If you’ve ever tried to clean the bugs and leaves from your radiator using a garden hose and amused yourself by spinning the fan with the stream of water, you understand how a torque converter works.
A torque converter is the coupling between your car’s engine and automatic transmission. Its job is similar to that of the clutch in a car with a manual transmission. But instead of a mechanical connection like the clutch makes, the torque converter makes a fluid connection.
Before we muddy the waters with a description of how a torque converter multiplies torque, let’s look back a few decades to the early fluid couplings. In the 1960s, we used to amaze our college buddies by sitting at red lights with the clutch pedal released on our 1950 Dodge Coronet.
The car had a Fluid Drive that still had a mechanical clutch as well a fluid coupling. We used the mechanical clutch when shifting gears, but when stopped, we could put the transmission in low, release the clutch and hold the brakes without stalling the engine.
The fluid coupling was a two-piece affair that resembled a sliced bagel, with one half attached to the engine’s crankshaft and the other attached to the clutch assembly. It resembled the chambers of a nautilus’ shell with the vanes sawed in half. In other words, it looked like bagel halves with the bread scooped out and replaced with lots of vanes.
The fluid coupling was partially filled with hydraulic oil and when the half that was connected to the crankshaft spun, the fluid would sling against the vanes of the half attached to the clutch assembly. This half of the coupling would then spin from the force of the fluid. The fluid would transfer the torque–the turning force.
You could create a simple fluid coupling by placing one household fan in front of another. Turn one on and the fluid (or in this case, air) from the running fan will spin the blades of the other fan.
The down side of fluid couplings is that they transfer quite a bit less torque than the engine develops, particularly at slow speeds. That old Dodge would crawl off the line when the light turned green and we measured its zero-to-60 times with a calendar.
The biggest problem we had with our car’s fluid coupling was its tendency to lose fluid, but Dodge conveniently provided a fill hole on its housing and an access place in the passenger floor, for replenishment.
The torque converter also is a fluid coupling, but it has an additional component inside the two halves that multiplies the torque delivered to the driven half.
There are three major parts to a torque converter:
– The impeller
– The turbine
– The stator
The impeller is attached to the engine. The turbine is splined to the transmission input shaft. So far, this is a kind of fluid coupling. But, remember that torque transfer is lame at low engine speeds–the fluid is not being flung with much force from the impeller to the turbine.
So, engineers devised a way to fling the fluid off the turbine, redirecting it at the stator between the impeller and turbine. The stator then flings the fluid leaving the turbine back at the impeller, multiplying the torque and providing that neck-snapping acceleration when you punch the pedal.
As the car gains speed, the impeller and turbine begin to spin at nearly the same speed, and the need for torque multiplication diminishes. As this happens, one-way clutches in the stator’s hub allow it to freewheel and the fluid passes from the turbine directly to the impeller. Because the turbine never catches up to the impeller’s speed, not all of the torque the engine generates makes it to the transmission.
To grab this last bit of torque, and increase fuel economy, the lockup torque converter was invented. This is a mechanical connection using clutches between the converter halves that gets them spinning at the exact same speed. When the lockup occurs, you may notice your engine running slower to attain the same road speed. As an added benefit, less friction is created in the transmission fluid, which lets it run cooler providing improved transmission life.
Early lockup converters relied on hydraulic pressure supplied by the automatic transmission to apply the clutches. More recently, the computer commands lock up via electric solenoids.
Torque converters are quite reliable, but they occasionally have problems such as noise, stator failure, imbalance, converter ballooning or hub wear. Noise is usually from worn components, particularly needle bearings, in the converter. You usually hear this when you are sitting still, and the noise goes away when you put the transmission in neutral.
The stator can fail by being constantly engaged or constantly freewheeling.
If the stator stays engaged, the car will have poor highway performance. Sometimes you may notice a lack of performance just before the transmission shifts gears. If the stator won’t release, the transmission fluid will get very hot, leading to premature transmission failure. The engine also may overheat.
If your car lacks power on acceleration, the stator may be freewheeling instead of multiplying the torque. Performance at highway speeds, however, will be normal.
Most torque converters have balance weights welded to them, and you usually will feel a vibration from the engine at certain engine speeds if something happens to cause an imbalance.
Sometimes, excessive internal pressure can cause the torque converter to balloon. Poor performance and noise are clues.
Hub wear usually results in a leak. You may see the red fluid dripping from the bell housing that surrounds the torque converter.
A lockup converter can be intermittent, causing a shudder while cruising, but the most common problem is a lockup converter that won’t disengage as you slow to a stop when you leave a highway. This causes the engine to stall. But the symptoms mysteriously go away after you restart the engine and may return only the next time you spend some time on the highway.
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Bob Weber is an ASE-certified Master Automobile Technician, having recertified every five years since 1978. Contact him at MMTribune@netscape.net.
