# How is a hybrid car NOT an over-unity machine?

Discussion in 'General Electronics Chat' started by strantor, Jun 2, 2011.

1. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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In case my title is misleading, let me start out by stating that I understand that over unity is a myth and this thread is not arguing the point. now, please read on...

Before hybrids first came out, I thought they were a crock of b.s. My initial thought was that you can't increase the efficiency of a mechanical system by adding in an equally bulky electrical system along with it. I put it on the same shelf with HHO and perpetual motion.

Later, after they hit the market I remember reading that a group of university students had gotten ahold of a prius and removed the entire electrical system; batteries, motor, wires, computer, everything, returning it to what it really is, a compact car with a tiny engine, and it got better mileage. I can't find that study on the web, so I assume it was a myth.

So, I am asking, how is it possible for the car to be more efficient with this hybrid system? I understand that there is regenerative energy reclaimed by braking, but it can't possibly be more energy than it took to accelerate. It can't possibly be more energy than it took to charge the batteries in the first place. The numbers don't lie; the prius gets better mileage than a compact car with an equally sized engine, so how can it be?

Last edited: Jun 2, 2011
2. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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The ENERGY required to move the car from point A to point B is the same, due to the inherent physics of the car(the friction and weight mostly). With the Hybrid car you have TWO energy sources, a gas tank, and a battery pack. If you are only looking at mileage(MPG), but you are utilizing the energy from both sources then, of course, you will get better gas mileage. Some of the energy that would have been supplied by burning gasoline will, instead, come from the electric power source.

3. ### #12 Expert

Nov 30, 2010
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I think the basic principle is that you can run a tiny engine more efficiently that a 300 Hp engine because you don't have all 300 horses eating gasoline when they aren't needed. It's like you are feeding 3 or 4 unneeded cylinders when you are cruising along at 30 MPH.

If a tiny 4-banger has to work full time to keep the batteries charged between accelerations, that's OK because the other 4 cylinders don't even exist.

That's my take on it.

4. ### SgtWookie Expert

Jul 17, 2007
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Ahh, there's only ONE energy source; the gas tank.

The batteries are not an energy "source" per se; they are there for "lossy" storage.
Whatever electrical power you put into the batteries to charge them, you get less electrical power out; and the rest is dissipated as heat.

5. ### steveb Senior Member

Jul 3, 2008
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I think it's just the fact (or, should I say, the theory) that a standard auto wastes some energy as heat in the break system. This energy is totally wasted from the point of view of getting from point A to B. The hybrid is able to recover some of this wasted energy.

Back in college, I drove for a week with no breaks because they were broken and I was low on cash. I planned my driving times to be in little traffic and drove very carefully, while timing the traffic lights perfectly so I wouldn't have to stop. I very rarely had to stop using the emergency break. The lesson here is, first of all that I was dumb as a kid, but more importantly that even a standard car can be driven much more efficiently than we presently do.

My uncle has one of these hybrid cars and I'm convinced that he saves more gas by watching the fuel usage monitor than from the break regeneration system. He is obsessed with maximizing the gas milage on the readout. Good habits count as much as good designs.

Last edited: Jun 2, 2011

Apr 30, 2011
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The basic theory underpinning hybrid technology is that most cars only need ≈40 hp most of the time and a gas engine is most efficient when run at a constant speed. Using the battery/electric system as a reservoir for peaking power, the overall efficiency is improved despite the added losses.

That's the theory at least. Does it work?

7. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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The battery is just a battery & it's stored energy ultimately comes from the engine. That's only one power source, so that's what confuses me. If the engine has to work not only to propel the car, but also to charge this massive battery, then how can the presence of the battery increase efficiency?

1.Engine burns gas to propel car>(heat loss) > car moves
2.Engine burns gas to charge battery>(minor heat loss)>battery runs motor to propel car>(heat loss)>car moves.
seems to me that adding the battery & motor would KILL efficiency

That makes sense, the electric motor only uses exactly as much power as needed to move the car, vs the engine which uses a minimum amount of energy just to maintain the chain reaction of running. I imagine that the engine is able to transfer a greater percentage of it's energy into the generator>battery than it is able to transfer into the drive train>to the road (especially at idle).

8. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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Interseting point; do you think that by adding a similar fuel usage monitor to a standard compact car, the driver could obtain near-hybrid fuel usage?

on the flipside, if driven like a sports car, a prius gets worse mileage than a BMW M3 (and it's more environmentally damaging to make the batteries alone than the M3)

9. ### tom66 Senior Member

May 9, 2009
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The engine can also drive the wheels, can't it? In certain situations, that is...

