Disappointing results - just 51 mpg - for converted PHEV Priuses in Seattle
Nearly a year ago the city of Seattle started a field test of 14 Toyota Priuses converted to plug-in hybrid capability. At the time, the expectation was that the cars would top 100 mpg overall and 150 mpg in urban driving. The problem is that these PHEVs, like all other hybrids, are particularly sensitive to driving style. If you try to accelerate at any rate above the bare minimum the system won't stay in EV mode and the mileage improvement is slashed. After 17,636 miles, the 14 cars in the city motor pool are averaging only 51 mpg. That is certainly excellent mileage by any standard, but not that much better than a regular Prius.
The problem is the $10,000 cost of the extra lithium ion battery. At $4/gallon the fuel savings amount to only $200 annually. Seattle's experience is by no means unique. Google's highly publicized experiment with PHEVs is yielding similar results with its fleet of Priuses averaging 54.9 mpg and a pair of plug-in Ford Escapes getting 37 mpg. For all practical purposes, existing hybrid vehicles can't reasonably be driven as electric vehicles except at excruciatingly slow rates of acceleration. Given the cost differential, it seems like PHEV conversions may be an economic dead end (One of Toyota's PHEV Priuses is pictured [UPDATE: image removed at the request of Toyota PR). The Seattle and Google fleets feature vehicles converted by third parties).
[Source: Seattle Times]
Reader Comments (Page 1 of 2)
why not the LS2LS7? 12:17PM (2/24/2009)
Serial hybrid is the only way to go for PHEVs. It gets you true zero-emissions range and thus far greater mpg too (as long as you can stay within that range).
But sadly, serial hybrids cost a ton too, they definitely underscore the difficulty in making any kind of plug-in make financial sense.
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Chris M 5:27PM (2/24/2009)
There are certain advantages and disadvantages to plug-in series hybrids, compared to "split path" plug-in hybrids like the Prius plug-in.
Advantages: Series hybrid is easier to implement, with fewer moving parts. Easy to implement various "range extender" configurations, such as diesel or gas turbine or stirling engines. Since it requires a powerful electric motor, it can travel on batteries at top speed.
Disadvantage: "split path" hybrids can transmit most of the power from the IC engine to the wheels through a more efficient mechanical linkage, meaning better mileage when the IC engine is running. Since the IC engine can provide mechanical power, "split path" hybrids can use a less powerful electric motor, and can use a less powerful battery pack that could be less expensive.
I'd say there are opportunities for both types of plug-in hybrids.
One of the problems with these after-market plug-in conversions is that it isn't fully optimized for plug-in operation. When Toyota designed their own plug-in Prius, they took advantage of the extra power available to use more powerful motors and changed the gear ratio of the power split device, which upped the top "EV" speed to 65 mph, and also optimized the control algorithms to optimize battery use and fuel economy.
why not the LS2LS7? 11:23PM (2/24/2009)
If you have an electric motor that can go 65, you've got a motor the size of a series hybrid.
And the ability to reduce the size of the battery is minimized if you make a vehicle a PHEV, because you have to have some place to store that power.
Eletruk 12:34PM (2/24/2009)
The real problem with plug-in hybrids, is that people forget to plug them in. If you drive a pure EV, then you quickly learn that if you don't plug it in, you don't go anywhere. With the hybrid, it's "Oh well, I still have gas". People that own the plug-in conversions easily get 75-100 mpg, and they remember to plug in and recharge the battery. If you are a fleet driver, you probably don't really care. Or at least that's what the results indicate.
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Mike Z 12:56PM (2/24/2009)
Do you have a sure that attributes this as the cause for the disappointing results?
Eletruk 1:34PM (2/25/2009)
This is according to the V2Green data that tracks the car usage. Tthe problem at the "bottom of it," as you put it is that these fleet cars are all too frequently dispatched with little or no charge in their supplemental Hymotion packs. The reason for this is that the drivers don't own the cars and don't pay the fuel and electricity bill.
kramerica5000 12:45PM (2/24/2009)
Seems to me that loading up a car with multiple drive trains would just make it heavy and inefficient. It's no wonder that acceleration returns poor result because you are always lugging around the dead weight of the gas engine.
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BoneHeadOtto 1:58PM (2/24/2009)
The prius already has multiple drivetrains. It can drive purely on electric, purely on gas, or using both. That does not in itself make it too heavy to be efficient. And yes adding extra batteries does add weight. But the problem with these conversions is that the Prius can only go up to 30mph on electricity alone. If you accelerate too fast it cant even do that. So the real problem is that the electric engine is not up to the task of lugging the car on electricity only, and people always drive faster than 30mph and probably try to accelerate too fast to keep the car in the sweet spot.
John Rowell 12:59PM (2/24/2009)
I would have thought that by now someone would have figured out how to hack the Prius's firmware to prevent the engine from kicking in so soon.
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Rich 1:12PM (2/24/2009)
I think that's the key. When you add more battery you have to change the software. Hacking it would be very difficult IMO. Toyota would have to hand over the source code. Or you could replace their computer with your own but the amount of work would be overwhelming for one individual.
Dave 4:36PM (2/24/2009)
The problem is that there is only so much you can do with the stock Prius hybrid system to keep the engine from starting up. And once it does start up, it's programmed to keep the engine running for a while to reduce emissions in case it has to start up again.
