FORD’S SPLIT TORQUE AUTOMATICS
During this period, Ford also revived the split torque concept, first for the AOD, Ford’s first four-speed overdrive automatic, and then for the ATX, the company’s first automatic transaxle for front-wheel-drive applications.
AUTOMATIC OVERDRIVE TRANSMISSION (AOD)
The Automatic Overdrive (AOD) transmission, introduced for some full-size FoMoCo cars in the 1980 model year, was derived from Ford’s older three-speed FMX automatic. It used a Ravigneaux compound gearset with three long and three short planet pinions on a common planet carrier, two sun gears (one with 36 teeth, the other with 30), and a single annulus (with 72 teeth) affixed to the output shaft.
As in the FMX, the torque converter turbine drove the smaller rear sun gear in first, second, and third gears. Two one-way clutches (supplemented in Low by a brake band) could alternately hold the carrier or the larger front sun gear, providing two forward reduction ratios. Where the AOD parted ways with the FMX was in the direct drive third gear. Instead of locking the front sun gear to the input shaft in high, as the FMX did, the AOD used an additional direct clutch to lock the planet carrier to a central shaft that passed through the main input shaft and was driven directly by the engine.
In third, engine torque was split between the rear sun gear, which rotated at turbine speed, and the planet carrier, which rotated at engine speed. Because the carrier always rotated faster than the rear sun gear, the long planets overdrove the larger front sun gear. This forced the short planet pinions to resolve the speed difference and drive the annulus forward at a speed slower than the carrier, but faster than the rear sun gear, demultiplying hydraulic slippage by 58.3%. (We’ll spare you the math, which with Ravigneaux gearsets is very cumbersome.) For example, if the engine was turning 2,000 rpm and converter slippage was 100 rpm, the annulus would rotate at 1,958.3 rpm, reducing net hydraulic slip from 5% at the turbine to 2.1% at the output shaft.
The AOD’s new overdrive fourth gear was created by releasing the forward clutch — thereby disconnecting the rear sun gear from the main input shaft and the turbine — while engaging a brake band to hold the front sun gear stationary. With the direct clutch still engaged, the carrier, rotating at engine speed, overdrove the rear sun gear, forcing the annulus to rotate at 1.5 times engine speed — an ratio of 0.67:1. The torque converter turbine continued to rotate in fourth, but was no longer connected to the planetary gears, so there was no hydraulic slippage. This lockup wasn’t available in any of the lower gears.
Ford took a different approach with the ATX transaxle, which became optional on the new FWD (Mk3) Ford Escort and Mercury Lynx for 1981 and the Ford Tempo/Mercury Topaz for 1984. Unlike the AOD and the earlier GM split torque transmissions, the early ATX used a separate planetary gearset specifically for torque-splitting purposes.
Known in Ford parlance as a “splitter gear,” the additional gearset was located within the torque converter torus housing, between the turbine and the flex plate. It was a simple planetary gearset with a single annulus (with 78 teeth) and three planet pinions surrounding a sun gear with 48 teeth. The flex plate and torus housing drove the splitter unit annulus at engine speed while the transmission input shaft, driven by the turbine, drove the splitter sun gear.
As we’ve explained in the preceding pages, this arrangement served to demultiply hydraulic slippage in the torque converter. Since annulus speed (VA) was engine speed while sun gear speed (VS) was turbine speed, the speed of the splitter gearset’s planet carrier (VC) was therefore:
VC = VS + ((VA – VS) / (1 + sun gear teeth / annulus teeth))
VC = VS + ((VA – VS) / 1.615)
For example, if the engine were turning 2,000 rpm and there was 100 rpm of converter slippage, carrier speed would be 1,961.9 rpm, demultiplying hydraulic slippage from 5% at the turbine to about 1.9% at the carrier.
