VWDiesel.net The IDI, TDI, and mTDI source.
Engine Specific Info and Questions => IDI Engine => Topic started by: fspGTD on May 12, 2004, 11:48:20 pm
-
All 45 pages of hardcopy are now in my hands, and scanning will commence shortly. :)
-
You have a PM... :D
-
me want :D . i'm going to rebuild one this summer (thats the plan :lol: )
-
This SAE paper is really the basis of research for all VW Diesel engines. It's ultra-cool, and also ultra-big. I scanned it into one .pdf file that's 20MB! Who wants it? :) I have no problems sending large attachments (broadband connection). But I can't put it online for you guys as currently I've only got 15MB free web storage space so it won't fit. Anyone have some web storage space they can host it at for the rest of the board to access?
Another option is I can re-scan it, separating the pictures from the text. Type the text in as HTML and scan the figures&illustration in as separate JPEGS. Then it might fit on my 15MB limit. But for now, I've just got one huge PDF file. (It was easier to scan in everything that way.)
-
Jake
I have about 150mb left on my site www.startercontacts.com
email me the PDF and I'll put it up there.
Larry
-
Larry - tried e-mailing it to you, but I guess I do have some limit for how large of an e-mail I am allowed to send! :(
Anyone have an upload point on the web where I can upload it directly?
Other idea is when I get the chance, I will scan the pics individually as JPEGs and write the text into HTML. (I'd like to eventually do this so we can link to individual images anyway, even though it will take a while.)
-
All 45 pages of hardcopy are now in my hands, and scanning will commence shortly. :)
Jake, see if the name Peter Hofbauer rings a bell. :wink:
I worked with him.
-
Jake
can you zip it?
Larry
-
All 45 pages of hardcopy are now in my hands, and scanning will commence shortly. :)
Jake, see if the name Peter Hofbauer rings a bell. :wink:
I worked with him.
Wholy moly!! Author of the paper is "P. Hofbauer and K. Sator, Volkswagenwerk AG"! He must have been a walking diesel encyclopedia of knowledge.
-
Jake
can you zip it?
Larry
That is a great idea! Don't know if the image data is compressed or not inside the .pdf but it's worth a shot! I will try that and maybe e-mail it, or maybe post it zipped on my website (if it's under 15 megs) for all to download.
Edit: Just tried zipping it and it must already be in a compressed internal format - as zipping it only lopped off about 200KB.
-
Dang!
I could set you up an FTP account for one of my sites if I knew how. I don't quite understand the whole FTP thing.
:(
-
If you have MSN you can send it to people.
[email protected] is mine.
-
If you have MSN you can send it to people.
[email protected] is mine
-
Ug. This is not working out very well... I tried uploading the file to a yahoo group, but then after a few minutes of progress, right when I thought it might work, it stops and comes back with the message "You may only upload files smaller than 5120K." :x
-
Jake, the reality is that almost no one is going to give you that much disk space for one file, other than on an FTB basis. I suggest you break the file up into about 10 subfiles and post them. that way it is easier on you as well as on all of us when we try to download them.
And my offer (see PM) still stands... :D
-
I have an HTTP/FTP server set-up on my personal machine, but uptime is not 100% as my computer (a desktop/portable hybrid) moves with me when I got on trips. I can do some wizardy with PDFs to make em smaller if you could send me raw scanned images as a single TIFF file, or a Postcript output, or even the finished PDF as you have it, in that order of preference.
I can have you upload the paper to my work FTP site and try to pare down the file size. If you're interested, send me a PM and I'll send you the log-in details. :)
-
Wow, all sorts of various people up here trying to be helpful... thanks for the offers folks but for the time being, I have a solution! I was able to find some additional web storage space on another e-mail account and so I put the file up there. Feel free to download it directly. Before you click to it, be forewarned that it is a 20MB PDF file. IE: You might want to save this link directly to your hard drive rather than re-loading it every time you need to look at it! OK, the link is:
http://home.comcast.net/~jakeru/15dsae.pdf
Enjoy!
