Carburetors

[wdfifteen]

In my research for a story I have been amazed at the difficulty inventors had with the problem of mixing air and fuel for early piston engines. There are well over a hundred patents for various kinds of mixers before 1920. The reason those crude hit-and-miss engines were so popular was the poor performance of the throttle governed type. It was easier and the engine actually ran better to simply hold the exhaust valve open for a few revolutions when the speed got too high than to make a carb that mixed the same ratio of air and fuel at different air flow rates. I encountered a bizarre invention that switched a hit and miss engine to a different mixer after the first "hit" if the engine was running slow enough to "hit" more than once in succession. Having two mixers and a bunch of valve linkage was easier than making a single throttle governed carb. I was reminded of International Harvester's infamous "Flaming Four" that had seven carburetors on a four cylinder engine.
I recall watching films of WW I aircraft landing and hearing the engines sounding like they were cutting out. The reason, it turns out, is that it was so difficult to mixing air and fuel properly at different volumes that the throttle didn't have a "low" setting. To slow the engine down they had a switch that shut the ignition off on half the cylinders and another one to shut the ignition completely off - just to control engine speed.

Oh well, back to work. As you were.

[asphaltgambler]

The biggest problem then (as I see it) was properly calculating 1)Air flow requirements for a given displacement at different speeds 2) Air speed at the venturi 3) Venturi size in relation to carb opening 4) Throttle bore size.

[emcon5]

The funny thing is, even when they got it is right is they ever did, it still wasn't all that good. Mileage wasn't great, and mixture across a range of operating conditions was adequate, but pretty far from ideal.

I am on a email list, "Sidedraft Central" that primarily deals with Weber sidedraft carbs, and the guy who sort of runs it is a machinist with imagination and a logging wideband O2 sensor.

He has been playing around with making his own jets, modifying them, creating shrouded versions and getting good results, increased power and mileage. Pretty interesting stuff.

He sells his parts here:
Webstore items for Member DCOE_Tuner

[sammyg2]

emcon5, many years ago I matched a 50 DCOE to the inlet of a turbocharger I mounted on a 2 litler 914 engine.
Talk about a tuning nightmare, took weeks to get it right and it still was really cold blooded to start.

While we're on the topic (sort of) ...
this is something I wrote up a long, long time ago in an attempt to edumacate a friend on how a carburetor works. It's a tad over-simplified but I think most of it is relevant.

