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“The Valve Overlap Conspiracy”
12 years ago
General
That’s what I'm going to call it anyhow. You're not going to find much in the way of websites or videos on this one because it’s so overly technical and very few people have put 2 and 2 together to make this conclusion.
I'm prompted to write this as I was watching a documentary about big oil, and subsequently was conversing with a friend.
I want to start off by explaining to you all what Valve Overlap is. Basically put, in the internal combustion engine where there is a piston, valves, spark plug, ect; there is a point in the cycle where both the intake, and exhaust valve are open at the same time. This period of time is only about 15 degrees of cam travel, but that’s quite significant when the higher RPMs are achieved. There are two reasons for this overlap, one major, the other minor. The first reason is that cold un-burnt fuel/air mixture is allowed to be sucked right out the exhaust to cool the valve. The second reason is to completely scavenge and remove any waste gasses from the combustion chamber after the exhaust stroke, by allowing the momentum of the exhaust gases shooting through the manifold to ‘pull’ anything remaining from the combustion chamber. However, at the top of the stroke there is so very little amount of space left, that any remaining burnt gasses would be very little.
Now, the cooling event is a valid reason, the second ‘scavenging’ is less so. There is a very simple solution to this problem that was arrived at back in the 1920’s that isn't really that well known. But anyone involved in Aviation is fully aware of.
Time for some history:
Back when engines, and more specifically aircraft engines, were being developed there was quite a lot of head scratching and more than a few boat anchors made in regards to the cooling systems required. Firstly, were liquid cooled engines. (The ones on the Wright Flyer were liquid cooled.) Liquid cooled engines have the advantage of being able to run at high combustion temperatures and compressions due to being able to wick the heat away and be diverted elsewhere. (Which still need to be dumped into the air further downstream.) The downside to this system is that they are heavy. Extra radiators, double walled cylinder construction, many gallons of water. And should there be one misplaced bullet in combat, within minutes your engine will be quickly overheating.
Air cooled engines on the other hand do not suffer from the issues of weight, or susceptibility to bullet ingress, entire cylinders could be shot out and the engine would still run without worry of overheating. (Initially anyway, oil loss is another matter.) But, the difficulties with air cooling are that a great deal of engineering trickery and forethought needs to go into how air will move throughout the engine, and scavenging the heat away from the combustion chamber as quickly as possible.
The worst part of this was the exhaust valve. In the first air cooled rotary aircraft engines, the exhaust valve, being the most highly stressed part of the engine, had a tendency to fragment, breakdown and generally speaking make a pain of itself for the rest of the engine. Usually destroying the engine in the process. The only solution to this was to change the valves, but the cycle time was an unreasonable amount so a solution had to be found.
Overlap was considered in these early days as a method of cooling the exhaust valve but when one is talking aircraft, fuel is more precious than life itself. It’s not an overly bright idea to design an engine that dumps a small measure overboard with every combustion cycle. So a better method needed to be found.
The idea was landed upon by a gentleman by the name of Sam Heron, who had noted the effect of sodium on metals when being used in heat treating operations. He realized that the way it ‘wetted’ against metals and melted at low temperatures. This would be an ideal material for carrying the heat away from the face of the exhaust valve, as sodium did so faster than the metal the valve was capable of radiating itself.
Thus is born ‘The Sodium Filled Valve’ For those of you where this is sounding familiar, yes, this is the exact same technology many of you have in your computers by way of ‘heat pipes’ which carry away heat from your CPU faster than simply having a large finned heat sink bolted to the surface of it. By using sodium filled valves, the heating problem was solved and overlap could be eliminated. Now the exhaust valves could be trusted upon to act reliably within a long enough time to make them usable.
In sodium filled valves, a portion of the valve head and stem are hollow and filled with powdered sodium. The stem is also thicker to allow for more of the element to do its job. Once the engine reaches operating temperature the sodium melts, flows and generally moves the heat away from the edges of the valve seat. (The most highly stressed part of the valve and what typically starts failing first.) Then back up into the stem so that it can be pulled away and into the block head. The number of fins machined into the casing and the blast of air does the rest of the work.
So, hopefully you've all been following along here. I've basically spelled out that in the 20’s they solved the problem of exhaust valves cooking themselves to death in /air cooled/ engines – which suffer from the problems not being able to run nearly as hot as liquid cooled engines because they rely on airflow and good engineering to draw the heat away.
