GooOnYou

Good point ya'll bring up.

Keep in mind that force is a vector. You can have negative force. Weight is defined using the normal force that the earth exerts on you due to gravity pointing down. There is the negative force of gravity and the positive normal force acting on you to determine your weight.

What does this have to do with torque? The reason you can apply force to a lever arm and not experience motion, for example when trying to loosen a bolt or a nut that is on tight, is because there is a negative frictional force opposing the centripetal force you are creating.

To calculate torque you have to factor in the NET force in one direction (centripetal force minus frictional force). Therefore, you have to overcome static friction in order to apply a net positive force and create torque.

If you have force you have to have movement UNLESS there is another force acting on it in the opposite direction to prevent movement.

I'm sorry i guess if the dictionary says tends then there must be a situation where you can have torque but no movement. I'll be removing two eggs from my face. But I spent a while thinking that over and can't see a reason why you can't have negative torque if you can have negative force? I haven't ever heard of negative torque though. If you do a google search, however, you get some results for negative torque.

If I made any other mistakes please point them out but I do believe the rest is accurate.

maxcooper

I think many of the posts here are way too hung up on component weight in their analysis of torque. The engine's power comes from burning fuel, not the inertia of the spinning engine parts. Power is not torque, but in steady-state mesurements (a brake dyno measuring torque at a single RPM), I would think that component weight would have little to no effect on torque. In dynamic measurements, like a chassis dyno, it is clear that heavier parts (like a heavy flywheel) give lower HP and torque measurements.

Maybe this is a confusion of cause and effect. Torquey engines tend to have a lot of rotational inertia because they have a large displacement and/or long stroke. Engines that rev high need to have light parts and short strokes to stay together. Those high-revvers tend to have less low end torque than low-revvers with similar power. But that doesn't mean that a lot of rotational inertia creates torque, but rather torquey engines tend to have a lot of rotational inertia mainly by coincidence (don't need to be light, tend to have long stroke and large displacement).

The 13B ingests the same amount of air per main shaft rotation as a 2.6L piston engine, so I think that size is the closest equivalent to the 13B from the piston world. So the "it's only 1.3L" excuse for the low torque is not a very good one -- though "it's only 2.6L" isn't such a bad excuse. The 13B is a small engine compared to the V6s that are in competitive cars. But there are engines with similar displacements that make more torque than the 13B. I think the explanation there is that the 13B has a small effective "stroke".

There are some very nice things about the 13B that perhaps make up for the lack of low-end grunt. It has a nice light rotating assembly, which to me feels a whole lot better than engines with a lot of rotating inertia. The 13B also revs high and makes very good power for it's weight or displacement (however you wish to rate it). It is delightfully smooth due to the inherently balanced 2-rotor engine design. The torque curve is very wide (compare it to similarly sized piston engines), even through it isn't very tall.

The best engine in the world would have lots of power, tons of torque, zero rotational inertia, and low static weight. Good mileage would be nice, too, but you've got to compromise some things.

-Max

GooOnYou

If inertia is a component of torque then how can you say that?

It is a fact that if you increase an object's inertia you increase the torque.

So, increasing the mass of the crankshaft and its radius then it should mean that its torque would also have to increase.

I also never said the engine's power came from inertia.

Stroke plays a part in torque that is true. The longer the stroke, the greater the distance from the axis of crank rotation, and the more the torque produced. Long-stroke engines make more torque at lower rpm than shorter stroke ones.

zoom44

ON HALF OF THE ROTATION THE INERTIA IS IN THE OPPOSITE DIRECTION RIGHT? SO isn't the increase offset somewhat but the "negative" inertia- on a piston engine?

GooOnYou

Whoa inertia isn't a vector you can't have negative inertia. It's just the property of an object (crankshaft) to resist motion. Mass and the radius.

zoom44

what i mean is that if evrything had more mass then on the down stroke of the piston you would have more to push down with but on the way back up you would have more to push up negating some of the benefit you had from the extra force on the way down.

neit_jnf

This is true exept that the increase in torque is not positive in this case. The more inertia the rotating assembly has, the more torque it requires to be able to move; so we have less available to use.

