Miller Cycle

[Steve Ravet]

At 08:34 AM 1/8/97 -0800, you wrote:
>
>I attended the LA auto show, and was suprised to find
>that Mazda has been selling a Miller Cycle supercharged
>V-6 in their Milenia for three years.  I had not even
>heard of the Miller Cycle previously.  The system delays
>intake valve closure timing, to provide backpressure to
>a supercharger.  This then appears to provide a higher
>intake pressure, and more efficient intercooler operation.
>Higher pressure, higher temp, higher heat transfer = better
>volumetric efficiency for the motor.  Have I got it right?

The Miller cycle works by making the intake stroke shorter than the power
stroke.  The idea is that in a normal engine, when the exhaust valve opens
at the end of the power stroke, there is still pressure (energy) left from
the combustion process.  Instead of opening the valve and releasing the
pressure, if you could make the power stroke longer you could make use of
that pressure.  Well, you can't really make the power stroke longer, but
you can make the intake stroke shorter.  Do that by either closing the
intake valve before the piston finishes the intake stroke, cutting off the
amount of fuel it sucks in, or closing it partway through the compression
stroke, so that some of the mixture is pushed back out of the cylinder.
That lowers the peak pressure in the cylinder, and means that when the
exhaust valve opens the pressure in the cylinder is closer to ambient.  You
make less power now, because you are burning less gas, but you make it more
efficiently.  Add a supercharger to force more gas into the short intake
stroke, and you end up making the same original power only more
efficiently.  You have 2 compression ratios, say an 8:1 compression ratio,
and a 10:1 decompression ratio on the power stroke.

Closing the intake early causes you to pull a slight vacuum in the
cylinder, which is inefficient.  Closing it late causes really strange flow
patters in the manifold.  I think calibrating the injectors for a Miller
cycle engine would be "interesting".

--steve

>
>I was also wondering if grinding cams to give a miller 
>cycle is an option for modifying cars to run superchargers,
>as it has the effect of reducing the compression ratio,
>so says Mazda.  They claim the equivolent of 8.0:1.
>Seems easier than changing pistons!

It certainly does.  What are the practical issues involved in converting an
engine to a Miller cycle?

>
>paul timmerman
>