These things really are toys...

[matt brown]

Yes, runaway from an "ideal" viewpoint. However, from a real viewpoint, as mass increases, so does leak potential and need for more heat input (per time). My basic premise is that any increase in charge mass (initial or during operation) will tend to increase engine pressure resulting in more output per cycle (rpm) AND more rpm. This reminds me of the SE throttling scheme where an external charge tank varies charge pressure to cold space vs old world throttling via source input. This idea was late in SE development, overlooked for decades. A later variable charge pressure scheme was simply a moveable "wall" similar some early submarines.

[Stroller]

The actual observed total volume while at operating temperature is 3.1cc for a total gain of .5cc!!!!!

This corresponds to a delta T of less than 80k!! And that's what makes these things toys. Not for being small, but for being shadows of their potential.

Wouldn't some of the difference between observed and theoretical volume be due to the inertial masses of the pistons/rods and angular momentum change of the flywheel? Two of these are getting accelerated from a standstill at TDC at 2500rpm. The third has significant mass. Those will apply a force against the gas volume. On the upside, the increased pressure will cause a temperature rise (or more likely a reduced temperature drop) as the motion towards TDC of the power piston increases the internal volume of the engine.

The alcohol flame is hot, but the key issue is how long the flame gets to transfer energy through the hot end wall over what size of surface area relative to swept volume. This is why Shane Pomeroy is getting peak power around 350rpm with is Mk3 and Mk4 engines - more time to get the working fluid hot. The toytown aspect of small model engines is the relatively slow heat transfer rate of borosilicate glass and its smooth surface. OK with tiny volumes but to run a bike you need more cubes for useful power, so Andy Ross went to the trouble and expense of making slotted surfaces to increase surface area.

[Stroller]

Apologies for the typo:
"as the motion towards TDC of the power piston increases the internal volume of the engine."
Should be BDC of course.

[VincentG]

Wouldn't some of the difference between observed and theoretical volume be due to the inertial masses of the pistons/rods and angular momentum change of the flywheel? Two of these are getting accelerated from a standstill at TDC at 2500rpm. The third has significant mass. Those will apply a force against the gas volume. On the upside, the increased pressure will cause a temperature rise (or more likely a reduced temperature drop) as the motion towards TDC of the power piston increases the internal volume of the engine.

All of the expansion testing I do is with a free piston at near static conditions. So there is essentially no effect from any forces other than buffer pressure.

The alcohol flame is hot, but the key issue is how long the flame gets to transfer energy through the hot end wall over what size of surface area relative to swept volume. This is why Shane Pomeroy is getting peak power around 350rpm with is Mk3 and Mk4 engines - more time to get the working fluid hot. The toytown aspect of small model engines is the relatively slow heat transfer rate of borosilicate glass and its smooth surface. OK with tiny volumes but to run a bike you need more cubes for useful power, so Andy Ross went to the trouble and expense of making slotted surfaces to increase surface area.

In the small high temp models it seems that heat goes in more than fast enough. It's getting the heat out that becomes the real bottleneck.

[Stroller]

All of the expansion testing I do is with a free piston at near static conditions. So there is essentially no effect from any forces other than buffer pressure.

Even free pistons have mass that has to be accelerated against their inertia. That increases pressure, and pressure compresses the working fluid. How much does your piston weigh? We could do a simple calc (ignoring small adiabatic effects).

In the small high temp models it seems that heat goes in more than fast enough. It's getting the heat out that becomes the real bottleneck.

Agree. It's when you try to scale up the toy size the problems start, because area only increases by the square as volume increases by the cube.

[VincentG]

Even free pistons have mass that has to be accelerated against their inertia. That increases pressure, and pressure compresses the working fluid. How much does your piston weigh? We could do a simple calc (ignoring small adiabatic effects).

I agree. But my epoxy engine is nearly reaching ideal gas law expansion and is fighting a vertical piston while doing so.

The small model that was the subject of this thread is nowhere near ideal and yet the piston is laying horizontally.