Design & Spec a 14-cyl Bristol Mercury with Fuel Injection (1 Viewer)

[tomo pauk]

We can debate about fuel mileage/consumption but it seems like the big savings for the Germans was when running rich. The German engines never really ran rich, they used only slightly more fuel per HP/hour at full power than they did when cruising. Many allied engines used 40-50% more fuel per hp/hour at high power. In part for cooling and in part to make up for poor fuel distribution (solved by fuel injection). But planes spent a lot more time cruising than they did at high power.

Running really rich was a way to achieve something else, like extra power, not something that people strove to do just because they can
If an engine does not need to be run very rich, even better.
The DB 601E used 240L/h in the cruise mode, 840 HP at 5.1 km. The V-1650-1 - engine with better carb than the float-type the British used until mid-war - used 238.5 L/h (65 US gals/h) in cruise mode, 758 HP at ~5 km (16000 ft). So we have the power surplus of almost 20% 12% for same consumption.

Some FW 190s just dumped raw fuel into the air intakes ahead of the supercharger inlet for emergency power. Extra cooling or trying to get some of the effect the British were getting from the carbs, or both? The German injector set-up could not handle the needed fuel flow. Perhaps a new pump set up with more fuel flow could have been made to work?

I'm not sure from where the notion that 'the German injector set-up could not handle the needed fuel flow' comes from.
Extra fuel dumped on the BMW 801 was a form of an ADI, or indeed the way to over-rich the mixture so the engine can handle the greater boost without detonation. Figuring out that such a way was very fuel-intensive (surprise!) and only working in the low gear, they switched to the simple over-boosting, in the fashion the Merlin III had years before.

Maybe I haven't got enough sleep

You are not the only one

 

[Shortround6]

So we have the power surplus of almost 20% for same consumption.

One of us needs to check their math, I an seeing about 12% which is still significant

 

[tomo pauk]

One of us needs to check their math, I an seeing about 12% which is still significant

Yes, 12%. I'll edit the post.

 

[loftonhenderson]

Would anyone like to hazard a guess around how "good" a Mercury could be by 1939/40 – with direct mechanical fuel injection, a 2-speed S/C, and other plausible refinements – at X altitude on both 87 and early 100 oct fuel?

 

[Bretoal2]

Would anyone like to hazard a guess around how "good" a Mercury could be by 1939/40 – with direct mechanical fuel injection, a 2-speed S/C, and other plausible refinements – at X altitude on both 87 and early 100 oct fuel?

Most likely those of the Gnome et Rhone 14R, with, say, 5% more owing to better breathing with 4 valves and, say, 5% less due to higher intake temperature (in a direct injection system, induction gases are not cooled by evaporation and acceleration of the carburetor venturi).

So, eventually, the same performances - see post #2.

 

[loftonhenderson]

Most likely those of the Gnome et Rhone 14R, with, say, 5% more owing to better breathing with 4 valves and, say, 5% less due to higher intake temperature (in a direct injection system, induction gases are not cooled by evaporation and acceleration of the carburetor venturi).

So, eventually, the same performances - see post #2.

Sorry, not a 14-cyl development. Just the actual 9-cyl historical engine with improvements maxing out

 

[Bretoal2]

Sorry, not a 14-cyl development. Just the actual 9-cyl historical engine with improvements maxing out

Sorry my answer lacked a paragraph.

So, eventually, the same performances - see post #2.

But with a 9/14 coefficient :

"In rated mode (87 oct., 950 mm Hg boost and 2,400 rpm), the 14R 04/05 gave 1,210 hp at takeoff (1st gear) and 1,230 hp at 6,000 m (2nd gear). In "overload" mode (roughly equivalent to the US "military" or WEP...), with 100 oct. fuel, at 2,600 rpm, the boost was 1,180 mm Hg and the power increased to 1,590 hp at take-off and 1,580 hp at 5,000 m" (metric HP)

becoming

"In rated mode (87 oct., 950 mm Hg boost and 2,400 rpm), the Mercury bis could give 767 bhp at takeoff (1st gear) and 780 bhp at 6,000 m (2nd gear). In "overload" mode (roughly equivalent to the US "military" or WEP...), with 100 oct. fuel, at 2,600 rpm, the boost was 1,180 mm Hg and the power increased to 1,008 bhp at take-off and 1,002 bhp at 5,000 m."

 

[Shortround6]

Would anyone like to hazard a guess around how "good" a Mercury could be by 1939/40 – with direct mechanical fuel injection, a 2-speed S/C, and other plausible refinements – at X altitude on both 87 and early 100 oct fuel?

A lot depends on "plausible refinements."
also we have never really defined the 2 speed supercharger.
Bristol was offering a 2 speed supercharger on the Pegasus in 1938 but I am not sure if it was a different supercharger (impeller/housing) from the Mercury or not.
At any rate the best Altitude for the Pegasus with 87 octane was 4724 meters and 885hp. In low gear it offered 1000hp at 914ft.
Pegasus ran at 2600rpm because of it's long stroke but that is 200rpm higher than the G-R 14 cylinder engines without center bearings.
The short stroke Mercury (sarcastic) ran at 2750 rpm to give 5.7% more rpm to make up for the only 87% displacement.