Exactly, the property of an object to resist motion, the more mass the more resistance to motion.

maxcooper

Think about measuring torque at a single RPM (like on a brake dyno). The crankshaft is not accelerating rotationally (RPMs are fixed). That means it's rotational inertia is out of the picture. What you are left with is the force of the expanding gasses pushing on the pistons or rotors to produce the torque. This is a bit theoretical since I don't think you could actually keep an engine at some precisely unvarying rotational speed, but I think the idea is still valid so it is a useful thing to imagine.

In the dynamic sitatution, like accelerating on an inertial chassis dyno, having a lot of rotational inertia actually hurts you. Since you are accelerating the crankshaft rotationally (i.e. revving it up), some of the engine's torque will be spent accelerating the crankshaft itself. The more torque required to accelerate the crankshaft, the less you will have to accelerate the dyno drum. A heavier crankshaft will result in lower measurements of power and torque. In reality, the difference will likely be small here since you dyno in higher gears and you aren't really accelerating the engine internals that quickly. But I once did some calcs on the effects of switching to a light flywheel and the numbers in 1st gear are surprisingly large: 50HP-ish for the particular car I was calculating for (it will vary from car to car). The differences dropped off to 15-5-3-2 or something like that for the higher gears. You only get a big difference when you are revving the engine up quickly (like 1st gear in a light, powerful car).

-Max

GooOnYou

I think I see what your getting at and its true if you have a heavier crankshaft, it will be harder to rotate and the angular acceleration will be less but torque may or may not suffer because inertia is increased as well, but if you have no inertia then you can't have torque. If there is no force being applied to the crankshaft then there is no torque at the crank.

You can't have torque either if there is no circular "twisting" motion. What we have been discussing is possible ways to increase/decrease torque on the crankshaft and the difference between how torque is created in piston engines vs. rotory engines.

Inertia is completly independent of acceleration or any motion. Any object has a moment of inertia moving or not.

The pistons/rotors are a means of creating torque on the crankshaft. I think maybe you meant the more inertia a rotating assembly has, the more angular acceleration/force it requires to be able to move. This is true and there is definately a delicate balance between the amount of force the pistons/rotors exert on the crank and the moment of inertia of the crank itself to produce max torque. You can't have an inertialess crankshaft nor can you have a crankshaft that is too bulky to spin. There are also other ways of increasing inertia without increasing mass.

Most of what I'm talking about is theoretical and just possible factors that could lead to the reason why the rotory has less torque than the piston engine. If you increase the inertia, torque should increase at the same angular acceleration but by increasing the mass of the crankshaft in order to increase inertia, angular acceleration suffers. Its a paradox, but if you could get the same force acting on the crankshaft through some sort of rod at a farther distance from the center of mass of the crankshaft then you could increase torque significantly. The best example of this is when changing your lug nuts, to get max torque and make it easier to turn the lugs, it is best to use a really long wrench to get the most leverage and torque. By increasing the length of the wrench, you increase the moment of inertia of the system which is the reason why the longer wrench creates more torque. It further increases torque if you apply the force perpendicular to the direction that the wrench is sticking off the lug nut.

I believe the lack of connecting rods in the rotory play a big part. They act as a "longer wrench" when trying to tighten a lug nut (same example I used earlier). They provide more leverage and increase the moment of inertia of the crankshaft without increasing the mass of the crankshaft.

I'm not completely sure what you mean here but I think you are trying to make basically the same point as neif. First of all it definately isn't a good idea to increase the mass of everything. It probably isn't even a good idea to increase the mass of the crankshaft because that would lead to a decrease in the angular acceleration and overall decrease of the force acting on the crankshaft as I discussed earlier.

The only force acting on the crankshaft from a piston engine occurs through the connecting rod on the down stroke. On the way back up, you have to overcome gravity which is almost negligable because the force of the piston is so great and that force continues through on the up stroke (Conservation of momentum and what not). So you do lose some of the benefit but I don't think it is enough of a loss to matter much.

I'm sorry if i repeat myself or ramble on. I probably made a mistake somewhere in this post, please feel free to ask more questions and point out anything you think could be a flaw in my logic. I'm not too good with words but I think we have all learned something through this thread. I know I have.

maxcooper

Here's an article from a circle track site that mentions the effect of crankshaft weight on the torque production of an engine:
http://www.circletrack.com/techarticles/84199/

-Max

GooOnYou

Nice article

Hymee

iTrader: (1)

Wow - there seems to be lots of bad info zoom zooming around here.

