Formulas for calculating the bore and stroke of a diesel engine

[peterl95124]

And is there a way to know what diameter and stroke is needed for a specific pressure and temperature?

for a flat topped piston and flat topped combustion chamber the diameter does not matter, only the stroke and TDC head clearance. The formulas for pressure and temperature are up above in previous posts, the two are physically / mathematically related, you specify one and the other is implied, you can't just pick these values independently. read. do the math.

 

[GreenTwin]

As Peter mentions, the formulas for pressure and temperature have been posted, but I would guess those are approximate values only, and you would get some static snapshot of what is happens in the combustion chamber at some instant, but some unknown instant ?
Could be at the moment of ignition, or maximum pressure could be at some point later when complete combustion occurs.

Seems like what is relevant to construct a model diesel is that there is enough compression for combustion of diesel, and we generally know what that is in diesel engines, so I am not sure what calculating the pressure and temperature would change, other than perhaps just being aware of what those variables may potentially be.

For a hypothetical discussion, one could calculate temperature and pressure.
.

 

[peterl95124]

As Peter mentions, the formulas for pressure and temperature have been posted, but I would guess those are approximate values only, and you would get some static snapshot of what is happens in the combustion chamber at some instant, but some unknown instant ?
Could be at the moment of ignition, or maximum pressure could be at some point later when complete combustion occurs.

Seems like what is relevant to construct a model diesel is that there is enough compression for combustion of diesel, and we generally know what that is in diesel engines, so I am not sure what calculating the pressure and temperature would change, other than perhaps just being aware of what those variables may potentially be.

For a hypothetical discussion, one could calculate temperature and pressure.
.

I think what Green is saying is about ignition / combustion / expansion stroke, yes those are unknowns.

but for compression the formulas are exact, the only things that can affect them are gas loss if the rings and valves aren't sealing correctly, and heat loss through conduction by the cylinder walls, both of these effects for an individual compression stroke are negligible.

what happens at ignition and during burning are anyones guess and probably not practical to measure, and the temperature and pressure after ignition isn't constant, the burn takes time and all the while the piston is moving, and you never know exactly how much fuel is injected and the burn is never complete so you can't use chemistry/physics to compute the heat added, you can only approximate and guess.

what we do know empirically, EG from Find Hansen and others, is that about 20:1 compression ratio is necessary for model diesel engines, he uses less compression in his hot-bulb engines, but true diesel at these scales requires this high compression.

the reason for doing the math on the pressure and temperature is to get an idea of how much stronger the con-rod and bearings need to be for a diesel verses a gasoline engine, the numbers are astonishing. If you think there's a big difference between low and high compression gasoline engines, you ain't seen nothin yet !!!

 

[kuhncw]

I believe valve events or valve timing will also affect actual/measured compression pressure.

Chuck

 

[peterl95124]

I believe valve events or valve timing will also affect actual/measured compression pressure.

Chuck

good point, absolutely true !!!