T.O.O., I'm calling you out on this one!

Posted by Sentinel on June 15, 1998 at 11:42:57:

On the 13th, on the engine geometry post you wrote "Honda uses a small bore and when coupled with a stroke that's
not terribly long, the rod angle is still "severe" but not as bad as it'd be if the piston were larger in diameter."

Umm, how is the piston diameter significant to the rod angle? The piston will have linear movement, colinear to the crank centerline. I see how the stroke and rod length come into play, with the severest angle with the rod journal perpendicular to the piston pin-crank journal cl, but the rod will be at the same angle no matter what the piston diameter. I can see where a bigger piston will allow a beefier rod, but not any geometric effect. What am I missing?

While I've got you on the line, I also have a short turn radius question! I'm picturing a side-view drawing of an intake port. The short turn radius will had a "focus", where if the radius was continued to make a full circle, the focus would be the geometric center of the radius.

Is a perfect arc what is needed, or is it even wanted? If so, by reshaping the radius I see how the focus will move - is there a prefered position for the focus? Or am on a dead-end on this?



Re: T.O.O., I'm calling you out on this one!

Posted by T.O.O. on June 15, 1998 at 16:27:46:
In Reply to: T.O.O., I'm calling you out on this one! posted by body on June 15, 1998 at 11:42:57:

The larger bore engine will have greater cylinder wall stress due to the distance from the pin center to the cylinder wall. I realize that this may be hard to understand, and harder to explain, as you're geting in the area where geometry and energy equations begin to mix.
If you build two engines of identical displacement. One is a small bore long stroke, the other is a large bore small stroke engine, and the rod length to stroke is equal= piston speed is the same, why will the long stroke small bore engine always demand greater port volume and flow rates? Remember that the displacement is equal at every degree of rotation.
Give me answers for both and I'll buy you a Coke, and I'll be considerably smarter.
...................................T.O.O. ............................................

So, is the rod angle "severe" or the side-load?

Posted by Sentinel on June 16, 1998 at 07:07:53:
In Reply to: Re: T.O.O., I'm calling you out on this one! posted by body on June 15, 1998 at 16:27:46:

What actually confused me, was that you said a larger bore would create a more "severe" angle. And it's pretty obvious the actual angle is based solely on the rod length and the stroke. If you meant that a larger piston might have more piston rock, o.k., I can see that.

As for your question, speed=distance over time, so which ever motor has the longer stroke (you said displacements are the same) will have greater piston speed.

The Art of Bullshit

Posted by andre on June 15, 1998 at 20:09:03:
In Reply to: Re: T.O.O., I'm calling you out on this one! posted by body on June 15, 1998 at 16:27:46:

I b.s.'d my way out of a lot of these kinds of questions in high school and college....let's see if I've still got the touch :).

Let's use the usual "take it to the limit" approach and analyze the extremes. Suppose you have a hugely oversquare piston (say 100 to 1 ratio of bore:stroke) with rod:stroke equal to another hugely undersquare piston with the same rod:stroke and the same displacement.

What's the same? As you said, the displacement at every crank angle is the same.

What's different?

I did some calculations that show that the piston speed IS different. Did I do something wrong? I also tried a variety of gedanken experiments and even when you keep the bore:stroke square and the rod:stroke constant, different absolute sizes yield different piston speeds...perhaps I misunderstood the post. Anyway,...

The size of the valves will probably be different but let's assume that doesn't matter for now.

The amount of work that's done to the gas by the atmosphere is the same in both cases since we could imagine an invisible balloon surrounding the intake charge. Its surface area is the same for both engines and atmospheric pressure is the same. Thus, both intake charges gain the same amount of energy. This energy can become a. kinetic energy as captured in the hopefully laminar flow of the fluid or b. heat energy because of internal friction of the fluid. Since the average distance a molecule in the long stroke small bore engine has to travel is longer than that of the short stroke large bore engine, the average velocity must be higher too (since the time of travel is the same in both cases at the same rpm). Thus, the intake charge needs to have higher average velocity when it enters the combustion chamber which explains why you would need better flow to achieve the same volumetric efficiency (and thus horsepower under conditions of ideal fuel atomization and combustion).

a good thought experiment for me was to imagine a timpani and a rifle barrel filled with air and then having one end opened in space. given the same volume it seems pretty intuitive that in the case of the rifle barrel, the atoms of the gas travel farther while being coerced into a pseudo-laminar flow by the walls of the rifle barrel and the pressure from behind it (as it flows out of the tip of the barrel), whereas in the case of the timpani, the pressure of the gas is wasted and never converted to a more useful form of energy like kinetic energy.

but like I said, just a bunch of b.s....


Re: You Da Man...................

Posted by T.O.O. on June 15, 1998 at 21:19:20:
In Reply to: The Art of Bullshit posted by body on June 15, 1998 at 20:09:03:

You have seriously managed to grab at several fixed assumptions and using your self professed art form, distorted each and every "component", and your conclusion is correct.
This is a real eye opener for me. You can teach an old dog new tricks. I've always said that the day you don't learn something, you better pack it in. I've been trying to explain this phenomenom for almost twenty years, and you have no idea how much I appreciate the understanding I've gained tonight. I'll be anxious to talk to the many people in the industry who've attempted to figure out why the small bore/long stroke requires greater port volume. The mixture has farther to travel....sometimes things are hard to see when they're too close to you, especially with "old" eyes.
....................................T.O.O. .........................................

Re: T.O.O., I'm calling you out on this one!

Posted by The Demon on June 15, 1998 at 11:58:50:
In Reply to: T.O.O., I'm calling you out on this one! posted by body on June 15, 1998 at 11:42:57:

Hope no one minds if I venture a guess here, but I'd assume that a larger piston diamater affords more clearance for the rod when it is at extreme angles....whereas a smaller diameter piston will only allow the rod to move so much side -to-side. Of course, that may not be TOO's reason, but I'm sure that that clearance does play a vital role.

The Demon