The inertia of components does nothing to make torque - its only effect would be to average out torque fluctuations.

You can have lots of torque with no "movement". In fact a steam engine produces it's maximum available torque at 0 RPM.

Torque and Horsepower are directly tied to each other. You can't have one without the other.

Torque is a measure of force - in a rotational sort of way (the leverage example)

Power is a measure of the RATE of doing WORK.

All of the below are ALL related, and changine one will affect the others:

Power, Torque, Mass, Acelleration, Time, Distance.

And to try to keep my post short - Power is the thing that makes the most sense to us car folk when we talk about performance. When our car is acellerating hard, it is really power we are feeling. Power is a product of torque. What matters to us is how quickly more torque is available.

I'll bow out of this argument before I get too involved. I have seen it on other forums as well. Same ol' same ol'.

My parting comment is to think about this:

A Tractor produces an absolute bucket load of torque. Much more than the '8. Will it beat the '8 in an accelleration run? Not likely.

Cheers,
Hymee.

PS - Has anyone here been to, or seen, a Tractor Pull competition? It is a great experience to help visualise torque and force.

D:D

wakeech

thanks Hymee, this thread is annoying.

GooOnYou

This is wrong there can be no torque without inertia.
If torque and distance and mass are related and inertia is derived from the mass and distance from the center of mass of an object to the tangent force acting on it then it has to be related to torque as well.

The article maxcooper posted explains a lot.

I'm not too sure what this means. Please elaborate.

This is true, I realize now. Possibly due to negative torque created from negative force pushing the opposite direction such as friction when trying to loosen a lug nut that won't budge.

This is also true but we haven't been discussing horsepower, power, or torque. Horsepower is definately the key to winning races. The yawpower article posted earlier has a lot of good info on that.

It may be that a lot of people think what I am talking about is totally off the wall and "bad info" but I want people to challenge it and not take what I say as fact so we can all learn a little bit more.

I've never seen a tractor pull but it sounds cool :D

zoom44

JUMP IN AND HELP OUT ANYTIME YOU WANT WAKEECH

zoom44

hymee and the others seemed to have missed the point- the point was how do the engines make T, what could cause an increase or decrease in T, and why does a rotary make less in comparison to a piston version.

maxcooper

That's true by coincidence, since you have to have some physical body (e.g. crankshaft) to exert a twisting force and thus you have some inertia by virtue of haing that physical body. But that does not mean that having a crankshaft with a large moment of inertia (= amount of rotational inertia) will increase torque output versus a "lighter" crankshaft. "The inertia of components does nothing to make torque." Both of your statements are true, and they are not in conflict with each other.

The basic point is that the rotational inertia of the crankshaft (or eccentric shaft) has nothing to do with how much torque an engine makes. Having a relatively "light" main shaft is not one of the causal reasons the 13B lacks low-end torque. The engine would have more torque if it was a 3 rotor or had greater eccentricity (stroke). And the shaft would likely be heavier in those cases, but the increased weight of the shaft would not be the cause of the torque increase. Rather, it is just another side effect of the change in configuration (another rotor or longer stroke).

As far as smoothing out the fluctuations, a heavy rotating assembly resists changes in speed (RPM) more than a light one. A shaft spinning at 3000 RPM wants to stay spinning at 3000 RPM -- it doesn't want to speed up or slow down. Over the course of one rotation of the crankshaft, the amount of torque produced by the engine varies. Consider a single cylinder 4-stroke engine -- the power stroke is only 25% of the complete cycle (180 degrees over two rotations of the main shaft). The output shaft will speed up during the power stroke and will slow down the rest of the time. And even during the power stroke, the amount of pressure exerted on the piston by the expanding gasses and the leverage of the connecting rod and crankshaft will vary, which makes the torque fluctuate. Having a large flywheel or simply a lot of rotational inertia in the crankshaft will reduce the amplitude of the fluctuations in shaft speed. The output will be smoother (less noticable power pulses) when the moment of inertia of the rotating assembly is large.

-Max

wakeech

...thing is, i don't wanna. it's kinda funny to see what some people have to say.

...and max is on the right track. inertia (i think this is an american word meaning "mass") has nothing to do with how much torque is produced. i've already covered the basics as far as why rotaries don't make so much torque.

