AC commits pre-meditated murder on MPG

The HVAC inverter takes 201.6 Volts DC and outputs 201.6 Volt AC, according to Toyota technical documentation. I'm not sure if this is 201.6 peak voltage, or rms voltage. It seems like it should be peak voltage, but it is never clarified.

Sometimes the DC-DC converter isn't using 201.6 Volts. For instance, if MG1 is outputting 400 Volts DC (after AC/DC conversion), and MG2 is using 650 Volts, then there is not a useful voltage there to directly run the HVAC inverter. So the HVAC inverter runs off of the battery.

This is because depending on its speed of rotation, MG1 can output a number of different AC voltages which will in turn correspond to a different rectified DC voltage.

As far as I know there is only one bi-directional high voltage DC-DC buck/boost converter, and one uni-directional 12 Volt DC-DC converter. So there isn't an extra converter to let the HVAC tap directly into the high voltage DC-DC bus when those voltages aren't compatible.

So you may wonder how MG1 can power MG2 and charge the battery at the same time. You can do this by spinning MG1 at a certain speed which in turn generates an AC to DC voltage that is slightly above 201.6 Volts DC. The slightly greater than 201.6 Volts can then be used to charge the battery, and then still be boosted to the voltage that MG2 needs. This way you don't need another highly expensive DC-DC converter.

BTW, the ORNL reports confirm that is how the Prius works in the above paragraph.

So in short, only when MG1 spins at a very specific RPM range is ~201.6 Volts present in the DC-DC bus. Most of the time MG1 does not spin at this RPM, because it needs to spin at an RPM to to allow the ICE to spin at its own most efficient RPM. When 201.6 Volts is not available, hardware limitations require that the HVAC inverter run off of the battery.

 

I can guarantee you have never seen 95 F and 95% humidity at the same time. That would be a heat index over 150 degrees F!

 

History : Weather Underground
95 degrees F and 100% Humidity on the same day, although not the same time.

 

Out of curiosity, I have monitored A/C watt usage with my SGII and seen it as low as 150 watts and as high as 2250 watts.

 

In matters of engineering and physics I am *never* sure

You may very well be right that the CoP for AC cooling is > 1.

 

Not quite correct.

The A/C inverter runs directly off the battery which may vary in voltage from about 180-250V (the 201.6 is the nominal). The bidirectional DC-DC converter then converts the battery voltage to the main DC system bus that can be anywhere from 200-500V (I don't think the Prius runs up to 650 V although other higher power Toyota Hybrids such as Highlander and Camry do).

The inverters that drive the motors (MG1 and MG2) can also step up the voltage from the motor to the system bus voltage whatever it may be - there is never a case that a motor/generator is producing too little voltage except at very low speeds (<7MPH for MG2). These inverters use the motor inductance rather than requiring a separate high-power inductor as in the case of the battery to system bus converter (that inductor is about a 4" cube I gather).

The main reason for the system bus voltage being varied is that it needs to be above the EMF produced by the rotation of the permanent magnet motors - so in general the system voltage increases with road speed. At the highest speeds the system uses field-weakening to enable MG2 to rotate a little faster than would otherwise be the case.

The more complex system used in the high-end Lexus hybrids have a two speed gearing to MG2 to extend the speed range.

kevin

 

That is what I previously stated. Thanks for proving my point!

 

I'm glad you can clarify this because I had a lot of trouble finding out some of these details. I was thinking about what I wrote previously and concluded I must be missing something.

How many DC-DC converters are there? Obviously at least one for the battery to boost its voltage.

Suppose when MG2 is running at max voltage, and MG1 is not producing a high enough voltage (due to a low RPM, which is dictated by MG2's RPM and the ICE RPMs), then there must be another DC-DC conversion. Is this correct?

 

There is one high power DC-DC converter connecting the battery to the system bus. It can only step up voltage from the battery or step down from the bus to the battery. This is rated at 25KW in the Gen III and 20KW in the Gen II. (125A or 100A from/to the battery)

There is an inverter from the battery to drive the A/C compressor. I expect this is rated at about 2KW.

There is a 1KW DC-DC converter from the traction battery to the 12V auxiliary battery (about 80-100A max).

