How to estimate HP of a compound marine engine

[Geoff StevensonParticipant @geoffstevenson56049]

Would anyone have a simple idea for calculating a rough idea of hp for a compound marine engine.? Most of the stuff that I have seen on the net requires one to have a Doctorate in Mathematics to even get a grasp on. I know the bores and the stroke and the working pressure. Alternatively some kind soul may be able to make a guess for me. I don't need the estimate to satisfy a Parliamentary Committee. Many thanks from downunder.

[Geoff StevensonParticipant @geoffstevenson56049]

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Hi Geoff ,

I’ll work it out for you . Send me all the details in pm .

MikeW

[RICHARD GREEN 2Participant @richardgreen2]

Michael,

Can we all see how you work it out ?

Richard.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Geoff – got pm ok .
Richard – yes .

Expect answer in a day or two .

MikeW

[roy entwistleParticipant @royentwistle24699]

You're taking me back 65 years but 2 PLAN /33000 springs to mind where P is mean cylinder pressure L is length of stroke in feet A is area of piston in square feet N is revs per minute the 2 is for double acting Note the pressure is NOT boiler pressure but the average cylinder pressure and is usually measured from an indicator diagram Do it for all cylinders then add together

1 hp = 33000 ft/lbs per minute

Roy Please don't ask about latent heat or properties of saturated steam I can remember having the lessons but little else

Roy

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Horsepower by a compound steam engine, whether it has two cylinders or several cylinders can be calculated in the same manner as that of a steam engine with a cylinder, in that it is estimated that the vapor total expansion takes place in the last cylinder and thence calculate horse power by the effective vapor pressure in this cylinder, its piston area and piston speed. Which will easily be appreciated that the only gain, which is obtained by applying several cylinders, the steam that is saved as one gets smaller cylinder condensation, while that one gets more crankpins on a shaft gets more steady operation of the steam engine.

[oldveloParticipant @oldvelo]

Hi

I guess that the question has been answered already so no formulae from me.

A point I remember about triple expansion marine engines with a condensor is that the condensor pressure is much lower than atmospheric pressure so a slight gain in power on the exhaust stroke by the negative pressure on the piston.

[Geoff StevensonParticipant @geoffstevenson56049]

Thank you Gentlemen for all your kind assistance. Much appreciated. Fortunately for me, one of your colleagues has generously volunteered to work it out for me. I'm completely at a loss when it comes to mathematics although I can turn out a very nice engine. Plain numbers yes but X over 2 by the power of a fleas instep.. I don't think so. I think my brain got math zapped back in the days of corporal punishment when I couldn't grasp the use of logarithms. That bit of schoolboy torture must have been invented by the Devil himself. Why anyone would want to use a set of tables to calculate what 2 and 2 equals is still beyond me. After wrestling with it for half an hour I'd be lucky to get any answer at all. But never 4. If you didn't get 6 out of 10 right it was whacko! The pain lingers on. A good warm up on a Winters morning but possibly not a good educational technique. . Thank you again and please forgive the levity.

[FMESParticipant @fmes]

Can you actually do the calculations without a cylinder indicator diagram?

[RICHARD GREEN 2Participant @richardgreen2]

So………………………………..no "plain English" calculations then ?

Richard.

[jason udallParticipant @jasonudall57142]

Ok given the OP is satisfied. Can anyone tell me if they following is a useful. “Estimation” of the upper limit

Per cylinder(assume circular)
Work done per stroke
(Bore*Bore /4) pi*Pressure*Stroke * (some constant to deal with units used )

Lets call that W..

Power (horsepower)
W*(2or1 depending on single or double acting)*rpm*60*(again a constant depending on units used)

Now add that up for each cylinder…

All easy enough but..
Pressure..just what do you use?
My estimate would be the difference between inlet and outlet pressure , again for each cylinder..
Yes I know the complete analysis would work from intergral calculus and the indicator diagramme

Edited By jason udall on 23/09/2013 09:28:26

[RICHARD GREEN 2Participant @richardgreen2]

Like I said…………………no "plain English" calculations.

Richard.

[jason udallParticipant @jasonudall57142]

Ok Richard.
But have I missed anything for a first cut estimate?

[jason udallParticipant @jasonudall57142]

Just thought..can you not just work from volume of steam consumed?

[roy entwistleParticipant @royentwistle24699]

Jason Steam is expanding after it leaves the boiler after cut off it is the expansion which drives the piston so which volume wouild you use

Roy

[John OlsenParticipant @johnolsen79199]

"Everything should be made as simple as possible, but not simpler"

We can make simple calculations for this sort of thing, but the more we simplify, the less accurate they will be. You may recall that early calculations of steam plant horsepower were often made on the basis of heating surface. (or was it grate area?) By the time these were applied to traction engines, the art had been improved to the extent that a 10 nhp traction engine could probably produce at least forty brake horsepower.

