Pre-heating the Prius Transaxle and Engine for better fuel economy

[MetroMPG]

Another really interesting thread, Marcus. Thanks!

Maybe I'm mistaken, but I think that despite the occasional chatter on the topic, this is the first thread that documents transaxle warming being done here.

So.... added to the project library.

Envious of your mild winter weather. (Then again, I'm not an Olympic snowboarder.)

[Christ]

Originally Posted by MetroMPG

Another really interesting thread, Marcus. Thanks!

Maybe I'm mistaken, but I think that despite the occasional chatter on the topic, this is the first thread that documents transaxle warming being done here.

So.... added to the project library.

Envious of your mild winter weather. (Then again, I'm not an Olympic snowboarder.)

So you should put something like "No, seriously this time..." in the title?

[MetroMPG]

Pretty much! We almost need a separate forum for ideas that continually come up but nobody really does. Like inflatable boat tails.

[orange4boy]

No no, thank you.

I just wish I could test this stuff properly but I'm stuck with what I have. I tend to do things that are pretty sure to have an effect. Usually after I've read one of your exhaustive tests.

If only I could make a living doing this...

Wait.... what did you say?...Inflatable boat tail?

[IMG][/IMG]

Inflatable Boat Tail... Thinking about but not doing since 1958.

(Image from a 1958 issue of Popular Mechanics)

[orange4boy]

I have been watching the Injector MS an they do jump a bit when you the ICE starts after it's been off for a bit. The numbers settle within a few seconds to a steady number though. This seems to suggests a bit of enrichment but it's short.

I wish I had a PC so I could graph some of this stuff.

I have been getting roughly 4.9 with preheat and 5.8 without on my commute. There seems to be enough difference to be certain it's not other variables.

To measure the transaxle difference would probably not reliably show up on my commute though, because the variability is +/- .15 L/100. I would expect the transaxle preheat to be in that area of improvement. at 2000 rpm, the difference between 28C and 60C was about 60 watts. Overnight ambient temperature here has been about 5˚C though so the gains must be substantially higher than this. I estimate 150w.

The real key to big mpg savings here is getting to the ice to 70 and into S4 asap.

Estimated watt savings:
Transaxle preheat: ~150W @2000 rpm
12V electrical load: ~500W continuous
Battery preheat: ??? Next project.

[mopo3]

As for coking the oil, that is why I'm interested in a steam heating solution. The temperature will never exceed 100C so the oil will be 'safe.'

For the set up you are talking about, this may as well be true. However just so you know steam can get hotter than 100C (superheated steam), just check a steam tablet to see how temperature is related to pressure. Also be aware that when the steam condenses that the condensate (liq water) will still be 100C, the heat lossed will be latent heat.

[orange4boy]

Well, I have been driving for a while with the transaxle preheat. It makes a noticeable difference in coasting between warmed and cool.

Testing will have to wait.

I have just finished installing a tank heater for the engine. I got a 750 watt unit for the transaxle and have swapped the 1500 watt unit to use with the engine. The nipple that came with the kit fits the rear block coolant drain just fine.

Instead of hooking into the heater core line, I plumbed into the top rad hose. Actually, I just removed the wimpy passive inline heater I had installed earlier and put in a T junction that I fabricated out of stainless. The heater return hose tap would have required me to take the inverter out completely but this way I just had to prop it up and I could leave it mostly still installed. Another down side to the heater hose is that it takes a steep upslope just as it leaves the engine which is a no-no for a tank heater set up.

I took some pictures this time so I will post those when I get a chance.

Just for testing , I also installed the factory block heater.

I also finally finished installing the last two aero wheel covers, but that's for a different thread.

[orange4boy]

Lots of pics as promised.

First, Here's the shot of the transaxle warmer. Now 750 watts. Fast heat! Plumbed into the drain plug with a custom nipple. This is the lowest point on the transaxle, beside the oil pan. Just a copper elbow brazed onto the drilled out drain plug.



There is the tank heater and the y it came with plumbed into one of the transaxle coolant lines. Look for the pink.



Now the engine heat:

This is the Y I made to plumb into the top rad hose.



This is the engine heater hot return installed. It will percolate back into the head from here.



This is the low/cold side nipple installed where the coolant drain used to be. There's room back here but not a lot of vertical so the heater will have to be mounted at a 45˚ angle. Should be OK.

[bwilson4web]

Originally Posted by orange4boy

. . .
First, Here's the shot of the transaxle warmer. Now 750 watts. Fast heat! Plumbed into the drain plug with a custom nipple. This is the lowest point on the transaxle, beside the oil pan. Just a copper elbow brazed onto the drilled out drain plug. . . .

That loop also handles cooling the inverter, which is at the top of the coolant column. The Graham scanner can report inverter temperatures and I would be interested in the final temperature after say 4-6 hours. But once the ignition is turned on, I would expect the inverter coolant pump to soon flood the inverter with relatively cold coolant. Normally power electronics like to be kept cool but 40-60C should be no problem. Since the coolant loop is at ambient pressure, I doubt it can exceed 100C.

It might be useful, if you have time, to see what the relative temperature of the engine, transaxle and inverter temperatures are after 4-6 hours. For example, I measured the block heater effect shortly after installation:

The formula in the upper left hand corner is the formula for how the temperature of an object increases as a function of the material, starting and ambient temperature. I used this formula to do some curve fitting and figure out how long it takes to reach steady-state with the ambient and the maximum value.

I used the following data to figure out the optimum block heater time:

Using local electrical rates and the impact on fuel savings, I've found anything from 30 to 60 minutes is optimum. Over 60 minutes begins to show diminishing returns.

If you get a chance, it would be useful to run the heaters for at least 4-6 hours and then measure every available temperature versus the battery temperature (ambient.) I'm thinking:

• engine temperature
• MG1 temp
• MG2 temp
• MG1 inverter temp
• MG2 inverter temp
• battery min temp (ambient)
• battery max temp (should be close to above)

If you have an IR thermometer, it might be useful to check the inverter radiator located in front of the regular radiator after 4-6 hours. If it is at ambient or close, good deal.

I've not done much successful brazing and wasn't terribly impressed with the long term, mechanical strength of the parts. I'm curious about how it works with the drain plug over time. Thermal cycles can lead to issues with dissimilar metals having different expansion rates. If the plug and fitting are both ferrous, no problem, any thoughts?

Bob Wilson

[orange4boy]

I'd like to figure out how to get the data off the scanner but I have a mac laptop so unless I can do it with a usb cable, I'm stuck with just sampling a few numbers manually.

I guess with data like this I have to do it manually anyway.