Bristol Pegasus XVIII (Hampden, Wellington, Sunderland)

[Greyman]

Anyone have a HP/altitude chart for this engine?

 

[Simon Thomas]

Second page of this: http://www.enginehistory.org/Piston/Bristol/Bristol/index.html

The XVIII was a pretty good engine for its time. It must have gone close to breaking the 1 hp/lb when on 100 octane, but I haven't found any performance figures for it on 100 octane.

 

[fubar57]

Nothing on altitude...

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From this book...

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[MiTasol]

Because they had no pressure lubrication to the top of the cylinders many of the Pegasus (and Mercury) versions required the valve gear to be lubricated with a grease gun. And the engine was rigid mounted to the airframe - no vibration isolators like on American engines.

 

[Greyman]

Fantastic Simon, thanks.

For what it's worth I have the following figures for 6.75 boost, 2600 rpm:

M Ratio: XXXXX​

S Ratio: XXXXX​

EDIT: seeing conflicting figures 6.75 boost. I'll look over more sources and add ...

 

[Simon Thomas]

I found this in 'Jane's Fighting Aircraft of World War II' 1996 edition p. 273. It shows the impact of 87 octane vs 100 octane for both the Pegasus XVIII and the Mercury XV and 25. Interesting that going from 87 to 100 on the Mercury had far more power increase than the Pegasus. I would put that down to Bristol not wanting to spend time optimising either engine when time was a precious commodity, and the sleeve valve was being a noisy infant that needed attention.
Unfortunately no information on boost.

 

[tomo pauk]

Anyone have a HP/altitude chart for this engine?

Courtesy Neil Stirling:

 

[Greyman]

Fantastic, thanks guys.

 

[fastmongrel]

Seems like the Bristol poppet valve engines were a missed opportunity for the RAF. Bristol obviously were spending most of their time, money and manpower fixing the sleeve valve engines and getting them ready for production. I have often wondered if the development of the poppet engines could have been passed onto another company like Alvis who were keen pre-war to get into aero engines licensing the Gnome Rhone 14 cylinder engines and working on their own small Leonides engine. Or Armstrong Siddeley who seemed to be coasting along building the excellent little Cheetah for trainers and utility aircraft and the egregious Tiger which was a waste of metal.

 

[Shortround6]

I am not sure how much further you could push the Pegasus (or Mercury) without extensive revamping.

Compare to the Wright R-1820, the Pegasus was an R-1750/3 engine. so it was about 96% in size.
It did use a 190mm stroke compared to 175mm so piston speed was higher.
The Pegasus was lighter.
While the Pegasus was rated quite as high for take-off in high gear it was rated at 885hp at 15,500ft max power while the R-1820G200 was rated 1000hp at 14,000ft.
To get the R-1820 past the 1200hp take-off and 1000hp at 14,000ft mark required a new crankcase, new crankshaft, new cylinders with totally new method of finning. new cylinder heads and even more bolts holding each cylinder to the crankcase.

You might get a few tweaks on the Pegasus but large improvements seem difficult and well beyond the abilities of any company that foisted the Tiger on the aeronautical community or thought that Gnome Rhone 14 cylinder engines with their lack of center bearing on a two row radial were the way to go.

 

[rinkol]

Seems like the Bristol poppet valve engines were a missed opportunity for the RAF. Bristol obviously were spending most of their time, money and manpower fixing the sleeve valve engines and getting them ready for production. I have often wondered if the development of the poppet engines could have been passed onto another company like Alvis who were keen pre-war to get into aero engines licensing the Gnome Rhone 14 cylinder engines and working on their own small Leonides engine. Or Armstrong Siddeley who seemed to be coasting along building the excellent little Cheetah for trainers and utility aircraft and the egregious Tiger which was a waste of metal.

Alvis licensed the GR engines, but had no success marketing them. Curiously, The GR radials were descendants of the earlier Bristol poppet valve engines, but differing in using 2 valves per cylinder. I don't think anybody managed to design a successful 2 row radial engine with 4 valves per cylinder, though a few makers tried. To the extent that this was a fundamental engineering problem, it would have been an issue in scaling up the Bristol single row engines.

 

[fastmongrel]

To the extent that this was a fundamental engineering problem, it would have been an issue in scaling up the Bristol single row engines.

If GR could design a 2 valve head to fit on what was basically a Bristol engine why couldnt Bristol do the same.

 

[Shortround6]

Spending too much time on the sleeve valve.