CARBURETION OPERATION AND THEORY
FUEL DELIVERY
The fuel pump pressurizes the fuel to around 3 to 5 psi for carburetors. The fuel flows through a filter (hopefully) and into the carburetor float bowl chamber. As this chamber fills, a float on a pivot rises with the level. As it gets high enough, it closes a valve, shutting off the flow of fuel. When the level drops, the float valve opens again and lets in more fuel.
IDLE CIRCUIT
The engine wants to run fast all the time. The throttle plate controls the engine by restricting how much air it gets.
The engine tries to pull more air in but the carburetor won’t let it at idle. This pulling causes vacuum, an area of low pressure in the manifold. There is a small hole in each of the throats of the carburetor just below the throttle plates. These holes are connected to a passageway that goes up, then back down to the float bowl. The vacuum pulls fuel from the float bowl, up through a restriction orifice (jet), and down into the manifold. Most carburetors have a needle adjusting screw that adjusts how much fuel flows into the throat. Many also have small holes in the passageway that let air in so that it mixes with the fuel and starts to atomize it so that it is well mixed as it enters the manifold. Some have idle mixture screws that adjust how much air is allowed into the passageway.
TRANSISTION CIRCUIT
As the throttle plates (butterflies) start to open, they expose a second set of small holes that are also tied into the idle jet passageway. As the butterfly passes these holes, the vacuum pulls fuel through them and mixes it with the higher velocity air that is rushing to squeeze through the small opening. Remember, there is low pressure below the throttle plates, but normal atmospheric pressure above them (except for full throttle high rpm conditions). Some carburetors have one transition port, some have as many as three or four. Their purpose is to try and correctly meter the amount of fuel so that the right ratio of fuel to air is maintained. Most driving is done in this circuit (Except for some sawz-all wielding friends of mine).
ACCELERATOR PUMP
When the throttle is opened rapidly, the manifold vacuum can suddenly drop to almost zero. No vacuum, no fuel gets sucked into the engine, no go. Just a big bog.
The accelerator pump is a band aid for this problem. As the throttle is opened, a piston or diaphragm is actuated, displacing fuel and pumping it right into the throat of the carburetor like a squirt gun. It mixes with the air and burns to keep the engine happy. Since the air is not moving very fast during this condition, the fuel has a tendency to puddle on the manifold floor or stick to the walls. V-8’S have this problem, but not as much as a single carburetor on a 914. The v-8’s use hot water or oil to heat the floor of the manifold to keep the fuel mixed up. It works, but is a major compromise. One of the hardest set ups to get dialed in is the Holley bug spray carburetor on a 914 4 cylinder engine. There is no heat riser to warm the manifold and the runners are long, so the fuel doesn’t stay suspended very well. Some people swear by this set up, some swear at it. Anyway, because the fuel doesn’t stay mixed very well during this low vacuum, low rpm condition, the carburetor has to add extra fuel so that we are sure to get enough into the motor. Not good for emissions or economy. Carburetor systems that have a single barrel for each cylinder and are mounted directly in line with the intake ports are the least susceptible to this, except for fuel injected motors. There are some problems with this type of carburetor set up (like reversion) but that is in the next class. Fuel injection is awesome.
MAIN CIRCUIT
As the engine rpm rises, the air that is flowing through the carburetor barrel has to go through a section that is a little smaller than the rest of It (venturi). The air really wants to get through, so it has to speed up a little so that all the air can get by. So it speeds up and squeezes though the tight spot, then the opening gets bigger again. The air is still going fast and straight. Right at the tight spot there is a small area that has lower pressure than the rest of the barrel. This is caused by the air trying to keep going straight, and the velocity makes it not want to bend back to fill the entire barrel. This is called the Bernoulli principle (I hope I spelled that right). Right there where the pressure is lowest, there is a small port, or passageway that is connected to the float bowl. The fuel is pulled from the float bowl, up the passageway, mixed with a little air, squeezed through the main jet, and goes into the carburetor throat. Some carburetors have a booster venturi, but they just do the same thing the same way.
Carburetors are simple things, but try to do an impossible job. That is, add just enough fuel to maintain a perfect air/fuel mixture, no matter how much the throttle is opened, the engine rpm, the air temperature, the engine temperature, The fuel level in the throat bowl, the list goes on and on. My father told me a long time ago that carburetor is a French word which loosely translated means “compromise”. He may be right. Early fuel injection systems had as much as 30 different "maps" or compensating circuits depending on input conditions, modern systems can have millions of "maps" due to the multiplication of all the variables.

[intakexhaust]

Way back in time while in high school, my science teacher assigned a week to make a carburetor. My mind was thinking way beyond of what he was soon to demonstrate.

So the day came when he ask if anyone made a carb. No one had so he chuckled and simply placed a plastic cup of water on the desk. Placed a straw at the edge and horizontal to the top of the cup and blew across it. What do you know, it atomized! LOL

[on2wheels52]

Bernoulli principle
Why carbs work and planes fly.
Jim

[romad]

I recall watching films of WW I aircraft landing and hearing the engines sounding like they were cutting out. The reason, it turns out, is that it was so difficult to mixing air and fuel properly at different volumes that the throttle didn't have a "low" setting. To slow the engine down they had a switch that shut the ignition off on half the cylinders and another one to shut the ignition completely off - just to control engine speed.

The ignition cut off was used on the rotary engine because you could not attach a carbs or manfolds to the rotating case. Gnomes and LeRhone had no thottle at all, just "blip" the ingition. The other conventional engines of ww1 had pretty developed carb systems, in fact the BMWiiia had altitude compenstating mixture control

[herr_oberst]

Originally Posted by intakexhaust

Way back in time while in high school, my science teacher assigned a week to make a carburetor. My mind was thinking way beyond of what he was soon to demonstrate.

So the day came when he ask if anyone made a carb. No one had so he chuckled and simply placed a plastic cup of water on the desk. Placed a straw at the edge and horizontal to the top of the cup and blew across it. What do you know, it atomized! LOL

Hah! My dad used this tool when he was a kid to shrink the fabric on the model airplanes that he would build. One tube in the dope, the other in the mouth and spray away!
I'm thinking those must've been high times.

[intakexhaust]