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AS NOTED BY edmunds.com, Nissan made news at the 2016 Paris Motor Show with its variable-compression-ratio VC-T engine scheduled for production in the 2018 Infiniti QX50 crossover. Not that it’s unique in having variable compression ratio: Honda displayed a similar concept at the SAE International World Congress in 2009. Saab had a completely different approach for variable c.r. back in 2000. And the CFR (Combustion Fuel Research) test engine has had variable c.r. since the 1920s.
Who cares and why?
Background. For a given four-stroke gasoline engine and its particular fuel, there’s a relationship between its efficiency and its compression ratio, how much its air/fuel mixture is compressed. Typically, higher c.r. gives more efficient operation–until inevitable detonation occurs. Detonation, aka knock and, in its less destructive form, ping, is uncontrolled ignition of the air/fuel mixture at the wrong time and place in the combustion chamber. For details, see the Coordinating Research Council’s Literature Review on this.
CFR test engine. A gasoline’s octane is defined by its anti-knock characteristics when operating a CFR test engine at increasing c.r. The CFR engine is a carbureted single-cylinder test rig with an adjustable cylinder head: Screw it downward for higher c.r., upward for lower.
Two hydrocarbons define the octane spectrum: isooctane as 100 and heptane as 0. A test fuel’s Research Octane Number is the percentage of isooctane in an isooctane/heptane blend producing identical anti-knock characteristics. (Pump octane, the number posted at the gasoline station, is RON averaged with Motor Octane Number, the fuel’s anti-knock characteristics measured on the CFR engine under another set of test conditions.)
On a modern automotive engine, for a given fuel octane, detonation is avoided by finessing the spark advance in response to knock-sensing. Generally, the more advance, the more efficient–until knock is sensed.
However, greater efficiency could be gained by mimicking the action of the CFR engine: adjusting c.r. during operation until just short of ping. But how to do it?
Saab SVC. In 2000, the Saab Variable Compression engine adopted an approach similar to the CFR engine’s. The SVC’s supercharged 1.6-liter double-overhead-camshaft 5-cylinder engine was split horizontally, its bottom end containing the crankshaft, its upper portion consisting of cylinders and head pivoting with hydraulic actuation.
At heavy load, the upper portion pivoted upwards, resulting in a relatively low 8:1 c.r. amenable to power-producing forced induction without detonation. Under light load, the upper portion pivoted downward, reducing the cylinders‘ clearance volume and yielding a fuel-efficient high c.r. of 14:1.
The SVC engine produced 220 hp, with a claimed 30-percent improvement in fuel economy with no loss of performance compared with a standard engine’s. Alas, it was developed during a period when GM exercised its option to increase its 50-percent ownership of Saab to full control. GM shelved the idea as too costly.
Honda Atkinson Cycle. At the 2009 SAE International World Congress, Honda showed a different approach–and intent–to variable compression. Its single-cylinder engine displayed a genuine Atkinson Cycle, in which the intake and compression strokes are shorter than the following power and exhaust strokes.
Briefly, the benefit of an Atkinson Cycle is in extracting more power from its extended cycles of operation. Other automakers have attempted to simulate these benefits with variable valve timing as opposed to the Atkinson Cycle’s (and Honda’s) actual variance of stroke.
Honda called this its EEGPE, Extended-Expansion General Purpose Engine, a prototype designed for portable generators, lawn mowers and the like. Its two strokes gave different displacements too: 135.2 cc for intake/compression, 203.6 cc for power/exhaust. The engine thus displayed variable c.r. Its compression ratio was 8.5:1; its expansion ratio, 12.3:1.
Nissan Harmonic Drive. The Nissan approach uses linkages similar to the Honda’s. However, whereas the Honda is designed specifically to produce two stroke lengths (and hence two different compression ratios), the Nissan’s c.r. can be varied continuously from its high-efficiency/light-load 14:1 to its high-power/heavy-load 8:1.
The Nissan design is a turbocharged 2.0-liter inline four producing 268 hp. It’s seen as replacing the company’s 3.5-liter V-6, with a 55-lb. reduction in weight and 27-percent reduction in fuel consumption. The engine will also have dual fuel injection, with a multi-point system for low-compression operation and direct injection for its high-compression mode. Both sets of injectors are used in conditions of high load and high rpm.
The Nissan engine is a far cry from the carbureted CFR test rig. Together, though, they’re excellent examples of the evolution of research to practice. ds
© Dennis Simanaitis, SimanaitisSays.com, 2016