ENGINES: PRESCHOOL

[Stewart G Griffin]

After being interested in cars since around age 4 or 5, thirty-two years later i'm finally getting it;  i finally have a basic understanding of how carburetors work, and all the magazines and books i've gotten over the years are finally beginning to make (some) sense.

What i want to get into now continues with carburetors:

If a cylinder displaces the same amount per stroke---(or at least from TDC to BDC), then it is taking in the same amount of air per stroke---it is physically impossible for it not to.

Then why do carburetors have throttle valves in the first place? (or fuel injection systems have air doors)

[HAULIN IT]

Stewart, I'm not sure I'm following you completely, but I'll give what I believe is a simple answer to your question. Lets look at a single cylinder engine to simplify it. The engine needs to be turned somehow to start the cycle (pushed in gear,cranked with a starter,rope,ect.) to begin taking in the air/fuel. The spark pushes the piston down,ect. ect. The more "gulps" of air/fuel the cylinder gets in a minute (Rpm's) the more "work" you get out of the engine. Without a throttle valve the engine would ingest as much air/fuel as it can all the time, ie: wide open throttle. What you are doing with a throttle blade is managing the full amount of air/fuel to a controllable amount for what "work" your trying to do. It would be hard to park your truck,drive through the store lot, ect. without a throttle valve. Think about any engine running at no restriction from the time you got it started until you shut it off. Chain saw,weed whacker, boat your trying to bring into the dock, ect. without the "restriction" on the Rpm's. I hope this helps a little, Lorne 

[VileZambonie]

S G G

Are you asking about the theoretical amount of air an engnie can displace in a one complete cycle vs what the engine can actually displace?If so this is called Volumetric efficiency.

As far as why a gas engine has a throttle valve: the engine speed is determined by the opening of the throttle plate and with a diesel engine there is no need for a throttle plate engine speed is controlled by adding more fuel. That alone will only control engine speed not VE. VE is determined by many factors, air pressure, elevation, intake design, port size, induction system, exhaust etc...

The analogy you are probably equating this to is the engine as an air pump.

A carburetor is just a fuel metering device. It's job is to deliver the correctly proportioned amount of fuel to be mixed with the air entering the engine (stoichometry) at a given speed and according to rate of acceleration. They aren't the best method of doing the job with today's technology and that's why they went bye bye.

[Stewart G Griffin]

i guess what i'm trying to ask is it seems to me that a cylinder moving from TDC to BDC with the intake valve open ALWAYS takes in the same amount of air regardless of how much or how little the throttle valves are open?

i know that air can be compressed (as with diesel engines), but it does not seem physically possible to vary to amount of air intake if the cylinder is moving the exact same amount every cycle.

[VileZambonie]

OK the analogy is this: On paper the engine is an air pump. It can pump in and out 350 cubic inches of air per cycle. This is only theoretical. Most engines especially stock engines have less than 100% Volumetric Efficiency.

Getting the air into the cylinder and filling it, compressing it, combusting it and expelling it at any given speed is going to vary. Not just by engine speed but by many other contributing factors, air pressure, elevation, mechanical timing, valve lift, duration, diameter, port sizes, multiple valve angles, combustion chamber design, intake design, exhaust flow, etc etc. Keep in mind as engine speed increases there is less time for each cycle to bring in and out that theoretical amount of air.

That's why performance enthusiasts spend all this time and money on their engines and their implements to help allow the engine to breathe better. Without forced induction you can exceed 100% volumetric efficiency. You won't exceed 100% all of the time but the goal is where and when you need it. You should research improving volumetric efficiency to learn more about the specifics.

As far as it's relationship to the engine taking in the same amount of air with the throttle closed vs open think of it like this. Turn on your shop vac, put your hand over the end of the hose, you'll feel a great suction (you can equate this to intake manifold vacuum and your hand is the throttle plate). Now allow some air to pass through your fingers, the suction pressure is reduced and the air being drawn into the vacuum increases and manifold vacuum decreases but the volume and speed in which the air is drawn into the vacuum is increasing. The vacuum is a negative pressure and the atmosphere is positive pressure now remove your hand and full volume can flow (WOT) Hope this makes sense. 

