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Old 01-18-2010, 01:30 PM   #1 (permalink)
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The problem with series hybrids

I've herd a few things lately that need more discussion. Namely with series hybrids like the volt and aptera appearing and folks assuming that running the engine at bsfc peak regardless of other "driveline" losses is the most efficient way to move a car. I don't believe it is, given a reasonable gearbox and a good driver, and a will and acceptable conditions (a personal decision) to drive for efficiency.

The thing is that the gas engine is billed as a "range extender", and to me that means usually a hiway trip for business/pleasure. I think a gas only solution would use less gas than a hybrid for this extended range mode, where an electric vehicle can handle the local "i.e. 40 mile radius" usage where there is a higher percentage of braking.

I've never seen anyone do any efficiency analysis on a series hybrid, as far as I can tell there are many losses to consider:

engine to generator
generator to controller
controller to motor
generator to charger
charger to battery
battery to motor
transmission/wheels


whereas a small gasser can use
engine to transmission/wheels

now that is a large number of energy conversions going on for the series hybrid, it is off to a real bad start. Lets also remember that these conversions are also not all at their peak efficiency all the time, motors and generators do not have a flat rpm/current/efficiency curve, nor do batteries have a flat efficiency curve.

So what of the benefits of bsfc peak? Well lets look at an example chart for a reasonably efficient engine:


Look at the small center "target" oval, @250. You can drive this car at within ~90% of bsfc peak from 1300 rpm to 4300 rpm!!! that is a huge rpm range. And the only equipment you need is a brain and a right foot, or from 40% load to %75 load if you have the rpm dialed in (i.e. coping w/hills). Of course this is best leveraged by pulsing and gliding around bsfc, and you do have other gears to climb hills with near bsfc peak and just glide down them.

So will all those conversions of the series hybrid outperform a "john henry" driver in terms of efficiency? I doubt it will, at least not before gas becomes obsolete anyway.

I do like the parallel hybrid short term, especially if it is just a small motor & wheel you stick in the hitch receiver of your electric vehicle, that is just sized to push you down the hiway near bsfc peak (the EV takes up the slack). To me that is pretty close to perfect (and similiar things have been done), as you still only buy one car and can go anywhere with it with minimal fuss, but move your daily commute to electric operation without waiting for the holy grail of batteries.

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Old 01-18-2010, 01:36 PM   #2 (permalink)
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The only thing is (and as you said, this has yet to be seen) is how much better BSFC could we get if the engine only had to be designed to run at a set rpm? Could you get it down to 200 or 150 (according to your chart)? I honestly don't know, but thats about the only place you could make up the efficiency losses from all the conversions as you stated.
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Old 01-18-2010, 03:52 PM   #3 (permalink)
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dcb -

I agree with Daox. From what I have read, if you design everything in the engine for a single RPM, you can optimize the intake and the exhaust to always operate at BSFC peak. The same optimization can occur for the emissions. Is it fair to compare an ICE BSFC to a series hybrid's (theoretical?) BSFC?

In your favor, maybe I am quoting R&D and university stuff that doesn't transition well to the real world. For example, it looks like the Volt's engine does *not* follow the single RPM rule. The engine does operate at different RPMs based on what's needed. Soooooo, it's not the kind of series hybrid I was hoping it would be. Orrrrrr, maybe the engineers saw the same thing you are describing and are also trying capture ~90% of BSFC.

From my POV, the whole point of hybrid drivetrains is that they are transition technology. Without them, you don't get to see electric motors, regen-braking, plug-in charging, and battery tech advancements being tested in the real world.

I agree that a good ecomodder can beat a regular driver in a hybrid under a broad range of driving conditions. But (again) the majority of people are not willing to take on this task.

