bajaman

Excellent post, Jimlab!

POM HB

Jimbo is always....da MAN!

KevinK2

As said, light rotors will act like a light flywheel, and make more power available at the wheels IN THE LOWER GEARS. No gain unless the tach is sweeping quickly.

Light rotors may allow a higher redline, and more hp.

jimlab: good job on clearing up long stroke = more torque. but ... "Lightening the rotors will not change the amount of torque the engine produces unless material is removed from the rotor face, affecting displacement."

don't think a face job changes displacement. would dished pistons change yours?

-------------------------------------------

Originally Posted by bajaman
"In a rotary....much, much different. And quite frankly I can't even begin to calculate exactly how the forces are generated"

As DamonB said.

Another view: It's like a piston directly connected to the crank arm. Watch a rotary animation ... slowly. The fired face creates a "force vector" that passes through the rotor center and also the apex seal on the far side. When the rotor rotates about 45 deg from the fire position, it's "force vector" is nearly at a right angle to the e-shaft pin, and is creating the most torque.

TT_Rex_7

If I recall right, the pockets on each face of the rotor give it more displacement. If you was to machine the pockets larger it should add displacement, but will also change the compression ratio from what I've gathered. Enlarging those pockets is another idea I have in my head.

-Alex

jimlab

Good point. The swept volume of the combustion chamber is what's being calculated, and it's assumed that the piston has a perfectly flat top for that calculation. Adding a dish or a dome to the piston changes the compression ratio and technically affects the volume of the cylinder, but it's not factored into the displacement calculation which is only concerned with swept volume, or in this case, 8 perfect cylinders the height of piston travel and the diameter of the cylinder bore.

As an example, I have 5cc valve reliefs (clearance) in the crowns of my pistons, but it only affects the compression ratio and not the rated displacement of my engine.



Yep. Pressure on the connecting rod journal (and therefore torque) also rises and falls in a piston engine as the angle of crankshaft rotation changes.

http://science.howstuffworks.com/fpte4.htm

tookwik

Theoretically it seems like it should create less torque at the wheel... but thats not what's producing the power and therefor not what you need to look at. A lighter rotor will let the drivetrain spin the wheels at a faster rate, if any noticeably difference, and therefor create more torque. The unsprung weight on cars is harder to explain... sorry if that explanation sucks.

dgeesaman

iTrader: (7)

It's easy to explain. Unsprung weight, flywheel inertia, rotor inertia, they are all like throwing sandbags on the back of the school's star running back. He has the same 'torque' (running power) as before, it just slows his acceleration and ability to change direction. And theoretically, suspension and drivetrain are best with zero response time.

Dave

7Langit

Lightweight rotors are what the RX-8 has. They are BETTER due to less inertia, just like lightweight flywheels.
Combustion physics has to do with rotor shape, injection rates, sparking, intake & exhaust timing (port shaping) and other things like 100 octane racing fuel vs the 91 octane crap they sell out West, but NOT heavy vs light rotors. "Light is right", in rotors as well as pistons. So good combustion characteristics along with light rotors is the ideal.
Lightweight flywheels, if TOO light, can affect street drivability due to not only fast spinUP but fast loss of RPMS (spinDOWN) too. Rotors affect this much less so. The power gained by having less weight to push around during combustion is better, now worse.

RotaryResurrection

iTrader: (8)

I think many of you are missing the forest for the trees. The weight of rotors/flywheel (i.e,. rotating assembly) does not change the torque or power PRODUCED by the engine, it just changes your perception of it, because it changes the INTERTIA of the assembly. The mass of the flywheel, other than providing a physical surface for a clutch assembly to grip and a ringgear to crank the engine with, is used for one thing...storage of energy. Same goes for the weight of the rotors themselves (again, other than their obvious physical properties and shapes). The more mass, the more energy you can store. This energy is sapped off the energy the engine produces during combustion. IF an engine with infinitely lightweight internals could make 500hp at the crank by transmitting ALL of it's generated power straight out, then that engine would make less as measured at the flywheel with a massive RA. Because some of that energy was invested into the RA's mass to begin with.

This helps you with low end driving. That energy is removed and transmitted to the drivetrain when load is introduced. When you introduce a load from the drivetrain and weight of the car, you drain the engine of energy at the crank. The flywheel, and to a degree, the entire RA's mass, has stored some energy to counteract this shock, so that it's easier to drive the car and maintain smoothness between shifts. Without this, you'd kill the engine every time you tried to pull out, and your revs would drop instantly during shifts, making driving a bitch.

The less mass the RA has, the less energy it stores, and the more energy gets transmitted directly to the drivetrain. It doesnt mean the engine is PRODUCING any more power because of the less massive RA, just that more of it is being transmitted to the car rather than into the RA. The less energy available to combat the effects of load being applied, the more you feel the effects mentioned above.

