bore and stroke

[quantexrox]

I know that what you say is logical, but the results seem to tell a different story. I know the heads differ quite alot. Also the BMW has 8v and the ford has 16v.
i have chosen engines with equivalent power to be as close as possible. other 1.6 engine seem to have similar stats. It's difficult to obtain clear results since we do not have engine production plants.

A stroked engine with identical heads are the 2.0l and 2.2l engines in the honda S2000. the 2.2 produces a little more torque, but is detuned to reduce max power rpm and that also increases torque. what's your opinion?

It looks like they increased the stroke (and consequently, lowered the max RPM,) to make a bit more low end torque probably to better comply with emissions standards. The more low end torque makes up for the loss of max RPM (engines are typically much more responsive at higher RPM's, especially the type used in the s2000,) so they can meet tightening emissions and not lose too much of the feel of the engine.

[darkjedi]

That may be the case. I saw that alot of people prefer the 2.0 version.

To me it seems the torque came from the higher capacity, but milder cams were used to lower the power band and increase low end torque further

[Bishop]

I think it has to do more with size than anything. diesels can operate oversquare, but will not work efficiently. diesel burns slow so it needs a long stroke to generate the necessary torque. also the pistons are much longer and needspace.
I used to belive the same about stroke on petrols, but after more studies i came to the conclussion that they are not related. maybe with carbureted engines, but newer ones seem to be unaffected.

diesels also operate at a much higher compression level than petrol engines and because of the slower burn they need the longer stroke to help take advantage of all the engergy.

[darkjedi]

that's exactly what i said.

[quantexrox]

That may be the case. I saw that alot of people prefer the 2.0 version.

To me it seems the torque came from the higher capacity, but milder cams were used to lower the power band and increase low end torque further

Peak torque is where the engine is basically at it's most efficient. The reason it drops off is that the amount of air going into and out of the engine starts to be too much for the intake runners/valves/exhaust manifold and they can't keep up. Think of it like trying to run a marathon breathing through a snorkel. At first, you are not tired and can breath pretty easily through it, but once you start getting tired and start huffing and puffing, you have a hard time sucking more air through the snorkel.

The air in intake runners moves through in waves, when the intake valve is open the piston moving down on the intake stroke sucks air in through the intake valve. This pulls air through the intake runners, when the valve closes the air is still moving through the intake runner due to lots of physics things, and hits the closed valve and creates a small pressurized area. The more air behind it, the longer it will hold that pressure in that area (think of it like waiting in a line, if someone moves forwards and stops, the person behind them runs into the back of them, and this lasts longer if you have more people in line). If you have long runners, there is lots of air behind it and the pressurized area sits there for a while and continues to build due to the air further down the runners continuing to push it's way in because of momentum until the valve opens again and makes a charging effect. If you're runners are too short, or engine running too slowly, this wave springs back and starts moving back out of the runners and into the intake plenum/intake piping/atmosphere. This means the intake runners are tuned to charge best above a certain rpm (the longer or thinner the runners, the lower the RPM,). The charging effect is great at low RPM's, but once you're speeding up the valve is opening much faster now and that pressure wave doesn't have enough time to build up as much, until the effect is gone at high engine speeds, and will dramatically decrease performance if you continue to increase engine speed. Make the runners shorter and bigger in diameter, and then it needs a lot more sucking to get that pressure built up. Once you're at high RPM's, you have the pistons sucking in enough air that the air is moving with enough velocity through the intake runners to start building up a charge.

Example:

Two identical engines, same length intake runners, one with 1" diameter runners, one with 2" diameter runners.

The 1" diameter engine is pulling the same amount of air through the intake runners at 2000rpm as the 2" diameter engine, but the air being pulled through the 1" diameter runners is a higher pressure as it has to rush through much faster. This creates that charging effect at low speeds where the size of the runner can keep up with the demands of the engine and increases power at low RPM's. Now at 5000rpm, the 1" diameter runners can't keep up and a vacuum starts to form at certain areas and the charging effect is gone. The engines computer will now inject less fuel to keep the air to fuel ratio the same and as you can imagine it falls on it's face performance wise (if you have a carb, it starts to run rich). If we look at the 2" diameter engine, the vastly larger runners have more potential to allow a lot more air to pass through, which is now moving though enough air at 5000rpm to start to get that charging effect. Now take that longer, 2" diameter runners engine and shorten them, now the charging effect doesn't happen until even higher RPM's. Excellent for racing conditions, not so much for drivability, fuel economy, emissions.

That is how intake runners effect how an engine breaths. Also, turbo/superchargers basically multiply this effect. Cam profiles change when, how long, and how big valves open. Increase the amount of time a valve is open and suck more air in, this means it will take longer for that charge to build up in the intake runners, but will increase HP/torque at top end. To increase your peak torque and lower the RPM it occurs at to a lower RPM using the same intake runners and head, you need to increase your stroke. That is where I was going with that, but it kind of went off in a tangent.

