# The future of plug-in hybrid cars **URL:** \ **Category:** In My Humble Opinion\ **Created:** [November 30, 2009, 2:24pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458 "2009-11-30T14:24:50Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [November 30, 2009, 2:24pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/1 "2009-11-30T14:24:50Z")
Last year, [I started a thread](http://boards.straightdope.com/sdmb/showthread.php?t=480022&highlight=hybrid) about the different approaches to plug-in hybrid cars: > [@](#): > > I am curious about the various types of [plug in hybrid vehicles](http://en.wikipedia.org/wiki/Plug-in_hybrid), and which design is the most efficient. [Charge depleting](http://en.wikipedia.org/wiki/Charge-depleting) designs can run on batteries alone, then switch to [charge sustaining](http://en.wikipedia.org/wiki/Charge-sustaining)mode by turning on a gas engine to generate electricity. > > But this seems inherently inefficient, because during charge depleting mode, the batteries are expending energy to lug around the gasoline engine, which is doing no work. Likewise, when it switches to charge sustaining mode, the gas engine is lugging around batteries that are doing essentially no work. > > Wouldn’t it be more efficient to use a smaller gas engine that runs constantly at peak efficiency RPMs, to provide only enough energy to extend the life of the batteries to a useful range? That way, the vehicle is lighter (smaller engine) which makes the batteries last longer. > > For example, if a fully charged battery, running as the sole energy source, can provide a 40 mile range, but by adding a small gas powered generator that runs all the time, the range is extended to 80 miles (hypothetical mileage). Would this not be more efficient than a similar hybrid that switched between charge depleting and charge sustaining modes? Well, it looks like my wish has been granted. Capstone Turbine will be showing [a concept car at the LA Auto Show](http://finance.yahoo.com/news/Photo-Release-Hybrid-Electric-pz-722592417.html?x=0&.v=1)beginning on Wednesday that is very close to my specifications: > [@](#): > > “The sleek-looking, low to the ground, high-performance supercar definitely raises hybrid’s cool factor on several levels,” stated Jim Crouse, Capstone’s Executive Vice President of Sales and Marketing. “The CMT-380’s design performance numbers speak for themselves: 0-60 mph in 3.9 seconds, 150 mph top speed, and an unheard-of driving range of up to 500 miles on a single tank of fuel, all with ultra-low exhaust emissions that rival any hybrid on the market today,” added Crouse. > > … > > The CMT-380 features lithium-polymer battery cells that can be charged at home or at a public recharging station. While driving, the sports car can operate on 100 percent battery power in zero emissions mode for a range of up to 80 miles. When the batteries reach a predetermined state of discharge, the Capstone C30 microturbine quietly fires up and recharges the batteries on the fly to extend the driving range up to 500 miles. The diesel fueled C30 microturbine requires less maintenance than traditional combustion engines and produces ultra-low exhaust emissions. Does that look sweet, or what? I want my turbine car! ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [November 30, 2009, 3:32pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/2 "2009-11-30T15:32:47Z")
> [@Fear\_Itself](#): > > But this seems inherently inefficient, because during charge depleting mode, the batteries are expending energy to lug around the gasoline engine, which is doing no work. Likewise, when it switches to charge sustaining mode, the gas engine is lugging around batteries that are doing essentially no work. If you want the benefits of both systems, you have to carry them around all the time. You only use a car’s brakes when you’re slowing down. Do you complain about having to lug around all that mass while they’re not doing anything? > [@](#): > > Wouldn’t it be more efficient to use a smaller gas engine that runs constantly at peak efficiency RPMs, to provide only enough energy to extend the life of the batteries to a useful range? That way, the vehicle is lighter (smaller engine) which makes the batteries last longer. Think of it as a car with two gas tanks; one that you can fill at 50-cents a gallon, the other that you can fill at 3-bucks a gallon. If you had a switch inside the car, you’d run the cheap tank almost dry before you’d start using the expensive gas. A gas tank and a battery really are the same thing; a portable means of storing energy. The difference is in the details. A gas tank can be recharged more quickly, for example. But you get more miles-per-dollar by running off the battery. Your best bet is to drive with the cheap option until its limitation (short range) makes you use the more expensive one. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [November 30, 2009, 3:43pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/3 "2009-11-30T15:43:52Z")
> [@Robot\_Arm](#): > > A gas tank and a battery really are the same thing; a portable means of storing energy. The difference is in the details. A gas tank can be recharged more quickly, for example. But you get more miles-per-dollar by running off the battery. Your best bet is to drive with the cheap option until its limitation (short range) makes you use the more expensive one. I don’t think that is the Capstone design however. The news release says it starts the engine when the batteries reach a predetermined level of discharge, not that it replaces the batteries when they are exhausted. The turbine/generator in the Capstone car is a [30kw generator](http://www.capstoneturbine.com/prodsol/products/index.asp), which by my calculations, equals about 47 horsepower, which is not nearly enough to achieve the listed specifications by itself. To get that kind of acceleration and top end speed, it will need power from both the batteries and the turbine. ETA: Apparently it can get those specs form the batteries alone, but I don’t think a 47 horspower engine will do it by itself. It will extend the range, however. ---
