Cavallino

I've just been reading a bit about the Miller engine cycle, and Im a bit pressed for time at the moment so Ill keep it short. As much as I've read, no one seems to have brought up any disadvantages to the Miller cycle. However, there have to be some kind of significant disadvantages to it over a normal combustion engine otherwise it would be more widley employed yes? So my question is what are the disadvantages and if there aren't any significant disadvantages, why isn't the Miller cycle more commonly used in modern autos?

454Casull

Cost. For example, Mazda's 3.0L Miller cycle engine is only 2.3L in use, so the 2.3L engine costs as much as a 3.0L engine.

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Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

Mendari

Yes. Cost is a factor because the Miller Cycle requires a supercharger in the compression cycle.

454Casull

Still haven't figured out why Mazda didn't use a turbo instead of a twin-screw on that engine. If they're going for efficiency that would have been the better choice.

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Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

Cavallino

Is it just the cost and the increased maintenance due to the forced induction that keeps the Miller cycle from becoming mainstream then?

454Casull

Basically. Also, I'm not sure whether Mazda advertised as 2.3L or 3.0L for their engine - as you know, displacement sells.

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Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

Mendari

Yup. In addition to the manufacturing cost of a supercharger, insurance companies raise their rates on forced induction cars. This cost, of course, trickles down to the consumer.
I'm not 100%, but the lobes of a roots supercharger must be used in order to compress on the intake stroke. The intake valve stays open while the piston moves up. Centrifugal superchargers and turbochargers have thin blades for conventional applications, i.e. all valves close when the piston moves up.

454Casull

You just lost me.

__________________
Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

Cavallino

Im guessing the blades on a roots supercharger are thicker and they're used because they have to deal with the additional stress of the pressure created by the piston compressing....?

454Casull

The intake valves for an Otto-cycle engine ALWAYS close during the piston's compression stroke.

__________________
Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

disco192

They didnt use a turbo because turbos do not create consistent boost with RPM like a supercharger. If they used a turbo it would be a very laggy car. Moreso than a regular turbo'd engine.

And it doesnt really sell because it was primarily designed for gas mileage over power if i am not mistaken. and it is a high price to pay for fuel economy.

FYRHWK1

well, my understanding of the miller cycle is very limited, but my understanding is that it requires the intake cycle to be under some kind of forced air, and if a turbo were to not be spooled (at idle or low RPM, after a BOV released, etc.) then it wouldn't have that, now what that would cause I really don't know. Someone wouldn't happen to have a paper or website explaining it would they?

454Casull

Oh, that's true. I suppose off the line the car would be pretty slow without instant boost.

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Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

454Casull

Nah. The 2.3L engine made more power than its conventional 3.0L sister engine, IIRC.

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Some things are impossible, people say. Yet after these things happen, the very same people say that it was inevitable.

Meyaht

could someone recap what the miller cycle is? i am un-enlightened