15:1 or How low can you go...?

RabbitGTDJoe

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I wish coatings for the head were in the budget this year. Costs alot more to have it done than the pistons did...so, no...not this year. Eventually though I'd like to have the exhaust chambers coated with the same TBC coating as well as the valves too. For this year though it'll be a very mild port matching/cleaning of the castings and a 3 angle. Nothing too fancy...

Thanks for the comments! I will keep things updated...


Joe
 

TDIMeister

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Bump. The newest 2.0 16V CR TDI will have piston bowl shown below.



Interesting to compare that geometry to that which I described as being ideal several posts back in this thread... the link to the picture I sketched is long expired, but I'll look for it in my local archive and host it on the pic server.

Other interesting information about the 2.0L 16V CR TDI engine: It has a 16.5:1 compression ratio, 8x 0.123mm hole piezo injectors, and variable intake manifold.

 

Scott_DeWitt

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At what point is power lost when lowering compression? It takes alot of power to compress the air. I know with gas/alcohol engines, around 13:1 you experience diminishing returns quickly.
 

TDIMeister

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Unfortunately it's more complicated than that. There is no rule-of-thumb that can be applied for all engines, but rather depends on a lot of factors. The Porsche 997 GT3 engine has a compression ratio of 14.6:1 using European Super-Plus spec pump gasoline (!!!) Other race cars in the same class have found their optimal CR at lower values.

Both gasoline and Diesel engines have optimal CR between 14-16:1, beyond which there are diminishing returns and even worsened efficiency due to proportionally increased heat-transfer and frictional losses. Production Diesel engines have higher compression ratios only because they have to start reliably in cold-weather, but even the general trend in production engine designs is to lower the CR for many reasons. VW's 1.5L IDI started with a 23.5:1 CR, down to 19.5:1 in the ALH TDI, down to 16.5:1 in the newest 2.0L 16V CR TDI. Toyota has a 2.2L Diesel engine with a 15.8:1 compression ratio, and purpose-designed race Diesel engines are even going down to 14:1, lower than some gasser engines!!! Crazy!
 

mrchill

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AhA! The Omega chamber piston! I have not been able to get a good pic of tht yet. I have seen design drawings though. If anyone has pisx, let me know. I'd like to see viually what the advantages are to the Omega design. Oh... and by the way, thanks for the thread resurrection Meister!
 

TDIMeister

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I have some pictures and technical background from VW on the piston bowl design that might answer what you are looking for, but I don't have access to the hard drive that contains them at the moment. When I do I will upload them to the pic server and post them here.
 

EddyKilowatt

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Meister, great stuff. Thanks for posting!

When you say 'optimum' CR do you mean for power, efficiency, emissions, or some real-world blend of the three?

I wonder to what extent NOx limits are driving the CR down...

Eddy
 
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turbo johan

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This is how i machined my pistons.
Runs fine.
I had troubles with cold start.
Running really bad for about 30 sec. after cold start with a lot of smoke.
Since i've changed the startup timing maps for cold engine and a bit longer glowtime it starts perfect and runs smooth cold.

Johan
 

TheoSweden

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turbo johan said:
This is how i machined my pistons.
Runs fine.
I had troubles with cold start.
Running really bad for about 30 sec. after cold start with a lot of smoke.
Since i've changed the startup timing maps for cold engine and a bit longer glowtime it starts perfect and runs smooth cold.

Johan
Nice Johan!

I'm thinking of do this to mine too.

Can there be some problems doing the CR lowering this way when using R783 injectors in a PD engine?
I'm thinking if maybee the spray angle is different or something, just want to be sure its working good before doing it.

All info about this subject is interesting!

Theo
 

StingrayRT

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turbo johan said:

This is how i machined my pistons.
Runs fine.
I had troubles with cold start.
Running really bad for about 30 sec. after cold start with a lot of smoke.
Since i've changed the startup timing maps for cold engine and a bit longer glowtime it starts perfect and runs smooth cold.

Johan

Johan that´s piston chamber profile is very bad.......you have touched the omega profile and bore out the upper part of chamber......a huge part of swirl is missing due the flame trajectory will be very short and it is reversed back to the cylinder head and not to the combustion omega chamber...... please do not make this anymore!
 

KROUT

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Stingray can you post pics of how your pistons look. I am sure you have before but if its not any trouble I would like to see them.
 

turbo johan

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@ stingray
There are 2 ways of lowering cr with stock pistons.
Or more space above the piston or machining the bowl.
So you meen you'll always need special pistons if you want to lower cr?

