Plug-in Hybrid Vehicles

Factual Questions
1

I am curious about the various types of plug in hybrid vehicles, and which design is the most efficient. Charge depleting designs can run on batteries alone, then switch to 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?

I understand this vehicle does not achieve the goal of operating without fossil fuel, but am I wrong to think it might be more efficient in its overall use of energy, both in charging the batteries and using fossil fuel? This also does not mean mixed mode hybrids could not be ideal for drivers who made mostly short trips within the range of the batteries; but in situations where the driver was consistently needing a vehicle that could go beyond the range of batteries alone every day, wouldn’t this be a more energy efficient solution?

I am envisioning a plug-in hybrid with a small gas turbine engine running constantly at peak efficiency RPMs, only large enough to provide enough electricity to extend the range of the battery pack, not replace it. Plus, a gas turbine is much more flexible in the type of fuel is uses, including biodiesel and ethanol.

I want my turbine car, dammit!

2

It would be more efficient, but you wind up with an extended range vehicle instead of an unlimited range vehicle. The existence of a maximum range, before a long recharge cycle, is considered a major downside to electric vehicles. Making the range longer helps, but it’s always lurking out there, waiting for you to screw up and get stuck somewhere you don’t want to be.

3

That’s good point, but only if you are using batteries with long recharge times. Altair Nanotechnology is supplying a battery to Phoenix Motorcar that can be recharged in 10 minutes, comparable to pulling into a gas station:

PG&E Corp. is buying four of the Phoenix rapid-charge trucks to evaluate for its fleet.

4

I really think you need to sit down and think about your energy storage + demand.

  1. Plug-in hybrids are designed to be plugged-in! The backup gasoline engine is only to extend range, and because of this, is much more expensive to run, per mile.

  2. You mention running an engine “…to provide only enough energy to extend the life of the batteries to a useful range?” What exactly, does this mean? In the Chevy Volt, the gasoline engine has enough power to run the car until it’s out of gas. If you have a less powerful motor/generator, than you can get into a situation where you have no battery power left, but still can’t run the car even with gasoline in the tank. If you plan on running the motor/generator all the time, you might as well eliminate the batteries (or make them very small). Wait! That’s what we have with the current crop of hybrids!

5

No, plug in hybrids are designed to be run on gas *and * be plugged in; that why they call them hybrids. Pure electric vehicles are limited by the relatively short range. That’s why the gas engine is added.

It means that if your batteries have a 40 mile range, and you want an 80 mile range, you install a gas-powered generator just large enough to supply the necessary electricity so that the total output of the batteries and engine is 80 miles. It is inefficient to have an engine that can power the car all by itself, because it is dead weight when you are running on batteries alone, resulting in lower efficiency during charge depleting mode.

So you pull into a recharge station and recharge in 10 minutes. How is that different than filling the gas tank on your hybrid?

A smaller engine that runs all the time at peak efficiency RPMs uses less gas & battery power to go the same distance as a larger engine that can carry the whole load.

6

I guess all I can say is: You must be a whole lot smarter than those engineers at GM…

You don’t think that someone has done all of the tradeoffs?

7

I’m sure they have, and settled on the one configuration that would meet the needs of the most buyers, not the one that yielded the best efficiency.

8

What does “best efficiency” mean?

You keep tossing around these nebulous terms. I’m sure it’s possible to make a hybrid that gets the equivalent of 200 MPG, but if it only seats 1 person, and can only go 5 miles, it’s not very useful. In your example above, you mention an 80 mile range. That might be fine for many commuters, but it’s worthless as an all-around vehicle. No one is going to take a 300 mile trip and want to stop 4 times to refuel.

Engineering is all about solving problems. What problem are you trying to solve?
Define your parameters - Vehicle range, weight, passenger seating, cost, etc. Then see what is the “most efficient” solution.

9

Precisely what is most efficient will depend on precisely what your usage patterns are. If, for instance, your car can go for 40 miles on battery alone, and most of your driving is a 15-mile-each-way commute, but with an occasional 600 mile trip to see family, then the current style of hybrid is exactly what you want: For most of your driving, you don’t need to burn gasoline at all, but you’re still able to burn gas for the occasional long haul.

If we had that same driving pattern, then the system you’re proposing would be a miserable failure: You burn gas on every commute, whether you need it or not, and you can’t go visit Gramma, either. It’s that visit to Gramma that’s the killer: If you want unlimited range, then the gas engine needs to be sufficient to fully power the vehicle.

The only place it seems to me that your system would provide benefits is for when your most-traveled-route is somewhat longer than the range provided by battery alone, but where you never need to go further than that most-traveled route. But even in that case, it seems like the better solution would be to just put in bigger batteries, and (if you’re certain you’ll never need a longer range) possibly remove the internal combustion system entirely.