As a real world illustration, if you take a stock car with a 25lb flywheel (like an s4 t2) and install a 9lb flywheel, and do back to back drives, it'll feel like you have gained hp. IN reality, your engine is identical as before, and is producing teh same power via combustion. Just that less of that power is tied up in rotating assembly inertia, and thus a bit more is transferred to the wheels, so the car feels faster. By the same token, at a stoplight or a hill, less energy is stored in that lighter flywheel, so when it comes time to present a load to the engine, you feel more of a shock and tendency to die because of that.

edmcguirk

There is one big difference between reciprocating angines and rotary engines. The reciprocating engine has to stop and start the weight of rods and pistons every revolution and that work is directly subtracted from torque/HP. That loss starts small at low RPM but increases at the square of the RPM (or is it cubed?). Rotary weight has no effect on a constant RPM, it only slows down acceleration.

The weight in a rotor also places stress on the bearings that have to keep it moving in the right direction. That weight will affect max possible RPM.

Since rotating weight only affects rate of acceleration, it really only affects the lower gears. It only really matters in gears that you can run through in less than 3 or 4 seconds. A lightened flywheel or light rotors doesn't really affect 4th and 5th gears. Probably not 3rd gear either unless you have a lot of HP on tap.


I would suspect that the SCCA guys want the lightened rotors not for any change in torque but to allow them to make the big HP that comes from running at higher RPM.

ed

jimlab

What's your justification for this statement? Do you not accelerate in higher gears also? And in fact, with less gear multiplication in higher gears, the heavier you make the components of the rotating assembly, flywheel and clutch assembly, or the rest of the drivetrain, the slower you will accelerate.

If anything, lightweight components are even more critical in higher gears and at higher speeds, where the available torque at the axles is far less and the weight of the car, drivetrain losses and rolling/wind resistance have far more effect on the rate of acceleration.

wanklin

iTrader: (2)

"If I have a V8 with a 4.00" cylinder bore and a 3.48" stroke, I've got 350 CID. (4.00 * 4.00 * 3.48 * 8 * 0.7854 = 349.9 CID). If I increase bore diameter to 4.03" and stroke to 3.75", now I've got 383 CID. (4.03 * 4.03 * 3.75 * 8 * 0.7854 = 382.7 CID) The difference, of course is that one has a longer "lever" on the crankshaft, but as a result, the piston is also traveling farther on each stroke (the distance of the stroke, FWIW), creating more displacement. More displacement means I can burn more air and fuel on each cycle, creating... more torque.

Now, let's say that I create an "all bore" 383 with the shorter 3.48" stroke of the 350. I'd have to use a 4.18" cylinder bore (4.18 * 4.18 * 3.48 * 8 * 0.7854 = 382 CID). Obviously, the 3.48" stroke crankshaft has a shorter "lever" than the 3.75" stroke crankshaft above. Would you expect torque to differ substantially? In fact, given equal intake components and cam profiles, the two engines will produce almost identical torque curves. Why? Because even with a shorter lever, the "all bore" engine is exerting more force on that lever, because it is burning the same amount of fuel and air, but has a larger surface area (piston crown) to "push on". Neat, huh?"

I find this very interesting and I have to admitt that it makes sense. Any chance of posting some SIM'd torque curves?

great post, Rob

turbojeff

iTrader: (30)

Torque available at the wheels is less at higher speeds but the POWER available at the wheels is the same. Power is what accelerates the car. The rate of change of the RA's angular velocity is very low at higher speeds.

At higher speeds the car isn't fighting accelerating the mass of the car. It is fighting the wind drag *and* where I believe I went wrong in the other thread was the increased rolling resistance from higher weight.

jimlab

Torque at the wheels, or more accurately, thrust at the tire contact patch (drive wheel torque divided by the rolling radius of the tire) is what accelerates the car. Torque available at the axles is not the same in higher gears, which is why you accelerate much more slowly in 4th gear than you do in 1st.

F = ma

a = F/m

Mass (m) is a constant, so as force (F) decreases in higher gears, acceleration (a) steadily decreases also, until the car cannot accelerate further.

Not necessarily. If you're cruising at 1,600 rpm in 6th gear (a valid example in the Corvette world), it's going to take much longer to get the rotating assembly and drivetrain to spin up if you just step down on the gas, despite the fact that you're already going 75-80 mph. You must downshift and increase gearing multiplication (and engine rpm) to spin up the mass of the rotating assembly and drivetrain faster and accelerate more quickly.

Rolling resistance and aerodynamic drag do increase at higher speeds, but those losses affect force, not mass. The available torque or thrust at the drive wheels is still fighting to accelerate the mass of the car further.

As you can see in Newton's formula above, mass is a constant, and you're still wrong.

edmcguirk

It's the RATE of acceleration that makes all the difference. If you can run through a gear in less than 3 or 4 seconds then your rate of acceleration is very large.

You should run through first gear in about 3 seconds and second gear in about 3 seconds. but it takes you much longer to run through 4th gear. It takes me about 25 seconds to run through 4th gear up the back straight at Watkins Glen. That's not a very high RATE of acceleration. Flywheel weight doesn't matter much there.

As an extreme example, a heavy flywheel will not affect your terminal velocity. At that point your wind resistance is perfectly balanced by your HP and you have no acceleration.