These factors can cause the engine to run more often, reducing fuel economy:
1. Hard acceleration (generators/battery aren't designed to provide too much acceleration power)
2. Cold weather (Engine takes longer to warm up if it start to start)
3. Running heat (cabin heat is drawn from the engine - see #2)
4. Limited EV only speed (32mph max unless really gentle on accelerator, 42mph max period)
Knowing these limitations of the Prius PHEV conversion might encourage drivers to attempt to work within those limits.
To get optimal results from "normal" drivers requires more modifications than adding a big battery pack and tweaking the ECU programming a bit.
Even then - Google's RechargeIT.org program has demonstrated that what a company fleet vehicle gets may not be representative for what your typical driver might achieve. For example, with their controlled test, their PHEV Prius gets over 90mpg. But their fleet vehicles' results are more in line with what Seattle got.
PeterG 1:03PM (2/24/2009)
I have always said these conversions make no economic sense whatsoever. They also make little functional sense in vehicle not intended for full time/full service electric drive mode.
I am also dead set against misleading "blended" mpg claims in PHEVs.
It should be electric range + sustaining mode MPG.
All around these conversions are a farce.
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Dave 7:07PM (2/24/2009)
Even the people who buy these PHEV kits agree that they don't make financial sense.
The people buying these are the early adopters who know what they're getting isn't perfect, and willing a premium to help fund technology that will one day make financial sense.
That said, I think they do make functional sense - even under fleet operations, the PHEV Prius used 50% less fuel that the regular Prius in this study. With a bit of user training, the fuel savings could be even higher.
paul 1:09PM (2/24/2009)
The problem with the plug-in conversions of the existing prius is that people are trying to make the car something it is not. The prius is designed as a primary gasoline engine powered car with an electric motor that helps the gas engine out during low-power/torque requirments. It does a great job of filling that niche. However, the electric motor is only designed to "help" the gasoline engine, it dosn't have the capability to produce the amount of torque required to move the mass of the prius quickly by itself. So, it dosn't matter how much electric capacity the batteries have, the motor can only supply torque up to its design capability. So, when you put the Li batteris in the prius, now you have an electric motor that can give you a low amount of torque over a longer period of time, but it still can't accelerate the car quickly - it can't provide the necessary torque. Plus, the plug-in conversion is still limited to 34 mph in EV-only mode. The only way that plug-in hybrids make sense is if you design the electric motor to be powerful enough to drive the car by itself without the engine. Of course, then you have put two fully capable drive-trains into the same car, the economics of which probably will not work out. As much as I love my Prius (great car for what it is!) I think Chevy has the right idea with the Volt - basically the exact opposite - the car is driven by the electric drivetrain and the gasoline engine "assists" the electric motor by providing juice when the batteris are used up. Whether or not Chevy can actually come through with the Volt is another matter all-together...
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marz 1:07PM (2/24/2009)
I read somewhere that these typicaly aren't being plugged in overnight. It won't do much for MPG if they're driving around with a couple hundred lbs. of empty Hymotion batteries! If that's the case then, they're doing pretty well averaging 51MPG. That's why the specifics of how these are being used are important.
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Paul 1:17PM (2/24/2009)
PeterG is correct. Why does anyone think that simply adding a bigger battery to a standard HEV with a small electric motor intended primarily for slow speed and acceleration assist will magically make a PHEV? It has nothing to do with plugging it in, folks. It has everything to do with trying to power a car with a lawnmower engine (a bit of an exaggeration, but a useful analogy). This is a con job and a waste of lithium. Wait for real PHEV's with adequately sized electric motors. Better, wait for a serial drive train (range extended electric) where the electric motor is the only means of propulsion. Then you'll see real benefits to plugging in.
PeterG, I agree as well that the combined MPG figures are a farce, especially for serial drive systems.
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MT 1:45PM (2/24/2009)
The electric motors in these were never intended to be used for primary propulsion. It doesn't matter how many extra batteries you stuff into the current hybrids, they're still going to have to use the gasoline motor for getting up to speed, climbing hills, etc with the flow of traffic. Therefore you're never going to see huge increases in mpg under normal driving conditions. Now if you swapped out the motor with a more powerful one, then you could make better use of extra battery capacity by staying in all-electric mode during most city driving. But like others have said, then you're wasting battery capacity to haul around an ICE and fuel in a pseudo-EV. You might as well go pure EV possibly with a very small gas engine for range extension..
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marz 2:21PM (2/24/2009)
The electric motor in the Prius is rated @ 67HP & 295lb.-ft. of torque. It's not the lack of a powerful motor, it's the lack of power to the batteries if the dopes aren't plugging them in overnight. The Hymotion packs can't charge from regenerative braking or the gas engine, like the stock Prius batteries, so if they don't get pluged in & charged overnight they're just driving around with dead weight. That's the problem with studies like this, if the users aren't educated in how the product works & willing to maintain it, the results will be meaningless like these. Like Electruk said, most people that have the Hymotion conversion are averaging 75-100mpg with normal driving. Those that are willing to modify their driving can do even better.
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jharlan 3:00PM (2/24/2009)
I agree with everyone. The manufacturers can do better than this. These cars make no economic sense, they are just a political statement made by those who value symbolism over substance.
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Dave 7:07PM (2/24/2009)
_________ make no economic sense. They are just a political statement made by those who value symbolism over substance.
Feel free to substitute anything you wish in the blank and it will work.
Try Ford Fusion (when compared to BWM 3-series), for example.
No one is buying PHEV conversions for economic reasons. Just like no one buys SUVs, luxury cars, sports cars, or any car for that matter for economic reasons.