Four-stroke engines, which are 99% of the engines on the road, need to let air in during intake. The combustion chamber needs to be sealed during compression and combustion, and we need to let air out during exhaust. This means we need a system that can seal and open the chamber thousands of times per minute while still 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 from hardened metals that offer impressive resistance to wear and temperature rise. As compression or combustion pressure acts on the valve head, it actually pushes it harder against the countersunk seat. The higher the pressure, the better the seal. Unfortunately, other than being great at sealing the chamber, poppet valves don't really have any other inherent advantages. From an engineering perspective, we could even say that they are a necessary evil that we managed to make reliable only with many technological advances. Did you know that in the 50s and 60s one of the reasons lead was used in fuel was to protect the valve seats? The intense hammering of the valve against the seat at high temperatures would cause micro welds between the two and when the valve would open again, these welds would break down, eventually leading to recession or failure of the valve seat. Only when we started phasing out lead in fuel did valve seats start to become more reliable and durable. Valve springs are also a problem. At very high rpm, the camshaft tries to open and close the valve so fast that the spring simply can’t keep up. So instead of closing and opening fully, the valve tends to float around the seat, leading to a loss of power or even contact between the valve and the piston in an interference engine. Valve float used to be such a limiting factor in engines that Ducati came up with the complex, maintenance-intensive 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, improved valve spring designs and materials, and we got engines with valve springs that can be revved to the moon. They went even further and invented variable valve timing and lift systems that can do almost everything that freevalve can do. And so the poppet valve stayed with us. But the big problem that no amount of technology can eliminate is that poppet valves are an impediment to airflow. But we solved that, too. We created clever intake manifolds with variable lengths and clever resonances to push air past the valve. We created forced induction in the form of superchargers and turbochargers to get more air into the chamber. We created long, complicated exhaust manifolds to help suck exhaust gases out of the chamber. But what if there was a better way? What if we just got rid of the valve instead of constantly trying to find workarounds? Of course, many engineers have asked this question over the years, and have actually come up with many alternatives. One of the most elegant and promising alternatives is called a rotary valve. Instead of a poppet valve, a valve seat, a spring, a tappet, a rocker arm, a lifter, and a camshaft, all we have is a rotating cylinder with cavities. As the cylinder rotates, the cavities in the cylinder line up with the cavities in the cylinder head to let air in and out of the engine. There is no valve spring for us to overcome, so this system doesn't consume the engine's energy to operate, which means more power and more efficiency. And since there is no valve spring, there can never be valve float at any rpm, so it is now much easier to achieve ridiculous rpm. So, it is better in every way than the poppet valve... ok... where is it then? If it is better in every way, why have we been using the poppet valve for the last 100 years and not this? #d4a #d4aenglish