Variable Compression—the Rest of the Story

Infiniti VC-Turbo

Over the course of a round-table discussion and a separate private interview with Alain Raposo, Alliance global vice-president, powertrain and EV engineering and Shinichi Kiga, chief powertrain engineer, gas engine group, we now know as much as these guys are allowed to tell us about Infiniti’s new variable compression turbocharged engine—which appears poised to become the first engine to reach production using technology that patents were first applied for in 1932.

Cost

No engineer sitting next to a PR guy will let slip the true cost or future price of a new technology, but the basic point of VC-Turbo technology is CO2 reduction, and the industry tracks the manufacturer cost of these technologies per gram/km of CO2 saved. Achieving compliance with today’s regulations has largely involved plucking all the “low-hanging fruit” in terms of cost, and many believe 48-volt hybridization to be the next big step forward in CO2 reduction. The cost of that technology is said to be $70-80 per gram/km, and Infiniti says the cost of VC-Turbo comes in at about half that. Raposo adds that the technology has some margin for further CO2 reduction/fuel efficiency built in as well.

Homogeneous-Charge Compression Ignition

One tool in the VC-Turbo box for further improving engine efficiency down the road is HCCI under certain optimal operating conditions. The high compression mode makes it possible to turn off the spark plugs and get the ultra-efficient all-at-once combustion that diesels enjoy using cleaner burning gasoline. And of course, further CO2 reductions can be accomplished much further down the road with hybridization.

Flex Fuel Compatibility

Things have gone mighty quiet on the flex-fuel/biofuel fronts of late, at least on the manufacturer side, but variable compression is a boon to adapting engine combustion to suit fuels of widely variable octane ratings. The knock limit varies considerably from pure gasoline to blends like E85 or E100 ethanol (and Raposo says E30 combustion is the most difficult to control), M85 methanol, biobutanol, etc., so the ability to tailor the compression ratio to suit the octane allows for more efficient combustion of the higher-octane fuels.

Turbo-Lag Reduction Tricks

I asked if the VC-Turbo engine was employing any of the tricks we’ve seen lately to reduce turbo lag—like Mazda’s acoustically optimized exhaust manifold (which pairs each exhaust runner with one that fired immediately before it to leverage acoustic pressure wave scavenging to improve flow) or the strategies to accelerate the flow of low-rpm exhaust gasses onto the turbine (like variable nozzle turbocharging or Mazda’s twin-path system that forces the low-flow through a small opening to accelerate the gasses). The answer was no, but obviously these are possible tools for future improvement, and of course, quickly dropping the compression ratio helps to hasten turbo spool-up.

Other Tidbits

• Infiniti’s VC system is not easily adaptable to V engines, so don’t expect V-12 performance out of a future VC-twin-turbo GT-R V-6.
• The company is contemplating use of a compression ratio gauge on the instrument cluster to highlight how variable it is (indeed, the full range of engine loads and speeds is mapped to assign the ideal ratio for every condition.
• Finally, we have BEGGED Infiniti boss Roland Kruger to dream up a more epic name than VC-Turbo for his ground-breaking engine—something that’ll stand up to Hellcat, Hemi, Small Block, EcoBoost, etc. in the engine hall of fame.

MCE-5/Dongfeng VCRi Concept

While in Paris I scheduled a meeting with Henri Trintignac, CEO of automotive powertrain R&D firm MCE-5, which is based in Lyon, France. The firm has been working on variable compression for over 15 years and is currently developing the technology for production “before 2020” with licensee Dongfeng, of China. I first covered this technology in June 2009, but to recap, the basic concept is identical to Infiniti’s, in that a mechanism alters the compression ratio by raising and lowering the piston’s stroke within the cylinder. The mechanism for doing this, however, is quite different.

Instead of a connecting rod pinned to the piston, the piston has a rigid vertical rack beneath it with teeth on two sides. One side engages a gear wheel that ensures the piston and rack remain perfectly aligned in the cylinder as it goes up and down. The other side engages a toothed sector—a small arc of a gear wheel—on a control arm. The center of this arm connects to the crankshaft via a traditional (but shorter) connecting rod. The end opposite the sector is pinned to a hydraulic piston. Moving this piston up or down changes the operating angle of the control arm with the sector, and this in turn changes the position of the piston’s range of motion within the cylinder. Here again, a picture is worth a thousand bumbling words of explanation.

Advantages of VCRi relative to VC-Turbo

• The range of compression variation is higher—8:1-18:1 (they’ve even experimented with 20:1) permitting full exploitation of Miller-Atkinson cycle operation (very late intake valve closing results in an effective compression ratio of 9:1 with an 18:1 expansion ratio in max-efficiency mode
• Piston alignment within the cylinder is even better, further reducing piston-to-cylinder-wall friction
• By controlling each cylinder’s compression individually, MCE-5 claims quicker ratio changes (as much as three points of ratio change per cylinder firing event), and hints at greater ease of achieving the fine-tune control required to support HCCI in the future

Disadvantages of VCRi relative to VC-Turbo

• Packaging the individual hydraulic control pistons adds 65-70 mm to the width of the block (although when the technology is used to deliver V-6 performance with an I-4, the packaging is still way narrower
• The pistons, hydraulic controls and gear wheels likely add a bit more mass and parasitic loss than the VC-Turbo’s shaft, electric stepper motor, and harmonic drive

Both companies claim a cost basis that’s essentially half that of 48-volt electrification, but Trintignac goes farther and claims a cost figure of $280-400 per engine depending on size, and that it might eventually even undercut the cost of diesel. When asked about the friction and durability implications of a design that routes full cylinder pressure through so many enmeshed teeth he claims that friction reductions elsewhere—like piston-to-cylinder-walls, and the inherent reduction that comes from downsizing an engine—help net a total system friction that falls somewhere between that of a gasoline and diesel engine of similar output. He also assures me that the engine has survived typical manufacturer durability testing cycles.

By exploiting the full benefit of the Miller-Atkinson cycle, Trintignac claims a base efficiency of 40 percent, which is up from what he claims is a range of 35-36 percent average efficiency for similar output gas engines today. Furthermore, 85 percent of the VCRi’s engine-operating map is said to top 32 percent efficiency. And high specific output enables greater downsizing and reduced need for transmissions offering more than six ratios in order to keep the engine operating in it efficiency sweet-spot. MCE-5 has a roadmap to improve that average engine efficiency to 44 percent by 2025 using future technologies like HCCI.

We look forward to following these two variable-compression competitors in their race to market, and to assessing the performance of each at our earliest opportunity. Stay tuned.

Four Expert Talking Points on Variable Compression:

• Permits lower (8:1) compression for high-boost, high-load turbo operation without knock—this is where the big-engine performance comes in
• Permits higher (14-18:1) expansion ratio to extract maximum power from small combustion events at lighter loads and while cruising—this is where the diesel fuel economy comes in
• Knock is prevented at those high compression settings by delaying intake-valve closure until the piston is as much as half way up its compression stroke, meaning that the air-fuel mixture is never compressed at 14 or 18:1
• The tech promises to allow the internal-combustion engine to meet tightening fuel economy/CO2 regs without costly electrification for another five years or so

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