**Author:** ![Keweenaw](https://avatars.discourse-cdn.com/v4/letter/k/da6949/32.png) [@Keweenaw](https://boards.straightdope.com/u/Keweenaw)\ **Post date:** [November 30, 2009, 3:54pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/4 "2009-11-30T15:54:05Z")
> [@Robot\_Arm](#): > > Think of it as a car with two gas tanks; one that you can fill at 50-cents a gallon, the other that you can fill at 3-bucks a gallon. If you had a switch inside the car, you’d run the cheap tank almost dry before you’d start using the expensive gas. With the Chevy Volt, that 'switch, is built into the control software. You’ll just need to input your destination. If it’s your home, or an equilivant recharging station the car will use GPS to map out how much electric it can use before the gas engine kicks on sustain change. If it can make it without kicking on the gas engine it will. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [November 30, 2009, 4:13pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/5 "2009-11-30T16:13:44Z")
> [@Fear\_Itself](#): > > I don’t think that is the Capstone design however. The news release says it starts the engine when the batteries reach a predetermined level of discharge, not that it replaces the batteries when they are exhausted. The turbine/generator in the Capstone car is a [30kw generator](http://www.capstoneturbine.com/prodsol/products/index.asp), which by my calculations, equals about 47 horsepower, which is not nearly enough to achieve the listed specifications by itself. To get that kind of acceleration and top end speed, it will need power from both the batteries and the turbine. > > ETA: Apparently it can get those specs form the batteries alone, but I don’t think a 47 horspower engine will do it by itself. It will extend the range, however. Which is the spec that concerns you? To run forever (or until the fuel runs out) the turbine only has to meet the average load instead of the peak load, with the electric system making up the balance when it’s needed, and recovering energy when it’s not. You need a lot of power for acceleration, but you can’t always be accelerating. You could do a maximum-performance 0-150, but the battery would be recharging while you were slowing down to do it again. There’s always a safety margin, but you could drain the battery down to where there was enough charge for two or three such runs and still meet the specs for acceleration. Top speed might be more interesting. Drag goes up with the square of velocity. Whether 47 horsepower could keep you cruising at 150 all day might be a bigger challenge that repeated 0-60 runs. ---
**Author:** ![Robot\_Arm](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/robot_arm/32/18280_2.png) [@Robot\_Arm](https://boards.straightdope.com/u/Robot_Arm)\ **Post date:** [November 30, 2009, 4:33pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/6 "2009-11-30T16:33:17Z")
> [@Keweenaw](#): > > With the Chevy Volt, that 'switch, is built into the control software. You’ll just need to input your destination. If it’s your home, or an equilivant recharging station the car will use GPS to map out how much electric it can use before the gas engine kicks on sustain change. If it can make it without kicking on the gas engine it will. That seems like a lot of complication for not much benefit. Do I really have to tell my car where I’m going every time I want to use it, even if it’s a route I’ve driven a thousand times?[sup]\*[/sup] Why not just use the battery down to its limit (not completely drained, of course) and then kick in the gas engine? Is there really such efficiency to be gained by squeezing those last few ampere-hours out of it if you know that home is just around the corner? What if I change my mind? Sometimes I stop at the grocery store on the way home. I could get a call with a last-minute dinner invitation. I can’t imagine the engineers would design a system that would risk leaving you stranded with a drained battery and gas still in the tank. The car must be usable on gas alone (plus some minimum charge), so why bother with mapping and prediction if the car can fall back to that mode? I’ve used in-car navigation systems. It’s kind of a pain to enter a location or an address, but it’s worth it if it helps me find my way. If you really need to enter the destination every time you drive the Volt, I’m sure the software would be optimized to make it easy to enter commonly used destinations, but that’s still an extra hassle to go through every time you drive. ---
**Author:** ![steronz](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/steronz/32/11765_2.png) [@steronz](https://boards.straightdope.com/u/steronz)\ **Post date:** [November 30, 2009, 6:47pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/7 "2009-11-30T18:47:43Z")
Does the hypothetical plug-in hybrid in the quoted section of the OP need to recharge after the hypothetical 80-mile extended range? Or does it just need to re-fuel? If it’s the former, then I’d leave the engine at home and just pack extra batteries. If it’s the latter, then it’s just a series hybrid, which is exactly what the Chevy Volt aims to do in non-supercar form. Note that it will still deplete the batteries, but then the 1.4L engine will kick on to continually charge the batteries for as long as is needed, making the range effectively limitless. However, if you never plug in a series hybrid, it’s not going to be as efficient as a parallel hybrid like the offerings from Toyota and Honda, due to losses in converting the mechanical energy from the engine into eletricity then back into mechanical energy. That’s why if the engine is going to turn on in a Prius, it might as well be used to turn the wheels. ---