However, for our purposes, a simple calculation that gives a rough idea of the power to be produced is probably close enough…unless you actually plan to put the engine on a brake later and actually measure the power?

So…people are worrying about indicator diagrams. Let's simplify things by assuming that we admit steam for the whole stroke. After all if we are trying to get maximum power we will go for a late cutoff. Let's also assume some pressures…we need to know the admission pressure, the intermediate pressure (or pressures for a triple), and the exhaust pressure..all should be absolute. Ok, so simple subtraction gives us the pressure difference across each cylinder. Don't bank on getting a vaccum better than 20" (About 5psi absolute) on a small plant

So apply the formula from Roy's earlier post for each cylinder, add the resulting powers together and you have a number. (This is also what Jason is suggesting.) This will be on the high side, since we did not allow for pressure drops across the valves or between cylinders. We also probably won't run at 100% cutoff! Note that things like cylinder condensation will not reduce the power when the cutoff is late, they just mean that more steam is used.

But bear in mind that you probably don't know how much power you will actually need for a given hull anyway. Any figures provided for driving the hull with a petrol engine are pretty irrelevant when comparing to a steam plant. This is because the steam plant will operate at much lower rpm, with a much larger and much more efficient propellor. So for instance the Leak compound, described in ME in 1983 and still available as casting, is recommended for boats over twentyfive feet long. My own back of an envelope calculations suggest that at the sort of pressure and rpm likely to be used this engine will probably produce about ten brake horsepower at the shaft. That does not sound like much, but if you check out the Steam Boat Associations register, you will see that plenty of people have successfully used this engine in boats that size and bigger. If you were going to substitute a petrol engine or small diesel you would probably need more than twice the shaft power to get the same performance, since the smaller high speed propellor would be much less efficient.

All this is by way of suggesting that comparing the proposed plant to others that are already working is probably a better way of deciding what hull size the engine can push than calculating the horsepower.

John

[jason udallParticipant @jasonudall57142]

Posted by roy entwistle on 23/09/2013 10:29:45:

Jason Steam is expanding after it leaves the boiler after cut off it is the expansion which drives the piston so which volume wouild you use

Roy

well.

if steam volume V1 at pressure P2..expands neither loosing or gaining heat..(steady state while running)

and is allowed to expand to volume V2 at pressure P2…

lets use PV=kRT..and simplyfy some what..{assume kRT applies anyway)

change in work per unit time is P1V1-P2V2…

but simpler.

.work into boiler = work into engin (else boiler pressure rises or falls)..thus if boiler delivers V in unit time at P then that is the upper limit on motor power…

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

To make short and clear answer: Horsepower is determined by the size of the boiler and not vice versa. Compound steam engine does not get more horsepower than a single cylinder steam engine. Difference between a single steam engine and compound steam engine: Increased steam economy and a smoother running steam engine in a compound steam engine. The simple method to measure horsepower to example by Prony's brake dynamometer or similar instrument to measure effect to example use difference propeller and a tachometer + a fixed size of the boiler to learn out how effective is steam engine.

[Sub MandrelParticipant @submandrel]

Lots of comment, nothiong that allows even an approximation of an answer.

Here's my method based on extensive (n=2) research.

At the risk of using some advanced maths, for an engine suitable for a small boat, take the mass in pounds and divide by 20 to get the approximate horsepower.

That should be ample if you just want to be in the right ballpark.

Now can anyone improve the accuracy of that without increasing eth complexity more than the accuracy of the answer?

Neil

P.S. Jens, your argument is circular, as the boiler has to be matched to the engine – a huge boiler won't make an engine more powerful if it can't use all the steam. They have to be matched so you need to know the spec of the engine before you can spec the boiler.

[Jens Eirik Skogstad 1Participant @jenseirikskogstad1]

Posted by Stub Mandrel on 24/09/2013 13:33:59:

Neil:

P.S. Jens, your argument is circular, as the boiler has to be matched to the engine – a huge boiler won't make an engine more powerful if it can't use all the steam. They have to be matched so you need to know the spec of the engine before you can spec the boiler.

The boiler is the source of the power; the engine is only a medium for converting the latent power in the steam into useful work.

It is better to have high pressure in boiler than the steam engine is designed to run at work pressure due there is loss of steam pressure from boiler to steam engine by condensation, resistant in steam pipe etc, etc..

[jason udallParticipant @jasonudall57142]

Neil..I usually go by froud..etal. at 3 hp per tonne
But that wasn’t the question asked

Edited By jason udall on 24/09/2013 17:13:25

[FMESParticipant @fmes]

… and the answer was?

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