Pushing the ICE Forward, Gradually

June 1, 2016

Lindsay Brooke

Free lunches don’t exist in the quest to improve vehicle efficiency and reduce emissions. No one knows this better than powertrain engineers whose work is a constant series of tradeoffs that must be tackled if the auto industry is serious about reducing CO₂.

Do you want enhanced drive-cycle fuel economy? It could cost you real-world performance. You say your latest dyno tests indicate efficiency improvements through charge dilution? Well, what about the impact to specific power and torque and full-load pumping losses? These are the arcane kinds of tradeoffs the industry grapples with as it seeks effective, low-cost production engine solutions, noted Dr. Terry Alger, who directs spark-ignition R&D at the Southwest Research Institute (SwRI).

Speaking at the 2016 SAE High-Efficiency Engines Symposium, Dr. Alger observed that as global emissions and fuel-consumption regulations have tightened, various engine technologies have evolved (i.e., downsizing/right-sizing) while others have gained popularity (boosting, valvetrain variability, late intake- and exhaust-valve closing strategies and waste heat recovery). All are moving forward within the confines of combustion ‘knock’ and stability.

“Improvements need to be on the systems level,” Dr. Alger told the SAE audience.

At no time in its 130-year history has the industry worked with greater urgency to investigate and develop such solutions.

Automotive Engineering editors track powertrain developments as part of their regular engagements with engineers and technical specialists. The following technologies are among the newest worth noting in the promising advanced-ICE space.

Mahle: Proving new Jet Ignition in F1

When Kimi Raikkonen’s blood-red Ferrari SF15-T crossed the finish line in fourth place at the 2015 Canadian Grand Prix, it carried an engine technology known only to Scuderia Ferrari leadership — and a small circle of engineers at Mahle.

The secret is Mahle Jet Ignition, previously known within the advanced-ICE development community and SAE magazines readers as TJI, or Turbulent Jet Ignition. Capable of lean-burn operation in excess of Lambda 2, the patented pre-chamber technology is improving the combustion efficiency (and thus reducing the fuel burn) of Ferrari, Mercedes and reportedly other F1 teams’ power-dense engines, which now must complete races with a limited quantity of fuel.

The intense F1 competition is crucial to Mahle, which hopes to productionize the technology in high volume.

In development since 2009, Jet Ignition has enabled a testbed 2.4-L production engine to achieve drive-cycle fuel economy gains of up to 25%, and has shown brake-specific fuel consumption (BSFC) of less than 200 g/kW·h with greater than 41% peak brake thermal efficiency.

“And we’ve seen significant reductions in engine-out NO_x — levels under 100 ppm in lean conditions,” noted Mike Bunce, a Mahle research specialist and expert on the technology based in Michigan.

Mahle’s unique design effectively decouples the main combustion chamber and pre-chamber air/fuel charges, with the smaller pre-chamber receiving (in production-engine testing) 3% of the injected fuel, the rest going to the main chamber. Both chambers are connected by a nozzle containing multiple orifices. Injection occurs at around 60° before TDC and spark reaches the pre-chamber at around 22° BTDC. Depending on engine application, up to eight high-pressure plasma jets shoot through the inter-chamber orifices and ignite the mixture in the main combustion chamber at between 12 and 5° BTDC.

The turbulent ignition event improves combustion — the inter-chamber orifices help create swirl and the decoupled chambers enable relatively segregated rich (small chamber) and lean (main chamber) mixtures, extending the knock limit and allowing higher compression ratio. This yields more power with significantly lower emissions, said Bunce.

Early last year, Mahle and Ferrari engineered Jet Ignition into the SF15-T race engines per F1 technical regulations. The system proved flawless in its first outing at the 2015 Canadian GP — reportedly months after Mercedes-Benz first used the Mahle system in some 2014 F1 events.

Mahle’s development of the technology for series-production engines continues. Certainly adoption of the concept requires dedicated cylinder-head designs, with boosting and aftertreatment systems tailored to it. But diesel-like thermal efficiencies are potentially in sight for gasoline engines using the Jet Ignition system — and Formula One has another “lever” to pull when the fueling regulations become tighter in the future.

Eaton: Cylinder deactivation for Diesels

The benefit of cylinder deactivation (CDA) technology for passenger vehicle gasoline engines is greater fuel efficiency through reduced engine pumping losses. Vehicle fuel economy can be improved by an average of 2-6% using CDA, while operating in light-load (typically steady-state) conditions. Effectively a “virtual downsizing” play, CDA allows larger-displacement engines with more cylinders to maintain their power and torque while delivering the fuel consumption of powerplants packing fewer cylinders.

Eaton Corp.’s valvetrain system expertise has put it in the forefront of CDA developments, with research engineers now setting their sights on a new opportunity: diesel engines. Their aim is to actively regenerate the diesel particulate filter (DPF) at higher rates, during steady-state cruise (65 mph, 1200 rpm, 7.6 bar brake mean effective pressure) without the efficiency-reducing solutions of a burner, fuel dosing, or a diesel oxidation catalyst.

“While there are many definitions for CDA in the industry, ours is: Shut off the air going in, shut off the valves letting air out, and shut off the fuel. If you let the exhaust valves open you’ll lose your ‘air spring’ [the exhaust gas charge remaining in the cylinder],” explained Jim McCarthy, Ph.D, Eaton’s Engineering Manager for advanced valvetrains. The air spring he refers to results in a reduction of piston-motion induced compression during the four-stroke cycle that’s worth perhaps 1% in fuel economy.

