Rotax 914 Fuel consumption MAP vs. RPM

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lion
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Rotax 914 Fuel consumption MAP vs. RPM

Unread post by lion » Sat Aug 01, 2020 11:14 am

What would return a better fuel consumption in the cruise with a Rotax 914 using a VP prop or setting a ground adjustable prop for the following.

4800 RMP at 31 MAP
5200 RPM at 31 MAP

Or

5200 RPM with 31 MAP
5200 RPM with 33 MAP
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Re: Rotax 914 Fuel consumption MAP vs. RPM

Unread post by delmur » Sat Aug 01, 2020 12:23 pm

According to my experience...

5200 RPM at 31 MAP
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Re: Rotax 914 Fuel consumption MAP vs. RPM

Unread post by TC » Sat Aug 01, 2020 12:31 pm

If the MAP remains the same, lower RPM will yield a lower fuel consumption. Fuel gets sucked into the cylinders fewer times per minute.
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Re: Rotax 914 Fuel consumption MAP vs. RPM

Unread post by lion » Sat Aug 01, 2020 12:35 pm

TC wrote:
Sat Aug 01, 2020 12:31 pm
If the MAP remains the same, lower RPM will yield a lower fuel consumption. Fuel gets sucked into the cylinders fewer times per minute.
Thanks TC, that is what I am trying to ascertain, ie., a high RPM or lower RPM at the same MAP, it seems that the lower RPM rather than a higher RPM at the same MAP will return a better fuel consumption.
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Re: Rotax 914 Fuel consumption MAP vs. RPM

Unread post by Mrb13676 » Sat Aug 01, 2020 3:39 pm

Although lower RPM may be more inefficient and slower so it would depend on how you were measuring fuel consumption. I.e l/hr or l/nm.

Liters per hour is useful for setting fuel burn but not really in the context of economy. The 914 doesn’t like high MAP against a low RPM - I run 5200/min at any MAP over 30” in cruise and 5500 in the aerodrome environment (pattern/approach) and in the climb.
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Re: Rotax 914 Fuel consumption MAP vs. RPM

Unread post by nomie » Sat Aug 01, 2020 6:02 pm

Mrb13676 wrote:
Sat Aug 01, 2020 3:39 pm
Although lower RPM may be more inefficient and slower so it would depend on how you were measuring fuel consumption. I.e l/hr or l/nm.

Liters per hour is useful for setting fuel burn but not really in the context of economy. The 914 doesn’t like high MAP against a low RPM - I run 5200/min at any MAP over 30” in cruise and 5500 in the aerodrome environment (pattern/approach) and in the climb.
Agreed. 4800rpm will be the lowest in terms of l/h but worst in terms of actual 'travel' efficiency measured in l/100km.

5200rpm @ 33map will actually be most efficient. Its also the closest to the Rotax recommended RPM vs MAP setting line.

See graphs below, courtesy of rotary forum.
l100.jpg
lh.jpg
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