High-Pressure Pumps: Weak Point of Thielert Engines ?
Thielert engines have been around for quite a while now - my personal flight time on Thielert equipped planes is close to 68 hours over two years. As far as I’m concerned these engine perform well. The only negative feed-back I can think of is about the high pressure pumps the engine uses to bring Jet-A1 fuel to the temperature and pressure conditions which make it explosive. The fuel is injected under a pressure of 1350 bars and at a temperature of 120 °C !
The planes I flew - a DA40 TDI and a C172 retrofit - both had some troubles related to the high-pressure pumps, which make me think that the pump could be the weak point of the Thielert engine, even if the troubles were of different nature.
No fuel starvation allowed
In a previous post I mentioned that the club I used to rent the Cessna from decided to forbid us to fly side-slips. This is decision was an extension of the AFM which mentions that side-slips with low fuel levels are forbidden. The reason is that the high pressure pump can be damaged after a fuel starvation of 15 seconds ! A prolongated side-slip could lead to such a condition. Any engine would probably stop running under such circumstances, but the pumps (electrical and mechanical) would not be damaged !
No water allowed
A DA40 also had its pump replaced because, according to Diamond, it had been damaged because too much water went through it. The hypothesis is that because of massive condensation and insufficient pre-flight draining left water in the tanks. It is not clear to me yet how water instead of fuel can be so bad for the pump, but I’m not an engine specialist.
Starting problems
One of the operational advantages of FADEC controlled engines is that they always start easily and at first attempt. No three hands procedure needed, nor variants for warm or flooded engine. Just press start or turn the key, and the engine fires-up… says the theory.
One of the planes I rented a lot was just unable to start when the engine was warm. A minimum cooling time was required before any successful restart. It went to the maintenance workshop several times before Thielert accepted to replace the full engine (or pump, I’m not sure now…). Measurements (the good side of FADEC controlling everything) shown that the pump was not able to reach the 300 bars needed during the starting phase.
Youth problems
None of these issues cause any engine failure in flight nor any incidents. Unexpected maintenance is always exasperating, but seldom dangerous. These problems are certainly youth problems that Thielert will cure rapidly. Don’t forget that the last “evolution” before diesel engines was injected engines ! And as far as I’m concerned, Thielert engines perform well.
14 Comments, Comment or Ping
Jess Sightler
This all sounds suspicious to me, honestly. A system that dies mechanically due to 15 seconds of fuel starvation shouldn’t pass certification.
The water incident is even stranger. Did the pilot have a forced landing due to this? Or switch to another tank? In either case, I don’t see how this would damage the pump.
But, what do I know.
I still wish that I could find more technical documentation from thielert on the problem.
Jan 5th, 2008
PlasticPilot
I don’t have Thielert documentation on this, but the DA40D AFM has a warning in the FUEL TRANSFER PUMP FAILURE section stating that:
“When the fuel pump takes in air (e.g. when the emergency fuel valve is not switched back and the auxiliary tank is empty), an inspection of the pump is necessary prior to next flight”.
In the “water incident” the decision to replace the pump was taken by the mechanic while doing a periodical inspection.
Jan 5th, 2008
Jess Sightler
A pump will tend to fail if run with no fluid for a significant period of time, as the fluid provides lubrication and possibly coolant. I expect that if someone were to leave the aux pump on, they could easily have done so for more than 15 seconds.
But as to water, that just doesn’t make sense. If there was enough water to cause a problem, the engine quits due to the excessive contamination.
I’ll have to look into this sometime, as it is kind of curious to me that this could be such a major failure mode.
Jan 6th, 2008
Ot
I have experience with the Ford diesel 7.3 Powerstroke (1998) and Cummins (1999 5.9L 24v ISB). The Ford diesel has run out of fuel 5 times, and was extremely hard to restart taking 20-30 minutes of cranking until I figured out a trick, which brought the restart proceedure time down to 1 minute. And if you know what the fuel starvation cackle sounds like, you can better that time by shutting down the engine prior to total starvation, this allows the engine to restart immediately once its refueled.
I have had 3 HP pump failures on the Cummins, due to low pressure, from a partially clogged filter, but none replaced with the 7.3 which had the 5 total fuel starves(3200 hours on it now). This is due to the superior design of the HP pump in the International 7.3 engine. Apparently the 7.3 uses alot of oil sump lubrication, and does not rely solely on fuel lubrication.
Additionally there is a water-fuel separator with a warning light when water in the fuel is detected. Should the indicator come on, the fuel drain is manually pulled for 10 seconds.
My point is Thielert needs to study and incorporate the rugged design of the 7.3 Powerstroke for their HP pump.
Mar 7th, 2008
PlasticPilot
Thank you Ot for the technical details. I don’t have the same detail of knowledge on pumps design, but your point about lubrication sounds interesting. I don’t know yet what Thielert intentions are.
