So then, if they're doing a non-ideal initial burn it would have to be a lean lower-compression burn, followed up by a higher-compression secondary burn?
It's a similar idea to the Atkinson cycle. You have dissimilar compression and expansion strokes. In normal engines, there's a limit to compression ratio because if it's too high, it causes knocking. But a bigger expansion ratio lets you extract more energy out of the combusted gas, which leads to higher efficiency.
The original Atkinson cycle idea was to use some complex linkage to get dissimilar compression and expansion strokes, but the way it's implemented in things like the Prius is to have a high compression engine, but mess with the intake valve timing such that you only use a small part of that compression during the intake phase so you effectively handicap your compression ratio to avoid knock, while still retaining the full stroke during the expansion phase.
I think it's safe to assume that the second stroke is burning incomplete combustion products left over from the first stroke. I think that the second compression stroke would have to be higher compression than the first in order to get more complete combustion of what's left behind.
Second stroke goes lower and there are some ports to add air which are not accessed during the initial compression stroke...so the stroke is longer (higher compression) and more air is added to help with the reburn.
Kinda sounds like combining the idea of the Miller cycle with a variable compression/stroke setup (see Nissan).
There are a lot of ideas out there that create gains individually...glad to see them being combined more and more in modern engines. (ex. VRIS, VVT, DFI,) I personally think there is still another few decades of playing around with ICE to be done...not sure it will be viable for the market...but the research will lead to a lot more interesting engineering.
Heh all along I'm wondering, what kind of thermodynamic cycle is it? By six strokes, does it mean there are actually distinct new phases to the PV graphs compared to Atkinson/Carnot/Miller?
Or is it just masquerading a well-known cycle underneath six strokes, only some parts are being optimised?
Seems to be like it is a Miller cycle (or could be) on the initial 4 strokes. Which would allow you to control the timing of the "Compression max point" in the stroke by varying the boost. That may also vary the amount of spent fuel remaining for the second "scavenge" stroke...which if I am reading comments above correctly it pulls air from ports lower in the cylinder which would help clear the cylinder for the next 4 stroke cycle.
Seems to me more focused on reburning/scavenging to make a "cleaner" burn than anything else though.
*Not an engineer...just a shadetree mechanic who reads too deep into engine papers.
So the first phase is like a regular 4-stroke engine, and the second phase is more like 2-stroke engine, where extra air (and possibly fuel) is introduced into the cylinder, like a 2-stroke, through ports located below the position of the piston during the bottom dead center of the first phase.
So I guess you have something like intake (high), compression (high), power (high->low), compression (low), exhaust (low->high), where in parenthesis is the adjustable piston height?
So a richer higher-compression first phase, followed by a leaner lower-compression second phase?
With direct injection they could even be injecting more fuel into the cylinder for the second stroke...but I suspect the second stroke F/A ratio is determined by the first stroke remnants combined with the extra air allowed in at the bottom of the second stroke.
All of this with the cam variators, timing control, boost control and fuel setup that VW already runs would be fairly easy to control with the proper sensors and code.
Just speculating at this point...but it makes sense to me.
Unless I screwed up my notation shift, the strokes from the patent are as follows:
1. Intake (low TDC -> high BDC)
2. Compression (high BDC -> high TDC)
3. Power (high TDC -> low BDC)
4. Compression (low BDC -> high TDC)
5. Power (high TDC -> high BDC)
6. Exhaust (high BDC -> low BDC)
So during the first power stroke, stroke 3, the cylinder moves from "high" to "low" and thus is the longer power stroke. Also during the second compression stroke, stroke 4, the cylinder position moves from "low" to "high". So technically leading to higher compression ratio. I was thinking it would cost too much energy to do so, hence dismissed that alternative, but I guess not.
The patent also notes that the extra scavenging ports are not needed, fresh air-fuel mixture can be introduced via the inlet valve(s) while the piston moves between the two BDCs.
Would be fun to try to simulate it in Ange's engine simulator[1].
That all makes sense. Pretty much what I assumed. The compression stroke at #4 is the longest stroke. Then it also has the benefit of the Power stroke at #5 being shorter which will mean it will have the benefit of a short(er) stroke motor on the power (more torque?) during that cycle.
The shorter stroke on #1 is desirable since you are using boost (I am assuming Miller cycle here) to control the beginning of compression as opposed to the intake valve.
Once the mechanical timing is down I assume it isn't too bad to keep all in line though...but a mess if it gets out.