How, exactly, does the J model fuel injection system work?

[RobertE]

I've looked online and not found a fine enough description.  Maybe I'm looking in the wrong place but I know this group will have the answers.

 

I know the system is constant flow and the injectors have a precisely measured orifice, so the amount of fuel delivered per cylinder is a product of a) the pressure in that injector line times the duration between intake valve openings.  What I don't completely understand is what the throttle controls, nor how the fuel mixture control operates.  Does the throttle simply restrict the intake opening or is there some mass flow sensor connected to it as well?  And is the mixture control as simple as a valve that constrains the flow of fuel to the spider?  Is the system is really that simple?

 

What partly drives these questions is the black art we all have of starting a warm engine.  I'm trying to understand the impact of different throttle and fuel mixture settings.

 

Thanks.

 

 

[carusoam]

I believe somebody (Byron?) covered some detail here somewhere..

Searching for the RSA flow divider and names that are associated should bring it up?

Hot starts are a good way to search for what and why things work...

Best regards,

-a-

[KSMooniac]

Happy studying!  

RSA_FuelInjection_15-812_b.pdf

[N201MKTurbo]

The PDF that KSMooniac posted is excellent. The FAA power plant manual also has a chapter dedicated to fuel injection and uses the RSA 5 as an example. It is also very good. You can find it on line.

[N201MKTurbo]

As far as starting the engine is concerned, all you need to know is that with the mixture at idle cutoff, no fuel is delivered. If you have fuel pressure and the mixture is not at idle cutoff some fuel will flow.

[mikefox]

For hot start, this procedure has never failed us:

 

Mixture:  idle cut-off

Throttle: open 1/2" (slightly more than for a cold start)

DO NOT PRIME

Start engine, and be ready to advance mixture as it catches, then reduce throttle for smooth operation.

[Guest]

Robert,

The fuel injection servo at the lower front of your engine(visible the roughy the ram air door) measures air flow by use of 4 impact air tube and a venturie behind the tubes. For each unit of air passing through the servo it delivers a set volume of fuel to the flow divider on top of your engine. The flow divider splits this volume of fuel into 4 equal volumes and deliveries it to the fuel injection nozzle in each cylinders intake port via the stainless steel tubes. The entire system is mechanical, the only electrical requirement is for the fuel boost pump required to prime the system for starting, after that the engine driven mechanical fuel pump supplies required fuel flow under pressure to the servo.

While fuel injected engines can be hard to start it is seldom the fuel system, however fuel nozzles should be cleaned every 200-300 hours. Lycoming has an s/b 1475 if my memory is correct, it details correct installation of the nozzles.

More likely your ignition system requires checking, high spark plug internal resistance, too wide an electrode gap setting, poor internal magneto timing (500 hour check), magnetos incorrectly timed to the engine, dirty ignition wire ends etc. all contribute to hard starting.

Also depending on the vintage of your plane s/I m20-59 may apply to your magneto switch.

Clarence

[carusoam]

Nice explanation Clarence!

I feel enlightened after reading that explanation.

Best regards,

-a-

[Guest]

I could likely give you a similar one for TCM fuel injection on your Ovation. TCM generally has more field adjustments available than Bendix, normally aspirated less so, turbo charged more so. I prefer setting up TCM systems.

Clarence

[Alan Fox]

I love the TCM system , it is simple compared to the Bendix ,   The reason they are so hard to start hot is because the spider , lines , and injectors are right on top of the engine and the hot engine boils all the fuel in the system out of the injectors and puts it into the cylinders .....It puts you into an almost ceartin flooding situation.......That is why you start a hot Lycoming at idle cutoff mixture , its already overprimed before you even get in it.....

[AndyFromCB]

I love the TCM system , it is simple compared to the Bendix ,   The reason they are so hard to start hot is because the spider , lines , and injectors are right on top of the engine and the hot engine boils all the fuel in the system out of the injectors and puts it into the cylinders .....It puts you into an almost ceartin flooding situation.......That is why you start a hot Lycoming at idle cutoff mixture , its already overprimed before you even get in it.....

 

As long as you've got three hands, it's all good ;-) 

[DrBill]

What baffles me in HOT START situations is this.  You shut the engine down by cutting the fuel supply.  Now, you come back 1 minute or more later and the engine will start with NO CHANGE in the mixture lever.  OK. it doesnt' run that way but where did it get the fuel to START ?  Remember, we shut off with the throttle set at 1000 RPM. Now get back in the plane,  TOUCH NO LEVERS and hit the key.   Mine starts EVERY TIME !.  Push in the mixture and I'm off !!

BILL

[jetdriven]

The fuel in the lines boils and then forces the fuel into the cylinders.  The cutoff is at the injector servo but all that fuel line from the servo to the sider and the lines from the spider to the injectors have fuel in them which heat soaks and boils after shutdown. You can hear something sounding like gurgling or bacon frying when you push the airplane into the hangar. Thats fuel boiling as it drips out of the injector nozzles. Which is why its very important to do a mag grounding check right before shutting it down.

