Maybe? But seems like it would hurt overall performance. Acceleration favors rear axle, regeneration favors front axle. Stability/ traction control wants all wheels active. Mechanical clutching (especially splined) hurts all that. Semi deals only with rear axles, so its dynamics are different.
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Investor Engineering Discussions
- Thread starter MC3OZ
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Maybe? But seems like it would hurt overall performance. Acceleration favors rear axle, regeneration favors front axle. Stability/ traction control wants all wheels active. Mechanical clutching (especially splined) hurts all that. Semi deals only with rear axles, so its dynamics are different.
Olle
Active Member
In the IPMSynRM's (that the above mentioned cars use), there will be a small loss due to the power supplied to keep reluctance and PM fields balanced while coasting. The range difference between Plaid and LR is probably a rough indicator of the drag created by adding one more of these motors, if we ignore the drag from the extra gearbox.
Olle
Active Member
In the video upthread, RJ Scaringe said that Rivian made a big effort to make the connect/disconnect quickly and seamlessly, so might be able to connect for regen and traction when needed, but of course not as well as a constant drive.
It appears as a temporary mechanical workaround for something that could be solved electrically.
Somewhat analogous of Taycan's two speed gearbox to enable "outstanding acceleration and top speed". Lucid and Tesla then went the extra mile in the electrical department and blew the doors of Porsche with just fixed gears.
Olle
Active Member
Particularly for Tesla investors in addition to the obvious benefit of 0 resistance, I find energy storage possibility interesting:
If such a storage system doesn't pan out, Andrew Cote's Scenario 2 high current density (>1000A/mm^2) superconductivity would mean big improvements even for li-ion batteries. Being able to run at 4V would eliminate the need not only for cell balance circuitry and compute, but also the cell failures resulting from loss of balance. ->300kA in a Plaid, <3cm^2 cable.
The next step, what Andrew Cote calls Scenario 3, where critical magnetic field and current are both high, you could have superconducting motors and run with e.g. a 4V pack mentioned above. Huge reduction in complexity and gain in reliability to just have one turn windings in the motor. Unfortunately requires 100x transistors in the inverter but maybe it's worth it.
Stretch2727
Engineer and Car Nut
The recent articles on the Tesla EPA ranges got me thinking about EV ranges compared to ICE. My thought is that EV's have much more variability due to wind, load, tire pressures etc. This is one reason EV's have to pay so much more attention to aerodynamics and rolling resistance etc.
Struggling to put this in true engineering terms. My thought is waste heat is such a large percentage of the overall energy used on ICE that the effect of a small change in load does not have a large effect on consumption. Where as with an EV a small change in load translates directly into a large change in consumption as there is very little waste.
Are my assumptions correct? Maybe EV's should give a window of expected range as the real challenge is not getting an accurate number but being able to account for all the variability in conditions that may occur. I know ICE have variability in range as well but my personal experience is it is not nearly as much as an EV.
Thoughts?
Struggling to put this in true engineering terms. My thought is waste heat is such a large percentage of the overall energy used on ICE that the effect of a small change in load does not have a large effect on consumption. Where as with an EV a small change in load translates directly into a large change in consumption as there is very little waste.
Are my assumptions correct? Maybe EV's should give a window of expected range as the real challenge is not getting an accurate number but being able to account for all the variability in conditions that may occur. I know ICE have variability in range as well but my personal experience is it is not nearly as much as an EV.
Thoughts?
Yah,Tl the more efficient a powertrain is, the more variable the efficiency will be in the real world. When 90% of energy goes into motion versus only 30%, a 10% shift in load results in 3x the impact.
Tesla: 350 Wh/mile vs 300 is a 14% drop in range
300 vs 250 is a 17% drop
ICE: 950 Whe/mile vs 900 is a 5% drop in range
ICE also has the advantage in winter of unlimited free heat.
EPA will never match people's actual drives, it only serves as a benchmark to allow relative comparisons. Publishing the 5 cycle results similar to the current city/ highway numbers would help people based on their commute. This would also cover thermal system efficiency.
dhrivnak
Active Member
I think that Tesla should under promise and over deliver. Easy to do by restating the Highway mileage. One can get rated range by driving at 55 with no HVAC. But few people will do that. Test at 70 mph using HVAC and use that number or just take the current number and advertise 90% of the rated range. It does not do EVs or Tesla any good to advertise a range that few any can achieve.
The EPA number (theoretically) allows comparison between vehicles. Higher speeds than the cycle test has will impact a higher efficiency car more than lower efficiency ones.
OEMs testing to, and EPA publishing the 5 cycle tests would be informative for purchasers. Tesla drivers may look at the high speed results, it's the only test over 60 MPH.
Hot cycle is 95F ambient, cold is a repeat of the city test at 20F.
Write up in the topic:
The Secret Adjustment Factor Tesla Uses to Get Its Big EPA Range Numbers
navguy12
Active Member
Publishing all five cycle EPA tests would be informative…to a small subset of educated/detail oriented purchasers.
Still a great idea, but IMO most purchasers would not bother to “wade” through the extra data as it would take more than about three seconds to digest.
dhrivnak
Active Member
Not sure if fully true, but my other EV friends in a Bolt, Mach E and Kia all report longer hiway ranges than reported by the Monroe sticker. They seem to under promise and over deliver in range.
navguy12
Active Member
My wife’s Kona EV also out performs the claimed ranges, in all but the worst snow covered/freezing precipitation scenarios.