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Old 01-18-2010, 04:25 PM   #4 (permalink)
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Quote:
Originally Posted by dcb View Post
I've never seen anyone do any efficiency analysis on a series hybrid, as far as I can tell there are many losses to consider:

engine to generator
generator to controller
controller to motor
generator to charger
charger to battery
battery to motor
transmission/wheels


whereas a small gasser can use
engine to transmission/wheels
To be fair I don't think an onboard genset will primarily charge the batteries. Most of the output will probably go straight to the motor and a little bit will go to the pack, which will be used for sudden/slight changes in output. Also, EV transmissions seem to be heading towards single gear reduction, so they'll be more efficient than a multispeed box, but you're pretty much right on regarding how lossy something like this is compared to a decent parallel setup ala the Prius.
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Old 01-18-2010, 04:31 PM   #5 (permalink)
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That is the problem, people claiming series hybrids are better and nobody has any source or specifics or any way to make an agreeable comparison.

lowest bsfc I know of is 155g/kwh, and it is from a 43 MegaWatt 2 stroke diesel.

can we at least agree that in the case where you have to go a long flat distance at a fairly constant speed that the lone gasser could be sized/geared/tuned to outperform an equivelant series hybrid? Even without much "technique"? Let's leave plugin out of the equation or at least account for the energy coming from the battery.

Yes, I want to see more electric used, but series hybrid is the electrical equivalent of an automatic transmission, which is probably why I dislike them the most. I think it is cherished for its convenience, while paying only lip service to efficiency.
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Old 01-18-2010, 04:42 PM   #6 (permalink)
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Quote:
Originally Posted by dcb View Post
can we at least agree that in the case where you have to go a long flat distance at a fairly constant speed that the lone gasser could be sized/geared/tuned to outperform an equivelant series hybrid? Even without much "technique"? Let's leave plugin out of the equation or at least account for the energy coming from the battery.
Oh, I definitely agree there. Throw enough technology (lean burn, atkinson, EGR, cylinder deactivation) into the gasser/diesel and you definitely can beat the conversion losses of a serial hybrid.
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Old 01-18-2010, 05:15 PM   #7 (permalink)
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IMO all that's needed is a parallel hybrid w/ an appropriately integrated transmission to beat a series hybrid. The biggest issue IMO, also why everyone is jumping on the EV bandwagon, is that Toyota has built a huge portfolio of hybrid patents, and anyone who wants to use something remotely close to their parallel system has to license through them, and if they don't then they tend to have a subpar system in terms of vehicle efficiency, eg the smaller Honda Insight/Civic hybrids get worse mileage than the larger Prius.
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Old 01-18-2010, 05:33 PM   #8 (permalink)
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Quote:
Originally Posted by cfg83 View Post
dcb -

I agree with Daox. ...From my POV, the whole point of hybrid drivetrains is that they are transition technology. Without them, you don't get to see electric motors, regen-braking, plug-in charging, and battery tech advancements being tested in the real world.

I agree that a good ecomodder can beat a regular driver in a hybrid under a broad range of driving conditions. But (again) the majority of people are not willing to take on this task.
i agree it's a transition technology.
i learned a ton from my driving habits when i bought a new 2004 civic hybrid. the cluster display told me so much. within a month i was consistently reaching 53mpg with hypermiling.

53 is great, but ecomodder.com has a handful of people reaching beyond that with older vehicles.
in 2004, i didn't have the time to mess with SG or aeromodding. but the civic hybrid opened my eyes and changed my opinion and habits.

btw, i learned about SG and hypermiling from here. i was hypermiling before i knew there was a word for it.
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Old 01-18-2010, 07:06 PM   #9 (permalink)
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http://www.innas.com/Assets/files/Hydrid%20brochure.pdf

Here is a series hybrid that beats them all, especially if you consider the vehicle they started with.

Hydraulic accumulator-up to 99% efficient
hydraulic motors-up to 95% efficient

Engine-pump-accumulator-wheel drive

Regeneration

wheel drive-accumulator-wheel drive

The key component is the wheel drive. It needs to be 95+% efficient. Most hydraulic motors loose a lot of efficiency when they run at higher speeds. This is why the motor needs to be in the wheel itself.

