For those not sick of the subject (as I am getting).
Series vs. parallel - a public effort at some quantification from Rotax
published data.
Page 50 of the 914 Installation Manual has a pressure-flow graph showing
a maximum flow rate of 120 l/h at zero pressure and a maximum pressure
of 1850 hPa at zero flow rate. The graph seems to be a redrawn copy of
the one in the Pierburg brochure for the E1F, pn 7.21440.78.0, with
stated rating of 95 l/h at 1 bar - thank you Gilles.
Whether the pump acts more like a "pressure source" (pressure little
dependent on flow) or "flow source" (flow little dependent on pressure)
depends on where it operates on this line. At low flow rates it acts
more like a "pressure source" at high flow rates more like a "flow source".
Estimating the maximum pressure that a pump must at least supply.
Page 9-5 of the 914 Operating Manual shows that sealevel airbox pressure
at 100% throttle is 1220 hPa, the boost above ISA pressure being 207 hPa.
Pages 10-1 and 10-2 of the 914 Operating Manual show that sealevel
manifold pressure at 100% throttle (some 40 hPa lower than the airbox
pressure) is maintained to an altitude of 4500 m where ISA pressure is
577 hPa, requiring an additional boost pressure of 477 hPa. Maximum
boost over ambient is then 207 + 477 = 684 hPa.
Page 9-5 of the 914 Operating Manual shows that sealevel airbox pressure
at 115% throttle is 1370 hPa, the boost above ISA pressure being 357 hPa.
Pages 10-1 and 10-2 of the 914 Operating Manual show that sealevel
manifold pressure at 115% throttle (some 50 hPa lower than the airbox
pressure) is maintained to an altitude of 2450 m where ISA pressure is
752 hPa, requiring an additional boost pressure of 261 hPa. Maximum
boost over ambient is 357 + 261 = 618 hPa.
It seems safe to assume a maximum boost pressure - airbox over ambient -
of 700 hPa.
Page 49 of the 914 Installation Manual specifies a maximum pressure loss
in the return line of 100 hPa.
Maybe the maximum pressure loss in the forward line (filters etc.) may
be put at 150 hPa.
Page 10-2 of the 914 Operating Manual states that fuel pressure should
be at least 150 hPa above airbox pressure (Installation Manual says
manifold pressure).
Not taken into account: hydrostatic difference and any ram tank pressure.
Pressure regulator assumptions:
It operates by adjusting its input pressure.
It adjust a restriction in the return line to set an operating point
(pressure, flow) for the pump(s).
The pressure drop from input to carburettor output lines is negligeable.
The maximum pressure a pump must be able to at least supply is then 700
+ 100 + 150 + 150 = 1100 hPa.
According to the pressure-flow graph it does this when regulated to a
flow rate of 90 l/h.
Even at the maximum fuel consumption of 36 l/h that makes for a 60%
return to the tank.
This is very close to the official Pierburg rating of 95 l/h at 1000 hPa.
The estimated pressure being the maximum required it follows that a
single pump operates to the left of the 90 l/h and 1100 hPa coordinate,
where it works much as a "flow source": flow little dependent on pressure.
It is interesting to see in the pressure-flow graph of page 50 of the
914 Installation Manual what are the effects of putting two pumps in
series and in parallel.
Series:
Say the one-pump operating point is 1100 hPa / 90 l/h.
Now double the graph in the horizontal direction - each flow rate
generating twice the pressure.
If the pressure regulator does nothing the line through the origin and
1100 hPa / 90 l/h finds a new combined operating point at about 1320 hPa
/ 109 l/h.
As the regulator regulates back to see 1100 hPa the combined operating
point then becomes about 1100 hPa / 111 l/h.
Note: the selected rightmost operating point is the worst case for the
series connection.
Parallel:
Say the one-pump operating point is 1100 hPa / 90 l/h.
Now double the graph in the vertical direction - each pressure
generating twice the flow rate.
If the pressure regulator does nothing the line through the origin and
1100 hPa / 90 l/h finds a new combined operating point at about 1350 hPa
/ 116 l/h.
As the regulator regulates back to 1100 hPa the combined operating point
then becomes about 1100 hPa / 180 l/h.
Note: the selected rightmost operating point is the best case for the
parallel connection.
Conclusion:
In the parallel setup the second pump causes a large extra flow to get
rid of. Switching it on and off causes larger pressure and flow changes
for the regulator to cope with.
In the series setup the main effect is that the second pump relieves the
first from half its load. Switching causes smaller changes.
The series setup actually saves some electric power too. Because the 2
pumps share the pressure between them, they each operate at 550 hPa in
the example (if identical - in practice one will do most of the work,
the other only a little). In the parallel case each pump supplies the
full 1100 hPa.
The graph on page 82 in the 914 Installation Manual (likely also redrawn
---From the Pierburg text) suggests a (maximum) savings of about 1 A for
both pumps together.
Cheers,
Jan de Jong
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