Friday 2010-12-10 Dev. video update and Discussion

[Richie]

Originally Posted by Sternjaeger

I wanted to post this before but just didnt have time:

As Oleg explained, the flame output of an exhaust is affected by factors like temperature and pressure. Flames visibility is due mainly to lighting factors (it'll be harder to see flames on a sunny day), but they're always there (bear in mind that what you see is the output of the avgas combustion straight from the combustion chamber!).
Exhaust fire shouldn't be confused with backfire though, which is an external combustion of a too rich mixture which causes flames like this


Oleg, I love the work you're doing, the attention to details as usual is surprising and comes from a man who obviously has experience with aviation, my humble suggestion is to keep the flames towards the red/blue spectrum more than yellow, since the temperatures involved are far higher than a "yellow flame" when it comes to exhaust output. Another important aspect is to keep them subtle (in terms of transparency/alpha channel) but visible from distance (thinking of the Zerstoerer night fighters spotting the engine exhausts of Lancasters at a certain distance).

This has nothing to do with the engine running normally it's just raw fuel on fire. That engine probably isn't even running yet. The wind from the prop would blow it out.

http://www.youtube.com/watch?v=EoyN29O8UoE

[Sutts]

Originally Posted by Spinnetti

So, some may know already, but "proper" color of flame is not just what you like, but depends on many factors including Fuel type and Mixture, and mixture varies by type and amount of boost. Actual flame color is relative to combustion temperature and oxygen concentration. Generally speaking, the leaner the mixture, the "bluer" the flame, and the richer, the more yellow it is. At full boost, you tend to run richer to avoid burning a piston, and it should be more yellow. At cruise, it should be leaner, and thus bluer. For game purposes, I would probably have a couple settings - yellower for start up and full power, and bluer for all other settings - Important to note, but generally the only time you see any flame at all is on a missfire. If the engine is running in tune, there should be little missfires, and thus not much flame (although even a 2% missfire rate is still about 11 missfires/sec on a 16 cyl at 2000 rpm if I did the math right). The flame occurs when the unburned mixture hits the hot exhaust and combusts in the exhaust stack.... This is the general truth, but I'm sure experts could refine the exact details.

That sounds all very good but please show us a video where these yellow flames are being ejected by a running engine or point to the entry in the WWII exhaust flame colour chart where yellow flames are referred to. That chart covers everything from weak to rich mixtures and a whole lot of other scenarios including engine damage. Strange how yellow flames don't even get a mention - orange on a damaged engine but everything else is red and blue. I know it's for an Allison engine but I can't believe it differs much from any other inline carburated aero engine.

The only yellow flames I've ever seen are from raw fuel burning in the stacks of a flooded engine and an initial burst of yellow/orange flame when the engine first fires up.

I'd be happy to be proved wrong as I like the pretty yellow flames but the overwhelming evidence so far is against yellow flames from a running engine.

[major_setback]

The big question is this: Will be be able to over-prime the engine in the game?

[Richie]

That's a very good question. I think because of the fuel injection of the German fighters the Allied aeroplanes will have more fun with the flames.

[WTE_Galway]

Originally Posted by Richie

This has nothing to do with the engine running normally it's just raw fuel on fire. That engine probably isn't even running yet. The wind from the prop would blow it out.

Half right ... once an engine fires the manifold vacuum and exhaust pressure extinguish any carby or stack fires.

The main, well documented, issue on WWII war-birds was the glow of the hot manifolds at night, interfering with pilot visibility (especially in high mounted manifold designs like the hurricane and spitfire) and making the aircraft a target to night fighters. A number of different design features were tried to mask this manifold glow.

[AndyJWest]

Yes, you have a point. It isn't the exhaust that is glowing, but the exhaust manifold. Inevitably this will be at a significantly lowe temperature than the exhaust itself, so where the exhaust is flame may be blue, the manifold will be yellow or red.

[Chill31]

Oleg,

If you havent already recieved enough info at this point, I want you to know that the flame color coming out of the exhaust of a running engine like that would be more blue! than red or yellow. The flame is burning hot and mostly clean, so it comes out with a nice blue flame at night. In the day, you wont see it.

On engine start, the only time you will get the yellow/orange flame out the exhaust is if the engine is too rich and it spits out unburnt fuel that burns exiting the exhaust stack. In that case you will get a lot of orange/yellow fire coming out for a short time. I was on the wing of a P-51 when that happened and it does get warm...

Note: its easiest to over prime with the electric fuel pumps found on P-51s and the like.

*edit The short stacks in most of the videos lets you see a lot of blue flame...more than if the full length stack was in place. If you leave the exhaust as is with no blue, you really will be misrepresenting the way exhaust looks coming out of the stacks at night...

http://www.youtube.com/watch?v=g2gef...eature=related

This video (previously posted) shows what I would expect to see coming from the exhaust at high power settings at night...see it at 0:30 video time

http://www.youtube.com/watch?v=UDZmq2VrcNQ

This video shows over priming at about 5:30. His first couple of attempts to start were underprimed and he didnt keep it running with additional priming.

Also, when you go from high power to low power rapidly, you will get some light popping and orange flame as the rpm comes down.

Good luck with your great project!
Chill31

*edit, looking back at all of the posts, it looks well covered!

Originally Posted by Richie

This has nothing to do with the engine running normally it's just raw fuel on fire. That engine probably isn't even running yet. The wind from the prop would blow it out.

well first of all is mixture, not raw fuel, on fire; second, if you READ what I wrote you might well see that that's exactly was I was talking about

Originally Posted by Richie

That's a very good question. I think because of the fuel injection of the German fighters the Allied aeroplanes will have more fun with the flames.

...what do you mean exactly?

[T}{OR]

Originally Posted by Sternjaeger

...what do you mean exactly?

When burning fuel in the cylinder, if we're talking about an ideal process - you need exactly the same amount of fuel and air in your mixture. Since space in cylinder is limited, there is only so much air and fuel you can put in there (if we're not talking about turbo or supercharging). And certain amount of fuel requires exactly certain amount of air (I forgot the exact ratio but I can dig it up if you want, I have it one of my books).

The biggest downside of carburetors is that they can produce such ideal mixture only on certain RPMs, or RPM ranges. While direct fuel injection is much more flexible and can provide the engine with better mixture on all RPM ranges.

I am talking theory here, but I believe this is what Richie meant with:

That's a very good question. I think because of the fuel injection of the German fighters the Allied aeroplanes will have more fun with the flames.