Continuously variable valve timing (CVTC) on Micra 1.6L HR16DE

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The 1.6L engine in the Micra uses continuously variable (within a set range) valve timing on the intake & exhaust camshafts / valves. (Information source)

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Above: at idle, the system enables an Atkinson-cycle approach by delaying closure of the intake valves to reduce compression load. The Atkinson-cycle allows more expansion than compression to improve heat efficiency.

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Above: in normal operation, EGR (Exhaust Gas Recirculation) is varied by overlapping the open cycles of intake and exhaust. By recirculating some exhaust into the cylinder, pumping loss is reduced.

While the Atkinson-cycle and EGR help improve fuel efficiency, neither achieves really stable combustion because of the lower compression and temperatures of the Atkinson cycle and the refeeding of unburned exhaust gas in the combustion chamber with EGR. Nissan uses a Dual Injector system to compensate for these drawbacks; together with CVTC the two systems boost fuel efficiency up to 4% compared with Nissan gasoline-powered engines in the same class, and reduce the amount of rare metal required in the catalyst.

CVTC operation

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Above: camshaft actuator housing detail shown on the intake side (left side in image) of a Nissan V6 engine with CVTC.

The twin (intake & exhaust) valve control system operates on the Micra's engine using hydraulic pressure -- engine oil directed through oil pathways in the actuator from ECU-controlled hydraulic valves.

This Youtube video explains how the system works. The presenter actually demonstrates with parts from an HR16DE engine:

http://www.youtube.com/watch?v=qZbaDZPrl_0

See also: http://micra-forum.com/showthread.ph...cifications%29

The atkinson cycle thing is cool. But yeah, it doesn't work well unless you bump up the compression ratio. You start getting misfires if you dilute the intake charge too much. At 9.8:1, I don't think they're really able to fully take advantage of it that much. At that point, anything to improve ignition helps, thus the dual fuel injection. Nice thinking Nissan. :)

I suppose that explains why they say they're only using Atkinson "at idle," according to the Nissan diagram. I wonder if it's meant to reduce idle fuel consumption or just improve idle smoothness. Maybe both, though idle-stop-start would be the biggest bang for the buck in reducing idle consumption!

Good question - I'm not sure.

It could be for air injection into the exhaust. I've heard of that. Otherwise the only other thing I'd think is EGR.

My VW GTI uses the Miller cycle and my former VW Tiguan uses the Budack cycle (Revized Atkinson cycle)
Budack: https://www.roadandtrack.com/car-cul...cle-explained/
Miller: https://www.sciencedirect.com/topics...g/miller-cycle
Interesting stuff. Lots of variations. https://www.sciencedirect.com/topics...o-cycle-engine