On Tue, 16 Aug 2016 15:05:36 -0500, Jon Elson <
jme...@wustl.edu>
Let me try the simple version and see if I can be clear: It is a
thermodynamics issue. The higher the compression ratio, the greater is
the Carnot efficiency of an engine. You don't need a thermodynamics
background to get the idea of the Carnot cycle and efficiency.
Wikipedia probably does it.
This is the main reason why diesels are so efficient: they always run
at full, nominal compression. There is no throttle on the air. It's
only the fuel that's varied as you advance from idle to full throttle.
On a spark-ignition engine, you keep the fuel/air mix as close to
uniform as you can, and you vary the amount of the mix that gets into
the cylinder, with the throttle. If you vary the fuel/air ratio by
much (the ideal is 14.7 pounds of air for a pound of gasoline), the
mixture won't ignite with a spark. So at full throttle, the engine
will be running at a high compression ratio, the nominal ratio --
maybe 10:1 for example. At part throttle, the lesser amount of
fuel/air mix produces a much lower effective compression ratio --
maybe 5:1 at some throttle settings. The Carnot efficiency goes to
hell.
So you can see why having a variable compession ratio is such a big
deal. Manufacturers have been trying to produce a variable compression
ratio system that works well and that doesn't cost an arm and a leg,
for close to a century.
The Atkinson cycle is something completely different, but it's another
thermodynamics issue. I'll give it a try:
The true, original Atkinson cycle involved a short intake and
compression stroke, and a long expansion stroke and exhaust stroke. A
true Atkinson did it by means of a complex crank mechanism. The
"pseudo Atkinsons," like the engines in today's hybrids, have the
same-length stroke for all four parts of the cycle. But they open the
intake valve late so the cylinder is less-filled. The expansion
stroke, therefore, is *relatively* long for the amount of fuel being
burned. You get more efficiency because the charge expands to a
greater degree than normal. The nominal compression ratio is very
high, but the *actual* compression is normal, because of the lesser
cylinder-filling.
This Nissan engine combines both, but it's the compression ratio
that's the big deal.
Tell me if this is as clear as mud. <g>
--
Ed Huntress