A theoretical question. Filter blockages and engine power.
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bencmaster
- Posts: 38
- Joined: Fri Jan 11, 2008 6:11 pm
A theoretical question. Filter blockages and engine power.
Does anyone have a view on how much a Gascolator screen would have to be blocked before one started to notice a loss of power on a Rotax 912 ?
Pete
Yes but....
I had an interesting and very much related experience in China last year. Running a fleet of 10 outboard powered boats we experienced loss of power in about one third of the fleet. I'm not an outboard service engineer and neither was anyone else who was there, so followed a very fast learning experience.
The engines in question have a series of three filters.
The first is a conventional fuel-water separator (spin-on canister type) with a 50 micron filter.
On the inlet side of the low-pressure fuel pump there is a smaller filter (very similar to a microlight in-line type filter) that also has a small water separator built in.
On the outlet side of the low-pressure fuel pump there is a much smaller filter with a porous membrane element. The filter size of this is not specified but discussions with the manufacturer suggest it may be as fine as 20 micron.
Finally, there is a porous-plug type filter between the injection pump and the injectors.
In each case the diagnosis was similar. The first filter had massive and visible lumps of grot in it. The second filter had very fine particulate in it. Both were capable of flowing fuel with a minimal pressure drop when pressurised with the hand-priming pump.
The third filter had no visible contamination but would not flow fuel…… Each filter had done its job in turn but the fuel was sufficiently contaminated with microscopic matter to invisibly block the finest filter in the line. Once all the filter elements were replaced the engines ran fine, though in practice, I suspect I only need to change the finest one.
The contamination was traced to a new fuel barge that clearly had not been cleaned properly prior to initial filling.
I appreciate that this may not be quite the case with a Rotax but there are some lessons here.
I have also had contamination problems with the Tilotson carburetor on my Tipsy Nipper. These always occur after maintenance that requires disturbing the fuel system (including checking the gascolator).
Tiny particles (visible only with a magnifying glass) get under the fuel delivery valve needle holding it slightly open and cause the mixture to go very rich (sometimes to the extent that fuel runs out of the exhaust). The good news is that when this happens, the engine will not start so the danger is minimal.
I have never knowingly had the problem in flight and I have always assumed that as the needle-valve is well open at cruise power settings it is unlikely to be a problem.
So in answer to your original question, lots – but it may not be as simple as that.
Yes but....
I had an interesting and very much related experience in China last year. Running a fleet of 10 outboard powered boats we experienced loss of power in about one third of the fleet. I'm not an outboard service engineer and neither was anyone else who was there, so followed a very fast learning experience.
The engines in question have a series of three filters.
The first is a conventional fuel-water separator (spin-on canister type) with a 50 micron filter.
On the inlet side of the low-pressure fuel pump there is a smaller filter (very similar to a microlight in-line type filter) that also has a small water separator built in.
On the outlet side of the low-pressure fuel pump there is a much smaller filter with a porous membrane element. The filter size of this is not specified but discussions with the manufacturer suggest it may be as fine as 20 micron.
Finally, there is a porous-plug type filter between the injection pump and the injectors.
In each case the diagnosis was similar. The first filter had massive and visible lumps of grot in it. The second filter had very fine particulate in it. Both were capable of flowing fuel with a minimal pressure drop when pressurised with the hand-priming pump.
The third filter had no visible contamination but would not flow fuel…… Each filter had done its job in turn but the fuel was sufficiently contaminated with microscopic matter to invisibly block the finest filter in the line. Once all the filter elements were replaced the engines ran fine, though in practice, I suspect I only need to change the finest one.
The contamination was traced to a new fuel barge that clearly had not been cleaned properly prior to initial filling.
I appreciate that this may not be quite the case with a Rotax but there are some lessons here.
I have also had contamination problems with the Tilotson carburetor on my Tipsy Nipper. These always occur after maintenance that requires disturbing the fuel system (including checking the gascolator).
Tiny particles (visible only with a magnifying glass) get under the fuel delivery valve needle holding it slightly open and cause the mixture to go very rich (sometimes to the extent that fuel runs out of the exhaust). The good news is that when this happens, the engine will not start so the danger is minimal.
