Charlie,
With respect (and agreeing that we are only talking about natural
aspiration) I again have to differ. The down stroke of the piston can only
act indirectly to induce the air fuel mixture to move by creating a vacuum
in the cylinder. Any restriction to flow (like throttle butterflies,
resistance in the runners or an undersized a port) will result in incomplete
cylinder filling. In fact without some very canny design tricks to optimize
inertial forces ideal filling is seldom approached.
On the up stroke, on the other hand, the mechanical force of the rising
piston is used directly to force the exhaust gases out of the cylinder so
the cylinder can be cleared even though the resistance to flow may be
higher. Thus the exhaust port can be made smaller than the inlet.
Of the two strokes the exhaust is much closer to being a positive
displacement event.
By weight the intake and exhaust should be identical. Every atom that goes
in has to come out. There may be a difference by volume since the combustion
process causes chemical changes, but there is no overall mass change in a
chemical reaction. The exhaust gases will possess a greater volume than the
intake because of thermal expansion in accordance with the ideal gas laws.
Jet engines do indeed have much larger intakes than exhausts, but it is not
because the mass of the air/fuel is being lost, but that its density is
being increased.
Later.
Bill
On 5/6/05 8:29 PM, "Charley Braum" <cbaustin@verizon.net> wrote:
> I think larger inlet ports might be due to the fact that more gas, by
> weight, is entering the chamber under lower absolute pressure, whilst the
> exhaust ports have to handle less total weight of gas under higher absolute
> pressure.
>
> Inlet is assisted by piston downstroke and exhaust the opposite, so the
> mechanical effect is nearly the same, with the volumetric change. There is the
> pressure drop through the 'inlet' and 'outlet' piping, which should have been
> designed into the engine performance characteristics.
>
> There is more gas by weight (fuel and air) going into the chamber which
> would require larger 'pipe size' than exiting (much less fuel and somewhat
> less Oxygen plus some combustion by-products) requiring smaller pipe size.
>
> Most 'pumps' which convert one form of energy into another have larger
> inlets than outlets (if normally aspirated; some jets and plasma engines, as
> well as forced fuel entry is another animal).
>
> Regards,
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