The Atkinson cycle in the Prius gasoline engine

[wheeler]

I was looking for some info on the Prius & noticed that its gasoline engine
has been classified as an Atkinson cycle internal combustion engine.
When I searched for more info on the Atkinson cycle, this webpage
http://www.keveney.com/Atkinson.html shows that the Atkinson cycle engine
has a different con-rod to crankshaft setup which allows the 4-stroke cycle
to be completed in one complete turn of the flywheel, but the Prius engine
has a conventional crankshaft like any regular gasoline engine.
I was then told that the Atkinson cycle in the Prius gasoline engine refers
to the late inlet valve closing technique it uses during the compression
stroke to reduce pumping losses and improve efficiency, but isn't that the
Miller cycle < http://en.wikipedia.org/wiki/Miller_cycle > ?
Then I came across this http://privatenrg.com website which has the
following statement:

"... one interesting thing that Toyota does with its highly touted Atkinson
Cycle ICE (Internal Combustion Engine) is seldom, if ever commented upon. I
have taken one of their drawings of the Prius Atkinson Cycle engine and
colored it up so that we can see that they use an off-cylinder-center-line
crankshaft position to help garner the incredible efficiency this engine
manifests. This off-cylinder-centering of the crankshaft allows the TDC
(Top-Dead-Center) of the piston to be achieved AFTER the crankshaft has
passed its rotational TDC and is in its downward motion ready to better
absorb and transfer the already improved Atkinson Cycle combustion forces."

The above-mentioned drawing is at http://privatenrg.com/index.42.jpg .

So, the Prius gasoline engine was designed to have an upward stroke
compression ratio which is less than its downward stroke expansion ratio and
this is what actually refers to the Atkinson cycle, and Toyota added in the
late inlet valve closing technique from the Miller cycle.
Without the battery and the electric motor, I wonder whether the Prius
gasoline engine is really more efficient than a conventional (Otto cycle)
gasoline engine.

[Don Stauffer in Minnesota]

On Jun 26, 10:40 pm, "wheeler" <whee...@rollin.net> wrote:

>
> So, the Prius gasoline engine was designed to have an upward stroke
> compression ratio which is less than its downward stroke expansion ratio and
> this is what actually refers to the Atkinson cycle, and Toyota added in the
> late inlet valve closing technique from the Miller cycle.
> Without the battery and the electric motor, I wonder whether the Prius
> gasoline engine is really more efficient than a conventional (Otto cycle)
> gasoline engine.

While I am sure a good, efficient engine is nice to have in a hybrid,
I was under the understanding that most of the efficiency boost came
from eliminating or greatly reducing part throttle operation, where
the normal throttled Otto cycle engine takes a big hit in efficiency.

Regenerative braking also helps of course, but is certainly not the
whole improvement.