Disadvantages to the Miller cycle?

[454Casull]

The Miller cycle basically keeps the intake valves open into the compression stroke, closing them a bit after the piston rises from BDC. Mazda's Miller-cycle engine displaces 80% of the nominal cylinder capacity.

Apparently, the Miller cycle gets more power and efficiency out of an engine.

[deuxexmachina]

Mazda also uses a Lysholm supercharger and not a regular roots style compressor. The difference here is that there is a male side with 3 lobes and a female side with 5 lobes. Part of the Lysholm's construction techniques reduce friction and leakage common to roots compressors.

The Miller Cycle is technically the same as the Otto Cycle except that a supercharger is used and the intake valve close timing is delayed so that the compression stroke starts with the piston compressing against the supercharger.

Mazda uses an intake valve timing of something close to 2deg BTDC open and 70deg ABDC close. Pretty late on the close.

Technically, the engine is set up for a static compression ratio of 10:1, but with the modified intake valve timing the actual compression ratio is a little under 8:1.

So, by using a smaller displacement engine with a shorter stroke Mazda can minimize the frictional losses and pumping losses for the engine itself. Then by using the Miller Cycle as opposed to the more common Otto Cycle it's easier to handle the most common issue of forced induction: engine knock.

It is a mixed blessing in that you have to pay more to get more, just as with any forced induction powerplant. The primary difference that Mazda touts is that it's cleaner too. So far I haven't been able to find anything different that any well tuned forced induction engine of comparable power would do.

Ironically enough, I've seen complete low milage used engines from '98 Millenias for about $1000. With the computer even.

That's about all I know :^)

[Stanimal]

Maybe I'll try and give it more of an overview.

An engine can be significantly more efficient if it had a larger expansion ratio (power stroke) then compression ratio (compression stroke). The larger expansion ratio allows the engine to extract more heat energy from the charge and the lower compression ratio makes it easier to manage knock. This, of cause, is easier said then done.

The Miller Cycle works on two fronts:
1) Efficiency: The Miller Cycle uses an engine with a higher then normal compression ratio, then delaying the closing of the intake valve(s) during the compression stroke, which in effect reduces the compression ratio, and retains a full power stroke. All else being equal, this would give you a more efficient engine when compared to a typical Otto Cycle gasoline engine and comparable power per displacement if we only measure the displacement from the point the intake valve(s) are closed. This design can theoretically improve the engine’s efficiencies by 15-20%. (The 2004 Toyota Prius hybrid uses their VVT-i system to achieve this)
2) Improved Power per Displacement: Positive displacement superchargers are reasonably efficient at low pressures. By using a low pressure positive displacement supercharger, power per displacement can be improved by 15-25%. However, the low pressure supercharger does rob a little bit of the efficiency. It’s 2-5% less efficient then a normally aspirated engine.

Altogether, the Miller Cycle can be 15% more efficient AND 10% higher power output per given displacement then a standard Otto Cycle engine.

However, as you can clearly see, the Miller Cycle is more complex then the Otto Cycle. Additionally, because it is better at extracting energy from the charge, the exhaust is measurable cooler. This makes it more difficult to design the catalytic converter to have it work effectively at the lower exhaust temp.

[SaabJohan]

A lysholm compressor is a screw-type compressor and it's completely different from a roots compressors. The best advantage from the Lysholm type compressor is its much higher effciency (roots have a really poor efficiency), the drawback is the higher cost.

Turbochargers are used to spool large two strokers, some also use a compressor and a turbocharger. That it isn't used here is probably because a mechanical driven compressor is easier to control.