Now, imagine what could be done if a conventional liquid cooled automotive engine was retrofitted with high efficiency sodium filled exhaust valves. It would be possible to /completely eliminate/ the valve overlap, thereby dumping all this excess fuel right out the exhaust. This would accomplish so many things. Greatly reduced smog, no more need for catalytic converters, oxygen sensors or constantly adjusting the fuel richness. And let’s not forget the biggest savings, fuel economy. If you're not dumping it out the tailpipe that means it’s now available for use.
Why wasn't this done? Well... Sodium filled valves are more expensive than normal valves. And throughout history big oil and automakers priorities in fuel economy were laughable. Simply put, one doesn’t make money by making sure you having a more expensive, and the most economical engine possible. Considering you're all chained to the fuel pump anyway, and you have to pay to use it – what’s wrong with a little fixing of the system to ensure you have to buy a little bit more each time.
:/
Yeah.
Anyway, just something to consider. To be completely honest I absolutely loathe the reciprocating engine (piston engines) as it’s a very, very badly designed device for creating rotational energy. It’s been surpassed many times over by other designs but the sad fact is that once something easy makes its way into the system, it’s very hard to get it out. But, I can guarantee you all that every recip engine on this planet, from your lawnmower engine to the car, to the motorcycle. If it’s a petrol engine, it has this overlap, and is /completely unnecessary./ If I was to take a shot in the dark, you could increase it’s efficiency by around 10-15% more by simply eliminating it.
-Kalte
I'm prompted to write this as I was watching a documentary about big oil, and subsequently was conversing with a friend.
I want to start off by explaining to you all what Valve Overlap is. Basically put, in the internal combustion engine where there is a piston, valves, spark plug, ect; there is a point in the cycle where both the intake, and exhaust valve are open at the same time. This period of time is only about 15 degrees of cam travel, but that’s quite significant when the higher RPMs are achieved. There are two reasons for this overlap, one major, the other minor. The first reason is that cold un-burnt fuel/air mixture is allowed to be sucked right out the exhaust to cool the valve. The second reason is to completely scavenge and remove any waste gasses from the combustion chamber after the exhaust stroke, by allowing the momentum of the exhaust gases shooting through the manifold to ‘pull’ anything remaining from the combustion chamber. However, at the top of the stroke there is so very little amount of space left, that any remaining burnt gasses would be very little.
Now, the cooling event is a valid reason, the second ‘scavenging’ is less so. There is a very simple solution to this problem that was arrived at back in the 1920’s that isn't really that well known. But anyone involved in Aviation is fully aware of.
Time for some history:
Back when engines, and more specifically aircraft engines, were being developed there was quite a lot of head scratching and more than a few boat anchors made in regards to the cooling systems required. Firstly, were liquid cooled engines. (The ones on the Wright Flyer were liquid cooled.) Liquid cooled engines have the advantage of being able to run at high combustion temperatures and compressions due to being able to wick the heat away and be diverted elsewhere. (Which still need to be dumped into the air further downstream.) The downside to this system is that they are heavy. Extra radiators, double walled cylinder construction, many gallons of water. And should there be one misplaced bullet in combat, within minutes your engine will be quickly overheating.
Air cooled engines on the other hand do not suffer from the issues of weight, or susceptibility to bullet ingress, entire cylinders could be shot out and the engine would still run without worry of overheating. (Initially anyway, oil loss is another matter.) But, the difficulties with air cooling are that a great deal of engineering trickery and forethought needs to go into how air will move throughout the engine, and scavenging the heat away from the combustion chamber as quickly as possible.
The worst part of this was the exhaust valve. In the first air cooled rotary aircraft engines, the exhaust valve, being the most highly stressed part of the engine, had a tendency to fragment, breakdown and generally speaking make a pain of itself for the rest of the engine. Usually destroying the engine in the process. The only solution to this was to change the valves, but the cycle time was an unreasonable amount so a solution had to be found.
Overlap was considered in these early days as a method of cooling the exhaust valve but when one is talking aircraft, fuel is more precious than life itself. It’s not an overly bright idea to design an engine that dumps a small measure overboard with every combustion cycle. So a better method needed to be found.
The idea was landed upon by a gentleman by the name of Sam Heron, who had noted the effect of sodium on metals when being used in heat treating operations. He realized that the way it ‘wetted’ against metals and melted at low temperatures. This would be an ideal material for carrying the heat away from the face of the exhaust valve, as sodium did so faster than the metal the valve was capable of radiating itself.
