An idea for more high-RPM power

[fspGTD]

I have a somewhat whacky idea to get more high-rpm power and efficiency on our IDI VW-Diesels.

I was browsing the IDI diesel patents, and I came across US Patent number 3,965,872. It's a patent about the design of the recess that is machined into the tops of the pistons.  Check it out - you can pull it up on http://patft.uspto.gov/netahtml/srchnum.htm - enter in patent number 3965872.

It basically presents 2 pre-existing designs, one that looks a heck of a lot like the VW IDI-Diesel piston (with the 2 "leafs" or circles), which it says has good combustion efficiency at low and mid RPM, and then a "slotted" design, which it says has good combustion efficiency at high RPM. It then presents a "hybrid" design, which looks like our VW IDI Diesel piston tops with a slot machined partway in, which it says offers efficiency over a broad powerband. It says depending how deep the slot is machined in, high RPM or low RPM can be biased. So machining it deeper in would then bias the high RPM efficiency more.

It looks like it might be a do-able modification to just take a VW IDI piston, and machine in one of these slots, thus converting it from a "low and mid RPM only" efficiency design into one that supposedly handles high RPMs better. To do this, one might remove some material below the white paint indicated here...


The author says on a test engine that was converted, both torque and power increased more than 8%, and smoke level was reduced as well as a result of the modification. He makes it sounds like it could be promising perhaps, on a VW IDI-Diesel.

What do you guys think - is this crazy or what.  Are there any diesel combustion chamber design gurus lurking on this list who might be able to chime in?

[Dr. Diesel]

wow Jake, that's something!  I almost wish my project engine's pistons weren't ceramic coated, otherwise I'd give it a shot! Perhaps it'd be worth trying on a stocker with a modded pump.
That's really interesting, as I've felt how willingly an IDI can rev the tach off the deep end given the right mods. I wonder how much improvement your slotted piston would give.... hmmmmmmmmmm......

[vwmike]

I don't really think I understand why you guys are so interested in high rpm power. It's a diesel and it's just not going to rev like a gas engine.

[fspGTD]

I don't really think I understand why you guys are so interested in high rpm power. It's a diesel and it's just not going to rev like a gas engine.

This thread from the old forum should help bring you up to speed on the high-RPM performance potential of the VW IDI-Diesel motors:
http://www.hostboard.com/cgi-bin/ultimatebb.cgi/ubb/get_topic/f/4037/t/242

In the future, please use the "search" feature of the forum to try and find the answer before posting.  Thanks.

[QuickTD]

I don't really think I understand why you guys are so interested in high rpm power. It's a diesel and it's just not going to rev like a gas engine.

There are 2 ways to make power from any engine. The first is to increase BMEP (Brake Mean Effective Pressure) which directly increases torque. This entails using more boost (or better flowing ports on an NA engine) and more fuel to increase the combustion chamber pressure. It works well, until you start blowing head gaskets and bending rods, which is where some of us have hit a wall. The second method of increasing power is to keep the same BMEP (and torque) but increase the frequency of the power strokes by raising the RPM. This is just as effective as increasing BMEP but is easier on engine parts, unless of course you get carried away. See the thread about fspGTD's over-revved head  .

 fspGTD's reasons for increasing the rpm range of the engine go beyond just the power advantages. Modifications that permit greater rpm allow the driver to hold a gear longer without shifting, very important in autocross. Also, governor mods, unlike boost mods, are permitted in the autocross class in which fspGTD is competing.

[web]

I don't really think I understand why you guys are so interested in high rpm power. It's a diesel and it's just not going to rev like a gas engine.

You are SO prejudiced. Think outside the box, man. We do, and it's brought us (especially Jake) amazing results.

[vwmike]

I don't really think I understand why you guys are so interested in high rpm power. It's a diesel and it's just not going to rev like a gas engine.

This thread from the old forum should help bring you up to speed on the high-RPM performance potential of the VW IDI-Diesel motors:
http://www.hostboard.com/cgi-bin/ultimatebb.cgi/ubb/get_topic/f/4037/t/242

In the future, please use the "search" feature of the forum to try and find the answer before posting.  Thanks.

I've read that before....besides, what I was referring to was the speed at which the diesel combustion process actually happens. Don't treat me like an idiot just because I don't have a big post count.

[RedRotors]

Please guys, stick to the subject, we are all here to learn and share what we know.. There's no 'stupid questions. I will write a 'Please read first' that i will stick on the top of the section..

Marc/
Admin.

[VWRacer]

Not to worry, Mike. Most of us used to think the same way about diesels as you used to... :wink:

But the truth is that diesels are inherently capable of revving as high as a gas engine of similar architecture. The primary restriction is that diesel has a relatively fixed ignition delay, but the flame front speed itself is pretty much as fast as that of gasoline.

The bottom line is that many of the tricks applied to raising the RPM, power and torque of a gas engine will work as well with diesels.

[RedRotors]

Let me some time to arrange something.. Ok, back to the crazy idea of Jake  

Marc/

[DVST8R]

Well if I was going to cut up any pistons in research those would be my first candidates :wink: , I would say tha tif it was worth the time effort and money to patent it it probablyhas some good merrit. In fact the next time I have the head off the old motor I will try it myself, or even on the "new motor" when ever it gets started or finished (another long story). My only consern here is how much CR are you going to drop by cuting that piston material away, and I didn't see any refrence (though I only quickly skimed it and the images wouldnt load on my comp) to the size, angle, or depth these cuts should be made. Other than those few things I am all for it and will have my die grindeer ready the next time I blow my head off  :roll:

[fspGTD]

This would be a "correct" way of adding volume to the combustion chamber to achieve a drop in compression ratio.  I would expect it to give much, much better results than thickening the head gasket, which adds volumne to the squish area.

