• There has been a recent cluster of spammers accessing BARFer accounts and posting spam. To safeguard your account, please consider changing your password. It would be even better to take the additional step of enabling 2 Factor Authentication (2FA) on your BARF account. Read more here.

Fuel mixture and engine temperature

kuksul08

kuksul08

Suh Dude
Joined
Mar 17, 2014
Location
Hoonville
Moto(s)
Whee!
I'm trying to understand this. When you start up a cold engine, on a carb'd bike you turn on the choke which lets more fuel in, and on an EFI bike it automatically runs richer. Most cars run richer in order to heat up more quickly so that the cats will heat up and the vehicle will run cleaner, evident by the strong raw fuel smell of a cold engine start.

At the same time it is said that a rich mixture keeps the engine cooler, and that going lean can be dangerous.


So does more fuel make the engine run hotter or cooler? Why and how etc?
 
I'm going to give this a shot as I understand it. A cold engines needs more fuel as it does not revolve as easily as a warm engine and -specially in carbureted engines- some of the fuel condenses as it hits cold metal and does not burn completely. The incomplete burn, creates less power and some of the smell.

Under normal conditions a lean mixture creates more heat and if it goes too lean can create so much heat as to burn a hole through the top of a piston. I believe it is the extra fuel absorbing some of the heat and reducing the extra oxygen that cools the combustion temp. I'd love to hear the science behind it as I may be off base.
 
More fuel makes your engine run cooler in two ways.

1. When a liquid evaporates, it has a cooling effect. Since you have more fuel than you need, some of the fuel going into your cylinder evaporates, and cools off the mixture, but does not burn.

2. If you are running rich, it means you have too much fuel, and you get incomplete combustion. Less than 100% of the fuel air mixture in your cylinder burns since there are not enough oxygen molecules. Since less than 100% of the molecules in your cylinder are used for combustion, the temperature is lower.

The hottest your engine can run is if your mixture is set to stoichiometric. This is the chemically perfect mixture where every oxygen molecule, and fuel molecule is used for combustion (theoretically). This is not the mixture that internal combustion engines are tuned to run at. Leaning the mixture too much can lead to hot temperatures, and detonation. Detonation is basically the fuel/air mixture exploding before it is ignited by the spark plug.

The reason that your engine runs rich when you start it is because fuel has a hard time evaporating in a cold engine. Liquid fuel does not burn, it needs to evaporate in order to burn. If not all of your fuel evaporates, it is like your mixture is lean. If you are too lean, your engine will not start, or it will run rough. By adding extra fuel, you get enough fuel vapor to support combustion, and for your engine to run smooth.
 
Last edited:
kuksul08 said:
I'm trying to understand this. When you start up a cold engine, on a carb'd bike you turn on the choke which lets more fuel in, and on an EFI bike it automatically runs richer. Most cars run richer in order to heat up more quickly so that the cats will heat up and the vehicle will run cleaner, evident by the strong raw fuel smell of a cold engine start.

At the same time it is said that a rich mixture keeps the engine cooler, and that going lean can be dangerous.


So does more fuel make the engine run hotter or cooler? Why and how etc?
Click to expand...

When the motor heats up, it heats up the air coming into the motor, leaning it out. The amount of fuel extra added at start is only to allow a fuel air mixture that is easily combustible to start the engine. It has nothing to do with the cat or running hot or cold. In fact rich motors tend to run cooler than lean motors.
 
Interesting! Thanks for the input.

For some reason I always thought the rich mixture when warming up a car was to warm it up quicker, but now I see that is not the case.
 
Rich mixture allows more fuel to vaporize at the colder temperature. If enough fuel doesn't vaporize, the charge will be outside of the flammibilty limits and won't explode(due to lean condition). Once the engine and intact tract etc are warm, a smaller amount of fuel is enough to vaporize enough fuel to be within flammability limits and burn.
There is more but I am afraid of geeking out.
There are more effects due to richer mixture including faster cat warm up.
Sometimes electric heaters are used to warm up the cats faster.
 
Last edited:
I don't know much about motorcycle carbs but every other carb I've ever worked on, the choke didn't add fuel but restricted the air intake so the mixture would enrichin.
 
Wow. Convoluted answers.

You choke out an engine when its cold because of the fact that your cylinder is so big and when a gas is cold you can cram more of it into some volume. So if it's really cold ten you don't want the carburetor to mix up the fuel and the oxygen as if the engine was at whatever temperature it's at when it's running all smooth and stuff. It's like making one of those flaming cocktails. Ideal gas law and something about effective collisions.
 
Try this link for a detailed explanation, and pros/cons: http://www.randakksblog.com/lean-vs-rich/

Net, it's about the fuel/air mix; a cold engine pulling in cold air through cold carbs and cold intakes needs a richer mix to run, and will dump the un-burnt gas out the exhaust (that strong gassy smell). A hot engine will vaporize the gas better and burn more efficiently so you can get by with less gas in your mix. Stick your hand in the exhaust gas coming out of a cold bike on startup vs a hot bike and smell your palm.

