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Overly complex or beautiful sophistication? The cranktrain of the industry’s first production Variable Compression Ratio engine, the Nissan KR20DDET, laid bare. (Nissan)
 

Nissan’s new VCR engine: worth the effort?

Announced at the 2016 Paris Motor Show, Nissan’s new Variable Compression Ratio engine—known as the VC Turbo—pushes the pause button, at least momentarily, on the company’s journey to a completely electrified-vehicle future. Fully productionized as the KR20DDET, the 2.0-L VC Turbo debuts in the all-new 2019 Infiniti QX50 compact luxury SUV. It will gain considerable production scale in its second announced application, the 2019 Nissan Altima.

As detailed in SAE Technical Paper 2018-01-0371, “Development of a New 2L Gasoline VC-Turbo Engine with the World’s First Variable Compression Ratio Technology” (https://www.sae.org/publications/technical-papers/content/2018-01-0371/), the VC Turbo can infinitely vary its piston stroke and effective compression ratio between 8:1 and 14:1. The low-compression mode is for high power demand and high-road-load conditions and the high-compression mode for cruising and light loads.

Several carmakers have explored VC technology, only to be thwarted by the hurdles of cost, complexity and reliability. Nissan is the first to put it on the road, a culmination of two decades of work. The fuel efficiency gained from VC operation alone is 8%, according to Shimichi Kiga, the Chief Powertrain Engineer.

Crossover showcase

As part of a complete road-to-roof redo, the new QX50 moves to an all-new platform. It drops the previous utility’s rear-drive layout and the transverse-mounted VC Turbo replaces the longitudinally mounted 3.7-L V6 and 7-speed automatic. The VC Turbo is matched to Nissan’s JATCO-supplied Xtronic continuously variable automatic transmission. Both front-drive and on-demand all-wheel drive are available.

The transverse powertrain is more space-efficient than the previous longitudinal one derived from the FM-platform G35 sedan. The new QX50 is 8.1 in (206 mm) shorter nose-to-tail than the previous version, yet gains rear seat legroom and cargo space.

Automotive Engineering piloted a VC Turbo-equipped 2019 QX50 over about 160 miles (257 km) of Los Angeles city streets, area freeways and winding Malibu canyon roads. Small luxury SUVs are as thick as thieves on most LA-area roads and the QX50 was definitely in its element. Crisp throttle response is highly prized there—highly Balkanized roads make for sudden merges, disappearing lanes, hidden intersections and overall traffic chaos. At a rated 268 hp (200 kW), peak output for the new QX50 is down by 59 hp (44 kW) from last year’s V6. But in stoplight drag races, torque is king. And as with competitive turbo fours from Audi, BMW, Lexus and others, the new VC Turbo-powered QX50 reaches its peak advertised 280 lb·ft (283 N·m) at a low (1600) rpm. It stays in the fat part of the torque curve to 4800 rpm, giving it healthy midrange and part-throttle responses.

A 0-60 mph (0-97 km/h) sprint now takes about 6.5 seconds, around a half-second down from last year’s model. Not that you feel cheated. Acceleration is still close to 2.0-L turbo-powered X3s and Q5s. The KR20DDET spools quickly and the CVT, although sometimes exhibiting a ‘springy’ quality, changes ratios without delay. The engine’s transitions between low and high compression modes are seamless and transparent. Even if you peek at the largely decorative digital meter between the speedo and tach that displays when the engine is operating in high compression (Power) or low-compression (Eco) modes, there’s no audible or tactile sensation of anything occurring under the hood.

There is, however, a split second of perceived turbo lag when punching the throttle from rest. The sensation could be a CVT belt rushing to the right place on the cone, or a traction control intervention to the throttle map.
The new VC Turbo has a different voice than last year’s V6 growl, with an exhaust note from the dual pipes (last year’s model had a single exhaust) a bit higher pitched and in the upper rev range. The song it sings is almost Italianate. The 2019 model sports both active noise cancellation working through the audio speakers to nullify unwanted rumbles when cruising at highway speeds at low rpm and active sound enhancement to emphasize the positive notes.
There are paddle shifters that allow the driver to summon eight simulated fixed gear ratios in the CVT. Four selectable driving modes--standard, sport, eco and personal—tailor the throttle and transmission ratio maps.

Big efficiency gain

Why did Nissan go to the trouble of the increased costs and complexity of the variable-compression engine? Fuel economy. The company is anticipating a 27-30% jump in QX50 fuel economy with the VC Turbo compared with that of the previous 3.7-L V6. Final EPA numbers were not yet available when this article was posted but Infiniti estimates that the 2019 QX50 with AWD will achieve a 24 mpg city/30 mpg highway/26 mpg combined rating.
The 26-mpg-combined number is just 1-mpg better than the combined EPA estimates for the 2018 BMW X3 xDrive30i and Audi Q5 and is 2-mpg more efficient than for the AWD 2018 Lexus NX300. Those competitors use turbocharged 4-cylinder engines but without variable compression.

In 100+ miles (161 km) of L.A. metropolitan driving, the 3952-lb (1793-kg) QX50 delivered 24 mpg, mostly operating in the high-load/low-compression mode. So, the real question: How much of the QX50’s fuel-economy improvement is due to downsizing to a 2.0-L 4-cylinder turbo and how much of it is attributable to the variable-compression feature?

While Kiga-san maintained that the VC feature alone is good for an 8% fuel-economy gain, other factors include the move to a CVT and overall vehicle weight savings of about 100 lb (45 kg).
Challenges in optimizing the VC mechanism included machining the central cantilever/actuators that vary the piston stroke, Kiga said. Tolerances are critical, he continued, and that’s one of the reasons the VC Turbo 4-cylinder is assembled at the same Yokohama plant that makes the high-performance Nissan GT-R V6.

The VC Turbo engine requires premium-octane fuel—a surprise to some who might think that the ability to switch to low-compression mode under higher road loads would permit the use of lower-octane ratings.
With Infiniti stating that its products will be electrified from 2021 forward, where does that leave an elegant and complicated mechanical solution to improved fuel economy such as the VC Turbo? It’s a shame buyers can’t see the fascinating workings within this machine that took Nissan engineers so long to perfect. Then again, how many luxury SUV buyers are apt to look under the hood at all? Continue reading »
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