Hard to start

by Robert Cunningham in 912iS Technical Questions 5 years ago

Flew out for a hundred dollar hamburger yesterday for the first time with my 10hour engine while still testing and I went to lunch and came back an hour later and did not hand prop to check the oil. The engine turned over and started, barely, then shut back down after one second. The Dynon said to land right away. I cycled the power and after several more attempts it started and ran well after a few seconds. I think it was because I had too much oil in the crankcase and I should have propped it and checked the oil after the long sit in 45 degree weather. Any ideas?

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Re: Hard to start

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

Please confirm we are discussing a 912iS fuel injected engine.

The Oil pooling should not make any difference.
It will clear in a few seconds.
An hour or so is Not considered a long time between starts.
Checking the Oil level is normally only done on the first start of the day unless you noticed a puddle under the engine.

Were both fuel pumps energized before the start attempt?
Did you have nominal fuel pressure before the start attempt?
It may help to get the fuel pumps running early in the checklist.
This gives time for the pumps to clear any vapor in the lines that may have formed shortly after shutdown when the engine was still hot.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

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Re: Hard to start

by Robert Cunningham » 5 years ago

Robert Cunningham

4 7 5

912is one pump. Didn't wait long after pump turned on before hitting the starter. Pressure looked good by the time I looked at it after a couple of start attempts. Will turn the pump on earlier next time. I will also look at the Dynon download in a couple of days when I am back home.

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Re: Hard to start

by Glenn Martin » 5 years ago

Glenn Martin

3 281 795

What size battery do you have ? The IS engine needs a bigger battery than a carbed 100. Rotax recommend a minimum of 16AH. Often people/manufacturers put in an IS with too small a battery and on cold days that can be a problem. If you have a lot of avionics you may need even bigger.

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Re: Hard to start

by Jim Flock » 5 years ago

Jim Flock

13 172 189

Were both fuel pumps energized before the start attempt?

What are your thoughts behind this question? Doesn't the OM say to start using only one pump?

Should both be energized first then use one to start?

Aviation Real Estate Specialist & iRMT

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Re: Hard to start

by Bill Hertzel » 5 years ago

Bill Hertzel

13 1.95k 7.96k

My thoughts are this...

The two pumps are Positive Displacement Pumps plumbed in series.
With both pumps operating, the same amount of fuel is going to be moved as a single pump and at the same pressure.
The normal difference would be that the two pumps would each do approximately half the work.

If when the engine was shut down, the heat within the engine compartment, in addition to the heat generated by the electric motors within the pumps themselves, raised the temperature of the fuel above its vapor point, the fuel would "Boil" locally and generate a vapor bubble in the fuel lines or the pump bodies themselves.

When restarting, this vapor bubble will need to be purged from the fuel lines for the engine to operate reliably.
The fuel pumps are capable of pumping liquid at nearly 100psi but can only pump vapor at ≈2psi (A confirmed value).
Until the pump(s) can get the fuel rail up to 45 psi, the only exit path for the vapor is through the relief orifice and the fuel return line.
Operating with both pumps can only accelerate the vapor pumping.
It may take 10-20 seconds for the pumps to move enough vapor that they regain their liquid prime and are able to pump the vapor bubble rapidly through the fuel regulator.
Additionally, if you start on a single pump and then energize the second pump sometime later, it is possible that you might introduce the vapor bubble that was within the second pump body into the fuel rail and experience the mystery stumble reported by some operators when activating the second pump.

Once full fuel pressure is established, the pumps will move ≈30 gallons per hour (≈1 liter every 30 seconds) through the fuel system.
This will be relatively cool fuel coming from the tanks and prevent any additional vapor formation.

Running a single pump prolongs the time it would take to clear the lines.
Operating both pumps simultaneously does Not draw twice the electrical power. Each pump works half as hard.

Whether you start in single or dual pump mode is fine.
You could also use Both Pumps initially to freshen the entire fuel rail and then revert to a single pump for the start.
In any case, it would be wise to confirm that your fuel pressure is nominal before the start attempt.
This may require waiting a few seconds for the pressure to stabilize, or modifying your checklist to energize the fuel pumps a few checklist items earlier.

Bill Hertzel
Rotax 912is
North Ridgeville, OH, USA
Clicking the "Thank You" is Always Appreciated by Everyone.

Thank you said by: Jim Flock, Andrew Gibson

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