 

 

heres the link to the precision RSA training manuals and other info.   http://www.precisionairmotive.com/rsasupport.htm

[DrBill]

Interesting since the mixture control shuts off the fuel at the servo and therefore the short time running after mixture cutoff should suck the fuel from those lines but I guess it doesn't get it all.  There must be enough there to get a quick start.   Thanks for the explanation.

BILL

[jetdriven]

The fuel stops moving when you go to ICO but it doesnt shut off the fuel completely, just enough to where it doesnt have enough to run. But now its trapped in the lines by capillary action. The boiling produces vapor which forces the liquid fuel and vapor out the injectors, flooding it.

[ArtVandelay]

The fuel stops moving when you go to ICO but it doesnt shut off the fuel completely, just enough to where it doesnt have enough to run. But now its trapped in the lines by capillary action. The boiling produces vapor which forces the liquid fuel and vapor out the injectors, flooding it.

I don't think you mean capillary action, which explains how plants get water to go upward, I think its more like a straw when you have a finger covering the top, preventing air getting in, which holds the water in the straw. There should be no air in your fuel lines, so it will stay there until forced out when some of it vaporizes.

It depends on the engine design, temperature, time, etc, but if enough fuel is forced out, then this could flood the engine, which explains why someone posted their hot start procedure using full throttle (same for flooded engine), by opening the throttle you allow more air to combine with the excess fuel. I saw someone else post they push the throttle 1" forward.

 

So we have at least 3 ways:

leave throttle at idle

open throttle 1"

full throttle

 

which way is best depends on how much fuel gets pushed out, maybe we should have a hot start contest :-)

[m20j]

Interesting since the mixture control shuts off the fuel at the servo and therefore the short time running after mixture cutoff should suck the fuel from those lines but I guess it doesn't get it all.  There must be enough there to get a quick start.   Thanks for the explanation.

BILL

 

Nothing gets "sucked out" of the lines.  The injectors are simply an orifice on the end of a fuel line that provides a continuous flow of fuel into the intake manifold when the engine is running.  When the mixture is pulled, the injectors stop squirting but the lines remain full of fuel, as the cutoff point is further upstream.

 

The residual engine heat (and lack of cooling airflow) raises the temperature of the fuel lines above the boiling point of the fuel. This bubbles it out of the lines and into the intake manifold.

 

There it vaporizes and is ready for the next hot start, if one understands the process and doesn't prime further!

[cliffy]

I also fly a Twin Mooney er, Twin Comanche and hot starts on the IO320s are easy. Throttle 1" open. Mixture cutoff, electric pumps on, start cranking and slowly advance the mixture, when it starts to fire advance the mixture all the way. Works every time.

[Bill_Pyles]

Hot start procedure:

Throttle full

Mixture idle/cut-off

Starter--engage

Simultaneously....retract throttle, enrich mixture and the engine starts. (this is where a third hand would be helpful)

[PTK]

Any comments on how much time, roughly, after shutdown can subsequent start be considered a cold start?

Is there an indicator temp to judge by? I'm thinking CHT because logically the fuel lines should cool down before the cylinders.

[carusoam]

Peter,

Interesting question...

1) What we want to know is the temperature of the fuel inside the lines.

2) The lines get indirectly heated by the cylinders...

3) The lines get cooled by air blowing through the cowl...

4) The important temperature to maintain is below the boiling point of 100LL...

5) According to the Statoil website, 100LL has boiling point range of 86F to 338F at one atm.

6) Since the fuel lines are open at the end, 1 ATM is a fair number to use.

7) Since the BP range is really wide it lends to an expectation that anything over 86dF could begin the challenge. The closer to 338 the lines get, the more likely the hot start procedure will be needed.

8) Using the CHT as a surrogate measurement for fuel line / fuel temperature is a good approximation if there is no airflow through the cowl.

9) airflow through the cowl will separate the CHT from the fuel line.

10) To improve the airflow through the cowl, point plane into the wind, open cowl flaps and oil door.

Combine that with the training I received at AAA...

1) If you just ran the plane hard and landed, it will be hot.

2) if you ran it LOP in the descent, it probably won't be hot.

3) if you let it sit for a period of time, it won't be hot.

Their method of hot start, then applies (mixture out, crank, mixture in when engine fires...)

Or go Continental, mixture out, fuel pump on, fuel circulates back to the tank, cools the lines.

Start as normal!

According to AAA (best I recall) there are actually few real hot start situations. I was under the impression that taxi to the fuel farm was a reason for hot start. That was a bit too conservative...

How does that sound?

Best regards,

-a-

[Guest]

I didn't know that Triple A had knowledge of aircraft!

Clarence

[carusoam]

The one at KSAT is pretty talented!

http://allamericanaircraft.com/Default.htm

David must have taught a thousand pilots this technique.

Best regards,

-a-

[jetdriven]

He taught me and it works. I showed a pilot this at the last Mooney safety clinic I taught at, he said that alone was worth the 800$. The quart of water I drained out of the left fuel tank at the end of the lesson was also worth 800$.