In really rough numbers:
In second gear you are accelerating by about 1000 RPM/second If you lost 1/2 second that would be a .000125 per 1000 RPM per second. Running fourth gear in 25 seconds is about 160 RPM per second. So ->.000125 X 160 X 25= the same 1/2 second lost over the entire 25 second distance.

You still lose in the upper gears, just not that much.

ed

jimlab

Flywheel weight does matter, because heavier drivetrain components slow your rate of acceleration in every gear. It may not be as noticeable in 4th because the rate of acceleration in 4th is much lower than in 1st, but it's still there. Note the green line in the chart below representing drivetrain losses through the gears.



About as much as in any other gear.

TT_Rex_7

I'm not sure if this is directed to me as far as you thinking I'm looking for an increase in torque from doing this, or if your just talkin in general, but I'm really not looking for any increase. I just simply had the idea of doing this and while I was searching through some old threads (2-3 years old) I noticed they all stated you would loose torque. Obviously I wouldn't want to do this and have a loss in torque since its not what our cars have alot of. I have decided to go through with this and am searching for a slightly used motor to mess with so I guess we'll all have an answer as to what will happen soon. As "RotaryResurrection" stated there won't be any gains from a lighter flywheel or lighter rotors at the crank. There will be a gain in the rear wheels though since less power is lost through the drivetrain due to lighter components. On another note, I really don't see lighter rotors making an extreme effect on daily driveability unless an extreme amount of weight is taken off. Obviously it will change the driving characteristics of the car but I don't see it being anything the driver couldn't adapt to pretty easily, like a lighter flywheel.

-Alex

TT_Rex_7

I'm with Jimlab on this one, how could you justify an increase in acceleration through lower gears, and then the increase in acceleration go away in 3rd and up. That simply doesn't make any since at all. An increase in acceleration wouldn't just vanish in higher gears, it just may not be as noticable, but it would def. be there.

-Alex

turbojeff

iTrader: (30)

Your wrong here Jim.

Torque available at the axles is NOT the reason you accelerate much more slowly in 4th gear than you do it 1st.

Look at your own "losses" graph and see that there is a dramtically increasing drag force, [I]that[I] is why you accelerate much more slowly in 4th vs. 1st. Weight/rolling resistance increases roughly linearly.


HP is what moves the car. Torque does not move anything. POWER moves things.

Like I said, torque at the rear wheels is less in the higher gears, HP available at the wheels doesn't change much at higher speeds.

edmcguirk

Rotational inertia does slow you in every gear but the amount is related to your rate of acceleration. Your graphs appear to be showing frictional losses not inertia losses. That graph does not show rate of acceleration anywhere. It could be equally valid for 10 seconds or 10 minutes.

Losing 1/2 a second in 3 seconds is a much bigger deal than losing 1/2 second in 25 seconds. When you reach terminal velocity the inertial losses go to zero.

ed

JebenKurac

i read through this whole thread and it made me think of two things. first, what would happen it the rotor face was cut into like that piston crown on page 2. also, what if there was weight added to near the tips of the triangles to add centrifical force?...i imagine it would lower the amount of power? that your engine needs to keep itself running when driving on the highway, but then you would be pushing your brakes and the engine wouldnt be able to slow itself down in stop and go traffic.

TT_Rex_7

From what I've gathered changing the pockets on the rotor faces will add displacement, but will change compression ratio. Its something else I have in my mind but 1 step at a time lol. I've also been thinking of a way to loose weight in the rotor and add/shift weight to the corners of the rotor. I'm not sure exactly how that will effect the motor either, and like I said, one thing at a time. I dont want to start altering everything at once and not be sure wich process made had it go all wrong. Once I get the motor I'll be able to tear it apart and see exactly how the rotor is constructed and I'll also find some bum rotors to cut in half to see how much room I have to play with and what not. I would rather not pull the motor out of my car and tinker with it since its in perfect running condition and less than 28000 miles lol.

-Alex

jimlab

Sorry Jeff, you are.

Have you ever seen a graph like this?



Correction, that is only part of the reason you accelerate much more slowly in 4th vs. 1st. See above.

Wrong again Jeff. A vehicle's rate of acceleration (g-force) follows the engine's torque curve, not the horsepower curve. Maximum acceleration is at torque peak in first gear, not horsepower peak. Do some research.

Horsepower is merely a calculation based on torque and rpm. If we converted torque at the axles to horsepower at the axles, you'd still see the same drop in POWER in higher gears.

jimlab

Good point, the graph does show frictional losses of the drivetrain, not inertia losses. The program isn't that detailed.

turbojeff

iTrader: (30)

I haven't seen anything that proves me wrong yet.

Torque does not move anything. Power moves the car, power in our case is torque x rpm. Plain and simple, PERIOD. Buy a Physics book and read up.

Where did that graph come from? What are the units? LB? That isn't a measurement of power.

550lb*ft/sec is one HP IIRC.

So take another look at the graph, make the units so they actually reflect POWER, to me it looks like your trying to show the *thrust* unit. That is NOT a correct way to characterize acceleration.