Starting and running an IO540-K1A5 w/CS prop
As a rule of thumb, injected Continentals have a return line. Leaving the mixture in ICO while priming** for 10-15 seconds will circulate cool fuel as far as the fuel control unit and displace the vapour back to a fuel tank. Continentals will tend to use mixture RICH for cranking.
Lycomings are usually the opposite ie no return line so priming with mixture ICO won't work. Prime mixture RICH which will shove fuel through the FCU, down the spider & out the injectors into the inlet manifold, displacing vapours on the way. This generally leaves the inlet manifold over rich so injected Lycomings tend to use Mixture ICO for cranking, until the fuel/air mix leans to a combustible ratio.
**Edited to add that 'priming' is probably not the correct word. You're not providing extra fuel to the inlet manifold to aid starting, just replacing hot, vapour locked fuel in most of the system with cool, liquid fuel. 'Real' priming is often done just while cranking with Mixture RICH ie start cranking, hit the primer & the ratio gets richer until eventually combustion can happen.
Lycomings are usually the opposite ie no return line so priming with mixture ICO won't work. Prime mixture RICH which will shove fuel through the FCU, down the spider & out the injectors into the inlet manifold, displacing vapours on the way. This generally leaves the inlet manifold over rich so injected Lycomings tend to use Mixture ICO for cranking, until the fuel/air mix leans to a combustible ratio.
**Edited to add that 'priming' is probably not the correct word. You're not providing extra fuel to the inlet manifold to aid starting, just replacing hot, vapour locked fuel in most of the system with cool, liquid fuel. 'Real' priming is often done just while cranking with Mixture RICH ie start cranking, hit the primer & the ratio gets richer until eventually combustion can happen.
Last edited by Tinstaafl; 5th Oct 2012 at 20:35.
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I wonder how this is dealt with when installing a fuel totaliser.
I also fly the DR400 Ecoflyer regularly and I assume it has the same setup. But I'm not familiar enough with the POH to say for sure that that is the case. And I don't have a POH copy here at the moment.
Tinstaafl, thanks, that makes a lot of sense.
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I think Tinstaafl has it exactly right. That makes sense with what I have seen.
That's amazing... are you 100% sure? What is the indicator? Is it just the G1000?
The reason I am amazed is because the return fuel flow is going to be very sporadic, with a lot of stop/start in it. It isn't going to drive the standard turbine transducer (most systems use the Floscan 201/201B) very well.
There are flow meters both in the main and in the return line, and the fuel flow indicator simply uses the difference between the two.
The reason I am amazed is because the return fuel flow is going to be very sporadic, with a lot of stop/start in it. It isn't going to drive the standard turbine transducer (most systems use the Floscan 201/201B) very well.
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Actually the DA40 that I flew had the analog setup, not the G1000. The indicator I was talking about was the Thielert/Centurion specific AED.
http://bp0.blogger.com/_G1BiqAsHwJU/...U/s320/AED.jpg
Furthermore, I don't think the return fuel flow was sporadic, but actually a steady stream, particularly at idle. After all, the return flow of the (heated) fuel from the common rail is used to heat the fuel in both tanks to above the "allowed take-off" temperature at very low temperatures. You would not be able to heat 30 USG of Jet-A using just a trickle of hot fuel.
(The fuel is heated mostly due to the compression in the common rail system.)
Edited: I just looked at the DA40 POH and it suggests the return flow of fuel into the tank is about 70-80 l/hr. But the POH doesn't specify where exactly the flow transducers are located.
http://bp0.blogger.com/_G1BiqAsHwJU/...U/s320/AED.jpg
Furthermore, I don't think the return fuel flow was sporadic, but actually a steady stream, particularly at idle. After all, the return flow of the (heated) fuel from the common rail is used to heat the fuel in both tanks to above the "allowed take-off" temperature at very low temperatures. You would not be able to heat 30 USG of Jet-A using just a trickle of hot fuel.
(The fuel is heated mostly due to the compression in the common rail system.)
Edited: I just looked at the DA40 POH and it suggests the return flow of fuel into the tank is about 70-80 l/hr. But the POH doesn't specify where exactly the flow transducers are located.
