Today’s combustion engines typically are only 20-30% efficient at part-load operating conditions, resulting in poor average fuel economy—and inspiring powertrain designers to reach further. Imagine new internal combustion engines (ICEs) that promise part-load brake thermal efficiencies of 50% (or more), with power densities approaching 2 hp/lb, and 33% lighter than a contemporary Otto-cycle unit.
Such engines currently under development—including opposed-piston two strokes, split-cycle types, variable-compression-ratio engines, and new twists on the Wankel rotary—also face significant technical and commercial challenges. This was stressed repeatedly at SAE International’s 2012 High-Efficiency IC Engines Symposium, held April 22-23 at the Westin Book Cadillac Hotel in Detroit. The second-annual event kicked off SAE World Congress week.
“No stone can be left unturned” in the quest to improve upon the 136-year-old spark-ignition (SI) engine and its only slightly younger diesel cousin, asserted Dr. David Foster, Professor of Mechanical Engineering at the University of Wisconsin-Madison. His keynote address focused on thermodynamics and the processes in the engine and fuels that cause losses and degrade its work potential, while examining the extent to which they can be made less destructive.
Keeping in-cylinder temperatures low is a key, Foster noted, as this directly impacts extractable expansion work for a specified expansion ratio, which works toward minimizing exhaust energy loss.
The symposium’s first-day session covered alternative engine designs, which are a theme with rich history at SAE Congresses. So it was appropriate to hear from the CEO of Achates Power, which has emerged as one of three prominent developers of opposed-piston engines. (The others include EcoMotors and Pinnacle Engines.) David Johnson discussed Achates’ 3.2-L opposed-piston two-stroke (OP2S) that in early testing and simulations has demonstrated 21% greater fuel efficiency and lower engine-out emissions levels than a benchmark Ford 6.7-L Powerstroke V8 diesel, while meeting U.S. EPA 10 standards with conventional aftertreatment.
Achates is designing engines of various displacements to suit the passenger vehicle (a 2.4-L) and commercial markets. According to Johnson, the advantages of the OP2S include two piston crowns per combustion chamber; multiple injection events per cycle using side-mounted injectors; variable swirl activity; simultaneous intake/exhaust events; and uniflow (rather than loop-type) cylinder scavenging, which helps reduce the pumping losses that have been a detriment to two-strokes. See video: http://www.achatespower.com/opposed-piston-thermal-efficiency-video.php
Fuel-specific oil consumption, another Achilles’ heel of traditional two-strokes, has been demonstrated at a very reasonable (at this early development stage) 0.18% across the Achates’ OP2S full operating map, he said. Testing thus far has been on what Johnson calls “good old number 2 diesel fuel.” See video: http://www.achatespower.com/opposed-piston-oil-consumption-video.php
He admitted that various hurdles remain before the OP2S would be market ready. “Thermal management is a challenge, as the highest heat is at the center of the cylinder liner, rather than at the end,” he explained. Packaging also will “take some more work, for sure,” he said, but the CAD illustrations in his presentation revealed a compact overall design.
MCE-5 varies compression ratio from 6:1 to 18:1
Using a variable compression ratio (VCR) to reduce CO2 emissions was covered by Vianney Rabhi, the self-taught inventor behind the MCE-5 VCRi engine. Rabhi patented his first variable compression ratio concept engine in 1991, and he self-funded initial development stages of the $18 million MCE-5 VCRi program.
The MCE-5 uses a fairly complex internal mechanism to provide continuous compression ratio control, ranging between 6:1 and 15:1 to each cylinder, according to Rabhi. The mechanism consists of a rod-crank assembly, drive gears, and unique actuators. A common cylinder head is used for both the combustion chambers and upper-control jack chambers, of which there is one for each combustion cylinder located on the inlet side of the head. See http://www.mce-5.com/english/mce-5_vcri_technology.html
Rabhi claims the MCE-5 VCRi can switch from minimum to maximum compression ratios in less than 100 ms. At the 6:1 ratio, the engine achieves 40-bar (580-psi) peak brake mean effective pressure (BMEP) at 1200 rpm with no irregular combustion. If peak BMEP is maintained below 35 bar (508 psi), fuel enrichment is no longer necessary, he said. Running at part load, the engine operates at up to 15:1 to minimize BSFC and maximize the “sweet spot” area on the map.