-
Wow, all sorts of various people up here trying to be helpful... THANKS for the gracious offers folks, but for the time being, I have found a solution!
I was able to find some additional web storage space on another e-mail account and so I put the file up there. Feel free to download it directly. Before you click to it, be forewarned that it is a 20MB PDF file. IE: You might want to save this link directly to your hard drive rather than re-loading it every time you need to look at it! OK, the link is:
http://home.comcast.net/~jakeru/15dsae.pdf
Enjoy!
This file will work for the time being - until I get to scanning in the individual pictures (which will be common JPEGS) and entering the text into HTML format. That way we can refer and embed specific, individual pictures when we post so everyone knows what we're talking about!
-
Here is a quote I like from the 1.5D SAE paper:
"Only swirl chambers will permit rated engine speeds in excess of 83 U/sec of 5,000 rpm and a specific power output of 34 BHP per liter (25 kw)."
-
Wow Jake! This paper is so cool. Thanks for taking the time to share this with all of us.
Larry
-
You guys are most welcome. I am reading through it again myself! Everything I look through it, I learn something new it seems. :o
-
Definatly worth the 1min download time 8) :P
-
aha! just in time to hit 'print' on the big office laser jet tomorrow AM with a coffee :D thanks jake 8)
-
Thirty-one seconds to download...
Thanks Jake! :D
-
Wholy moly!! Author of the paper is "P. Hofbauer and K. Sator, Volkswagenwerk AG"! He must have been a walking diesel encyclopedia of knowledge.
Peter Hofbauer is Senior Technical Advisor at FEV Engine Technology in Auburn Hills, MI, where I worked between October 2000 until January, 2003. I didn't have a chance to work directly with him on any projects, but I was well aware of a number of his activities. Most of my time was spent on a project designing a new family of locomotive Diesel engines (4500 HP V12 and 6750 HP V16 :shock: ), although I did spend some time at Ford in Dearborn and also working on some EPA/DOE engine design projects.
-
great read jake... i'm hardly into it, can't wait to read more.
TDImeister... are you the same guy who posted before/after pics of your silver passat on the vortex? if so.. tasteful, i like your car. i'll take 1 of those 6000hp jobbies!
-
I'm still slowly reading my way through the paper, but am already struck by a few passages.
Here are a couple...
The compression ratios which permit attaining optimum fuel consumption in small swirl-chamber engines range between 16 and 18.
What do you think about that? Other texts on diesels I have remark that fuel efficiency goes up in NA diesels until the CR is about 26:1, when increasing mechanical stresses overcome further gains. I have to wonder about the effect of turbcharging.
and...
The 1.5l 50 BHP Diesel and the 1.6l 110 BHP spark ignition engine use the same internal engine components (crankshaft, pistons, conrods and bearings).
So why does it say that diesels are "necessarily" more expensive than their gassers?
-
regarding your second point...
the 1.6L 110hp engine they refer to i believe is the herron head, early european GTI engine, that we never got this side of the water... it certainly isn't a regular old late-70s watercooled 8v, and i'm sure it would cost more to produce than an 80hp 1.8L GTI engine that we got for instance.
-
Good points, BlackTieTD, but IIRC we did get a 1.6l GTI here in North America. Don't know if it was the same engine, though... :?
In any event, one of my diesel engine books slams VW for using gasser parts in their early diesel engines. Compared it to GM's failed effort employing the same economizing. Unfair, AFAIK, since the 1.6TD in particular has a great rep.
-
I'll take a crack at these...
I'm still slowly reading my way through the paper, but am already struck by a few passages.
Here are a couple...
The compression ratios which permit attaining optimum fuel consumption in small swirl-chamber engines range between 16 and 18.
