Hello,

I am a mechanical engineer and haven't touched anything that has to do in thermodynamics for 15 years (gosh I just realized how long ago I graduated).

I am working on a steam engine and its been a bit rusty to get back into the whole subject. One Thing that I am not able to figure out is how to figure out the temperature and pressure at the outlet side of the piston.

Can anyone help out?

so from high school you should remember P x V = n x r x T, the relationship that says Pressure and Volume are inversely proportional to Temperature (for a fixed quantity of gas, EG steam).

you do know the volume change of your cylinder, but that's one equation in three unknowns so is useless by itself, sadly they don't teach the other equations until you take a course in thermodynamics specifically, which is

P x V^gamma = const (for a fixed quantity of gas/steam)

the exponent gamma depends on the type of gas, 1.4 for diatomic molecules like N2 and O2 which comprise about 99% of air, but steam, H2O, is triatomic so use 1.333 for steam,

knowing that equation you can then derive

PR = TR^(gamma / gamma-1), VR = PR^(-1 / gamma), TR = PR^(gamma-1 / gamma)

PR = VR^(-gamma), VR = TR^(1 / 1-gamma), TR = VR^(1-gamma)

these are ratios equations, PR = Pressure Ratio (after vs before)

and the all important (also not taught until thermo class) energy equation

E = T[in] x Cp x (1 - PR^(1-gamma / gamma))

Cp = coefficient of heat at constant pressure for the gas, which is fairly

constant for air but varies a bit for steam depending on its temp

Cp for air is about 1000 J/kG/K, for steam is about 2000 J/kG/K

(MKS metric units, Joules, kiloGrams, degrees Kelvin)

and its inverse

PR = ( E / Cp / T[in] - 1 ) ^ (gamma/(gamma-1))

so now in theory you have all the equations required to solve the problem, except...

...you don't really know how much steam actually got into the cylinder because of valve timing events

you might want to start by assuming that at cut-off the cylinder is at boiler pressure and temperature (it will actually be less), and then for your VR (volume ratio) assume it all exhausts as soon as the exhaust port starts to open (again, not true), and you'll find you haven't expanded it down to atmospheric pressure (hence the "puff" at the exhaust) and it will still be quite hot (wasted energy).

happy computing !

Pete.