The splitter gearset’s planet carrier drove a hollow sleeve shaft (which for reference we’ll call the “splitter shaft”), passing through the main transmission input shaft. (The sleeve shaft was also hollow because it contained the solid driveshaft for the transmission oil pump, which was located on the opposite side of the transaxle from the torque converter.) Both input shafts carried power to the main planetary transmission, a Ravigneaux gearset with a single annulus (with 86 teeth), two sun gears (one with 52 teeth, the other with 29), and three short and three long planets (each with 17 teeth) on a common carrier. The carrier drove the differential input gears.
The splitter shaft was ineffective in first and reverse, rotating idly while the main shaft drove the smaller input sun gear at turbine speed. In second gear, the multi-disc intermediate clutch engaged to connect the splitter shaft to the annulus of the Ravigneaux gearset. Since the larger rear sun gear was held by a brake band, this caused the smaller sun gear to overrun the input shaft and spin idly on its one-way clutch. Since the splitter shaft was now driving, hydraulic slippage was demultiplied by 61.9% (1 / 1.615).
In the direct drive third gear, the multi-disc direct clutch engaged, again allowing the main input shaft to drive the smaller sun gear, and the brake band was released, allowing the large sun gear to rotate freely. The intermediate clutch remained engaged, so the splitter shaft continued to drive the annulus at the same time. With both the annulus and small sun gear driving, the planet gears overdrove the large sun gear, forcing it to rotate faster than the engine. This in turn forced the planet carrier to rotate faster than either input shaft, albeit still slightly slower than the engine.
Although the math is again very cumbersome, this arrangement demultiplied converter slippage by a whopping 93.4% in third. For example, if engine speed were 2,000 rpm with 100 rpm of converter slippage, carrier speed (discounting mechanical losses) would be 1,993.4 rpm, reducing hydraulic slippage from 5% at the turbine to a mere 0.003% at the differential. That was efficient enough that the ATX could forgo a lockup clutch without a noticeable sacrifice in fuel economy, an important consideration for Ford’s cheapest U.S.-market models.
THE TRIUMPH OF ORTHODOXY
Ford’s split torque revival was relatively brief. Later versions of the ATX transaxle abandoned the splitter gear and dual input shafts for either a conventional hydraulic lockup (which Ford abbreviated FLC, for “full lockup clutch”) or, in some applications, a centrifugal lockup clutch (CLC) similar to that of the C5 transmission. The change coincided with the availability of more powerful engines in the Escort/Lynx and Tempo/Topaz lines, which suggests that the rationale may have been to facilitate increases in the transaxle’s torque capacity. (Tempo/Topaz versions of the ATX already had extra clutch plates in each multi-disc clutch.)
Similar concerns led to the elimination of the split torque feature of the four-speed AOD in the early nineties. The AOD was adequately strong for early eighties engines, but as power and torque increased throughout the decade, the secondary input shaft became a notable weak link. When the electronically controlled AOD-E debuted in 1991, it had only a single input shaft with no third-gear torque split. The same was true of the closely related 4R70 and 4R70W that replaced the AOD-E in 1993.
By then, all or nearly all domestic and most non-U.S. passenger car and light truck automatics had lockup clutches, most of them hydraulically operated and electronically controlled, differing only in minor details. The centrifugal lockup clutch eventually went the way of the split torque units, since its purely mechanical operation didn’t offer the fine-tuned control of electronically controlled electro-hydraulic clutches, whose operation can be better tailored to different operating conditions. For example, an electronically controlled clutch can be programmed to remain unlocked during warm-up or if either the engine or transmission is in danger of overheating, which a centrifugal clutch cannot.
This period of comparative orthodoxy — mostly four-speed overdrive automatics with electro-hydraulic lockup clutches — turned out to also be relatively brief. Subsequent automatic transmission design has diverged along several paths, including relatively conventional planetary transmissions with an ever-growing number of gears, at least three types of continuously variable transmissions, and dual-clutch semiautomatic transmissions. All these are beyond the scope of an article that is already considerably longer and more complicated than we originally intended, so we’ll just say that for automatics that have torque converters, computer-controlled lockup clutches are now the established norm and are likely to remain so.
As for split torque layouts, those have become common for hybrid electric vehicles, but that subject, like many others, will have to wait for another day.
NOTES ON SOURCES
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