My design allows infinite variations in displacement in the wheel drive itself. The linked design requires a transformer to control flow and pressure to the wheel drives. Mine requires no transformer, and replaces the friction brakes on an equal weight basis.

In doing so it eliminates all the rest of the power train components.

Power from the engine to pump--- 95% out of the pump---99% out of the accumulator---95% at the tire.

That's 89.3475% of the engine power to the wheel.

Regeneration

Wheel motor 95%---accumulator 99%----wheel motor 95%.

That's also 89.3475% regeneration efficiency

Now you can run the engine at only it's best BSFC, because you are using it to charge the accumulator in every mode of operation, except when you are climbing a sustained grade that would deplete the accumulator.

Take your conventional vehicle and throw away.

Induction system
Transmission
axles
brakes
differential
Starting system
Propeller shaft

Use that weight saved to install an accumulator that allows you one 0-70 acceleration event. The size depends on the weight of the vehicle, but in every case it would be less than what you no longer need, listed above.

The engine starts by using hydraulic pressure from the accumulator. It shuts off when the accumulator reserve is topped off. It starts up when accumulator pressure is at a predetermined minimum that is driver adjustable.

The "engine" could be any form or design that gives you hydraulic pressure.

Electric
Steam
Diesel
Gas

It doesn't matter how you get the pressure as long as you have pressure.

The amount of energy required to stop from 70-0 will run your car almost a mile at 60 MPH. That is all the reserve you need.

In the linked design over the European test cycle, the engine only ran 11.9% of the time the vehicle was moving through the cycle, and only at its vest BSFC range.

Want a big honking engine, I don't care, it would just run for a smaller percentage of the time. Smaller engine, again I don't care, it just runs a longer percentage of the time.

The EPA estimated that fuel consumption could be improved by 80% through power train improvements alone. The INNAS design gives a 100% improvement with the same engine, in the same vehicle, through the same test cycle.

Instead we hypermile cars to do exactly the same thing the INNAS design does without any more driver input than the normal person driving normally.

Make the car hypermile itself.

Every out of system improvement:
Better aero
Lower rolling resistance
Weight reduction

Makes the engine run less to go the same distance and directly improves fuel efficiency.

Pop was a computer systems analyst beginning in 1960. He always told me to blame the system not the people. We all know people will always be the weak link in efficiency. When we make cars "people proof" by making them self hypermiling, we have solved the problem.

The biggest problem I see is that when we do make cars self hypermiling, then we will have to find another obsession!!!!!!!!!!!!!!!!!!!!!!!!!!!

Two power modules, one pure electric, one pure IC. Switch them when you need to take a road trip, or travel further than your electric only range.

One car does both jobs. When battery technology gets better you use the battery nodule more and the engine module less.

regards
Mech
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Old 01-18-2010, 10:18 PM   #10 (permalink)
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A serial / series hybrid is much more efficient than an ICE -- because the electric motor is so much more efficient than the ICE. It is about 85-90% vs 5-20%. And the clutch and transmission only lower the ICE's efficiency.

Any serial hybrid should be a plug-in (heck any hybrid should be!) and the other efficiencies of an electric car apply: you can get 25-45% of your energy regenerated in city driving; and somewhat less than this on highway driving. There are no idling loses. There are no warm up time (for the electric motor).

The serial hybrid motor has a fixed load, at a fixed torque. It can be much smaller because the peak torque is what the generator requires, and it only has to keep up with the average demand on the battery. And so, the engine can/should be tuned to peak efficiency at that known output.

We have several known examples of serial hybrids: diesel/electric trains are serial hybrids -- I wonder how it is they can move a ton well over 400 miles on just one gallon of diesel? The other example is the early Mini hybrid: it had four 160HP hub motors (for a staggering 640HP total!) and it only needed a 250cc ICE to charge it's batteries -- and it got ~80mpg in charging mode.

I can only imagine how much better FE it could get if it "only" had four 40HP electric hub motors -- 160HP from electric motors and four wheel drive, traction control, regenerative braking are all simple to implement -- this Mini hybrid has 'em.

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