Formulas for calculating the bore and stroke of a diesel engine

[ruben]

Hello, I would like to know if there is a formula to calculate what diameter and stroke a diesel engine should have with a compression ratio of 20 to 1 or with a compression value of 25 bars.

 

[GreenTwin]

For an engine with a 1" diameter bore, the area of the top of the piston is pie-r-squared, so 0.785 sq.in.

Assuming a clearance of 1/16", then the volume at TDC would be 0.04906 cu.in.

For a 20:1 ratio, then the volume at BDC would be 0.98125 cu.in.

So 1.25" stroke.

My math is off somewhere.
Somebody help me.

.

 

[minh-thanh]

Compression ratio depends on 2 factors: r (radius) and h (length)
There are 2 types of h: h1 is the piston stroke and h2 is the combustion chamber
If you choose compression ratio 20:1
From there you calculate the ratio of h1 and h2
If you choose h1 stroke then you can calculate h2 and vice versa
Note - the combustion chamber is not cylindrical so to calculate its volume you have to measure everywhere or you can measure it by filling the combustion chamber with liquid and measuring the volume of liquid in the combustion chamber

 

[peterl95124]

My "Hansen" Diesel has a stroke of 40mm, and a head space at TDC of 2mm, so ~20:1, diameter doesn't factor in, its that simple !!! (technically its 42:2, but 40:2 is close enough for gov't work !)

 

[petertha]

My math is off somewhere.
Somebody help me.

Using your example which assumes the combustion chamber is perfectly cylindrical, the CR calculates to 21. Bore & Pi drop out of the equation so you can get the same CR value just knowing stroke (s) & clearance (h) using CR=(s+h)/h. But typically combustion chamber is domed or angled or otherwise 'non-cylindrical' so you have to compute or otherwise measure its actual volume Vc & use formula CR=(Vd+Vc)/Vc

https://en.wikipedia.org/wiki/Compression_ratio

 

[petertha]

(for the simplified cylindrical combustion chamber example only!) if you know CR and stroke (s), h can be determined using h =s/(CR-1).

But for a 'non-cylindrical' head it becomes a convergence type solution. Example Vc was measured directly at 0.080 in2. Stroke was iterated until the target CR of 15.0 was achieved using the same CR formula. Similarly you could define the stroke & vary the bore to converge.