10

It means, what design will yield the most mileage for the same amount of energy, both electricity to charge the battery and gas to run the engine.

I used those figures as hypotheticals only. The Volt is rated at 40 miles on electricity only, so your 300 mile trip is going to be using the gas engine for 260 miles. As you pointed out, that is not much of a savings.

I am trying to design a hybrid that will be the most efficient at longer distances, not the best car for the most people. Clearly, the Volt fills a niche if you drive less than 40 miles per day, but it is not the electric car that will meet everyone’s needs. The more your daily driving exceeds 40 miles, the less efficient (by my definition above) the Volt becomes.

11

I agreed totally. I am just asking if my idea would be a more efficient hybrid design if you regularly exceeded the battery only range. I cede the less than 40 miles per day drivers to the Volt design.

12

Then you probably don’t want a plug-in hybrid. The batteries are just dead weight for long trips. What you want to design is an efficient internal or external combustion engine.

Note that this is not a typical driving scenario for the vast majority of drivers, who do most of their trips within 5 miles of home.

13

So for the vast majority of drivers, isn’t the engine a just a dead weight for short trips?

14

The answer is, “it depends.” Not particularly satisfying, I know, but realistic all the same.

What the answer depends on are a few basic things:

  1. How long is the car in use between charges? This one’s fairly obvious. If John’s trip back-and-forth to work takes 10 miles, and Jane’s trip takes 60 miles, John runs in charge depleting mode all the time, while Jane is forced to segue into charge sustaining mode.

  2. How does the driver actually use the vehicle? There’s more to driving cycle than the length of trips. If Jane’s 60 mile trip is stop-and-go, Joan’s 60-mile trip is 35mph on suburban streets, and Jenn’s 60-mile trip is on the highway, both the total amount of power required, and the instantaneous amount of power required, is going to be different among the three.

  3. How efficiently are the batteries charged and discharged? Here’s the interesting bit. Charging and discharging the battery means changing energy from one form to another, which is inherently lossy. Rate of charge and discharge also affects charging efficiency.

  4. How efficient is the engine, and how is it used? Here’s the key factor. This is a little more obvious in the “traditional” mixed-mode operation, because the engine isn’t used at all in the first 40 miles, and it provides whatever power is needed for the rest of the trip. However, even in this case, the existence of the batteries means the engine can load level, and doesn’t need to instantaneously put out the max power required (merging onto a freeway, for example), and so can still be smaller than the engine in a non-hybrid. In your “always on” scenario, you might be able to down-size the engine (if you’re willing to accept a gradual draw-down of the batteries) and run it more efficiently yet, but the available margin is much smaller.

  5. How “costly” is gasoline vs. the plug on the wall? (for some definition of “costly,” which might include “efficient use of fuel source.”)

So, putting those all together, is it more “efficient” to run 40 miles on wall power, and then run X miles on the engine, or is it more “efficient” to run (x + 40 miles) on a combination of wall power and a smaller engine? Well, it depends.

15

Absolutely.
For most drivers, 80% or more of their trips could be made on electricity alone (with the Chevy Volt). For example, I work 6 miles away from home. Including travel for lunch, I drive less than 20 miles per day, 5 days a week. I take longer trips on weekends, but most are less than 40 miles round trip.

**However, I would never buy a car that I couldn’t drive any distance I wanted. **

I think the Volt is pretty well thought out. It might not be the most “efficient” vehicle for some special case, but it seems to be pretty much no-compromise.

16

One other solution might be to own multiple vehicles for multiple purposes. You might, for instance, keep an all-electric with just as much battery as is needed for your commute (or probably a little more, to allow a margin of error), but then also keep an all-ICE vehicle for trips longer than your commute. Or, for that matter, you could even walk or bike for your commute, if it’s short enough, or take public transportation, if available.

17

My point was not that my idea is better than GM’s, but that in the future, there might be a variety of designs available that would appeal to different drivers, depending on their needs. I just wanted to do a thought experiment to see if there was a niche that would be best served by a design like mine.

18

It’s going to come down to: how big of a niche is that? You are looking at people who drive over 40 miles on their daily commute, but don’t need to go over about 80 miles between fill ups.

Until this spring, I would have been your target market. I drove 40 miles each way to work, and we mostly use my SOs car for longer trips.

Unfortunately, no matter how good the idea is, if there aren’t enough peopel in the target market, there won’t be enough demand to allow economy of scale to bring the product price down. In short, I don’t think there would be enough demand to lower prices to the point where it would make economic sense to buy this car.

19

What external combustion engine would you reccomend? :wink:

20

I realize that you’re joking, but there is such a thing as an external combustion engine (used on cars, even), and there have been a few tentative efforts at making a modern alternative-fuel car based on them. The biggest advantage, so far as I can see, is that with the combustion chamber entirely separate from the moving parts, you can easily refit the engine to run on a wide variety of fuels or heat sources.