The future of diesel engines for passenger vehicles is promising, despite the high cost of exhaust aftertreatment technology. Even though gasoline engines require less aftertreatment equipment, the inherent combustion efficiency of diesel engines means the technology will remain attractive even as U.S. and European CO₂ emission regulations are tightened.

That’s the view of Pierpaolo Antonioli, GM's Executive Director of Global Diesel Powertrains. Based in Turin but with engineering staff in Russelsheim and Pontiac, Antonioli’s teams are working on powertrains and electronic controls for all the major world markets including India and China.

According to Antonioli, there are several engineering developments that will aid diesels as emissions standards, including the looming Euro 6 regulations, become more restrictive. Overall, European standards are moving towards those in the U.S. as global emissions regulations start to converge, says Antonioli.

“The main difference between the U.S. and Europe,” he noted, “is that emission test cycles in U.S. are much more focused on high loads than in Europe."

This means that diesel powertrains developed for European applications do not necessarily meet North American requirements. One example is the Chevrolet Cruze diesel launched in the US last year. The 2.0-L engine was already in service in the Opel Astra in Europe but needed extensive development to meet U.S. emissions limits and OBD-II requirements, says Antonioli.

Underscoring the ongoing importance of state-of-the-art diesel engines in the European market, GM used the 2014 Geneva Motor Show to debut the latest version of its most advanced compression-ignition powertrain. The new 1.6-L turbodiesel—Opel’s first all-aluminum diesel engine (AL head, block, and die-cast bedplate)—launches first in the high volume Astra sedan and hatchback models. Offered in SAE-rated 110 hp and 136 hp (82 and 101 kW), the diesel is coupled with a low friction, six-speed transmission.

The 110-hp version returns a claimed 63.5 mpg on the European combined cycle. With CO₂ emissions of 97 g/km, it meets the initial requirements of the Euro 6 regulations. It uses lean-NOx-trap-based aftertreatment. (See http://articles.sae.org/11758/). At Geneva, GM executives also promised that diesel engines would be coming to expand the appeal of several Cadillac models in the European market.

Meeting the new RDE cycle

With the advent of Euro 6, which take effect in September 2015, the aftertreatment equipment on diesels are going to have to improve in efficiency from the current 50-80% range to 90%. Starting from 2018, the second step in Euro 6 will involve a new test called the "real driving emissions" (RDE) cycle. This, explains Antonioli, introduces higher loads and higher speeds, similar to the U.S. cycle, that are typically not covered in the normal EU testing.

Another challenge is new European legislation that involves portable devices that authorities can use in cities to check vehicle emissions.

“For diesel it is relatively easy to meet CO₂ requirements, because fuel consumption is better than gasoline, as combustion is more efficient,” says Antonioli. “The big issue is developing for diesel a really sophisticated combustion control system, in order to reduce engine out emissions as much as possible. Then we focus on EGR (exhaust gas recirculation) which is very sophisticated to further clean exhaust gas, to reduce the production of NOx."

The quandary with diesel aftertreatment is that making the engine itself more efficient produces more NOx, which in turn requires NOx-reduction technologies.

For particulate reduction GM has two aftertreatment strategies: one with lean NOx traps without selective catalytic reduction (urea), and one with SCR. The bottom line, he asserted. is that a diesel powertrain carries with it a big "chemical lab," including an extensive sensor suite for onboard diagnostics.

Given this assessment, it’s fair to ask how diesel powertrains can expect to be cost competitive in the future. However Antonioli points out that diesel fuel's combustion efficiency advantage over gasoline offers a compelling edge.

For the short term, he acknowledged the industry's challenge to meet the 95g/km CO₂ emission rule for new cars recenty approved by the European Parliament. Its one-year phase in period begins in 2020, with full implementation by 2021. The law allows super credits, to apply from 2020-2022, whereby the cleanest cars in each OEM's portfolio count for more than others.

Big focus on reducing friction

The cost delta between diesel and gasoline engines is narrowing due to gasoline engines adopting exhaust particulate traps to meet more stringent global PM standards, thus making them more expensive, while engineers drive cost out of diesel aftertreatment systems, Antonioli said.

One cost-saving development focuses on the SCR system, which is currently mounted in the exhaust system under the vehicle. GM Powertrain is moving to close-coupled SCR systems that are located directly on the engine (and thus more efficient and less expensive), he explained.

Other GM technology paths aimed at making diesels more efficient in terms of emissions (other than CO₂) include advanced charging systems (for improved combustion control), and internal friction reduction. A lot of work focuses on improving bearing efficiency, and new lubricants.

Such solutions will be developed by GM and other OEMs and suppliers over the next three years and enable diesel powertrains to lead the way in meeting CO₂ targets, not only in Europe and the U.S. but also in China, where diesel passenger cars sales are still very low.

In the U.S., Antonioli points to Volkswagen’s success with diesels and believes customer resistance to diesels will be overcome as buyers realize the powertrain’s superior fuel consumption and low end torque advantages.

Looking further out, Antonioli foresees that the emissions limits envisioned for 2025 will be impossible for diesel or gasoline powertrains to meet without some degree of electrification.

“What I expect then is some level of hybrid, including diesel hybrids which are being developed by more and more OEMs including Mercedes and BMW.”

Beyond that stage, Antonioli says the focus will shift to fuel cell development as anticipated legislation calls for even lower tailpipe emissions.