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.
Later,
Jerry Christiansen
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.
Later,
Jerry Christiansen