Moment of inertia

Started by Petrus, May 31, 2022, 21:31

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Was researching the moment of inertia of the ice engine and tripped over a representative value.
The 2006 Honda 1.8 i-VTEC engine including the drive train has an MI = 0.1782 kg/m2
This seems low but it is the weight of the rotating parts in relation to the radius. For a disc that is the weight times half the diameter. For say a camshaft or a crankshaft it is less than half the diameter of the rotating circle of course.

With the value for the Honda engine we have a nice benchmark reference for the 1ZZ-FE. With this benchmark it is now easy to see the effect of a lighter flywheel, crank pully etc.
Assuming that the flywheel is a disc with the weight evenly distributed, with half the radius being 7.5 cm. = 0.075 m. then a 3.5 kg reduction is 0.005625 kg.m2 amounting to 3% total of the engine´s MI.
Do note the assumption about the flywheel whereas in reality the weight reduction is mostly outsíde of half the radius. A q&d gives me close to 4%.

Maybe 4% does not look impressive but it is. Just by fitting a 3.5 kg lighter flywheel, the WHOLE rotating inertia of the entire drive thing has been reduced by 3% which can directly be related to the power needed = 4% less power loss.

You can do the same for a lighter crank pully, ditto alternator pulley and slowing the rotor down by say 30% for the MI equals reducing the weight by 30%.

For the SMT the effect is larger still because that is with the engine uncoupled from the gearbox.

This proza is not about the 3 or 4 or however much %.
It does go to show that it is quantifiable as well as the order of effect.

Taking this to driving reality the effect on acceleration of the cár is obviously reduced by the mass of the whole thing and depends on the reduction, the gear you are in.
The effect on lighting up the rubber though is not diluted by the car mass and in the lowest gears the effect is massive.


MR2  8)

Fínally! With thanks to @Ardent


Rotating inertia is a keen topic @Petrus but only affects the rate of acceleration so is purely relevant to accelerating performance. This is why wheel size, diameter and weight are keen tuning topics. 4% is equivalent to nearly a 4% increase in performance so effect of ~6bhp on a good day, worth it between the traffic lights or on a race course.

And of course can aid a proportional reduction in fuel consumption (during acceleration only) either requiring the same performance requirement or greater acceleration meaning shorter period requiring acceleration. In the long run a small benefit for daily drivers but for weekend fun potentially a small benefit. I'd not suggest it warranted the cost of purchase and installation of a lightened flywheel (though I'd do it anyway).

However the crux is you have to consider rotating inertia from the valve train right through to the tarmac and the larger mass or diameter has the highest impact so starting at the wheels, then flywheel, then reducing any rotating mass adds benefits. So in order of impact:

Rotating drag such as transmission oil, engine oil etc. Extra gears and synchros etc for example (love a 5 speed Racer)
Gear drag, friction, misalignment. A whiny final drive for example is a concern (very small drag impact but it adds up and warns of other issues).

This is a subjectively educated list and I haven't done exact maths but the physics is sound and good for conversation  ;)

RIPieces: Sable \'05 Roadster \'Red\' edition.
Project: MR-S import (JDM) now my toy and mule for:
SC build OR a 2AR-FXE swap (ref: Frankenstein Motorworks) OR Electric conversion
AND rebody with a fibreglass replica of Porsche 550 Spyder.



Apart from highway cruising most driving involves ´constant´ accelerating/decelerating. The effect on fuel economy is of little importance to me though; I more than ´waste´ that scrubbing the wubbah anyhow.

It is not really about performance either as I generally adhere to the speed limits, cértainly in built up areas and/or with traffic.
I don´t care about quicker acceleration times and nevvah evvah do drag races with anyone.
Ditto top speed. Either from added lightness or added drag by the wing.

In mý, strictly personal, frame of things my MR2 is a strictly fun experience even when it has a transport role.
For mé the car is all about MR2 and that applies from the car itself; the way it responds to changes of direction; on steering input, to how the thing responds to the loud pedal = engine MR2.
As such to mé the engineers´ choice of the 1ZZ over the 2ZZ makes total sense as does a supercharger over a turbo as does ány reduction of drive train inertia.
Yes I have lightweight rims and would have gone Volk CE28s if I could have sourced them even at double the price of the Enkeis.

Deleting the pas, fitting a wooden steering wheel are a good example too. That is about the directness of the feedback.
Ditto hard shell seats. Rather obviously less damping of the seat in the pants feel.
Both enhancing the feedback at nó performance increase. I could harp on about increased safety becaus of increased feedback but nâh, it is all about the fun of it.

The effect of the lighter flywheel on the drive train in especially 2nd and 3rd is definitely a thing I enjoy feeling through the hard shells and ditto steering  :))
The calculation is a hard backing up of it having a solid reality.
And yes this coin too comes with a different side. Like the increased nimbleness off adding lightness can be perceived as instability. Like most car users ´want´ the luxury crap. Perception is reality and I like my perception to be undiluted  8)
The MR2 is one of the very few hard(ish) core things still legally to enjoy. Even recreational sex for fun is discutable nowadays  :o


I agree in the whole with you Petrus and share your sentiments about the MR2. I'm also not interested in high speed or power, just enjoyment within the limits of legality and my own skills and the MR2 is one of few ideal options to do so.

PS Enkei are great wheels but I was lucky enough to find some CE28s for a bargain, almost scrap price and rather pleased with myself. I can't say that the full price difference is worth it though, I'd never pay for new Rays forged wheels anyway, don't expect to be able to afford it any time soon either ;)

RIPieces: Sable \'05 Roadster \'Red\' edition.
Project: MR-S import (JDM) now my toy and mule for:
SC build OR a 2AR-FXE swap (ref: Frankenstein Motorworks) OR Electric conversion
AND rebody with a fibreglass replica of Porsche 550 Spyder.