In general, a bit rich is safer than a bit lean, since the one may foul your plugs and the other may crater your pistons. On carbed bikes, especially 2Ts, a lot of folks tuned slightly rich for safety margin.
 
bcv_west said:
Try this link for a detailed explanation, and pros/cons: http://www.randakksblog.com/lean-vs-rich/

Net, it's about the fuel/air mix; a cold engine pulling in cold air through cold carbs and cold intakes needs a richer mix to run, and will dump the un-burnt gas out the exhaust (that strong gassy smell). A hot engine will vaporize the gas better and burn more efficiently so you can get by with less gas in your mix. Stick your hand in the exhaust gas coming out of a cold bike on startup vs a hot bike and smell your palm.

In general, a bit rich is safer than a bit lean, since the one may foul your plugs and the other may crater your pistons. On carbed bikes, especially 2Ts, a lot of folks tuned slightly rich for safety margin.
Click to expand...

correct, and the poor man's solution to detonation on built four strokes is to run them rich.
 
dravnx said:
I don't know much about motorcycle carbs but every other carb I've ever worked on, the choke didn't add fuel but restricted the air intake so the mixture would enrichin.
Click to expand...

ya most carbs are like that. thats y its called a choke, it chokes the airflow down.
but high performance engines like in sportbikes w/ 1 carb per cylinder prob make it pretty hard and not that efficient to have some linkage moving something in the airflow.
 
hattusilis said:
I think that you people need to review basic fluid dynamics...

https://www.youtube.com/watch?v=Wokswr_KHXQ

https://www.youtube.com/watch?v=wyspAHrMbb8
Click to expand...

ELxNgow.jpg
 
The "choke" on a bike carb does not work the same as a car carb. On a bike carb, it us really a enricher because it's adds fuel when starting and not restricting air like on a car carb.

When you pull the "choke" on, you are opening up a circuit in the carb that lets more fuel in the airflow as it goes threw the carb.

The reason for the extra fuel for a cold engine is because cold air is denser than warm air and therefore the the more fuel is needed to start the motor.

As the air heats up, plus the carbs, as the motor heats up, it makes the air less denser as it enters the cylinders and therefore less fuel is needed.

If you ride a carb bike long enough, you wil notice on certain days the bike will just run better than others. All because of the weather.

Oh, get the current Rider cause there tech section goes through a carb rebuild.

Recently, I've been dealing with a bike that had one cylinder running good 100-120 degrees cooler than the others at idle, plus it just wasn't running right.

Turned out to be that cylinder carb fuel mixture screw was about a full turn too rich. Set the mixture screw correctly, cylinder came up to temp, and with a good carb sync, which is critical to do on carb bikes, the bike sounds like a Singer sewing machine now.

IR temp gauge is a must have in a tool box.
 
Last edited:
RickM said:
lol.
Have you read the original post? Both those links have nothing to do with the answering the question in OP
Click to expand...

Actually, they do. Bernoulli principle contributes to the cooling effect of the entering mixture.

On a cold engine, you want to enrich the mixture for two (or more) reasons:

For a given volume, more O2 molecules will be present for combustion (Charles Law).

Gas vapor condenses on the cold cylinder walls (Henry's Law).
 
dravnx said:
I don't know much about motorcycle carbs but every other carb I've ever worked on, the choke didn't add fuel but restricted the air intake so the mixture would enrichin.
Click to expand...

As others have mentioned, most motorcycle carbs unless they're really, really old, use an enrichening circuit to feed additional fuel into the intake tract. On carbs with an actual choke, the intake to the carb becomes restricted which increases the speed of the air entering the carb which then sucks more fuel into the intake tract.
 
El Feo said:
Actually, they do. Bernoulli principle contributes to the cooling effect of the entering mixture.

On a cold engine, you want to enrich the mixture for two (or more) reasons:

For a given volume, more O2 molecules will be present for combustion (Charles Law).

Gas vapor condenses on the cold cylinder walls (Henry's Law).
Click to expand...

I am not disputing your post. I do have a good understanding of Thermodynamics and Fluid dynamics.

As far as engine design goes, the primary reason more liquid fuel is needed on a cold engine vs a hot engine is rate of vaporization. It's true colder engine will have denser air (in turn more O2). But that effect pales in comparison to needing more liquid fuel to vaporize enough.

If the presence of more oxygen was the major reason to need more fuel. there wouldn't be so much unburnt gasoline out of the tailpipe on a cold engine running the richer mixture. The excess gas to compensate the extra oxygen should just burn!

I could go on and say, if the additional fuel is only to compensate for the denser air, then it's not a richer mixture as A/F ratio is maintained? :) But that's not the case. The mixture is quite a bit richer with the choke/enrichener activated
 
You must log in or register to reply here.
Back
Top