4WD MODIFICATIONS - TECH TORQUE
Heavy trucks have been using up to 20-speed gearboxes for many years, so multispeed transmissions aren’t new in the automotive world. However, they’re relatively new to the 4WD scene and the main reasons are economy and emissions reduction.
Diesel heavy truck engines have always had a relatively narrow operating band, typically between the peak torque point around 1000rpm and maximum power, around 1900rpm. Medium truck engines are more flexible, but still have a relatively narrow band between around 1500rpm and 2600rpm.
Keeping an engine operating in those bands has meant using multispeed manual transmissions, with small gaps between successive ratios. In recent years many of these manual transmissions have become electronically automated.
Engineers of heavy vehicles have had no choice but to use multispeed boxes, but it’s a relatively new procedure for car, SUV and 4WD engineers. In smaller vehicles a multispeed transmission allows the engine to operate in a narrow band, where it can be tuned for reduced fuel use and emissions. More ratios with smaller steps also results in ‘seamless’ shifting, for a smoother driving experience.
All new 4WDs come with a minimum six-speed manual or automatic choice and many have seven-, eight- or nine-speed automatic options.
Do modern engines need multispeed transmissions? Of course not. All of them could get by with a simple five-speed box with a single overdrive gear, but economy measurements as measured in a laboratory situation – and that’s what’s published as ‘combined fuel consumption’ – would not be as favourable.
When we test vehicles we find that some cannot meet the maker’s economy claims, but some can – if they’re very lightly loaded. Bring the vehicle up to its rated gross vehicle mass and fuel use increases by 50 to 60 percent. Towing also increases fuel consumption, sometimes doubling unladen-vehicle
That said, in back to back testing we’ve done with Ford’s 10-speed behind the two-litre turbo diesel engine and the six-speed behind the 3.2-litre diesel the 10-speed came out as the economy winner – both unladen and when towing. How much of that result is down to the multisided box and how much to the more efficient, smaller engine we can’t say.
Are there any downsides to the use of multispeed boxes in serious 4WDs? One is obvious from experience with VW’s Amarok auto and that is the temptation for makers to delete a two-speed transfer case. On firm ground at mild operating loads the Amarok auto copes very well, but load it up and run it on very soft beach sand and the transmission overheats quickly. Also, it struggles when manoeuvering a heavy caravan.
VW has announced an auto model with a two-speed transfer case to counter these issues.
The only way a multispeed box could eliminate the need for a two-speed transfer case is if the multispeed ratios were stepped to provide the same degree of gearing spread as a main box plus two-speed transfer combination and no transmission maker has done that to date. The box would need an overdrive top gear and a first gear with around 15:1 reduction. You can get that ratio spread in a heavy truck transmission, but not in a 4WD box.
The other problem with a multispeed automatic box behind a diesel engine is that the engine operates at the lowest possible revolutions, which is good for economy, but not so good for keeping the diesel particulate filter hot enough to reduce soot buildup. It’s as if the transmission designer has gone in one direction and the engine designer in another!
It’s easy to see why many vehicle makers are looking to the petrol hybrid rather than the diesel to satisfy the demands of low emissions and improved fuel economy.
In the meantime, we’re certain to see more multispeed autos as band-aids for engines that are struggling to meet emissions and economy targets. Here’s GM’s latest:
The new GM 10-speed
Although the latest multispeed auto is intended initially for use in the 2017 Chevrolet Camaro ZL1 it will doubtless find its way into other vehicles, including SUVs.
The 10-speed is an all-new design, with a wider, 7.39 overall gear ratio spread, that enables the ZL1’s supercharged engine to remain at optimal engine speeds during upshifts.
“With world-class shift times on par with the world’s best dual-clutch transmissions and the refinement that comes only from a true automatic, the 10-speed delivers incomparable performance,” said Dan Nicholson, vice-president, GM Global Propulsion Systems. “It also leverages the experience of our other multispeed transmissions to deliver that performance with greater efficiency as its use expands into other vehicles.”
The wider overall ratio enables a lower numerical top gear ratio – an attribute that reduces engine speed on the highway, which contributes to greater fuel efficiency than a comparable eight-speed transmission. Improvements in spin loss complement the optimised gearing, further enhancing efficiency.
And while the Camaro ZL1 will be the first GM vehicle to offer the new 10-speed automatic, designed for rear-wheel-drive applications, it was scheduled for eight additional vehicles by 2020.
The 10-speed is approximately the same size as the six- and eight-speed transmissions, minimising changes to vehicle interfaces.
Thanks to only two non-applied clutches – the same number as the eight-speed – as well as other design features, the 10-speed automatic has lower friction that contributes to greater fuel efficiency over GM’s six- and eight-speed automatics. New ultra-low viscosity transmission fluid also reduces friction, while an internal thermal bypass allows the transmission to warm up faster – attributes that enhance fuel efficiency.
It is the latest transmission to use an all-new, GM-developed control system, with performance calibrations tailored specifically for different vehicles.