I have never knowingly had the problem in flight and I have always assumed that as the needle-valve is well open at cruise power settings it is unlikely to be a problem.
So in answer to your original question, lots – but it may not be as simple as that.
Hope the guys or at least the journos on PPrune spectulating on the 777 crash at Heathrow don't make yet another meal out of this, "China, Fuel, Contamination".
I would have thought that as Pete says it takes a lot of gunge to block a filter and unless it was a sudden mass of gunge I would expect that you would require increasing amounts of throttle for equivilent performance as the filter blocked up.
I would have thought that as Pete says it takes a lot of gunge to block a filter and unless it was a sudden mass of gunge I would expect that you would require increasing amounts of throttle for equivilent performance as the filter blocked up.
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bencmaster
- Posts: 38
- Joined: Fri Jan 11, 2008 6:11 pm
Well then I will relate my experience:
The aircraft arrived in June. I did lots of flying in it to revalidate my old licence. Performance was 'ok'...just 'ok'.
80kts was hard to achieve except by flat-to-the-firewall throttle and a long wait. Take-off seemed lacklustre.
Every day I drained the fuel and checked.............nothing.
It even went through an engine service, although I doubt that the Gascolator was even located.
So eventually, as the performance, or lack of, started to irk me, I got stuck in.
The Mark Ingegno cartridge type Gascolator had a teaspoon of swarf and husk-like material in it. The inner o rings had perished and the same crap was inside the cartridge.
Further investigation revealed similar quantities of swarf in each tank.
Rectifying this resulted in a plane that would easily hit 80kts with 75% throttle, take off was also transformed.
.
Some difference.
It was so marked that I actually began to doubt my eyesight!
The aircraft arrived in June. I did lots of flying in it to revalidate my old licence. Performance was 'ok'...just 'ok'.
80kts was hard to achieve except by flat-to-the-firewall throttle and a long wait. Take-off seemed lacklustre.
Every day I drained the fuel and checked.............nothing.
It even went through an engine service, although I doubt that the Gascolator was even located.
So eventually, as the performance, or lack of, started to irk me, I got stuck in.
The Mark Ingegno cartridge type Gascolator had a teaspoon of swarf and husk-like material in it. The inner o rings had perished and the same crap was inside the cartridge.
Further investigation revealed similar quantities of swarf in each tank.
Rectifying this resulted in a plane that would easily hit 80kts with 75% throttle, take off was also transformed.
.
Some difference.
It was so marked that I actually began to doubt my eyesight!
.Blocked filter on a petrol engine does not cause loss of power it results in stoppage, when the flow drops below that of demand the float chamber empties and it stops. only by reducing throttle setting and hence, demand will you keep it going. The place you find blocked filter trouble is going into the trees at the end of the takeoff run, when the float chamber runs out.
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Barry Plumb
- Posts: 29
- Joined: Wed Jan 02, 2008 10:14 am
- Location: Leighton Buzzard
In order for fuel to pass through the fuel filter there will be a pressure loss. It may be small but there will be a pressure loss. Assuming you have done a fuel flow check to start with you will be assured that enough fuel will pass through the system including a clean filter to satisfy the demands of the engine at full throttle, plus a margin.
Let us assume that you have a gravity feed through the filter and at full flow the pressure loss is equivalent to 0.5 ft head of fuel.
Pressure loss due to flow through a restriction in a fuel system has a square law function with flow rate. Therefore if you halve the available flow area through a filter (due to half being blocked) the flow through the remaining half will double its velocity. This would result in four times the pressure loss, or be roughly equivalent to two feet head of fuel. This would be enough in a Taylor Monoplane to reduce the fuel flow to the point where the engine would start to run lean. This would mean loss of power, high CHT, high EGT, high oil temps, onset of detonation and ultimately engine stoppage. Ouch.
Welshman is absolutely correct, start with the finger strainer. Check the gascolator filter regularly, and remember some fuel pumps and carbs have small filters as well. Even if you get a good flow when you drain the gascollator this does not mean the filter is clean, as the filter is downstream of the drain point.
