[Diy_efi] Intro, and q's about fuel economy

Patrick Cahill patc at opposition.tv
Mon Dec 16 23:42:10 GMT 2002



On Mon, 16 Dec 2002, Alexei Pavlov wrote:

> pedward at apsoft.com wrote:
> > The only way to eliminate throttle losses is to have no throttle at
> > all.  You then do the throttling via modulation of the valve timing.
>
> Could you explain why there are no pumping losses with variable valve
> timing or clearance ?
> I see variable valves exactly like throttles, the piston still has to
> pump the vacuum to reach the BDC. The energy loss is still here.

Are you guys talking about variable valve timing? or variable valve lift?

You'd need some large amount of timing variance to be able to control
cylinder filling with timing alone.

Continously variable valve lift is probably more than beyond the scope of a
uni team.

On a more related note - with no pumping losses a cylinder will fill
completely with its volume at ambient pressure - is there a way, other
than making it harder for the air to enter, to limit the filling mass?
More to the point - is it possible to have cylinder filling control with
*no* pumping losses.
Letting it fill completely then releasing some before compression?

Mos.


Check out the way the current GDI systems work. The throttle body is usually
quite large but is wide open except for low rpm operation (<2000rpm) in the
interests of driveability. The reason is increase efficiency due to a
drastic reduction in pumping losses. The restriction from the throttle is
the cause of most of the pumping losses at anything but WOT, so they have
done away with it and limit engine torque via some very tricky stratified
charge injection and clever manipulation of the ignition timing. However, it
was found that trying to limit torque via the timing alone to that extent
was terribly inefficient and results in atrocious emissions and even worse
economy.

Either variable valve timing or lift will only have minor influence on
pumping losses in comparison.


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