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I have heard many times that four-wheel drive is less efficient than two-wheel drive. Many car producers have a normal drive mode where only two wheels are powered, and turn the four-wheel drive on only when needed, to "save energy".

I have never understood why it should be so. You need to transfer only half the energy to each wheel (which must also save the material wear), so the total energy consumption must be the same.

Please answer the question also for the case of electric cars with four separate small engines each attached to a single wheel.

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  • $\begingroup$ Tesla currently claims that their dual motor configuration is more efficient than a single motor for AWD. But as both badjohn and Userman said, it is rather an engineering question. $\endgroup$
    – lmr
    Apr 3 '19 at 8:55
  • $\begingroup$ in the case of the question about electric motors - are you trying to ask if 2 500W motors are more or less efficient that 1 KW motor? Remember that motors are heavy; and cars don't care about power, but power per weight. $\endgroup$
    – UKMonkey
    Apr 3 '19 at 16:08
  • $\begingroup$ @UKMonkey I am not sure if there is a clear answer to your question. I tried to search for "how does power-to-weight-ratio changes with the size of electric motor" and I could not find any good results. $\endgroup$ Apr 5 '19 at 11:13
  • $\begingroup$ In that case I would suggest you search for the weight for a 1kW motor, and the weight of a 500W motor; and compare - you can even make a graph if you want to put more effort into it; there are plenty of places that sell motors and include this information $\endgroup$
    – UKMonkey
    Apr 5 '19 at 11:16
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Simply put, power transmission is not free. Every time you have some energy over here, and you want to move it to over there, you are going to lose some of it in the process. In this case, if your engine is in the front and you want to drive the back wheels, you'll need some combination of gears, bearings, and shafts to transmit that power. Gears are pretty efficient, but it's not 100% efficient. Maybe 97-98% efficient. Look at the schematic of a 4 wheel drive train (good one here: https://www.artofmanliness.com/articles/gearhead-101-how-part-time-four-wheel-drive-works/) There are 2 differentials and a transfer case. All of those contain gears, and every single gear costs you a little bit of efficiency. Each one is not a large loss, but it does add up. With 2 wheel drive, you'd just have 1 differential and no transfer case.

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  • $\begingroup$ Does this apply also to electric cars with 4 (2) separate engines? $\endgroup$ Apr 3 '19 at 13:28
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    $\begingroup$ Another factor is tire-to-ground friction is higher, at least on some vehicles, where wheels on an axle are locked and "scrub" in turns. $\endgroup$ Apr 3 '19 at 15:21
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It is an engineering question. Weight, space, and friction are a problem when you have a four-wheel drive. There are more mechanical (rotating) parts, which produces lots of friction. If I remember correctly, there is about 1/4 energy loss in a typical four-wheel driven car.

The engineering problematics hold true for electric cars, despite the fact that there are fewer moving parts. However, I don't think that there is a basic physical principle underlying.

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As others have pointed out, this isn't really a matter of physics so much as the choice of how to implement the 4WD system. With modern ECU governed AWD systems, the mileage penalty is much less than it used to be with the hydraulically controlled AWD units. A common system was to hang a small hydraulic pump on the front and rear shaft, piped in a loop. If the machine was FrontWD normally, and everything was hooked up, no pressure would develop in the system. If the front axle's tires started to slip, it's pump would pump more fluid than the rear and transfer some torque. If the imbalance was large, the pressure would engage a clutch and transmit power directly though the shafts. These systems had appreciable parasitic losses. The two pumps can be eliminated now and the entire thing controlled with sensors and an ECU. They can also talk to other ECU's and as a coordinated group comprise a dynamic stability control system.

See this short video on the Honda CRV comparing the pre 2012 system with the post 2012 system. Honda CRV AWD systems

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