Despite low gasoline prices making larger vehicles more popular, automakers continue to invest in fuel-saving technologies to satisfy regulatory mandates and to be ready for a potential change in consumer tastes if prices begin to rise at the pump.
Two General Motors (GM) engineers developing advanced powertrain technologies say a key to creating fuel-efficient vehicles in the current market is to focus on one word – and. Instead of offering fuel-efficiency or more-responsive driving, it’s critical to offer both. Offering increased power and 52mpg highway fuel economy ratings is an easier sell than focusing on the environmental benefits of green technology.
Mike Siegrist, assistant chief engineer for GM’s Ecotec 1.6L diesel engine, says when engineers began adapting the Italian-designed engine for North American use, they had three goals – good fuel economy; high power density; and excellent noise, vibration, and harshness (NVH) performance.
“You can’t succeed by only focusing on fuel economy. We wanted to deliver an engine that would be transparent. You wouldn’t notice it’s a diesel except for the extra power,” Siegrist says.
Engine design tools
Unlike GM’s other diesel engines, the 1.6L Ecotec features an aluminum block. With trucks, the higher compression ratios of diesel engines have typically mandated heavy cast-iron blocks, but Siegrist explains that better design software and finite element analysis (FEA) systems allowed GM engineers to spec the lighter-weight metal while still crafting a durable, powerful engine.
“Our analytical tools have improved,” Siegrist says. “In the past, when we’ve had engines that had to withstand high loads, we didn’t have the tools we have now to analyze the structure of the system to really keep it from failing.”
Engineers at GM’s engine design facility in Turin, Italy, created the base 1.6L diesel. It’s the only major European operation that the automaker plans to keep as it sells its Opel and Vauxhall brands to the PSA Groupe this year. Siegrist’s team in Michigan then developed the system for North American emissions rules and driving patterns. He adds that his team worked closely with engineers who developed Duramax diesels for GM trucks, especially on selective catalytic reduction (SCR) filtration systems as U.S. diesel rules are tougher than those in Europe.
“It’s got an aluminum block and an aluminum bed plate that sandwiches the steel crank. The timing drive, instead of being designed in the front of the engine, is in the back to encapsulate the drive behind the transmission and provide noise abatement,” Siegrist explains. “There are a lot of designs in the engine to keep the noise down.”
To maintain the engine’s green cred while making it attractive to non-eco-minded buyers, it was critical to offer lots of torque – 240 lb-ft.
“The thing that makes the Cruze diesel really fun to drive is that it will make 219 lb-ft from 1,500rpm to 3,250rpm. So you get power through that really wide torque band. The power comes on early and keeps coming as you rev the engine higher,” Siegrist says.
Tighter transmission tolerances
Scott Kline, assistant chief engineer for the Hydra-Matic 9T50 9-speed automatic transmission, says his goal was similar – deliver a fuel-saving transmission that drivers would never notice. Other automakers have failed with continuously variable transmissions (CVTs) and dual-clutch transmissions (DCTs) – powertrain options that save fuel, but are intrusive and annoying to drivers.
Reviews of the 2017 Chevrolet Malibu hardly mention the 9-speed, according to Kline. “That’s a transmission designer’s goal, that nobody recognizes we exist.”
Kline says one of the toughest requirements was squeezing three more gears into the same basic space as the 6-speed automatic. There were several technological enablers for shoe-horning more gears into the same space, with higher-precision machining near the top of the list, closely followed by better computational tools.
Hydraulic valves on the 9-speed have 30µm tolerances, a tiny fraction of the acceptable ranges used in earlier generations of GM transmissions. Kline says the automaker invested in machining equipment, adding “all-new assembly lines for control bodies, valve bodies, and solenoid bodies.”
Because the valve openings are smaller with tighter tolerances, they’re more susceptible to damage from sediment, so engineers added fine filtration systems for transmission fluid to ensure durability.
Kline explains that the switch to 30µm tolerances was critical to getting more gears into the transmission.
“We went from variable-flow solenoids to linear-force solenoids in the controls,” Kline says. “Variable-flow solenoids work by regulating pressure to a shift valve that then regulates pressure to the clutches. A linear-force solenoid acts directly on a valve, so it’s actually pushing the valve mechanically. That takes a lot of space out of our controls, so we can fit more solenoids into a tighter space.”
Another technological enabler to the tightly packaged transmission was the development of a selectable, one-way clutch to control gear selection.
“Currently, first gear and reverse in the 6-speed are provided by a diode-type clutch and a traditional clutch pack,” Kline explains. “The selectable one-way clutch gets rid of that clutch pack. That takes almost an inch from the transmission. We can do first gear, braking, and reverse with just a diode.”
Even with low gasoline prices making fuel-efficient vehicles less popular, Kline and Siegrist say teams of young engineers flocked to the diesel and transmission development projects.
“You’re talking to two former Corvette engineers here, and I can tell you that we had the same kind of enthusiasm working on this project,” Siegrist says. “They have a lot of similarities. They’re exciting new products that we’re launching out on the cutting edge of what Chevrolet is all about.”
Both the new diesel engine and the 9-speed transmission are pushing into new market areas for GM. Diesel passenger car sales have not been a big part of the North American market, other than a very brief blip during the 1980s, so a successful launch of the 1.6L could create new market opportunities for Chevrolet, Siegrist says.
Kline adds that the 9-speed is pushing the theoretical limits of transmission efficiency. “We think 9- and 10-speed transmissions are about the edge of that progress, but then again, we said that about 6-speeds a decade ago. Technology may change, and we may find more ways to squeeze efficiency out. The whole purpose of a transmission is to keep the engine operating at its most efficient place. With the ratios we’ve set up with the 9-speed, we think we’ve identified the right parameters.”
The young engineers who worked on the project prove that stereotypes about young engineers only wanting to work on software or self-driving car technologies aren’t true, Kline says. GM has hired large numbers of engineers and designers throughout the past five years, and many of them are excited to work on transmissions and other components that drivers only notice when they’re not working properly. While there are some concerns within design studios of a skills gap between retiring employees and new hires, he says GM’s recent experience has been the opposite.
“We’ve been bringing in a lot of fresh talent, and I’ve been extremely impressed with the talent coming out of the universities,” Kline says.
General Motors Co.