[HAULIN IT]

Very well put Vile, The sweeper analogy is a good down to earth explanation! A person reading your answer can (or should) be able to tell you teach theory. Stewart, I'm not sure your true question in your mind is " why do engines have throttle valves?" I think you are really trying to get around to " if a piston moves the same amount each stroke(swept volume), how can it not help but ingest the same amount of air?" Is this right? If so, Vile answered that in his last post. Engines are not as efficient as in your "thought" & none are completely efficient ALL of the time (that I know of). I think you asked the question different than what you are thinking, maybe something like " How does a throttle valve control the amount of air comming in if the cylinder pulls in the same amount each cycle"? If so, the answer is the same as Vile's last post. I'm gathering your real questions are with the basic engine function (efficiency,cam opening events, ect. I think your looking at "the engine is spinning ingesting air/fuel" & really the engine ingests air/fuel & then spins, as you control the air flow the engine slows down. Sort of like the tail wagging the dog. This is why when you have a vacuum leak (really an air getting in leak, other than the "throttle valve") the engine speeds up. That is why I asked go to a single cylinder engine (one carb for one cylinder, one exhaust pipe) to not get caught up in 4 hole carbs,but 8 cylinders & exhaust scavenging, ect. Hope this helps, back to the books, we can never learn enough, Lorne

[Stewart G Griffin]

I think you are really trying to get around to " if a piston moves the same amount each stroke(swept volume), how can it not help but ingest the same amount of air?" Is this right? ............................... " How does a throttle valve control the amount of air comming in if the cylinder pulls in the same amount each cycle"?

This is precisely what i'm asking;  Well, (apparently?) cylinders don't suck in the same amount of air all the time.  i would have thought it to be the same as liquids.  For example, if you take a syringe and put the tip of it in a sink full of water and then pull the syringe's plunger all the way up, it will pull in the same amount of water each and every time.

But air(gases) doesn't work like that? 

[ccz145a]

Don't forget the dimension of time... while the engine is breathing the same volume/per cycle, going faster means more cycles per time unit -- hence more volume per time unit.

[VileZambonie]

You can't compress a liquid.

[85MudMonster]

however if you compress some gases they become liquids......

[VileZambonie]

Are you talking about diarea?

[ccz145a]

Are you talking about diarea?

rof lmao

[Stewart G Griffin]

PT.2

Alright, it is apparent that cylinders do not suck in the same amount of air on intake stroke all the time.  i have also thought about the vacuum cleaner experiment and when you do put your hand completely over the hose, the vaccum cleaner does not stall out (in fact some rev a little higher) so that sort of proves that air is different, physics-wise, then liquids.  i guess when you close up the throttle instead of sucking in the same amt of air past the crack in the throttle, you get more vaccuum?  So that's why they call it a vaccuum cleaner?  i'm still a little fuzzy on the physics of it all, but the  bottom line is, apparently, gas engines don't suck in the same amount of air at all times because if they did, they would run at the same speed all the time?

Anyways, my next burning question concerning engines is with fuel pumps and the fuel system:
i noticed when looking at pictures of small blocks and big blocks (although the question could apply to any carbed engine) that sometimes some of these fuel pumps have no return line----it's just one line from the tank to the pump and then another line from the pump to the carb.

Now, knowing that a stock fuel pump puts out at least 4-5 lbs of fuel pressure, what if you just sat there an idled for say a half hour.  Wouldn't you flood the engine?  i know that a carb has floats to control fuel level in the bowl(s), but since you can't compress liquids and at 5 lbs of fuel pressure, i don't think the float springs can overcome the pressure?

2) Why do some carb setups have a return line?

[ccz145a]

This explanation is for the stock diaphragm pump (before the advent of fuel injection)

In the case of no return line, the pump is flailing away, trying to push but cannot overcome the head pressure. Thats ok, it was designed that way.

Someone discovered that if you put in a return line in the loop, the pump will last longer. Now if you have a return line, then you can integrate a charcoal canister to recover the vapors and eliminate the tank vent, too.

Newer centrifugal pumps need the return line to release the unused fuel and lower the head pressure. They will fail without it.

[HAULIN IT]

Stewart, I'm glad your getting a little better handle on things, if we all talked about the "theory" everyday, sooner or later digging deeper into it we would find little things to argue about,based on different peoples thoughts & interpretations, however  the basics are allways the basics. Your thinking about this stuff & probably have a sucker bite on your hand from the sweeper, this is GOOD! To answer your question about the fuel pump, they have an internal by-pass, just like your engine's oil pump or your floor jack,ect. when it comes up against the immovable, it bleeds off by a check ball/spring set-up,over & over until it is called on for more... The reason for the external by-pass is this "inefficient work" builds heat, the return system removes the heat by helping eliminate the buildup. How they picked which ones (with a mechanical pump) remain a mystery to me, usually the A/C vehicles had them, but not all across all of the cars made.This is probably due to the idle compensator raising the idle while the A/C is on. Some "performance" vehicles had them in the '60's, later the emission vehicles had them? It really is a good way to remove heat & maintain steady flow.       Have Fun, Lorne