Liquid cooled Napier Dagger

[wuzak]

There has been some discussion in other threads on the merits of the Napier Dagger.

The Dagger, in its ultimate version, could spit out around 1000hp @ 4200rpm. This from an air-cooled engine of a 1027 cubic inch (16.8l) capacity.

The downside was that it weighed as much, or more, than a single stage Merlin and, though it was narrower, it was longer and taller than the early Merlin.

The opinion seems to be that the main weak point of the Dagger was its cooling. It isn't hard to see why - the intake plumbing takes up much of the space between the cylinder banks, where cooling air might be ducted.

So, would a liquid cooled version prove to be better and more useful?

Or is the Dagger just too small, especially when compared to its weight?

 

[wuzak]

Some quick calcs:

Dagger with the same BMEP (180psi) with the same piston speed as Merlin (15.2m/s vs 13.3m/s): 1140hp @4800rpm

Dagger with the same BMEP as the Merlin XII on 100 octane fuel (1280hp @ 3000rpm = 205psi BMEP) and Merlin piston speed gives 1275hp @ 4800rpm.

This shows that the Dagger has almost exactly the same piston area as the Merlin even though it has only 5/8ths the capacity.

Calcs for a Dagger scaled up to Merlin capacity (27l):

4.465" bore (3.813" for the dagger) x 4.39" stroke (3.75").

Using the Dagger's piston speed and BMEP (13.3m/s, 184psi) I get 1370hp at just under 3600rpm.

If we up the speed to the same as the Merlin (15.2m/s) I calculate 1570hp @ 4100rpm.

If you could them up the BMEP to the equivalent of a Merlin XII on 100 octane fuel (1280hp @ 3000rpm) the result would be 1750hp @ 4100rpm.

 

[yulzari]

Whatever one might do to the Dagger, (and I am one who thinks it would have been a success with an improved cooling air exit) Napiers were well into the Sabre at the start of the war and could barely cope with that so the Dagger was never going to get any attention.

For liquid cooling one might as well make another engine which is what Napiers did. The ultimate limitation on a developed Dagger could be the 2 valve head, whereby the valves will be unable to transfer as much heat to the head as the smaller 4 valves of the Merlin. The period motor racing fix was to use an alcohol based over rich fuel to spray over the combustion chamber and this is what alcohol/water injection did in aero engines; but you can only carry enough for a few minutes. This may well be why Napiers then went on to the sleeve valve in their liquid cooled Dagger successor.

 

[nuuumannn]

I think the limited future potential of the Dagger meant that Halford and Napier abandoned it for the Sabre, a bigger engine with a lot more grunt in its basic form than the Dagger could ever produce.

 

[wuzak]

I was thinking along the lines that the liquid cooled version of the Dagger was developed instead of the air-cooled historic version. This would have put it ahead of the Sabre, was less technically risky than teh Sabre, and was in a different power class to the Sabre.

 

[Shortround6]

The Dagger was a 16.8 liter engine, the Peregrine was 21.2 liters

The Dagger has to do some making up just reach Peregrine levels.

The 1000hp Dagger was 1000hp engine at 8,750 ft.
The Peregrine had 885 at 15,000ft.
By the time the Dagger gets to 15,000ft how much power does it have?
The Dagger is already several hundred pounds heavier than the Peregrine but the Peregrine has nor radiators/coolant.

Weight of liquid cooled Dagger with radiators/coolant?
The Dagger was already using 6lbs of boost but was using 7.5:1 compression ( a benefit of the small cylinders) and could have swapped some compression of more boost.
The BMEP for both engines were 3 psi apart and piston speed for the Dagger was 2625fpm vs 2750fpm for the Peregrine.

As far as the valves go?

Dagger had a 97mm bore and a 95mm stroke, Perigrine had 127mm bore and 140mm stroke and the Merlin had 137mm bore and 152mm stroke.
Piston area of the Dagger was 73.86 sq cm and the Merlin was 147.33 sq cm. I don't think the valves would have been that different in size to make heat transfer a big deal.

The Dagger needs a bigger supercharger and some work just to beat the Peregrine, what it needs to equal a Merlin III (1030hp at 16,250 ft) is a lot more.

 

[wuzak]

I thik you've only counted one piston for piston area SR6.

Total piston area for the Merlin is 1773cm².
Total piston area for the Dagger is 1768cm².

 

[nuuumannn]

The other issue, Wuzak, is what would this liquid cooled Dagger go in? Look at its frontal area; not very streamlined, therefore not ideal for fighters, and did pre-war or early war British bombers need anything that was going to slow them down any more?

 

[wuzak]

It was slightly taller (5-6") than the Merlin, but narrower (by about 7-8"). Why isn't it streamlined?

 

[Shortround6]

I thik you've only counted one piston for piston area SR6.

Total piston area for the Merlin is 1773cm².
Total piston area for the Dagger is 1768cm².

The question was about valve sizes and valve cooling. If the cylinder head had a similar size to the piston top the Merlin had twice the area to stick 4 valves in compared to the Dagger and it's 2 valves. The Daggers valves wouldn't have been that different in size to give either engine a real advantage in dissipating heat though the valve seats.

