Powertrain experts detailed the technologies they see as most promising to enable light-vehicle engines to meet global CO2 regulations through 2025, at the 2016 SAE High-Efficiency IC Engines Symposium in Detroit. The list includes growing use of the Miller and Atkinson thermodynamic cycles, cooled EGR, and water injection, all of which are currently in production.
And progressing rapidly due to greater industry development focus and investment are a new range of high-octane fuels, various methods of waste heat recovery, variable compression ratio (V-CR) systems, divided exhaust boosting, and dedicated-EGR (d-EGR). The latter technology employs one or more of an engine’s cylinders for mixing a high octane hydrogen-and-carbon-monoxide reformate for recirculation (at rates up to 25%) back into the combustion chambers.
Now in development at Southwest Research Institute (SwRI), d-EGR enables the highest possible (over 12:1) compression ratios and ratio of specific heats—two key metrics for optimum engine efficiency—within a significantly expanded knock/stability window. In-vehicle testing using a turbocharged Buick Regal has demonstrated claimed city and highway fuel efficiency improvements of 13% and 9.2%, respectively, with NOx + NMOG emissions (31 mg/mi) at virtually LEV3 levels and a 1-s improvement in 0-60 mph acceleration.
“Improving engine efficiency is hard; we struggle to achieve 1%—and there is no Moore’s Law for engines,” noted Dr. Terry Alger, Director, Engine & Vehicle R&D, Engine, Emissions & Vehicle Research, at SwRI. Dr. Alger kicked off the two-day symposium, now in its seventh year, which precedes the SAE World Congress. He asserted that despite test drive cycles being not representative of real-world vehicle use, the industry “must optimize engine emissions outside those cycles to really clean the air”—while delivering customer-pleasing performance.
According to BorgWarner CTO Chris Thomas, who closed the day’s session with a provocative look at how to predict automotive powertrain’s future, an exceptionally efficient gasoline engine today that is claimed to achieve 42% brake thermal efficiency (BTE), actually averages only about 30% BTE across the vehicle fuel-economy cycle. So much work needs to be done in optimizing the powertrain within the overall vehicle.
ORC for heat, energy recovery
During the first-day (April 10) meeting, engineers and researchers from Bosch, FEV North America, IAV Automotive, the New A.C.E. Institute in Japan and BorgWarner, in addition to SwRI, said the advanced technologies in the pipeline will mostly be optimized as systems. These “bundles,” as some called them—one combination might be a Miller cycle engine with variable compression ratio, Organic Rankine Cycle waste heat recovery, and water injection, for example—are vital for achieving vehicle fuel consumption reductions averaging 6% per year.
Such aggressive progress cannot be achieved, the SAE experts agreed, without powertrain electrification, in order to hit the EU’s mandated 95 g CO2/km fleet target in 2020 and the U.S. 2025 CAFE, said Marc Sens, Department Head of Thermodynamics and Boost Systems at IAV.
The Organic Rankine Cycle (ORC) is a closed-loop thermodynamic process where heat is transferred to a fluid at a constant pressure. The fluid is vaporized then expanded in a turbine or piston-type expander to drive a generator, producing electricity. The spent vapor is condensed to liquid and recycled back through the cycle. Sens noted typical exhaust gas energy losses of 32% that can be harvested. He reported that IAV is developing a mechanical ORC that features two heat exchangers along the engine’s exhaust manifold and a 250-cc piston expander. The ORC produces 9kW, he said.
H2O injection and variable CR return
Water injection, a decades-old technology first proven in WWII fighter aircraft and later in racecar engines for mitigating combustion knock and thus enabling higher compression ratios (http://articles.sae.org/14176/ ), is now in active discussion in production-vehicle circles. Both IAV and Bosch experts highlighted their companies’ work in this increasingly viable area.
Li Jiang, Director of Advanced and Systems Engineering at Bosch and holder of 14 patents, noted that “the real efficiency of water injection is unlocked by higher compression ratios.” But she admitted that a typical system adds mass and requires a fairly extensive bill of material, including a unique double-acting pump needed to draw water out of the lines and tank to avoid freezing in cold ambient temperatures.
There’s also the not-insignificant issues of added cost and of where to source the system’s water: from the onboard tank (tap water or distilled), or from A/C system condensate, or from exhaust-gas condensate. Liang reported an extensive Bosch customer survey, conducted with over 3000 people in the U.S. and Germany, regarding acceptance and expectations of a vehicle water injection system and proposed refill intervals.
Variable compression ratio engines are another technology that has been investigated and prototyped for decades, and one that again is under development focus, the SAE experts noted. Both Marc Sens of IAV and Rob deBrujin, Director of Gasoline Engines at FEV North America, showed data on the benefits of V-CR for improved knock resistance with a variety of fuels, combustion stability, and significant fuel-economy gains. V-CR technology works best with long-stroke (1.3 to 1.4:1 bore/stroke ratios), according to Sens.
FEV’s deBrujin detailed his company’s novel 2-step VCR featuring a sophisticated and complex connecting rod design. He said the set-up has been tested to 200 bar (2900 psi) peak firing pressure.
Divided exhaust boosting (DEB) is a BorgWarner concept presented by CTO Chris Thomas. DEB is an elegant, efficient, and relatively low cost strategy of bypassing the turbocharger using bifurcated exhaust ports and a series of valves that direct exhaust gas directly to the aftertreatment catalyst. The concept enables faster cat light-off, use of a smaller turbine, and an 18° shift in the knock limit at 4000 rpm wide-open throttle, among other benefits.