Crankshaft Modification 6

[mindenm]

Hi all,

I am neither an engine nor automotive engineer but recently using CAD I came up with a quite simple in my opinion addition to a crankshaft. It shows in that CAD model that power and intake strokes will last for 190 degrees of the crankshaft rotation but exhaust and compression ones- for the rest 170 degrees. At the same time a connecting rod deviates for up to 3 degrees from the direction of the piston center line during high pressure in a cylinder from TDC for about 130 degrees rotation of the crank. And only at the end of the power/intake stroke and the beginning of the exhaust/compression stroke that angle goes to about 16-17 degrees.
I would appreciate opinions of engineers in the field if those features would help to improve performance of an engine and if it is advisable to try to build a prototype.
P.S. It is a much simpler addition to the crank then Honda’s EXlink.

 

[Metalguy]

What is your addition? Rod angle depends on the length of the rod.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[patprimmer]

If you are moving the big end pin location in the crank arm or the crank centre line in the block as the crank rotates by moving the main bearing bore or by moving the little end pin in the piston, you will need a VERY robust mechanism. I very much doubt you can fit anything robust enough in the space available.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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[MiketheEngineer]

Make one - see what happens

 

[BrianPetersen]

The effects sound the same as those of moving the crankshaft centerline slightly off the cylinder centerline. If that's the case, or something equivalent to it, there are already several current production applications.

 

[mindenm]

Thanks to everyone who responded for your interest.

Looks like I did not provide enough information.
I tried to work against a schematic I found on the web: a crank throw of 2" i.e 4" stroke and a connecting rod 6.1" long.
Practically 1 component is added to the crank. But the throw is smaller, Connecting Rod is 4.5" long and cylinder offset is about 5". The engine comes out looking shorter but wider.

I hope you are still interested. Then let's try again.

 

[140Airpower]

Please give the URL of the schematic.

 

[mindenm]

Hi 140Airpower
Here is URL of the original schematic I compared my idea against.

http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm

I tried to upload the schematic of my idea to ENGINEERING.com.
Hope you will be able to get it this way.
It is only a schematic even though there are some modifications.

mindenm

 

[140Airpower]

Nice diagram. I suggest you diagram the motion if you haven't already. Without seeing that I couldn't comment except to say what looks like your crank is too small for the forces.

 

[mindenm]

Hi 140Airpower
This time I'll try to show it to you in motion but again it is only an idea not a design in any way.
I hope it will help.
About the crank - it is only a diagram but I admit that quite a bit of work is needed to make it perform and that's why I asked the question and looking for any help. As I said at the beginning I have not worked with engines but know enough to look for help instead.

 

[LionelHutz]

I find it hard to believe the sliding side of that lever would last very long.

 

[ivymike]

I've actually seen that mechanism before... I think Honda had prototyped one about 10 years ago for a variable compression ratio engine.

 

[ivymike]

here's the one I was thinking of

http://www.google.com/patents/EP1496219A1?cl=en

 

[ivymike]

yours looks more like this one

http://patentimages.storage.googleapis.com/EP1143127B1/00180001.png

http://www.google.com/patents/EP114...a=X&ei=n5IGUZ20JMe3ywH5s4CgAQ&ved=0CDkQ6AEwAA

 

[140Airpower]

mindenm,
Very nice animation. I assume your measurements of stroke durations are accurate. Your rocking link has a "counter center pivot" that appears to move strictly vertically. I assume it is in a slide channel with no vertical constraints -otherwise there is further mechanism to show. This is, indeed simple. I assume the relative dimensions of everything are critical to determine the piston stroke parameters you are trying to optimize. Thorough analysis and/or prototyping can show the advantage gained vs the weight and complexity increase.
I might suggest a lateral link to locate the "counter center pivot" instead of a slide. I think it's bearings will wear better.

