Light Vs heavy pistons (ie: ACL specs)

[-GT-]

Yeah, it's a lot. Which is why i'm still not convinced. Sorry i cut that last reply off a bit suddenly, i had to go out.

The reason i said 7.5kW at the end there is that what i was getting at was that the 4 stages of the piston's cycle were 'powered' by a different cylinder's combustion, so it didn't give the total per cylinder, and hence shouldn't have multiplied by 4. I've now gone back on that, as if i'm calculating overall i should've added the 4 piston's mass difference. I go back to saying the original calcs are correct, but i don't believe the answer, for some reason still beyond me.

There's got to be somebody around here who can tell me why i'm wrong and show me the right numbers?!

[oldeskewltoy]

Arias 8.6 CR(82mm nore) 4AGZE16V slugs have a mass of only 303 grams... saving you an addition 42 grams...

That might be worth looking into.........

HOLD the phone.... Arias has a NEW 20V low compression forged slug! "Stock head" - doesn't say which.... CR =9.0. Mass is ONLY 288 grams!! a further savings of 15 grams over their 16V slug... and a MASSIVE 121 grams savings(PER PISTON!) over OEM slugs!
Arias slugs therefore save you OVER 1 US pound in piston mass alone!!!
http://www.ariaspistons.com/ARIA2007cat.pdf

[Jasper]

But I also think the main point of lighter internals is lower load on the crank and rod bolts. Therefore a higher safe rev limit.

This is on the money.


However, if you insist on quantifying the difference that piston weight makes, you need to calculate the difference in rotational inertia (J).

To express the difference in kW, you then need to assign an (arbitary) angular acceleration to multiply the rotational inertia by.

This is the change in engine speed, expressed in radians per second, per second (rad/sec^2). Hand timing a second gear acceleration from 3000-6000rpm should give you some numbers to start with, just remember to use the correct units.

[-GT-]

Thanks Jasper. I think i was working along these lines, but trying to simplify the calcs a bit. Seeing as the pistons were the only thing different between the 2 systems figured i should be able to simplify it to a linear relationship, as the pistons oscillate rather than rotate. The whole system has rotational inertia, but if all the rotating parts are the same, i was hoping i could cancel them out of the equations and just look at the difference. I think you can, as the only difference in the rotational inertias is the pistons' linear oscillations?

[Sam_Q]

arnt Arias meant to have some quality control issues though?

[Grega]

nice one guys
anyone know what the formula atlantic pistons weigh in comparison. that would make a good benchmark.
rep all round

[oldeskewltoy]

nice one guys
anyone know what the formula atlantic pistons weigh in comparison. that would make a good benchmark.
rep all round

http://www.hasselgren.com/contact.html - nothing on their webpage.....

[Killswitch]

Very interesting thread this one, be interested to know the outcome of some further calculations to more precisely gauge any power gains there might be from lighter pistons.

Are there any engine builder computer simulations where you can alter piston masses?

[oldcorollas]

The reason i said 7.5kW at the end there is that what i was getting at was that the 4 stages of the piston's cycle were 'powered' by a different cylinder's combustion, so it didn't give the total per cylinder, and hence shouldn't have multiplied by 4. I've now gone back on that, as if i'm calculating overall i should've added the 4 piston's mass difference. I go back to saying the original calcs are correct, but i don't believe the answer, for some reason still beyond me.
There's got to be somebody around here who can tell me why i'm wrong and show me the right numbers?!

regardless of the crank input, the rods, or the combustion, the pistons still have to go up and down, so you can calculate them as a seperate system..

there are only 2 phases for the pistons. accelerating and slowing.
1. accelerating from TDC or BDC to centreline.
2. slowing from centreline to TDC or BDC.
so...all the pistons are in the same phase at the same time, so you add the efforts of each piston together. ie, when 1 and 4 are approacing TDC, 2 and 3 are approaching BDC, so they all slow together. (actually, they will be slightly out of phase due to rod geometry stuff but..)

so... for 1/2 of 1 total rev, the crank is accelerating all 4 pistons and for the other half, it is slowing them all down...

after thinking about it, you may be right that with no friction or losses, the system will spin forever, BUT it will not do it with a consistent crank speed.. it will speed up during the piston slowing stage and the crank will slow down when the pistons are accelerating (force and reaction etc)

so... the next step to thinking of that is.. to keep a consistent crank speed, you need to apply a torque to the crank above and beyong its closed system kinetic energy, in order to maintain the speed during the times the crank wants to slow down or speed up...

