Engine RPM - finite speed?

[Q]

Ok, this one is for all you propeller heads out there! :biggrin:

My friend and I were having a talk about the amazing engine rpms that F1 is producing, and then the conversation digressed into the dizzying speeds that model airplane engines and such can generate. These fantastic engine RPM speeds raised a question for me.

Is there a point where the fuel in a reciprocating internal combustion engine cannot burn fast enough to sustain RPMs? You can get around this by advanced timing and such, but even then you can only advance so far until you start encroaching upon the other cycles in the combustion process. Fuel takes a finite time to burn as I understand it. Is there some kind of catalyst to decrease burn time? Let's suppose the engine wouldn't fly apart, and our only variable is RPM. Can an engine have a theoretical unlimited top RPM speed? My answer would be no, but I would like to hear what others think. This question has gotten the best of me for the last few years.

 

[liftshard]

Take a gander at how top fuel dragsters work. Answers will be there.

 

[JoeSchmoe]

Yes, the engines will max out at some point. The flame front needs to burn faster than the piston is moving, or else it will produce vacuum and slow the piston down. There are limits to how fast gasoline will burn in an engine - perhaps 100-200 FPS. To to get higher expansion speeds, you will need to detonate the fuel/air mixture. Eventually this will max out at 20,000-30,000 FPS. You will need to build the engines stronger/heavier if you want to rely on detonation (ie, like diesel engines). Otherwise, this will destroy your engine.

This does not mean you can make an engine go 100x faster because you still have to solve the engine flying apart problem. You can accelerate the piston back and forth only so fast before you exceed the strength of the piston, rod or crank material.

FYI, if you think F1 engines go fast, you should check out RC car engines. These regularly max out at 30,000-40,000 RPMs.

-- Joe

 

[Q]

Take a gander at how top fuel dragsters work. Answers will be there.

Maybe my question was a little vague. Are you talking about nitromethane? I understand how it assists the combustion process, but how does it make it burn any "faster"? You can burn more quantity of fuel, but not any quicker. My point is that a piston could be traveling so fast that it infringes the time in which the fuel mixture needs to ignite.

 

[Q]

FYI, if you think F1 engines go fast, you should check out RC car engines. These regularly max out at 30,000-40,000 RPMs.

-- Joe

Yes, my friend and I were talking about that. That's insane RPMs! Thanks for your comments, that makes sense.

 

[JoeSchmoe]

Yes, my friend and I were talking about that. That's insane RPMs! Thanks for your comments, that makes sense.

Here's an example of RC insanity:
RC engine

SPECIFICATIONS
Displacement: 0.211 cu in (3.5cc)
Bore: 0.654 in (16.6mm)
Stroke: 0.630 in (16.0mm)
Practical rpm: 3,000-42,000
Output: 2.5 hp @ 34,000 rpm
Weight: 11.1 oz (315.5g)

I think it works out to 700HP/liter. Earplugs are not included.

-- Joe

 

[Q]

That's a great link, thanks! I love that radial, WOW!!!

 

[White92]

What if we weren't talking about a piston engine? Would things be different if you used say a rotary?

 

[JoeSchmoe]

What if we weren't talking about a piston engine? Would things be different if you used say a rotary?

I think it will suffer from the same limitations. The rotary still works on the principle of expanding gasses pushing against something to do work. Since the gases expand at finite rates, the engine RPM will be similarly limited.

-- Joe

 

[wswen]

Formula 1 V-10 engines at 18000RPM have more than 300 up and down cylinder movements per second! Overhead valves are a limiting factor in reaching high RPM. 2 cycle engines can obtain higher RPM because of half the cycles needed and do not have valve movement.
Wswen