2010 Prius Plug-In - 60 MPH EV, 13 Mile Range, 1.5 Hour Recharge

Nice, they implemented my idea of pre-cooling the car using the plug:rockon:

 

Nice for the owner, bad for the planet. I can't help thinking that it may cost more energy than you save by plugging in.

 

I was thinking the same thing. When I read about the pre-heating and pre-cooling, I thought "Cool! I can set the heater to turn on at 7:00am (about ten minutes before I usually get into my car), while the car is still plugged in and the battery is full." But then as I kept thinking, I thought about the fact that it would be electric heating, and adding to my home electric bill. But then again, it's a small space; just the interior of the car. I guess we'd have to experiment with it.

 

Price? another spec of great interest.

 

It's a heat pump, which is generally more efficient than traditional resistive electric heating. In fact it seems to be almost a magic device from what I can see, being quoted at producing a heating power of 2.5 times as much as the electrical power required to run the pump.

It's basically an air-conditioning compressor being run the other way around: instead of cooling the air going into the car by heating up the surrounding air, it cools the surrounding air and heats the car. The specification describes it as both a heating and cooling device.

In any case, it's far more efficient than burning petrol to heat the car - even with the Exhaust Heat Capture system, most of the fuel wasted with the engine idling (over 80% of the heat content of the fuel) just goes straight down the exhaust pipe.

 

Doesn't really matter what it is when it is used to preheat or precool the interior. The energy is essentially "wasted" compared to not preheating or precooling the interior. Interior heating or cooling on the fly is likely more energy efficient. This is particularly true of heating the interior while accelerating.

On another note, the petrol based efficiency is about the same as that of the typical coal plant.

 

Yes, certainly it's wasted compared to not pre-heating. However, I disagree that interior heating 'on the fly' is more energy-efficient. If it uses regular resistive electrical heaters, it will use more power (the Coefficient of Performance, ratio of useful work to input power, is given as 2.2x for heating from -20°C to +35°C, and the CoP increases for lower temperature differences - CoP of a resistance heater is 1.0 almost by definition). Also, it's power coming from the battery, so there have already been losses in power conversion and charging.

It does raise the question of why the on-board A/C compressor cannot be used as a heat pump, though. Possibly that it's still needed for dehumidifying the air.

Edit to add: duh, the heat pump is on board. This is also the primary heater when the car is moving, running electrically rather than firing up the engine to produce heat.

At peak efficiency for the petrol engine, yes, but it doesn't usually operate at that point because average demand is much lower than the point of peak efficiency. BSFC chart:



The peak value of 220 g/kWh can be translated as follows: gasoline has an energy content of around 44.4 megajoules per kilogram, so one gram has 44.4 kilojoules (kJ). (Data from Gasoline - Wikipedia, the free encyclopedia). One kilowatt-hour is 3600 kilojoules (one watt = one joule per second). So we burn 9,768 kJ of energy to get 3,600 kJ of output, or efficiency of 36.8%.

Note, though, that this region runs from about 1,200 to 2,700rpm, peak efficiency being somewhere in the middle of that. When cruising at moderate speeds, the engine doesn't need to turn nearly that fast. The other graph from the same page shows the power outputs:



At peak efficiency we're producing 20kW at the output shaft. That's way more than we need to cruise comfortably - the power needed to overcome air resistance at 70mph is only about 14.5kW on Gen 2, and that's an overestimate, I've used the Gen 2's overall height and width multiplied by Cd. Gen 3 is 20mm wider but marginally lower, and a lower Cd, giving about 14kW.

The power required to overcome air resistance varies proportionally to the cube of the speed: at 50mph it only needs 5kW which on both Prius generations is on the vertical line on the left-hand side of the graph (there are of course other losses, due to rolling resistance etc, but I don't know how large they are).

In practice the car runs the engine at the lowest speed it can (more revolutions = more air pumped = more fuel needed per second, in general). That means less efficiency, however it actually consumes less fuel overall. In the end miles per gallon is just miles per hour divided by gallons per hour, so the speed you're actually travelling does affect your fuel consumption in that way too.

Note: the whole BSFC graph shows the efficiency at any torque and speed output that's possible on the engine. The thick black line, I believe, shows what the HV ECU will actually ask for once it's fully up to temperature - it does deviate a bit before being up to temperature, generally revving higher with the throttle further closed, moving down and right on the graph to achieve the same power but with less efficiency = more heat. If it just wants to charge the battery it follows the curve up and to the right which generally gives greater efficiency!