The problem with 100 octane fuel is that it really needs a lot more air flow to really show it's extra oomph.
The Pegasus with 100 octane fuel was rated in high gear at 965hp at 3962 meters. And in low gear it was 1065hp at 380 meters

Max pressure was +6.75lbs vs 5.5lbs with 87 octane. The question is if was a cooling problem (not enough fins) or a connecting rod/crankshaft problem.
Pegasus used the same bore as the Mercury and pretty much the same cylinder head and valves.
The Mercury XV and 25, according to one source, gave 995hp 2820 meters on 100 octane instead of 840hp at 4270meters on 87 octane. It needs the thicker air to flow enough mass to take advantage of the 100 octane fuel. But it does not a make any more power near or above 4270meters with 100 octane fuel.
A two speed Mercury might have been given a low gear to increase take-off and low altitude power rather than high altitude power. Pulling 5-5.5lbs pressure at 14-15,000ft was better than most anybody else was doing except RR in 1939-40. A two speed Mercury could pick up an easy 100hp for take-off and low altitude even on 87 octane fuel as shown by the medium supercharged Mercuries.

A major problem for the Mercury Bis is the crappy cooling. The designers can draw whatever they want, if they can't cast/forge/machine the heads and cylinder fins deep enough, thin enough and close spaced enough it doesn't matter.

Note the top cylinder with the 90 degree exhaust elbows
Now this shows, with aid of all the yellow dots, hottest parts of the engine for the air flow over. The Exhaust ports/pipes heating the air up before it flows over the valve seats and the intake valves, seats and ports. Note also the huge jump in fin size from the cylinder barrel to the head. Easy machining but does anybody think that the cylinder barrel was uniform in temperature all the way down?
American 2 valve radials had one exhaust valve and one port on one side of the cylinder and the cool intake port and cylinder on the other side. Even in the late 20s or early 30s they offered the choice of having the exhaust exit to the side or rear to get the heat away from the rest of the head.
Bristol was ideally suited for artic flying

as it used the exhaust heat from 9 cylinders to pre warm the air before they tried to cool off the cylinder heads and engine
Anybody flying from Egypt to India and beyond (South Africa?) were on their own.

The Mercury was 24.9 liters, the Pegasus was 28.7 liter. The R-1820 was 29.9 liters and the R-1830 was 30 liters and divided up the heat load between 14 cylinders.
The Fuel injected DB 601 started at 33.9 liters. Slapping a fuel injection unit on a Mercury was not going to give you a 1st class engine, it was just too small.
And a 14 Cylinder Mercury was only 38.7 liters, which is too small to really compete with the DB 601 and Jumo 211 unless you use a lot of other modifications.

 

[loftonhenderson]

A lot depends on "plausible refinements."
also we have never really defined the 2 speed supercharger.
Bristol was offering a 2 speed supercharger on the Pegasus in 1938 but I am not sure if it was a different supercharger (impeller/housing) from the Mercury or not.
At any rate the best Altitude for the Pegasus with 87 octane was 4724 meters and 885hp. In low gear it offered 1000hp at 914ft.
Pegasus ran at 2600rpm because of it's long stroke but that is 200rpm higher than the G-R 14 cylinder engines without center bearings.
The short stroke Mercury (sarcastic) ran at 2750 rpm to give 5.7% more rpm to make up for the only 87% displacement.

The problem with 100 octane fuel is that it really needs a lot more air flow to really show it's extra oomph.
The Pegasus with 100 octane fuel was rated in high gear at 965hp at 3962 meters. And in low gear it was 1065hp at 380 meters

Max pressure was +6.75lbs vs 5.5lbs with 87 octane. The question is if was a cooling problem (not enough fins) or a connecting rod/crankshaft problem.
Pegasus used the same bore as the Mercury and pretty much the same cylinder head and valves.
The Mercury XV and 25, according to one source, gave 995hp 2820 meters on 100 octane instead of 840hp at 4270meters on 87 octane. It needs the thicker air to flow enough mass to take advantage of the 100 octane fuel. But it does not a make any more power near or above 4270meters with 100 octane fuel.
A two speed Mercury might have been given a low gear to increase take-off and low altitude power rather than high altitude power. Pulling 5-5.5lbs pressure at 14-15,000ft was better than most anybody else was doing except RR in 1939-40. A two speed Mercury could pick up an easy 100hp for take-off and low altitude even on 87 octane fuel as shown by the medium supercharged Mercuries.

