Out of gas, engine speeds up, Old Cars question

[trimacar]

There was a question in a recent Old Cars Weekly, why does a gasoline engine speed up slightly right before it runs out of gas?  This would apply to normally aspirated engines only of course, with carburetors, not fuel injection.

 

Interesting question, so I asked my buddy Greg Cone, who builds engines from scratch. 

 

He says the old guys in the speed world talked about "maximum lean power".  This is the point at which the air/fuel ratio is perfect for the engine, and it produces the most power, and thus runs faster.

 

The problem with the air/fuel mixture being at the MLP setting is that the engine won't live long running that lean.  We all know that running an engine too lean can burn valves, too much heat and not enough cooling effect from the incoming air.

 

So, apparently what happens is that at the very instant of running out of gas, the mixture goes from slightly rich to a point which equals the maximum lean power point, and it speeds up momentarily.

 

I thought I knew a lot about engines, but this is an interesting tidbit!!

[leon bee]

On old motorcycles it was often part of all the rituals to run the carbs dry after using the bike. I learned that the exhaust gets real hot at that speed-up point...........and if you have brand new shiny chrome pipes this habit will help turn them blue up next to the head.

[Spinneyhill]

I always wondered why the string trimmer (2-smoke) did that when it ran out of fuel. Thank you.

[Guest]

Just pure physics.

[intimeold]

1 hour ago, trimacar said:

There was a question in a recent Old Cars Weekly, why does a gasoline engine speed up slightly right before it runs out of gas?  This would apply to normally aspirated engines only of course, with carburetors, not fuel injection.

 

Interesting question, so I asked my buddy Greg Cone, who builds engines from scratch. 

 

He says the old guys in the speed world talked about "maximum lean power".  This is the point at which the air/fuel ratio is perfect for the engine, and it produces the most power, and thus runs faster.

 

The problem with the air/fuel mixture being at the MLP setting is that the engine won't live long running that lean.  We all know that running an engine too lean can burn valves, too much heat and not enough cooling effect from the incoming air.

 

So, apparently what happens is that at the very instant of running out of gas, the mixture goes from slightly rich to a point which equals the maximum lean power point, and it speeds up momentarily.

 

I thought I knew a lot about engines, but this is an interesting tidbit!!

 

Your buddy Greg was spot on: About the maximum lean condition.

 

I would add if the vehicle automobile,  "Motorcycle, chainsaw, generator, etc)", is low on fuel; turn the engine off.

 

For several reasons, possibility of pulling contaminate, from the bottom of the fuel tank; and don't run an engine in a too lean condition, may cause damage.  

 

 

Edited December 13, 2016 by intimeold (see edit history)

[trimacar]

4 hours ago, Tinindian said:

Just pure physics.

That's true, but it's not intuitive physics as many things are....it's obvious that a bowling ball drops on your foot due to Mean Mr. Gravity, but a lot of other things are not quite that obvious....

 

But, basically, yes, you are correct, the engine is following basic laws that govern how our world works....