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The mechanics of steam driven pistons

  1. Oct 25, 2011 #1
    Hi, I've recently gotten some simple plans to build a hobby kit steam engine running on 2 pistons that drives a crankshaft. It runs on saturated steam instead of superheated steam. One can attach a small generator to the end of the crankshaft to generate some power. Looking at these plans, I have a few questions:

    1) Using a standard steam volume input, if I increase the no of pistons to 4, 6, 8 etc, will that lead to an increase in horsepower because of a higher rpm of the crankshaft?

    2) If I double the volume of steam input, will that mean double the horsepower too?

    3) If I double all the physical dimensions of this 2 cylinder steam piston hobby kit, will that lead to double the horsepower too? Do I also need to double the steam input in this respect?

    My understanding of engineering is limited, so any help would be highly appreciated. Thx
  2. jcsd
  3. Oct 26, 2011 #2


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    1. If 1 cylinder gives 0.1 HP than 2 cylnders give 0.2 HP but not due to increased rpm but increased torgue - you are doubling the torque.
    2. HP = torque x rpms( rotational velocity) ( be consistant with your units though when calculaing ) Not really an answer to give here - except if you double your rpms at a given torque then HP doubles also.
    3. Doubling the piston area will double HP. since this also doubles the volume of the clinder, and agian double the torque, then steam input can be doubled at the same rpm to give double HP. Doubling the whole engine is more than doubling piston area ie A= pi r^2.

    Note that entrance and exit losses of the fluid (steam) may change the doubling. Friction losses of the cylinder and piston are also not scalable.
    1. should double , 2 ?? 3. you may get less or more than double.
  4. Oct 26, 2011 #3
    Hi, thx for the informative reply. According to the piston designs, the 2 cylinder model is supposed to output 10 hp, whereas a 6 cylinder design claims 25 hp. This is the highest rated outputs for both models running at max speed of 1500 rpm. Needless to say, the 2 cylinder design is much cheaper than the 6 cylinder design. But I have another question about sizing an alternator/generator with these units.

    I know both these unit are capable of reaching a max speed of 1500 rpm. I assume the 6 cylinder will get there faster. For a generator, does the power output also depend on the torque? For example, if I have generators that runs at rated speed of 1500 rpm, what makes one generator be able to generate more power and the other generator less power, although they both run at the same speed of 1500 rpm? Is it also the torque? Or is it the number of poles in a generator? In the above scenario, can I just get a 2 cylinder 10hp (7.5kw) model which is cheaper, and attach a 1500 rpm rated generator, increase the number of poles if necessary, so as to generate as electricity as much as the 6 cylinder unit? I am intending to use this to generate power to sell back to the grid, so things like acceleration, etc is not important. As long as it generates as much electricity as it possibly can, as we are paid by utility companies for total number of our kwh that we generate.
  5. Oct 27, 2011 #4
    Doubling the piston diameter will give 4x the area for the reason you said :). Leading to 4x the torque with consistent steam pressure. Doubling the stoke as well will give a 8x increase in swept volume.

    Power = Torque * angular velocity. (the exact equation depends on your units of choice)
    So you can increase power by spinning the engine faster or using more steam pressure (higher torque).

    It's easier to think of the engine as simply a pump (which is basically what it is), the more throughput you can get the power you have. If you want more power you need more/higher pressure steam.

    Generating your own steam will take more energy than you'll get in electrical power to sell back. Power generation benefits from economies of scale in terms of efficiency, it's why power stations are so big.
    Last edited: Oct 27, 2011
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