 

[mindenm]

140Airpower
Thanks for taking time to look into it.
Yes, there is no constrain in the guide-channel and the roller/fulcrum is moving up and down only. By manipulating dimensions of the crank's throw, arms of the rocker/lever, position of the guide and piston's offset I got the results which are presented at the beginning. That far and a little bit more I managed on my own in solidworks. I think at list a preliminary analysis of this kind of an engine should be done to decide on feasibility of building a prototype. That was the reason for my post.
If you can and would like to get involved you can find me at comcast.net

 

[kirrer]

One of my biggest concerns is with the force transmission angle during the compression stroke. See attached diagram --> in order to achieve a required force on the piston, the reaction loads through the intermediate arm will be much greater, and therefore, the torque required to compress will be much greater (green arrows represent resultant forces on conrod pin large end}. You can demonstrate this effect using what I call the principle of virtual work - neglecting friction, the work required to rotate the crankshaft one unit (say, 1 degree) is equal to the work required to move the piston that same amount. Similarly, if I know the forces on the piston and I know the kinematics of the crank mechanism, I can calculate (neglecting friction and even inertial effects for a first estimate) what the torque on the crankshaft will be. The gas forces on the piston ideally would come from a pressure trace; even a 'motored', no-load pressure trace would be helpful but to demonstrate the effect you can even assume a constant pressure (and therefore, constant force) for the compression and power stroke respectively - ie, 1000 lbf into the piston during compression and 1000 lbf into the piston during expansion. Then, the principle of virtual work works as such - Work_in = Work_out, therefore Force_piston [lbf] * change_of_stroke_piston [in] = Torque_crankshaft [lbf-in] * change_of_angle_crankshaft [radians]. Solving this equation gives you a first look at the torque required to compress the piston and the mechanical (dis)advantage that provides.

 

[140Airpower]

kirrer, It appears to me that the forces on the crank are higher approximately by the ratio of the moment arms from rod pin to fulcrum vs crank pin to fulcrum in the highest pressure phase of the piston's motion. That means he has to have that much stronger parts. Also, the side forces on the slide channel are as bad as on the crank. That is why I suggested a lateral link. Other than these concerns do you see any deal breakers here?

 

[kirrer]

140Airpower, just from the animation alone it looks critical but it's hard to tell whether it will be a dealbreaker or not. The large force on the sliding cylinder bearing certainly will generate a lot of friction and has the potential for creating too much wear; of course, as mentioned it is just a concept right now and the geometry can change if necessary. A first step (IMO) would be looking at forces based on a mechanism analysis (within CAD using virtual work principle as described earlier), or with a bit more work, developing a slider-slider-crank mechanism analysis spreadsheet for which the lengths, angles, geometry could be modified on-the-fly and 0-360 deg reaction loads at all joints could be analyzed. I did this for another project and although it's not trivial, it's also not overly complicated. To see the example I used, you can look up Norton's Design of Machinery, Chapter 11 (Dynamic Force Analysis), Section 5 --> Robert Norton takes you through the methodology for developing such a system for a simple slider-crank mechanism (as in a typical 4-stroke ICE); the same methodology could be applied to a system with more unknowns, as is the case here.

The virtual work method is quicker, especially for only one geometry, but is limited in that the user must redesign the parts, run the analysis, read the results into excel and apply the equations as I mentioned.

 

[mindenm]

Kirrer, Thank you very much for bringing to my attention this useful verifying tool. This whole mechanism has a rocker/lever which works as a wheelbarrow. The wheel is moving up and down in a narrow guide/channel and works as a fulcrum. The first attached file shows a diagram which I used for calculating forces/moments. It works in a power stroke and should as well work in a compression stroke. I built a small wooden fixture and used a bicycle torque wrench to verify it. Your force diagram does not consider a working lever. I ran the principle of virtual work analysis as you suggested using force diagram as in the attached file and the result was fine Work In = Work Out both ways. By the way, the wheel shifts sides in the channel only at TDC and BDC so that there should be no impact. I agree with both of you that the wheel and reaction forces should be carefully considered. One more thing: because a throw of the crank is smaller than 2" (in there current model) overall friction path in the mechanism is smaller but with higher friction forces. Volume change in the cylinder with this mechanism is also slower comparing to an engine with the same stroke which should influence pressure inside. I found it running my CAD model.
But I still do not have any idea what kind if results to expect from significantly reduced friction in the cylinder and 190/170 power-compression split.