Today's modern diesel engines operate efficiently in a relatively narrow speed range, usually from 1,200 rpm to 1,800 rpm. There are heavy-duty engines rated at 2,100 rpm, and medium-duty diesels that go to 2,400 or even 3,600, but we'll look only at the typical heavy-duty engine. The newest ones can easily operate briefly below peak torque, but the low end of the normal operating range is the point of maximum torque, usually 1,200 rpm. The high end of the normal operating range is defined as the point where horsepower is rated, usually 1,800 rpm. This gives us useable engine speeds between 1,200 and 1,800, or a power band of 600 rpm, one-third of the maximum engine speed.
To keep the math simple, we'll assume that the fastest we would ever want to go would be 75 mph. We know this may not be realistic for your truck, but calculating one-third of 75 is easier than one-third of 67, 83, or some other number. And since the engine makers and experts tell us we get better fuel mileage at lower engine rpm, if we gear for a maximum of 75, we can run more economically at 62 or 67. In fact, overdrive ratios are designed to lower engine rpm compared to vehicle speed, but we are getting ahead of ourselves.
Getting back to our math, if we gear for a maximum of 75 mph at 1,800 rpm, we'll be driving 50 mph at 1,200 rpm, the speed of our peak torque rating and the slowest engine rpm we want to maintain. If we could instantly and magically reach 50, and could safely and legally drive only forward between 50 and 75 all day, we wouldn't need a transmission. But in the real world, we do need some kind of mechanism to allow us to drive slower than 50, down to as low as crawl speed. That's what a transmission does. It houses gears and provides a mechanism allowing those gears to be engaged with one another in various combinations.
When a gear with 24 teeth engages another gear with 24 teeth, they both will turn at the same speed. But when a gear with 16 teeth engages a gear with 24 teeth, the one with 24 teeth will turn only 2/3 of a revolution for each full revolution the smaller driving gear turns. (Overdrive gears make the driven gear turn faster than the driving gear.) This speed reduction (or increase) allows the truck to run slower (or faster) and still stay within the efficient operating range of the engine. Using our one-third of maximum rpm power band as the gear reduction of each gear set in our transmission, we can go from 50 to 75 mph in top gear, 33 to 50 mph in the next gear down, 22 to 33 mph in the next lowest, and so on. With ten evenly spaced gear sets, we can crawl at about one-and-one-third miles an hour at 1,200 rpm in first, or go 75 mph at 1,800 in tenth.
The more gears we have, the greater the control we can have on the engine. More gears allow for narrower reduction ratios, perhaps 17 percent instead of 33 percent. That means we can choose to be at high rpm at a given speed to increase compression braking when descending a hill, or at lower rpm at the same truck speed, to allow for acceleration when climbing. But more gears complicate the transmission, making it large and cumbersome to operate. They add weight, too.
To avoid having too large, heavy and complex a transmission, years ago truck builders started using two transmissions. The engine would turn the input shaft of the first, and its output shaft would turn the input shaft of the second. Each had its own shifter coming through the floor, giving us the description "two stick." An early 15-speed compound transmission might have a forward gearbox with three speeds. First might handle start to 10 mph, second was for 10 to 25 mph, and third might go to a top speed of as much as 50. In those days, truck engines had very narrow power bands, perhaps from only 1,800 to 2,100; one-seventh the engine's speed range. The second transmission, with five speeds, could keep an engine in its power range within the limits set by the front transmission. Typical combinations were three by four, three by five, four by four and four by five.
It took a practiced driver to work a four by five or an occasional four by six gear set. To simplify things, Fuller invented the range selector transmission. It incorporated a gear set that functioned like a second transmission. Coupled with overdrive gears, we can now have as many as 18 speeds inside one gearbox. Range selectors and splitters add versatility while keeping transmissions relatively simple to operate, compact in size and lightweight.
Splitters are reduction gears that change ratios by about half what a normal shift would, increasing driven gear speed. They are overdrive gears. Working only when the transmission is in high range, a nine-speed
becomes a 13-speed. Allow the splitter to work in low and high range, and the nine-speed becomes an 18-speed. Ten-speeds use either splitters or range selectors on five-speed main gearboxes.