A major problem for the Mercury Bis is the crappy cooling. The designers can draw whatever they want, if they can't cast/forge/machine the heads and cylinder fins deep enough, thin enough and close spaced enough it doesn't matter.
View attachment 840944
Note the top cylinder with the 90 degree exhaust elbows
Now this shows, with aid of all the yellow dots, hottest parts of the engine for the air flow over. The Exhaust ports/pipes heating the air up before it flows over the valve seats and the intake valves, seats and ports. Note also the huge jump in fin size from the cylinder barrel to the head. Easy machining but does anybody think that the cylinder barrel was uniform in temperature all the way down?
American 2 valve radials had one exhaust valve and one port on one side of the cylinder and the cool intake port and cylinder on the other side. Even in the late 20s or early 30s they offered the choice of having the exhaust exit to the side or rear to get the heat away from the rest of the head.
Bristol was ideally suited for artic flying
View attachment 840945
as it used the exhaust heat from 9 cylinders to pre warm the air before they tried to cool off the cylinder heads and engine
Anybody flying from Egypt to India and beyond (South Africa?) were on their own.

The Mercury was 24.9 liters, the Pegasus was 28.7 liter. The R-1820 was 29.9 liters and the R-1830 was 30 liters and divided up the heat load between 14 cylinders.
The Fuel injected DB 601 started at 33.9 liters. Slapping a fuel injection unit on a Mercury was not going to give you a 1st class engine, it was just too small.
And a 14 Cylinder Mercury was only 38.7 liters, which is too small to really compete with the DB 601 and Jumo 211 unless you use a lot of other modifications.

Really interesting. Okay, what about the same exercise for the Pegasus since it was larger and in theory had more room for improvement?

Also clarifying question: are you saying the Mercury is too small to bother with fuel injection, or that simply adding fuel injection to an engine as small as the Mercury isn't going to magically push it into first-class engine performance?

 

[Shortround6]

what about the same exercise for the Pegasus since it was larger and in theory had more room for improvement?

I am impressed that the Pegasus did as well as it did. It had a 190mm stroke and at 2600rpm it had a Piston speed of 3250fp (Merlin piston speed was 3000fpm). It was also light at 1135lbs (515kg). Basically a Pegasus was just a Mercury with 25mm more stroke (165mm for the Mercury) while the Wright R-1820 used a 175mm stoke (the R-2600 and R-3350 used the same 155mm bore but cut back 5mm on the stoke). The 1200hp R-1820 maxed out at 2500rpm with a piston speed of 2865fpm and used higher pressure in the cylinders to make power. It also weighed about 1320lbs for the strength needed and for the cooling fins needed. The Wright engine may have had a longer time between overhauls which also means more weight. The 1200hp R-1820 was on it's 2nd forged steel crankcase design to get the needed strength so when we start talking about "improvements" we can be talking about some major changes. The Mercury and Pegasus got the minimum to keep them in the market as most of the engineering was going into the sleeve valve engines. We can also think of the Pegasus as an improved, modernized Jupiter and the Bore & Stroke date back to the Cosmos Jupiter of 1918. At some point stop flogging the dead horse.

simply adding fuel injection to an engine as small as the Mercury isn't going to magically push it into first-class engine performance?

That is basically it. Sticking fuel injection on a small engine to get a 920-925hp engine (10% gain) at 14,000ft is not going to get you a first class fighter. Japanese used a 970hp/3400 meter engine in the early Ki-43s but they were happy (sort of) with no protection, a top speed of 308-310mph and two 7.7mm machine guns (later one 7.7 & one 12.7mm).

Was the MC. 200 Saetta really a first class fighter?
The Italians were requesting more powerful engines for the Saetta in 1938, The Isotta Fraschini engine wasn't powerful enough and the 1010hp Fiat didn't work. Depending on your opponents to screw up is really poor planning

 

[loftonhenderson]

I am impressed that the Pegasus did as well as it did. It had a 190mm stroke and at 2600rpm it had a Piston speed of 3250fp (Merlin piston speed was 3000fpm). It was also light at 1135lbs (515kg). Basically a Pegasus was just a Mercury with 25mm more stroke (165mm for the Mercury) while the Wright R-1820 used a 175mm stoke (the R-2600 and R-3350 used the same 155mm bore but cut back 5mm on the stoke). The 1200hp R-1820 maxed out at 2500rpm with a piston speed of 2865fpm and used higher pressure in the cylinders to make power. It also weighed about 1320lbs for the strength needed and for the cooling fins needed. The Wright engine may have had a longer time between overhauls which also means more weight. The 1200hp R-1820 was on it's 2nd forged steel crankcase design to get the needed strength so when we start talking about "improvements" we can be talking about some major changes. The Mercury and Pegasus got the minimum to keep them in the market as most of the engineering was going into the sleeve valve engines. We can also think of the Pegasus as an improved, modernized Jupiter and the Bore & Stroke date back to the Cosmos Jupiter of 1918. At some point stop flogging the dead horse.

That is basically it. Sticking fuel injection on a small engine to get a 920-925hp engine (10% gain) at 14,000ft is not going to get you a first class fighter. Japanese used a 970hp/3400 meter engine in the early Ki-43s but they were happy (sort of) with no protection, a top speed of 308-310mph and two 7.7mm machine guns (later one 7.7 & one 12.7mm).

