Automatic Transmission ND

[Mr Morlock]

I raised it before but what on earth does Skyactive mean- I suggest its marketing only. The guy in the street probably thinks it's the USA's next space program.

[KevGoat]

I raised it before but what on earth does Skyactive mean- I suggest its marketing only. The guy in the street probably thinks it's the USA's next space program.

Copied/pasted from Google search ... "SKYACTIV is a brand name for a series of technologies developed by Mazda which increase fuel efficiency and engine output...."

[StillIC]

This all means that more of a low speed/high torque design is required for the transmission and propshaft. This does not make a lot of sense as rotating component durability is reduced exponentially more by increased load than by increased revs.
That means the transmission and driveshaft are heavier than they need to be, dríven by higher maximum and mean output shaft loadings.
The engine and diff mounts need to be heavier also. (Axle torque change is not a factor being dependent on overall ratio only, which would not necessarily change.)
Seems strange to go from a high speed/low torque drivetrain to a low speed/high torque drivetrain, when we know weight saving is a key design goal

True, but, like all engineering, there is a trade-off....

It makes sense from an NVH perspective to have rotating items operating at lower rpm. For the same centrifugal force, a lower speed device can have greater imbalance.

It also makes sense from the point of view that in a bevel drive assembly it is easier to get a ratio of 2.9:1 to be more efficient than a 4.3:1 ratio. I think this is one of the reasons that Mazda are quoting much more efficient transmissions as part of their Skyactiv marketing push. Oh, and for the same size input gear as a 4.3, the 2.9 output gear is smaller, meaning lighter and meaning lower mass moment inertia. Win win.

[Mr Morlock]

thanks Kev for clearing that up- the Space program is out- drat - Mazda might have put one in orbit- that would be a first.

[KevGoat]

Mazda might have put one in orbit- that would be a first.

Haha ... mental picture of astronaut sitting in drivers seat of an ND, roof down, floating in space ....

[97 MXV]

It makes sense from an NVH perspective to have rotating items operating at lower rpm. For the same centrifugal force, a lower speed device can have greater imbalance.

Agree higher speed shaft imbalance force (mass times acceleration) increases as a square of speed ie frequency squared. However minimum isolator stiffness needed also increases with shaft frequency squared and rubber isolator damping effectiveness increases with shaft frequency.

Also bevel gear tooth pass frequency lowers with a 2.9 diff suggesting softer mounts might be required.

So softer mounts having to control more torque could be going in the wrong way in respect of NVH isolator design which also has to handle inertia loads from bumps etc.
In other words I am not sure that a slow speed/high torque driveline that lowers the forcing frequency necessarily makes it easier to deal with transmissibility aspects of the resulting NVH, even if lower magnitude.

It also makes sense from the point of view that in a bevel drive assembly it is easier to get a ratio of 2.9:1 to be more efficient than a 4.3:1 ratio. I think this is one of the reasons that Mazda are quoting much more efficient transmissions as part of their Skyactiv marketing push.

Marketing initiatives which "Save the Planet" are really good if they do. My problem is I do not imagine a 2.9 diff will run that much cooler than a 4.3.
They would have to go straight cut gears to get a measurable difference I would have thought.
It would be interesting to run an ND auto 4.1 and an ND manual 2.9 on the same road and measure the diff temperatures at the destination.
Actually I think most of the small amount of heat generated in a diff relates to the stirring of the bath of oil, and the speed of that is the same in both cases.
Maybe a dry sump diff might be a better way to Save the Planet.

[StillIC]

Marketing initiatives which "Save the Planet" are really good if they do. My problem is I do not imagine a 2.9 diff will run that much cooler than a 4.3.

I believe the heat generated (Joules) by the 2.9 will be lower, whatever the resultant operating temperature (degrees).

They would have to go straight cut gears to get a measurable difference I would have thought.

I don't believe there is any efficiency difference between straight cut and helically cut gears, but the helicals are quieter (and more expensive).

It would be interesting to run an ND auto 4.1 and an ND manual 2.9 on the same road and measure the diff temperatures at the destination.

This would be interesting. If someone can do this this would be great. I assume they both have the same case and they are equal in all other respects?

Actually I think most of the small amount of heat generated in a diff relates to the stirring of the bath of oil, and the speed of that is the same in both cases.

If the gears were 100% efficient they would not need any oil at all. The gear drive needs enough lubricant to keep it efficient, without so much as to make the assembly less efficient. I do accept that for longevity there is a trade off here because the inefficency mostly comes from friction between sliding surfaces, where slight imperfection in the machining accuracy means the rolling of one gear tooth on the next is not quite perfect. This increases with wear and use, proving positive feedback (vicious circle)...more wear means more sliding which means more wear etc. So I suppose lots of oil will help delay/circumvent this vicious circle and perhaps make your statement true. But I am not convinced.