May be one difference in airplane engines is the wider range of operating temperatures, but I don’t know if this is a factor or not in these failures.
Mar 7th, 2008
Karoliina
>None of these issues cause any engine failure in flight nor any > incidents
Not entirely accurate maybe:
Our flying club’s diesel-C172 (with Centurion 1.7) came down near Helsinki last autumn to a highway because of failure of the high pressure pump.
Mar 14th, 2008
PlasticPilot
@Karoliina: thank you fot this information. Do you have any link to investigation report, or a tail number so that I can search ?
Mar 14th, 2008
Karoliina Salminen
I was just yesterday at SIL winter days (Finnish EAA-like organization) and the all the accidents were presented there.
The tail number of the plane was OH-CAU. It had the latest revision of the Thielert installed last summer. According to the preliminary findings there are multiple reasons why the high pressure pump failed leading to a crash:
- The gasoline tanks had ingested some water when the plane was outside. The high pressure pump does not like that. Despite all the care of draining the water out prior each flight, apparently it was not enough.
- Another finding was that the JET-A1 sold in Finland does not meet the lubrication requirements set by Thielert. If I remember correctly, the Centurion 1.7 was approved for up to 780 (lower number is better) and the Finnish JET-A1 had this number averaging somewhere around 870 (I do not know exactly what this value is though). With auto-diesel this accident potentially could have been avoided, however, ironically the Centurion 1.7 is not approved for auto-diesel.
- There is a experimental builder who is doing a diesel conversion out of VW 1.9 TDI engine and he said that he plans to use a small amount of two stroke oil mixed into the JET-A1 because otherwise he would end up with similar issues than Thielert.
By the way, our other flying club’s plane Diamond DA40TDI (OH-FDA) went to the sea near to Helsinki last summer as well, because of malfunction of the overload clutch. According to the accident investigation, the overload clutch simply is not so good design and after it starts to slide, it gets destroyed quite quickly which can lead to loss of power. The clutch is rated to only 50Nm higher torque than the maximum output of the engine and it is functioning on its limit all the time. Oil leakage to the clutch contributed to the accident. According to one diesel specialist, it could be fixed simply by putting one O-ring on one place (I can’t remember because I am not engine specialist myself) to avoid the oil leakage to the clutch, but apparently Thielert has been in high hurry with getting the engines done and shipped. DA40 is by the way, according to this forced landing on the sea, pretty safe for landing to water. It did not flip upside down and it did not sink, the pilot and passangers were sitting on a wing and waiting for rescuers to come and the only thing that happened, they got their feet wet. This accident was pretty reassuring that if malfunction happens, I will attempt to land the plane to water instead of trees or other not so good place which could be fatal. The plane got only minor damage to the fuselage, but right wing’s skin was badly damaged. The avionics got wet and was considered as destroyed. Diamond did not want to fix the plane and it was claimed as totalled.
Our flying club purchased a C172SP (OH-SRH) and is going to install the Centurion 2.0 engine on it. The other flying club is going to purchase a DA40TDI to replace the previous Diamond. To my understanding, these come with the Centurion 2.0. I hope that engine lasts and is not functioning like the two-stroke engines on early ultralights…
Mar 17th, 2008
PlasticPilot
Thank you Karoliina for this extensive and excellent answer. I’ll do some research based on the tail number to see if I can find something.
Mar 17th, 2008
Antoine Eddé
In my club, the mechanic recently issued loud reminders that both tanks AND the central drain of the DA40 TDIs must be drained THREE times, even if the first two drainages show no water.
He explained that the residual water in the fuel circuit caused damage to the injectors (that may be close enough to the high pressure pump issue in this thread and maybe more likely).
Before that communication took place, I have myself experienced and reported several unexplained alerts whereby the ECU would issue a warning and the engine was still running.
This was disturbing and scary. The warnings are gone now, but not the feeling that this engine is a tricky animal…
I have recently read that Diamond intend to offer an “upgrade” to the new Austro engine for existing DA40 and DA 42s. I hope the new engine is better… And I look forward to a couple horsepower more!
Antoine (Switzerland)
May 13th, 2008
PlasticPilot
Antoine, I guess I flew there as well, and know about these warnings. The question is then how massively must one drain the tanks ? Three times depends the size of the fuel tester you use, and so on… Not exactly a precise procedure. And why three and not four then ?
May 13th, 2008
KM
I also have some problems with the c172 diesel am flying. The fadec goes on when the power runs at 100% for more than 2 mins. The problem according to FADEC service tool lies in the High neg dp rail and the switch B b/c rpm. The parts were replaced and the problem still persist.At 80% max power the engine runs with no fadec warnings. Does any one have any relevant info on what the problem might be? I must starting to suspect the High pressure pump is faulty. Any insight?
May 20th, 2008
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