What do you think about that? Other texts on diesels I have remark that fuel efficiency goes up in NA diesels until the CR is about 26:1, when increasing mechanical stresses overcome further gains. I have to wonder about the effect of turbcharging.
I'd say I've got no reason to doubt the optimal CR's as presented for these motors, but I haven't read other sources that give conflicting information either. Have read some that supports it. I'd be surprised if the optimal CR were above 23:1, because the paper goes into a lot of discussion about "engineering tradeoff" of raising the CR to 23:1 to promote good cold starting and cold idling. I think the optimal CR depends on how much frictional losses increase as the CR increases, which is not the same for every engine. It would be increased on an IDI motor with all the combustion chamber surface area where more heat can be lost.
and...
The 1.5l 50 BHP Diesel and the 1.6l 110 BHP spark ignition engine use the same internal engine components (crankshaft, pistons, conrods and bearings).
So why does it say that diesels are "necessarily" more expensive than their gassers?
Where does it say that? The original 1.5 diesel engine block I think may be from the same casting at the 1.5l gasser, or might even be the same machined part number (not really clear on that), but I think may be different (they talk about reinforcements in certain areas for the diesel?), so I am not sure, would have to compare part numbers. The conrods are I think the same and I wouldn't be surprised if the bearings were as well. It does say that component tolerances are held smaller by selecting the ones most blueprinted for the diesel, also by using multiple head gasket thicknesses. This additional manufacturing complexity seems like it would add to the cost. Other sources of cost difference could be in the fuel injection components (injection pumps, etc) also there is the benefit of no spark system. The pistons are definitely different with a metal reinforcement inside the dieel version that's not in the gas, also a different shape on the surface. The paper goes into great length describing their development. Machining tolerances are higher with the diesel - they even matched pistons to bores according to "honing groups" to achieve the tight piston-wall clearance required in the Diesel. None of this is necessary in manufacturing the gas engine.
-
The compression ratios which permit attaining optimum fuel consumption in small swirl-chamber engines range between 16 and 18.
What do you think about that? Other texts on diesels I have remark that fuel efficiency goes up in NA diesels until the CR is about 26:1,
Could the key be "small swirl-chamber engines"? Perhaps they are different than direct injection.
-
TDImeister... are you the same guy who posted before/after pics of your silver passat on the vortex? if so.. tasteful, i like your car. i'll take 1 of those 6000hp jobbies!
That would be me 8) .... so can you make it to the GTG in London in June? Date is tentatively the 19th. The tentative host is a Diesel nut and the head tech at a SW Ontario VW dealership.
-
Good points, BlackTieTD, but IIRC we did get a 1.6l GTI here in North America. Don't know if it was the same engine, though... :?
In any event, one of my diesel engine books slams VW for using gasser parts in their early diesel engines. Compared it to GM's failed effort employing the same economizing. Unfair, AFAIK, since the 1.6TD in particular has a great rep.
VW did sell a 1.6 GTI but it was for one single year only: 1980. Before that, the `79 GTI (of which my family owned one -- it's my all-time fondest car) had a 1457cc CIS motor that made some pathetic excuse for horsepower :).
BlackTie is right, there was a so-called Heron head for the Euro GTI that had very high compression (in it's time) -- something like 10.5:1, in an era of points ignition and no knock sensor. Definitely not for North American 91-octane pump gas.
Anyway, this being a Diesel site.... I have an excellent book in German entitled "Modernste Dieseltechnik (State-of-the-art Diesel Technology) by Christian Bartsch, published in 1998 by Motorbuch Verlag. It goes through a pretty detailed histories of the VW Diesel- and TDI engines along with mention of some pretty key figures in their development like Peter Hofbauer, Joseph Steinwart and Richard van Basshuysen. There are also early sketches and prototypes of early engine concepts.
-
I'll take a crack at these...
I'm still slowly reading my way through the paper, but am already struck by a few passages.
Here are a couple...
