Four-stroke motorbike
In my four-stroke motorbike user’s manual something like this is written in the engine tuning section. "turn the air_fuel mixer control screw fully clockwise to make the mixture rich (with fuel). Now turn the screw anti clockwise two and a half times to make the mixture leaner. The engine RPM will increase. Adjust the idling speed just enough to keep the engine running . . .”
Now the question is, why does the engine runs faster when you supply leaner mixture meaning fuel)?
Now the question is, why does the engine runs faster when you supply leaner mixture meaning fuel)?
Labels: thinktank
posted by Rajesh Lal at
6:54 AM
8 Comments:
Engine RPM is a matter of the acceleration of the explosion, not necessarily just the volume of fuel.
The right mixture of fuel and air maximizes the explosive force within the cylinder and therefore releases the most energy. This also implies the greatest reduction in mass of the remaining exhaust gas, and therefore lowest exhaust gas density.
Too much fuel does not burn entirely and produces a smaller explosion and higher density exhaust to push out of the engine while the other cylinder is on the power stroke. Too little fuel creates too small an explosion, and the engine stops.
As the explosive force is maximized and the exhaust becomes easier to push out, the engine RPM increases.
The richer fuel doesn't have enough oxygen to help it burn completely. As you add more oxygen, the fuel burns more effeciently and the rpms increase.
Ever notice how an idling chainesaw motor speeds up just as it runs out of fuel?
I should also have mentioned that with more liquid in the mix, the compression cycle is going to be far more difficult for the engine, so that is going to reduce RPM compared to a leaner mix.
I don't think that the additional liquid would be enough to change the compression. The key is the percentage of fuel and oxygen. When there's to much fuel, then there's not enough oxygen. Air is only about 20% oxygen, but it would be interesting how it would run at a higher percent oxygen. But I would suspect the you could increase the oxygen and then run with the richer amount of fuel.
For the last week, I think ToM has been rather luke warm. Where's all the great ToM posters?
Ragknot-
Thats why they make higher octane fuels.
If you slowly lean the carburetor, the rpms will increase because you are approaching the optimum fuel/air ratio. However, this optimization is occurring under "no load" conditions. If you attempt to ride the bike with the carb leaned out, you will find there is not enough fuel to provide adequate power under load (or runing conditions). Also running the engine too lean generates more heat and could actually seize the engine. Ideally, you lean the carb until you find the maximum rpm ... then open the jet to allow a little more fuel as per manufacturer's recommendation.
If you slowly lean the carburetor, the rpms will increase because you are approaching the optimum fuel/air ratio. However, this optimization is occurring under "no load" conditions. If you attempt to ride the bike with the carb leaned out, you will find there is not enough fuel to provide adequate power under load (or running conditions). Also, running the engine too lean generates more heat and could actually seize the engine. Ideally, you lean the carb until you find the maximum rpm ... then open the jet (per mfg. recommendation - often a 1\4 turn) to allow enough fuel to perform adequately under load.
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