[alfa] Intake airflow facts

[C M Smith <cmsmith@xxxxxxxxxxxxxxx>]

Couldn't let this howler go by unmarked. Intake air does NOT stop flowing 
into the engine just because the intake valve closes. The inertia of the 
air ensures a continuous inflow while the engine is running (single venturi 
carburetors would not work were it otherwise). It is crazy to think that 
the closing of the intake valve 15 times per second (at 2,000 rpm  say) 
could interrupt the flow of air. Continuous is by no means the same thing a 
constant and clearly the flow cannot be constant or there would be nothing 
to create the pressure waves which can be so beneficial if exploited. No 
engine properly designed starts spitting air back out of the intake (though 
pressure waves can and do pop out as well as in), and by deduction it is 
easy to see that no engine can stop the inflow of air. CIS fuel injection 
relies on this actually, as well as carburetors.

What does travel back up the intake when the valve closes or opens is a 
pressure wave. Pressure waves may move up and down the intake but that 
doesn't mean the air moves with it. On the contrary, it is the pressure 
waves that cause tuned intakes (and tuned exhausts for that matter) to work 
allowing natural supercharging of the intake even in atmospheric engines.

As for port flow and cam timing etc, all I remember from reading simple 
stuff is that the smaller the port relative to the cylinder capacity, the 
higher the velocity into the intake and theoretically the higher this 
natural supercharging could be. Of course, at some point the port size 
restricts the flow volume so reduces the total air ingested by the 
cylinder. Bigger ports mean nothing without knowing what the cam timing and 
valve size and lift are, as well as the desired optimum rpm for the maximum 
torque is. Naturally, for a racing engine the requirements are different, 
but there are many examples of very powerful racing engines that could not 
win races because the power band was in the wrong place or too narrow.

Modern Formula One engines must be getting pretty close to theoretical 
limits for cylinder filling which ultimately is limited by the inertia of 
air at atmospheric pressure.

Cheers


Michael Smith
White 1991 164L
Original owner
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