especially in a reduction gear, because of the mechanical connection between the engine flywheel and the transmission input shaft neither can overrun the other. When you coast in gear, the transmission attempts to drive the engine crankshaft, whose inertia causes a braking effect. Turbine speed tends to fall behind impeller speed any time load increases (for example, when going up a steep grade) and an increase in engine speed doesn’t immediately produce a corresponding increase in turbine speed.įluid clutches also don’t allow much engine braking. Second, the nonlinear relationship between impeller and turbine speed can be troublesome. First, that slip wastes some fuel - you’re “paying” for more engine revolutions (and more power) than reach the driveshaft. Hydraulic slippage is much less desirable at cruising speed. Also, as we explained in the sidebar of our article on GM’s other early automatic transmissions, torque converters utilize the speed difference between the driving and driven torus members to multiply engine torque. Hydraulic slippage also offers some advantages during acceleration: As with a plate clutch, a certain amount of slip makes for a much smoother takeoff. The speed difference between impeller and turbine is simply lost to heat within the operating fluid.Īt idle and very low road speeds, slippage is desirable because it keeps the engine from stalling or lugging in gear. ![]() Unlike a reduction gear, this speed reduction doesn’t multiply engine torque. For example, if engine speed is 2,500 rpm, the turbine might only turn 2,375 rpm: hydraulic slippage of 5%. However, even at coupling stage, the turbine still turns somewhat slower than the impeller. Once the vehicle is moving at a constant speed, the speed difference between the impeller and turbine diminishes, eventually reaching a minimum point known as coupling stage. When starting, the fluid clutch’s driving torus (the impeller) may reach a speed of 2,000 rpm or more before the driven torus (the turbine) begins to move at all, a point known as stall. When the vehicle is at rest, the fluid clutch slips enough to allow the engine to idle with the transmission in gear without stalling. If the engine isn’t free to turn faster than the driveshaft at rest or extremely low speeds, the engine will stall!īy contrast, a fluid clutch always slips at least a little. This is why a disc clutch must be disengaged or the transmission shifted to neutral whenever the vehicle stops. Once the clutch plate is fully engaged against the pressure plate, both must turn together at the same speed. A healthy plate clutch slips only briefly when engaged or disengaged. One of the fundamental differences between a fluid clutch (a fluid coupling or torque converter) and a mechanical plate clutch is slippage. In this installment of Ate Up With Motor, we take a look at how GM, Ford, Chrysler, Packard, and Studebaker have approached this slippery problem from 1949 through the late eighties. Since the 1940s, automakers have come up with a variety of strategies for reducing or eliminating that slip, including series parallel “split torque” transmissions and different types of converter lockup clutches. Fluid clutches - fluid couplings and torque converters - have many advantages for automotive transmissions, but with those benefits comes a cost: fuel-wasting hydraulic slippage even at cruising speed. ![]()
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