Lowest RPM and V6 engine or any engine can go 2

[hafele]

Does anyone know the lowest RPM that a combustion engine can go?

 

[franzh]

Hate to stir up the mess, but when I was doing some research on air and fuel mixtures, one limiting factor was the air velocity entering the intake valve. Liquid fuel tends to "drop out" of the air stream at slow speeds, whereas a vapor fuel tends to stay relatively homogenized, regardless of the air/fuel velocity.

Case in point: I took an old irrigation engine, a Chrysler 413 (hey Evelrod, remember those?) and had problems running it down to 400 rpm without very erratic performance (no-load). Installing a simple Propane carburetor (Impco 200 if anyone cares) and it easily idled down to 200 rpm, where I suspect the motoring friction losses exceeded the power production.

One thing more to add. The engine rpm has little to do with spark intensity (contemporary distributor, either electronic or contact point primary, not the magneto system), as I read in “rmw’s” post above. What you may have been feeling is one impulse every five or ten seconds.

Franz

 

[bettonracing]

Does the Carnot engine count?...

*exit stage left before bottles and tomatoes start flying*

 

[evelrod]

You did that on purpose! How did you know that thermodynamic theory gave me bad headaches?

Rod

 

[obanion]

franzh, so you think gaseous (propane) fuel, since it doesn't suffer from drop out, would allow a engine to run slower?

 

[franzh]

Obanion;
Yes, done so many times. In one of my former personal vehicles (import 4 cyl 2.4L w/ std) easily idles down to 300 rpm. Low speed engine performance was also smoother than on gasoline.

Using an optical engine, and watching the air-fuel mixtures propagation and homogenization when changing from liquid fuels to vapor fuels also confirms this. If you can change fuels in steady state mode, it is readily apparent. In some cases, you can even hear the difference.

Next, using pressure transducers, vapor fuels show a lower overall pressure during combustion, but the combustion pressure remains longer on the power stroke than with gasoline.
Franz

 

[obanion]

Oh that third point I didn't know about (lower peak pressure, longer sustained avg pressure). That's a big bonus to me. Trying for lots of power, and lower peak pressures means a higher upper limit on hard parts breakage. Water injection does the same thing as well, which I am also using.

 

[franzh]

Without delving too deeply into combustion physics, using water injection and vapor fuels are two entirely different things.

Vapor fuels, LPG or Methane, are combustible hydrocarbons, water is not. LPG and Methane have higher octane ratings than conventional gasoline. It’s the octane that allows for a longer burn duration, thus a longer combustion pressure, albeit lower, than gasoline. Both LPG and Methan have lower BTU’s than gasoline, thus a lower peak combustion pressure.

Water injection tends to suppress combustion temperatures and pressures. It takes chemical energy to convert the liquid water droplets to steam. There is relatively little (if any) energy derived from the steam expansion process. The chemical energy required to convert water to steam is derived from the air-fuel mixture. Consider the first and second laws of thermodynamics.

My experience has shown that if water injection is needed, there is a shortcoming somewhere in the engine dynamic process. Either compression is too high for the fuel provided, the fuel is of an insufficient octane, the timing is too aggressive, the air-fuel mixture is incorrect, the combustion chamber is not set up properly, and so on.

Before I get lambasted for saying water injection is bad, there are circumstances where water injection can indeed prove beneficial, such as in stationary turbine power generation systems.

Franz