SAE’s 2012 High Efficiency IC Engine Symposium in Detroit—the second as an immediate precursor to the SAE World Congress—opened on Sunday with a consideration of several alternative engine designs: opposed piston 2-stroke (Achates Power); variable compression ratio (MCE-5); split cycle (Scuderi); and advanced rotary (Liquid Piston).
Dr. David Foster, the Phil and Jean Myers of Mechanical Engineering at the University of Wisconsin - Madison, a Principal Investigator at UW Madison’s Engine Research Center (ERC), and co-director of the General Motors - ERC Collaborative Research Laboratory, set the stage with a brief discussion of maximum possible theoretical work, losses, and different processes.
The internal combustion engine is a chemical process, not a thermodynamic cycle, Foster said, and all of the energy in the fuel is theoretically available to be converted into work.
If that is the case, why do we fall so short? Well, the engine processes are not ideal. Two concepts need to be kept in mind. Non-ideal processes actually destroy the work potential of the energy in the fuel. And many engine processes require a work interaction that detracts from the work output of the engine.
...irreversibilities of combustion are not inefficiencies. Combustion efficiency is a measure of the completeness of the overall chemical reaction. One can have high combustion efficiency and low engine efficiency. Combustion efficiency in most engines on the market today is very high.—Dave Foster
As a summary of basic principles to underlie substantive actions to improve the efficiency of internal combustion engines, Foster noted that:
Losses, which are efficiency decrements, can be identified.
Compression and expansion are really wonderful processes thermodynamically—especially expansion.
Combustion irreversibilities of fuels are unavoidable.
Keeping in-cylinder temperatures low is thermodynamically very advantageous.
Heat transfer is a difficult loss to contend with. Low in-cylinder temperatures help.
Gas exchange work is a necessary expenditure because the engine is a chemical process.
An increase in pumping requirements for any reason carries a fuel economy penalty.
No stone can be left unturned: crevice volumes, friction, rotating inertia, etc.
Approaches to higher efficiencies using more conventional engine designs are the topic of the symposium’s second day, as well as the topic of multiple tracks in the SAE World Congress itself.
Achates Power. Achates is the developer of a two-stroke, compression-ignition (CI) opposed-piston engine (OP2S); Dr. Foster is on the Technical Advisory Board. CEO David Johnson said that to company continue to improve on its fuel efficiency (earlier post), citing latest fuel consumption figures for a medium-duty configuration of 189.9 g/kW-hr, compared to a Ford 6.7 liter turbodiesel at 239.9 g/kW-hr, with a BTE of 46.3%.
Johnson also noted that the raft of technologies that can be applied to increasing the efficiency of conventional engines—such as improved turbocharges—can also be used with the Achates OP2S.
The packaging of the engine is different, Johnson, said, but “we can make it work.”
MCE-5. MCE-5 is the developer of a gear-based variable compression ratio engine. (Earlier post.) Vianney Rabhi, director of strategy of MCE-5 Development SA, said that with a 6:1 minimum compression ratio, the current version of the MCE-5 VCR achieves 40 bar peak BMEP at 1200 rpm with no irregular combustion. If peak BMEP is maintained below 35 bar, fuel enrichment is not necessary. When running at part loads, the engine operates at high compression ratios to minimized BSFC and maximize the sweet spot on the map.
The company is working on a net-generation configuration combining VCR with stoichiometric charges and cooled EGR to improve part-load efficiency by means of both a reduction in heat and pumping losses. MCE will hold the three-cylinder engine to 33-35 peak BMEP as a way to remain in the best part of the BSFC map, Rabhi said.
MCE-5 expects that the combination of a highly boosted VCR engine with start-stop functionality, variable valve actuation, cooled external EGR and high-energy ignition will deliver an overall efficiency close to that of current full hybrid systems at more affordable costs. Expected CO2 emissions for a 90 kW system should be around 91 g/km.
Scuderi. The Scuderi Group is the developer of a split-cycle engine (earlier post). Stephen Scuderi presented an overview of applying Miller expansion techniques to the Scuderi split-cycle; two more papers to be presented at the World Congress will go into details on the specifics of implementing Miller cycle in the Scuderi engine, while another describes a model-based analysis of a Scuderi engine in a European vehicle.
Scuderi explained that “Millerizing” the Scuderi split cycle means downsizing the compression cylinder to get the over-expansion technique that is the core of the Miller approach—achieved in a convention engine by using early or late valve closing. By being able simply to downsize the compressor, the engine avoids some major pumping losses across the intake valve that would occur to get more aggressive in a Miller application with a conventional engine.
The simulation of a Millerized Scuderi split cycle engine in a 4-seat compact created from an average of the characteristics of eight OEM vehicles resulted in CO2 consumption of 89.2 g/km; the combination of the Millerized Scuderi with an air hybrid system resulted in CO2emissions of 81.6 g/km.
Liquid Piston. Liquid Piston is the developer of an efficient naturally aspirated rotary engine that can fire on both gasoline and diesel (earlier post) that is based on a thermodynamic cycle called the “High-Efficiency Hybrid Cycle” (HEHC), which borrows elements from Otto, Diesel, Atkinson, and Rankine cycles.
The HEHC cycle consists of 1) a high compression ratio; 2) constant-cycle combustion; and 3) over-expansion. At a compression ratio of 18:1, the HEHC cycle delivers an ideal thermodynamic efficiency of 74%. To embody the HEHC cycle, said Alexander Shkolnik, President and CEO (and co-inventor of the engine along with his father) Liquid Piston has developed two different rotary engine architectures: the M and the X series. He said that in practice, the company expects to be able to deliver 57% efficiency at full load and more than 50% at partial load, with a power density of 2hp/lb.
The company is currently testing 20 hp and 40 hp (15 and 30 kW) versions of the M engine, and a 60 hp (45 kW) implementation of the X engine. Sealing, heat transfer and combustion efficiency issues need to be addressed.