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Author Topic: How do I compare two internal combustion engines?  (Read 12446 times)

lyner

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How do I compare two internal combustion engines?
« Reply #25 on: 22/08/2008 22:11:57 »
Sounds OK so far. You need to make sure that the 'stroke' is consistent with the geometry of the cam, of course; the engine design needs to be 'possible'. You will, presumably, have the same swept volume for both engines.
 

Offline xengineguy

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How do I compare two internal combustion engines?
« Reply #26 on: 25/08/2008 02:17:34 »
This one is a crankshaft engine,is this correct?
 

Offline xengineguy

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How do I compare two internal combustion engines?
« Reply #27 on: 25/08/2008 04:05:21 »
The bore/stroke are the same.Posting work based on your information.
 

lyner

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How do I compare two internal combustion engines?
« Reply #28 on: 26/08/2008 11:04:13 »
The geometry looks ok so far  but I am not sure what your columns of numbers mean (I assume they are right). You have only drawn one section of the cam; The stroke will be (?) the difference between the maximum and minimum radius of the cam; the profile in between will affect how the volume changes through the cycle.
You may need to go over the whole cycle of 'your' engine and make sure that the cam radius and angles actually produce the same stroke length as the cranked engine.
Are you sure that the cam on the engine in the picture is as steep as 450? In the picture, it seems a lot less pitched.
I think that the  force normal to the cam will be the pushrod force modified by a factor of Cos(angleACB). This means is gets less and less as the cam gets steeper and needs to be factored into the calculation each time. You will get more 'leverage' but less effective force. I think that you are being a bit over optimistic about the new design and that the advantages you are looking for will, in fact, 'all come out in the wash' and prove to be marginal.
I have a feeling that, as you can't get something for nothing in this life, you will find that the only advantage of this engine is that it gives you a non-sinusoidal piston motion and the right choice of motion could improve efficiency. I am not dismissing it because that, in itself, could be a significant advantage. I just don't know enough about engine cycles to have an opinion. The problem is that, with the exception of the Beam Engine, all piston engines that I know of use a crank and the PV etc. diagrams all take that into account. (Wankel Engines can also have this feature; you may find some info about how effective it can be.)
Of course, it does give you four strokes per rev, unlike the cranked engine. You could also imagine having a different number of cam lobes / cylinders. You could also allow your induction/compression phases to have a different profile to the ignition/exhaust phases (different cam profiles each side); another possible improvement.
 

Offline xengineguy

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How do I compare two internal combustion engines?
« Reply #29 on: 26/08/2008 18:30:25 »
The columns of Numbers are A-C distance and the adjusted arm of A-B at different points on the ramp. After that the % of change.
  The prototype engine is very different than the engine in the question.The ramp in the prototype is not 45 degrees.
I don't think the ramp angle makes any difference in total work,45 degrees is just easy to use in an attempt to understand. From what I see as the angle is increased to say (60 degrees)more torque is produced but less motion. So in the end the horse power is the same. 45 degrees is just 1 to 1 .
I have from the beginning been interested in the leverage differences,from the axis to the ramp surface and how it compares to the crank engine. I think it may be a wash also but would like to prove it one way or another. My original calculations seemed to good,I guess what I'm asking is what am I missing,or how do I prove it with an equation based on the difference in leverage.
 

lyner

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How do I compare two internal combustion engines?
« Reply #30 on: 26/08/2008 18:59:22 »
When you get down to it the work done by the piston will be set of forces times a set of distances all added together. That is as much as you can possibly get out of the engine. Levers and gears etc. will still involve forces and distances which must come out more or less the same (less friction etc.) They merely present the engine with the opportunity to work 'at its best'.

I think the cam system may well do a bit better than the crank system because you can tailor things to get the best out of the combustion process. It may be possible, for instance, to make sure that all the combustion occurs with the piston near the top of its stroke so that more of the energy can be extracted. But the practicalities of the implementation may nullify this advantage.

Have fun with your sums!
 

Offline xengineguy

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How do I compare two internal combustion engines?
« Reply #31 on: 05/09/2008 18:11:27 »
Thank you for your time and responses.I understand more now than I did before. I still have a few questions,but thats the for another time. For now I will just build another prototype and do some dyno runs. This should give me the information needed to compare with other engines of similar displacements. THANKS again Mike
 

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How do I compare two internal combustion engines?
« Reply #31 on: 05/09/2008 18:11:27 »

 

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