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 Post subject: Budak cycle in a slant?
PostPosted: Thu Nov 30, 2017 9:18 pm 
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For 2018 VW updated one of its engines from the traditional otto cycle to what they call the budak cycle (here's an article about it https://jalopnik.com/heres-how-volkswag ... 1818632265). Nothing ground breaking here, basically the same principles as the atkinson cycle but rather than the intake valve closing late it closes early. Both approaches improve efficiency by limiting the dynamic compression ratio such that it is less than the expansion ratio.

Ok enough background, time for a crazy idea. Imagine 2 near-identical engines... engine 1 has a SCR of 10:1 and DCR of 8:1, on the other hand we have engine 2 with a SCR 12:1 and a DCR 8:1. What would be the relative performance characteristics of these 2 engines.

Since their DCRs are the same would they produce the same power?
Since engine 2 has a greater expansion:compression ratio, would it run cooler?
And finally, if engine 2 runs cooler where does that difference in energy go? Do I actually gain more power?

I've read several articles covering atkinson and budak cycles and they generally mention the trade off of power for efficiency, I can't really tell where the power is being traded off though. For example if your engine already has a DCR of 8.4:1 you're at the limits of what you can run on pump gas, so increasing its SCR but leaving its DCR the same should yield similar (or greater?) power numbers I would think.

So with that crazy idea out there, can some experienced engine builders weigh in and explain why it doesn't work that way? Anyone done this or similar in a slant?

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PostPosted: Fri Dec 01, 2017 3:27 am 
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This whole cycles is just based on valve timing from the cam lobes correct?


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PostPosted: Fri Dec 01, 2017 4:34 am 
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I don't think this is anything new, except that engineering terms have been added. A 'radical' cam will help you run an engine with high static compression ration because it bleeds off pressure at low rpm's - due to 'cam timing' (overlap etc.). Likewise, 'RV' cams help power and torque because they help raise dynamic compression on engines with low static compression. With the high static compression ratio/radical cam you get improved drivability, with the low static compression/RV cam, you get improved torque and low end power.
I am paraphrasing things as I am not an engineer.

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PostPosted: Fri Dec 01, 2017 9:23 am 
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Greg Ondayko wrote:
This whole cycles is just based on valve timing from the cam lobes correct?

Yes. Usually, they use variable valve timing to run the more efficient cycle during low load situations (highway cruising, etc). As Badvert65 pointed out this is not really a new idea.

Badvert65, I've never really understood cams very well so I really appreciate your knowledge! I thought a radical cam was usually just a high lift, high duration cam usually with a good amount of overlap. No clue what an RV cam is, but I've seen some RV cam grind recommended on this forum before. Would be interesting to hear what specifies an RV cam.

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PostPosted: Fri Dec 01, 2017 12:10 pm 
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Concerning the trade off of power for efficiency,,,

engines only make power on the combustion stroke, the other 3 strokes: exhaust, compression and intake all consume power.

the Budak cycle motor is all about closing the intake valve early. Closing the intake valve shuts off the incoming flow of air and hence, reduces the horsepower consumed by pumping air in.

The reduced air intake nets a lower power output, as stated in the article "the latter is used during low-load cruising conditions' with the latter being the early closing intake.

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PostPosted: Fri Dec 01, 2017 8:12 pm 
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Quote:
Imagine 2 near-identical engines... engine 1 has a SCR of 10:1 and DCR of 8:1, on the other hand we have engine 2 with a SCR 12:1 and a DCR 8:1. What would be the relative performance characteristics of these 2 engines.


