ls1evan

Exhaust Valves Larger than Intake Valves:

I hope the extremely intelligent forum members of this site can answer this intellectually challenging question.



On all engines I know of, the intake valves are larger than the exhaust valves.
This is because:
-the pressure differential is greater across the exhaust valve and therefore a smaller valve can flow equal air

My question is:
On engines designed ONLY for high efficiency, is it ever advantageous to have MUCH larger exhaust valves than intake? According to my knowledge, it would have the following advantages:
-higher fuel atomization at low RPM (low rpm is where a high efficiency engine always is)
-larger exhaust valves, therefore better exhaust flow (this decreases pumping losses)

A counterpoint would be that it reduces intake flow. THIS IS NOT A VALID POINT BECAUSE intake flow is always limited by the throttle in high efficiency driving. By that logic, driving at full throttle would be more efficient.

The described engine would have the following characteristics:
-less maximum power out due to worse intake flow (again, not relevant on a high-efficiency engine)
-throttle valve open more for same power out (less pumping loss)
-less intake vacuum (less pumping loss)

Since HONDA is an industry leader in fuel econ and high-technology, I would expect them to use this if anyone does. Have they ever, like on a first generation Honda Insight or something? Has anyone else?

Thanks so much! Honda rocks!

BryanM.

iTrader: (1)

Well in a way, Honda has already designed something like this, only better. It's called VTEC-E.

At low rpm, only one intake valve opens (creating swirl and therefore a better fuel/air mixture) while both exhaust valves open normally. So in a sense, the exhaust valves are bigger, since only one intake valve is opening.

An engine with bigger exhaust valves/ports than intake valves/ports probably wouldn't perform very well at higher rpm. Honda addressed that too with VTEC-E. At around 2500 rpm (iirc) both intake valves open normally, allowing for normal power production at higher rpm.

ls1evan

cool, that is awesome. That does solve the low horsepower problem of my suggested engine with tiny intake valves. Still, it does not have the larger exhaust valves so it does not receive the full benefit.

I was thinking about it and my engine would be optimal where you needed a small amount of constant power. (Maybe a Chevy Volt-like vehicle except the engine only powers a generator.) Maybe in this case, it would be more efficient and cheaper to just have a smaller displacement/ less cylinder count.