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Prime with Throttle

fbrewer

Well Known Member
Our starting checklist has the following step:

Prime with Throttle - Prime!

I think I know what is happening, but would like your comments.

What does this mean?

How do I Prime?

What happens when I Prime?
 
Probably what you think, with the fuel pump on you can pump the throttle all the way open and closed a few times to inject fuel into the engine for starting. Too much and you’re likely to flood the engine, probably will take some experience to find out how much your particular engine likes depending on how long it’s been sitting. I assumed you a talking about procedures for starting a fuel injected engine.
 
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A word of caution is required here, but first some background info.

Our carbs are updraft carbs - air, mixed with fuel, moves upward through the carb into the cylinders. Many carbs have what is referred to as an "accelerator pump" - a small piston pump designed to squirt raw fuel into the air flowing through the carb. Normal activation of the throttle does not invoke the accelerator pump. Rapid operation of the throttle causes the pump to squirt fuel into the carb.

Now for the word of caution. Squirting raw fuel into the carb can be a very bad thing. Fuel in liquid form will follow the laws of gravity, eventually dripping downward and pooling in the air intake. A backfire during starting provides an ignition source to light that pooled fuel. An engine fire of this nature can be very nasty indeed.

If one is going to use the accelerator pump it is best to do so after the starter is engaged. The cranking engine is sucking in lots of air - this air will pick up the raw fuel being squirted in by the accelerator pump, carrying it to the cylinders where it will do the most good.

Operating the accelerator pump before cranking is an invitation to an engine fire. Operating the accelerator pump during cranking is a recipe for good engine starts. For those of us who operate in cold weather conditions, the accelerator pump may not be a reliable enough solution so a separate priming system which injects fuel directly into the cylinder heads is often installed.
 
What Canadian JOY said. In this photo, you can see the accelerator pump discharge nozzle aiming up inside the venturi. That's where the fuel squirts out when you pump the throttle:

MgPweE.jpg


On my plane, I had to cap off the primer because it was leaking through. Now I use the accelerator pump, but I can only get the engine started by pumping the throttle down to around 40 F. Below that, the engine is very difficult to start even by pumping furiously. I came close to draining the battery trying to get her started at 30F.
 
Have a fire extinguisher handy if you try using the throttle.....:rolleyes:

Plenty of accounts of a dull whoomph followed by smoke rising.
 
So how does the fuel react to various throttle applications? Is it always the accelerator pump that adds fuel to the carb inlet? Does it matter how fast or much you apply the throttle? That is, can you crank the engine and just crack the throttle open a little, or does the throttle need to be opened all the way to get any fuel to squirt out? I always wondered about this.
 
The accelerator pump is basically a squirt gun. It only squirts fuel as you advance the throttle.

Edited to add: The amount of fuel squirted is proportional to the distance you advance the throttle. The rate at which you advance the throttle drives the velocity of the fuel spray.
 
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The accelerator pump is basically a squirt gun. It only squirts fuel as you advance the throttle.

To elaborate on Kyle's comment, it's like a squirt gun where the trigger is the pump - the harder you pull the trigger, the bigger the squirt.

In the case of these carb's, the pump is a piston pump which is mechanically tied to the throttle linkage. Move the throttle fast and you get a good squirt of fuel. Move the throttle slowly and the pump doesn't make enough pressure to squirt and fuel.

(Thanks for posting that great picture, Kyle - it really clearly shows the accelerator pump outlet!)
 
Despite what some say and as detailed in Paul's blog post, throttle pumping to prime can be done safely but it must be done correctly (I am pretty sure that induction system fires are caused by incorrect priming whether it be from throttle pumping or using an actual priming system).

- It must be done with a battery in a condition that allows normal/brisk cranking speed.
- The throttle should only be pumped (briskly / about one stroke in and out per second) while the engine is cranking (briskly).
Note - On aircraft that have a more traditional priming system (whether it be a pressure plunger or a valve that is electrically opened) I only use it while the engine is cranking as well, in order to reduce the risk of induction system fire that still exists.
 
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Well dang, I was less than clear in my original post.

We have an IO360 - it is fuel injected -- so NO carburetor. So back to the original question

Our starting checklist has the following step:

Prime with Throttle - Prime!

I think I know what is happening, but would like your comments.

What does this mean?

How do I Prime?

What happens when I Prime?
 
Here's a little video I shot of the accelerator pump in action:

https://youtu.be/VtMB-3U6sc8


For a description, you can see my blog post from a few months back on Kitplanes.com:


https://newsline.kitplanes.com/2017/07/05/priming-with-the-pump/#more-10964

Just checked out the blog. I do not have a prime system. Temps have been below +30 deg F for some time now.