I've seen cars that've been machined the top of the piston.
I don't like how they run.
They don't make as much power as with machined bowl and smoke more easy.

By making the space bigger above the piston your combustion will be not completly in the bowl.
Imo the best combustion will be with the piston really close to the head.
Than the air above the piston will be squeesed into the bowl.
Then you'll archive the highest air speed and the most air in the bowl and the best combustion.
But i like to hear other options and reasons why.

How should a lowered cr piston look like in your opinion Stingray?
Than i will test the difference.

Johan
 
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StingrayRT

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turbo johan said:
@ stingray
There are 2 ways of lowering cr with stock pistons.
Or more space above the piston or machining the bowl.
So you meen you'll always need special pistons if you want to lower cr?

I've seen cars that've been machined the top of the piston.
I don't like how the run.
They don't make as much power as with machined bowl and smoke more easy.

By making the space bigger above the piston your combustion will be not completly in the bowl.
Imo the best combustion will be with the piston really close to the head.
Than the are above the piston will be squeesed into the bowl.
Then you'll archive the highest air speed and the most in the bowl and the best combustion.
But i like to hear other options and reasons why.

How should a lowered cr piston look like in your opinion Stingray?
Than i will test the difference.

Johan

This is correct design of tdi piston.......best position to remove a material is lower part of omega chamber - you can see that I have made there a big radius.......I´ve run 48psi boost so the piston design has to be a key factor for me......



 
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Rub87

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What about valve piston clearance, so we want piston as close to the head as possible, so a 1 hole gasket, and a good shaped deeper bawl, but what if you use this combo whit a decent cam with more lift and duration at the exh side so you create minor overlap, should there be the danger of piston hitting the valve at the end of the exhaust cycle?
 

turbo johan

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I've made one today, is this the way you make them?
Diameter of the lip and omega chamber both bigger.
Kept the stock design of the lip.


And this is one of a R10 audi, i've made it a bit like that :)


I like to hear opions ;)

Johan
 

StingrayRT

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yeeep this design has to be much better.......but I see a very sharp tip in the omega chamber......make it with radius R3

Please polish the omega chamber to mirror shine
 

TdiRacing

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I dont think that picture above is a R10 piston. They look more like this:



This a a REAL custom made racing piston for a diesel engine. Not some modified production piece.

The Koln Motorsports expo had a lot of profession people there that a talked to and was permitted to take this picture for reference only. This piston is a very special design and composition. If you want a piston for racing, this is the way to go. Higher lift cam, deeper valve pockets and maintaining the correct bowl shape. The average engine may run fine without it, but every engineer i talked to said that it is Essential to maintian the correct bowl shape.

This piston is much stronger than a production piston and about 20% lighter.
 

turbo johan

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Thank you Stingray!
I will try it on a engine i'm gonna built next week.
I'll post the results :)

@ tdiracing
Nice pic of the R10 piston!
My pic came from audi R10 promo clip which i found on youtube.
To bad i didn't went to Köln last week, there was a lot to see.

Johan
 

TDIMeister

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Nice piece there (the piston, that is). Awesome opportunity to take that picture, Oliver! Too bad you didn't have a set of calipers on-hand to measure the diameter. :)

It's amazing to compare this piston to an OE one. The lack on an Alfin top-ring groove reinforcing insert and just how thin some of the sections are show this is clearly not designed for a production engine with an expected 300,000 km lifetime.

I agree, this show was excellent. Being located so close to Köln, and with a free visitors pass, I'll definitely try to make it to future shows. Oliver, you and Jeff should consider to sign-up for and attend the Engine Expo in Stuttgart next May. I've already signed-up myself. It would be great to see you guys then.
 

turbo johan

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I hope i go to stuttgart in may to.
Looks like a nice show.
In Köln Dynostar did demoruns with new single roller dyno.
From that one i've got the prototype.

Johan
 

GoFaster

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Note how the R10 has a central dome that comes to a point - not like the wide radius some people seem to be insisting on. My money is on that R10 piston being optimized rather well!

I cannot believe how thin the top of that piston is.
 

MarkoP

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just found this wonderful topic =)

I can see how Omega lip will make thumble in the swirl in conbustion chamber during injection phase and thats really good thing with economy and noice.
the lip will also reduce the amount of shock wave hit by the combustion process to cylinder head and it reduces some noise too.

