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Author Topic: The Original 1.5 Diesel SAE paper  (Read 23835 times)

Reply #30May 17, 2004, 07:17:13 pm

DieselsRcool

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« Reply #30 on: May 17, 2004, 07:17:13 pm »
Quote from: "VWRacer"


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.

Reply #31May 17, 2004, 08:08:52 pm

TDIMeister

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« Reply #31 on: May 17, 2004, 08:08:52 pm »
Quote from: "BlackTieTD"
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.

Reply #32May 17, 2004, 08:20:01 pm

TDIMeister

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« Reply #32 on: May 17, 2004, 08:20:01 pm »
Quote from: "VWRacer"
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.

Reply #33May 17, 2004, 08:49:06 pm

TDIMeister

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« Reply #33 on: May 17, 2004, 08:49:06 pm »
Quote from: "fspGTD"
I'll take a crack at these...

Quote from: "VWRacer"
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.

Quote

Quote

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.

Reply #34May 18, 2004, 01:28:12 am

fspGTD

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« Reply #34 on: May 18, 2004, 01:28:12 am »
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.
Jake Russell
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Reply #35May 18, 2004, 08:52:41 am

BlackTieTD

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« Reply #35 on: May 18, 2004, 08:52:41 am »
great discussion guys. nice to see some good stuff brewing in the new forum!

Quote from: "VWRacer"
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.



Quote from: "TDIMeister"
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



Quote from: "TDIMeister"
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:

Reply #36May 18, 2004, 09:20:22 am

VWRacer

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« Reply #36 on: May 18, 2004, 09:20:22 am »
On the question of diesels being more expensive than gas engines, Jake wrote
Quote
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
Quote
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
Quote
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)
Stan
C-Sports Racer

Reply #37May 18, 2004, 03:51:52 pm

fspGTD

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« Reply #37 on: May 18, 2004, 03:51:52 pm »
Haven't seen the old Ricardo stuff!  Would love to get my paws on a copy of that...
Jake Russell
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Reply #38May 18, 2004, 03:58:09 pm

fspGTD

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« Reply #38 on: May 18, 2004, 03:58:09 pm »
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!
Jake Russell
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Reply #39May 18, 2004, 05:26:57 pm

VWRacer

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« Reply #39 on: May 18, 2004, 05:26:57 pm »
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.
Stan
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Reply #40May 18, 2004, 06:14:20 pm

TDIMeister

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« Reply #40 on: May 18, 2004, 06:14:20 pm »
Quote from: "VWRacer"
Quote
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.

Reply #41May 18, 2004, 10:19:15 pm

VWRacer

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« Reply #41 on: May 18, 2004, 10:19:15 pm »
fspGTD wrote
Quote
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
Quote
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
Stan
C-Sports Racer

Reply #42May 19, 2004, 02:08:45 pm

fspGTD

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« Reply #42 on: May 19, 2004, 02:08:45 pm »
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.
Jake Russell
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Reply #43May 19, 2004, 02:11:21 pm

fspGTD

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« Reply #43 on: May 19, 2004, 02:11:21 pm »
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.
Jake Russell
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Reply #44May 20, 2004, 12:48:36 pm

VWRacer

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« Reply #44 on: May 20, 2004, 12:48:36 pm »
TDIMeister wrote
Quote
...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.
Stan
C-Sports Racer

 

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