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Why an electric motor is used in hybrid cars

  1. May 7, 2013 #1
    I've been wondering about hybrid car technology lately and I'll start with just the most basic question to try and gain some clarity. I'll ask more specific questions if they come to mind after someone explains this first question. So, I was wondering why an electric motor is used in hybrid cars if it's just being charged by an IC engine anyway. Why not just use that small IC engine alone to power the car and eliminate the electric motor?
     
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  3. May 7, 2013 #2

    berkeman

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    One reason is regenerative braking. What can you tell us about how that works, and why it is an improvement?
     
  4. May 7, 2013 #3
    So, is regenerative braking one of the main reasons? Correct me if I'm wrong , but the bulk of great fuel economy can be attributed to the really small combustion engine, right?

    I'm no mechanical engineer (preparing to study EE), but from what I read briefly today, regenerative braking is thermal energy captured from the brakes by the electric engine and stored back in the batteries.

    Does regenerative braking really contribute much to energy and fuel economy? And are there any other reasons why the electric engine is used instead of a small IC engine alone?
     
  5. May 7, 2013 #4

    berkeman

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    Wikipedia lists 3 main reasons that hybrid electric vehicles are more efficient...

    http://en.wikipedia.org/wiki/Hybrid_car#Environmental_issues

    .
     
  6. May 7, 2013 #5
    It seems to me that most of the energy used by the electric motor comes from the combustion engine, though. So is the electric motor just responsible for regenerative braking and extra power when it's needed? The combustion engine seems like it's being used the most in a hybrid car, which means most of the lower emissions and fuel economy can be attributed to just a small IC engine.
     
  7. May 8, 2013 #6

    SteamKing

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    Shhh. You're asking too many questions. Next, you'll be wondering if hybrids are cost effective in terms of fuel savings v. price differential for a hybrid over a regular power train.
     
  8. May 8, 2013 #7

    russ_watters

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    A couple not mentioned:
    Electric motors are efficient over a much broader range of power outputs than ICE's.
    Electric motors start faster and therefore can be shut off when not in use.

    These two factors are a lot of the reason ice cars are so inefficient at city driving but hybrids are efficient.

    [Edit]Would be nice to see an accounting of the energy benefit to f each feature.
     
    Last edited: May 8, 2013
  9. May 8, 2013 #8
    I think I understand the role of the two motors now. The small combustion engine can charge the batteries much more efficiently than it can run the car because of the different work loads. Then the electric motor can be used to run the car off the energy stored in the batteries because it operates more efficiently than the IC engine. I can see the whole picture now, thanks for the clarity guys!
     
  10. May 8, 2013 #9

    OmCheeto

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    This article may be almost 2 years old, but I think it somewhat answers your question:

    http://www.engadget.com/2011/11/19/jay-lenos-chevy-volt-still-has-original-tank-of-gas-11-000-mil/

    The Chevy Volt is a hybrid. Leno drove it over 11,000 miles, without refilling his gas tank, implying that his electric motor was almost never charged by his IC engine.

    Electric motors can be plugged in when you get home. They can also be plugged in when you get to work. They can also be plugged in when you get to the store. If none of the previous three sentences are true where you live, then you don't live where I do.

    IC engines are at the most, 25% efficient, and in the real world, less than 5% efficient. (Based on my research of the data available from the Carnegie Melon Univ. "Charge Car" program.)
    E-engines don't suffer from this inefficiency.
    and E-engines, although minuscule, can deliver neck breaking acceleration.

    https://www.youtube.com/watch?v=GDHJNG2PngQ

    I once saw the Killacycle in real life. I got whiplash*, just from watching it.

    ----------------------------
    *Ok, slight exaggeration..... It just seemed like a neck snapping event. But wow.
     
  11. May 8, 2013 #10

    russ_watters

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    Wow, really? An electric car doesn't use gas? That's profound! Plug-in hybrids - and in particular that article - make the efficiency situation cloudier, not clearer because you typically aren't even told how much energy is being used in them and from what source (or are only told the gas and the electricity is treated like it magically appears in the car, from an eternal fountain!), much less how it is being used!
    I'd like to see where exactly you got that, because it really doesn't compute. It doesn't even actually make any sense: they can't be both at the same time. 25% sounds about right for real-world efficiency of a car engine on a highway. And city driving varies widely of course, but as rated tends to be something like 2/3 what the highway efficiency is.

    And in any case, if an article on the issue makes no comparison with electric motor driven efficiency, it is completely useless anyway! What is the energy use/cost difference between the two? Hypothetically, if the energy comes from gas (oil) either way, how much less oil does an electric car actually use? These aren't questions that can be handwaved away with the utterly pointless statement that an electric car doesn't use gas.