In contrast power plants run a lot closer to their peak efficiency, most of the time. To do otherwise would be uneconomic. Coal plants do take a lot of effort to start up if they've been shut down, so plants that are easier to vary (e.g. natural gas) are usually shut down first.

 

I don't think a heat pump will operate below 20F. So the PTC heating elements will have to be used for very cold temps.

Wayne

 

But who would live in such a cold place?

 

Those of us who have a penchant for being creative in the snow.

wayne

 

For Prius PHV, high efficiency at low load would be critical in the blended mode. 220 g/kWh (near peak efficiency) at about 1,280 RPM is a must. It is a well thought out and nicely tuned ICE. It shows Toyota thought well ahead for the PHV version.

 

Mike

I do what I can to keep the engine running in that efficient range (GII). I don't baby it like some do as it became pretty obvious early on that it was counterproductive to mileage. I want it either pulling efficiently or off, not "lugging" in low efficiency mode.

As it is I rarely run the heat except when the car has been sitting outside (not garaged) and it is very cold (below 20F where a heat pump won't work...and I doubt it's COP or capacity is suffient at even 30F), or when defog/defrost is needed. Note that for outside start that usually means a commute from a parking lot...so using the existing charge would be a questionable trade because of range shortening. And what I do is campaign the heat/defrost. When sitting idle I tend to shut it off, I run it when accelerating or pulling.

But I need to remind myself that It's academic anyway, as my electric source is 100% coal, so the CO2 footprint of EV is inferior to the Prius in regular hybrid mode.

 

Well, when you put it that way, it costs FAR more energy to drive to work than to walk....however we have decided as a species that comfort is more important than most things. All I wanted this idea for, was so that the car would be kept at a more moderate temperature. So if I get in the car in the middle of the summer, the steering wheel is not too hot to touch; or too cold to touch in the middle of the summer.

In any case, my state is implementing a whole lot of green energy technologies - see Renewable Energy in Texas - Wind, Solar & Biofuels in Texas - Popular Mechanics .

Granted, Houston probably gets most of its electricity from coal and gas plants, but I still have to think that pre-heating the car for 10 minutes via electricity might be more efficient than running at a much lower fuel economy range, due to the engine having to run longer, to provide cabin heat.

 

Ok Toyota, enough with the tease. How about letting us know when the Prius plug-in will be available to purchase and how much extra will it cost (as an option)?

 

The way I look at it is if you can have your petro engine turned OFF for an entire 13 miles of EV mode (for some that means never having to turn your engine on at all), perhaps that 9 minutes of pre-heating (just so the interior is tolerable and not beyond that) is an energy bargain!

Toyota could improve things for everyone by doubling the capacity of regenerative braking. As I've suggested in the past, having an additional generator that only engages the drivetrain when braking exceeds the regen capacity of MG should be experimented with. With as many aftermarket phev conversions we've seen pop up around the world, I'm surprised no one has advertised an additional generator to double the regen capacity.

Perhaps you could make a home-made version.
(go to autozone or pep boys and buy a bunch of alternators and couple them to your half-shafts using starter-motor solenoids that are activated by the brake pedal. Hahaha.)

(I'm assuming the reason Toyota has not added an additional generator is because the magnets that Toyota uses are EXPENSIVE! So, they should make it an option and charge extra)

 

That's a very good point. With additional batteries the ability to absorb a higher overall amp load should be no more stressful to them than in a standard Prius. Start by connecting it to the rear. But beware that it will add a lot to cost, traction control and whatnot. The rear end would become considerably more expensive to build.

If I'm not mistaken another factor would be the inverter. It is sized for the present amp load. Double that instantaneous load and the inverter will have to be enlarged or otherwise augmented, correct?

If this also could be used for drive power then the acceleration of the vehicle could be very impressive as well...

 

In another thread, Doug Coleman (Prius Team) said about the PHV:
'we are planning to launch retail sales in 2012. So there is already a follow-on phase on the books that won't just be "limited release, fleets, lease plans".'

Didn't specify cost, but a statement about retail sales in 2012 is more than I'd seen before.

 

There is no need for additional generator. HV NiMH pack can take about 25kW. MG2 alone should have capacity to generate around 60kW.