Miller-cycle question

[wizbummer]

I'm learning more about the miller-cycle vs. otto-cycle, but I'm finding very little information that i'm looking for on the net. Anyways, I have 2 questions:

1) How much intake pressure is required to keep air coming in during the part of the compression stroke that the intake valve is open?

2) Could any forced-induction engine be converted from otto-cycle to miller-cycle with just a change in the vavle and ignition timing?

 

[Retracnic]

Wizbummer,

I am by no means an expert on the Miller cycle engine, but I can answer your questions.

1)How much intake pressure is required to keep air coming in during the part of the compression stroke that the intake valve is open?

Inlet pressure does not have to overcome cylinder pressure. What you are doing is using the increased charge density to compensate for the reduction in volumetric efficiency. In over simplified terms: A miller cycle engine takes in half as much air that is twice as dense.

In a classic Otto-cycle engine, the intake valve would be closed the entire duration of the compression stroke. With a Miller-cycle, we leave the intake valve open for a portion of the compression stroke. In a standard engine, this would tend to cause reversion (the air/fuel charge to runs back into the intake tract). Yet in a Miller-cycle engine, this is not possible because the intake tract is pressurized as well. So what we get, in effect, is the piston pushing against the supercharger's compressor. This has the benefit of compressing the charge between them. Thus neither one incurring the full pumping loss associated with compressing a volume of gas.

So almost regardless of their respective starting pressures, the supercharger and the piston will always work to compress the charge between them.

2)Could any forced-induction engine be converted from otto-cycle to miller-cycle with just a change in the valve and ignition timing?

The short answer is no. For a Miller-cycle engine to be effective, the compressor must be more efficient at compressing the charge than the piston is. Turbine type compressors are only efficient over a very narrow range of speeds, making them unsuitable for Miller-cycle use. The compressor also has to be efficient at producing boost at relatively low engine speeds as well.

Good Luck,

Bryan Carter