predicting power of a steam motor?

[havoc]

I was wondering how you can have a "reasonable" estimation of the power a steam motor (just a basic engine, not a loc) will give without actually having to build the thing.

The basic dimensions you have or can put at some number:
- stroke
- bore
- number of cilinders
- double or single acting

You design for a given pressure of steam and pray the boiler can hold it And you can make some assumptions about filling ratio's.

But all the formulas I found so far assume you also have the rotational speed in it... And I don't see a way of pulling that out of the air.

[chris vine]

Hi,

In simple physics, power = force x speed.

If you want power in watts then the force is in newtons and the speed in metres per second

What pressure (average) do you expect to achieve on top of the piston during its stroke?
perhaps 25% boiler if running fast.

The force in newtons will be the area of the piston in metres squared x the pressure in newtons per square metre.

15psi or 1 atmosphere is 100,000 newtons per metre squared.

The average speed of the piston is the number of revolutions per second x the stroke in metres

That brings you to the question of RPM (or per second) which you raised. That depends on how fast your load lets your engine run!!!
If you stall it then the speed will be zero and the power output will be zero.
If you let it run free then the speed will rise until the pressure on the piston drops to the point when the force it is producing is only able to overcome the friction in the engine (and pushing the exhaust out etc,) Again power output will be zero.

if you want the answer in horse power then 746 watts make up one GG.

You can look at it another way and say that the power developed is the torque x the rotational speed. In this case power in watts = torque in newton metres x rotational speed in radians per second. (there are 2 x pi radians in a revolution)

Again if you stall it the torque will be large but the rotational speed (in any unit) is zero so the product and the power is zero.
If you let it run free, the speed is high but the torque it is exerting on the load (there isn't one) is zero so again power is zero.

How fast do you want it to go??!!

Have fun with a calculator
Chris.

[havoc]

Sadly enough, I do understand the physics of it... It is the mechanical engineering bit that is missing.

Maybe I better rephrase the question.

What is a reasonable rpm for a given engine? I mean, you can assume (and calculate) the power of your engine at 3520rpm. But is this a reasonable speed to ecpect from a two cilinder double-acting engine of 10mm bore and 14mm stroke with a boiler at 3 bar?

[burnettsj]

Reading Chris's post - the engine speed depends on the load and thus the power output depends on the load (as it depends on speed)!

So your question is - what power can I generate at a given speed?

We have 2 equations:
(1) W = Ap.P.u
(where W is Power, Ap is piston area, P is steam chest pressure, and u piston speed)

and :
(2) W = T.w
(where T is torque and w is the rotational speed in radians per sec.)

Thus equation 3:
(3) Ap.P.u = T.w

piston speed is a function of rotational speed and engine stroke (I'll let someone else work out the trig. my brain isn't up to that today)

Therefore Torque (and then power from equation 2) can be calculated, by knowing rotational speed, piston size (and hence area), steam chest pressure and the engine stroke.

You then get your trusty calculator out (or excel) and work out torque for a range of values of engine r.p.m. (engine r.p.m = w divided by 2 pi)

this would be fine for a single cylinder double acting engine. Ap would need to be modified for multi cylinders or single acting.

OK everyone - or did I have one too many pints last night?

[chris vine]

Hi,

Keep it simple!

I would think 2000 rpm might be about right for the sort of engine you are looking at.

Now work out the area of the piston, double it for two cylinders,
make a stab at the average pressure you might expect on the top of the piston during its stroke,
divide 2000 rpm by 60 to get rps and get your calculator going.
Remember to use the exact units I gave in my first reply or you will get a joke answer and will be writing to Ferrari suggesting they use your new engine design!!!

Chris.