"Jon Elson" wrote in message
...
Ed Huntress wrote:

On Mon, 15 Aug 2016 15:53:20 -0500, Jon Elson
wrote:


I think you're misreading it, Jon. This engine is truly
revolutionary.
Varying valve timing as you describe just produces a
pseudo-Atkinson
cycle, used on today's hybrids:

Right.


What this new Nissan engine does is to actually change the
compression
ratio of the engine.

The nominal compression ratio of an ordinary engine (say, 10:1) is
only achieved at full-throttle operation. At any other condition,
the
actual compression ratio is less, because less air-gas is let in.
This
is the chief reason that gasoline engines can't achieve the
efficiency
of diesels, which are always running at their nominal compression
ratio.

Well, getting rid of pumping loss is a really good thing, so maybe
this
accomplishes the variable output without a throttle, at least under
the
driving range of operation (might still be needed for idle).
If you can vary the compression ratio, you can approach the nominal
ratio even at part-throttle operation. To achieve it, you actually
have to increase the ratio *above* the nominal amount (say, to
14:1).
When you do that, the part-throttle operation, which lets in less
air-gas, causes the *actual* compression ratio, or effective
compression ratio, to be returned to the optimum 10:1. (These
values
are just examples.)

You wind up with diesel-like efficiency. Then, the Atkinson cycle
extracts more, by effectively extending the expansion stroke.

The Atikinson-cycle part of the operation is incidental to the
variable compression ratio. It's the variable compression ratio
that's
the big deal. A true Atkinson cycle is quite efficient, but the
pseudo
Atkinson cycle of today's hybrids is less so.

OK, that probably requires a degree in thermodynamics to understand.

Jon

Engine efficiency increases with higher combustion ratio, limited by
preignition and Knock.
https://en.wikipedia.org/wiki/Engine_knocking

Knock varies with combustion conditions and can be sensed with a
microphone sharply tuned to the block's resonant frequency (thus
filtering out other sounds) and controlled by backing off the spark
advance until it nearly disappears. The old vacuum advance did this
open-loop, advancing the spark further at light throttle when intake
vacuum is high and releasing it back to the RPM-controlled centrifugal
advance position when you floor the pedal. However this is the easy
but not the best way.

SAAB's conceptually simple system moves the cylinder block up or down
relative to the crankshaft and piston to vary the combustion chamber's
size to maintain maximum allowable pressure and the best efficiency at
any power demand.
http://www.autozine.org/technical_sc...h_engine_4.htm
Look at the positions of the orange eccentric shaft to the right of
the connecting rod, and the red combustion chamber.

--jsw