Quote:
Originally Posted by jpd80
I fail to see how this is much of an advance over what VW showed a few years back
when it used aggressive VCT with higher compression in a turbo and supercharged engine.
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UPFRONT APOLOGIES: This post is longer than intended. jpd80 got me on an interesting topic regarding the VW Budack cycle engine. Again another Miller cycle engine re-badged, because, well, in modern days that is what "we" do.
Unless I am missing something, the more I think about it, over all this system seems to be a bit of shell game the patent office fell for.
The are a lot of misconceptions surrounding many of the components.
For example the Atkinson cycle used today is nothing but a Miller cycle with a name change. I'll just list a couple reasons:
1. The true Atkinson cycle, though conceived for a different purpose, resulted in a similar outcome as the Miller cycle in that the compression ratio (CR) was less than the expansion ratio (ER) by way of jointed "crankshaft/connecting rod", whatever you want to call it. The result is less power, however greater efficiency of the fuel consumed. This is one way to reduce internal hp loss by way of reducing "pumping losses". The fuel consumed in the intake and combustion strokes spend less effort compressing the next intake charge. So, although the engine produces less power this way, a greater percentage of the available energy from the combustion process is converted to mechanical rotational force of the crankshaft known as torque.
Along comes modern days and a misunderstanding of what the Miller cycle was. Somehow or another along comes this idea that using a conventional crankshaft and connecting rod arrangement in conjunction with the delay in closing of the intake valve long enough to push some of the intake charge back out of the cylinder past the intake valve, thus reducing the intake charge volume relative to the expansion volume (lower CR compared to ER), is now all of a sudden an Atkinson cycle, simply because there is no supercharger/turbocharger involved.
Actually, this misunderstanding is "the devils work".
2. The intent of the Miller cycle was to reduce combustion temperatures at TDC while an engine is under "full load" to prevent pre-ignition and allow a more complete burn of a rich fuel mixture under full power. Which it does. Anyone who has ever spent time testing on an engine dyno will know the ideal air/fuel ratio (
AFR) or stoichiometric for petrol in an engine under full load is around ~12.5:1 and not the ~14.7:1 often taught in school (at least in my day). The 14.7:1 is basically for light load conditions.
How did Ralph Miller accomplish this? With variable valve timing, of course. This was patented back in the early-mid 1950s. Two of Miller's patents cover the aspects of the Miller cycle; one in 1954 and the other in 1956. Under these patents, application of the Miller cycle using variable valve timing applies to enhancements of the following engine types:
- 4 stroke diesel
- 4 stroke spark ignition (petrol, alcohol, gas, etc)
- Naturally Aspirated
- Boosted (supercharged and/or turbocharged)
The Miller cycle in fact does reduce combustion temperatures at TDC. How? This is probably the 3rd time this is mentioned here, please forgive me, by reducing the CR by way of variable valve timing relative to the ER.
It is interesting, Ralph had 2 valve timing methods of doing this.
- Closing the intake valve very early
- Clising the intake valve very late
Ralph liked option 1 for heavy use applications. By closing the intake valve early, before the piston reaches BDC, the intake charge now has a window of time to expand as the piston continues to travel downward to BDC. If anyone has seen a compressed gas cylinder, such as a CO2 cartridge, rapidly decompress they will have noticed how cold it became. Even covered in frost. The same principle applies to an internal combustion engine when the intake valve is closed prior to BDC. The expanding intake charge provides what Ralph called "internal cooling".
Option 2 above is exactly what the modern "Atkinson" cycle engines of today do. Remember Millers patents were not limited to boosted applications.
The sum of this Miller cycle vs Atkinson cycle discussion is Miller cycle engines have been used in a lot of applications for many years, so so GM can not put a patent claim on this particular component of the system presented in the article.
This has grown longer than I anticipated. If I have time I will add another comment or so about the myth that twincharging has only been around for 24 years, why flat nosed cams are nothing new, maybe I will think of something else...
Forgot some interesting references:
https://www.dieselnet.com/tech/engine_miller-cycle.php
https://www.dieselnet.com/tech/engine_miller-cycle.php