Rotary Valves Make Normal Valves Look Silly - Why Aren't We Using Them?
Four stroke engines, which is what 99% of the engines on the road are, need to let air in during intake. The combustion chamber must be sealed during compression and combustion and we must let air out during the exhaust. This means that we need a system that can seal and unseal the chamber thousands of time per minute while at the same time withstanding the incredibly harsh conditions present in the combustion chamber.
And this is exactly what poppet valves do. They are great at sealing the chamber because the conical shape of the valve face fits into the conical shape of the valve seat creating a positive seal. Both the face and seat are made of hardened metals which offer impressive resistance to wear and increased temperature. As compression or combustion pressure acts on the valve head it actually pushes it harder against the countersunk seat. The greater the pressure the better the seal.
Unfortunately other than being great at sealing the chamber poppet valves realistically don’t have any other inherent advantages. From an engineering perspective we could even say they are a necessary evil that we managed to make reliable only with a lot of technological advancements. Did you know that back in the 50s and 60s one of the reasons why fuel was leaded is to protect the valve seats. The intense hammering of the valve against the seat under high temperature would cause microwelds between the two and as the valve opened again these welds would tear eventually leading to valve seat recession or failure. It is only when we started to phase out leaded fuel that valve seats began to become more reliable and longer lasting.
Valve springs are also a problem. At very high rpm the camshaft is trying to open and close the valve so fast that the spring simply can’t keep up. So instead of fully closing and opening the valve tends to float around the seat leading to a loss of power or even to contact between the valve and the piston in an interference engine. Valve float used to be such a limiting factor on engines that Ducati came up with the complex and maintenance-heavy desmodromic valve system just to get rid of the valve spring. Koengseeg came up with the extremely complex freevalve system to get rid of the camshafts.
But engineers persisted, they improved valve spring designs and materials and we got engines with valve springs that can rev to the moon. They pushed even further and invented variable valve timing and lift systems that can do almost anything that Freevalve can. And so the poppet valve stayed with us.
But the big problem that no amount of technology can ever eliminate is that poppet valves are an impediment to airflow. But we got around this too. We created clever intake manifolds with variable lengths and clever resonances to ram the air past the valve. We created forced induction in the form of super and turbochargers to stuff more air into the chamber. We created long and complicated exhaust manifolds to help suck the exhaust gas out of the chamber.
When you think about it a lot of the development of the internal combustion engine is actually an effort to work around the valve. To overcome its limiting factors. If you observe an engine you will see that the cylinder head and the intake and exhaust manifolds actually take up more space than the heart of the engine which is the engine block, where the crankshaft, pistons, and rods are. We need more space for the breathing equipment of the engine and we need it because the poppet valve makes breathing hard.
But what if there was a better way? What if we simply got rid of the valve instead of trying to constantly work around it.
Of course many engineers asked this question throughout they yerars and they did indeed come up with many alternatives. One of the more elegant and promising alternatrives is called a rotary valve
Instead of a poppet valve, valve seat, spring, retainer, rocker arm, lifter and camshaft all we have is a rotating barrel with cavities. As the barrel rotates the cavities in the barrel line up with cavities in the head to let air in and out of the engine.
There is no valve spring we need to overcome so this system does not drain the energy of the engine to operate which means more power and more efficiency. And because there is no valve spring there can never be any valve float at any rpm, achieving ridiculous rpm is much easier now.
This system is also very simple and it has much less moving parts which means a very low chance of failure and reduced engine size and weight.
So it’s better in every way than the poppet valve….ok…..where is it then? If it’s better in every way then why have we been using the poppet valve for the past 100 years and not this?
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