 

[wuzak]

The question was about valve sizes and valve cooling. If the cylinder head had a similar size to the piston top the Merlin had twice the area to stick 4 valves in compared to the Dagger and it's 2 valves. The Daggers valves wouldn't have been that different in size to give either engine a real advantage in dissipating heat though the valve seats.

Sorry - took your statement out of context.

 

[Shortround6]

No worries

 

[nuuumannn]

Why isn't it streamlined?

Yes, its slender as an air cooled engine, but it's a rectangle; it doesn't taper at the front, not only that, but the location of the cooling accessories; glycol tank, ducting etc has to go alongside the engine, therefore bulking it up a bit and increasing its frontal area.

The Dagger needs a bigger supercharger and some work just to beat the Peregrine, what it needs to equal a Merlin III (1030hp at 16,250 ft) is a lot more.

Based on what Shortround has produced, what would this engine go in?

 

[wuzak]

Yes, its slender as an air cooled engine, but it's a rectangle; it doesn't taper at the front

You mean top (for an inverted vee) or bottom (upright vee)?

I would suggest that the frontal area at the narrow area of a V-12 is balanced by the wider part. Also, fighter fuselages tended not to mimic the shape of the engine.

Take the Spitfire, for example, the fuselage is wider than the bottom end of the Merlin, allowing space for engine mounts, coolant and oil lines, etc. This would be no different for the LC Dagger. The Spitfire is much closer to the Merlin's width at the top, requiring bulges to cover the cam boxes.

The Spitfire fuselage is deep enough to fit a Griffon - which is deeper than the LC Dagger.

The Dagger is longer than a single speed Merlin, but that is largely because of its air-cooled construction.

not only that, but the location of the cooling accessories; glycol tank, ducting etc has to go alongside the engine,

I would imagine that the glycol tank (header tank) would fit above the reduction gear, as it did for the Merlin.

Based on what Shortround has produced, what would this engine go in?

I am continuing the theme from the what-if-no-Merlin thread.

I imagine that the LC Dagger woul dbe an alternative for aircraft like the Spitfire and Hurricane.

 

[nuuumannn]

I am continuing the theme from the what-if-no-Merlin thread. I imagine that the LC Dagger woul dbe an alternative for aircraft like the Spitfire and Hurricane.

Aah, starting to make sense. Still, would rather have a Peregrine than go all out and modify the Dagger to produce the same power output. A guy I know who rebuilds custom car engines once told me that if you want performance you need cubes, baby.

 

[Shortround6]

Pictures of the Dagger;

http://www.enginehistory.org/G&jJBrossett/Napier Dagger.JPG

Not a lot of space around the reduction gear not already being used.

This is NOT going to be a cheap engine to make or service.

To be a viable engine even in a MK I Spitfire or Hurricane it needs to make close to 1000hp at 15-16,000ft. which means it needs to make closer to the 'equivalent' of 1100-1200hp at 8,000ft. It needs more power to drive the supercharger and it needs more "power" to overcome increased internal losses (friction, etc) while doing so.

Now you could drop the compression a bit and use higher manifold pressure, up to 15% could be gained this way, NOT counting power to run the supercharger but fuel economy suffers and you are loosing part of the advantage of the smaller cylinders.

Now what happens when you try to push beyond Merlin III performance to Merlin XX/45 performance? Granted you get Hooker to work on the supercharger and you have the 100/130 fuel but you now are trying to pull almost 1.5hp per cu. in. and you are trying to do it at altitude.

 

[yulzari]

Not a lot of space around the reduction gear not already being used.

This is NOT going to be a cheap engine to make or service.

To be a viable engine even in a MK I Spitfire or Hurricane it needs to make close to 1000hp at 15-16,000ft. which means it needs to make closer to the 'equivalent' of 1100-1200hp at 8,000ft. It needs more power to drive the supercharger and it needs more "power" to overcome increased internal losses (friction, etc) while doing so.

Now you could drop the compression a bit and use higher manifold pressure, up to 15% could be gained this way, NOT counting power to run the supercharger but fuel economy suffers and you are loosing part of the advantage of the smaller cylinders.

Now what happens when you try to push beyond Merlin III performance to Merlin XX/45 performance? Granted you get Hooker to work on the supercharger and you have the 100/130 fuel but you now are trying to pull almost 1.5hp per cu. in. and you are trying to do it at altitude.

That was Napier's conclusion too, hence they decided to design the Sabre to be able to run the revs and boost necessary to double the period Merlin power. The faults of the Dagger were lessons for the Sabre.

The main Sabre fault was Napier being unable to concentrate on production engineering. The early problems lead to over ordering of Sabres so that, by 1945, the RAF had far more spare Sabres in store than they would ever need as the production faults were overcome and engine replacement was far less frequent.

 

[fastmongrel]

Just looking at the photo showing the magnetos splayed in an V made me wonder would the Dagger have been better as an X type engine. More room for air ducting and a lower engine ?