Why pump the throttle?

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92128

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The 172RG "starting engine" checklist states that the throttle should be pumped once or twice and then left open 1/4 inch for engine start. What I am trying to figure out is: what does pumping the throttle accomplish? If the prop is not turning, there is no air being pulled through the induction system? There is no airspeed obviously, so no air is being rammed through the filter and into the venturi. So this being the case, it seems that fully opening the throttle plate would not cause any fuel to be pulled into the venturi because decrease in air pressure to pull the fuel into the venturi. The throttle plate never actually closes all the way (even at idle), so with the engine stopped, ambient air pressure would already exist inside the intake manifold. No more air or fuel has been added, correct? So what is pumping the throttle accomplishing? I understand leaving it open 1/4 inch, but I don't get the pumping. There must be a reason. Any ideas?
 
:confused: - http://forums.jetcareers.com/1591336-post8.html

http://en.wikipedia.org/wiki/Carburetor#Accelerator_pump

The greater inertia of liquid gasoline, compared to air, means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, which can cause a temporary "lean" condition which causes the engine to "stumble" under acceleration (the opposite of what is normally intended when the throttle is opened). This is remedied by the use of a small mechanical pump, usually either a plunger or diaphragm type actuated by the throttle linkage, which propels a small amount of gasoline through a jet, wherefrom it is injected into the carburetor throat. This extra shot of fuel counteracts the transient lean condition on throttle tip-in. Most accelerator pumps are adjustable for volume and/or duration by some means. Eventually the seals around the moving parts of the pump wear such that pump output is reduced; this reduction of the accelerator pump shot causes stumbling under acceleration until the seals on the pump are renewed.
The accelerator pump is also used to prime the engine with fuel prior to a cold start. Excessive priming, like an improperly-adjusted choke, can cause flooding. This is when too much fuel and not enough air are present to support combustion. For this reason, some carburetors are equipped with an unloader mechanism: The accelerator is held at wide open throttle while the engine is cranked, the unloader holds the choke open and admits extra air, and eventually the excess fuel is cleared out and the engine starts.
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Do all carburated airplanes have accelerator pumps? I've always wondered about this because it does make sense that opening the throttle and reving up the engine quickly would lean the mixture temporarily but in airplanes you usually go from 1000 rpm to 2400 rpm (say C172) in about 3 seconds. Thats only a 1400 rpm increase in 3 seconds. Is that a "rapid" acceleration at all?

I mean bikes can go from 1000 to 9000rpm in a second when "off gear" (Neutral). Thats rapid acceleration to me.
 
LV-ARG said:
Do all carburated airplanes have accelerator pumps? I've always wondered about this because it does make sense that opening the throttle and reving up the engine quickly would lean the mixture temporarily but in airplanes you usually go from 1000 rpm to 2400 rpm (say C172) in about 3 seconds. Thats only a 1400 rpm increase in 3 seconds. Is that a "rapid" acceleration at all?

I mean bikes can go from 1000 to 9000rpm in a second when "off gear" (Neutral). Thats rapid acceleration to me.
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I guess they wouldn't have to... especially if you just start them and leave it at one rpm and then shut it off... like a generator, or mower.

I don't know of any carbs on vehicles that don't. Also, a bike is typically 1L or under. Even an O-200 is 3.2L. There is considerably more mass, and the latter makes like 1/2 the HP, so it's going to take longer to speed up... but yes 1000 to 2400 in 3 secs is plenty fast to create a lean condition if the engine isn't already running rich.
 
I do it simply because it makes me feel like I'm doing something useful, whether I am or not. :D
 
Pumping the throttle if the engine isn't turning will just pool up a bunch of fuel on the carb and start a fire if it backfires. Use the primer instead.
 
Do all carburated airplanes have accelerator pumps?
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No they dont. The Cessna 140 that I fly does not have a accelerator pump. I didnt know this the first time I flew the airplane with my instructor. It didnt start right away and he suggested priming it. Me being the know it all I am told him hold on I'll just pump the throttle a few times and this puppy will start right up. He allowed me to humor myself for about 10 minutes trying to start it this way till he informed me there was no accelerator pump on this airplane. Which then led to my next question of what the hell is a accelerator pump? I learned alot that day lol

This experience led to me to want to start studying to get my A&P because it made me releize how little I knew at that time about how airplanes really worked mechanically.
 
I wonder why some similarly sized engines need an accelerator pump and some others dont. I've flown a couple of airplanes on which as you slowly advance the throttle they sputter a little bit as engine revvs up trough 1500rpm. I've wondered if that is due to a crappy/non existent/broken accel pump or has to do with one of the carb circuits.
 
LV-ARG said:
I wonder why some similarly sized engines need an accelerator pump and some others dont. I've flown a couple of airplanes on which as you slowly advance the throttle they sputter a little bit as engine revvs up trough 1500rpm. I've wondered if that is due to a crappy/non existent/broken accel pump or has to do with one of the carb circuits.
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Could be a handful of things. Anything that causes a lean condition would do it. You also could just jet the thing rich under normal conditions and that would take care of needing a pump, but be expensive.
I didn't know they didn't use them in some aircraft, but if it's old, that makes sense.
 
mojo6911 said:
Pumping the throttle if the engine isn't turning will just pool up a bunch of fuel on the carb and start a fire if it backfires. Use the primer instead.
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+1. WTF did Cessna install a primer for anyways? I know it's not in the checklist, but it should be.
 
"means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, which can cause a temporary "lean" condition which causes the engine to "stumble" under acceleration (the opposite of what is normally intended when the throttle is opened)..."

I'm familiar with the "stumble" when adding power quickly in a 152, but does anybody know why this does not happen in the older 172's?
 
Chief Captain said:
"means that if the throttle is suddenly opened, the airflow will increase more rapidly than the fuel flow, which can cause a temporary "lean" condition which causes the engine to "stumble" under acceleration (the opposite of what is normally intended when the throttle is opened)..."

I'm familiar with the "stumble" when adding power quickly in a 152, but does anybody know why this does not happen in the older 172's?
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I'm also curious as to why this happens in fuel injected airplanes as well?
 
mojo6911 said:
Because the mechanical fuel injection in most aircraft don't distribute the fuel that well.
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Yeah, mechanical fuel injection is equivalent to putting a carb on each cylinder. You get slightly better performance, but it's hardly better.
 
gotWXdagain said:
I'm also curious as to why this happens in fuel injected airplanes as well?
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At my shop, we have several different types of aircraft, some injected, and some have carbs. the ONLY time I get that sputter is in a 152.

...Yet another reason I'm not a C152 fan...
 
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