Capture a car's lost energy

I have a friend that wants to put several alternators on a car. However, he wants to gather the power from the wheels rather than the engine in order to "capture lost energy" and "save" fuel by running the AC and other electrical components off these alternators. I've already explained that this is not possible because the engine is driving the wheels, therefore, the source of energy is still the fuel in the car. Furthermore, getting the power from the wheels is less efficient than getting it from the engine as there are energy loses through the transmission and the driveline. Also, the new alternator(s) would decrease efficiency because it, itself is not 100% efficient and it would add to drag forces. He's really stuck on this idea, so I need to show him how it's wrong with the math. I am a bit stuck on this part. I know I need to do energy balance by using the first law of thermodynamics but that's as far as I'm getting.

I looked up the loses due to transmission and driveline and those are estimated to be around 5.6%. I also looked at several alternator efficiency curves and their efficiency is usually between 40% with peak efficiency going as high as 60%. I also looked up AC power draw and found it to be around 720 watts.

I haven't done a thermo or physics course in a long time so any help to get the ball rolling here would be much appreciated

davenn
Gold Member
hi there
welcome to PF
You gave your thread a really bad title, I almost reported it. PPM discussion isn't allowed on PF, not even to debunk it

Maybe the moderators can rename the thread to something more appropriate ? @berkeman

I have a friend that wants to put several alternators on a car. However, he wants to gather the power from the wheels rather than the engine in order to "capture lost energy" and "save" fuel by running the AC and other electrical components off these alternators.

power can be gathered during braking or down hill coasting .... google regenerative braking as used by electric cars.
recovered power is low but is used to do some recharging of the battery ... never enough to fully recharge it tho
You can never generate enough power tho and most of the power is still being supplied by the fuel

There is no point trying to recover power during normal flat ground/up hill driving, as the energy lost to the drag caused by the load of the alternator will always be more than the power generated. And you are just going to use more fuel to overcome that loading

Dave

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hi there
welcome to PF
You gave your thread a really bad title, I almost reported it. PPM discussion isn't allowed on PF, not even to debunk it

power can be gathered during braking or down hill coasting .... google regenerative braking as used by electric cars.
recovered power is low but is used to do some recharging of the battery ... never enough to fully recharge it tho
You can never generate enough power tho and most of the power is still being supplied by the fuel

There is no point trying to recover power during normal flat ground/up hill driving, as the energy lost to the drag caused by the load of the alternator will always be more than the power generated. And you are just going to use more fuel to overcome that loading

Dave

I had no idea this was a rule (Assuming PPM is perpetual motion) I can't really change the title now though, or can I?

As for your answer, I completely get that and know the only way to get some "free" energy is while going downhill or braking. However, this guy is pretty hard headed and "needs to know for sure." I think if I can just show him the math and why it would never work, I might get through

Mark44
Mentor
welcome to PF
You gave your thread a really bad title, I almost reported it. PPM discussion isn't allowed on PF, not even to debunk it
Thread title changed to "Capture a car's lost energy"

davenn
davenn
Gold Member
I had no idea this was a rule (Assuming PPM is perpetual motion) I can't really change the title now though, or can I?

you should have read the rules when you signed up
As you can see I tagged a mentor to hopefully change it ( no you cannot)

davenn
Gold Member
Thread title changed to "Capture a car's lost energy"

Thanks @Mark44

davenn
Gold Member
However, this guy is pretty hard headed and "needs to know for sure." I think if I can just show him the math and why it would never work, I might get through

there are ALWAYS losses in a system .... and in any mechanical system, friction accounts for most of it

so power recovered can never come close to power generated

Mark44
Mentor
Diesel-electric locomotives generate electricity when they're going downhill, but this is to assist in braking, not to make up for the fuel used by the engine(s).

Hard-headed? My cousin who thinks he can run a water pump from an inverter attached to a battery to run a water wheel to power an alternator/generator which then charges the battery back and he thinks he can create energy. Explained 1st and 2nd thermodynamic laws, friction loss etc. and he refuses to believe. Tried to joke with him saying I was running a battery charger charging a battery that was running an inverter powered by the battery charging -- in other words, the same type of circle just without the extras. He didn't get it.

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russ_watters
Mentor
It's unfortunately a common problem. But since it looks like the OP had it right all along and the discussion isn't really acceptable here, I think it can be locked.

davenn and berkeman