Was the MC. 200 Saetta really a first class fighter?
The Italians were requesting more powerful engines for the Saetta in 1938, The Isotta Fraschini engine wasn't powerful enough and the 1010hp Fiat didn't work. Depending on your opponents to screw up is really poor planning

A Mercury improved to reliably give 1000-1100 hp @14k ft would make the Blenheim a lot more viable in the early war. Similarly, while it wouldn't make the F.5/34 a premier interceptor, it could make it a decent colonial or fleet defense interceptor. A Pegasus giving 1100-1200 hp could fill the Perseus/Taurus niches (and offer an engine that doesn't melt itself like a Taurus) making the Albacore and Beaufort much more reliable. Could make the early Wellington more survivable too, and go on the Skua and other airframes instead of the Perseus. That makes a lot of sense to me in a non-sleeve world, but then again you clearly know a hell of a hot more about engines than I do.

Meanwhile, of course Bristol should be working on the 14 and 18 cylinder types, but better 9-cyl could have filled the gaps. But are you basically saying that a clean sheet poppet 9-cyl would be better than a Mercury or Pegasus? Because I hadn't understood the Pegasus to be much worse than an R-1820 frankly.

 

[Shortround6]

Because I hadn't understood the Pegasus to be much worse than an R-1820 frankly.

Which R-1820??
Because there were 7 of them.
The
E
F
F-50
G
G100
G200
H
Each series had a number of subtypes. Like reduction gears and supercharger gears. There were also changes in the amount of fin area with each new series.
The G of 1935/36 were usually around 900-1000hp for take-off but did not carry the power to higher altitudes unless fitted with two speed superchargers or early turbos.
They maxed out at 2200rpm.
The Gs had introduced automatic valve gear lubrication (the F-50s had used oil feed but not oil return). Wright was offering 6 different supercharger gear ratios, how many were actually sold?
1937 was the big change in the late 30s with the G100s with the first steel crankcase, 30lbs heavier but 50% stronger. Valves and ports were redesigned for better flow of both intake and exhaust. RPM went to 2300rpm and max power to 1100. Many small changes. It was not just new fuel and higher boost.

Here is where the Pegasus stagnated. They didn't change much except see if it would tolerate slightly more boost. Kept the same grease gun lubrication for the Valve train and the same breathing from the early 30s.
The Wright G200s were the 1200hp engines at 2500rpm and they kept the bore & stroke and some nuts and bolts and changed everything else. Like the new forged steel crankcase that was the same weight as the G aluminum crankcase but was even stronger than the G100 crankcase and was easier to machine/produce than both. Intake and exhasut ports were made larger, Larger roller bearings on the crankshaft and many, many other changes. They built just under 86,000 G200s.
The H was another completely new version starting in late 1942. It was used in the FM-2s. Power went from 1300-1350hp to start and then to 1425-1525hp at the end of the war or post war.
This required water injection or 115/145 fuel. They were also used as helicopter powerplants due to their light weight but the high power levels needed by helicopters for cruise shortened time between overhauls.
The Wright R-1820 is one of the better documented aircraft engines and seeing how the changed it so much just to get increases of 100hp at time does make one wonder about some other engines. The R-1820s and other American engines were also increasing their time between overhauls substantial in the 1930s and 40s. Bristol did get the Hercules to very high standards eventually and RR roughly added 50% from 1939 to 1945 despite the increases in power. Some was better materials, some was better oil/lubrication and some was better quality control/manufacturing.
The standards of hours flown for the number of hours of maintenance were changing. Both for warplanes and commercial.

 

[loftonhenderson]

Which R-1820??
Because there were 7 of them.
The
E
F
F-50
G
G100
G200
H
Each series had a number of subtypes. Like reduction gears and supercharger gears. There were also changes in the amount of fin area with each new series.
The G of 1935/36 were usually around 900-1000hp for take-off but did not carry the power to higher altitudes unless fitted with two speed superchargers or early turbos.
They maxed out at 2200rpm.
The Gs had introduced automatic valve gear lubrication (the F-50s had used oil feed but not oil return). Wright was offering 6 different supercharger gear ratios, how many were actually sold?
1937 was the big change in the late 30s with the G100s with the first steel crankcase, 30lbs heavier but 50% stronger. Valves and ports were redesigned for better flow of both intake and exhaust. RPM went to 2300rpm and max power to 1100. Many small changes. It was not just new fuel and higher boost.

Here is where the Pegasus stagnated. They didn't change much except see if it would tolerate slightly more boost. Kept the same grease gun lubrication for the Valve train and the same breathing from the early 30s.
The Wright G200s were the 1200hp engines at 2500rpm and they kept the bore & stroke and some nuts and bolts and changed everything else. Like the new forged steel crankcase that was the same weight as the G aluminum crankcase but was even stronger than the G100 crankcase and was easier to machine/produce than both. Intake and exhasut ports were made larger, Larger roller bearings on the crankshaft and many, many other changes. They built just under 86,000 G200s.
The H was another completely new version starting in late 1942. It was used in the FM-2s. Power went from 1300-1350hp to start and then to 1425-1525hp at the end of the war or post war.
This required water injection or 115/145 fuel. They were also used as helicopter powerplants due to their light weight but the high power levels needed by helicopters for cruise shortened time between overhauls.
The Wright R-1820 is one of the better documented aircraft engines and seeing how the changed it so much just to get increases of 100hp at time does make one wonder about some other engines. The R-1820s and other American engines were also increasing their time between overhauls substantial in the 1930s and 40s. Bristol did get the Hercules to very high standards eventually and RR roughly added 50% from 1939 to 1945 despite the increases in power. Some was better materials, some was better oil/lubrication and some was better quality control/manufacturing.
The standards of hours flown for the number of hours of maintenance were changing. Both for warplanes and commercial.

