Noise Filter                                                                                                                                                       

Motor noise filter.

I added three ceramic capacitors across the hair dryer motor and the USB motor noise interference is gone. The following link was very helpful:

http://www.beam-wiki.org/wiki/Reducing_Motor_Noise

I heated the engine up to 80ºC and the engine is not quite self sustaining. I can tell that I am close. One problem is that the cold side of the engine warms about 10ºC for every five minutes of use. By the time the hot side has warmed to 80ºC, the cold side is at 35ºC. Without active cooling I can only get, at maximum, a 45ºC temperature gradient for a few minutes. The engine was designed to run on a 60ºC temperature difference and that 15ºC is sorely missed. Power of these engines seems to increase exponentially as the temperature gradient increases.The whole challenge of this experiment was to see if usable work could be extracted from a low temperature gradient. I must start thinking about installing water cooled tubing.

I am going ahead with the building of Jackrabbit’s second engine. The second engine’s power stroke is offset from the first engine’s by 90 degrees. The combination should supply continuous torque to the flywheel, through the entire engine cycle. I will experiment with adding a cooling coil to the second bucket. If I have success, I will retrofit the first bucket with a cooling coil.

We  are thinking of installing eight new windows in the front of the house. When I start working on that, this engine project will have to be delayed.

Pressure Tests                                                                               

The bucket was easily made air tight by placing a 2mm bead of modeling clay on its lip before sealing the lid. The allthread clamps hold the lid on securely.

The programming and interfacing of the Arduino Uno was challenging but not impossible. I found Beginning Arduino Programming by Brian Evans to be an excellent text. An Arduino C-like program gets written and uploaded from your computer, though a GUI supplied by Arduino.  The Arduino program environment consists of a global variable definition section, a setup section that is executed only once and a loop section that runs continuously thereafter.

Arduino sketch

The Arduino program is designed so it can run the engine independent of the desktop computer, but information and control is available when it is connected to the desktop graphical interface. The Arduino program keeps track of the engine timing, reads the sensors and actuates the fan and heater relays. Each time a crank sprocket blocks the timing sensor (48 times per revolution), telemetry is sent to the desktop interface;

Telemetry example : “44 15 85 010 #”

            Crank position (0-47)  zero = TDC

            Engine speed  (milli-seconds since last crank sprocket detection)

            Engine relative air pressure (PSI times 100)

            Relay values as a binary string (fan switch 1 & 2, heater switch)  

            End of data string character (#)

 I wrote a VB6 graphical interface that receives and graphs the real-time engine telemetry and also sends single character commands to the Arduino program over the connecting USB cable.  The commands are:

            “0” for stop engine and exit test mode

            “1” for start engine

            “2” for change the baud rate to 38400

            “3” for turn on fan and heater*

            “4” for turn off fan*

            “5” for turn off heater*

            “6” enter test mode

*The fan and heater timing commands are time dependent and are sent only once,  just prior to the desired change.  The change is remembered by the Arduino program and is used during subsequent cycles.

Piston pressure real-time graph.

In RUN mode, the Arduino program uses the crank angle position to control the fan and heater relays. This real-time graph is of pressure (black), speed (red) and switch values (magenta), as I manually turn the crank with no heat supplied and the valve between the piston and bucket open (connected). The changes in pressure are due solely to the pumping of the piston. There is a pressure range of about 0.6 PSI to -0.5 PSI.

Piston pressure simulation.

 It is gratifying that the graph of the real data looks remarkably similar to the pressure (blue) predicted by my engine simulation program. The simulation gives a pressure range of positive 0.77 PSI to -0.72 PSI.

Regenerator pressure real-time graph.

In TEST mode the Arduino simulates the crank movement at a rate of one sprocket position every 15 mS.  I ran the engine in TEST mode with the heater and fan cycling on and off and the valve closed to the piston (a completely closed bucket).  The change in pressure is due solely to the action of the air moving back and forth through the regenerator. Temperatures at 60ºC and 23ºC.  Pressures at +0.6 PSI and -0.3 PSI

Regenerator pressure simulation.

It is interesting that the real-time pressure curve increases and decreases linearly as the fan moves the air through the regenerator, instead of the sinusoidal curve as predicted in the engine simulation program. Simulation pressures were at +0.8 PSI and -0.6 PSI

The engine has not produced enough power to run by itself yet. I am having some very frustrating fan motor problems that interfere with the USB cable data transmissions. I keep loosing the interface when the fan runs. I must reopen the bucket to add noise filtering capacitors to the hair dryer motor before doing more tests.

Jackrabbit Engine                                                                                 

Jackrabbit Layout.

I am making an engine for the Jackrabbit out of a five gallon open-head HDPE bucket. I wanted to use five gallon steel pails but none are available locally and I cannot afford the online shipping fees. HDPE is pliable when over 100ºC and 82ºC is listed as the maximum safe working temperature. I am going to try to keep temperatures under 80ºC. Most of the heat is concentrated in the top ¼ of the bucket. In that part, I have doubled the wall thickness of the plastic and I have also reinforced the plastic lid  and bucket bottom with sheets of wood. 

Regenerator and fan in bucket.

Sitting ¾ of an inch below the lip of the bucket is a donut-like regenerator made from alternating layers of aluminum screen and nylon netting. Poking through the center of the regenerator is a three inch diameter metal tube containing an electric hairdryer motor and heating element.

 

Engine animation.

The polarity of the energized fan, determines the direction of the air blowing through the regenerator. Hot air blows though the center fan tube into the top of the regenerator and cold air comes out the bottom of the regenerator. In the bottom ¾ of the bucket, the hot air and the cold air volumes are kept separated by a plastic bag partition. The fan repeatedly fills and empties the bag with hot air within the surrounding space of cold air. The amount of air in the bucket is fixed, so the pressure of the air in the bucket increases and decreases each cycle. A flexible hose runs from the cold side of the bucket to the piston.

One engine simulation

I have not, as of yet, added water tubing for the heating and the cooling of the engine air, but I have left room for copper coils above and below the regenerator. For my initial tests, the electric heating element and the ambient air temperature should give me a temporary temperature gradient of about 60ºC.

Bucket clamped down.

A pressure test of the HDPE bucket fails to keep even one PSI of pressure from leaking out from under the lid within a few seconds. I am thinking that I can make the bucket airtight if I seal the lid with aquarium glue. Each PSI of over-pressure inside the bucket produces 94 pounds of upward force on the lid. I am using 3/8” allthread clamps to hold the lid tightly down on the bucket with about 150 pounds of force. During the vacuum part of the engine cycle, an additional 118 pounds of downward force is felt by the lid. I am hoping the hot HDPE bucket can withstand the stress.