Gord96BRG

Boy, have you got your fundamentals mixed up!

Read that circle track article again - it's diametrically opposed to what you're claiming. You suggest that more inertia (via heavier engine components) makes more torque - that's wrong. From the article: (Note also that the article is referenced mostly towards balancing.)

My mechanical engineering and internal combustion engines training is over 20 years old now, but still - torque requires no movement nor inertia. There definitely exists static torque - force times distance, that's all. As mentioned, steam engines and electric motors make maximum torque at zero rpm. If you introduce time as a factor, then you're talking power, not torque. Re inertia - unless you're changing the speed of rotation, it's a complete non-issue. It is entirely possible with an engine dyno to keep rpm constant and vary load and throttle opening - so, for example, you have a test bench engine running at 3000 rpm, at 20% throttle. Say it's producing 50 lb-ft of torque. Open the throttle to 100%, increase the load to keep the rpm at 3000 - there is absolutely zero change in inertia since the rpm didn't change, correct? How come the torque output increased to 200 lb-ft then? Because inertia is NOT a factor in torque at all.

Here's another example to consider - take a V8 with heavy steel rods and cast pistons, real slugs. Put it on a test bench and measure torque at, say, 3000 rpm and full throttle. Now rebuild that same engine with titanium rods and forged aluminum pistons that are otherwise identical (same crown shape and compression ratio), and rebalance the crankshaft for these, so the crank counterweights will be lighter as well. Much lighter engine internals - put the engine back on the test bench and measure the full throttle torque at 3000 rpm - according to your theory, it should make less torque. However, you'd find that the torque would be exactly the same. Also, the lighter internals engine would accelerate much quicker due to the lower inertia. In performance engines, lower inertia is always better.

Regards,
Gordon

klegg

This is a little off topic, but I replaced my engine with three gerbils on crack running around endlessly on a wheel, attached to my drive shaft...No matter how many times I shock their little ***** with my cattle prod, the car will not move,

Do you think reduction gears will help? Is anyone making a hamster mod?

wakeech

are you sure you're not holding that cattle prod backwards and trying to think and chew gum while typing??

klegg

No, I have been drivin mad by

1 A inflamation of my right eye which is very painful and has left me half blind

2: The pain meds for the above

3: The toe on my right foot that I fractured on tuesday when I triped over a toe that my kids left on the floor, and I could not see due to my damaged eye

4: The poopy diaper I had to change when my baby had a massive blowout

5: Yet anothet thread whining about MPG/MPH/HP/DYNOS/car x is better...

6:the strange photoshoped combo of you and a dog ...truly disturbing

T-von

Sorry for bringing this back guys. I notice a lot of differant opinions on this subject. I'm trying to understand this the best I can. So instead of us comparing the rotarys torque to a piston, why not compare the torque of the Renesis to the 13b found in the 3rd gen Rx7?

I understand that one is turbo charged while the other is NA but if the Rx7 has 255hp and 217 lbs why does the NA Renesis with 238hp have a torque peak of only 158? I mean both engines have the same displacement, similar rotating masses(Renesis is slightly lighter because of the lighter rotors for higher rpm's) and produce similar amounts of hp. Why is the torque of the Renesis so much lower by comparison? Does it come down to the 50% larger exhaust port area of the Renesis? I ask because I've always heard that some exhaust back pressure will add torque. Thats why we Fd guys tend to loss some low-end when replacing a cat with a midpipe. To add to this....look at the 20b of the Cosmo. That engine was tuned for torque because in stock form it had more restrictive exhaust sleeves. With these sleeves in place, the engine would produce higher torque figures than hp figures but if the engine was ported in the exhaust area that engine would also loose some torque. Someone help me out hear so I can feel more educated.

wakeech

again, my first post.

force = pressure * area. the 13BREW's air/fuel charge is almost double what the 13B-MSP's is because of all that mechanical compression (the turbos). so even though the compression ratio is lower, the amount of pop is understanably higher.

their horsepower outputs are still fairly close because, as in my first post, at maximum output they're still burning about the same amount of air and fuel, just in different ways.

the internal mass of the componentry has nothing to do with how much torque is being made. i have no idea where this stuff is coming from.

neit_jnf

There's also the difference in redlines
3rd gen is lower and since torque and hp are related by rpm this accounts for part of it.