Question stands though: are you basically saying that a clean sheet poppet 9-cyl would be better than any effort to further develop a Mercury or Pegasus? Could a Mercury be improved to reliably give 1000-1100 hp @14k ft and a Pegasus to give 1200 hp?

Furthermore, all the many versions of the R-1820 indicate to me that with that level of investment, the Pegasus could have been developed into a significantly better engine. The question is, are you saying it's too old a design to be worth bothering?

 

[Shortround6]

Question stands though: are you basically saying that a clean sheet poppet 9-cyl would be better than any effort to further develop a Mercury or Pegasus?

A clean sheet engine in the mid/late 30s would be better than any major effort. The Perseus was that attempt except it didn't work out as planned.

Could a Mercury be improved to reliably give 1000-1100 hp @14k ft and a Pegasus to give 1200 hp?

Anything can be improved if you spend enough time and money on it. The Question is should you or would the time and money be better spent elsewhere.

Furthermore, all the many versions of the R-1820 indicate to me that with that level of investment, the Pegasus could have been developed into a significantly better engine. The question is, are you saying it's too old a design to be worth bothering?

The Pegasus had some problems that were due to it's age.
However a lot of engine design is a compromise.
A lot of engine designs were the way they were do to the materials and manufacturing ablities of the times.
WW I engines mostly had narrow bores and long strokes. It was more than just fashion. There were limits on how big you could make the piston and still have the middle stay cool (not melt). With plain steel valves there limits on how big you could make the valves and not have them overheat or warp and not seal which lead to failure very quickly. Valve springs often broke even at 2000rpm or less. And a bunch of other problems.
Money was often scarce in the 1920s and at least the first 1/2 of the 30s. Aircraft engine companies very often tried to save money buy using existing tooling or parts on several different engines. This sort of worked in the 20s with low rpm engines (around 2000rpm, give or take). For Bristol using 3 Mercury size cylinders (or short Jupiter) for a 120-140hp engine looked attractive to the accountants. Lots of common parts and a way to get into the small aircraft market. But a 3 cylinder 8 liter engine was both not comfortable to fly behind, they tended to vibrate things loose in the aircraft structure. What was saved in engine maintenance was canceled out by airframe maintenance. There were 5 & 7 cylinder engines using the same cylinders although this saw a little more success French or Italian license production?
Bit of sidetrack there but many companies held on to existing tooling and designs too long or tried to make new designs fit old tooling which limited the new designs. Wright had merged with Curtiss and at the time the corporation had their own airline/s. Which meant they had their own market for their own engines. But even Curtiss-Wright didn't have enough money to do everything, in the early 30s they had at least 6 different engines in two different factories and finally decided to simplify things after the merger. Curtiss had been a leader in liquid cooled engine design in the 20s but they were limited in sales by the surplus Liberty engines in several ways. Why buy a new 400-440hp Curtiss D-12 when you can buy an in crate, brand new 400hp surplus Liberty for a fraction of the price? Turns out many of of Liberties (and early radiators) leaked water so bad than many US military aviators, once they had moved up in rank, swore never to buy liquid cooled engines again With the Market disappearing for the D-12 and the 25.7 liter Conqueror C&W shifted resources into the R-1820E (enlarged and improved R-1750) and just trickled improvements to the Whirlwind 5,7,9 cylinder engines. This market shift plus the loss of their chief designer stopped development and sales of Packard liquid cooled engines for nearly 10 years which meant that Wright only had to compete with P&W for the US market and many foreign markets.
Again a side track but it explains why C&W had some of the money or were willing to invest in rapid engine development in the 30s.