that make sense so far?

however, the extra energy added (with no friction) also balances out (cos you need same force for speeding or slowing.. not true for the different slowing phase of each piston, due to different rod leverage, but it evens out between odd pairs of pistons)
so say for example.. you had a flywheel, with a torsional spring.... you could have the flywheel speeding and slowing and maintain the same crank speed... or vice versa? hmm now i'm getting confused

so what does this mean for piston weight? it means that the extra energy needed to maintain crank speed is, at any one time, larger than it would be for lighter pistons..
but does it make a difference (assuming no losses)??

when pistons are speeding up.. yes it makes a difference for the worse.. absorbing energy from the car kinetic energy to move them.. but, when the slow down, they are contributing energy to the car to increase it's kinetic energy.....

i'm not sure how that translates to losses in the drivetrain etc...... however, having more energy needed to be transferred through the big and little ends will absorb a bit more energy, since it is just oil film etc.... and while the friction is low, it is still there and you will definitely have more frictional losses... but these losses are probably much lower than the energy needed to accelerate the pistons....

remember.. a 7.4kw energy loss...is a lot of heat!!!..... you can work out the temperature increase of the pistons based on that heating rate...

anyway.. its 1.30 here.. time for sleep..

discuss

[oldeskewltoy]

regardless of the crank input, the rods, or the combustion, the pistons still have to go up and down, so you can calculate them as a seperate system..

there are only 2 phases for the pistons. accelerating and slowing.
1. accelerating from TDC or BDC to centreline.
2. slowing from centreline to TDC or BDC.
so...all the pistons are in the same phase at the same time, so you add the efforts of each piston together. ie, when 1 and 4 are approacing TDC, 2 and 3 are approaching BDC, so they all slow together. (actually, they will be slightly out of phase due to rod geometry stuff but..)

so... for 1/2 of 1 total rev, the crank is accelerating all 4 pistons and for the other half, it is slowing them all down...

after thinking about it, you may be right that with no friction or losses, the system will spin forever, BUT it will not do it with a consistent crank speed.. it will speed up during the piston slowing stage and the crank will slow down when the pistons are accelerating (force and reaction etc)

so... the next step to thinking of that is.. to keep a consistent crank speed, you need to apply a torque to the crank above and beyong its closed system kinetic energy, in order to maintain the speed during the times the crank wants to slow down or speed up...

that make sense so far?

however, the extra energy added (with no friction) also balances out (cos you need same force for speeding or slowing.. not true for the different slowing phase of each piston, due to different rod leverage, but it evens out between odd pairs of pistons)
so say for example.. you had a flywheel, with a torsional spring.... you could have the flywheel speeding and slowing and maintain the same crank speed... or vice versa? hmm now i'm getting confused

so what does this mean for piston weight? it means that the extra energy needed to maintain crank speed is, at any one time, larger than it would be for lighter pistons..
but does it make a difference (assuming no losses)??

when pistons are speeding up.. yes it makes a difference for the worse.. absorbing energy from the car kinetic energy to move them.. but, when the slow down, they are contributing energy to the car to increase it's kinetic energy.....

i'm not sure how that translates to losses in the drivetrain etc...... however, having more energy needed to be transferred through the big and little ends will absorb a bit more energy, since it is just oil film etc.... and while the friction is low, it is still there and you will definitely have more frictional losses... but these losses are probably much lower than the energy needed to accelerate the pistons....

remember.. a 7.4kw energy loss...is a lot of heat!!!..... you can work out the temperature increase of the pistons based on that heating rate...

anyway.. its 1.30 here.. time for sleep..

discuss

my brain hurts.........

[LeeRoy]

Also keep in mind youll be able to trim some weight off the cranks counter balances also since your lowering some piston weight. Now your doubling the weight saved

- LeeRoy

[Hen]

OC - Flywheels work to maintain consistent engine speed throughout the crank revolution. Any variations from average are dulled by the relatively large rotational inertia of the flywheel.

Also, light weight internals will not cause a change in peak power. End of story.

Lower weight means less energy required to spin the engine up from say 1000rpm to 9000rpm. So it will accelerate quicker. Just like a lightened flywheel.