225 slant six #1 can be acheived relatively easily using the "Lou" build parameters of a 10:1 engine and a 270-280 advertised duration cam (about 220-230 at .050) to bleed the cylinder pressure down at the overlap event... typically this build is about 190hp at peak rpm... peak torque is around 3-4000 rpm.... gearing with OD for highway use to match can work for street/strip use. With proper ignition timing this could be run on 87 octane... (best power though on 89-92)

225 slant six #2 can be acheived but the cam to bleed the cylinder pressure would be too big for the build and the engine would be doggy under 3000rpm and would like some drag gearing.... the cam selection appropriate for this build would land it closer to 9:1 DCR (250@ .050)... or higher if 110 race gas is available....for best power... at 9:1 it will have better torque and HP than the stocker engine and somewhat better than engine #1 above but not by too much (this proper cam match would weigh in at the 225-250hp range at high rpm, with peak torque in the 3500-4500 rpm range)

225 slant six #3 would be the "Doc" or "Dan" build where a cam with slightly more duration and lift than stock is used on a reasonable compression engine (9-9.5:1), the cam advertises in the 250-260 range and has a tight overlap event that doesn't allow the cylinder to bleed off on the overlap event... with the best amount of advance degreed into the intake lobe this cam would be about 9:1 SCR and an 8:1 DCR which would provide best low RPM streetable efficiency (low rpm torque is only slightly under #1 by a fair margin but peaks about 2500-3500 rpm, lower depending on advance and LSA).

Quote:
if your engine already has a DCR of 8.4:1 you're at the limits of what you can run on pump gas, so increasing its SCR but leaving its DCR the same should yield similar (or greater?) power numbers I would think.


This is horseradish from the slant perspective... I have a 12:1 fat pak slant in my duster and it runs perfectly fine on super... the DCR is 9.2:1.... (just to be safe for racing I'll spike the tank with something over 100, and not use the vacc advance).

This is all academic as we don't quite have a set up for VVT.... At one time the other brands had played with hydraulic lifters with faster bleed down rates... the theory being that at low rpm they partially collapse and provide a low duration low lift profile similar to the RV cam mentioned above, at high rpm where the lifter doesn't have time to bleed down is will have a better profile similar to the cam in slant #1 above... unfortunately Rhoads and Shubeck lifters are kind of a thing of the past... and because of the way the slant six hydro lifters are configured neither of the afore mentioned companies made a lifter that would be compatible with anything close to an AMC/Slant Six. Similarly we don't have any Powerball winners to step forward with an adaptation of the F1 poppet system, or the magnetic coil driven GM poppet system that was experimented with a partial decade back...

This isn't all inclusive, but have built #1,2, and 3.... and will settle eventually on 2 1/2 in my final build (and there is such thing as #4 for the guys having fun in the 13+:1 SCR realm...).


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PostPosted: Mon Dec 04, 2017 10:55 pm 
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DadTruck, you're right the power stroke is where the magic happens. The reason exhaust gas is hot is because the engine wasn't able to extract 100% of the energy generated during combustion, if it were the exhaust would be the same temp as the air/fuel that went in. The reason our engines don't extract as much power as they could is because combustion creates so much pressure that there is still tons of pressure even when the piston reaches the bottom of the power stroke. Gasses cool as they expand, and because the combustion gases still have pressure to expand but have no where to go, it retains it high temp and moves into the exhaust. This is the source for a bunch of wasted power, and the reason atkison and budak cycles create more power for a given cylinder filling amount.

DI, thanks for bringing tons of knowledge and real world experience to the discussion. Initially, my thinking was if the engine can produce similar power for less fuel, just crank up the cylinder filling and create more power. But after looking at your examples and DadTruck's comments its clear that any modification to start with the fuel efficiency of these cycles I could always be making more power by using the traditional otto cycle.

Final thought experiment.

Stock 198: 26.4cu-in cylinder filling, ~125hp (advertised)
Stock 225: 30.0cu-in cylinder filling, ~144hp (advertised)
Budak 225: 26.4cu-in cylinder filling, hp for this engine is a bit trickier to calculate but it probably makes around ~130 (based on 4% efficiency gain claim of budak cycle)

So better than a stock 198, but not as good as a stock 225. I was hoping I could increase the displacement of a 225 but limit cylinder filling to 225 and make more power, but its way better to use the extra displacement (which should have been obvious in the first place). Well, maybe still a good discussion for someone who wants a really fuel efficient slant.

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