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Don't know in other airplanes, but in mine (Continental engine) the fuel flow meter is located just before the spider, and the fuel return lines (one per fuel tank) are located on the fuel selector valve. Therefore, the fuel flow metered by the sensor is the actual one, not being influenced by the fuel returning to the tank since the return is done well before the ff sensor.
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the fuel flow meter is located just before the spider
Most "analog" flow indicators just measure fuel pressure, which given the fixed size and permanently open injectors should result in a reasonable "flow" indication.
The turbine transducers are sensitive to turbulence and vibration and are normally mounted away from the engine.
I can easily see someone having a pressure gauge takeoff just before the distribution spider.
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Well, despite Peter chewing my ear 
it's certainly provoked a better understanding of the theory behind the injection-system. This in turn has enabled the LOGICAL extension of WHY a particular "routine" works and also enables one to see which steps are,indeed, totally superfluous and continue to be used for no better reason than that they were in the original routine that worked.
WRT fuel pumps, AFAIK all internal combustion engines have a healthy over-capacity.....allowances have to be made for wear,back-leakage on valves,etc. whilst still delivering adequate pressure and volume for maximum-demand operation.
An engine-driven pump (normally camshaft-eccentric) will have an output proportionate to engine-speed. An electric pump is "all or nothing." to sustain intermittent operation, a pressure-accumulator would be the normal solution, allied with a pressure-switch.....much simpler,cheaper and more reliable to have a continuous -run pump which dumps surplus output back to the inlet or the tank.
IMHO any system which can vapourlock is lacking. The constant -circulation system eliminates this problem, though the return -line to the tank is another thing to go wrong, also, warming and evaporative losses of petrol is not a good thing, though at altitude it's probably a big advantage.
The low-pressure injection system sounds quite primitive, the sort of thing Bosch was upplying for automotive use in the early 1960's.
thanks to all the knowledgeable contributors, who have broadened our understanding of the alternative to carburettors.
it's certainly provoked a better understanding of the theory behind the injection-system. This in turn has enabled the LOGICAL extension of WHY a particular "routine" works and also enables one to see which steps are,indeed, totally superfluous and continue to be used for no better reason than that they were in the original routine that worked.WRT fuel pumps, AFAIK all internal combustion engines have a healthy over-capacity.....allowances have to be made for wear,back-leakage on valves,etc. whilst still delivering adequate pressure and volume for maximum-demand operation.
An engine-driven pump (normally camshaft-eccentric) will have an output proportionate to engine-speed. An electric pump is "all or nothing." to sustain intermittent operation, a pressure-accumulator would be the normal solution, allied with a pressure-switch.....much simpler,cheaper and more reliable to have a continuous -run pump which dumps surplus output back to the inlet or the tank.
IMHO any system which can vapourlock is lacking. The constant -circulation system eliminates this problem, though the return -line to the tank is another thing to go wrong, also, warming and evaporative losses of petrol is not a good thing, though at altitude it's probably a big advantage.
The low-pressure injection system sounds quite primitive, the sort of thing Bosch was upplying for automotive use in the early 1960's.
thanks to all the knowledgeable contributors, who have broadened our understanding of the alternative to carburettors.
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I don't think I chewed your ear the way you thought 
I was saying that in the PPL training one doesn't learn any of this, and one certainly doesn't open any "technical manuals".
Very few people understand how this stuff really works, and that includes me. Fortunately the procedures in my POH always seem to work - although a fast starter always helps a lot if you get it a bit wrong
I was saying that in the PPL training one doesn't learn any of this, and one certainly doesn't open any "technical manuals".
Very few people understand how this stuff really works, and that includes me. Fortunately the procedures in my POH always seem to work - although a fast starter always helps a lot if you get it a bit wrong
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Well, the discussion certainly provoked a number of additional questions with me.
In carbureted engines we always worry (perhaps too much) about carb ice. Carb ice, AFAIK, occurs because of two things:
a. The pressure drop behind the throttle butterfly.
b. The evaporation of fuel.
An injected engine has a throttle butterfly and I assume the fuel that is injected in the injection chamber (in neat or atomized form) will evaporate too, to a certain extent, before it gets into the cylinders properly. So why then is there no chance of icing in a fuel-injected system?
