# Tesla thought experiment



## stubby5000 (Feb 19, 2020)

Hello all, I'm new here. This post may well be in the wrong section; if so, moderator, please feel free to move it with my apologies. This post is for anyone out there who's good with math and has the grasp of electrical engineering that I lack.

Let's imagine that a Tesla somehow has an onboard electric generator that runs on gasoline (I'm picturing these things: https://www.electricgeneratordepot....00eh-dual-fuel-portable-gas-propane-generator ). What sort of combustion engine/generator would be necessary to keep a Tesla running with no net change in the battery's state of charge? I understand driving conditions matter a lot in this hypothetical scenario, so let's just say the Tesla is driving at 60 MPH on level ground in perfect weather. Is there any way to estimate what MPG the Tesla would get? This suggests it might do better than a conventional ICE car:

https://reneweconomy.com.au/tesla-e...or-still-cleaner-than-conventional-car-61942/

I assume this is because while an EV running on a generator still has the inherent inefficiencies of an ICE, you don't have the losses at the transmission, driveshaft, etc. Isn't this the way the Volt worked once it depleted its battery?

Thanks for any thoughts!


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## potatoee (Aug 26, 2018)

Well, if you want to take the very simple approach and neglect a bunch of stuff... Take the rated efficiency of 250Wh/mi and a speed of say 60mi/h, multiply the two and you get 15kW. You'd need two of these things to run your car (again, neglecting a bunch of things and just spitballing it). I don't think they'd fit in the frunk


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## garsh (Apr 4, 2016)

Ah, pretty interesting thought experiment. 

At 60 mph, we'll say the car averages 240 W•h/mile.
I think that comes out to 240*60 = 14.4 kW of power.
Is there a generator that can generate this much power?

It looks like some whole-house generators can.
https://www.bestgenerator.org/home-generator-reviews/
There's not a whole lot said about efficiency, but one of them says 2.6 gallons per hour.
I guess that's liquid propane.
So that would give you about (60 mph)/(2.6 gallons/hour) = *23 miles per gallon*.

Again, liquid propane, not gasoline.
The gasoline engine in a Chevy Volt is designed better for this kind of situation than this kind of ad-hoc solution of using a generator.


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## msjulie (Feb 6, 2018)

These guys kinda did it https://electrek.co/2020/01/06/tesla-model-3-hybrid/


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## Long Ranger (Jun 1, 2018)

I'll just throw out another way to think about it that doesn't get into generator efficiency questions, but maybe this isn't truly following the intent of the thought experiment.

From an energy equivalency point of view, the LR Model 3 battery is equivalent to about a 2.25 gallon gas tank, and the car can go about 300 miles on that.

When the EPA comes up with its MPGe numbers for EVs, those MPGe numbers are based upon equivalent energy usage, not equivalent cost. A US gallon of gasoline has approximately the same energy content as 33.7 kWh of electricity. Therefore, a 75kWh battery is equivalent to a 2.23 gallon gas tank. 
MPGe


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## stubby5000 (Feb 19, 2020)

Thanks for the replies guys! How does voltage figure into this? I think those generators are made to produce electricity at a certain voltage; the higher voltage ones (120/240v) seem to produce fewer watts. Would whatever comes out of the generator need to be stepped up to 300+v for the Tesla? Surely there would be some inefficiencies introduced by doing that...?


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## bwilson4web (Mar 4, 2019)

Source: https://www.ebay.com/itm/2017-BMW-i...311515&hash=item2137285150:g:oI0AAOSwR8haX1X3

Source_2: https://www.ebay.com/itm/Engine-Gas...227580?hash=item2f3d51b3fc:g:gPcAAOSwbPleOjTw

You'll need a radiator, exhaust pipe, and gas tank. Best start with a rear-drive Model 3 and use the front motor space and probably the frunk.

Bob Wilson


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## garsh (Apr 4, 2016)

stubby5000 said:


> Thanks for the replies guys! How does voltage figure into this?


For a simple thought experiment, it doesn't matter. Just match the generator's power output (watts) with the power used by the car at a steady highway speed.

But if you're actually thinking about _implementing_ something like this, then...








The cars (indeed, all EVs) have an AC charger built-in. The problem is that this AC charger is limited to 48 amps at 240 volts (for all current Teslas). That gives you 11.5kW, which is short of the 14.4kW required to maintain a steady 60 mph on the highway.

If you want to charge at higher rates, you'll need to bypass the AC charger and use DC charging. Which means that you'll need to rectify the generator output to 480v DC. Good luck trying to find off-the-shelf equipment to do that. 

And then you still have to figure out how to update/bypass the Tesla's charging circuitry to allow the car to both charge and drive at the same time. Fun times!


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## bwilson4web (Mar 4, 2019)

I'd start with the range extender engine and generator from a crashed BMW i3-REx:

25 kW including the generator and output DC electronics
two-cylinder, liquid cooled engine
I would tune the engine:

new cam with delayed intake valve closing (Atkinson)
post catalytic converter, cooled exhaust recirculation
tuned exhaust at optimum efficiency, rpm speed
Locate the engine in the lower trunk. Radiator(s), exhaust plumbing, and wiring would be another challenge. Putting the gas tank in the front area where the front drive would go would also work. Recover the CCS-1 hardware, then the car could be both Supercharger, CCS-1, and a hybrid.

NOTE: an interesting technical challenge, it would be a great answer for a salvage Model 3 that had fast DC charging disabled.

Bob Wilson


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## Ed Woodrick (May 26, 2018)

stubby5000 said:


> Hello all, I'm new here. This post may well be in the wrong section; if so, moderator, please feel free to move it with my apologies. This post is for anyone out there who's good with math and has the grasp of electrical engineering that I lack.
> 
> Let's imagine that a Tesla somehow has an onboard electric generator that runs on gasoline (I'm picturing these things: https://www.electricgeneratordepot....00eh-dual-fuel-portable-gas-propane-generator ). What sort of combustion engine/generator would be necessary to keep a Tesla running with no net change in the battery's state of charge? I understand driving conditions matter a lot in this hypothetical scenario, so let's just say the Tesla is driving at 60 MPH on level ground in perfect weather. Is there any way to estimate what MPG the Tesla would get? This suggests it might do better than a conventional ICE car:
> 
> ...


That's kinda called a hybrid.


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