The 1920s had a lot of different ideas floating around. Wright had the experimental P-1 floating around in 1923/23 with 6in X 6.5in cylinders 1654cu in (27liters) but the cylinder head, while using very sloped valves uses a rather strange pushrod/crank arrangement that was soon changed. In was in the mid 20s that Radials began to use "mixing fans", very low boost superchargers that mainly assured a somewhat equal amount of fuel/air mix went to all cylinders. Long skinny cylinders and more cooling surface for the volume and short distance from center of piston to the cylinder walls. But skinny cylinders restrict valve area. Then we hit the problems of flame travel in the cylinder and scavenging. Theory says that the fuel burn should be most completed when the piston is about 20 degree past TDC for efficiency. Expanding burnt gases push the piston the rest of the way down. Everybody was closer to 5 to 1 compression ratio than they were to 6 to 1 so this was important. I haven't done the math to see if long skinny cylinders had less scrubbed surface (area the piston/rings rub over each stroke, roughly 80% of the internal friction in an engine) than short fat ones, may depend on exact measurements. But the long stroke introduces the increase acceleration loads at higher rpm( pistons have to start/stop and each end of the stroke and the accelerate to high speed and slow down again). Forces are usually considered to go up with the square of the rpm, a real reason that C&W kept changing the crankcases and shafts every time they increased the RPM.
The 4 valve head Bristol used was a good idea at the time, 4 small valves gave more area than 2 if they were all vertical or nearly so in the cylinder. 4 small valves were easier to keep cool and didn't warp as much as two larger ones. But this was with the early 20s valve materials and valve seat materials and even with the head materials and casting/forging techniques. A lot of the new materials were introduced over 3-5 years so there was not a single "AH-HA" moment. It even took well into the 30s to get to really good valve seat materials. Wright and P&W had bet in the mid/late 20s on shorter stroke (still had longer stroke than bore, just not as skinny as the British) and two valve heads with a lot of angle between the valves. Might have been failures if the Salt and Sodium filled exhaust valves had not worked out.
Now I have not addressed (because I don't know) any problems the Bristol engines may have had with oil or bearing failures. We do know that some engines were not supposed to be flown at certain rpm bands due to vibration but some Wright and P&W engines had similar restrictions. Sometimes later versions fixed that (new vibration dampers?)

By 1943/44 the Wright R-1820 was running at 2800rpm and had a slightly lower piston speed than the Pegasus did running at 2600rpm. It is not quite as bad as that because the fatter/heavier piston of the R-1820 create more load but you get the idea. I will also repeat that Wright shortened the stroke (slightly) of their 14 & 18 cylinder engines to reduce the piston speed at the higher rpms. Post war Wright just used 7 cylinders from an R-2600 to make a R-1300 to fill in that part of their product line. Most model changes had to do with vibration dampeners.

A lot of companies liked to stick with old dimensions not just because of existing tooling but in the days before computers figuring out vibration problems took a lot of work, both doing calculations and running experiments to see they were right. Vibration problems go up exponentially with the rpm. Engine might be fine 2400rpm, going to 2600rpm introduces a harmonic that breaks the crankshaft in short order.
Story about the First Griffon engines. They rotate backwards from Merlins. They broke the first two crankshafts in short order. Then somebody noticed (or said out loud) that instead of making the crankshafts the same as a Merlin and trying to run them backward, they make the crankshafts like a mirror image of the Merlin crankshaft with the crank throws doing the mirror image. They tried it and the new crank ran for hundreds of hours. They had only been studying vibration problems for around 20 years at the start of WW II. In WW I they were just worried about getting the engine to run at all for 20-30 hours.

I don't have a real good idea if Bristol could have improved the Pegasus substantially by throwing enough money at it. There are certainly theoretical problems with it's basic design that needed changing to make it easier to get improvements. But a lot of those improvements also require substantial sums of money, like new, shorter, fatter cylinders with much improved fins. New cylinder heads with more fins (new valve gear?) improved bottom end, and so on. May keep the Pegasus name but new engine.

 

[z42]

I'm not sure from where the notion that 'the German injector set-up could not handle the needed fuel flow' comes from.
Extra fuel dumped on the BMW 801 was a form of an ADI, or indeed the way to over-rich the mixture so the engine can handle the greater boost without detonation

IIRC it was explained in Calum's book that the existing pump design ran out of capacity. Adding a 15th pump injecting into the supercharger was a quicker solution than developing a new higher capacity pump design.

 

[z42]

The V shape for the valves is not great being on the shallow side. He was also a little late compared to Wright and P&W ( who also stated out with shallow Vs).
The R-1750 Cyclone of 1927 used valves inclined 37.5 degrees from the center line.

We had a discussion about combustion chamber shape and valve angles in Combustion chamber shape and valve angles in WWII aero engines

The modern "consensus" seems to be a 4V head with a fairly flat pent roof style combustion chamber with valve angles in the vicinity of 30 degrees. That is, 30 degrees between the valve stems, so if you measure against the cylinder center line that'd be around 15 degrees.

Now, if you have a fairly low revving engine, which is fairly undersquare, and has a relatively low geometric compression ratio, like most WWII era radials, a "hemi" style 2V combustion chamber with highly splayed valves will be fine. And in the context of radials, might even offer some nice additional advantages, like making the engine diameter slightly smaller, and provide more room for cooling fins.

 

[z42]

A major problem for the Mercury Bis is the crappy cooling. The designers can draw whatever they want, if they can't cast/forge/machine the heads and cylinder fins deep enough, thin enough and close spaced enough it doesn't matter.

Note the top cylinder with the 90 degree exhaust elbows
Now this shows, with aid of all the yellow dots, hottest parts of the engine for the air flow over. The Exhaust ports/pipes heating the air up before it flows over the valve seats and the intake valves, seats and ports. Note also the huge jump in fin size from the cylinder barrel to the head. Easy machining but does anybody think that the cylinder barrel was uniform in temperature all the way down?
American 2 valve radials had one exhaust valve and one port on one side of the cylinder and the cool intake port and cylinder on the other side. Even in the late 20s or early 30s they offered the choice of having the exhaust exit to the side or rear to get the heat away from the rest of the head.
Bristol was ideally suited for artic flying
as it used the exhaust heat from 9 cylinders to pre warm the air before they tried to cool off the cylinder heads and engine
Anybody flying from Egypt to India and beyond (South Africa?) were on their own.

