With the recent EPA-imposed greenhouse gas regulations already beginning to bite and even tighter limits set for 2017, innovations which promise reductions in fuel consumption and CO2 emissions are being looked at with ever-greater interest. Downspeeding, in particular, is a concept which is fast finding favor, especially among producers of heavy machinery operating for extended duty cycles and clocking up many thousands of running hours each year.
On the face of it, downspeeding provides the perfect win-win: the engine turns more slowly and closer to its most efficient operating point, noise is reduced, wear and tear are minimized and significant savings are realized in both fuel consumption and CO2 emissions. Dropping the engine’s operating speed by just 100 rpm is enough to boost fuel efficiency by a full 1 percent, offering operators useful benefits in annual fuel costs and, in some markets, taxation rates. Indeed, between the typical products of 2010 and 2013, linehaul truck highway cruise rpm values have fallen from 1,400 rpm to just over 1,200, and with the aggressive downspeeding moves planned for the next few years, rpm values could fall to 1,000 or even 900.
According to Lubrizol experts, other measures, such as low-viscosity oils, can also help. These oils are thinner and easier to move around inside an engine, and because they are thinner the energy needed to pump the oil and also the churning losses related to parts moving through a fluid are reduced, resulting in increased engine efficiency.
The potential fuel savings are there for the taking — so what is there not to like?
Engine downspeeding is not quite as simple as it seems, warns Bob Ostrander, chief engineer for drivelines at Meritor, of Troy, Michigan. Because the engine is turning at a lower speed, the rear axle ratio has to be correspondingly faster to achieve the same cruising speed on the road. Yet, says Ostrander, this seemingly insignificant change has important consequences for the rest of the driveline and means that different components and systems may have to be specified.
Addressing a webinar audience in November last year (2015), Ostrander explained that because of the faster ratio in the axle, the torque multiplication performed by the axle is less. “The truck needs that torque,” he said, “so we have to find it someplace else.”
The “other place” referred to by Ostrander is, of course, the driveline itself. More torque needs to be provided through the propeller shaft if the torque at the road wheels is to remain the same — and that, in turn, means that the rest of the driveline has to be specified for the higher torque rating.
With respect to lubrication, Lubrizol notes that higher torque equates higher load on the parts, and thus more demand for wear protection in a fluid. There is little sense in making the parts larger: larger parts have higher inertia and this would negate the fuel economy benefits of downsizing. And while lower rotational speeds do allow the fluid to run cooler, the cooler fluid then becomes thicker and thus raises churning losses again.
Meritor’s Ostrander cited the example of a traditional linehaul truck of pre-2010 engine specification, with an overdrive transmission and an axle ratio of 3.55 or slower. This rig, he calculated, would typically need a driveline torque rating of 18,500 lb.ft (25,000 Nm) to maintain a steady 62 mph (100 km/h) cruising speed at 1,390 rpm; even then, this torque might not always be sufficient to hold the required speed.
By contrast, he continued, a modern emission-controlled engine with direct-drive transmission and a 2.47 (or faster) axle ratio would require a 21,500 lb.ft (29,150 Nm) drivetrain torque rating and could hold the 62 mph cruise at 1,300 rpm, reducing fuel consumption by almost 1 percent in the process and keeping more torque in reserve.
Still not that simple
Yet even that is not quite the end of the story, reveals Ostrander. Not only does the downspeeded drivetrain have to handle higher absolute values of torque, it has to deal with a very different set of transient torque characteristics, too. “Pre-EPA 2010 engines have a much slower ramp-up of their torque,” he told his webinar audience. “Typically, the engine might achieve its peak torque in 5 to 10 seconds. But with post-EPA 2010 engines, the torque ramps up much faster. Maximum torque can be reached in less than half a second, and this sends a huge spike of torque into the drivetrain components.
“In a situation where the torque comes up fast,” he warned, “the peak loads can overshoot because of the inertia in the system. These momentary transient conditions can fracture any of the components from the clutch to the rear axle, though there are ways of mitigating these peak transient conditions.”
Every component in the drivetrain has its part to play, whether good or bad. Modern ceramic clutches, for instance, are “almost digital” in their fast responses, and their lack of buffering slip can have an amplifying effect on the engine’s torque output. If the driver steps off the clutch, warned Ostrander, high shock loads will be transmitted through the system; equally, direct drive transmissions, with their greater torque multiplication effects, can further amplify the torque overshoot sent through the system. This risk is especially pronounced when maneuvering in first and reverse gears, or when hitching up a trailer. Further exacerbating the problem is the fact that modern truck tires have better traction on the ground, which limits their opportunity to slip and to dissipate torque spikes in that way.
Torque increase path
The challenge faced by the powertrain engineers is that torque values are higher at every point in the driveline. In Ostrander’s line haul example, the downspeeded engine needs to be rated at 1,750 lb.ft (2,375 Nm). This, driving through a 14.80 to 1 first gear, imposes a torque of almost 21,000 lb.ft on the drive shaft. The traditional solution, on the other hand, needs under 16,000 lb.ft torque capacity for its drive shaft.
The extra engine torque required to maintain startability is some 23 percent higher, calculates Ostrander, a figure which translates into a startling 44 percent rise in drivetrain torque, exposing every component to significant additional stress. “This means the driveline components may have to be up-sized,” he noted, giving transmission gearing, drive-shaft universal joints and the rear axle pinion stems as examples.
Wet clutches, says Lubrizol, offer the opportunity of superior cooling to help deal with the consequences of torque spikes. Clearly, the need for the fluid to cool and protect the clutch material increases as the torque spikes increase.
Other solutions, or combinations of solutions, are also on hand to lessen the risk of damage and to reduce the peak loads on the system. Maximum torque can be electronically limited in the starting gears and also in the steady state cruise condition, and peak torque control — again, electronic — can control the speed of torque ramp-up, as well as lowering the engine speed at which the clutch is engaged. This, said Ostrander, could reduce peak torque by 37 percent when needed.
“These peak torque mitigation measures can be put in place without the driver realizing it,” he revealed. “It could be in winter, for instance, when the brakes are frozen and the driver gives it a big tug. In other circumstances that [technique] could actually fracture components.”
By way of conclusion, Ostrander confirmed that while downspeeded drivelines were highly suitable for linehaul type trucks where annual mileages were over 60,000 (100,000 km), four-fifths of driving was on highways and there were at least 30 miles (50 km) between stops. The innovation was less appropriate for vehicles with fewer highway miles, closer stops or where most of the driving was in the lower gears.
Overall, said Ostrander and his co-authors in a Meritor White Paper on downspeeding, truck OEMs, engine manufacturers and drivetrain component manufacturers need to continually work together to develop and implement control strategies that satisfactorily protect the drivetrain components.
The trend to continue downspeeding for fuel efficiency gains has limitations for drivetrain components. Trucks equipped with manual transmissions become more difficult to drive, Automated Manual Transmission (AMT) becomes more limited, dual clutches become more necessary for operation, and rear axle and drive shaft components become more stressed.
“A control strategy is more effective than simply up-sizing fractured drivetrain components,” wrote the engineers. “Up-sizing, though it will solve the problem with that particular component, will only transfer the problem to the next weakest component in the drivetrain system. The outcome is larger, heavier and more costly components to overcome the unintended effect of downspeeding for fuel efficiency gains and greenhouse gas emissions standards.”