    Here's an article that actually discusses the issue. And shares my sarcastic exasperation about the pointless, hand-wavey, worshipey treatment the issue typically gets: http://www.motortrend.com/features/consumer/1010_chevy_volt_the_real_efficiency_number/viewall.html

    Here's what it says about the volt:
    Then they go on to derate the Volt's number a little more due to the charging inefficiency that isn't picked up in those numbers:
    That's actually surprisingly poor to me. Since a car engine is at best about 25% efficient, I would have expected an electric to be at least 3x the efficiency, so if under gas power it is a 40mpg car, I would have expected at least 120mpg equivalent electrical operation. And remember, that electricity comes from a power plant which if it runs on fuel is probably only 40% efficient anyway. Now that 73mpg is more like 30mpg! It means a plug-in hybrid or electric isn't really any more energy efficient than typical gas driven car, much less a gas hybrid!

    In terms of cost:
    Old numbers and both are low, so the ratio is probably about the same. As an electric, it costs half as much as as a strictly gas powered car.

    So if there is no actual energy benefit, why would there be a cost benefit? The fossil (and other) fuels used in generating electricity (coal and natural gas, 35% of our power each) are much cheaper than gasoline, that's why!

    For example, what if we just used the natural gas in a car?
    http://www.caranddriver.com/reviews/2012-honda-civic-natural-gas-test-review

    1.4:1, so not as good as the 2:1 for electricity, but I suspect that is because of the low penetration of CNG results in a huge mark-up. That cost quoted is much, much higher than the low pressure natural gas people burn in their homes/businesses: more than 50% higher for the home retail price alone and that's even higher than the commercial price.

    But even at that, a natural gas hybrid (instead of natural gas ICE alone) would necessarily make up all of the rest of that difference, being as cheap and efficient to operate as an electric. Good news because it means in the short term, while natural gas is plentiful, we never really have to suffer the limitations of full electrics. Natural gas hybrids will provide exactly the same energy performance without the limitations.
     
    Last edited: May 8, 2013
  12. May 8, 2013 #11

    AlephZero

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    That isn't new technology, for IC engines. There have been IC engine cars marketed in Europe with engine shut off for 30 years or more. The interest has increased in the last decade. In 2011 Bosch (the auto components and subsystems supplier) forecast that 50% of new Europoean car sales would have the technology by 2013 (but I don't know how that forecast turned out).

    It integrate nicely with manual transmissions as the "default" in Europe. The engine stops when the transmission is in neutral and the clutch pedal is up, and restarts when you press the clutch. The extra fuel cost of a "hot start" is claimed to be less than 1 second of fuel consumption at idle.

    The modern systems are smarter than the old ones - e.g. they system disables itself if the battery charge is low, since obviously the electrical load of lights, aircon, etc is not being met from the alternator when the engine is off.

    Future developments are "early engine stop" when the car is predicted to be slowing to a stop (possibly including sensors to detect other vehicles). and "auto coasting" to disengage and stop the engine at any speed where no power is being supplied to the drive train. IIRC Porsche are working on "auto coasting", which probably means it will soon filter down to VW and Audi.

    Given the "baseline" fuel consumption for medium size diesel engined cars in real driving conditions is the EU is "60 mpg at 60 mph" these days, even without that technology hybrids aren't a no-brainer option.
     
  13. May 8, 2013 #12

    AlephZero

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    It's interesting the way small gasoline engnes are developing these days. I recently had a courtesy car with a 1200cc engine. One of the "features" (on the cheapest model in the range) was a display of the recommended gear for best fuel consumption. It was a bit surprising to see such a small engine recommending the highest gear at only 1500 RPM (and speeds below 30 mph) but for town driving the performance was perfectly acceptable. I guess the engine management system was set up to run that way.
     
  14. May 8, 2013 #13

    russ_watters

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    That's great: I was aware the technology existed (as does the ability to do it yourself, of course), but didn't realize it was so widely used in Europe. It is my understanding that there is virtually none of it in the US. Perhaps that explains the always perplexing to me higher fuel efficiency of European cars vs American cars (that and the smaller engines of course)?

    Lots of cars have an "Eco" button now in the US. They should include that as an optional feature. It would do more good than the usual meaningless tactic of softening the gas pedal. For an automatic car, it would have to be based on brake pedal use.
    I can't see how that would provide much benefit at all, since it is my understanding that a gas engine doesn't use any gas when coasting anyway. I suppose if you are coasting but not stopping it isn't slowing you down as much as coasting against the engine, but if you are coasting to a stop anyway, all it should do is wear out your brakes faster.
     
    Last edited: May 8, 2013
  15. May 9, 2013 #14

    OmCheeto

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    Much has changed in the last 5 years

    https://www.physicsforums.com/showpost.php?p=1776095&postcount=19

    It does to me. Or did. I re-ran the numbers just now on my commute, from the numbers at CMU, and the efficiency was nearly 23.62%. I was almost ready to concede that you were correct.

    But as I recall, they had a maximum regen number. I'm pretty sure I ran all the numbers myself, and determined, that if I could ixnay their regen numbers, I could invalidate my current foible.
    It is, absolutely.
    And that's where everything falls apart.
    I'll try and find my non-hand wavy numbers. They are on my old laptop.

    She still has some battery power left in her. And if not, I have another i-charger.
     
  16. May 9, 2013 #15
    You boys have to be careful here. As this:

    Is 'dodgy' use of numbers, especially the bold bit. As you've now taken MPG and MPGe which are comparable, as they are tank-to-wheel numbers (inc. charging loss which is acceptable to include). Then applied a factor that is then taking into account upstream efficiency on the EV but not on the conventional fuel.