The Bristol design with the exhaust valves at the front may have been motivated by a desire to have the hottest part of the combustion chamber around the exhaust valves get the best cooling?

But this then raises the question of what then? Put an exhaust collector ring right in front of the engine? Or make a 180 degree turn and route the exhaust to the back? Or both? Neither seems particularly attractive.

That being said, looking at American radials with their side mounted intakes and exhausts, seems the manifolds are routed very close to each other there as well. It's pretty crowded no matter what you do.

 

[z42]

2 - The 'intake cooling' by the carb is perhaps a kool aid by the RR people?

Maybe. Surely there was an effect, but OTOH direct fuel injection like the Germans did provided some cooling of the combustion chamber giving a bit extra knock margin.

A carb itself is a net loss for the engine power since it restricts and messes up the air flow, be that if installed in front of the S/C or behind. The British float-type carbs were especially bad in this regard. One also has to be sure the carb is ice-proof, or the ice guard must be installed, further hurting the perfomance. See also the elaborate air intakes on aircraft like the P-51, to handle the different air temperatures, the feature not present on the aircraft with fuel-injected engines.
Carbs don't allow for the increase of valve overlap - that can add ~10% to the power, as shown already in the early 1930s in the British tests, that people in charge decided to neglect - and will offer a better fuel mileage, again as noted by the British when the Jumo fuel injection was shoehorned on the Merlin (that again was brushed aside by the powers that were). Fuel injection was considered improvement by anyone that put their effort to it - Wright, Shvetsov and Japanese, plus the Germans.
Last but not least, the cost of a fuel injection system was hardly noticeable vs. the cost of an aircraft.

Persisting with trying to make the float carburetor work adequately was arguably one of the big engine development mistakes the British (and Americans) did in the interwar years. A mistake which could have been relatively easily fixed, even.

That being said, direct injection definitely has some tricky issues to be handled as well. If the atomization isn't good enough, you'll have liquid fuel hitting the cylinder walls, washing away the oil film and contaminating the oil. Germans spent a lot of time to perfect this. Probably a contributing factor in the 601N saga as well?

Port injection could have given most of the benefits at lower risk?

Or if you want to save money, just use a single point injector. Which I guess is what they eventually essentially ended up getting in the form of the pressure carburetor.

 

[loftonhenderson]

A clean sheet engine in the mid/late 30s would be better than any major effort. The Perseus was that attempt except it didn't work out as planned.

Anything can be improved if you spend enough time and money on it. The Question is should you or would the time and money be better spent elsewhere.

The Pegasus had some problems that were due to it's age.
However a lot of engine design is a compromise.
A lot of engine designs were the way they were do to the materials and manufacturing ablities of the times.
WW I engines mostly had narrow bores and long strokes. It was more than just fashion. There were limits on how big you could make the piston and still have the middle stay cool (not melt). With plain steel valves there limits on how big you could make the valves and not have them overheat or warp and not seal which lead to failure very quickly. Valve springs often broke even at 2000rpm or less. And a bunch of other problems.
Money was often scarce in the 1920s and at least the first 1/2 of the 30s. Aircraft engine companies very often tried to save money buy using existing tooling or parts on several different engines. This sort of worked in the 20s with low rpm engines (around 2000rpm, give or take). For Bristol using 3 Mercury size cylinders (or short Jupiter) for a 120-140hp engine looked attractive to the accountants. Lots of common parts and a way to get into the small aircraft market. But a 3 cylinder 8 liter engine was both not comfortable to fly behind, they tended to vibrate things loose in the aircraft structure. What was saved in engine maintenance was canceled out by airframe maintenance. There were 5 & 7 cylinder engines using the same cylinders although this saw a little more success French or Italian license production?
Bit of sidetrack there but many companies held on to existing tooling and designs too long or tried to make new designs fit old tooling which limited the new designs. Wright had merged with Curtiss and at the time the corporation had their own airline/s. Which meant they had their own market for their own engines. But even Curtiss-Wright didn't have enough money to do everything, in the early 30s they had at least 6 different engines in two different factories and finally decided to simplify things after the merger. Curtiss had been a leader in liquid cooled engine design in the 20s but they were limited in sales by the surplus Liberty engines in several ways. Why buy a new 400-440hp Curtiss D-12 when you can buy an in crate, brand new 400hp surplus Liberty for a fraction of the price? Turns out many of of Liberties (and early radiators) leaked water so bad than many US military aviators, once they had moved up in rank, swore never to buy liquid cooled engines again With the Market disappearing for the D-12 and the 25.7 liter Conqueror C&W shifted resources into the R-1820E (enlarged and improved R-1750) and just trickled improvements to the Whirlwind 5,7,9 cylinder engines. This market shift plus the loss of their chief designer stopped development and sales of Packard liquid cooled engines for nearly 10 years which meant that Wright only had to compete with P&W for the US market and many foreign markets.
Again a side track but it explains why C&W had some of the money or were willing to invest in rapid engine development in the 30s.