I haven't run any calcs yet to see what the compresion ratios would drop to, but it would depend how deep and long the pockets are made.

There is one thing I noticed that might throw a monkey wrench into the idea, and that is on the VW IDI diesel combustion chamber design, I noticed in between the valves there is a semi-circular relief already machined into the head... you can see them in this picture:


In the combustion chamber, this relief is in the same general area as where the groove would be machined into the piston, except the VW IDI groove is in the head rather than in the piston top.  So I wonder if VW has already incorporated this functionality into our engine's design, but just in a different way (by putting the void in the head rather than the piston?)

The patent papers don't show if they expect the surface of the head at this location to be flat or not, and nothing is mentioned about it that I saw.  But it might be assumed.  Since it's patented by someone working for Toyota in the mid 70's...  it makes me wonder what the diesel engines Toyota produced in this era looked like... anyone know if Toyota made any diesel engines in this period (maybe Japanese market only type deals?) and if so how we might find out what their combustion chambers looked like?

I tried out filing and dremeling grooves into the edge of a scrap chunk of aluminum by hand just to experiment a bit, and I just wasn't happy enough with the results to warrant trying doing this on my pistons with them installed in the block.  I think to give it a decent shot, I'd need to remove the pistons and take them to a milling machine, which could machine in the slots precision-style and repeatable from one piston to the next.

[DVST8R]

For those of us running greater then stock boost lowering the CR will probably quite quickly become a advantage and mabey even a necessity, however in your app, I see this more as a hinderance then a help, as you arn't able to fill the added voulme wiht extra pressure. But I maybe wrong and I sure some one will fill me in if I am :wink:

[DVST8R]

Oh I just thought of some more infoi, the only three road vehicals that toyota imported into north america in the 70's were the corona, the corolla, and the p/u none of which had a diesel avalible until the early 80's.... oh scrap that thought the Landcrusier had the toyota 3b as i recall and that might have been as early as '76, but through further research the diesel engine wasnt an option until '81 dont know of any mediium or heavy duty toyota trucks from the '70s that were imported, the only other thing that I can come up with is forklifts, or somthing on the JDM.

Also now that I am on a role they said the deeper or was it bigger the cut in the piston the more it moved the rpm scale up. So even if VW has already taken in some account in the semi cirlces in the head they were never forseeing, or designing  for some one to be racing this motor and wanting to extend the rpm band further, so there is a good posibilty that there are still some benifits to doing this.

[fspGTD]

"Balancing" the shape of the piston tops exactly so the combustion patterns and power from each piston to the next is also important.  Figure 24 of the 1.6lTD SAE paper shows a cross-section of the piston.  The piston crown is a solid chunk of aluminum from the piston top to the cavity inside, it's a half inch or thicker.  (The cavity starts at about the same depth as the top ring.)  So it looks like it could take a little machining into it's top to me, no problem.

Dvst8r - the CR for optimal efficiency is significantly lower than what VW went with at 23'ish:1... at least that's what they have stated in the SAE Papers (here we go... 1.5D SAE paper, p.83 states "The compression ratios which permit attaining optimal fuel combustion in small swirl-chamber engines range between 16:1 and 18:1."  Therefore if it were to get dropped down the correct way (IE: by enlarging the pre and/or main combustion chamber, not by enlarging the squish area which is what happens when you install a thicker headgasket) this means we might expect to see an increase in power and fuel efficiency.  The downside is that cold starting would be harder.

If found a figure in the 1.5lD SAE paper (figure 8, p.82) that shows a cross-section of the VW IDI Diesel combustion chamber.  It clearly shows the semicircular "blip" in the cylinder head between the valves and the relationship of this feature with the "clover leaf" recess in the piston tops.  Interestingly, at TDC, there is not a very open connection between the two volumes except through the thin clearance between top of piston and bottom of cylinder head!  As the piston moved down slightly, it seems like the gasses may be channeled upward and into this relief in the head however.  Make me wonder what's going on in there... IE: is it a feature that comes into play only after the piston moves down somewhat...  (kind of like the high-rpm piston channel described in the patent paper we are discussing...)

Studying some figures in the 1.5D SAE paper (Fig 9, p.83) of the cylinder pressure over crank angle, it looks like most of the combustion (the pressure rise) happens between TDC and 40 degrees crank angle.  At some RPMs though, it looks like some combustion trails off until around 60 crank degrees.  It would be interesting to do a study and see what shape the combustion chamber is when combustion is happening...

PS - to the guy who said that combustion in a diesel was too slow for it to be able to revv high...  here in the 1.5D SAE paper there is a comparison (in figure 9, p. 83) of the 110HP GTI motor vs the diesel...  and in the GTI motor, the combustion is spread over about a 80 crank angles.  This is much broader than with the diesel, which is concentrated more narrowly, like over more like a 40-50 crank angle area.  Just goes to show that with a diesel, combustion happens fast!  A big challenge for the diesel engine designers indeed seems to be trying to get the combustion to slow down and getting it controlled, not trying to get it to speed up...  This explains the existence of pre chambers in diesels, and also of the significance of the pilot injection feature (which is what VW pioneered, with Bosch's help, for their TDI motors.)