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Efficiency Losses in a Reciprocating Engine

  1. Oct 3, 2006 #1
    We are developing an engine with pistons and cylinders that do not reciprocate... in other words the inertia of the pistons is constant at a given engine speed. More on that latter if you wish.

    We are looking for a method to calculate the efficiency losses in a conventional internal combustion reciprocating engine due to the starting and stopping of the pistons and piston rods (inertial changes) during each revolution of the crank shaft.

    Any ideas?
     
  2. jcsd
  3. Oct 3, 2006 #2

    brewnog

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    The causes of inefficiencies in reciprocating engines are as follows:

    Pumping losses (induction and exhaust)
    Frictional losses (in bores, bearings, and ancilliaries)
    Thermal losses to exhaust, coolant (jacket, charge and oil), and surroundings

    There are no losses sustained directly due to 'starting' and 'stopping' the pistons. Neglecting the above inefficiencies would allow all this inertia to be taken from the flywheel.

    Have I understood your question correctly?

    Also, I'd be interested in hearing more of your idea of pistons which do not reciprocate!
     
  4. Oct 9, 2006 #3
    Are you sure?

    Moving any mass from a static state to a kinetic state requires an input of energy. Stopping that mass requires an equivalent amount of energy (ignoring all losses such as friction).

    In a reciprocating engine, the energy required to start the piston moving from "top dead center" immediately after combustion is energy not applied to the flywheel. Likewise, the energy required to stop the piston at the bottom of its stroke is energy derived from the kinetic energy of the crankshaft and flywheel. There must be ineficiencies involved here compared to, say, a purely rotary engine. We want to, at least, estimate what those inefficiencies equate to.

    If you'd like, I can send you a motion diagram of our engine concept that is currently being built as a prototype. However, the file is too big to send as an attachment in this forum. I'll need your email address.

    Regards, Tolojim
     
  5. Oct 9, 2006 #4

    russ_watters

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    Yes.
    Correct.
    Correct.
    Also correct.

    What you are missing is that that energy is exactly what the flywheel is looking for! That's the energy that turns the crankshaft. The kinetic energy of the flywheel increases while the kinetic energy of the piston decreases.
    Nope. In essence, the energy is put in at combustion and taken out at the crankshaft by the act of opposing the motion of the piston.
    Well, we can certainly help with the concept of your engine, but why can't you just post a jpg? In any case, because of the above misunderstanding, I'd be concerned that you are missing something with the engine concept you are working on.
     
    Last edited: Oct 9, 2006
  6. Oct 9, 2006 #5

    russ_watters

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    Indeed, as that is a self-contradiction!
     
  7. Oct 10, 2006 #6
    Hmmm,
    OK, then. Why are light weight pistons more efficient than heavy pistons?
     
  8. Oct 10, 2006 #7

    Bystander

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    They aren't. Look at marine diesels. Light pistons accelerate more rapidly for the racing crowd, but are no more, or less, efficient at transmitting the energy of expanding gas to the crankshaft.
     
  9. Oct 10, 2006 #8

    wolram

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    There maybe some very small loss due to crank deflextion??? other than that i can not think of any thing.
     
  10. Oct 10, 2006 #9
    You guys aren't half bad.
    Nonetheless, we continue our development.
    I am posting a video diagram of our engine on the internet for the curious (too large to post here). Standby. It won't be long
     
  11. Oct 11, 2006 #10

    brewnog

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    Has everyone gone slightly potty today?!


    Not only do lightweight pistons and conrods reduce the rotating inertia of the engine (like with a lightened flywheel, allowing faster acceleration), but they can decrease vibrations due to less imbalance.

    They in no way whatsoever improve efficiency.

    Looking forward to seeing the video of this engine!
     
  12. Oct 30, 2006 #11
    OK, everyone. All engineers here now agree with the above feedback regarding no efficiency losses related to a piston's changing inertia in conventional reciprocating engines. Thank you for your input. But as you'll see, a piston that does not reciprocate is not a "self contradiction".

    Just a quick, down and dirty website to view what we've been talking about here.

    Please go to www.circlecycleengine.com

    Any and all comments are welcome. See any muck in this brass?
     
  13. Oct 30, 2006 #12

    Bystander

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    Reciprocating pistons and reciprocating cylinders --- you have "one-upped" the WW I Gnome rotary.
     
