What are the pros and cons of series and parallel hybrid engines (e.g. Prius vs. Volt)? I have heard that electric motors are good for acceleration but not as good as ICEs for constant speed. If this is true, it seems like a parallel hybrid makes the most sense, but what do I know.
Thanks,
Rob
The big advantage of a series hybrid is that it can be run as a purely electric vehicle. If you do all of your driving around town, you may never need to buy gas.
Electric motors have good low-end torque and offer easy regenerative braking. ICEs have better high-end torque (to a point), so the combination makes sense from a complementary standpoint. However, as beowulff points out, the series hybrid provides the possiblity of zero gasoline use for short commutes while retaining range. The other factor is that the cost of electricity for those short commutes is less than half the cost of gasoline for the same distance driven. The plug-in hybrid electric vehicle, such as the Volt, has the potential for quicker and greater payback than the traditional hybrid, such as the Prius. I’m getting pretty excited about PHEVs.
In that case, is Toyota planning to make a series hybrid?
Thanks,
Rob
I’ve always been repelled by the complexity of the parallel hybrid, and wondered why we didn’t simply make an electric with a range-extending ICE powered generator which needn’t run all the time, and would allow the ICE to run very efficiently at a constant speed when it did. It was later that I found out that such a thing is called a “series” hybrid, and it still seems to me to be the way to go. I really hate the complex drive trains in a parallel hybrid to allow motive power to come from two separate power plants, and until convinced that such an arrangement isn’t a nightmare, I won’t own one.
One of the advantages of an electric is that you don’t need a transmission, and can make do with a bog simple power train. Why screw that up? The serial idea simply provides a generator as an auxiliary part to remedy what’s probably the biggest (though not ONLY) problem of electrics - the limited range.
I can see a series electric commutermobile being a very good idea - let’s say you have an electric with about a 20 mile range on the batteries alone. You buzz into work, having charged up in your garage, and use no gas. You buzz home, and the generator kicks in enroute, or maybe not, if you have a short commute or the ability to plug in at work. The point is that you don’t use gas or any other combustible fuel for the majority of your short trips, but you can still drive to grandmother’s house on Thanksgiving by burning some.
BTW, a few years ago, there was a company which actually made a comical seeming device. They sold a gas generator mounted on a little trailer, and intended to be towed behind your battery electric, so you could kick in the generator and recharge the batteries when they started running down. A series hybrid conversion kit for pure electrics, if you will.
I don’t think the Hybrid Synergy Drive has a transmission either, but rather a planetary gearing system to switch between the ICE and the electric motor or some combination of the two.
Thanks,
Rob
No. They are claiming their plug in parallel approach is superior.
http://www.greencarcongress.com/2007/09/toyota-takes-a-.html
Looks like they’ll be the ones playing catch-up if the Volt is successful.
My point wasn’t just the lack of a transmission in electric cars, but the simplicity of the drive train overall. You can hardly call the hybrid synergy drive “simple”, whether it has a transmission or not. “Sophisticated”, okay, but “simple”, no.
Another point with the serial hybrid is that you can accept a fairly low battery-only range, and only require a smaller battery pack. It becomes a design trade-off you can play with - how much battery weight are you willing to carry vs. how far do want to be able to go without burning gas.
I’m somewhat annoyed that GM doesn’t seem to want to push the volt until they have fancy-schmancy automotive LIon battery technology available. I don’t understand why the idea cannot be applied to age old lead-acid battery technology.
I think this is because lead-acid batteries don’t last very long when deep cycled regularly.
From a theoretical engineering point of view, the difference between parallel and series hybrids boils down to energy storage and conversion efficiencies.
As yabob points out, in a series hybrid, the engine is decoupled from the wheels, which means the engine is free to run at its most efficient speed and load. Energy conversion from gasoline to shaft work tends to be most efficient at high engine torque levels; in a “standard” drivetrain, the engine rarely runs at high torque levels, so there’s a lot of efficiency gain to be had here.