McCarthy’s Eaton team, in collaboration with Cummins Engine and Purdue University’s Herrick Laboratory, is busy these days, investigating the effects of early and late intake- and exhaust-valve timing on diesel aftertreatment system thermal performance. A flurry of SAE and other technical papers reveal the promising findings of their work. Catalysts are typically effective between 250 and 450°C — and “waking them up” to clean the exhaust is a major challenge at lower engine loads, during cold start and at idle (where diesel exhaust temperatures hover between 110° and 130°C) and in colder ambient conditions. Using variable valve actuation (VVA), intake throttling can increase exhaust-gas temperatures at the turbocharger outlet and deliver reductions in fuel consumption, NOx and engine-out particulates.

Eaton’s switching-roller finger followers (SRFF) and the company’s classic Type 2 valvetrain, proven in millions of light-duty gasoline engines, serve as baseline hardware and are capable of response times in the 12 to 18 ms range. The SRFF design is scalable for light-, medium-, and heavy-duty diesel applications. (see SAE Technical paper 2015-01-2816.)

The diesel CDA work is being conducted on a Cummins 6-cylinder testbed at Purdue; the engine also is equipped with a variable-geometry turbo and cooled EGR. “The testbed is fitted with ‘camless’ VVA giving fully independent authority of all valve events, cycle-to-cycle” McCarthy told Automotive Engineering. “We’re using the camless to figure out what VVA functions work well, what the benefits are and why it will work for production. We’re looking at scalable light- and heavy-duty diesel applications, in concert with aftertreatment.”

While the team has hit its share of setbacks, there are many reasons for optimism. Cylinder deactivation does increase the rate at which the particulate filter heats up, and deactivation (through valve motion and fuel injection shut-off) of two of the six cylinders enables engine-outlet temperatures up to 520°C due to the reduced air/fuel ratio. Deactivating three of six cylinders — “and we have capability to deactivate more than three,” McCarthy said — at loaded idle enables a rise of 190° to 310°C at the turbine outlet with only a 2% fuel economy penalty compared with the most efficient six-cylinder operation. But a 39% reduction in fuel consumption was shown versus six-cylinder operation achieving the same 310°C turbine outlet temperature.

“By reducing airflow through the engine with our valvetrain technology, we get higher exhaust temperatures, faster aftertreatment warm-up and reduced pumping work compared with non-deactivated operation,” McCarthy said. His extended team continues its work with additional focus on transient operation and control-algorithm development: transitioning groups of cylinders in and out of activation needs to be as seamless in a Class-8 diesel as it is in a passenger-vehicle V6, Eaton insists.

BorgWarner: Divided Exhaust Boosting

Chris Thomas paused for effect as he clicked to the next PowerPoint slide, an X/Y-axis graph loaded with data and an ominous-looking trend line. He was giving Automotive Engineering a preview of his upcoming talk at the 2016 SAE High Efficiency Engines Symposium.

“There’s no way around it,” said Thomas, BorgWarner’s Vice President and CTO. “Only electrified vehicles can meet the U.S. 2025 CO₂ targets. Okay, maybe a few extreme lightweighted vehicles with non-electrified powertrains will squeak through. But even the best engines today will need ‘diesel like’ brake-thermal efficiency — about 42% BTE, or about 30% BTE on a cycle average — coupled with precisely calibrated state-of-the-art transmissions and drivelines, energy recuperation, ‘sailing,’ engine load shifting, and more.

“If the U.S. fleet achieved an average efficiency equal to the current top 1% of SI powertrains, they wouldn’t get past MY2020 regulations,” Thomas noted. “And diesel-like efficiency would provide compliance only through MY2023. We’re at about 21.5% propulsion system efficiency today, which is about a 14% increase from 2005. That’s a huge gain — but we need to be at about 29% by 2025. That’s why BorgWarner spent $1.2B acquiring Remy International — for the electrification of the propulsion system.”

Thomas discussed various promising technologies aimed at getting there, including one that’s elegant and comparatively low-cost: divided exhaust boosting (DEB) is a concept born of a conversation among Thomas and a few other BorgWarner engineers while waiting for an outbound flight at Frankfurt airport. It uses bifurcated exhaust ports in the cylinder head, a second integrated exhaust manifold and a few simple valves to split the engine-out exhaust gas stream into two routes: to the turbocharger and to the catalyst.

The DEB concept enables faster catalyst light-off, use of a smaller turbine and an 18° shift in the knock limit at 4000 rpm and wide-open throttle, among other benefits.

“We’d had this concept for years, valve-event modulated boost (VEMB), but it was far too complicated. It needed to be simplified dramatically. In short, we arrived at DEB using a valve that resembles the EGR valves we make. When it opens, all of the hot, blow-down gas is sent directly to the turbo,” Thomas explained. “And all the exhaust gas that the piston has to push out, the residual, is directed by another valve to bypasses the turbo and go directly to the catalyst.”

While the DEB concept helps Miller-cycle operation as well as enabling dedicated EGR, lean-burn and gasoline compression-ignition strategies, it’s basically combustion-system agnostic. The significant change to current engine architectures would be the additional cast-in exhaust manifold and coolant jacket.

“The benefits of this are interesting,” Thomas said of the DEB. “We’re throttling it, but only on cold start. With all of the exhaust gas routed directly to the catalyst it lights off much faster — in fact, faster than a naturally-aspirated engine. We had to resize and rematch the turbo and we actually get a pumping benefit and about 25% more low-end torque with up to 4% higher fuel economy.”

Thomas reckons that between 2025 and 2030, the majority of gasoline ICEs in the market will be running either Miller or Atkinson cycles, as they’ll be part of electrified vehicle systems. The majority will be boosted (Miller-cycle), he said: “When you do really early intake valve closing you need boosting to get sufficient air into the engine.”

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