Also, don't forget the other optimisations: CVT gearbox, modified Otto cycle (IIRC?) and perhaps other tiny things (don't own one so can't tell.) Electric motors are very efficient, in the 80-90% range. ICEs are only about 30%.

Also remember an ICE has the greatest efficiency in a certain RPM band, whereas electric motors are very flat in terms of efficiency (resistive and frictional losses being the major factors, and those matter a lot at high speeds, but not so much at city speeds.)

Oct 3, 2010
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Apr 30, 2011
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12. ### Kermit2 AAC Fanatic!

Feb 5, 2010
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I start off on a trip. The battery has been charging all night and I have a full tank of gas. You saying I can take the 800 pound battery out of the car and just leave it at home because it ISN'T an energy source?

I understand that while you are traveling the car is continually putting some charge BACK into the battery, but that initial charge IS an energy input for the car whether some is put back into it during use or not.

This obviously wasn't a physics question, but a 'usage style' question for hybrid enthusiasts. I'll just sideline myself and enjoy the show.

13. ### strantor Thread Starter AAC Fanatic!

Oct 3, 2010
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What you're describing is a "plug-in hybrid". People have been converting prius's over to plug-ins for a while and I guess the new nissan leaf or chevy car is a plug-in hybrid, but I was referring to the normal type of hybrid which is only charged from the engine/generator. I should have stated that, sorry.

14. ### k7elp60 Senior Member

Nov 4, 2008
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My wifes car is prius. It gets very close to 50 miles to the gallon. When going down hill the battery is charged by the movement of the car. When accelerating up a hill or an on ramp to a freeway the engine helps with acceleration. It has cruise control that work really well. It has a display thet indicates what is happening. Typically on the freeway at 75mph it is burning gas on level ground or up hill, down hill the motor goes back to an idle and the movement of the car charges the battery with more power than it needs for movement. At least thats what the display shows. It has a 10 gallon gas tank.

As a side note: Compare the physical size of a gallon of gas to a normal car battery. One can see that the gas has a great deal more energy than
a battery. Another thing to think about is that 746 watts = 1 horsepower.
Sure a car battery may provide about 300 amps for a short time to start the engine of your car. I estimate the capacity of a normal car battery to be 70 amp/hours. So at 12V that is 900 watts just a little over 1 horse power.
The prius over unity..............no way.

15. ### tom66 Senior Member

May 9, 2009
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My dad has a normal diesel 2 litre HDI turbo-charged Peugeot 206 and it does 56 mpg with no fancy electronics...

I know a couple people who have a new VW Passat which can do about 68 mpg, in other words from Basingstoke to Belgium and back on a single tank.

I like the idea of a Prius, but it is not a solution for reducing the damage we do the environment. I think the solution is a full electric car, but battery tech is behind the times, unfortunately.

16. ### Wendy Moderator

Mar 24, 2008
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I disagree a little. If the fuel is biomass or produced from solar electricity it is very green, it becomes a transport mechanism. Frankly the tech isn't there yet for electric, and what there is is expensive. This is a tech killer.

Jul 26, 2010
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18. ### steveb Senior Member

Jul 3, 2008
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Interesting, but I would double check the data to be sure the source is not playing any statistical games. Personally, I drive a Jeep Liberty which weights well over 4000 lbs (plus I carry about 500 lbs of stuff in it always) and I get over 20 mpg with a good mix of city and highway driving. This is not a huge outlier, but the article claims there are no outliers on the plot. Well, if you don't put them there, then of course there will be no outliers.

Also, I looked up the Jeep Patriot at 4435 lbs with 23 mpg city and 29 mpg highway. - Big outlier.

Of course, the correlation between mpg and weight is expected based on simple physics. I would be more interested in a plot that shows all commercial automobiles, rather than a hand picked set. How do I know that the guy who made the plot isn't selling something, ... like a car?

Last edited: Jun 3, 2011
19. ### shortbus AAC Fanatic!

Sep 30, 2009
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Total electric cars are really not going to save the Earth until they come up with a new way to power them. The plug-in electrics while they don't air pollute themselves still need a source of electricity to charge from. This source of electricity still takes energy and pollution.

Until some form of fuel cell or reactor is developed that doesn't pollute its always going to be trading one kind of pollution for another.