Auto Tech: Clutches and new transmissions

By Jim Kerr

I recently drove a Porsche 911 with a six-speed manual transmission. It made me realize how much I enjoy driving manual transmissions, even though the Porsche PDK seven-speed automatic (a computer-controlled manual transmission) will provide better performance. Most drivers prefer to drive without a clutch pedal, however, and it is seldom that auto manufacturers will sell more than about ten per cent maximum of any model optioned with a manual transmission. I understand this, as driving in stop and go traffic can be tiring on the clutch pedal leg, and automatic transmissions are usually much smoother on initial launch and can even provide better fuel economy.

Many people think a manual transmission is more efficient, and gets better fuel economy than an automatic. This is sometimes true, but you have to take the gear ratios into consideration. Manual transmission equipped vehicles often have lower final drive ratios so the vehicles are easier to get moving from a stop. This means that the engine is revving faster on the highway, hence lower fuel economy. Automatic transmissions use a torque converter, which can multiply the engine torque almost twice, so the final drive ratio can be higher. Lower engine rpms at highway speeds provides better fuel economy, as long as the engine rpm and torque are matched to the vehicle rolling resistance.

In recent years, we have seen a new style of automatic transmission – one that combines the efficiency of a manual gearbox with the convenience of an automatic. Volkswagen refers to it as a DSG automatic; Porsche calls it PDK; BMW has named theirs DCT (Double Clutch Transmission). These transmissions have two input shafts, one inside the other, and two clutches to engage the transmission. One transmission input shaft is connected to the odd gear ratios (1st, 3rd, 5th etc.), while the other input shaft is connected to the even gears (2nd, 4th, 6th). By engaging one of the clutches, the car starts out in first gear. The computer has already pre-selected 2nd gear on the other input shaft and all it has to do is release the 1st gear clutch and apply the other clutch for 2nd gear. This continues, with the computer selecting 3rd gear and changing clutches again to engage 3rd, and so on. Shifts are extremely fast, much faster than even a racer could do with a manual gearbox.

These transmissions use multi-plate wet (they run in oil) clutches, similar to what you would find inside an automatic transmission. An oil pump in the transmission is used to lubricate, cool and apply hydraulic pressure for clutch engagement. Now Ford has a new automatic transmission, with a computer that uses two electric motors to shift the gears, but instead of a multi-plate wet clutch it uses two single disc dry clutches.

Ford calls it the PowerShift transmission and it is a fully automatic system offered on the compact Ford Fiesta. This “intelligent” mechanical gearbox doesn’t use an oil pump, making it more efficient and less complex. Compared to a wet clutch setup, it can cut vehicle weight by more than 13 kilograms.

Gear selection by the computer is similar to other units. The dry clutch assembly is truly innovative. An electric motor is used to engage the clutch, just like drivers have done with the clutch pedal in the past. The faster you step on the gas, the faster the clutch is engaged – all automatically. When it comes time to shift, the electric motor moves the clutch mechanism to release the one clutch disc and apply the other. The description sounds similar to the wet clutch operation, but the mechanism is much different. The two clutch discs are mounted in a compact housing, with one clutch disc on each side of the apply plate. By moving the electric motor and linkage one way, the clutch disc closest to the transmission is engaged. Moving the electric motor the other way engages the other disc closest to the engine. It is a simple push-pull method of engaging two clutch discs, one at a time.

The computer controls the slip speed of the clutch during engagement to maximize performance, fuel economy and clutch life, but if ever necessary, the compact clutch assembly is as easy to replace as a clutch on a conventional manual transmission. The convenience and comfort of automatic shifting with the performance and economy of a manual transmission is now available in economy cars too.