    You've also got to be to be very careful about where you getting figures from. Strictly speaking you can't compare 'real world' driving, as it doesn't provide a comparable test. As flawed as drive cycles are, they do provide a direct comparison between.

    As an example, if you are real world test of an EV and ICE in stop start town driving. You are playing to the strengths of the emotor. If you go on a long steady cuise you are playing to the strengths of a combustion engine.

    So all we can safely conclude is: it is difficult to conclude anything just from a numbers excercise.
     
  17. May 9, 2013 #16

    OmCheeto

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    I would highly recommend that everyone ignore this comment.
    As one of my hero's says; "We blow things up, so you don't have to."

    I performed this experiment on my last vehicle, where if I thought I was going to be sitting for at least 20 seconds, I shut off my engine. My starter lasted a week. It may have been a coincidence, but I don't think so. Starters on regular vehicles are simply not designed to start your engine every 5 minutes. Of course, if you doubt me, you are welcome to do the experiment yourself. My starter wasn't really that difficult to replace. And we can always use more data points. And just as a reference, the Charge Car data of my trip to work indicates that I stopped 20 times over the 10 mile distance.

    It may have been Charge Car's analysis of the cost of my commute where I came up with my numbers yesterday. That was back in November of 2009, so my memory is a bit fuzzy.
    Of course, it's all theoretical.

    One thing I find funny is to what extent some people will go to, to prove that electric vehicles are not a viable alternative. Charge Car requested people upload, from their GPS devices, the data from their trips to and from work. Check out this persons daily commute:


    He doesn't need an EV, nor even an ICE. That dude need a nuclear reactor!
    I'm curious what he does for a living(newspaper route for the entire state?), and how much a year he spends on amphetamines and coffee.


    I've done other experiment that have gone wrong. I once over-pressurized my tires by about 10%, and one of them had what I would call a "dissociation" failure. It started bulging, which gave the car a somewhat Dr. Seuss kind of ride. It had to be replaced, and I didn't do that again, either.


    I'll interpret this as; "It was after midnight and my brain wasn't working right."

    Unless you turn a gas engine off, it is using gas.

    Didn't you say a while back that you'd purchased one of those fancy aftermarket fuel consumption gauges? Does it read out in gallons per hour? Or liters per minute. I really need to get one of those.
     
  18. May 9, 2013 #17
    Injected engines don't fuel on overrun. The advantage of stopping an engine and coasting (ie disconnecting it from the drivetrain) is that you don't have pumping losses.

    It's the next step to 'stop start', the car acutally coasts with the engine fully off. Though with EPAS and vacuum storage and other electric driver aids you don't lose steering or assisted brakes. Which is nice.
     
  19. May 9, 2013 #18

    OmCheeto

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    I pretty much agree with everything, except your last comment.

    This will probably be the 10th time I've quoted this guy's lecture:

    Now, I'm not sure where he got that number, as he was only the energy secretary. :wink:

    But taking that number, the number of gallons consumed(~4) by Jay Leno driving 11,000 miles, the cost of solar panels(~$1/watt woo hoo!)... Well, you do the math. People keep whining about how taxes are too high. I say BS. Taxes get recycled in this country. That $430 billion(every freakin' year!) does not. It's money down the drain, or out the tailpipe, whatever.

    The sooner people figure out that getting 200 mpge is kinda cool, the better.

    -----------------------
    ps. 386 dtg, and the Koch brothers, OPEC, Exxon, etc., will be out to kill me..... :tongue2:
     
  20. May 9, 2013 #19

    OmCheeto

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    Apparently it was my brain that wasn't working right.
    Such things (DFCO = Deceleration Fuel Cut Off) do exist.

    I'll blame my ignorance on being old. I didn't have that on my 61 beetle, and if I shut the engine off while driving, it did all sorts of weird things.

    This technology must require some healthy automation. With the foot off the accelerator, the system would shut off the fuel, but this would create significant braking, so the system would have to open the butterfly valves, but that would reduce the vacuum available to the power brakes, making it a bit more dangerous. Ah! Sometimes technology scares me.

    hmmm.... I wonder......

    272 hp? That's more than my truck, two gliders*, and all my outboards put together.

    On one website I visited, someone mentioned that DFCO was more an emissions thing than a fuel saving thing.

    Anyways, thank you everyone for educating me. :smile:

    *glider is an EV enthusiast term for a car that doesn't have a working ICE.
     
  21. May 9, 2013 #20

    SteamKing

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    Not all of the crude imported into the U.S. is turned into gasoline has everyone forgotten diesel and aviation fuel?). There are a myriad of other uses for crude besides fuel production. Even if 100% of the passenger cars in the U.S. were totally electric, this would not negate the need to fuel whatever was generating the electricity. Everyone talks about using solar and wind, but very few are willing to have these types of electricity generators sitting in their backyards. A lot of people naively think that electricity is obtained by plugging whatever into the closest available outlet, blissfully unaware of the source of this power, because the generating plant may not even be within the same city or state in which they reside.
     
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