The compression ratios which permit attaining optimum fuel consumption in small swirl-chamber engines range between 16 and 18.
What do you think about that? Other texts on diesels I have remark that fuel efficiency goes up in NA diesels until the CR is about 26:1, when increasing mechanical stresses overcome further gains. I have to wonder about the effect of turbcharging.
I'd say I've got no reason to doubt the optimal CR's as presented for these motors, but I haven't read other sources that give conflicting information either. Have read some that supports it. I'd be surprised if the optimal CR were above 23:1, because the paper goes into a lot of discussion about "engineering tradeoff" of raising the CR to 23:1 to promote good cold starting and cold idling. I think the optimal CR depends on how much frictional losses increase as the CR increases, which is not the same for every engine. It would be increased on an IDI motor with all the combustion chamber surface area where more heat can be lost.
The answer to this question, and the trade-off being mentioned, is that from a purely theoretical thermodynamic analysis, thermal efficiencies increase logarithmically (i.e. diminishingly) as a function of compression ratio according to roughly (compression ratio)^(n-1). This "First-Law Analysis" doesn't take heat transfer during compression and expansion into account, but in reality, this cannot be ignored. As compression ratios increase, geometrically the ratio of surface area-to-volume also increases, which therefore increases heat transfer, and reduces efficiency, among other effects.
In an IDI engine, compression ratios are higher than is really needed for optimum thermal efficiency due to cold-starting issues. Jake's paper, and contemporary common knowledge has it that the optimum compression ratio for thermal efficiency is closer to between 16-18:1, and most modern Diesel engines are being designed into this direction. For example, the PD TDI engines have CR between 18-18.5:1, while the state-of-the-art BMW, VAG and M-B Diesels have CRs between 16.5-17.5:1 This range is high enough for good first-law efficiency without excessive heat losses due to the high surface-to-volume ratio.
The 1.5l 50 BHP Diesel and the 1.6l 110 BHP spark ignition engine use the same internal engine components (crankshaft, pistons, conrods and bearings).
So why does it say that diesels are "necessarily" more expensive than their gassers?
Where does it say that? The original 1.5 diesel engine block I think may be from the same casting at the 1.5l gasser, or might even be the same machined part number (not really clear on that), but I think may be different (they talk about reinforcements in certain areas for the diesel?), so I am not sure, would have to compare part numbers. The conrods are I think the same and I wouldn't be surprised if the bearings were as well. It does say that component tolerances are held smaller by selecting the ones most blueprinted for the diesel, also by using multiple head gasket thicknesses. This additional manufacturing complexity seems like it would add to the cost. Other sources of cost difference could be in the fuel injection components (injection pumps, etc) also there is the benefit of no spark system. The pistons are definitely different with a metal reinforcement inside the dieel version that's not in the gas, also a different shape on the surface. The paper goes into great length describing their development. Machining tolerances are higher with the diesel - they even matched pistons to bores according to "honing groups" to achieve the tight piston-wall clearance required in the Diesel. None of this is necessary in manufacturing the gas engine.
The VAG Diesel engines have the same critical feature sizes such as cylinder bore-to-bore spacing, main-bearing diameters and widths, and con-rod big-end dimensions and wrist pin diameters to their gasser breathren, but this does not imply that they're the same part. The first 1.5L Diesels and early 1.6Ds did indeed have SOME identical components like cranks, con-rods and bearings to their gasser breathren, but as the power ratings increased with turbocharging and what-not, components were beefed-up as required.
The early VAG Diesels were not completely troublefree, but they had nowhere the bad reputation as the GM 5.7Ds. This is because VAG recognized that they were already starting out with a stout basis of an engine design. From its earliest days, many VW cranks and con-rods were forgings rather than the cheaper and inferior castings that GM used and still use to this day... VW's excellent modern Diesels -- which still very much have the same roots and resemblances to the `70s' ancestors -- give testament of the basic engine design, and that's why VW has largely resisted any major changes to a winning formula.
