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Mousetrap Betterment: Keeping Combustion Contemporary - Technologue

Motor Trend logo Motor Trend 6/26/2015 Frank Markus

Fuel cells, EVs, HEVs, and PHEVs make headlines, but experts will tell you that the suck-squish-bang-blow engine will be moving most of us for the next several decades despite efforts to kill it. They trust the industry’s pocket-protected elite to continue developing dazzling new twists on combustion, two examples of which caught my attention at the 2015 SAE World Congress.

Research

Engineering consultancy FEV showed a variable compression ratio (VCR) system that doesn’t involve dramatic hinged blocks, wandering crankshafts, multilink connecting rods, or cylinder head plungers. Those concepts required radical redesigns of engine architecture, manufacturing processes, or both, but FEV’s simple, smart connecting rod concept allows for a step change between two ratios that are typically three or four ratio points apart.

Mousetrap Betterment: Keeping Combustion Contemporary - Technologue

SAE 2015© Provided by MotorTrend SAE 2015 Up at the rod’s “little end” where it connects to the piston, there’s an eccentric bearing that can raise or lower the piston by approximately 2mm relative to the rod. Intake suction, mass forces, and combustion pressure forces move the piston up or down, and engine oil pressure supplied to one of two hydraulic rams inside the rod maintains either position. A simple shuttle valve located across the bottom of the “big end” beneath the crankshaft controls the switch between these positions as each crank throw passes bottom dead center. It takes a few revolutions to switch all cylinders over to the new position, but this helps smooth the transition.

The auto industry’s pocket-protected elite should continue developing dazzling new twists on combustion.

Power and efficiency both benefit. During part-throttle cruising, high-compression operation (12:1 or 13:1 with gasoline) improves thermodynamic efficiency for a consumption drop of around 5 percent. Floor the accelerator, and compression lowers to a level that won’t provoke knock with aggressive spark advance (8:1 or 9:1), boosting output by a similar 5 percent or so relative to a fixed compression ratio engine. This concept is a boon to flex-fuel vehicles capable of running higher-octane fuels such as CNG or E85 that desire higher compression to fully use the energy in the fuel but are currently limited to what gasoline can tolerate. To date most emphasis has been on gasoline applications, but there are potential benefits for diesel engines, too. Higher compression operation (17:1) increases cold-start performance, reduces hydrocarbon emissions, and improves tolerance of low cetane fuels, and lower compression (14:1) reduces particulate-matter emissions and permits more turbo boost under high load operating conditions.

Panama-based Lugo Developments proposes a bigger revision to the crankshaft that delivers mechanical Atkinson cycle operation, with shorter suck/squish and longer bang/blow strokes shown at right. (Current quasi-Atkinsons fake a shorter compression stroke by closing the intake valve late.) Lugo connects the piston rod to an eccentric bearing driven by the crank and geared to a pinion gear on the block—kind of like an elaborate Spirograph ring combination. The result imparts a small up-down motion to the piston in 340 degrees of rotation and a larger one in the next 380 degrees. The big plus is that this composite linkage starts delivering positive torque to the crankshaft 15 degrees before the piston reaches the top of its travel, enabling dramatic ignition advance and allowing operation on gasoline with compression as high as 18:1. The expansion ratio of 36:1 extracts way more work from the fuel. This arrangement makes HCCI gasoline compression-ignition a more viable option and promises to boost engine efficiency as high as 50 percent. The Lugo Crankshaft was adapted to a 2.0-liter Duratec I-4 engine and successfully ran at speeds up to 3,500 rpm with minimal vibration despite having no counterbalance weights. (The out-of-phase piston motions help.) The actual crankshaft weighed 22 pounds less than the Ford unit, and fully assembled the rotating mass was close to that of the Ford. Furthermore, because the strokes are not all 180 degrees each, the rotating masses are non-periodic, avoiding the harmonic vibrations ordinarily addressed by the counterweights, and the Lugo Duratec reportedly ran very smoothly.

FEV has a fleet of I-4 engines running and targets production by 2020. The rods weigh about 50 percent more than a standard rod, but testing so far suggests there’s no problem spinning them to 6,800 rpm. Suppliers seldom talk price, but we’re assured the fuel consumption improvement cost per percentage point is dwarfed by that of electrification, and engines with and without VCR could be built interchangeably on the same line. What’s more, the technology is fully complementary to other efficiency-boosting technologies such as VVT, Miller cycle supercharging, cylinder deactivation, and externally cooled EGR.

The Lugo crank adds a lot of gear and journal-bearing friction and will cost more because it must be forged, due in part to its smaller diameter and to withstand the higher compression forces, and lubricating all these additional rotating elements will be a challenge, but the reduction in pumping losses and increased cycle efficiencies more than compensate. Mechanically rotating those pinion gears slightly even provides VCR within a range of 8:1 to 22:1! Cost is hard to estimate at this point, but even with lean combustion, engine-out emissions are expected to be clean enough to enable big savings in after-treatment and little revision of the manufacturing process is required. Keep the great ideas coming, fellow engineers.

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