How much does an exhaust reducer affect horsepower?

Discussion in 'Prony Brake' started by Jerry Christiansen, Nov 7, 2007.

  1. Jerry Christiansen

    Jerry Christiansen Mega Poster WMSTR Lifetime Member

    Last summer, two different steam engine owners asked how much effect a reducer on the exhaust nozzle had on the horsepower their engine could produce.

    When the PLAN formula is used to calculate an engine’s indicated horsepower, P represents the pressure difference between the high pressure side of the piston and the exhaust side of the piston. If a reducer is in the exhaust nozzle one would expect that the pressure on the exhaust side of the piston would be greater than without the reducer. A reducer would cause the value of P to decrease for a given boiler pressure, thus decreasing the engine’s power output. If an indicator was used, and pressures were measured; the theoretical differences of horsepower could be calculated.

    As we all know, the power predicted by the PLAN formula is always greater than the belt horsepower because of frictional loses in the engine. Based on that and the fact that I don’t have an indicator, we decided to test the difference using a Prony Brake.

    We used the Red Prony Brake on the WMSTR show grounds for the testing. One test was done with Jeff K’s 20Hp Minneapolis in June 2007, the other was done with Tim R’s 40Hp Case during the WMSTR show Labor Day weekend that same year. Each engine was tested with and without a reducer without being unbelted from the Prony Brake. The same engineer fired the engine for both runs and the Brake was run by the same operator.

    A series of readings were recorded for each run. The data table below shows a particle list of each run.

    1917 40Hp Case Steam Engine Operating Pressure 150psi

    With reducer in nozzle ------------------------------Without reducer in nozzle

    Pounds........ RPM........ HP------------------------ Pounds........ RPM........ HP
    of Force .............................-------------------------of Force

    190 ...............242 .........45.9 ----------------------210 ...............242 ..........50.8
    200 ...............241 .........48.2-----------------------220............... 241.......... 53.0
    210 ...............240 .........50.4 ----------------------230 ...............240 ..........55.2
    220............... 236 .........51.9 ----------------------240 ...............238 ..........57.1
    230 ...............233 .........53.5---------------------- 250 ...............236 ..........59.0
    240 ...............227 .........54.4* ---------------------260 ...............232 ..........60.0
    250 ...............207 .........51.7 ----------------------270 ...............229 ..........61.8
    260 ...............187 .........48.6 ----------------------280 ...............223 ..........62.4*
    290 ...............209.......... 60.6
    300 ...............195 ..........58.5
    310 ...............180.......... 55.8

    1924 20Hp Minneapolis Steam Engine Operating Pressure 175psi**

    With reducer in nozzle------------------------------- Without reducer in nozzle

    Pounds ....PM ............HP ----------------------------Pounds ......RPM .......HP
    of Force ..........................------------------------------of Force

    300.......... 249 ..........74.1 ------------------------------320 ..........250 ..........80
    310 ..........247.......... 76.6 ------------------------------330.......... 249......... 82.2
    320 ..........244 ..........78.1 ------------------------------340 ..........247 .........83.7
    330 ..........242 ..........79.9 ------------------------------350 ..........242 .........84.7
    340 ..........234.......... 79.6 ------------------------------360 ..........237 .........85.3
    350 ..........230.......... 80.5------------------------------ 370 ..........231......... 85.4
    360.......... 225 ..........81.0* -----------------------------380 .........225 .........85.5*
    370 .........213 ...........78.8 ------------------------------390 ..........217 .........84.6
    400 .........210........... 84.0
    410 ........400 ............82.0
    420 ........190 ............79.8

    * indicates the highest horsepower
    ** Jeff’s engine originally operated at 150psi. The new boiler is rated at 175psi

    This data shows that the Case Steam Engine gained about 8Hp and the Minneapolis Steam Engine gained about 4.5Hp.

    These results bring up the question, “Why run an engine with a reducer in the exhaust if the reducer will also decrease Horsepower?”

    Usually engines are not at maximum horsepower when they are doing work such as threshing. At a lower power output the volume of steam going up the stack will be less and its velocity will also be less. The lower velocity of the exhaust would mean a decrease in the Bernoulli Effect and less draft.

    The reducer will increase exhaust speed up the stack. The increase in exhaust speed will increase the Bernoulli Effect thus increasing the draft. The increase in draft could make the engine easier to fire.

    I would like to thank Jeff K., Tim R, and Brian H. for their expertise at firing an engine underload and for letting the engines be a part of this test. If other engineers are interested in a similar test, the Rollag Prony Brake Crew would be happy to work with them.