  14. Oct 30, 2006 #13

    Mech_Engineer

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    While it's a very interesting idea, there are several problems with the overall idea that I see... Most of all, I don't really think you have removed "reciprocation" in its most general sense from the engine, you've added more! But, that's neither here nor there... from what I see in the animation, some BIG problems with the idea:

    1) Lubrication. Not only will it be very difficult to lubricate all of the parts, it will be nearly impossible to lubricate the cylinder without burning off all of your lube each stroke. If you thought 2-stokes or wankels burned a lot of oil, you ain't seen nothing yet. And don't forget, burning the oil will kill your emissions. This engine would be one gas and oil slingin' machine.

    2) Compression: It will be very difficult to achieve a good seal with a piston that is completely removed from the cylinder on EACH stroke, and any non-metallic materials these are made out of will have generally poor wear characteristics. This is a problem for people trying to make ceramic reciprocating engines. Also, friction on the seal around the piston from constantly exiting/entering the cylinder will severly increase wear, obviously degrading engine life significantly.

    3) Emissions Control: You will not be able to even marginally separate intake and exhaust gases (since they are mixed in the same area), making intake of clean air for combustion nearly impossible, and emissions will be horrible since a catylitic converter will not be possible. It's stated that the cylinders will be completely air cooled, which means they will need large heatsinks on them, making them far larger than presented in the animation.

    4) Sound deadening: If you ever get the engine to run it will sound like a shotgun going off each time a cylinder finishes its power stroke, as you have no possiblity of muffling short of encasing the entire engine. Encasing the entire engine to add sound deadening would add significant weight and size to the design, as well as make it even more unlikely you will be able to get clean air into the engine each stroke, and destroy any chances of your "efficient" air cooling.

    5) Control: How do you plan to attach fuel lines or plug wires to a cylinder which is constantly rotating? The plugs, wires, and fuel injectors will have to spin WITH the cylinder, increasing rotating mass. I don't think there are any fuel lines or plug wires that would be able to withstand the centripetal accelerations of a couple of thousand rpm's while remaining functional and lasting for many hundreds of hours of operation.

    6) Size/Cost/Weight: The added structure that would be required to hold this thing together with the strength needed would be large and heavy, unless made out of more expensive materials. In fact, the total size of this engine would easily surpass a comparable IC engine.

    That's just off the top of my head, I'm sure I'll think of many more problems later on this evening. This is a very unique and innovative design, but I think if you want to remove reciprocation from the automotive engine, you need to develop a turbine engine.
     
    Last edited: Oct 30, 2006
  15. Oct 30, 2006 #14

    russ_watters

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    Well, technically speaking, I guess they don't "reciprocate", they "revolve", which essentially is reciprocating in two dimensions instead of one!
    http://dictionary.reference.com/browse/revolve
    http://dictionary.reference.com/browse/reciprocate

    Surely you must be able to see that if you let your animation run twice, the piston that started in the middle is now directly opposite where it started in the same axis. In fact, if you were to graph that motion along the x-axis, you'd find it to be exactly the same as the motion of a regular piston. As far as the piston knows, you haven't changed anything - you just made your camshaft bigger and put the piston inside it! (*caveat)

    But, as Bystander hinted....you've changed a lot for the cylinder....

    *The net result is that your power is now just split between two reciprocating parts, with the camshaft outside of the reciprocating/revolving piston and clylinder.

    Clearly, the pistons and now cylinders (and the whole camshafts) are accelerating and decelerating: they are accelerated by the push of the combustion and they are decelerated by the compression stroke - just like a regular engine. Now, you've got a lot of mass there, so you probably wouldn't need a flywheel anymore, but you would need to oppose several of these things to avoid vibrating the crap out of your engine (like a one cylinder engine vibrates).

    edit: Also, while you might think your scheme "softens" the acceleration, you're comparing one cylinder to four: your scheme has exactly the same x and y axis forces as a radial 4-clyinder engine with the cylinders 90 degrees out of phase with each other. Except, of course, in a radial 4cylinder engine, the cylinders fire twice as often as yours do.

    Here's a great animation of resultant vibration forces in an engine, which helps visualize what is going on. A radial 4 cylinder works pretty much the same as two 180 degree 2-cylinder engines at 90 degrees from each other (ie, perfectly balanced). http://www.dinamoto.it/DINAMOTO/on-line papers/twin motors/twin.html
     
    Last edited: Oct 30, 2006
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