Hen

[Sam_Q]

can someone work out the difference in load in the big ends from running the lighter piston? or how about working out how many extra revs in theory is possible to have the same force going from the ZE to the ACL?

[-GT-]

But if the engine accelerates faster from 1k to 9k rpm, surely the car it is attached to must have accelerated faster up to the equivalent road speed too???

How this can happen without a discernable difference in measured power at the wheels is what i'm struggling to get my head around? It may not be a significant number, but conservation of energy is telling me it must go somewhere. If not to the wheels, where?

OC: Thanks for explaining where i went wrong in the calcs, i was just adding it all together.

[oldcorollas]

But if the engine accelerates faster from 1k to 9k rpm, surely the car it is attached to must have accelerated faster up to the equivalent road speed too???

How this can happen without a discernable difference in measured power at the wheels is what i'm struggling to get my head around? It may not be a significant number, but conservation of energy is telling me it must go somewhere. If not to the wheels, where?

if the combustion produces a constant force (or at least the same for light and heavy pistons), then the acceleration of the AVERAGE speed of the light pistons will be faster.
although each piston is accelerating at all times.. the average speed rises with engine speed, and so the average kinetic energy decreases.
this reduced kinetic energy requires less force to produce, so there IS extra power at the wheels. just not very much.

same with flywheel, reduced inertia results in faster acceleration in lower gears, where the rotational inertia of the flywheel is a significant proportion of the "effective weight" of the car (ie, inertia + rotational inertia)

i guess, as a rough calculation, you could work out the average speed of the piston (not peak speed.. so it's an integration of a sine curve.. or just distance and time ), and multiply that by the weight to get the extra energy required to go from say, idle to 6000rpm.

what is stroke for 4AG? 77mm?
77mmx2 =154mm movement per rev. 15.4cm 0.154m

6000rpm = 100rps, so movement of piston = 100x0.154 = 15.4m/s average piston speed

KE=0.5MV^2
KE=118.58 x M joules (where M is in kg)

so
for ACL345gm = 40.9 joules
for standard 384gm = 45.5 joules
for GZE 409gm = 48.5 joules

so the difference between ACL and GZE is 7.6 Joules, thats the difference in average kinetic energy at 6000rpm

at 1000rpm, the numbers are
for ACL345gm = 1.14 joules
for standard 384gm = 1.26 joules
for GZE 409gm = 1.35 joules

at 8000rpm, the numbers are
for ACL345gm = 72.7 joules
for standard 384gm = 80.95 joules
for GZE 409gm = 86.2 joules (so obviously you should go for GZE and 8000rpm.. it's Karma )
so GZE has about 19% more kinetic energy... but is it significant?

say you wanted to rev your GZE pistons from 1000rpm to 8000rpm in 1 second (makes it easy for me )
86.2 J per piston, = 344.8 J for all 4 pistons
minus 1.35 J x 4 = -5.4 J

so the change in average kinetic energy of the pistons (the energy needed to be introduced to the system just for pistons)
= (344.8 - 5.4)/1sec
= 339.4 watts. 0.34kw

for the light ACL jobbies = 0.286 kw
the difference is about 0.054 kw


cliffnotes...
to accelerate the GZE pistons from 1000rpm to 8000rpm in one second, will take 0.34kw
the amunt of energy saved by using light pistons is 0.054kw

if you accelerate from 1000-8000 in 2 seconds, that becomes 0.027kw

compare that to a say.. 5kg flywheel, uniform thickness (not true but) and around? 300mm diameter?. and 1000 to 8000rpm (33.3Pi, 266.6Pi Rad/s)
KE = 0.5 I w^2 (I is moment of inertia, w is radians/sec?)
I = k*M*R2 (M=mass; R=radius); k = inertial constant (depends on shape)
for solid disk of uniform thickness; k = 1/2

so....
KE = 0.5 * k * * M * R^2 * w^2
KE = 0.5 * 0.5 * 5kg * 0.15m * (33.3Pi)^2 or (266.6Pi)^2

1000rpm KE = 2052joules
8000rpm KE = 131,529 joules

so to spin flywheel up in 1 second from 1000 to 8000rpm = 129kw?
in 3 seconds = 43kw
make that a 2kg flywheeel, and then... it is 17.2kw (for 1000-8000rpm in 3 seconds)

perhaps the numbers are off for the flywheeel example (or i might have it totally wrong please check....) but... it shows the relative importance of the piston vs flywheel weights

that help?