If there are any large particles at all in the gascollator then assume that these have some smaller cousins and keep looking. Actually debris in the bowl of the strainer will not cause any pressure drop. It is only the crud that is caught in the filter that matters. However if large stuff is there then look for the small stuff.
If you do find some crud in the fuel system be thorough and clean it all out. When you have the system clean, go the extra mile and carry out a fuel flow check to reassure yourself all is OK.
Remember to write it all up in the maintenance log and get your inspector to sign it off.
Happy Landings
Barry Plumb
Let us assume that you have a gravity feed through the filter and at full flow the pressure loss is equivalent to 0.5 ft head of fuel.
Pressure loss due to flow through a restriction in a fuel system has a square law function with flow rate. Therefore if you halve the available flow area through a filter (due to half being blocked) the flow through the remaining half will double its velocity. This would result in four times the pressure loss, or be roughly equivalent to two feet head of fuel. This would be enough in a Taylor Monoplane to reduce the fuel flow to the point where the engine would start to run lean. This would mean loss of power, high CHT, high EGT, high oil temps, onset of detonation and ultimately engine stoppage. Ouch.
Welshman is absolutely correct, start with the finger strainer. Check the gascolator filter regularly, and remember some fuel pumps and carbs have small filters as well. Even if you get a good flow when you drain the gascollator this does not mean the filter is clean, as the filter is downstream of the drain point.
If there are any large particles at all in the gascollator then assume that these have some smaller cousins and keep looking. Actually debris in the bowl of the strainer will not cause any pressure drop. It is only the crud that is caught in the filter that matters. However if large stuff is there then look for the small stuff.
If you do find some crud in the fuel system be thorough and clean it all out. When you have the system clean, go the extra mile and carry out a fuel flow check to reassure yourself all is OK.
Remember to write it all up in the maintenance log and get your inspector to sign it off.
Happy Landings
Barry Plumb
Barry,
I agree completely with your statement regarding the need to be diligent in checking out the system, I don't agree with your figures on the likely effect of dross in a filter.
In the case of a very small filter what you say would obviously be true, however typical gasolator has a surface are of perhaps 6 sq inches, so if we assume that the filter fabric reduces this to say 2 sq in, it will have negligible effect when the cross sectional area of pipe work is perhaps 1/16 sq in.
I agree completely with your statement regarding the need to be diligent in checking out the system, I don't agree with your figures on the likely effect of dross in a filter.
In the case of a very small filter what you say would obviously be true, however typical gasolator has a surface are of perhaps 6 sq inches, so if we assume that the filter fabric reduces this to say 2 sq in, it will have negligible effect when the cross sectional area of pipe work is perhaps 1/16 sq in.
Peter Diffey
029340
029340
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Barry Plumb
- Posts: 29
- Joined: Wed Jan 02, 2008 10:14 am
- Location: Leighton Buzzard
Pete.
It does not make any difference how many square inches a filter has. If it has half of its passages blocked then in order for a 100% flow rate to pass through the remaining clear openings the pressure loss will be four times the pressure loss when clean.
Fundamental square law.
Happy Landings
Barry Plumb
It does not make any difference how many square inches a filter has. If it has half of its passages blocked then in order for a 100% flow rate to pass through the remaining clear openings the pressure loss will be four times the pressure loss when clean.
Fundamental square law.
Happy Landings
Barry Plumb
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cardiffrob
- Posts: 370
- Joined: Wed Jan 16, 2008 3:56 pm
So what is the practical solution to this ever present 'crud' problem? If Gascolators don't have filter screen material in them, is it possible to buy an appropriate material to filter out particles of a size that would get caught in float chamber valves etc. or would it merely move the problem upstream?
How big are the particles in question? Enough to be seen? Who makes the best / lightest / safest filter units? IIRC, those plastic cheapo auto filters are a no-no (so I'll have to replace mine before the test flight!)
How big are the particles in question? Enough to be seen? Who makes the best / lightest / safest filter units? IIRC, those plastic cheapo auto filters are a no-no (so I'll have to replace mine before the test flight!)