Intellectual engine questionnnnn...

[LEANE30]

Sounds like you're way too smart for the rest of us.



Well, actually it does a little bit, since it will alter the piston's stroke pattern (smaller rod ratio will make the piston dwell more at tdc/bdc); however, that's not what I said.



:rofl:

I stand by my statement. Rod length and thus rod ratio has no effect on your definition of dwell, only crank stroke length and rotational velocity calculate your dwell duration. Please do not pretend to know what you are talking about while propagating misinformation to the community. Maybe your post count is reflecting the quantity yet not your contributing quality. I have learned a lot from the posts for many respectable members over the years, and I am sure there are many who actively read to enrich their knowledge through the posts of wiser members, which hopefully is quite obvious to all that you are not.

[kronus]

can we agree on what we think dwell is before descending into ad hominem?

[pantelones]

Man you guys are dumb. Even when the motor isn't running it is moving..... Hahaha Lets see if anyone can figure it out? It has to do with sail boats and god. Again LOL

Anyways, this pistion, rod, crank, or what ever you are pondering about is moving at all times. Even if it is a .00000001", it is moving. I will not go through the mathematical calculations to prove this, but I understand it in my head and that is all that counts :)

[pantelones]

Never mind, easy way to describe this...

Think of two circles on a right angle, and a dot that is rotating at the same speed, in the same direction on both circles. One will appear to be making in a circle, and one will be moving up and down. So I guess it depends on what circle you are looking at :) Again going back the idea of perspective. Which way do you want to look at it?

[Wh33lhop]

I stand by my statement. Rod length and thus rod ratio has no effect on your definition of dwell, only crank stroke length and rotational velocity calculate your dwell duration. Please do not pretend to know what you are talking about while propagating misinformation to the community. Maybe your post count is reflecting the quantity yet not your contributing quality. I have learned a lot from the posts for many respectable members over the years, and I am sure there are many who actively read to enrich their knowledge through the posts of wiser members, which hopefully is quite obvious to all that you are not.

Calm down.

You are thinking of a simplified version of an engine, where the rod's vertical length is the same throughout the entire stroke and the piston moves in a perfect sinusoid. This is a good approximation for most purposes but it's not entirely accurate. In fact, looking at my previous post I was only partially correct, as it effects differently at TDC and BDC.

Forgive me if this is a bit unclear, I'm writing on a shitty old iphone with no diagrams to assist me.

Anyway, in reality, the rod is only straight up and down at TDC and BDC. Elsewhere it's at a diagonal, which effectively "shortens" the rod verically. So if you have a crank with 100mm stroke and a 150mm rod, let's look at the stroke pattern.

Let's call the piston position at BDC 0, and the position at TDC 100. Where is the piston when the crank is in the middle of the stroke, at 90 degrees? It will be at 50, right?

Well, not quite.

The crank is at 90 degrees, which means the bottom of the rod is offset 50mm from the centerline of the engine. The rod is no longer straight up and down, it's now tilted 18.5 degrees in the diagonal. This shortens the rod's effective length to 142.2mm, which puts the piston at vertical position 42.2, as opposed to the 50 you were probably expecting. What this means is, the piston will spend less time in the upper half of the stroke than the lower half, and will dwell more around BDC than TDC. A longer rod (or a shorter stroke) will tilt less and give less of this effect.

[Ryann]

I'm going edit my original statement.

If there was zero play in the rod bearings, wrist pin, and piston to cylinder clearance AND you had really accurate measuring devices..

Ha! That's funny. I've observed what you said in your first post myself, and after pondering it decided it must be the oil-less rod bearings.

[CHIF8008]

our eyes deceive us. its like watching a wheel spin- at first its going forward, but then it appears to start to rotate the other way and so on and so forth

[Bimmerman325i]

There are two types of people in this thread, those who understand the linear and angular equations of motion and how x(t), v(t), a(t), and j(t) are related, and those who don't.

The answer's been said a few times. YES, the piston momentarily stops at TDC and BDC. Linear movement momentarily stops, velocity is zero with nonzero slope thus accel isn't zero, thus force isn't zero. Modelling a sin/cos/-sin relationship, for those who understand them and derivatives, explains all of this. Real-world will introduce some variation in the answer, but YES the piston still stops momentarily at TDC/BDC even if it's tilted somewhat due to wobble/tolerances. No, it doesn't matter a whit how the wobble or tilt happens.

This problem is MUCH easier to mentally grasp from a momentum refrence frame. At TDC and BDC, the linear momentum of the piston goes to zero and experiences a sign change. Throughout, the angular momentum of the crank is unchanged, which causes the sign change at T/BDC.

Actually, best way to describe this, for folks who have been on the amusement park rides that shoot you up vertically. When you reach the top of the ride, you are momentarily stopped, but there are still forces acting on you, yes? AKA, your stomach is trying to escape your mouth? The ride platform/chairs/tracks/wheels are still wobbling a bit side/side/etc, yes? You just did an analog to the BDC-TDC-BDC journey, just you have rotation of the crank rather than hydraulics and gravity causing the sinusoidal motion.

[Wh33lhop]

Calm down.

You are thinking of a simplified version of an engine, where the rod's vertical length is the same throughout the entire stroke and the piston moves in a perfect sinusoid. This is a good approximation for most purposes but it's not entirely accurate. In fact, looking at my previous post I was only partially correct, as it effects differently at TDC and BDC.

Forgive me if this is a bit unclear, I'm writing on a shitty old iphone with no diagrams to assist me.

Anyway, in reality, the rod is only straight up and down at TDC and BDC. Elsewhere it's at a diagonal, which effectively "shortens" the rod verically. So if you have a crank with 100mm stroke and a 150mm rod, let's look at the stroke pattern.

Let's call the piston position at BDC 0, and the position at TDC 100. Where is the piston when the crank is in the middle of the stroke, at 90 degrees? It will be at 50, right?

Well, not quite.

The crank is at 90 degrees, which means the bottom of the rod is offset 50mm from the centerline of the engine. The rod is no longer straight up and down, it's now tilted 18.5 degrees in the diagonal. This shortens the rod's effective length to 142.2mm, which puts the piston at vertical position 42.2, as opposed to the 50 you were probably expecting. What this means is, the piston will spend less time in the upper half of the stroke than the lower half, and will dwell more around BDC than TDC. A longer rod (or a shorter stroke) will tilt less and give less of this effect.

I just found a nifty wikipedia page with a diagram that illustrates this better. Also pictured are that the piston acceleration is not necessarily the greatest at BDC depending on the rod ratio of the motor:

[quickervicar]

That IS nifty!!

[z31maniac]

Oh god. So much lulz in this thread.

Keep it up kids.

[fronton]

Mathematically it never even gets to TDC or BDC, it just gets infinitely closer to that point.

[unevolved]

[nando]

I have an excel spreadsheet somewhere that models piston dwell and acceleration based on rod length. it's interesting to experiment with.

it is pretty amazing how many people are completely wrong in this thread, and even some of the people I'm very surprised at..

[LEANE30]

That is a very nice graph. Lol! I do retract my statement. Well played sir. Nando, do you care to share said spreadsheet? I apparently need to do some more reading and less posting !