Piston Compression Reduction: How & Why

Hello,

I'm thinking about how to run my AFN engine with a consistent boost pressure of 1.5 bar or more . I'm coming to the conclusion that there's no way around reducing the compression ratio. I don't want to use an intermediate plate because I think it would change the position of the injectors relative to the combustion chamber. Why not just have the pistons machined? Are they somehow specially coated? I hit an old piston with a cheap screwdriver and left quite a few craters. The surface doesn't seem to be particularly hard. Or is the coating meant to protect against heat? Does anyone have any experience with this?

Regards,

Maciek.

Hello Maciek!

These are aluminum pistons, so don't be surprised if you can easily leave scratches on them with a screwdriver (chromium vanadium steel).
Coatings primarily serve as "wear protection" because the mechanical stress on the piston skirt is relatively low. Much higher mechanical stresses occur in the area of the piston pin.

Besides a thicker head gasket, a larger piston dome recess or a shorter piston overall could also help reduce compression. You could cut your old piston in half to examine the wall thickness at the piston crown (and also check the distance to any potential oil cooling channel!).
Perhaps the firing bridge could be shortened by a few tenths of a millimeter, or even a full millimeter. However, the problem then is that the coating is missing in the bent area. I wouldn't change the geometry of the recesses, as they are of particular importance for the combustion process.

Regards,
Alex.

Hello Alex,

I suspect that slightly 'undercutting' the top of the piston (by a maximum of 0.5mm) should be enough to reduce the compression ratio to 17.5. Alternatively, one could mill the valve pockets deeper. I'm not sure which would be better. I don't want to modify the combustion chamber. My main concern with all of this is that the piston might melt. As far as I know, valve pockets are often reworked in gasoline engines. Aren't the temperatures in a gasoline engine higher? I would appreciate any opinions on this.

Regards,

Maciek.

Here's what else I can think of. Planning the cylinder head is also not unusual in TDI engines. This surface is partly part of the combustion chamber and is exposed to high temperatures and pressures. Shouldn't that also be possible on the pistons?

Regards,

Maciek.

Hello Maciek!

It's a common misconception that combustion chamber temperatures are higher in gasoline engines than in diesel engines. No, it's actually the opposite!
Only the exhaust gas temperature is higher in gasoline engines, because the gas cannot expand as much (lower compression ratio). Overall, a diesel piston is subjected to significantly higher thermal stress. Therefore, TDIs are also equipped as standard with oil spray nozzles on the connecting rods, which aim at the bottom of the piston for cooling .

Your idea about machining the pistons or re-milling the valve recesses sounds feasible. Perhaps a reworked piston could be re-coated on those surfaces. Then there shouldn't be any problems.
However, this should also work well because the piston cavity is the area subjected to the highest thermal stress.
I would be very interested to see your results!

Good luck.

Regards,
Alex.

Hi.

I also had that idea, but for now, I'm settling for less boost pressure because I don't want to open up my engine.

'As far as I know, there are different strengths of head gaskets available, and I would have tried to solve the problem of nozzle position by using thinner gasket rings. These have a thickness of 2 mm; 1 mm should be sufficient for sealing. Based on what I measured when the nozzle was removed, the nozzles can still be positioned a little deeper into the combustion chamber and piston crown without hitting. This would also help to mitigate the 'fuel spraying behind the piston' problem with excessively long injection times.'

Furthermore, it is likely to provide significantly more benefit if, when using 'aggressive' tuning chips, the injection timing is adjusted to reduce peak pressures.
The problem isn't the absolute boost pressure itself, but rather the peak pressure that results from the combination of boost pressure, compression, fuel injection volume, and injection timing. If you reduce any of these factors, the situation improves significantly.

Yes, exactly, it's about the peak pressures. These not only damage the cylinder head gasket (ZKD), but also other components over time. I was wondering why the old swirl chamber diesels, which have much higher compression ratios, can withstand up to 1.4 bar. The explanation is the different injection process. The short injection time for a relatively large amount of diesel in direct injection (DI) results in such high peak pressures. I don't want to shift the start of injection further before top dead center (OT) and use larger nozzles for that. Nevertheless, the problem remains with larger increases in fuel quantity. The pistons are hard anodized. I'm not yet sure if this process can also be applied retroactively to the machined areas. There's definitely some risk involved.


Regards,

Maciek.

One last thought: Why not mill a groove into the cylinder head?
Sure, here is the translation:

'Yes, cylinder head gasket replacement is planned, and a re-coating will not be done. It doesn't really take much to reduce compression. A cylinder head gasket failure would be more tolerable than burned pistons. Does anyone have a sectional drawing of a VE TDI cylinder head?'

Regards,

Maciek.

Hi,
Two fundamental considerations:
- The piston, specifically the recess (or cavity) and the sharp, circumferential edge of the recess, are anodized to increase their thermal resistance. I would not use any tools on that area.
"Indentations in the cylinder head or piston should only disrupt the flow characteristics if they are located within the piston bowl area. A key focus in the development of the very first TDI engine was reducing parasitic clearances (or something similar). Therefore, the only area where indentations would likely be acceptable is the cylinder head above the piston bowl. Alternatively, one could look for pistons that happen to be a suitable fit."

Hello,

Re-eloxalizing a machined piston bottom is actually not a problem. You can either take it to a plating company or, if necessary, do it yourself.

Best regards,
Christian.

chris11 wrote:

Re-oxidizing a machined piston crown is actually not a problem.

In principle, no. However, one should definitely know how the original manufacturer proceeded. I'm not very familiar with that area myself, but I can imagine that different results, at least in terms of layer thickness, can be achieved there.

Okay, before you further weaken those pistons, I would suggest shortening the connecting rods or using a crankshaft with a shorter stroke. The engine has to be disassembled anyway, so I think the crankshaft solution is the most practical.
The simplest solution is to place a metal plate between two cylinder head gaskets.

I'll leave the pistons alone for now. Instead, the cylinder head will be modified. To reduce the compression ratio from 19.5 to 17.5, the combustion chamber needs to be increased in height by 0.62mm. Either a recess (or two) with the corresponding volume will be milled into the cylinder head, or the entire combustion chamber area of the cylinder head will be machined down in a circular pattern by 0.62mm, and the valves will be ground or, if possible, set deeper. I still need to discuss this with an expert. If everything goes according to plan, I should have some initial results in about two weeks.

Regards,

Maciek.