    Jerry Christiansen
  2. Lee

    Lee Mega Poster WMSTR Lifetime Member

    This is something I have always thought about. Great tests. Is there different size reducers? I thought I had read once about different reducers for different fuels burnt (coal, wood,straw). Could you notice any difference in the stack output between no reducer and reduced? Could a special reducer be made to get the maximum velocity out the stack? You know, a spark show special.
  3. Jerry Christiansen

    Jerry Christiansen Mega Poster WMSTR Lifetime Member

    Hi Jeff,

    You asked some good questions. Perhaps others can help answer some of them.

    When we were doing the testing, I was not looking for a difference in the stack output. I think that would have to be "measured with the eye" and would be difficult to do.

    I have wondered about a reducer for Spark Show purposes. I wonder if an engine that has lost pressure over the years would benefit more than an engine that can still carry high pressure.

    Jerry Christiansen
  4. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member

    Seems you and Todahl always make some pretty good attempts at the "eye measuring" thing during the spark shows. I have even seen some pretty shaky looking tools used to measure sparks, and plenty of "explanation" on the PA to explain how it all works!

    You're never going to catch us Jeff! :biglaugh: :hide:

    If we find a belt that can hold all that HP!
  5. Lee

    Lee Mega Poster WMSTR Lifetime Member

    "You're never going to catch us Jeff! :biglaugh: :hide:"

    So if I port and polish the exhaust ports, bypass the preheater, put in a Super Turbolater Exhaust Reducer, and the special Lee-Bernoulli stack insert, do you think I might stand a chance against the "Mighty Avery"?
  6. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member


    You will need to add another cylinder and about 80 lbs of pressure! :D
  7. 40avery

    40avery Mega Poster Super Moderator WMSTR Lifetime Member


    Mark should not be so quick in shutting you down. There are things that can be done to enhance the spark production on an engine not all are in the smoke box but it is one of the key areas. I have worked with a very good steam engineer on getting a 32 states Reeves to fire better with better draft and to some degree it has helped but there are other factors too. I look forward to seeing what things you can come up with that might help. One of my favorite sayings is "Don't ever give up". Keep pondering you never know what you may come up with.

  8. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member

    I need a "tongue in cheek" smilie!;)
  9. Colin

    Colin J I Case Steam Enthusiast

    Interesting comparisons Jerry. Good job everyone who had a part in it.:congrats:
    They always said to throw the nozzel away when pulling hard or going for maximum power. so it looks like thats holds true.
    I have a picture attached here of Kory Andersons 1915 65 Case on the Rollag brake in '06. taken the last day of the show i believe...the only day it didn't monsoon.:banghead3: I was co-engineer and Kory fired. i can't remember who was running the brake that day. i believe we got 100hp at 150lbs. I remember Gary Bradley standing there the whole time with a big grin on his face watching us. and after we were done he came up to us and the first thing he said was why didn't you take the exaust nozzel off.:eek:... Doh! :bonk: Kory had bored it out a bit bigger, but having it right off would have been better. it was still a good pull though, we were happy with 100:thumb:
    I also wanted to ask what the bore and stroke of Jeff's 20hp Minneapolis is?. the 40 Case is 8 1/4 x 10. 45 and 50 are 9x10, 60 is 10x10, 65 10x11 and 75 and 80 11x11.
  10. CaseyD

    CaseyD B & R Sawmill Crew WMSTR Lifetime Member

    Don't forget that the 110Hp Case has a 12"x12" cylinder!

    "The sawmill is where I'd rather be"
  11. P.D.

    P.D. Intermediate Poster WMSTR Lifetime Member

    Hi Jerry,
    Just digging through some archives (I've had enough of the snow) and thought I'd add some observations of my own from Jeff K.'s Minne on the brake last June.

    Jeff was trying to do a power run and the engine just wasn't respond to the firing. The fire was lazy & got too thick. Jeff asked for an opinion and we just couldn't seem to get the fire to burn down properly. Robert H. noticed the exhaust sounded pretty mushy. A quick look in the smokebox revealed 2 reducers on the nozzle.
    He pulled one reducer out and the fire immediately livened up. We were able to get it to burn down to and maintain a proper thickness. We were also able to put a feather on the pop and Bob D. at the brake held about a 75 HP load (if I remember correctly). The injector was able to just make water against that load so everything was pretty close to a balance. We were able to maintain those conditions until we came to the end of the coal in the bunker.
    With a reload of the fuel, Robert took the other reducer out of the nozzle and fired a second power run. He was able to maintain a feather on the pop under load with the injector on but I don't remember what his horsepower figure was. He had to eventually back off as they were using more water than the injector could supply and the duplex pump decided to blow a water side gasket.

