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How efficient is?

Posted by kdc2 
How efficient is?
October 02, 2009 02:07PM
143.45 hp @ 23.053 cfm @ 15.859 lb of water @ 320 psi @ 425 deg f ?

And do these numbers work for you?

Re: How efficient is?
October 02, 2009 09:23PM
You are opening up a can of worms, with that question! The data you added is provided is backwards, to how I would calculate efficiency.

First I use basic rough calculations, to figure efficiency derived from actual steam engines, that is standard the same.


First figure out boiler efficiency, higher pressure and higher temperature going hand in hand, lower the heat conversion efficiency. Now you are not going to run your engine full open, rather it will be throttled down, forcing the steam to expand reducing thermal losses more and decreasing efficiency.

Now your going to have three major steam losses, that going to reduce volume of steam, percentage steam used, and pressure of working steam. One is thermal heat losses, from all the radiating surfaces, from condensation reducing volume that is being replaced by boiler. Two the size of passages and the bends to flow pattern, plus the length from boiler to steam chamber, all reduce pressure by resistance as flow rate demand increases. Three the throw away steam, that never is used to produce any power, this is steam that occupies the intake passage that dumped each cycle.

So with all these losses, the rule of thumb is 1/5 loss in boiler efficiency, 1/5 loss in volume of steam, and 1/5 in the pressure of steam before you figure engine losses.


Now steam engines you have lots of mechanical losses, losses to advantage of the crank were steam is doing the work, and back pressure losses because of undersized valves or to high of run speed.

Lets start back pressure, this is any increase of pressure in cylinder during exhausting up to compression, this back pressure robs of work done by pressurized steam on work cycle. The smaller valves are, the more back pressure your likely to have.Mechanical looses are every where on a engine, from ever place that making contact with another part and is moving like bearings, piston, seals, sliders, and etc. Now advantage of crank is where most steam guys ignore, if steam is put in early under full pressure very little energy is sent to turning the crank, most of pressure is put on the main bearing doing nothing.

So on Mechanical losses you can figure 1/5 with all robbing friction, back pressure loses for most is 1/5 of engine output, and on crank advantage losses to pressure supplied figure you only get massive 1/3 to 1/2 work done from the steam you put in.


These just basic losses, multiplied up all together in order 1*.8*.8*.8*.8*.8.*.5 = .16 or 16% efficiency, boiler 1*.8*.8*.8 = .51 or 51%, engine 1*.8*.8*.5= .32 or 32%, and again .51*.32 = .16 or 16%
Re: How efficient is?
October 02, 2009 10:47PM
Hi All,

You would do better by posting to this thread-


Im not quite sure what Jerry is up to on that thread, however, I can best relate to the question, as "steady-state" vs "intermittent-state". Dont forget, the Carnot, and Rankine cycle benchmarks for total possible efficiency.


Re: How efficient is?
October 02, 2009 10:59PM
Backwards wouldn't be the first descriptor used in my business.

My question is much simpler than anticipated.

Rephrased: (I may have 2 boiler systems, one nuclear power and one wood fired. They are out of the equation as are valve restrictions number of cylinders and orientation to the world. smiling smiley

Enigne A can produce above stated specs. Engine B produces half the horsepower at the same input. Engine B would be 50% less effecient as A.

What I am trying to gauge is how either engine would compare to others based on the same available power source.
Re: How efficient is?
October 03, 2009 08:24AM

After assuming that you have a counter flow engine with 8% clearance and a frictional resisting torque of .10 ft-lb per cubic inch and a condenser suction of 14.7 psia a primary performance assessment yields the following:

In order to avoid steam stretching and overcoming piston friction at expansion end, you need a minimum cutoff of about 12 percent. This yields a steam rate of 22.2 lb/hp/hr with an engine(cycle) efficiency of 9.3 percent.

The performance curve is very flat between 20 and 26 percent cutoff with the optimum occuring at 24 percent. This cutoff yields a steam rate of 19.3 lb/hp-hr with an engine (cycle) efficiency of 10.7 percent.

Re: How efficient is?
October 04, 2009 05:58AM
Thanks for the reply. Condenser vacuum will help. This engine has higher compression with longer cuttoffs. Cutoff @ 50 deg =22.1 %. That gives me an advance of 14.7%. B side cylinder is 26.3% with the same degree of cutoff (I can and will change that) and the advance is only 8.6%.

Anyway, without correcting for these 2 errors (yet), still @ about 13.2 lb/hp/h. Engine does have variable cutoff, but the advance is linked. With 70 deg cutoff I get 216 psi in compression, 50 deg only 153 psi. Even so the shorter cutoff and lower compression still is more efficient (less hp though). I'm targeting 320 psi operating. clearance is a whopping 15%.

Can of worms...I really should be working in absolute pressure and not gauge. Flipping signs isn't very clean in excel. Also wondering if the port area is too large. Can't be optimal. I'm treating port area like a full flow, but the valve is only fully open for 30 deg. Target is 1.34 in^2. Maybe i can cut down the ports. Very hard time increasing the valve opening more than 1.34.

Edited 1 time(s). Last edit at 10/04/2009 10:20AM by kdc2.
Re: How efficient is?
October 04, 2009 05:33PM
Whew..I feel better now. Tone things down (1800 rpm), apply vac adjust b side timing and I get 6.3 lb/hp/hr. Going to allow 25% losses and just call it 8. Also ran port numbers through some valve calculators for ic intakes (vac drawn) which seemed to provide a safety margine over the pressure driven steam valve. Referencing Marks' Standard p 9-58 seems to match up pretty well.
Re: How efficient is? or how fun is it?
October 05, 2009 04:22PM
I believe that if someone were to take the time and resources to make a steam car or other project, the efficiency should be measured by how fun the project is,the pride of showing something that was created, how many people stop what they are doing to watch it, how many people say "wow",etc.

For example the new Locomobile aka Likamobile or Lykamobile can only cruise about 15 miles and is not an inexpensive vehicle,

For such a steam auto , who cares about efficiency, gas milage, thermodynamic-isms.
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