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Piston Charts


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I made this a few weeks ago and figured I would post it up. Basically this shows some principles at work especially how the connecting rod angles and leverage play a key role in everything. This is only showing the thing compressing and then decomopressing, such as if no fuel was injected. I am not about to try and calculate that :stuned: These charts assume no boost, 100% volumetric efficiency, 14.7psi atmospheric, 100% sealed off rings, and all the other crap that I don't care to figure out at this point. Took enough time to do with all perfect conditions so I am not going to factor in all this other crap. The goal was to see the effect of the crank to connecting rod angle and stuff and it is clearly seen and very interesting.

This is using a 12V compression ratio of 18:1

Ideally I only needed a few charts but I kinda went all out so I'll just post them all. Remember, this is perfect world crap......and this will be obvious in a lot of charts since they are a lot higher than real world tests show.

My reference point is 0* TDC. So the cycle starts there and rotates clockwise 360*, so 180* is BDC..

Oh and I have all the specs for every big 3 diesel except the 6.7 cummins. I never found a connecting rod measurement for it. So although I only show 4, I can change them around if someone wants something else.

First is piston travel. Obvious as it sounds, it is just the distance the piston moves away from TDC (since that is my reference point). That is why 180* (BDC) is the actual stroke of each engine. Notice it isn't linear...at TDC and BDC there is very little movement in the piston as the throw just pivots the connecting rod but does very little piston movement.

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Now we have exposed volume. By this I mean the clearance volume (volume above the piston at TDC), plus the volume that the piston exposes as it moves down. Not 0 because there is always clearance volume.

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Then we have chamber temp. Just as it sounds, it is the temperature of the air in the combustion chamber. From this point on, the degrees have been swapped. The last 2 charts were 0-360 from left to right, now it is 180-180. This is because the peak is now at 0 rather than 180 and I ended up having a split peak on the sides of the graph otherwise which wasn't as nice to look at, so just be aware of that. The IAT was 100F so thats why it starts at that. The 6.0 is the same as the 5.9 because the compression ratios are the same (so the red line is also the cummins).

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Then we have chamber pressure which is obviously pressure in the combustion chamber. Notice it starts at 14.7psi because this is psia or absolute pressure. For those that don't know, boost gauges and such are gauge pressure, meaning they are zeroed at 14.7, otherwise you would have 14.7psi of boost without the truck even on and its a lot easier to just look at addition pressure rather than start at 14.7 and try and add/subtract. As far as I know, boost gauges measure the difference, so at altitude they won't be reading low or anything. Anyways just something neat. Same as before the 6.0 is the same as the 5.9 compression so red line is also the cummins.

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Jumping off of pressure in the chamber, you can calculate the actual force exerted on the piston based on the surface area, since after all, it is pounds per square INCH. Remember, this is just force exerted on the piston or basically the wrist pin, don't get ahead of me like some of you might be thinking :evilgrin: Oh and the blue and green lines are on top of each other..

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So now you're wondering how much force actually makes it to the crank. Well since it will be easier to let you scroll back and forth between the 2 pics if they are on top of each other, I will post is now and explain it after.

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Alright so you just saw that there is a LOT less force that makes it to the crank, and a weird dip in the middle. How can there be no force in the middle when there is peak force on the piston at that time? Well at TDC the piston has absolutely 0 leverage. The only way it moves is from momentum. All of the force on the piston +/- 10*ish of TDC (or BDC) is pure waste. That is why engine builders aim to get max pressure around 15* since as you can see, that is where the leverage starts to come back and the piston is able to do some actual work. In this next chart I plotted out the actual percentage of power from the piston that actually gets delivered to the crank. Notice that 100% happens around 72* (or 288* coming up on the compression stroke) as that is the point of 90* leverage. You would think that point would be half the stroke but because of the angles that the connecting rod is at, it changes to 72.

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And just for kicks, here is the actual angle that the crank throw to connecting rod actually is. Again, you can see the 90* optimum point is at 72* crank angle.

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Here is the spreadsheet if anyone wants to play with it. Cell F1 is a drop down so you can change between 0-360 or 180-180 because of the thing with the first 2 charts.

Piston Acceleration 03.xls

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Yeah it really is amazing to see how much power is lost simply from the connecting rod angle. The rest is kinda boring. I want to make one showing all the pistons since this isn't exactly apples to apples having big cummins pistons vs. v8 ones since they make up for it with 2 more.

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