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VARIABLE COMPRESSION RATIO ENGINES
This holy grail of engineering may have come too late, we think.

 

Infiniti presented a VC-T (Variable Compression -Turbocharged) engine at the Paris Motor Show on 29 September 2016. It was the world’s first production-ready variable-compression-ratio engine.

 

The last time we saw a show car with a
variable-compression-ratio (VCR) engine was in 2000, when SAAB displayed a prototype, but the Swedish company got into financial difficulties that precluded any production developments.

What is a variable-compression-ratio (VCR) engine and why is it a goal for many engine makers?

All current production engines have a fixed compression ratio – the degree to which air is compressed by the rising piston on the compression stroke. The ratio is fixed by the stroke length, piston height above the gudgeon pin and the combustion chamber space.

However, engine makers have long known that a variable compression ratio is desirable: lower compression at high power demands and higher compression at light loads, for improved fuel efficiency.

In the case of a naturally aspirated engine that fuel efficiency increase would be only 4-5 percent, but can be much greater if employed in a reduced displacement engine with high supercharging or turbocharging pressure.

A conventional four-stroke petrol engine is most efficient (maximizing the energy in the fuel) when it is running at a high load. A small engine must work harder and run closer to full load if it is to perform the same work as a bigger engine, which utilises only part of its maximum capacity. The small engine often extracts more energy from every drop of fuel.

One reason for this is that the pumping losses are lower in a small engine. Pumping losses arise when the engine is running at low load and when its fuel consumption is relatively low. In order to maintain the ideal air-to-fuel ratio (14.7:1), the air supply must be restricted by reducing the opening of the butterfly valve in the air intake, creating a slight vacuum during the suction stroke.

The extra energy needed for pulling the piston down is pumping loss.

Since a small engine frequently runs at full load and the throttle is therefore more often fully open, the pumping losses in the small engine are usually lower than they are in a big engine.

Additionally, a small engine is lighter, has lighter internal reciprocating mass and has lower frictional losses. Therefore, a small engine is generally more efficient than a big engine.

Although a small engine is efficient, it is not powerful enough to be used for anything other than powering small, lightweight cars. By supercharging or turbocharging the intake air, thus forcing more air into the engine, more fuel can be injected and burned efficiently. The engine then delivers more power for every piston stroke, which results in higher torque and horsepower output.

By forcing in air only at greater throttle openings when extra power is really needed, the fuel economy of a small engine can be combined with the greater performance of a big engine.

The energy in petrol fuel is better utilized if the compression ratio is as high as possible, but if the compression ratio is too high, the fuel will pre-ignite, causing ‘knocking’, which can damage the engine.

That’s where the attraction of the VCR engine comes in.

Due to its variable compression ratio, this engine can be run at the optimum compression ratio of 14:1 at low load in order to maximize the use of the energy in the fuel, and the compression ratio can then be lowered to 8:1 at high load to enable the engine performance to be enhanced by supercharging without inducing ‘knocking’.

Although most variable-compression-ratio research has been directed at petrol engines, there are advantages for diesels as well.

A VCR diesel engine can have substantially reduced peak firing pressures required for a given power density without negatively affecting the engine’s part-load behaviour. This facilitates a reduction in friction losses and weight and allows more freedom in the design of intake and exhaust ports.

A heavy duty VCR diesel can more easily comply with legislated limits for NOX emissions at high loads. The technology makes using high EGR rates possible without exceeding limits for particulate matter (Pm) and without resort to higher peak firing pressures.

 

 

Infiniti made the first production model

 

After more than 20 years of research and development by parent company Nissan, Infiniti’s four-cylinder turbocharged petrol VC-T engine represented a major breakthrough in internal-combustion powertrain technology.

It was also interesting that this new technology was released firstly in the Infiniti product range, not mainstream Nissan vehicles. That suggested a cautious release, to test the real-world experience across a smaller vehicle sample and also hinted that the cost may be considerable and better absorbed by a higher-profit marque.

The Infiniti VCR method was one of the most complex approaches we’ve seen, but it did provide ‘infinite’ variable-compression-ratio changes, between 14:1 and 8:1. Some other approaches provided an ‘either or’ choice of 8:1 or 14:1.

Roland Krueger, president of Infiniti Motor Company said:

“VC-T technology is a step change for Infiniti: a revolutionary next-step in optimizing the efficiency of the internal combustion engine.

“This technological breakthrough delivers the power of a high-performance two-litre turbo gasoline engine with a high level of efficiency at the same time.”

The ingenuity of VC-T engine technology lay in its ability to raise or lower the height the pistons reach. As a consequence, the displacement of the engine changed and the compression ratio could vary anywhere between 8:1 (for high performance) and 14:1 (for high efficiency). The sophisticated engine control logic automatically supplied the optimum ratio, depending on what the driving situation demanded.

VC-T technology claimed to deliver multiple
customer benefits, including significantly reduced fuel consumption and emissions, and reduced noise and vibration levels. It was also lighter and more compact than comparable-output, conventional engines.

Infiniti patented more than 300 new technologies in the VC-T engine.

The Infiniti VCR design replaced a conventional connecting rod with a multi-link system, operated by a ‘Harmonic Drive’ actuator that rotated a control shaft at the base of the engine. That shaft altered the angle of the multi-link and that varied the effective distance between the crankshaft and the piston, hence varying displacement and compression ratio.

The VC-T engine used individual cylinder ignition timing control and valve timing for high-precision control over combustion characteristics. Electronic valve timing control was used for the intake valves, contributing to more responsive acceleration, and enabling greater fuel economy under Atkinson-cycle
combustion conditions. The exhaust valves had conventional hydraulic valve timing control.

