On Mon, 15 Aug 2016 15:53:20 -0500, Jon Elson
wrote:
Howard Beel wrote:
New engine to hit the market in 2017.
http://www.autoblog.com/2016/08/14/i...gine-variable-
compression-official/
Best Regards
Tom.
Hmm, my 2009 Honda Civic hybrid has variable intake valve timing, which
seems to pretty much accomplish the same thing. They've been doing this for
some time, probably dates back to at least 2006 on that model.
By reducing the charge drawn into the cylinders, it reduces the peak
pressure, so that seems to be varying the compression, too.
A pretty low-tech way to accomplish it, it doesn't make any chages to the
lower end. This Infiniti scheme seems to add a LOT more complexity to the
lower end. I wonder if this is some way to get around a Honda patent?
Jon
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:
https://en.wikipedia.org/wiki/Atkinson_cycle
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.
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.
--
Ed Huntress