Gears play another important role. As they allow trucks to operate at lower vehicle speeds relative to given engine speeds, they also multiply torque, so the truck has more pulling power when it needs it most. If peak torque in our example is 1,500 lb. ft. at 50 mph in top gear, by gearing down 33 percent, the transmission will increase torque output to 2,000 lb. ft. at 33 mph, one gear down. Downshift again, and you can have 2,660 lb. ft. at 1,200 rpm, which will be at 22 mph. The net effect is to multiply available torque by the same ratio as vehicle speed is reduced, which is the ratio of the gear you're in. Torque is used to accelerate and climb hills while horsepower keeps us at the speed we choose to drive. Torque is the raw turning force, while horsepower is a measure of work performed over a given time. Many newer engines can develop 800 lb. ft. of torque at clutch engagement speed of 800 rpm. With first gear ratios of around 12 to 1, that torque is multiplied so that 12,800 lb. ft. are available to get the truck started and to keep it rolling even at engine speeds below peak torque speed.
From the time when Fuller developed his range selection system until the '90s, the only heavy-duty transmission development was the adoption of synchronized transmissions in Europe and elsewhere. In North America, we stayed with straight-cut gears and non-synchronized transmissions, partly from tradition and partly to save weight and cost. Among many drivers, the mark of a true professional was the ability to shift gears without using the clutch (floating gears). Experienced drivers could match engine and transmission rpm accurately, and the gears just slipped together like a hot knife in butter. However, when a driver missed a shift, a few steel shavings wound up in the transmission oil.
In 1990, Mercedes-Benz offered trucks the first automated manual truck transmission, which helped win it the coveted European "Truck of the Year" award. It had a paddle shifter, which the driver could push forward or pull back; once to go up or down one gear, and twice to skip-shift. A computer matched speeds and activated the shift, which was easier with the synchronized gears. If the transmission could not complete the shift without drastically over-revving or lugging, it would stay in gear. That was a great safety factor on hills.
By the mid '90s, Dana-Spicer and Eaton-Fuller had automation on their 10-speeds, but only between 9th and 10th, and later 12th and 13th on 13-speeds. Semi-automated manual transmissions appeared about a year later, with the Eaton AutoSelect and Rockwell ESS (Engine Synchro Shift). Both required manual input, like the Mercedes. Neither the AutoSelect nor the ESS was very popular. Eaton already had its AutoShift in field test while Meritor (formerly Rockwell) was putting the finishing touches on its SureShift. Both used the clutch for starts and stops, but could shift gears like an automatic at all speeds. The driver could override the computer with both transmissions, provided the shift called for kept the engine within its safe rpm range. The new FreedomLine from ZF Meritor, the transmission-making joint venture between Germany's ZF and Meritor, is based on proven European technology using 12- and 16-speed gearboxes. It is the first fully automated heavy truck transmission that has no clutch pedal. A computer has taken over that function, too.
Automated transmissions still carry a hefty up-charge, but many smart operators find them cost-effective. Large fleets that originally opted for automated transmissions to help attract new drivers and to shorten their training time, have found veteran drivers are asking for them. And many owner-operators are opting for automation as well. As OOIDA members Bob and Suzanne Stempinski, Skokie, IL, told me when I test-drove their truck, "We wish we had this years ago. We're not half as tired when we're done for the day, and our fuel mileage has definitely improved compared to what we expected."
As the Stempinskis are finding out, because every shift is perfectly timed and smoothly made, shocks on the drive train are virtually eliminated, minimizing damage and thus lowering maintenance costs.
One of the greatest advantages automated transmissions offer is safety when climbing and especially descending hills. Ask anyone who has ever missed a shift going down a steep hill and had to rely solely on the service brakes. During my test drive, I manually overrode the AutoShift to downshift on steep downgrades, just to see if the system worked. It did, perfectly, and it helped the engine brake perform better, too. The same with climbing. No missed shifts. No finding yourself in neutral, hoping your momentum would get you to the shoulder, so you wouldn't be stopped in traffic.
Factoring in increased residual value with improved safety and lower fuel and maintenance costs, the future of transmissions lies in automation. For those old veterans who feel the mark of a true professional is how well you handle a transmission, you may just have to find another standard. And though you may not want to admit it, you just might find you like the ease of operation of an automated transmission.
Paul Abelson serves Land Line as technical editor and freelances from his office in Lisle, IL.