Sane Minimum Piston Crown thickness ?

Hi

I need to drop CR of 8.9:1 down to 7.9 : 1

Only option left is to remove material from the dished area. Currently, crown thickness is 7.2 mm. The pistons are forged Wössner, 84 mm dia.

We are aming for 2.1 bar boost, 90 - 100 HP per each of the 500 cc cylinders (Motor 2000 cc, Inline 4)

How much minimum crown thickness would you advise to safely and reliably run?

TIA

Dom

I would suggest the people to ask would be the piston manufacturer, because anything we say is going to be 100% a guess, and they will know the limitations and safety margins built into the pistons. That said, machining the rest of the deck to the same height as the recessed area should work OK, as it should be too wide a clearance to cause detonation issues, "should".

Even then, I'd be looking at gasket options and perhaps some unshrouding of the head to increase the unswept volume. first

There are two formulas that help calculating crown thickness (I can share them here If you need them) but to me the best way to handle it is exactly what Gord advised- talking to professionals...

Hi Guys

Thx. Yes Shota, please share them here.

Thanks in advance

D

Methods for Determining Piston Crown Thickness:

· Strength (Grashoff's Formula):

This method focuses on the piston's ability to withstand the pressure exerted by combustion gases. The formula is:

· t = D * √(3p / (16σ))

· Where:

· t is the piston crown thickness (mm).

· D is the cylinder bore diameter (mm).

· p is the maximum gas pressure (N/mm²).

· σ is the permissible bending stress (N/mm²).

Permissible Bending Stress:

This value, typically between 30 and 90 N/mm², represents the stress the piston material can handle before bending or breaking

·

· Heat Dissipation

This method ensures the piston crown can efficiently transfer heat away from the combustion chamber. The formula is:

· t = H / (12.56 * k * (Tc - Te))

· Where:

· H is the heat flow through the piston head (W).

· k is the thermal conductivity of the piston material (W/m°C).

· Tc is the temperature at the center of the piston head (°C).

· Te is the temperature at the edge of the piston head (°C).

·

Determining the Final Thickness:

1. Calculate Thickness: Both strength and heat dissipation calculations are performed.

2. Comparison: The larger of the two calculated thicknesses is chosen for the piston head design. This ensures the piston is robust enough to handle both mechanical stress and thermal load.

Example:

For a diesel engine, the heat flow through the piston head (H) is calculated, then the thickness (tH) is determined using the formula tH = H / (12.56 * k * (Tc - Te)). Comparing this to the thickness calculated using Grashoff's formula, the larger value is selected.

Thank you very much indeed Shota !

You are welcome anytime...