Can’t we apply technologies such as VVT to make it perform just as well as a car with 4 valves per cilinder?
In: Engineering
There are a couple of reasons.
Most importantly, many smaller valves provides a greater total area for air to flow through when compared to two large valves. Air can only flow around the edges of a valve. So by using small valves, you get more “edge” per unit of area. This provides a more efficient pathway for air to flow.
An engine runs best when the air flowing into the cylinder is moving rapidly. If you move a little air through a large opening, it will move slowly. If you move a little air through a small opening, it will move rapidly. With four-valve engines, you can route the air into the engine so that only two of the four valves are used at low RPM. This causes the air flowing through two out of the four valves to move much more rapidly. This creates turbulence in the cylinder that is useful for mixing fuel, which leads to more efficient combustion.
With more valves, you can put more air into the cylinder more efficiently. With more air, comes the ability to cleanly burn more fuel and because of the more fuel burnt per power cycle, you create more power for a fractionally increased amount of work
Having two openings spaced apart allows better airflow in and out of the engine than one larger valve with the same area. This increases the engine’s efficiency by reducing wasted power and potential energy from hot gasses. It’s also easier to manufacture precise valves today than it was 30 years ago.
The LS V8 engine family is a good example of how a 2-valve architecture can still be viable.
Larger valves mean more mass, which means lower rpms before you start floating them. Smaller mass valves allow for higher rpms.
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