    My "seat of the shovel" observations (with no scientific data to back them up) are that the double reducer probably significantly increased the velocity of the exhaust but caused it to wire draw enough to reduce the overall volume enough to reduce the draft through the fire. I think that is why the fire was so lazy.
    Also, with both reducers removed the engine probably produced more overall HP but required more water (steam) to transfer the heat from the boiler to the cylinder in producing that HP. That may be why they couldn't keep up with the water in the third instance.

    I hope this may help but this is one reason it is all so much fun - trying to learn something else! Besides, it is more fun to think of something warm.....PD
  12. M Kerkvliet

    M Kerkvliet WMSTR Past President Administrator WMSTR Lifetime Member

    Interesting observation Pete. It would seem that increased velocity (smaller reducer) up the stack would increase the draft. I have never heard the term "wire draw" but I think I know what you are trying to say... not using the full capacity of the stack for draft because of the reduced amount (diameter) of the stream being shot up the stack?

    I help run an engine that we have a lot of trouble with... similar to what you are describing with Jeff's Minneapolis. I am going to have to ask my peers about this theory and see if we can do some experimenting next summer.
  13. Jerry Christiansen

    Jerry Christiansen Mega Poster WMSTR Lifetime Member

    Hi Pete,

    Thank you for the information about the reducer affect on the fire and how well the fire did or did not draft.

    In an earlier post I pointed out that higher gas speeds will increase the Bernoulli Effect and therefore could increase the draft and make firing easier. As with most things in life, this idea runs in to the ‘law of diminishing return’. As you pointed out, too much restriction could make the stream of exhaust gas too narrow to fill the stack and reduce the draft.

    Some different tests come to mind that could be investigated. (If we did the research, I am certain we learn that we are not the first to contemplate these things!) 1) Perhaps the additional reducers raise the height of the exhaust too close to the stack and that is why the stream of exhaust is too narrow to fill the stack. We could experiment with short, thick walled reducers that would all maintain the same exhaust to stack distance. 2) Perhaps each diameter of reducer has its optimum distance from the stack. We could experiment with reducers that have the same diameter, but different lengths. 3) Perhaps different loads on the same engine would best be served by a particular diameter and length of reducer. For example: Jeff’s 20Hp Minneapolis might work best with a 1.5” diameter, 3” tall reducer when producing 40HP, but would work best with a 1.75” diameter, 2” tall reducer when producing 60HP. Not only do we have to test power and ease of firing, we should also do economy testing to determine the most economical reducer.

    If we aren’t careful, we could use up all next summer and the entire coal pile testing Jeff’s engine. After that, we would have to proceed to the other engines and redo the entire series of tests to learn the optimal reducer for each engine on the grounds. Perhaps we need to write a grant proposal to some large industry or the government and get funded!!

    We are also interested in optimal reducer size for Spark Shows. Some engines that have low pressure ratings because of their age and/or boiler construction seem reluctant to participate in Spark Shows. Perhaps if we experimented with different reducers we could find one that would result in better Spark Shows and allow more engines to get in on the fun.

    Jerry Christiansen
  14. 40avery

    40avery Mega Poster Super Moderator WMSTR Lifetime Member

    I had the pleasure for three years running to talk with three brothers. Two that lived in Ireland and had many steam engines and one that lived in Minneapolis. They were VERY well versed in creating draft and what it took to get an efficient condition for the fire to react properly. One of the things they stressed was to get a "seal" at the base of the stack. That means to get the exhaust steam to fan out to the place that it hits the entire area of the base of the stack. Getting it to fan out too much starts to defeat the effect as well as having it too narrow. They gave us a lot of other ideas that also have helped us. It is one of the truly exciting facets of Rollag to have people like this that will hang out and share the knowledge that they have.
  15. Jerry Christiansen

    Jerry Christiansen Mega Poster WMSTR Lifetime Member


    I like the term 'seal' being used in this case. Did the fellows you talked with have any literature that explained how to achieve the seal?

    It seems to me that the location of the nozzle would depend on the amount and pressure of the exhaust steam. Both the amount and pressure of steam depends on the load on the enigne. The pressure also depends on the boiler.