Next-generation MCE-5 VCRi engines will combine VCR and stoichiometric charges, diluted with high rates of external cooled EGR (up to 60%), to improve part-load efficiency by reducing heat and pumping losses, and optimizing the compression-expansion ratio. This strategy, combined with downsizing-donwspeeding, requires a high-energy ignition system to promote fast, repeatable, stable, and complete combustion, Rabhi explained. The design would appear to present NVH challenges compared with a conventional SI or diesel engine, according to engine experts in the audience.
Combining split-cycle and Miller cycle
The Scuderi Group’s split-cycle engine program is no stranger to SAE World Congress attendees in recent years. A naturally aspirated proof-of-concept unit has been running in a test cell at Southwest Research Institute (SwRI) since 2009. Recently, the Springfield, MA-based company added Miller cycle operation to the engine, according to Stephen Scuderi, the project’s chief engineer who also is his company’s lead patent attorney.
Scuderi noted that his company is presenting two SAE technical papers at this year’s Congress related to the “Millerization” of the Scuderi engine, including one detailing a vehicle simulation in a European subcompact car. In the modeling, the vehicle was projected to emit 81.6 g/km of CO2 on the NEDC cycle, comfortably below the 95 g/km target set by EU emissions regulator for 2020. The projected data showed that the Scuderi-powered vehicle could potentially achieve U.S. 65 mpg. For information on Scuderi Group’s patent application for the new engine design, see: www.scuderiengine.com/assets/Uploads/WO2012040431-A1.pdf.
The basic split-cycle engine uses two paired cylinders to divide the four strokes of a conventional Otto-cycle engine. One cylinder, known as the compressor, handles the intake and compression strokes, while the other cylinder, called the expander, handles the power (expansion) and exhaust strokes. The transfer of compressed gas is done through a crossover port connecting the compressor and expander.
For the turbocharged Miller-cycle version, the Scuderi engine uses a compressor that is downsized by 65% related to the expander. A further iteration, which Stephen Scuderi calls “the extreme Miller” and is configured to run on compressed natural gas (CNG), has a compressor that is about 50% the volume of its expander.
Stephen Scuderi told the SAE audience that simulations showed “diesel-like” thermal efficiency (around 49%) with significantly improved BMEP compared with typical CNG-fueled engines, and indicated that Scuderi Group is targeting the stationary power market.
Rotary redux with high thermal efficiency
Dr. Alexander Shkolnik is betting he can push the Wankel engine farther than Mazda, Curtiss-Wright, Norton, or any other rotary maker has taken it. Shkolnik is co-inventor of the LiquidPiston engine, and CEO of the company bearing the same name that he and his father launched in 2004. He has a PhD in computer science from MIT, and he’s using his modeling, optimization, motion planning and electronic-control skills to unlock new potential in Felix Wankel’s brainchild. See LiquidPiston's SAE technical paper: http://papers.sae.org/2008-01-2448/
Shkolnik has developed the High-Efficiency Hybrid Cycle (HEHC), an improved thermodynamic cycle that he claims optimizes each process (stroke) of the engine operation, with the aim of maximizing fuel efficiency. For SAE technical paper, go to: http://papers.sae.org/2010-01-1110. The cycle consists of a high compression ratio, constant-volume combustion, and over-expansion. At a compression ratio of 18:1, the HEHC offers an ideal thermodynamic efficiency of 74%.
For this, LiquidPiston has developed two rotary-engine architectures. Dubbed the ‘M’ and ‘X’ engines, they offer operational flexibility, a compact package, and low mass. Both 20- and 40-hp (15- and 30-kW) versions of the ‘M’ engine, and a 60-hp (45-kW) ‘X’ engine, are currently under test. Shkolnik noted the modeling of the engines has demonstrated 57% brake efficiencies are possible at full load.
The constant-volume combustion made possible by the rotary engine, combined with the HEHC, is the ‘holy grail’ according to Shkolnik, enabling 30% greater efficiencies than current diesels, he told the audience, adding that the engines’ dressed weight ranges from 50 to 80 lb (23 to 36 kg). He admitted that the Wankel’s traditional challenges—rotor sealing, heat transfer, and combustion efficiency—must be addressed before LiquidPower can demonstrate a production-viable engine.
The company’s objective is to demonstrate part-load brake efficiencies greater than 50%, in a low-NVH engine that approaches 2 hp/lb.