Two different B.H.P. values in british engines??

[Keke]

In many british documents I have seen two different values for engine power ratings in same contitions.

For example (Flight: Feb. 16, 1939, in table page 155) :

Bristol Perseus XIIc:

Max. cruising: + 1.25 lb./sq.in., 2,250 rpm. 655/685 B.H.P @ S.L.
Max. climbing + 1.25 lb./sq.in., 2,250 rpm. 680/710 B.H.P @ 4,000 ft.
Max. take-off + 3 lb./sq.in., 2,700 rpm. 865/890 B.H.P @ S.L.
Max. eco. cruising - 1 lb./sq.in., 2,200 rpm. 530 B.H.P @ 9,000 ft.
Max. level flight (5 min) + 1.25lb./sq.in., 2,600 rpm. 815 B.H.P. @ 6000 ft.

Does somebody knows why first three power figures is given with two different values?

If I am thinking correct, it can't explane with a different fuel grade, because the boost figures is same and there is no two values for the Max. level flight power values. It can't explane at power with/without supercharger because then the difference of power must be higher in the take-off ratings.

Thanks for any help!

Regards

Keke

 

[Aaron Brooks Wolters]

I'm just guessing here but the second hp rating may be an emergency rating (water injection). Again, it's just a guess.

 

[Keke]

Nice guess, Aaron but I think that they are not WEP figures.

If these double values is only on take-off and 5 min. level flight power figures I could agree you, but this is also in max. cruising values and I haven't seen any usable of water/metanol injection at cruising.

If my collected data for Bristol Perseus XIIc is correct this type is used on DeHavilland D.H. 95 civil passanger plane and I think this engine don't have injetion. It has equipped with 1-speed/1-stage supercharger.

Keke

 

[PStickney]

You're referring to the "Max Cruising" and "Max Climbing" values that you
have listed, correct?

They're not the same conditions - the Cruise rating that you show is at Sea Level, and the Climb rating is at 4,ooo'.
For a given set of Manifold Pressures and RPM, power will tend to increase slightly as altitude increses. This is due mainly tp the decrease in air temperature as altitude increases, making the air in the cylinders more dense, and a slight contribution from less back pressure on the exhausts, as the outside air pressure devreases.

 

[Tempik]

BHP and HP its various power. BHP had more watt than HP. HP = PS (my idea)

 

[Kurfürst]

BHP and HP its various power. BHP had more watt than HP. HP = PS (my idea)

It seems to big of a difference. IIRC 1 PS = 0.986 BHP.

I am thinking rammed/static figures.

 

[Keke]

Kurfürst, best explanation this far... It could be static/rammed values.
And many thanks for all those great documents, that you have posted here.

Regards

Keke

 

[peril]

RAM increases alt at which the boost level chosen is available.

It does not increase hp if measured at the same alt as static on SC engines.

 

[Keke]

RAM increases alt at which the boost level chosen is available.

It does not increase hp if measured at the same alt as static on SC engines.

In first, I though same way, but I find theoretical difference between rammed/static situation in same altitude with same MP. Correct me, if I am wrong.

In ramming air situation air pressure in a intake of supercharger is higher than static situation, so supercharger don't need make work so much to achieve same manifold pressure. With ramming air we maybe can take more power from propeller shaft because supercharger don't "eat" so much power.

Regards

Keke

 

[jerryw]

"Bristol Perseus XIIc:

Max. cruising: + 1.25 lb./sq.in., 2,250 rpm. 655/685 B.H.P @ S.L.
Max. climbing + 1.25 lb./sq.in., 2,250 rpm. 680/710 B.H.P @ 4,000 ft.
Max. take-off + 3 lb./sq.in., 2,700 rpm. 865/890 B.H.P @ S.L.
Max. eco. cruising - 1 lb./sq.in., 2,200 rpm. 530 B.H.P @ 9,000 ft.
Max. level flight (5 min) + 1.25lb./sq.in., 2,600 rpm. 815 B.H.P. @ 6000 ft.

Does somebody knows why first three power figures is given with two different values?

Keke"

I think what is going on here is that Bristol were in the habit of quoting their engine horsepower figures as a RANGE rather than a specific figure.
Note that the difference between the two figures is always about 30 HP.
However, where two figures are given for UNSUPERCHARGED JUPITER engines, they refer to low compression (about 5.3:1) and high compression (about 6.2:1) versions.
Another example below:-