Rabbit                                                                                                                      

The Rabbit engine layout

The Terrapin project was so slow and bulky it would not run. I hope the single engine Rabbit project runs a little faster. I may add a second engine and call it Jackrabbit,  Lepus californicus.

Click images for bigger views.

Piston and Cylinder

The piston cylinder is made from a 12 inch piece of ABS 4” drain pipe. I picked a pipe that was fairly smooth on the inside. I glued a ½” PVC barbed fitting into the 4” cap and pushed a 5” piece of 13/32” brass tubing though holes drilled in the cap’s sides. The cylinder will pivot about this point on a 3/8’ steel axel as the connecting rod moves on the crank.

Piston Seal

The piston head is made from three Plexiglas discs (turned on a router table) and a 4” o-ring sandwiched together. The o-ring is held to the cylinder wall by the pressure (or vacuum) inside the piston and it slides well with the help of a slathering of petroleum jelly. The o-ring seal is the only moving part of the project where I have to contend with air leaks. If the finished piston leaks, then the project will be postponed until I can make a functioning piston.

SMPP-03 Calibration Graph

I ordered several pressure sensors and temperature sensors from Mouser electronics. I mounted a SMPP-03 sensor ($4.50) and a small op-amp board ($0.49) inside a PVC fitting.  At ambient pressure, the unit outputs 1.85 V. The voltage changes linearly from 0.5V to 3.75V over the entire +7 PSI range. I am working on PVC overpressure and underpressure relief valves. I have $5 worth of 5/8” rubber balls, coming from Smallparts.com.

PVC Parts

Ardunio Uno R3

I ordered an Ardunio Uno R3 micro controller on eBay for $16.49. With it, I should be able to control and monitor the engine by way of a USB cable to an old desktop XP computer. I will be attempting to write a VB6 interface program to send engine commands and collect data. I have a steep learning curve ahead, but I have wanted to learn about this supposedly excellent product for a long time, and this project is a good fit for this device. Even if the engine does not run, I will have learned something new.

An Engine Simulation Program                                                                         

I've been thinking about a new Stirling engine project that uses a fan to blow cold air through the regenerator into the hot space and visa-versa. The vessel would contain a plastic bag to keep the hot and cold air from mixing.

Simulation Program. Click for bigger view.

I wrote a VB6 simulation program that (I hope) models what happens when the engine runs. The program calculates the instantaneous torque produced by the engine for each degree of movement of the flywheel. For each position of the flywheel, I know the piston volume , the vessel volume, the dead space volume and the total volume of the engine in cubic inches. Also for each position of the flywheel, I know the ratio of hot and cold air in the vessel and I can calculate what that associated hot/cold air volume would be if it were unbound so as to expand (or contract) it’s volume to keep the air pressure at an ambient 15 PSI. From the ratio of the total volume and the unbound volume I calculate the pressure in the vessel that fluctuates positively and negatively about the ambient pressure each cycle. The force on the piston head is calculated, as is the torque delivered to the flywheel by the connecting rod. It turns out that a wheel driven by a connecting rod does not exactly follow a sinusoidal curve. With a bit of creative programming I think I now accurately calculate the reciprocating piston position and the true force the connecting rod imparts tangentially to the flywheel.

Pressure-Blue,     Force-Red,     Torque-Green

Because  power is transferred to the flywheel as the air is expanding and again as the air is contracting, two peaks per cycle are seen on the torque curve (green). For bigger piston volumes, the torque can momentarily become negative, although curiously, the average torque is still larger than for a smaller piston.

Torque-Green ,  2nd Torque Curve-Orange,   Sum-Purple

If two of these engines are coupled at a phase of 90 degrees, then the torque curves add to make a relatively constant positive torque value. I 

incorporated torque curve: duplication, offset, summation and scaling functions, into the program by mouse clicking on the graph. There are a lot of bells and whistles in this program and I believe it does a good job of simulating the kind of engine I have in mind to build. You may download the simulator from the following link. It is called RabbitSimulation.exe.

https://docs.google.com/file/d/0B9fsJB6CcZqrVW9fMVBEVVJTZVE/edit?usp=sharing

A New Concept                                                                                    

The view from the doghouse today.

I’ve dismantled the Terrapin, cleared off the garage workbench and cleaned out the doghouse; time for a new project...

I must be a gluten for punishment but I’m not done with the Stirling engine bug yet. I really want to know if usable work can be extracted from a low temperature gradient, of say, 80C. I am tired of patching air leaks in wooden boxes and of complex mechanical linkages. What I want to do is to build a simple experimental engine from which I can collect experimental data. I have some concepts that I am thinking about:

1. Keep this engine mechanically simple so that a minimum amount of energy is lost to friction.
2. Use a calibrated electric heat source so I can track the amount of energy going into the engine.
3. Use an off–the-shelf air tight containment vessel.
4. Instead of using a displacer to move air between hot and cold chambers, use a fan to move air though a stationary regenerator. An air-tight vessel will use a plastic bag partition to keep the hot and cold air volumes from mixing.
5. Experiment with making a piston from a metal can.
6. Gearing the piston output so that the flywheel turns many times each engine cycle.
7. Use a small generator coupled to the flywheel to measure output work.
8. Make over-pressure and under-pressure safety relief valves.
9. Use a computer to monitor and control the engine in real-time.
10. Use mostly recycled or scrounged materials with a budget of $100.
11. Write a simulation program that models what I might expect in the way of output power.

I am going to start with the simulation program and then see what develops.