My RV 0320 has been starting ok, usually 2-3 turns, however I've been pre-heating using 4-each hair dryers. Two in the intakes and two in the bottom of the cowl through the exhaust openings. Cylinder head temp show above 100 deg by the time the preheat is done. Removing ice/frost takes 1-hour: whole time preheat is on-going.

Have not been using the "prime" method of pumping throttle. Just cracking open the throttle about 1/4".

Will try the "prime" method later today.

Thank you for the blog!

Understand the OP has modified his question explaining "IO 360". But wanted to say thanks, learned something from the answers provided for the "carb prime method".

Best regards,
Mike Bauer
 
Caveats: #1, it does not apply to the OP because he clarified he is asking about FI; caveat #2, for the life of me I can not find the original material.

OK, short of the above, sometime in the past two weeks I’ve read an article, seen an authoritative video, or attended a webinar where the topic was:

Using the throttle’s accelerator pump during engine start

The critical point of the presented material was “hit the starter and THEN quickly jab the throttle to activate the accelerator pump AND QUICKLY pull the throttle back.”

The presented material went on to say, “pumping the throttle without the starter activated means the fuel will quickly pour back out of the carburetor into the airbox. If the engine should backfire, the airbox is a volatile chamber and fire is a real potential.”

UPDATE: the source I was looking for was the EAA webinar “Fly the Easy Way”
 
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Transition training

I did transition training with Mike Seager in N666RV. (O-320) The standard procedure he taught for N666RV was 5 pumps on the throttle, then hit the starter key.

I've used that same procedure for my airplane, except that I only use 3 pumps on the throttle.

I'm always puzzled as to the diversity of opinions about these kinds of things. But Paul's video certainly convinces me that I should change my procedure.

Michael-
 
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My 0320 fuel injected takes 6 seconds, WOT and full rich, of boost pump then turn it off and crank. Starts on first swing of prop every time.
I learnt about the amount of time with boost pump for prime when I overdid it and had fuel pouring out and lost my K&N filter red oil.
 
Just to add a little more clarity...

Somebody mentioned priming 'into the head'. I've seen posts in the past (other threads) where people thought that the primer (or injection nozzles) injected into the combustion chamber. That is not the case. Both the carb'd engines with primer lines, and injected engines using fuel pump for priming, are squirting fuel into the intake path, short of the intake valve. Since Lycs have their intake tubes on the bottom, the fuel will still run downhill, toward the intake. So....any of the three techniques can end up with fuel pooled in the bottom of the intake system, if priming is done incorrectly.

As an aside, I don't often feel old, but when the majority of males in a conversation about internal combustion engines don't know how a carb (with accelerator pump) works, I start to wonder if I'm past my sell date. :)

Charlie
 
I suppose the real question is why we haven't moved to direct injection like the rest of the technological world. It seems to solve a lot of problems, and improve efficiency (and emissions) significantly.

If my tractor can manage it--at less total cost than an aircraft engine alone--why can't we get with the times?

(Serious question, BTW--what am I missing here?)
 
I suppose the real question is why we haven't moved to direct injection like the rest of the technological world. It seems to solve a lot of problems, and improve efficiency (and emissions) significantly.

If my tractor can manage it--at less total cost than an aircraft engine alone--why can't we get with the times?

(Serious question, BTW--what am I missing here?)

To a large degree it is design/development costs

When the cost of a new design is going to be spread among a sales volume in the 10's of thousands (in the case of cars, 100s of thousands), it doesn't have nearly the impact on selling price that it would if the sales #'s are only a fraction of that (as they would be with an aircraft engine).
 
Depends on what you mean by 'we'. :)

Regular electronic injection isn't too big a hill for a 'cottage industry' vendor to climb, but direct injection is a whole other mountain. A lot more complicated than just figuring out how much fuel to squirt. A primitive diesel just does a metered squirt, but newer computer controlled diesels (I'm told) vary timing of the injection, both start timing and rate. Same for gasoline direct injection. Software is complex, and hardware even more so, with injection pressures measured in thousands of psi. Pretty tough to manage in a low volume industry.

Now if you're willing to use an actual alternative engine, instead of just alternative fuel delivery....
 
Duly noted, and thank you for the education. Still, it seems like a (mostly-)solved problem, and if we could just lay hands on the patents, there ought to be enough aircraft engines around to make it worth trying.

But then, I started in engineering and finished in law; what do I know about business?
 
Alternate method

For the original poster I have another suggestion to offer. In the Glasair (IO360 with fuel injection) I start with the mixture off, bring on the boost pump until pressure stabilizes at full value (about 28 PSI in my engine), then shut off the boost pump. When ready to start push mixture full rich. Fuel pressure will go to zero as the fuel primes the engine. Engage starter. Once running, don't forget to lean a bit until ready for run up or takeoff as fuel injection systems are set to run overly rich at idle.
 
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