**Author:** ![iamthewalrus\_3](https://avatars.discourse-cdn.com/v4/letter/i/258eb7/32.png) [@iamthewalrus\_3](https://boards.straightdope.com/u/iamthewalrus_3)\ **Post date:** [November 30, 2009, 7:05pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/8 "2009-11-30T19:05:31Z")
> [@Robot\_Arm](#): > > That seems like a lot of complication for not much benefit. Do I really have to tell my car where I’m going every time I want to use it, even if it’s a route I’ve driven a thousand times?[sup]\*[/sup] Why not just use the battery down to its limit (not completely drained, of course) and then kick in the gas engine? Is there really such efficiency to be gained by squeezing those last few ampere-hours out of it if you know that home is just around the corner? > > What if I change my mind? Sometimes I stop at the grocery store on the way home. I could get a call with a last-minute dinner invitation. I can’t imagine the engineers would design a system that would risk leaving you stranded with a drained battery and gas still in the tank. The car must be usable on gas alone (plus some minimum charge), so why bother with mapping and prediction if the car can fall back to that mode?[/size] This is much ado about nothing. _If_ you input your destination, then the car will use that information to plan out a more efficient use of battery power. If you don’t, it will just do something reasonable that will likely be very slightly less efficient. There’s absolutely no down-side to this, it’s just the engineers realizing that, since many people use the GPS units, the info is there anyway, and they might as well use it to squeeze a little more efficiency out. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [November 30, 2009, 9:08pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/9 "2009-11-30T21:08:02Z")
> [@Fear\_Itself](#): > > Well, it looks like my wish has been granted. Capstone Turbine will be showing [a concept car at the LA Auto Show](http://finance.yahoo.com/news/Photo-Release-Hybrid-Electric-pz-722592417.html?x=0&.v=1)beginning on Wednesday that is very close to my specifications: Wait, what? You originally thought that cars with “[c]harge depleting designs [that] can run on batteries alone, then switch to charge sustaining mode by turning on a gas engine to generate electricity” are “inherently inefficient.” Instead, you wanted to “[add] a small gas powered generator that runs all the time.” But now you think that a car that “can operate on 100 percent battery power in zero emissions mode for a range of up to 80 miles [after which] the Capstone C30 microturbine quietly fires up and recharges the batteries on the fly” has granted your wish. So what’s the functional difference between a car that runs on batteries alone and then turns on a gas engine to generate electricity and a car that operates on 100% battery power and then fires up an engine to recharge the batteries? Not that I necessarily think that any of these are bad ideas, but I don’t understand how you’re categorizing these cars that makes the Capstone fundamentally different from what you were skeptical about last year. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [November 30, 2009, 9:35pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/10 "2009-11-30T21:35:37Z")
It is fundamentally different in that it is a series hybrid, not a parallel hybrid, and it is not designed to run solely on the gas engine. I think that is a good idea, so the batteries don’t have to be unreasonably big to accommodate a useful range, and the gas engine doesn’t have to be large enough to pull the car all by itself. That is the difference I am applauding. ---
**Author:** ![zut](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zut/32/2875_2.png) [@zut](https://boards.straightdope.com/u/zut)\ **Post date:** [November 30, 2009, 9:56pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/11 "2009-11-30T21:56:03Z")
So then your original thread was about wanting series rather than parallel hybrids? In that case, what separates the Capstone from the Volt, which has (conceptually, at least) been around a lot longer? ---
**Author:** ![UncleRojelio](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/unclerojelio/32/3160_2.png) [@UncleRojelio](https://boards.straightdope.com/u/UncleRojelio)\ **Post date:** [November 30, 2009, 10:04pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/12 "2009-11-30T22:04:12Z")
I don’t have a dog in this fight because I’m holding out for my (long overdue) flying car, but, can’t a 47hp diesel turbine be significantly lighter in weight than an ICE? ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [November 30, 2009, 11:39pm UTC](https://boards.straightdope.com/t/the-future-of-plug-in-hybrid-cars/519458/13 "2009-11-30T23:39:54Z")
> [@zut](#): > > So then your original thread was about wanting series rather than parallel hybrids? No, that’s not what my original thread was about. My original thread was about using both the batteries and the gas engine simultaneously, thereby increasing efficiency by decreasing the dead weight of the batteries when the engine is running, and the dead weight of the engine when the batteries are discharging. The Capstone approach uses smaller batteries and a smaller gas engine, and I think that is a good idea.