I still believe that 2.9 will be more efficient than 4.1, especially as they wear.

[97 MXV]

I believe the heat generated (Joules) by the 2.9 will be lower, whatever the resultant operating temperature (degrees).

Actually surface temperature change has always been a reliable measure of heat being generated inside.

I don't believe there is any efficiency difference between straight cut and helically cut gears, but the helicals are quieter (and more expensive).

Well that's a treasured myth busted

inefficiency mostly comes from friction between sliding surfaces, where slight imperfection in the machining accuracy means the rolling of one gear tooth on the next is not quite perfect.

Now because gear teeth complicate things, I will think of the drive as friction drive of shaft against a disc like a model T Ford with the same disc speed (wheel rpm).
Now we know sliding friction force x sliding speed = power lost.
I will further assume that sliding is a percentage of rolling speed so concluding from the formula that the inefficiency would be the same.
However if sliding is not proportional to rolling speed, than inefficiencies would be different.
So how teeth geometry would affect all that I am hopefully to find out.

[StillIC]

I will further assume that sliding is a percentage of rolling speed so concluding from the formula that the inefficiency would be the same

Well this is where I need to be careful about speaking generally or specifically, and I should have been careful previously, I now realise.

Comparing the ND 2.9 to the ND 4.1, I assume the crown wheels have the same number of teeth? If so, your statement seems correct.

But more generally, an optimised 2.9 drive versus an optimised 4.1 for what is in all other senses the same application, the 2.9 drive would have a smaller number of teeth meshes per revolution output. And, if the power lost per tooth mesh was the same in both (arguable?!), then more power is lost in the 4.3 per revolution of output. Herein lies a large portion of my argument.

I am at the point where I feel I should re-read some of my engineering text books!

[97 MXV]

If the gears were 100% efficient they would not need any oil at all. The gear drive needs enough lubricant to keep it efficient, without so much as to make the assembly less efficient.

OK then, lets imagine an experiment.
Lets hypothesise that the rolling of the gears is practically 100% efficient, better even than a train wheel on a track. Deflections in the hard steel in contact being perfectly elastic with no scrub. The work done by the wheel in deflecting the track under the weight is minuscule and what there is, is perfectly stored and restored as elastic energy of the resulting deflections as governed by Young's Modulus.
Perfect rolling.


Now lets submerse the tracks in thick oil and run the hard wheel over the hard track again. This time we will find that the wheel will need a bit more power to maintain speed due to viscous drag from continually pushing through the oil bath and squeezing the oil out from the line of contact.
Now the key thing here is viscous drag is independent of load, it has likely linear and possible exponential functions of speed (or distance traveled in the same time), depending on the Reynold's number that the oil chooses to go with.
Now a 4.1 diff must have a "longer track" than a 2.9 diff irrespective of teeth numbers.

Now in a diff the difference in length of the rolled path causing the extra drag is independent of load.
Because load increases with speed due to wind resistance, tooth load is lowest at steady state low speed.
This is where drag is a relative factor. Figure 1 of this study demonstrates the effect on cumulative carbon emissions.
At low temperature/viscosity/speed and load, the impact of drag can be seen, but at higher high temperature/viscosity/speed and load it is hardly measurable.

This hypothesis supports a view that the 2.9 diff is measurably more efficient than the 4.1 diff at low temp, speed and loads, but not at high temp, speed and loads.

So the question is how much power is lost squeezing out the 4.1/2.9 or 40% extra oil, namely:
- The bulk oil filling the teeth cavities in a submerged diff
- The last drops of oil in the tooth boundary layer in a dry sump diff
- Extra oil displaced and squeezed out in the longer path taken by the pinion bearing rollers

[StillIC]

Nice work Keith. After my last post I thought I should have mentioned the elastic deformation of the gear teeth causing scrub, but you did anyway!

Yesterday I was browsing the Mazda Technical papers, specifically the section on the 6 speed manual driveline.The functional allocation outline included in the paper suggested that increases in fuel efficiency were made by a number of key technologies. In no particular order....

Reduction in spin loss, including:
-Direct drive in 6th gear
-Low viscosity synthetic oil
-"FD Torque" (I guess Final Drive) from "Reduction in MD gear speed" (Main Drive?)

Reduction in gear mesh loss:
-Reduction in CS speed (I assume counter shaft speed?)

Of note "Reduction in MD gear speed" and "low viscosity synthetic oil" were key factors to achieve "Change RPM" to reduce rattle noise, to improve quietness. I am not sure how the oil helps here...any guesses?

[Apu]

julius-sumner-miller-09.jpg

[97 MXV]

Exactly my point, just imagine what a drag that guy's experiment would be if his balls were dipped in diff oil.