Exhaust ring

Next to me in the big BP hangar, is a really nice little by-plane made by Louis McNair, his dad Robert and grand-dad Wallace:

McNair Aeroplane Co

What I want to talk about is what Wallace had to say  !!  🙂

He has lot of knowledge of engines, so started talking about the optimum valve timing etc. Straight away we were checking mine 🙂

Exhaust closing about 20 or 30 deg past TDC, and mixture opening about 5 to 10 deg before TDC. Yes he was happy with that.

Talked about a few more things, and then the discussion switched to the exhaust collector ring.

What I have is basically a collector, picking up all the exhaust and spitting it out down below. All nice, BUT

 

The big problem engine people have is that when the piston get’s to TDC position (Top Dead Centre), the exhaust valve is still closing and the mixture is already opening (see above)

When a piston is in this position, and another cylinder is just pushing all the exhaust gas out, into the exhaust collector, you have the situation where “foreign” exhaust gas is entering another cylinder that is just about to start sucking up a fresh amount of mixture (Air and Fuel).

This action basically stops the mixture from entering its cylinder for a while, and even worse, this exhaust gas will stop it from burning properly I think….

So basically you need to make sure that each cylinder has a long enough exhaust pipe, to contain 2 exhaust cycles I think (calculated using volume, and speed, and …..) before they can be combined into one big exhaust pipe.

This of cause is very impractical, so on old engines you often see stubs on each cylinder. No common  collector, just a short bit of pipe, and off it goes into the air.


Now to prove that my engine has the same problem, I made this Excel spreadsheet showing the exhaust and air intakes of all 5 cylinders. And yes theoretically I have this same problem. Not to bad for some of the cylinders because sometimes the distance from the exhausting cylinder to the cylinder getting mixture and the distance to the collector outlet make it unlikely that exhaust gas will go the wrong way. but there are definitely situations where it will go wrong:

Piston position for Exhaust and Mixture cycle only

You can see that at the TDC of every piston movement/ position, were the mixture valve is opening and the exhaust is still closing, another exhaust valve has already opened (exhaust valve is open before, during and a bit after the rising curve and the mixture is open just before, during and a bit after the falling curve).

And because that exhaust valve is opening early, the exhaust gas is already flowing out of that valve and into the collector ring.

So for every cylinder where a mixture valve is opening, another cylinder is already exhausting gas.

Not every combination will cause a problem as the opening exhaust valve could be on the other side of the collector ring somewhere.

Below is a list of cylinders with opening mixture vales and exhaust valves blowing exhaust gas into them:

So maybe not such a problem having an exhaust ring but worth to try without the whole ring collector.

Just a comment on the Velie that was on the Bleriot picture I send to Jack in the US, that one had stubs !!!! See below 🙂

You can clearly see one stub on the left, in the shade two on the top right side by side for the top cylinder and the one to the right. And then there will be two at the bottom, but I can’t see them !!   🙂

I always thought that was, because it was cheaper and easier that way, but maybe there was more to it !!

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