Sorry if this seems to be going off on a tangent, I really love talking about the physics (and once you get me started...) and such of how engines work and hope to one day design engines as a job. I mean no disrespect and am not trying to deliberately confuse anyone or anything like that.

Edit: Grammar, some physics that clears some stuff up

Edit2: Added some more things about pressure waves in the intake runners

[Daffyflyer]

^All that stuff is quite correct.

Also note that long stroke engines are typically tuned for good low RPM power, reason being that they don't survive well at high RPM due to high piston speed, so that might provide the illusion that stroke has an impact on peak torque RPM.


And indeed larger bore does allow more valve area and more power, an effect that isn't currently modeled but soon will be (next update). One reason to run longer stroke is that it doesn't make the engine much physically larger, and also in the UK and some other places, cars are taxed on bore, not capacity, hence why long stroke British engines are so common.

[Moth]

I'm aligning with the longer stroke helps make more torque side. A longer stroke is of course not the only, and not the largest influencing factor; but I believe it is one. Without even going into theories; if you look at most engines produced; it holds true. It's not a massive difference, but there is one. Generally undersquare motors produce about 10 ft-lbs of torque more than their horsepower rating, and oversquare motors produce about 10 horsepower more than their torque rating. 10 is just a rough number, but it's about the average for what I'm talking about. Of course there are exceptions; as with anything in the world.

You are making it sound like there is no point in making undersquare motors at all; and if that were the case manufacturers wouldn't produce them.
I think it would make for a good game mechanic as well; if you want a motor with more torque and at a lower speed, then you build an undersquare motor. If you are going to impliment the larger bore = larger valves mechanic, then there would be no point whatsoever to build undersquare motors.

[Daffyflyer]

The only thing is that none of the engineering textbooks that we reference play out that theory, so we'd just be making it up unless we can find something that quantifies the effect.

[vantil]

Longer stroke means longer crankshaft rotation. (what ever you call it?)
Longer crank rotation means more torgue?

Why isn't it that simple?

[darkjedi]

because gasoline burns very fast and power is apllied for a short duration of the stroke, making the longer lever somewhat useless. it could be used to produce more torque by retart ignition and intake.

[Water77]

Well from what I have learned in physics, the stroke does not really act like a lever. The piston is only going up and down in the cylinder and is applying no horizontal force. That is if we think of vertical being up and down in the cylinder and horizontal meaning left and right. The length of the stroke would only increase the torque if it were to be moving left and right. Lets think of it like this, if you have a pole the is 1 meter long that is attached to something that you are pulling, lets say 20 pounds. If you increase the length of the pole to 2 meters, nothing happens, you still need to use 20 pounds of force to pull it. That is because the force you are using to pull it is not torque since torque is rotational force. Now if you use the same 1 meter pole to pry up something that is 200 pounds it will be difficult, but if you use the 2 meter pole it will be much easier since this time you are using torque or rotational force to pry this 200 pound object up. So stroke really just increases the height of the engine and the volume, since the volume of a cylinder is pi * radius squared * height. stroke being the height, bore being the diameter. I do not see how stroke could increase the torque. This is just how I think of it, I do not know if I am right as I am no expert with this stuff by any means.

[Killrob]

The never-ending discussion of torque... *sigh* Water77 gets most things right, but in general I think this thread has so many misconceptions that it's painful to read.

Let me start off by showing how stupid it is to even talk about torque in the first place. Weee!
On my bike (me being the motor) I get 130 Nm of torque, ain't it amazing! Ain't I accelerating fast!? Well... no because I only have a power output of ~300W while doing so. The only thing this torque accomplishes is bending the frame.

Talking about torque in regards to performance is like talking about speed in kilometers (or miles). Hey, he did 300km! Ehhh, per what? 5h? Not that impressive.

How does stroke affect torque? Simple answer: linearly. If you increase stroke by x%, at the same time you increase capacity by x%, which in turn is directly proportional to torque in first approximation.

If you want to read some more about it, I did a little write-up about this stuff a while back:
viewtopic.php?f=13&t=198

Cheers!

[Bishop]

http://www.d-series.org/forums/engine-b ... -care.html

[Daffyflyer]

http://www.d-series.org/forums/engine-building/141308-advanced-tech-what-makes-torque-why-do-i-care.html

Great article!

[darkjedi]

killrob, you are saying that if you increase stroke on an engine and bore stays the saame, it produces more torque , explained by you to be a result of larger capacity.
what if you inrease stroke and decrease bore to mantain the same cap.? In the article posted by bishop it tells the exact thing i said; peek burn and peek pressure happen long before 70-80 degrees ATDC, therefore the longer stroke has very little influence on torque