# Tesla drive unit and generator. Explain why this won't work.



## trucker21r (Mar 25, 2021)

I can build/fix about anything. Overhauled diesel engines, tractors, race engines, etc. But, my knowledge on electrical is limited and thats where my questions are. I can do simple things like wire trailers, replace receptacles in my house, replace fuses, etc. but I do not have a great understanding of watts, ohms, volts, etc. 

What are the difficulties in buying a Tesla drive unit, and powering it with a small diesel/gas generator?

I'm reading online that estimates to run a Tesla drive unit, 65 miles per hour for 1 hour, will consume around 20kwh of energy. So does that mean I would need a generator, that is making a constant 20kw?

Could I put a small gas/diesel powered 20Kw generator under the hood, and wire it to the tesla drive unit, and use the efficiency of the small gas/diesel generator to power the Tesla drive unit down the highway? Simple example, if I can spin a 20Kw with a diesel motor under 1/2 load, at lets say .8 gallons per hour, traveling 65 miles per hour. That would be roughly 81.xx miles to the gallon. If there are any electrical reserves to recharge a battery bank, could possibly go further. I understand I'll need a drive control and whatever else (similar to an electric golf cart). Please tell me what I'm missing. I appreciate the education I'm surely about to receive. Thanks for any input.


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## OR-Carl (Oct 6, 2018)

trucker21r said:


> I'm reading online that estimates to run a Tesla drive unit, 65 miles per hour for 1 hour, will consume around 20kwh of energy. So does that mean I would need a generator, that is making a constant 20kw?


This is like saying "I only need 25 HP to keep my car moving at freeway speeds, can I just replace the engine with a 25 HP predator engine from harbor freight?" Sure! but your acceleration will be pretty slow, to say the least.



trucker21r said:


> Could I put a small gas/diesel powered 20Kw generator under the hood, and wire it to the tesla drive unit, and use the efficiency of the small gas/diesel generator to power the Tesla drive unit down the highway?


You could; I believe this is how diesel powered trains work, more or less. The question is, do you want a car that accelerates like a train? Do you want to slow down to a crawl when you need to climb a hill? If you want to build a road-train, by all means, go for it. If you want to get 80mpg, buy a moped.


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## brian_ (Feb 7, 2017)

That works, and it is called a series hybrid. 

There's absolutely nothing special about Tesla vehicles or motors. Yes, electric cars (of all brands) use roughly 20 kWh per 100 kilometres or 60 miles, so at 100 km/h or 60 MPH they are using about 20 kW.

There's no point in using the big Tesla motor if there is only 20 kW available. On the other hand, if you size the generator to suit the peak power demand, you've lost the advantage of the small motor, and you still have the inefficiency of the generator and motor (which are much less efficient than a transmission), so a series hybrid normally also has a battery. With the battery, the motor can use energy from the battery for bursts of power requirement (such as acceleration), then recharge the battery by continuing to run the engine at an efficient power level when the vehicle needs less power. Without the battery it is, as Carl mentioned, the same as a diesel-electric train (or off-highway truck).

Current series-hybrid cars include the Mitsubishi Outlander PHEV and Honda Accord Hybrid, except that both of those also have a single-ratio mechanical connection (controlled by a clutch) to bypass all the electric stuff at highway speed for efficiency. The old Fiskar Karma, BMW i3, and Nissan ePower vehicles (such as the Note) are pure series hybrids, but they don't work very well (the Karma and i3 are notoriously inefficient when using the engine).

The electric motor and it's controls are the same in a series hybrid as they would be in a plain battery-electric car. The battery is the same except that it can be very small.


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## Electric Land Cruiser (Dec 30, 2020)

Firstly, yes, you can. From my experience with fuel-electric hybrid drones, you get about 50% efficiency. So in order to make 20kw from your generator, you need roughly a 40kw gas engine to drive it. It certainly is possible and from what I understand the components to turn an AC motor into a generator are pretty simple.

If you want to do it then you should build the vehicle as a standalone EV first with enough battery to drive around, like 5+ kwh minimum.


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## brian_ (Feb 7, 2017)

Electric Land Cruiser said:


> Firstly, yes, you can. From my experience with fuel-electric hybrid drones, you get about 50% efficiency. So in order to make 20kw from your generator, you need roughly a 40kw gas engine to drive it.


The lower efficiency of a generator plus motor (plus controllers) compared to a mechanical transmission is an issue, but 50% would be horrendously bad. Both generators and motors should run at least 90% efficient anywhere near their optimal operating point.



Electric Land Cruiser said:


> It certainly is possible and from what I understand the components to turn an AC motor into a generator are pretty simple.


There are no additional components required. An AC motor (at least as implemented in an EV) is a generator; it acts as a generator during regenerative braking.


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## Electric Land Cruiser (Dec 30, 2020)

brian_ said:


> The lower efficiency of a generator plus motor (plus controllers) compared to a mechanical transmission is an issue, but 50% would be horrendously bad. Both generators and motors should run at least 90% efficient anywhere near their optimal operating point.


That was real world performance on 4 and 6 rotor drones using 1, 2, and 4 cylinder gas engines. Hobby grade components. There is likely a lot to be gained going up in size. The drones were in the 1000-2000 watt range only.


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