Continued thoughts on Miller Cycle

[Whittey]

I've been thinking more and more about this and have some thoughts....

The camshaft on a miller closes much after BDC. Between BDC and valve closing, what kind of pressure is created on a n/a engine?

The howstuffworks.com website shows the miller cam in the mazda engine to keep the intake valve open for 30 more degreess. That drops the compression ratio from 10:1 to 8:1. That seems like a drop that can be taken care of. Now that i'm sitting here thinking of numbers, lets say that extra 30 degrees subtracts 30psi from your compression. If you added, say, a big twin screw supercharger to this bad boy that puts out over 30psi, what do you lose in the translation? Or what if you use the supercharger just to keep it initially manageable and a turbo blowing through the supercharger (I want that kind of setup just for the cool factor) for the boost in the upper rpm's?

Help me, i'm going insane (-:


-=Whittey=-

 

[MaxRaceSoftware]

The camshaft on a miller closes much after BDC. Between BDC and valve closing, what kind of pressure is created on a n/a engine?
============================================

racing engines , especially drag race engines
typically close Intake valve 60 to 75 degrees ABDC
.....sort of like Miller-cycle effect ???

my old dragster's Chevy SBC 355 cid
actual measured 13.7:1 CR
Int valve closes 70.75 deg ABDC .037 Int lash 1.75 rocker
Cranking PSI = 225 psi after between 7 to 10 pulses of compression tester's needle movement

my computer program results were=
Cranking PSI = 225.9 psi
Dynamic Cranking Compression Ratio = 9.84:1
Intake Opens = 37.25 BTDC
Intake Closes = 70.75 ABDC

Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[franzh]

Knock me around if you wish, but if I remember correctly, the Miller Cycle engine intake valve closes much SOONER then a NA engine. This reduces the intake volume, making the engine appear smaller.

And, yes, I have seen the real neat and often referenced

http://www.howstuffworks.com

website, and use it frequently.

The forced induction setup is required to increase the intake charge density when RPM's increase, restoring engine power. This is how a small engine has the economy and lower internal friction, with the power of a larger engine when the throttle is opened.

A NA Miller engine will work, but not efficiently, and it won't have any benefit since there is reduced intake volume.

Rebuttal?

 

[ivymike]

Miller cycle engines close before OR after TDC, depending on what the designer felt like.

From a theoretical standpoint, the Miller cycle improves efficiency with or without forced induction. If you implement a Miller cycle scheme with a large difference between compression and expansion ratios, on a N/A engine, you'll likely have undesireably low specific power output.

If you can use VVT and eliminate the throttle, then you're going to get a bigger improvement in efficiency.

 

[MaxRaceSoftware]

many years ago, after reading either a Popular Science or
Mechanics mag , i thought all the "Miller Cycle" was about
was closing the Intake very late ABDC ....in conjunction
with using a Blower

Are there different "Miller Cycles" ??
--------------------------------------------
from a website ;
A Miller-cycle engine depends on a supercharger.
A Miller-cycle engine leaves the intake valve open during part of the compression stroke, so that the engine is compressing against the pressure of the supercharger rather than the pressure of the cylinder walls. The effect is increased efficiency, at a level of about 15 percent.

The late closing of the intake valve eliminates the substantial amount of energy normally required to overcome friction (as well as pumping losses), in the process of completing a normal compression stroke.

While this sounds good in theory, the usual result of blowing half the intake charge back out the intake valves would be a reduction in volumetric efficiency.

In the Miller-cycle engine, however, this is where the compressor comes to the rescue. Any loss of intake charge through "back flow" is more than compensated for by the density of the charge provided by the compressor. Under these circumstances, the Lysholm compressor is more efficient (lower pumping loss) at carrying out the job of filling the cylinders than a reciprocating piston.

Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[Whittey]

Looking at this feedback I can't help but think of possibilities for a diesel/miller engine. Unthrottled direct injection, supercharged by a whipple/sprintex/whatever, low rpm's..... Hrm... Of course diesels run much higher compression so the supercharger would have to use a ton of boost to overcome and i'm not exactly sure you can create that kind of boost, depending on how far ABDC it closes.


-=Whittey=-

 

[franzh]

My ME Prof gave me a heads up on the Miller cycle during a lecture once.

The rational for early intake valve closing was to allow the intake cycle to lower the pressure inside the combustion chamber, allowing the gasoline vapor to partially condense, absorbing latent heat.

This would increase the density by providing partial charge cooling.

This makes more sense than allowing the intake valve to close during the compression stroke, as the compression is far higher than the blower pressure.

Still, the Miller cycle engine has proven its merit. I drove the Millenia "S" a couple years ago and was very impressed.
Franz

 

[JayMaechtlen]

The article about Miller Cycle on Howstuffworks leaves me unconvinced, as did some of the materials on the Sterling cycle (for different reasons)

we have seen that Engineers (even professors) can be as confused as anyone else...

is there any difference between a "miller cycle" engine and a supercharged Otto-cycle engine, other than funny valve timing?

when you look at an engine with lots of tweaks from the ordinary, its real hard to tell what is really going on-

It seems to me, that during the intake cycle, air/fuel will flow into the cylinder as long as
1. the effective pressure in the inlet is higher than the effective pressure in the cylinder, and
2. the inlet valve is open.

leave the valve open too long on compression stroke, will get backflow from cylinder into inlet system. This would seem to be very inefficient, the energy cost of blowing the gas across the valve costs fuel. Maybe some other kinds of compressors (superchargers) are theoretically more efficient than recip piston, but in addition to the piston that is already there? I have my doubts...

closing the valve early to reduce effective volume- how much different is this than the usual, where we make a full stroke against a low density mix? (due to the throttle being not wide open...)

how about the engine shown a while back that varied the crankshaft/cylinder relationship? (variable compression/expansion ratio) Was it Saab?
comments?

Jay

Jay

anyway,