The 1920s had a lot of different ideas floating around. Wright had the experimental P-1 floating around in 1923/23 with 6in X 6.5in cylinders 1654cu in (27liters) but the cylinder head, while using very sloped valves uses a rather strange pushrod/crank arrangement that was soon changed. In was in the mid 20s that Radials began to use "mixing fans", very low boost superchargers that mainly assured a somewhat equal amount of fuel/air mix went to all cylinders. Long skinny cylinders and more cooling surface for the volume and short distance from center of piston to the cylinder walls. But skinny cylinders restrict valve area. Then we hit the problems of flame travel in the cylinder and scavenging. Theory says that the fuel burn should be most completed when the piston is about 20 degree past TDC for efficiency. Expanding burnt gases push the piston the rest of the way down. Everybody was closer to 5 to 1 compression ratio than they were to 6 to 1 so this was important. I haven't done the math to see if long skinny cylinders had less scrubbed surface (area the piston/rings rub over each stroke, roughly 80% of the internal friction in an engine) than short fat ones, may depend on exact measurements. But the long stroke introduces the increase acceleration loads at higher rpm( pistons have to start/stop and each end of the stroke and the accelerate to high speed and slow down again). Forces are usually considered to go up with the square of the rpm, a real reason that C&W kept changing the crankcases and shafts every time they increased the RPM.
The 4 valve head Bristol used was a good idea at the time, 4 small valves gave more area than 2 if they were all vertical or nearly so in the cylinder. 4 small valves were easier to keep cool and didn't warp as much as two larger ones. But this was with the early 20s valve materials and valve seat materials and even with the head materials and casting/forging techniques. A lot of the new materials were introduced over 3-5 years so there was not a single "AH-HA" moment. It even took well into the 30s to get to really good valve seat materials. Wright and P&W had bet in the mid/late 20s on shorter stroke (still had longer stroke than bore, just not as skinny as the British) and two valve heads with a lot of angle between the valves. Might have been failures if the Salt and Sodium filled exhaust valves had not worked out.
Now I have not addressed (because I don't know) any problems the Bristol engines may have had with oil or bearing failures. We do know that some engines were not supposed to be flown at certain rpm bands due to vibration but some Wright and P&W engines had similar restrictions. Sometimes later versions fixed that (new vibration dampers?)

By 1943/44 the Wright R-1820 was running at 2800rpm and had a slightly lower piston speed than the Pegasus did running at 2600rpm. It is not quite as bad as that because the fatter/heavier piston of the R-1820 create more load but you get the idea. I will also repeat that Wright shortened the stroke (slightly) of their 14 & 18 cylinder engines to reduce the piston speed at the higher rpms. Post war Wright just used 7 cylinders from an R-2600 to make a R-1300 to fill in that part of their product line. Most model changes had to do with vibration dampeners.

A lot of companies liked to stick with old dimensions not just because of existing tooling but in the days before computers figuring out vibration problems took a lot of work, both doing calculations and running experiments to see they were right. Vibration problems go up exponentially with the rpm. Engine might be fine 2400rpm, going to 2600rpm introduces a harmonic that breaks the crankshaft in short order.
Story about the First Griffon engines. They rotate backwards from Merlins. They broke the first two crankshafts in short order. Then somebody noticed (or said out loud) that instead of making the crankshafts the same as a Merlin and trying to run them backward, they make the crankshafts like a mirror image of the Merlin crankshaft with the crank throws doing the mirror image. They tried it and the new crank ran for hundreds of hours. They had only been studying vibration problems for around 20 years at the start of WW II. In WW I they were just worried about getting the engine to run at all for 20-30 hours.

I don't have a real good idea if Bristol could have improved the Pegasus substantially by throwing enough money at it. There are certainly theoretical problems with it's basic design that needed changing to make it easier to get improvements. But a lot of those improvements also require substantial sums of money, like new, shorter, fatter cylinders with much improved fins. New cylinder heads with more fins (new valve gear?) improved bottom end, and so on. May keep the Pegasus name but new engine.

So basically it's a cooling issue that requires a clean sheet engine. I know less about the sleeve valve lineage -- where did the Perseus stumble?

 

[loftonhenderson]

Maybe. Surely there was an effect, but OTOH direct fuel injection like the Germans did provided some cooling of the combustion chamber giving a bit extra knock margin.



Persisting with trying to make the float carburetor work adequately was arguably one of the big engine development mistakes the British (and Americans) did in the interwar years. A mistake which could have been relatively easily fixed, even.

That being said, direct injection definitely has some tricky issues to be handled as well. If the atomization isn't good enough, you'll have liquid fuel hitting the cylinder walls, washing away the oil film and contaminating the oil. Germans spent a lot of time to perfect this. Probably a contributing factor in the 601N saga as well?

Port injection could have given most of the benefits at lower risk?

Or if you want to save money, just use a single point injector. Which I guess is what they eventually essentially ended up getting in the form of the pressure carburetor.

Is there a downside to the pressure carb solution compared to direct mechanical?