But the stock bowl is designed for stock injection penetrability and duration.. so I would also assume that when larger injectors are used you might need deeper or wider bowl (depends how you look at it) or more swirl with denser air. But because swirl creation is balancing with quality and quantity (mass flow) if you dont want to get huge boost pressures I think the smarter bet would be to enlarge the combustion chamber even though it also reduces the swirl.
My thought is still that the denser the air is the more it has mass and inertia and you can cut some swirl because of more boost pressure and still get decent swirl levels to cut smoke away.
When injection duration is increased then injection starts when piston is on its way up and continues when its going allready down.
If there is piston lip close to injector, then the injection would hit to te wall and then it would not burn so well and that would cause smoke.

I have allways recommended to cut the lipp off to reduce CR, one reason is melted lips and one reason is that that point is at least somehow involved in combustion process when comparing to guys who just put in thicker head gasket and gain nothing else than reduced startability.
The main function with reduced CR should be to fit more air in to chamber / combustion process without excessive combustion peak pressures.. not to just lower the combustion pressure.
I would also quess that at higher RPM levels the Quench effect is acting much like omega chamber lip to get some thumble motion in to swirl motion and burns the oil faster / more completely reducing soot.
Now in low rpm production engines this might not be the case and the lip has a huge effect in soot production at low / moderate RPMs.

So if we dont have good visibility in to combustion chamber or really good programs which use to calculate combustion process, then at least with production pistons with different component combinations (tbo, injectors etc.) there is really only trial and error method and maby some one can make some educated guesses, though I dont coun't my self as a such =)
 

TDIMeister

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TdiRacing said:
I dont think that picture above is a R10 piston. They look more like this:



This a a REAL custom made racing piston for a diesel engine. Not some modified production piece.
Oliver, please level with us. Is this picture that of an actual R10 piston as we have come to understand from your post? I went pretty carefully through almost every noteworthy stand at the show, and didn't see any R10 pistons being offered for show.

And it would seem crazy that despite Audi Sport's top-brass being present at the show, someone would be so cavalierly showing-off the deepest-technology of a current and top-flight OEM racing program! This just doesn't happen in Motorsports! You must have some impressive connections!!!

IIRC Capricorn had a stand with some Diesel pistons.
 

StingrayRT

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hhahhaha I know only one company which make a steel racing pistons for diesel but this picture seems diferent and non-real.......seems that this design has nothing to do with internal combustion engineer......both Scania and Volvo start to using hybrid pistons upper part high quality steel - lower part alluminum based alloy on his trucks........

Ok guys stop the dreaming about this picture......and try to find how high is a peak cylinder pressures and temperature in combustion bowl and look at the thickness of materials.......you look at the ring grooves......hahhahahaha so little support for this grooves......
 

TdiRacing

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TDIMeister said:
Oliver, please level with us. Is this picture that of an actual R10 piston as we have come to understand from your post? I went pretty carefully through almost every noteworthy stand at the show, and didn't see any R10 pistons being offered for show.

And it would seem crazy that despite Audi Sport's top-brass being present at the show, someone would be so cavalierly showing-off the deepest-technology of a current and top-flight OEM racing program! This just doesn't happen in Motorsports! You must have some impressive connections!!!

IIRC Capricorn had a stand with some Diesel pistons.
No, they did not have an actual piston on display. Given the politics involved in Motorsports, nobody would do that. So you just snoop around and see who else is making stuff similar.

The picture i posted is a prototype piston. Very "similar" to the R10 psiton I was told in confidence. Not that there is that much of secret. That engine does not produce that much more HP/L than production BMW diesel engine. ..

And yes, picture is of a steel racing piston. It was suprisingly light.
 

TDIMeister

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GoFaster said:
Note how the R10 has a central dome that comes to a point - not like the wide radius some people seem to be insisting on. My money is on that R10 piston being optimized rather well!

I cannot believe how thin the top of that piston is.
I was waiting for Tdiracing's confirmation of my suspicions to address your post, Brian. That's not a picture of an R10 piston, and just because one piston is shown with a sharp center tip doesn't mean it's the best or even the only solution.

Different in-cylinder airflow characteristics, injection pressure and spray geometry will result in optimization leading to- and converging to a certain direction. I can find pictures of the latest BMW and Mercedes-Benz Diesel pistons showing similar trends in bowl design.
 
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