Transmissions: A New Environment
The latest engines have less forgiving fuel curves that put a new emphasis on transmission specs.
Steve Sturgess
Senior Editor
It's probably no surprise that transmission specification in the new, lower emissions engines environment has become more critical. But the issue is not with increased heat from exhaust-gas-recirculation engines, as one might have thought. "If you had asked me about heat a year and a half ago, I would have been nervous about a response," said Gerard DeVido, product line manager for Eaton Fuller heavy-duty products.
Indeed, Eaton engineers "went out and did a lot of work" in the interim, ensuring that various engine/transmission combinations would not exceed the 250-degree maximum temperature that is the cutoff for a manual box. So they're confident that heat will not be a problem.
But what has transpired is that transmission specification has become critical to maintaining fuel economy, because the fuel maps for the new engines are far less forgiving than previously.
"We must make it clear to the customer that they have to match road speed to transmission and axle selection. For example, for the Maxidyne engine, the sweet spot is 1,700 rpm<->—plus or minus 50 rpm," says Steve Ginter, vocational product manager for Mack. DeVido says you only have to stray 10 to 20 rpms outside the new engines' sweet spot and the fuel consumption dramatically increases.
The cause is the much narrower "fuel island" for the EGR engines. The brake specific fuel curves for an engine are mapped on a graph three dimensionally as a series of contours. Europeans refer to these as a mussel chart, because they closely resemble a mussel shell as the rounded contour lines spread out from the central sweet spot. That central area is where the engine needs to operate to gain the best economy. Straying outside gets into higher fuel usage. With the EGR engine curves, it gets into higher fuel usage faster.
If you look at a two-dimensional specific fuel curve—often included with other engine performance graphs—you'll see the same minimum point. This is where the engine should operate as much as possible. At the low end, this curve starts to rise quite quickly; at the high end it takes off almost exponentially. It's what some engineers used to call the curve's scorpion's tail. And like that arachnid's stinger, it's a place you don't want to go.
Ratings have changed too. Ginter points out that some of the Mack power curves have migrated upwards 100 rpm, which means what used to work in the old environment needs rethinking when spec'ing today's ratios.
Ginter points to the T308 and 309 transmissions in the latest Mack triple-countershaft line-up as examples, where the overdrive top gear ratio has been dropped to 0.71 from the earlier T200 transmission's tall 0.60 to one. He's quick to point out that the Mack T300 has an aluminum case for better heat transfer, allowing the transmission to shed heat better.
Transmission Technologies (TTC) Spicer transmissions have a lightweight option of aluminum case or aluminum case and bellhousing under the Opti-Lite tag. According to Jim Averill, TTC marketing manager, it is a popular option in weight-sensitive applications and it does handle heat better. But heat is not an issue for the Spicer product because it has a forced lubrication system, the Opti-Lube, which reduces temperatures by up to 50 degrees. "A gerotor pump on the countershaft directs oil though galleries to the bearings to keep them cool," he said. "In fact, we were able to reduce the total amount of lubricant in the transmissions by 30%, which saves windage and improves fuel economy with no penalty whatsoever."
Averill says that there has been a learning process over the last couple of years, weaning fleets off oil coolers with the Opti-Lube transmission because they run so much cooler. "It saves money and eliminates issues with lines and leakage."
Allison's response to EGR heat has been entirely opposite. Steve Spurlin, chief engineer for the 3000/4000 series says the company looked at the issueof heat rejection from the perspective of what they could do to help. Since the torque converter generates heat that is normally discharged through the cooling system, Allison changed its guidelines to give back some cooling capacity to the engine. According to Spurlin, the converter efficiency change was relatively small and easy to accommodate, since trucks today perform so much better and the automatic stays in lock-up more of the time, lessening the heat that must be discharged from the transmission.
While Eaton's DeVido is confident in the current line-up of engine/transmission combinations and the lubricant specs, ZF-Meritior's director of sales and engineering, Charlie Allen says that at some point we may be pushing the capability of the oil. "The oils we've got are doing a good job, but I think we're starting to see the ceiling. Back in the days of 1,450 pounds-feet peak torque we didn't use the oil's full capacity. Now, though, the lubricant is as important a part of the spec as the gears and bearings." He argues that fleets must look at gear lubricants as a component of the transmission. Maintenance must follow recommendations for change intervals and for checking levels if fleets are to enjoy the long warranties offered by the component manufacturers.
TTC's Averill points out that vocational customers should carefully match their maintenance to their operation to maintain warranty. "Some of the warranties are complicated when it comes to vocations. For Spicer transmissions we have a simple five-year/750,000-mile warranty, matching other over-the-road transmission manufacturers." He councils fleets to look at the vocational conditions and the maintenance required when spec'ing out transmissions.
While Eaton's DeVido sees more opportunity for the 13-speed, given the shorter, less-forgiving engine operating ranges, Averill and ZF-Meritor's Allen say there will continue to be good demand for the 9- and 10-speed, because there's less for a driver to get wrong. "A 10-speed gives you good performance and less opportunity for the driver to get in the wrong place," says Allen. "With a multi-speed, a driver often has a choice of gears to run at a given speed."
Everyone agrees that it is critical to spec the transmission for the application. Mack's Ginter says part of the process is being completely honest with the dealer when drawing up the specs. "It's no good telling the dealer you'll run the truck at 65 mph because that's the state speed limit, then going out and running the trucks at 68 mph." Political correctness doesn't cut it when spec'ing gear ratios, he says. You have to be completely upfront with the sales professional who is helping with the process and using tools like Mack's MackTraq computer-based spec'ing tool to get the most out of the integrated drivetrain, he says.
As important as matching the transmission to the engine's torque and fuel island charts is the training of the driver. DeVido says the multi-speed transmission will allow the driver to keep the engine in its "sweet spot" but it takes a good driver. The best way to get all drivers to optimize their shifting is to take the decision away from them — and provide automation for the transmission. That way, there's no facility for the driver to put the lever into the wrong gear: the transmission makes the decisions and optimizes the whole powertrain.
Allen sees the automated transmission as the way forward. Citing ZF-Meritor's FreedomLine product, he says providing an automatic puts the driver into his real role as a vehicle manager. "When you ask the driver to make decisions about which gear to choose you're making him the powertrain manager. That's not his job. He's there to manage the vehicle. He — or she, if a fleet is looking to broaden its driver pool resources — shouldn't be making decisions about how to optimize performance and economy. The driver should only have to push one pedal to go and the other to stop."
And reports back from fleets show that the FreedomLine transmission will get between a 3% and 5% savings on fuel consumption — which makes up for the economy loss the EGR engines are anticipated to have. "Automated is the way to get back the performance you had before," he says.
Eaton's DeVido agrees. The automatic is the way forward for fleets because it has the ability to bring all drivers to the same level of economy as the best drivers. And, at the same time, the automated shift is protective of the entire drivetrain. Citing the low level of automation offered by just the Top 2 transmission, DeVido says fleets have reported savings of 4/10 mpg.
At the March Mid-America Trucking Show, Eaton is due to unveil the UltraShift 10, a new automatic that matches the FreedomLine's two-pedal automation by using a centrifugal clutch. It takes the driver a step further than the current Eaton Autoshift technology and makes the driver interface much more car-like.
The overall concensus is that fleets should pass the decision for spec'ing the transmission to the drivetrain experts, and drivers should pass the decision for which gear to choose to transmission automation.
