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From blog:

Jonathan short | fatfuck@sover.net | IP: 216.114.136.157

I read what you wrote about your engine ideas with interest. I agree with you that the possibility for integrating electronic valve timing with a traditional steam engine. When I looked into the most obvious solution, solenoid valves such as you suggested, I found that the requirements for the solenoid are not trivial. If you pursue this, you will want to look closely at the performance of the solenoid, both in terms of duty cycle and actuation time. The results suggest that you need a very powerful, very fast solenoid with nearly 100% duty cycle. This is not inexpensive. I tried reducing the weight of the valve to improve response time by actuating a very light pilot valve, but the problem is that the iron in the solenoid plunger has a lot of mass. You can increase the power of the solenoid by increasing the magnetic field, but greater the magnetic field, the longer you need to apply a given current to generate the field, thus the slower the response time.

The problem is not insoluble. I know that the automotive industry is looking at solenoid valving, which is an even more demanding application. But they are looking at a highly engineered solution. I do seem to spend a lot of time dishing out bad news in alternative energy circles, but I also wanted to let you know that if you wish to pursue your steam engine (and I hope you do), you will need greatly reduce your efficiency expectation below the 19% you quoted. I think you are about four times too optimistic. The sort of engine that you sketched can be expected to produce about 5% efficiency assuming a reasonably efficient boiler. You quoted the Skinner unaflow engine at 19% peak efficiency. Taking that as a benchmark, here are the ways in which an engine such as yours falls short. Ignoring boiler efficiency as outside the scope of this discussion:

1. The Skinner used pressures around 800 psi, much higher than the 100-200 that is common for most smaller setups.
2. It operated at several hundred degrees of superheat, which would be difficult to achieve without expensive controls and high alloy superheater tubes.
3. It was Unaflow and condensing. Unaflow is a great way to go, but if you are not operating condensing at a low condenser pressure, unaflow operation requires complex auxiliary exhaust valve which opens only after the unaflow ports are opened. In any case, a non-condensing engine will be less efficient than a condensing engine. One advantage that marine engines had is that they were sitting in a body of water and could thus easily cool their condensers and keep them very low pressure. If you don’t use unaflow valving then compounding becomes VERY important for efficiency.
4. your engine is smaller and therefore you will have less mechanical and thermal efficiency.
5. You do not jacket the cylinder and head with high pressure steam. Again, more condensation losses.
6. Unless you have a very constant load, you cannot obtain good efficiencies without variable cutoff (varying the expansion ratio to accommodate different loads). One of the advantages of electronic valving is that you can more easily implement variable cutoff, but your electronics would be more complex and you would need some form of sensing for the electronics to determine the right cutoff.

Those are the major efficiency factors that you would want to consider if you want to achieve better than 5% efficiency. As you can see, there is, as always, a tradeoff between efficiency and manufacturability.

Good luck with your work. I look forward to hearing more about it in the future.

P.S. This comment has been floating around my hard drive for a long time. I first started writing it when I read your initial posting. I see that my comments about the performance requirements of solenoid valves have already been discussed. Have you found something satisfactory?

Jonathan Herz

May 1, 7:45 PM — [ Edit | Delete | Unapprove | Approve | Spam ] — Steam Age meets the Digital Age: Open Source Steam Engine