Engines Again

RE: Engines Again

[updated:LAST EDITED ON 01-10-02 AT 07:57 PM (GMT)]Keith

Thanks for that info. I remembered the “SilentKnight” as soon as you mentioned it – it was buried away in the memory banks from years ago. I now recall that it was fitted with sleeve valves and can recall wondering at the time what the hell they were.

Your comment regarding the maintenance of poppet valves on aero engines makes sense – I can imagine they would be labour intensive, particularly as those engines had mechanical valve actuation instead of hydraulic lash adjustment. But the feeling that Bristol had about poppet valves reaching the end of their development is interesting too. We are talking about the 1940’s, when the reciprocating engine was still quite crude compared to today’s engines. The interesting thing is that since the 40’s, engine development, particularly with motor cars, has adhered for the most part with multiple poppet valves and the volumetric efficiency of today’s engines, combined with the vastly improved breathing afforded by multiple camshafts and valves and fuel injection, tends to indicate that poppet valves still had a long way to go. Obviously though, such applications do not apply to aviation and I think that the turbo-prop engine was really the death of conventional piston engined aircraft and sleeve valves.

Your articles on the technical aspects of aviation are probably the most interesting I have seen on these forums. Keep them up – for those of us with any mechanical aptitude, they are fascinating.

Regards

Wombat

RE: Engines Again

Hi Wombat. The original idea was not a Bristol one. A chap called Charles Y Knight of Chigago actually invented it for use in motor cars and because of the very quiet operation compared with poppet valves they were known as “Silent Knight” engines. Later the system was improved by a Scottish Engineer called Peter Burt and a Canadian named James McCollum. It became known as the Burt-McCollum valve. But it was Bristols that really got the system up and running.

Why you ask, well it was believed that the poppet valve was reaching the limit of it’s development, and new ways were being looked at to maintain or improve the volumetric efficiency of the engine. Poppet valves are also very maintenance intensive compared to sleeve valves which are virtually maintenance free. I’ve spent many hundreds of hours adjusting valve clearances on Wasps and Twins Wasps and it’s the sort of job you just hate!! So I just love sleeve valve engines.

KeithMac

RE: Engines Again

>I think you’ve covered most of it Steve. So I’ll just add a
>few points. The action of the drive gears acted on a crank
>at the bottom of the sleeve, so it went both up and down and
>side to side, uncovering the inlet and exhaust ports as
>required. On the cylinder there were 5 ports, 3 inlet 2
>exhaust, but only 4 on the sleeve – this sometimes causes
>confusion! However there were 2 inlet ports, 1 exhaust and 1
>which was common, so during induction the two inlets ports
>plus the common port were open, and on exhaust 1 exhaust
>port plus the common port were open.
>

Thanks Keith, that’s a relief to know I’m in the right ballpark, and you were spot on when you said it causes confusion! 🙂 I’ve just spent the last fifteen minutes trying to digest what you’ve written above, and reconciling it with the picture I posted – I *think* I understand it…

Since becoming involved with the Beau I’ve always been intrigued as to how the Herc works, we’ve got a sectioned early Herc and a few loose pots in the hangar, so, armed with your explanation, I’ll have to take a very close look at them when I’m next in. Thankfully, I won’t be the person who has to make our engines work – the cowl gill rings are as close as I’m getting to that particular task!

RE: Engines Again

Keith

I find it fascinating that Roy Fedden and Bristol ever developed such a complex breathing system. Do you know where they got their inspiration? How did anybody think up this system when poppet valves were so much simpler to design and manufacture (and I imagine, a lot bloody cheaper!)

Anybody with any engineering understanding would look at the principles employed by Bristol and wonder that they ever got it to work, but what on earth prompted them to develop such a system in the first place? Were they the first, or just the first to make it work successfully?

As you said, getting a cylindrical valve to run up and down inside a cylinder and then in turn have a piston thumping away inside the valve, must have take a lot of endless nights to perfect. It’s no wonder that when the engines on racing Sea Furies, for example, karked it, they were replaced by P&W 2800’S or other large US engines.

On another note, how did rotary engines such as those fitted to Camels (Le Rhones et al) in WW I work, when the crank was stationary and the cylinders rotated around it?