And about these injection chambers. Do these have a special shape, for instance incorporating a venturi, so that the fuel that is injected (neat or atomized) is actually evaporated before getting into the cylinders, or does the fuel essentially enter the cylinders in vapor/atomized form?
In carbureted engines we always worry (perhaps too much) about carb ice. Carb ice, AFAIK, occurs because of two things:
a. The pressure drop behind the throttle butterfly.
b. The evaporation of fuel.
An injected engine has a throttle butterfly and I assume the fuel that is injected in the injection chamber (in neat or atomized form) will evaporate too, to a certain extent, before it gets into the cylinders properly. So why then is there no chance of icing in a fuel-injected system?
And about these injection chambers. Do these have a special shape, for instance incorporating a venturi, so that the fuel that is injected (neat or atomized) is actually evaporated before getting into the cylinders, or does the fuel essentially enter the cylinders in vapor/atomized form?
There certainly is a potential for icing in injected engines - it's just not so likely. Fuel is introduced a long way downstream from the throttle plate, unlike carbies, so avoids the double whammy of pressure reduction + evaporative cooling occuring at damn near the same place. Also the injectors are right next to the hot cylinders so there's some warming for that area. Many injected engines are also turbocharged with an associated compression heating of the induction air prior to the throttle plate.
Impact icing eg flying in snow showers is still a problem though.
I can't think of an injected engine I've flown that didn't have alternate induction air.
The injectors introduce fuel in a finely atomised form into the induction manifold just near the intake valve. The injectors themselves are designed to atomise the fuel, often having an air intake as part of the injector. The fuel flows through a venturi which is vented to atmosphere. The pressure drop sucks in air which is mixed with the fuel as the fuel atomises.
Impact icing eg flying in snow showers is still a problem though.
I can't think of an injected engine I've flown that didn't have alternate induction air.
The injectors introduce fuel in a finely atomised form into the induction manifold just near the intake valve. The injectors themselves are designed to atomise the fuel, often having an air intake as part of the injector. The fuel flows through a venturi which is vented to atmosphere. The pressure drop sucks in air which is mixed with the fuel as the fuel atomises.
Last edited by Tinstaafl; 5th Oct 2012 at 20:50.
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Are you sure there is a pressure drop, Tinstaafl?
AIUI, a carb works by creating a pressure drop, using a venturi, and this sucks fuel upwards from the float-controlled chamber underneath. It is this pressure drop which gives rise to the temperature drop which gives rise to carb icing.
However a fuel servo doesn't need to drop the air pressure (and thus the temperature) to suck the fuel in, because the fuel is pressure fed from the pump(s) upstream. All the fuel servo has to do is sense the amount of air flowing, and open up a fuel valve according to that.
A fuel servo can still suffer from icing but it is very unlikely and AFAIK needs ice particles in the incoming air, which can then get stuck to the injectors.
AIUI, a carb works by creating a pressure drop, using a venturi, and this sucks fuel upwards from the float-controlled chamber underneath. It is this pressure drop which gives rise to the temperature drop which gives rise to carb icing.
However a fuel servo doesn't need to drop the air pressure (and thus the temperature) to suck the fuel in, because the fuel is pressure fed from the pump(s) upstream. All the fuel servo has to do is sense the amount of air flowing, and open up a fuel valve according to that.
A fuel servo can still suffer from icing but it is very unlikely and AFAIK needs ice particles in the incoming air, which can then get stuck to the injectors.
Injected systems still have a throttle plate that variably obstructs the airflow and the engine is still sucking air in. No different to a carby in that respect. Think what happens to MP pressure as the throttle is closed.
As for the FCU, in Lycoming's system (RSA) an air pressure differential between inlet & a venturi is used to schedule fuel delivery using a diaphragm.
As for the FCU, in Lycoming's system (RSA) an air pressure differential between inlet & a venturi is used to schedule fuel delivery using a diaphragm.
Last edited by Tinstaafl; 6th Oct 2012 at 07:48.
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That does suggest that flying with a wide open throttle (usually the case at about 8000ft plus) means icing risk reduces from tiny to more or less zero, whereas with a carb it is always present due to the deliberate venturi.