Buckminster
Well-Known Member
Doggydogworld
Active Member
That's sound logic, but it's not how the math works. A 90% efficient EV can meet a 270 Wh/mile road load with 300 Wh/mile of battery energy whereas a 20% efficient ICE needs 1350 Wh/mile of heat energy. All else equal, increasing the road load 10% to 297 Wh/mile increases both the EV battery energy and ICE energy requirement by 10%. And range for both will decrease by 10%.
There are exceptions. Winter, for example. As Mongo mentions, the ICE gets "free" cabin heat in winter. And the battery actually loses capacity when cold, while gasoline does not. Of course a cold engine block might burn that gasoline less efficiently, but the block heats up quickly and again mostly using "free" waste heat. The more massive battery, on the other hand, might require active heating. You can also lose regen when starting out cold. These small hits add up.
It's also possible Tesla super-tuned their aerodynamics for straight-ahead airflow and, as a result, their Cd degrades more rapidly in cross-winds. I kinda doubt this, but it would increase the gap between EPA and real world.
There's one other EV vs. ICE issue. I used 20% in the above example, but ICE efficiency varies a lot more with road load than EV efficiency. Most conventional ICE today are ridiculously overpowered for highway cruising. If my 10% increase in road load comes from driving 5% faster, that's a 15% increase in power. That might move the ICE to a better spot on the BSFC map, maybe 20.5% efficient instead of 20%. In this case my 10% increase in road load does not increase fuel consumption by 10%, but by only 7.3%. (All made up numbers, but you get the idea).
My final point (I promise
is that range is much more psychologically important in a BEV. Gas stations are everywhere are fill-ups are fast. So even if both ICE and BEV suffer a 20% range hit due to higher speed or whatever, it will "feel" a lot worse in the BEV.Yes and no.
10% more load needs 10%/efficency more gross energy to produce. But 10% increase in an external load doesn't necessarily correlate to 10% more total load.
Part of ICE efficency is chemical to mechanical conversion, but part is engine losses. Depending on initial operating point, it might even get more efficient at the higher load (not that the shift would likely be sufficient to offset the additional power). Water pump may consume 7 HP on its own (at high RPM).
Where the Energy Goes: Gasoline Vehicles
Consider headwind. Rolling resistance is the same, RPM is the same, but power is increased.
If a vehicle has high pumping loads, rolling resistance, drivetrain friction, the additional aero is less of the total load than vehicles with less or entirely eliminated quantities of those factors.
Stretch2727
Engineer and Car Nut
I tend to agree. I got curious on this when the Model 3 came out and there was a statement that the 18" aero covers are 10% more efficient at highway speeds. Various real world test never showed 10% but it was substantial, like 7-8%. On an ICE car you would never see that much improvement from aero covers.
Buckminster
Well-Known Member
Cool tailgate but no rain protection when loading:
Tesla's aerodynamic shape has the added benefit of not needing this solution because the tailgate doesn't stick out much behind the car in a space with limited length.
Tesla's aerodynamic shape has the added benefit of not needing this solution because the tailgate doesn't stick out much behind the car in a space with limited length.
Hm, using up all that internal luggage space for the rear opening.
With all that engineering effort wouldn't it have been easier to create a rear hatch that opens up left, right and up both hinged and as a sliding door...?
With all that engineering effort wouldn't it have been easier to create a rear hatch that opens up left, right and up both hinged and as a sliding door...?
navguy12
Active Member
…and I was strangely reminded of the complexity of a 1961-1967 Lincoln Continental convertible mechanism. And it wasn’t noted for long term reliability.
Olle
Active Member
Looks almost identical to the BMW Z1 doors. An eye catching novelty but also hugely inefficient in almost every other aspect. There is probably a reason BMW didn't put this in their mass market cars...
Doggydogworld
Active Member
You're mushing different things together. Aero drag and rolling resistance are both part of what auto engineers call "road load". Obviously if you increase aero 10% and hold RR constant, road load will increase by less than 10%. But it's the same for EVs and ICE, thus has no bearing on this discussion.
The engine, drivetrain and parasitic losses in your graphic are not part of road load. They are a side effect of producing power. Increase power and you increase those losses.
A full hybrid like Prius has all the losses shown in your graphic. The engine losses are a bit lower, say 61-65% vs. your 71-75%. So your logic should still apply. But it doesn't -- Prius suffers almost exactly the same range hit due to headwinds, higher speeds, etc. as a BEV.
Why do Prius and BEV suffer similar range hits while conventional ICE sometimes suffers a lesser hit? It's not the absolute level of tank-to-wheel efficiency -- BEV is vastly more efficient than either Prius or conventional ICE. It's due to the way that efficiency changes with power output. Prius and BEV efficiency is pretty constant over a wide range of highway conditions. Conventional ICE efficiency can vary meaningfully at highway speeds, though, and it generally improves as road load increases. It's a second order effect, e.g. my 7.3% vs. 10% range hit example. But it can be noticeable. And in fact that was an early criticism of Prius -- MPG dropped off more dramatically at very high speeds than conventional ICE drivers were used to.
TL;DR - it's not the difference in efficiency that causes EV range hit to sometimes be worse than conventional ICE, it's how that efficiency varies. If you had a vehicle with a constant 1% power train efficiency it would suffer the same range hit as a BEV.
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