The VC-T engine was able to switch between Atkinson and regular combustion cycles without interruption. Each cycle enabled greater combustion efficiency and optimal engine performance as the compression ratio changed.

The VC-T engine had multi-point injection (MPI) and direct injection (GDI), balancing efficiency and power in all driving conditions

Because there were minimal piston lateral forces on the cylinder walls, the VC-T in-line four-cylinder engine needed no balance shafts to damp out vibrations.

 


Since its introduction in the Infiniti range, Nissan’s variable-compression technology has been used in the post-2022 Qashqai and X-Trail, 1.5-litre, three-cylinder engines, giving them unprecedented running smoothness and outputs of up to 150kW and 330Nm.

 

 

VCR alternative technologies

 

SAAB’s year-2000 SVC engine had a cylinder
head with integrated cylinders (a ‘monohead’) and a lower portion consisting of the engine block, crankshaft and pistons.

The compression ratio was varied by adjusting the slope of the monohead in relation to the engine block and internal reciprocating components. This altered the volume of the combustion chamber with the piston at top dead centre and changed the compression ratio.

The monohead pivoted at the crankcase by up to four degrees, using of a hydraulic actuator. The monohead was sealed at the crankcase by a rubber bellows.

The first SAAB patent application was lodged in 1990. The first usable experimental engine had a displacement of two litres and delivered higher torque and power output than was necessary.

Actual testing began with a 1.4 litre, in-line six in the mid-1990s. Interestingly, SAAB collaborated with FEV Motorentechnik in Aachen, who confirmed that the engine met the desired targets and that it was also possible to make further advances by continued development work.

The year-2000 SAAB display SVC engine was a five-cylinder, 1.6-litre prototype.

That’s as far as it went for SAAB, but FEV has been working on VCR engines ever since.

 

 

FEV Motorentechnik

 

For more than two decades FEV was involved in the design, development and testing of various prototype VCR concepts. As a result of continuous evaluation and research on VCR concepts, FEV’s VCR solutions were advanced.

The company had its VCR engine spark ignition and diesel engine solutions on test with several vehicle makers.

FEV pursued a variable-length connecting
rod to achieve variable compression ratios. In the FEV system, compression ratio adjustment was achieved by installing the piston pin bearing in a rotating eccentric.

The rotation was achieved hydraulically and mechanically, with small pistons and rods incorporated in the connecting rod design.

The mechanism operated using forces and moments related to movement of the piston for actuation and no external energy was necessary.

However, unlike the Infiniti VCR system the FEV design had two settings: high and low compression, with no graduation in between. FEV said the system achieved VCR transition within 0.2 to 0.6 seconds.

It was a simple system, requiring hydraulics and a two-way valve to lock the rod into either of two positions. The FEV con-rod incorporated two small hydraulic pistons, each within a dedicated chamber. One chamber drained and the other filled with low-pressure oil that came through the crankshaft and the con-rod
into that chamber.

The upside was modular and compact design, so that the FEV VCR con-rod suited the trend towards multi-purpose engine platforms. VCR connecting rods could be used across all engine platforms, including petrol, diesel and flex-fuel and in configurations that included in-line and ‘boxer’, with bores down to 70 mm.

For several years FEV operated a Lotus Elise MK1 demonstrator vehicle. The demonstrator featured a 1.8-litre TC PFI engine downsized to 1.65 litres, a six-speed manual transmission and VCR con-rods that produced compression ratios of 8.8:1 and 12.0:1.

In addition to this in-house demonstrator, FEV’s two-stage technology was also used in various demonstrator vehicles at leading OEM sites.

FEV displayed a working cutaway at the 2016 SAE Conference in the USA and Dean Tomazic, executive vice president of FEV North America, said that European OEM testing had been very successful to date. FEV had 50 operating engines in European makers’ test fleets, back then.

FEV’s sophisticated con-rod wads much more expensive than a conventional steel rod, but the emissions and fuel trade-off alternative that engine makers face was a hybrid electric drive fitted to an existing non-VCR engine. The FEV con-rod was a cheaper method.

 

 

Porsche and Hilite

 

Porsche and Hilite patented another
design of variable-length con-rod.

A solenoid allowed small oil-pressure-driven rods and an eccentric adjuster to raise or lower the bearing supporting the piston.

Hilite International made components used in variable valve timing controls (VVT) that ran reliably despite their complexity.

 

 

MCE-5 Development SA

 

The MCE VCRi engine operated for some years under the hood of a Peugeot 407. The 1.5-litre engine produced 165kW and 420 Nm of torque, with test fuel consumption of 6.7 L/100 km.

That compared with 155kW and 290 Nm of torque, and 14.7L/100km for the Peugeot 407’s three-litre petrol V-6. A 2.7-litre turbodiesel also was offered in the production car, delivering 150kW and 440Nm of torque and 11.8L/100 km.

 

 

Gomecsys

 

Dutch engineering company, Gomecsys, developed its own variable compression ratio technology.

The complete VCR system was integrated in the crankshaft and virtually any four-stroke engine could be upgraded by replacing a normal crankshaft with a Gomecsys VCR crankshaft.

A central splined shaft ran through the crankshaft and engaged with gears that were meshed with ring gears around each crankshaft big-end journal. When the central splined shaft was rotated by a worm-drive actuator the geartrain moved the con-rod big ends up or down, thus varying the stroke and compression
ratio of the engine.

Like the Infiniti VCR engine the Gomecsys engine had infinitely-variable compression ratios, between upper and lower limits.

Additional fuel saving technologies incorporated in the system boosted the overall CO2 reduction to a claimed 18-percent, without downsizing.

 

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