Alternative & Renewable Energy Blog

Having proven that my engine design was practical (and actually worked), the next step was to test its power output in order to determine engine efficiency. I did not have access to or the ability to build a boiler, so my only option was to run it on compressed air as a power source, and measure its power output with a DC electric motor hooked up as a dynamo.

Once again returning to the reliable scrap bin, I scrounged up some aluminum rails to use as motor and dynamo mounts. I also acquired a spare flexible coupling from another team working in the shop. The coup de grace came when I was loaned a brushed DC motor as my generator, and the use of a large electrical resistive load to test its output.

Before beginning, I measured the line pressure and flow rate of the compressor line in the shop that would serve as my power supply. This came out to about 100 psi (7 bar) and a 0.972 liters per second flow rate. Using the equation

Watts = [(V*100*ln(P))]

Where V is flow rate and P is pressure in bar, I calculated the total available power. (Approximately 189.14 Watt hours)

Next, I ran the engine and measured the output voltage and amperage at various resistances across the load bank to create a rough power curve, and compared the maximum value against the input power. The curve peaked at 48 watt-hours, which gave an approximate energy efficiency of 25.4%. This is only half the equation, however, since it does not take the Carnot efficiency of steam into account.

Because I had not included provisions for measuring the exhaust pressure of the air, I assumed that maximum energy was used and the steam exhausted out at ambient pressure. (Note: I know this is not the case, but this is only for rough estimate purposes.)

Carnot Efficiency = 1 – [(T out)/(T in)]

Assuming that the temperatures would equal those required for the input pressure (165 Celsius) and the output pressure (100 Celsius), I calculated an efficiency of 39.4%. Multiplying this by the measured energy efficiency of the engine gave me a decent estimate of the system's total efficiency.

By chance, I found a discarded AC motor in the recycle bin after I had returned the DC motor to its proper owner. Although the AC motor doesn't work yet, I had already completed my measurements and mounted it purely for display purposes. Around this time, I was getting ready to return to America. So I paid the extortionist prices to ship my parts home and prepared to embark on part 2 of the project: ironing out all the problems and proving its practicality.

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Last time, I described the design and construction of the cylinder and crankcase for the steam engine. This time, I'll be wrapping up the basic design with the cylinder head and cam drive. Without getting into too much detail (I may yet try to patent this thing), the valve alternately allows steam into the cylinder and vents it out again during the upstroke.

Combined with the cylinder exhaust ports, this configuration is referred to as a semi-uniflow engine. If I did not vent the upstroke and allowed it to recompress, which would maintain the temperature gradient of hotter at the top and lower at the bottom, then it would be a true uniflow engine.

I felt, however, that the loss of thermodynamic efficiency was acceptable if it reduced the risk of hydro-locking the engine (where a liquid, which is basically incompressible, enters the cylinder and stalls it with potentially damaging results). I would rather have it work with a high degree of reliability than be highly efficient, but prone to breakdowns.

The Cylinder Head

The cylinder head was the most difficult part of the engine to design, and went through multiple revisions until I was satisfied with the result. Yet the first design still failed miserably, which prompted further redesigns. All told, I spent at least a month just sketching different configurations before I came up with one robust enough to work (although it was much more difficult to machine). It would be another four months until I came up with a design that was both robust AND simple to make, but I will discuss that during a later installment.

The Cam Drive

The final part to build for the motor was the cam drive. Given my lack of money and the large number of abandoned bicycles on campus, I was able to repurpose two of the rear sprockets from a cannibalized 18-speed. As luck would have it, the largest and smallest sprockets on the rear wheel were a perfect 2:1 ratio. I made up some adapters to attach the two, cut an old chain to length, and finished assembling and timing the engine. (Note: I do not recommend doing this for a cam drive. It is insanely dangerous and WILL take a finger off if you get caught).

Amazingly, the motor ran under its own power on compressed air after only 15 minutes of fiddling. After testing it and making sure it worked, the next step was to build a test stand.

Follow this link to see the motor in action.

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The preceding article is a "sneak peek" from Alternative & Renewable Energy, a newsletter from GlobalSpec. To stay up-to-date and informed on industry trends, products, and technologies, subscribe to Alternative & Renewable Energy today.

The next part of the project was to design a uniflow-style cylinder for the engine. This design bears many similarities to a two-stroke cylinder, especially in the sense that the exhaust port is machined into the cylinder wall itself. But I couldn't use a cylinder from a two-stroke engine for a variety of reasons. First, because I had no need for intake or transfer ports, a two-stroke cylinder would have been unnecessarily complicated for my purposes. Also, according to period steam literature I've read, the optimal exhaust timing for a uniflow engine is 15% above bottom dead center (BDC) of a piston's stroke.

Because of the radically different nature of internal combustion (IC) engine physics, the ports are much larger and open much earlier in the stroke than would be efficient for the project. Assuming that the piston skirt covers the port when at top dead center (TDC) to prevent excess steam in the crankcase, the only consideration is maximizing the size of the port while maintaining the exhaust timing. In the case of this engine, I was able to machine four ports with a 6-mm diameter into the cylinder.

Of course, the only option was to once again make the parts myself. Cast iron is still the gold standard for cylinder liners, so I attempted to find a source. (I know I'll receive comments telling me all about coated aluminum liners, but bear with me - I'm a college student with basically no budget.). A funny thing about Denmark, though, is there aren't a whole lot of cast iron pipes lying around. I even tried going to the on-campus foundry, but they required me to make my own molds, specify the exact alloy desired (I had no idea), and wait for them to actually perform the production run.

Instead, I decided to use a piece of carbon steel, machine it myself, and have it honed by the campus shop since it was a prototype and longevity wasn't an issue at that point. For future designs, I would prefer something less prone to oxidation, such as a Nikasil lining or stainless steel. After machining the liner, I shrank-fit it into an aluminum cylinder to reduce the weight and provide a place to run the mounting studs through. In my next blog entry, I will describe the creation of the cylinder head and timing chain.