Regards

Wombat

RE: Engines Again

To tackle Ant’s query on direction of rotation. – Well I don’t have a definitive answer. Most British engines rotate clockwise (viewed from the front). I suspect this developed from the early days were many engines were started by hand swinging the propeller. Most people at right handed, and it’s easier to hand swing a prop which goes clockwise if you are right handed. This of course stopped being the primary starting method very early on, but once you’ve set the fashion, why change? It’s a bit like the discussion we had on helicopter first pilots sitting in the right hand seat – more a convention than a necessity.

KeithMac

RE: Engines Again

I think you’ve covered most of it Steve. So I’ll just add a few points. The action of the drive gears acted on a crank at the bottom of the sleeve, so it went both up and down and side to side, uncovering the inlet and exhaust ports as required. On the cylinder there were 5 ports, 3 inlet 2 exhaust, but only 4 on the sleeve – this sometimes causes confusion! However there were 2 inlet ports, 1 exhaust and 1 which was common, so during induction the two inlets ports plus the common port were open, and on exhaust 1 exhaust port plus the common port were open.

It took Bristols 10 years to develop the sleeve valve to a production standard where it could be mass produced, an it was quite an achievement. Getting a thin steel sleeve to run inside an aluminium cylinder, and have an aluminium piston with cast iron rigs running inside it was a bit of a challenge. All the metals have different expansion rates, and the whole thing needs to be lubricated. Some people would have given up, but not Roy Fedden and his team.

KeithMac

Sleeve valves

>Hi Galdi. We did the sleeve valve a while back, but not
>probably as well as I would have liked. I’m trying to find a
>decent drawing and some pictures to explain it better. So
>keep looking and I’ll see what I can come up with.
>
>KeithMac

Not wishing to steal Keithmac’s thunder at all, but I’ve taken a bit of an interest in sleeve valve engines (Bristol Hercules specifically), and have a limited understanding of how they work, so I’ll have a go a this one if I may…

Essentially, the valves are not the conventional push-rod type, but consist of a steel sleeve, either rotating or moving up and down inside the cylinder, and between the cylinder and the piston. In the case of the early Hercs, the fuel / air mixture would be fed in from the rear of the cylinder, and the exhaust gasses would exit throuh the front – the attached photograph shows the two exhaust ports on the front of the cutaway pot. The exhaust gasses would then run forward to the collector ring which was mounted on the front of the engine cowling.

One of the benefits of a sleeve valved engine was more power and less drag. This was achieved by having the valves situated on the front and rear of the cylinders instead of at the tops, thereby a) increasing the stroke and therefore power of the engine, and b) reducing the diameter of the engine unit, thereby reducing the size of the engine cowling required to house it in.

Hopefully that makes sense, although I’m sure KeithMac can give you a much better explanation that that!

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RE: Engines Again

I thimk you’re correct, the sun gear or as we call it an planet gear reduces rpm dramaticly in a small space or small amount of gears.
It is also used in helicoptes who uses turbine engines to reduce the engine rpm. Because the rotor rotates much slower than the engine RPM. This happens in the Maingearbox.

RE: Engines Again

Gold dust Keithmac, keep them coming. There must be an army of piston fanatics (like me) who are simply not mechanically minded, so any stuff like this is much appreciated.

RE: Engines Again

Hi Kieth,
Thanks for the explaination,this feature is a good idea methinks! I’ve got an Air Ministry manual called ‘Aircraft and Engines’,which is highly detailed and well written,but it can be very heavy going! It’s always good to have someone who can explain things in layman’s terms to help you along.
Just out of curiosity,why did Bristol always make thier radials left handed?What dictates the direction an engine/prop is designed to turn?

RE: Engines Again

Hi Galdi. We did the sleeve valve a while back, but not probably as well as I would have liked. I’m trying to find a decent drawing and some pictures to explain it better. So keep looking and I’ll see what I can come up with.

KeithMac

RE: Engines Again

This is really a great idea!!!! I´m fairly new to the board, and I seem to have missed your earlier series, I´ve got to start looking at the old posts. And by the way, thank you for the reduction gear explanation.

There is one little thing about Bristol aeroengines, which my little bone head has never understood and that is, what on earth is a sleeve valve??? Can you give me a little clue? You don´t have to simplify too much, as I´ve got a little bit of understanding of how engines work.

Regards,
Galdri