The drawback is the conversion and storage efficiencies in the additional drivetrain elements. In a series hybrid, all the motive energy has to be converted from gasoline to shaft energy (in the engine), then to electrical energy (in a generator) then to chemical energy (in the battery) then back to electrical energy, and finally to shaft energy (in the motor). Each step decreases the energy available. How much the energy decreases depends on a lot of things, because these efficiencies are not constant; they depend heavily on speed and torque or voltage and amperage. Batteries, for example, tend not to like heavy power draws.
In addition, a series hybrid would typically require some substantial energy storage–large batteries, in other words–to store all the power the engine is producing. Exactly how large is an engineering tradeoff: a very large battery pack stores a lot of energy, likely stores it more efficiently than a smaller pack, and allows the engine to cycle on and off only occasionally. However, a large battery pack is heavy, and that requires energy to haul around.
Then, of course, there’s the additional practical considerations: Series hybrids don’t require a transmission (or if they do, it would be a simplified one), so they’re simpler in that sense.
Finally, to point out: diesel-electric locomotives, which have been around for years, are series hybrids. In addition, not all series hybrids need to be electric; hydraulic hybrids can also be series or parallel, and have different sets of advantages and disadvantages.
I was reading recently about a car being launched in India that runs on compressed air. Anyone working on a pneumatic hybrid yet?
Thanks,
Rob
Sounds like the same problem as with flywheel energy storage - not too great in an accident. Also, air compressors are necessarily noisy.
Sorry, but a 5 mile range before recharge isn’t going to fly commercially. I drive nearly 20 miles to work, which with all the other issues (Living in Minnesota = hot summers, extreme cold in winter) leaves me doubting even the viability of the planned 40 mile range of the Volt.
Heavy, expensive, don’t last long, cost big money to replace and more big money to recycle the old ones. No one wants to have to pay a couple of thousand dollars to replace their entire battery pack every two years or less (I can’t recall the exact lifespan, but it wasn’t longer than this).
I wasn’t thinking THAT low. The 40 mile range of the Volt is more like it, though I used 20 as an example earlier. Note that you can keep driving - you just start burning gas. If you didn’t use any gas on your commute to work, and got good mileage out of it coming back, you may have a car that gets 80 mpg or something like this. The Volt supposedly gets 50 mpg when the generator’s running. Parallel hybrids which have only a very small emergency electric-only range are being accepted in the commercial marketplace.
Could I get a quick education on the differences of series and parallel hybrid engines.
My guess. The parallel uses both the ICE and the electric motor to provide motive force. The series would only use the ICE to run a generator to power the electric motor and charge bats. In the series, all motive power is provided by the motors like a locomotive.
Is that about it?
Yup.
Close enough. It’s really all about where the energy goes.
In a series hybrid, all the energy from the engine is converted to electricity (I say electricity for simplicity; it could be some other form) which is used to power a motor which turns the wheels. In other words, the energy follows a series path from the engine through the electircal system to the wheels. Like a locomotive, as you say.
In a parallel hybrid, the engine is still connected to the wheels, so some of the energy flows directly from the engine to the wheels, and some is converted into electricity. (This energy can come either directly from the engine or through regenerative braking.) In other words, the energy follows a parallel path from the engine, then either directly to the wheels or routed through the electrical system.
A plug-in hybrid complicates the scenario somewhat, because there’s two sources of energy: the engine and the plug. However, so far as I know, everyone’s comfortable considering the engine as the “primary” source, and defining parallel or series by the path the energy from the engine follows on its way to the wheels.
And, lastly, there’s no theoretical reason you couldn’t have a combination parallel/series gasoline/electric/hydraulic/pneumatic/human powered vehicle. It’s unlikely the extra complexity could be justified by any performance increase, but as I said, theoretically.
Why can’t a parallel-hybrid be a plug-in, too? AFAIK, the Prius already only runs the electric motor at low speeds, and clearly can use the ICE to charge the motor. Why can’t you just change the logic for when the ICE kicks in (based on amount of charge left in the batteries, instead of vehicle speed)? Is there some reason that plug-ins are easier with series hybrids?
Looks like Toyota is planning a PHEV in 2009.
ETA: Beaten to it by Keweenaw. Teach me to read a thread fully…