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Car Forum / Driving, Maintenance, Tuning / Maintenance and Repair / June 2005multivalve engines
Hi all, I am a car laic trying to understand reasons for some design decisions. So far the best explanation I found for multiple valves is based on the assumption that valves must be round; then, to cover more area of a round cylinder head with circles, it's better to use more smaller circles. My question to the group is, why do valves need to be round? With "D"-shaped valves there could be one intake and exhaust valve that has maximum possible flow. Thanks! > Hi all, > [quoted text clipped - 9 lines] > > Thanks! If the valves are not perfectly round, then you have the added complexity and likely friction of a mechanism to keep the valves from rotating. Regular round valves do rotate, and indeed it is desirable for them to do so, to keep deposits from building up and causing the valves to burn and/or beat imperfections into their seats (which will eventually cause them to burn...) in fact many heavy-duty versions of various engines have exhaust valve rotators built into the retainers. It's a good question, but unfortunately not a good idea. nate  Signaturereplace "fly" with "com" to reply. http://home.comcast.net/~njnagel They are usually round so that they can rotate and prolong the sealing properties against the seat. A D shaped valve might work for a while, but would soon lose its ability to seal efficiently. There are other types of valves which have been used commercially and experimentally. To get the most power from an engine, you want to get the maximum gas mixture into the cylinder and the exhaust out, efficiently. Multiple valves help attain this. > Hi all, > [quoted text clipped - 9 lines] > > Thanks! How would you design a machine that could cut D shaped valve seats that perfectly matched D shaped valves to such fine tolerances that they sealed properly? Clearly you can't rotate them to lap them in and you also have to build in a mechanism that stops them rotating in use. If you work it out then patent it quick because no one else can manage it. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) > Hi all, > [quoted text clipped - 9 lines] > > Thanks! Valves need to rotate in order to maintain a good sealing surface. You can't rotate non-round valves. > > Hi all, > > [quoted text clipped - 12 lines] > Valves need to rotate in order to maintain a good sealing surface. You > can't rotate non-round valves. Valves seat just fine without rotating. > > Hi all, > > [quoted text clipped - 12 lines] > Valves need to rotate in order to maintain a good sealing surface. You > can't rotate non-round valves. Some diesel engines have valves with a shroud on one side to direct the airflow and are pinned to stop them rotating. There's no absolute necessity for rotation to maintain a good sealing surface. -- Dave Baker - Puma Race Engines (www.pumaracing.co.uk) >>Valves need to rotate in order to maintain a good sealing surface. You >>can't rotate non-round valves. [quoted text clipped - 4 lines] > -- > Dave Baker - Puma Race Engines (www.pumaracing.co.uk) One advantage to round valves is doing a valve job. Valve and seat grinding, lapping, to get a good seal when doing a valve job requires a round seat. It would be very hard to do this, but not impossible, without a round valve. One would almost have to replace the valves, and have a thread-in seat, for non circular valves. Not impossible, but complicated. All in all, a round valve is nice. Of course, there have been sleeve valve engines, and other methods of porting, but these have gradually become dead ends. Although I respect the opinions of a previous poster, round valve rotators are designed to help lengthen the time that the valve will function well. The design is intended to help prevent warpage, lessen deposits, extend the period of positive seal. Odd shaped valves can certainly seal efficiently at first, and possibly with advanced design can last a long time. Design and metallurgy of this degree of refinement are seldom a part of production automotive engines. If you are willing to spend the money to do it right, you can get by with a lot of things. |
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