As to the cost deltas between Diesels and gassers in general, they do exist. I can list just a few components that Diesels have that gasser's of the same class and power ratings generally don't:
- Turbocharger, intercooler and associated plumbing
- Injection pump and injectors (highly precise components compared to gasser components)
- Under-piston oil jet coolers
- Forged cranks (some VWs, notably the contemporary 2.slow gassers, have gone to cheaper cast cranks :roll: )
- Cooled EGR
- Etc.
Also keep in mind that the price premium you see on the sticker of a car NEVER reflects an automaker's true cost. For example, that $2k price premium you see listed for a Diesel option in a new car truely only costs a fraction of that in terms of the component price differences to an automaker, but they (the OEMs) also have to amortize development expenses and qualifying/certifying powertrains for the markets in which they are to be sold, plus of course the obligatory profit margin.
-
Fascinating info and discussion... we've got some smart cookies on this board, it seems. :wink: PS - I wish I could read German, I'd go and look for a copy of that book!
I've seen some other SAE papers discussing I think development of the 5- and 6-cylinder VW Diesel motors (changes from the 4-cylinder) and also some on emissions stuff. Neither of which I obtained a copy of. But I do have 2 more on TDI motors - one on the "original" TDI engine development and another on the development of changes for the "New Bettle" TDI motor. My favorite SAE papers are the 1.5lD - because it's the root of all others - and of course the 1.6lTD paper (because it's the engine I race with of course ;) )
I kind of wish I got a copy of that 5- and 6-cylinder SAE paper... had info on enlarging/beefing up the components as they switched from the early IDI (1.5l) components to the later IDI (1.6l) components, IIRC.
-
great discussion guys. nice to see some good stuff brewing in the new forum!
Good points, BlackTieTD, but IIRC we did get a 1.6l GTI here in North America. Don't know if it was the same engine, though... :?
ahhh, yes. the 1980 GTI. i always forget about that one... i think that it was only canadian market, not USA, but I could be wrong. not sure if its the same european 110hp 1.6L heron head engine that we got here...... but somehow i doubt it. if it is the same engine, i want one.
That would be me 8) .... so can you make it to the GTG in London in June? Date is tentatively the 19th. The tentative host is a Diesel nut and the head tech at a SW Ontario VW dealership.
tentative june 19th... sounds like it won't be a problem. let us know when there is a set date and i'll write it on my calendar. i don't want to miss out on this, seems like it will be more of an educational experience for me as much as a recreational one. :D
VW did sell a 1.6 GTI but it was for one single year only: 1980. Before that, the `79 GTI (of which my family owned one -- it's my all-time fondest car) had a 1457cc CIS motor that made some pathetic excuse for horsepower :).
BlackTie is right, there was a so-called Heron head for the Euro GTI that had very high compression (in it's time) -- something like 10.5:1, in an era of points ignition and no knock sensor. Definitely not for North American 91-octane pump gas.
was that 1979 GTI imported, or did canada get a few GTIs in 1979 as well?? i thought the first year for GTIs here was 1980. super-rare... i've never seen a true european GTI in person. that would be a great stock resto project, early GTI with the heron head 8v.. oh wait, this is the diesel forum :lol:
-
On the question of diesels being more expensive than gas engines, Jake wrote
Where does it say that?
On page 78 of the document, near the bottom of the right column, it says, Lastly, we wanted to keep the cost of production, though necessarily higher than that of a spark ignition engine, as low as possible.
Good stuff, TDIMeister. Thanks for weighing in! :D
On the subject of common parts between the 1.5 gasser and diesel engines, you wrote but this does not imply that they're the same part.
I'm sorry if I gave the impression I was implying that. In fact, I was quoting from page 83 of the article, where it states it as fact. I just found it curious.