    Maybe adjusting the exhaust will make some of our engines easier to fire and help them produce a bit more power.

    Jerry Christiansen
  16. 40avery

    40avery Mega Poster Super Moderator WMSTR Lifetime Member


    They had nothing written down. They just knew what worked and what did not. The whole concept sounds simple but is actually pretty complex. It is not simply how big the nozzle is. The shape of the hole and how the nozzle is profiled and where it is positioned in relation to the stack directly influences how fast the steam fans out or does not fan out. A nozzle that is not designed correctly can cause turbulence that will inhibit the draft. Any nozzle that is off center is very hard to get to draft well.

    The engine Mark speaks of is a twin that carries relatively low pressure and has two exhaust nozzles. It is very difficult to get them set correctly. We have tried many things with the nozzles and have improved things. We can now thresh with it but there are other complications that inhibit it plowing. With good water so it will not prime it will be fun to get it on your mobile prony and fire for a while and try different things to see what really helps it.

    They also mentioned that any leakage of cold air into the smoke box can really muck up the draft.

  17. P.D.

    P.D. Intermediate Poster WMSTR Lifetime Member

    Hi Again Folks,
    I wasn't ignoring anyone, just sometimes spare time evaporates.
    This very discussion was going on with the railroads for nearly a century and if you dig there is some information available on line. I'll post one link when I get to my work computer as I don't have it at home. The thought of getting the exhaust to seal on the inside of the stack is central to most thoughts of a proper front end arrangement (in railroad terms). Most of what I've read called for the steam to hit the stack 15% to 25% of the total height up from the bottom. Rick, that goes right along with what the Irish fellows were telling you.
    Also, a plain pipe reducer is about as poor a nozzle as you can find (according to the oldsters) because the ragged edge caused by the threads causes turbulence which disrupts the smooth flow of the escaping steam. We have been told the nozzle should be a smooth bore with what amounts to a countersunk bore in the end to create the diverging nozzle. If I remember correctly (and please feel free to correct me) the angle of the counterbore should be nearly what is needed to draw a straight line to where you want the steam to strike the inside of the stack.
    Jerry, you hit the idea I had right on the head about the small nozzle possibly not allowing the exhaust to completely seal the inside of the stack. It might have been just enough off center that with a larger nozzle it didn't matter. It also may have not sealed the stack at all or at least too far up to be effective.
    Mark, what I meant by wire drawing is trying to force a larger volume of steam through an opening than the cross section will allow without building up excessive back pressure. Yes you get tremendous velocity of the steam escaping but with the smaller orifice the steam will build pressure in the exhaust piping (back pressure) and kill your horsepower. Not only does the steam being admitted to the cylinder have to do the work of pushing the piston it also has to push the exhaust steam on the other side of the piston out of the way with the exhaust nozzle causing a restriction (taking the illustration to the extreme). That's why the Case manual cautions to use the largest nozzle possible while still getting enough draft for the fuel used.
    Wire drawing is also a good thing when you consider your throttle valve. When you crack the throttle open the boiler pressure is maintained behind the throttle valve and you admit a small volume of very high velocity steam to the valve chest and can control the speed of your engine.
    This is a good discussion and Jerry, if done right the grant writing should be able to support an entire team for 3 to 5 years:dancingparty:......PD
  18. Jerry Christiansen

    Jerry Christiansen Mega Poster WMSTR Lifetime Member


    Thanks for the information. I am looking forward to learning what the railroad knew about exhaust nozzles and sealing the stack.

    Perhaps we should talk to the show's budget committee before we write the grant. Maybe we should write the grant large enough to cover the entire budget for the next couple years . . . . .

    Jerry Christiansen
  19. Lee

    Lee Mega Poster WMSTR Lifetime Member

    I was reading a few years ago on the web about someone that was trying to improve efficiency on locomotives by changing the shape of the nozzle and stack. They used a special computer program and came up with a square shaped stack and four small nozzles if I remember right. Kind of building a 21st century steam locomotive. They had fabricated both a nozzle and stack and tried it on a running engine. It made quite an improvement as it reduced the "dead zone" compared to a conventional stack/nozzle.
  20. P.D.

    P.D. Intermediate Poster WMSTR Lifetime Member

    Hi All,
    One good reference on front end arrangements is
    and the link for the table of contents is

    Lee, I think you may be talking about L. D. Porta who never quit working on steam locomotive improvements. He developed the Lempor Exhaust Ejector which is what you describe. There is information on the Ultimate Steam Page under designers at

    More things to take your time!.......PD

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