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  #1  
Old 02-01-2012, 04:11 AM
Bastien's Avatar
Bastien Bastien is offline
 
Join Date: Jun 2010
Location: Vannes, FRANCE
Posts: 146
Question Would it be a good solution to avoid hot start issues?

Hi,

As a student, I had a project which consisted in failure mode analysis of an automotive fuel injection system "Bosch K-Jetronic". It was installed in the WW Golf 1 and 2GTI and also the Porsche in the seventies. It was the first competitive and reliable alternative to the carburetor and it had a component which I believe could be usefull for our engines.

It is a simple accumulator mounted after the electric and mechanical pump and before the fuel injection servo.



The b position is when you cold start the engine : The spring is unloaded
The a position is when you are running your engine : The fuel pressure load the spring.

When you stop your engine, the fuel pressure normally drop but with this system the fuel pressure is kept for a short time (maybe half an hour or an hour) and this is told to avoid vapor lock and hot start difficulties.

I believe in this case engine shut down should not be by cutting the mixture but by putting off the ignition as in a car. Also this implicate that we should have an "anti return" to fuel tank valve.

I think I need help to know if it would be really useful or not.

I apologize for my english and so hope this will be understandable.

Have a good day
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Van's RV-8 2513 F-PTGV
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  #2  
Old 02-01-2012, 06:39 AM
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Walt Walt is offline
 
Join Date: Aug 2005
Location: Dallas/Ft Worth, TX
Posts: 6,003
Default

Almost all aircraft FI systems are of the continuous flow type (the injector is just an orifice with no valving), adding an accumulator would flood the engine with addtional raw fuel. The hot start issues aircraft engine experience now is due to too much fuel gets dumped into the cylinders on shut-down.
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  #3  
Old 02-01-2012, 07:24 AM
Don at Airflow Don at Airflow is offline
 
Join Date: Jun 2006
Posts: 316
Default Hot starts and FI systems

Understanding the systems can make trouble-shooting systems easier and more effective. The Bosch K-Jetronic uses injector nozzles that have pintal valves in the end of the nozzle. This is what atomizes the fuel but also shuts off the fuel and keeps the nozzle line pressurized when the engines shut off. The accumulator keeps pressure in the system as long as the pintal valves seal. (I had an 80 Rabbit and after a few years the nozzles got dirty and allowed the fuel pressure to bleed off. Then it was a pain to start the car when it was hot). With a Bendix system the injector nozzles are air-bled type. This type of nozzle uses the differential pressure of manifold pressure and ambient pressure to atomize the fuel. The nozzle uses a fixed orifice; the size based on the available fuel pressure and the fuel flow that is to go through it at max power. There is no check valve to keep the nozzle line pressurized. There fore as Walt said having an accumulator in the system will only flood the engine when it is shut off. The fact is there is a type of accumulator in the system already. The diaphragm fuel pump used on most Lycoming?s acts like a fuel accumulator due to its design. There is one stroke of fuel that will be discharged in the system when the engine is shut off.

So the issue with hot starts using a Bendix system is the fact that the engine heat boils the fuel from the fuel system components. Since the system does not have a return system the only path for the vapor to escape is through the fuel metering system and into the engine. The effective way to solve this problem is to circulate fuel through the system but not allow the fuel into the engine. A device like our purge valve accomplishes this task. Some customers do not want the additional control required to operate this component, nor the return line required in the system. For those installations we offer the FM-150 or FM-250 fuel control to satisfy that requirement.

Just a reminder to all; Fuel Injection 101 Class is the first weekend in March 2012.


Don
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  #4  
Old 02-01-2012, 07:45 AM
Bastien's Avatar
Bastien Bastien is offline
 
Join Date: Jun 2010
Location: Vannes, FRANCE
Posts: 146
Smile Thanks

Thank you very much for your answers which are really precise and quick.
I don't know much about injection system but I'm interested in. I didn't know that our injector nozzle were made like that. Now I understand why it is not viable.
Don, would it be possible to have an exploded view of your fuel injection system ? (If it is, here is my E-mail address : bastien.le-roux(at)wanadoo.fr).
I have a "precision airmotive" FI on my RV8 (under construction) and I wanted to know if it would be possible to change the fuel restrictors (when I will be flying) if there is a large difference (0,5/1GPH?) between the first and the last cylinder to peak ?
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Van's RV-8 3040 F-PVRB
Aerosport IO-375 turning WW200RV
Dual Dynon SV1000
SJ Wheel pants/plenum/cowl
Flying since 07/27/12

Van's RV-8 2513 F-PTGV
Lycoming IO-360 B1B
Purchased as a project--Finishing kit
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