You also wrote Jake's paper, and contemporary common knowledge has it that the optimum compression ratio for thermal efficiency is closer to between 16-18:1, and most modern Diesel engines are being designed into this direction.
This is very old knowledge, predating VW's use of it by at least 50 years. In fact, the whole of VW's diesel head and piston design (up until the TDI) was lifted directly from Ricardo patents in the 1920's (as noted on page 82). My 1927 copy of Ricardo's book has what appears to be the exact same diagrams from page 82 of Jake's paper. The design of the pre-chamber, piston head, everything except the Bosch injector and glow-plug.
Ricardo agrees that cold start considerations are the principal reason IDI engines require higher CRs than optimal. Furthermore, his experiments in the 1920's showed that a preheated engine can start and run efficiently as low as 12:1, and 10:1 with very good fuel (peanut oil?)!
Great stuff...keep it coming! Gotta run to work. 8)
-
Haven't seen the old Ricardo stuff! Would love to get my paws on a copy of that...
-
By the way, I can envision people reading the above SAE paper quotes about optimal CR as saying: "Gee, well I'm going to just optimize my CR by maching my piston tops / increasing my head gasket thickness. That will rock." Well let me save you some time: reducing it in this manner will not help with the engine efficiency. I know this because I have tried reducing the CR with a larger head gasket and the results were disappointing!
My conclusion is that when the engineers mention that the optimal CR is lower than the selected 23:1, they must be assuming that any extra volume added must be put primarily into the swirl chamber volume. Putting additional volume into the main chamber will not add any additional efficiency or power!
-
You're right, Jake. The head needs quench for optimal mixing, and quench is dependent on proper piston-to-head clearance. Place a too-thick gasket in there and you can kill the quench, with predictable effects on performance.
-
Jake's paper, and contemporary common knowledge has it that the optimum compression ratio for thermal efficiency is closer to between 16-18:1, and most modern Diesel engines are being designed into this direction.
This is very old knowledge, predating VW's use of it by at least 50 years. In fact, the whole of VW's diesel head and piston design (up until the TDI) was lifted directly from Ricardo patents in the 1920's (as noted on page 82). My 1927 copy of Ricardo's book has what appears to be the exact same diagrams from page 82 of Jake's paper. The design of the pre-chamber, piston head, everything except the Bosch injector and glow-plug.
The 1920's patents you allude to surround Ricardo's "Comet" swirl chamber, which as you know went through decades of evolutionary refinement to the latest "Comet V" that you see incorporated into the VW IDI. Therefore, to say that the VW IDI was borrowing 50-year-old technology at that time would be analogous to stating that the TDI borrowed direct injection technology can be said to date back to Rudolf Diesel's very first prototype in the 1890s -- correct in principle, but not completely true in light of the state-of-the-art during the periods of both engines' developments. Therefore, while the Ricardo Comet's underlying technology dates back to the `20s, the VW swirl-chamber was quite state-of-the-art at the time of the engine's development.
I should have also clarified that the "optimum" compression ratio was "known" long before this latest apparent bandwagon, but I should have expounded to say that this could only be realised in practice with direct injection and recent advancements in cold-starting aids like glow plugs and electronic control of injection timing.
You are 100% right about the fact that thicker head gaskets, while lowering the compression ratio, have the undesirable effect of altering the quench area and thus is detrimental to efficiency and performance rather than helping either.
-
fspGTD wrote
My conclusion is that when the engineers mention that the optimal CR is lower than the selected 23:1, they must be assuming that any extra volume added must be put primarily into the swirl chamber volume.
Ricardo recommends that the volume be evenly divided between the pre-chamber and the piston dome cavities.
fspGTD wrote Haven't seen the old Ricardo stuff! Would love to get my paws on a copy of that...
I see Ricardo's The High-Speed Internal-Combustion Engine go on eBay for $200-$300 a few times a year, depending on edition and condition. Or about twice that in a "classic book store"... :?