The two motor inverters are rated at 30KW an 50KW respectively (MG1, MG2). They don't have a separate inductor or transformer but use the motor inductance to step-up from the EMF that the motor is producing as a result of its rotation up to the system voltage. Even when the road speed is as low as 7MPH the MG2 voltage (which is only about 35V at that speed) can be stepped up to the ~200V of the battery. That is why friction brakes take over below 7MPH.

kevin

 

Heat pumps are more efficient than A/C. However, it wont be able to drop the temperature as effectively as an A/C unit.

I did a little experiment today. In order to get the fan on 3 with A/C auto, I had to set the temperature at 85 degrees. And this was already after 15 minutes of A/C cooling. MPG has also dropped from 46.4 to 45.7 from running it continuously. Battery dropped to 3 bars. Not quite pink, but also not ideal.

I think A/C off during stops, and A/C auto at highway speeds will maintain the best compromise in terms of mpg and comfort.

 

Aww, kwitcherbellyaching. You are trying to over think things. If you really do not want to use the A/C, then sweat if you will. For the rest of us, A/C is a godsend.

 

I was able to test it today, no camera though.

93 degrees outside.

starting cold after sitting for approx 3hrs.

Auto set at 83 fan speed was 4.

6 bars on battery SOC

6min to 3 bar, <1 additional minute to 2 bars and engine start.

 

History : Weather Underground
Last night at 9 PM, it was 85 degrees and 88%

 

I drive slightly more than 70 miles daily on my roundtrip commute to work in my 2010 Prius IV. It's been between 97 - 100 pretty regularly here. Tinted windows and the custom sun shade sold here help to keep the car noticiably cooler than what I'd experienced with my previous car. I have the a/c temp set on 76. I drive with the cruise control on 71 - 72 when traffic permits. I'm getting a little more than 53 MPG on the current tank after driving almost 400 miles.

 

I found a system diagram for the inverter assembly. I believe it shows that the MG2 inverter uses whatever rectified DC that MG1 outputs. I suppose that it is not an issue if MG1's voltage is lower than vehicle speed would normally indicate it should be, as MG2 can use field weakening when the required torque is low. The ICE can change RPM so that MG1 can ramp up in RPM (either in one direction or the other) to increase voltage when MG2 requires more torque. Usually this means the ICE spins up.

Interestingly at high vehicle speeds it seems the ICE will spin slower in order for MG1 to speed up in reverse.

 

You and my wife have the same logic when it comes to AC. Full on or off no middle ground. Using the fan to control temp... LOL On the prius this effectively gaurantees that the AC compressor is running full speed all the time using more electricity and wasting gas. Just like every other car. The prius is different.

If you park in the shade you reduce the AC load on the car when you get back in. If you have the solar sunroof this also helps amazingly well. If you use a window shade in the front window this also helps keep the car cooler inside while parked in the sun. Once rolling and the AC is on, use the auto mode, and let the AC control the temp to a comfortable temperature level for you and the compressor will slow down or turn off occasionally once that temp is reached. You might actually find if you do this that it does not take such a big hit on the MPG's.

I got my best tank yet with the AC on set to 74 for the wife, 95 degrees out, and the car fully loaded driving all Interstate going 70 Mph. Drove from Cocoa Beach Florida to Atlanta on one tank. 51.5 MPG calculated, not displayed. The display showed around 56.5. Driving 80 Mph on the way there I got 43.7 calculated which I was not so happy with... BUT, the car was fully loaded for a two week vacation on the beach and I just wanted to get there. SO over all it was an amazing trip. It showed me there is a lot you can do to increase your mileage without sacrificing your comfort level whatever that may be.

 

Actually it doesn't. If you step out of the car while the ac is running you can hear the compressor change speed.

 

Actually, A/C is just a specific configuration of a heat pump, such that the heat pump is providing cooling. If you change which coil in the system is the condenser and which is the evaporator, your A/C system becomes a heat pump. So neither one is inherently more or less efficient, since they're effectively the same thing.

 

Really? My brother is fond of showing a picture of a thermometer at 160°F, in the shade. That is a heat index over 150 without even considering humidity.