Burning better—Advanced combustion technologies presented at symposium24 April 2012
With continuing pressure to increase fuel economy with ever lower emissions, industry specialists and researchers presented key findings in new, combustion schemes and research.
Five companies used the SAE 2012 High Efficiency Internal Combustion Engines Symposium to present new and novel approaches through combustion to increase fuel economy and performance.
Gasoline direct-injection compression-ignition (GDCI)
Looking at alternatives to spark-ignition (SI) gasoline engines, Mark Sellnau of Delphi Corp. described its work on a gasoline direct-injection compression-ignition combustion system. Delphi is developing this using RON 91 octane gasoline. “GDCI is somewhere between HCCI and diesel [in terms of efficiency,]” explained Sellnau.
It was developed with low-to-moderate injection pressure. Low-temperature combustion was achieved using multiple late injections, intake boost, and moderate EGR. He described both tests on a light-duty, single-cylinder engine and 3-D CFD simulations as generating positive results. He showed data with low indicated specific fuel consumption—less than 190 g/kW·h at about 2000 rpm and 11 bar (160 psi) IMEP (indicated mean effective pressure). Engine-out NOx was less than 0.2 g/kW·h, and PM emissions less than 0.1 FSN.
“We are controlling the mixture stratification so that the onset of autoignition is about top-dead center,” he said. Also, if the burn is within certain limits, they avoid soot, he pointed out. He described two technologies as key enablers—fuel injection and valvetrain designs.
More information is in SAE paper 2012-01-0384, available here.
Turbulent jet ignition
Dr. William P. Attard of Mahle Powertrain presented an advanced SI system for pre-chamber combustion, a system for otherwise standard SI engines. He noted in his presentation that divided chamber combustion systems were first introduced in 1918 and versions were seen as recently as the Honda CVCC in the 1970s. The system he described features combustion in the main chamber as initiated by jets of partially combusted (reacting) pre-chamber products. This provides a high energy ignition source. “Jet ignition assembly simply replaces the spark plug in a contemporary engine design,” he said.
The turbulent jet seeds the main chamber with partially combusted pre-chamber products. These then entrain and ignite the main chamber charge. The widely distributed ignition sites allow relatively small flame travel distances. This means short combustion durations and high burn rates.
He described results from a single cylinder, experimental, modified General Motors Ecotec LE5 that uses port fuel injection modified to include pre-chamber direct injection. Compression ratio (CR) was 10.4:1 with a four-valve, DOHC with variable valve timing. He noted experimental results achieved high load (> 10 bar IMEPn), high efficiency (> 40%), and low NOx (< 25 ppm.)
More information is in SAE paper 2012-01-0386, available here.
Using what we have intelligently
“While market share of hybrids and electric vehicles will grow in all car markets, the ICE will remain the dominant power source to the year 2030,” said Mark Christie, Vice President for Engineering and Operations for Ricardo’s U.S. operations in his presentation. The most cost-effective solutions are likely to be combinations of technologies that in isolation might be limited in their contributions to more fuel-efficient ICEs, he noted.
He describes his company’s HyBoost concept as a combination of low-cost technologies used with synergy to deliver micro-hybrid operation. According to his presentation, Ricardo is combining downsizing, electric boosting (electric supercharging and electric turbocompound), low-cost energy storage, and micro-hybrid technologies.
Energy storage includes lead acid batteries plus supercapacitors to get lower cost. Micro-hybrid technologies include stop/start and mild regenerative braking.
“The work on this [HyBoost] program is supported by the U.K. Technology Strategy Board, with Ricardo as the lead and partners Ford, Valeo, Imperial College of London, CPT Power,” he said.
A Ford Focus provided a testbed that included a highly downsized GTDI engine, electric turbo-compound unit, conventional turbocharger, electric turbocharger, and a low-cost ultracapacitor pack. He explained the electric boosting mitigated transient delay. Combined with conventional turbocharging, it allowed for extreme downsizing of about 50%. Regenerative braking, stop/start, and torque assist were also featured. This test vehicle achieved 95 g/km of CO2 emissions over the NEDC test drive cycle.
He also presented Ricardo’s Spray Guided DI Concept (T-SGDI) With the addition of boosting and advanced injection strategies, he showed data that a test system achieved brake specific fuel consumption of 205 g/kWh at 2500 rpm. “The cost-to-benefit ratio for HyBoost looks favorable compared to diesel and full hybrid,” he concluded, with further potential by adding T-SGDI
Variable compression ratio GTDI
AVL explored through experiment the benefits of adding variable CR to a GTDI. The concept, according to Paul Whitaker, of AVL Powertrain Engineering, demonstrated aggressive downsizing with in-cylinder pressures above 40 bar (580 psi) brake mean effective pressure using a two-stage turbocharging system with 95 RON octane gasoline—without severe knock or low-speed pre-ignition.
Best BSFC was improved over a broader operating range, that is the "sweet spot" for fuel economy. Other conclusions AVL found as a result of its experiment included reduced cyclic variability and predictable knock characteristics that allow for calibration of spark advance closer to the knock border.
He also noted that using cooled EGR allowed an increased CR due to the knock suppressing benefits of cooled EGR operation. “Due to this, a further reduction in BSFC can be achieved in the sweet spot of the engine map,” he said.
Other points include that VCR allows for an adaption of the engine to different fuel qualities and ambient conditions. Finally, variable CR provides the capability to increase free spray length for improved particle emissions during catalyst heating.
Transonic combustion—a novel approach to new combustion
Chris De Boer of Transonic Combustion presented a truly novel combustion process his company developed for supercritical injection-ignition using gasoline. His company calls it Transonic Combustion (TSCi.)
“We are stuck where we were 100 years ago with SI and diesel,” he said in his presentation, explaining why his company decided to try and develop such a novel approach. In TSCi, supercritical fuel at 300°C (572°F) and 200 bar (2900 psi) is injected into the cylinder. Benefits of the approach include rapid mixing of the cylinder contents, high cycle efficiency, better lean burn tolerance, and represents "ideal heat release," according to De Boer. It also can be used with low-octane gasoline.
However, he was quick to note there is little science that documents the mixing process, ignition characteristics, and combustion behavior of gasoline-like fuels in supercritical conditions or the fluid transport properties. “We needed to do fundamental work on the properties of supercritical fluids,” he said. While simulation demonstrates a potential for 25% fuel economy improvement, validation is continuing.
More information can be found in SAE Papers 2012-01-0155 and 2012-01-0702, available here and here, respectively.
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