The differences between Ricardo's "Comet" of the early 20's and the "Comet V" of the early '70s are pretty minor, based on sitting here looking at drawings from Ricardo and the SAE document. Mainly details of the water cooling jackets and the injector and glow plug, as well as slight changes to the piston crown cavities. But the IDI geometry is all there.
BTW, in the interests of accuracy, it should be noted that Diesel's first prototype ran on coal dust (or variously on ammonia or gasoline vapors) injested with the air, and relied on detonation to run. It wasn't until his third prototype engine that he used direct injection (of peanut oil!) and achieved controlled combustion, perfecting the "contant pressure" process.
Man, talk about coming full circle! :D
-
Can you guys describe to me what "quenching" is and why it gets worse when you increase the main cylinder volume?
Does the ricardo paper describe the shape that is machined into the top of the VW IDI Diesel piston? I always thought it was curious design, and must have been based on some testing done at some point.
I have seen rudolph's first diesel engine in person! I even saw it running (OK, it wasn't running on it's own power, but was being driven by a small electric motor.) Very cool! If any of you are ever in Germany, I highly recommend visiting the museum where it is held at, the Deutche museum in Munich. There are also all sort of other VW Diesel stuff there! It's all in their diesel engine exhibit. They have all sort of other very interesting and well done exhibits as well. If you go, plan on spending a whole day in there, it's HUGE! I spend several hours in their underground mining exhibit. And there was so much I didn't get to see.
-
How about the shape of the port that the passageway between main chamber and swirl chamber? I have wondered if easing the edge of this port might in getting more power. I was wondering if it was an abrupt sharp angle because of economical manufacturing reasons.
Edit: in the 1.6l motor, VW increased the overall size of this port and found improved efficiency.
-
TDIMeister wrote
...to say that the VW IDI was borrowing 50-year-old technology at that time would be ... not completely true...
Yes, you're correct. I engaged in a bit of hyberbole in stating that they had lifted a 50-year-old design and dropped it into their engines. I bought my first VW diesel in 1981 (a 1980 demo car from the San Antonio VW dealership), and am now on my third VW diesel. In all that time I never had an inkling that theirs was not original research. Not that it is necessarily bad, mind you, just that it was news to me. Obviously the design evolved, as you say. And the TDI is a completely different design yet. Who knows where it come from.
fspGTD, quench is the space between the top of the piston and those flat portions of the head which overhang the cylinder bore and correspond to flat sections of the piston crown. The TD head is essentially flat over the bore, except where the valves intrude. Likewise, the top of the piston is essentially flat except for the "swirl pots". Ideally, these corresponding flat sections would have very little (less than 10-thou) clearance between them at TDC. By very nearly closing they force the air that would normally be trapped out on the edges of the piston and head to squirt violently into the swirl pots, creating turbulence and adding oxygen to the mix. The result is more power and torque, along with reduced smoke and other emissions. The precise top of the pistons determines which gasket to use to ensure proper static and dynamic quench (normal practice is to provide about 40-thou of static quench -- the rod and piston grow about 30-thou under high RPMs, producing the proper 10-thou of "dynamic" quench). That's why VW provides 3 different head gasket thicknesses -- to ensure the engine builder can get proper static quench. Don't just throw the 3-notch gasket on there and be done with it...as you found out! :shock:
You asked about the shape and size of the edge of the port. As far back as 80 years ago Ricardo's patent drawings show a sharp edge. My guess is that the sharp transition ensures turbulent flow into the swirl pots cast into the top of the pistons. Turbulence ensures the fuel and air are properly mixed while burning, to assure the fuel has the maximum opportunity to find free oxygen and efficiently combust. It stands to reason that the size of this opening would grow with increasing displacement. I am confident that there is an optimal velocity for the igniting mix to exit the pre-chamber. Perhaps VW's research found a better solution to the specific port used on the 1.5l engine.