Thus is born ‘The Sodium Filled Valve’ For those of you where this is sounding familiar, yes, this is the exact same technology many of you have in your computers by way of ‘heat pipes’ which carry away heat from your CPU faster than simply having a large finned heat sink bolted to the surface of it. By using sodium filled valves, the heating problem was solved and overlap could be eliminated. Now the exhaust valves could be trusted upon to act reliably within a long enough time to make them usable.
In sodium filled valves, a portion of the valve head and stem are hollow and filled with powdered sodium. The stem is also thicker to allow for more of the element to do its job. Once the engine reaches operating temperature the sodium melts, flows and generally moves the heat away from the edges of the valve seat. (The most highly stressed part of the valve and what typically starts failing first.) Then back up into the stem so that it can be pulled away and into the block head. The number of fins machined into the casing and the blast of air does the rest of the work.
So, hopefully you've all been following along here. I've basically spelled out that in the 20’s they solved the problem of exhaust valves cooking themselves to death in /air cooled/ engines – which suffer from the problems not being able to run nearly as hot as liquid cooled engines because they rely on airflow and good engineering to draw the heat away.
Now, imagine what could be done if a conventional liquid cooled automotive engine was retrofitted with high efficiency sodium filled exhaust valves. It would be possible to /completely eliminate/ the valve overlap, thereby dumping all this excess fuel right out the exhaust. This would accomplish so many things. Greatly reduced smog, no more need for catalytic converters, oxygen sensors or constantly adjusting the fuel richness. And let’s not forget the biggest savings, fuel economy. If you're not dumping it out the tailpipe that means it’s now available for use.
Why wasn't this done? Well... Sodium filled valves are more expensive than normal valves. And throughout history big oil and automakers priorities in fuel economy were laughable. Simply put, one doesn’t make money by making sure you having a more expensive, and the most economical engine possible. Considering you're all chained to the fuel pump anyway, and you have to pay to use it – what’s wrong with a little fixing of the system to ensure you have to buy a little bit more each time.
:/
Yeah.
Anyway, just something to consider. To be completely honest I absolutely loathe the reciprocating engine (piston engines) as it’s a very, very badly designed device for creating rotational energy. It’s been surpassed many times over by other designs but the sad fact is that once something easy makes its way into the system, it’s very hard to get it out. But, I can guarantee you all that every recip engine on this planet, from your lawnmower engine to the car, to the motorcycle. If it’s a petrol engine, it has this overlap, and is /completely unnecessary./ If I was to take a shot in the dark, you could increase it’s efficiency by around 10-15% more by simply eliminating it.
-Kalte
http://egaa.home.mindspring.com/engine3.htm
As revealed above, sodium filled valves in the O-320 engine did not eliminate valve distress problems. To the contrary, our question is whether or not they either cause or increase these problems. Sodium filled valves are an extremely expensive component that we believe are greatly to blame for valve/guide distress incidents. They do not make heat magically disappear, as some would have you believe. All these valves do, at great monetary expense, is transfer heat from the valve head to the valve stem, or more generally, from the combustion chamber to the cylinder head via the valve guide. They merely move heat from point "a" to point "b." It still has to be eliminated from the cylinder head by either air cooling or oil cooling or both. The problem is that valve guides are wearing out prematurely and are doing so in spite of operators keeping CHT levels in the proper range. Excess heat is the primary cause of premature guide wear (in a properly assembled cylinder), as most engine shops will tell you. The problem with sodium filled valves is that they serve to import even more heat into the guide by transferring it up from the valve head. Lycoming's long history of valve/guide failure incidents in the parallel valve cylinders has shown that there is simply no way that the guide can shed all of its higher heat load via the cooling fins alone, and Lycoming's design provides for very little oil to aid in that process. The irony here is that Continental uses solid stem valves that dissipate most of their heat into the valve seat. Relatively little comes up the stem and into the valve guide and yet Continental has an abundance of oil in this area to aid in heat transfer. If sodium filled valves are needed at all, one wonders why Continental doesn't use them, since their barrel style hydraulic lifters provide substantial oil for additional cooling of the guide and valve. Lycoming's mushroom style lifters do not. We think that without any change in oil flow to the rocker boxes, Lycoming valve and guide longevity might well benefit from simply going to solid stem valves. Unfortunately, these are not available.
Seriously though, thanks for the long history lesson write-up! Very cool.
Food for thought.