Question on ICE warm-up

Phil:

Welcome to the club. I've had mine for almost nine months -- still much to learn. I finally could not resist, and installed a CAN View system, so here's what I drive around with on the MFD most of the time:


One thing that has become clear -- the car works hard to keep itself up at operating temperature, a priority that will eventually come into conflict with the owner's desire to see the ICE off. With ICE temp displayed, you can clearly see the surge in temp from the inflow from the thermos, and then you can see how, after the temp starts to decay, the ECU eventually "insists" on a run to keep the temp up. I don't know where you are in Canada, but I can only assume that your ambient temps make it harder to keep the ICE warm than mine do.

 

I also would be very interested in finding out this extreme fuel usage during warming up. Just by hearing ICE I would not expect so high fuel usage. Under normal operation when speeding up the consumption is more or less 5 - 20 l/100km but warming up mode takes 10 - 100 l/100km. You can check my pics posted in this topic: http://priuschat.com/index.php?showtopic=3...mp;#entry420812

I am not an expert at all... just can't understand why the values are so high instead of 10-20% hit.

- Piotr

 

What you describe is an artifact of the math calculation of miles per gallon, or liters per 100 km:

When the car is still warming up, the engine must run to get everything hot enough, which is why it does not shut off. Because at a stop you are not moving, it would require an infinite amount of gas to get anywhere. You are getting zero miles on a gallon, or burning an infinite amount of gas per 100 km. Again, this is merely because you are not moving. However in both case, in terms of time interval, you are burning nearly no gas. If you looked at gallons burned per hour, the number would be near zero.

It is one of the pitfalls of instantaneous mileage.

 

Would it be better just to leave the car sitting while it's warming up? I mean, you do waste gas while it's warming up, but at least you aren't taking a big hit from the cold start when you do that.

 

<div class='quotetop'>QUOTE(Alexstarfire @ May 4 2007, 10:31 AM) [snapback]435433[/snapback]</div>

I don't think prius takes a big hit from the cold start because it keeps the hot coolant in a thermo pack for cold start the heat the engine up quickly.

 

<div class='quotetop'>QUOTE(Alexstarfire @ May 4 2007, 11:31 AM) [snapback]435433[/snapback]</div>

I personally believe it is better to let it sit during warm up. It's correct that there's a themo to keep the hot coolant. Depending on your ambient temperature, it still takes a while before the ICE reaches operating temperature.

For those of you who have CAN-View, you can see this. If you start driving during the warm up period, given that you don't floor the pedal, the engine RPM stays below 1500, while the battery is providing most of the energy (huge current draw). Due to conversion loss (mechanical -> electrical -> chemical -> and back), it is almost always less efficient to use energy from the battery. That's why in the P&G technique you accelerate with ICE only and glide with no arrows to avoid using the battery.

 

<div class='quotetop'>QUOTE(Alexstarfire @ May 4 2007, 08:31 AM) [snapback]435433[/snapback]</div>

Yes, but not for the reason you suggest. See below.

<div class='quotetop'>QUOTE(ScottY @ May 4 2007, 10:54 AM) [snapback]435536[/snapback]</div>

Correct. This was obvious even before the CAN-View arrived. Even without Can-View you can see from the MFD that during the first minute or so after starting, the car draws from the battery in the sort of light acceleration and slow-speed driving when the car (if not in electric mode) would normally be charging the battery rather than drawing from it.

The computer wants to go easy on the engine while it is cold, so it draws from the battery, which is less efficient. So that hard driving immediately after starting is inefficient. I do not believe it actually hurts the engine. But I like to drive gently during that first minute.

<div class='quotetop'>QUOTE(CO2 miser @ May 4 2007, 04:42 PM) [snapback]435721[/snapback]</div>

It's not your imagination. However it's related to the RPMs of the engine, not to the power output. When the engine is under very light load it is burning very little gas, even though it is turning at around 1,000 rpms. The feeling seems severe because we are not used to a car's engine shutting off by itself. I think you get the same sort of shudder when you shut off any car engine. But you're expecting it when you turn the key off. The Prius does it when you're not really expecting it.

 

Daniel, would your 5 stages writeup help the OP, at least to the point of better understanding of the car's warmup gyrations?

 

The engine is also running a richer-than-normal fuel mixture. This causes the catalytic converter to quickly heat up as it consumes the unburned hydrocarbons.

Running a normal air-fuel mixture would take longer to heat the converter, so the engine would run longer---and burn more fuel overall.

 

<div class='quotetop'>QUOTE(JimboK @ May 4 2007, 06:39 PM) [snapback]435793[/snapback]</div>

I'm not sure how relevant it would be to the OP's question, since Stage 1 is described in the article as the phase during which the engine will not shut off, which may or may not be congruent with the phase during which the car is using as much electricity as it can to avoid putting a load on the engine.

I have no personal technical knowledge of how the car works. I wrote the article by abstracting a more technical article that one of the early Japanese Prius adopters had posted. I have since been unable to find that article.

The behavior of the car which I described in my first post in this thread is my own observation. But most of the information in the knowledge base article about the 5 stages of Prius hybrid operation comes from the Japanese source. (And BTW, I have since come to believe that the criteria for Stage 5 operation may be more complicated than any of us believed back in those days.)

 

<div class='quotetop'>QUOTE(daniel @ May 5 2007, 11:06 AM) [snapback]436018[/snapback]</div>

OK, just a thought. I knew it didn't answer the question directly; I was just wondering whether it might be helpful as background.

Regarding your closing parenthetical comment, I tend to agree. I've thought about starting a new post with my own thoughts and observations, but I want to get them together first.

 

Be aware, just like non-hybrids, the Prius uses a richer fuel mixture (choke mode) when the engine is cold. I don't yet have experience with the Prius, but the anti-Prius (see below) uses 2-3 times as much fuel when in choke mode, measured with a Scangauge as L/hr when stopped. I see 3.6 L/hr or more (it's a rapidly moving target to measure), but once it's warm I see 1.2 l/hr.

When warming up the car, it's better to be moving on a purely efficiency basis. Then the engine has to run at least some of the time anyway, so the waste heat of providing motion is warming it up. When stopped, running the engine just to warm it up is a total waste of the gas.

Note also that just like other cars, the Prius must warm up both the engine and the cat. Once the engine is warm, it still has to maintain the cat. temp. On some cars, keeping the cat. warm really lowers mileage (RX-8 for example). In effect they produce more CO2 to generate less HC and NOx by injecting more fuel than necessary. It's quite a dramatic difference as well - noticed when we were "tuning" the RX-8 for more power.

 

<div class='quotetop'>QUOTE(Alexstarfire @ May 4 2007, 10:31 AM) [snapback]435433[/snapback]</div>

[Just as David wrote] No. Idling without moving gets you zero MPG; you might as well be moving and get *some* use from that burning fuel. The best way to warm up any car, including a Prius, is to drive it, gently for the first coupla minutes. It's completely pointless to warm up a car without moving unless you live in some subarctic climate and just can't stand to drive it until some heat comes out of the cabin vents.

 

<div class='quotetop'>QUOTE(CO2 miser @ May 4 2007, 10:44 AM) [snapback]435178[/snapback]</div>

My bet is you're in S3, idling check mode.
http://techno-fandom.org/~hobbit/cars/five-stages.txt

<div class='quotetop'>QUOTE(richard schumacher @ May 16 2007, 11:48 PM) [snapback]443317[/snapback]</div>

With Prius, it's a different story.

During the first 57 seconds and when you drive, Prius ICE runs only for warming up the cat and you're in EV mode. You'll consume the HV battery a lot.
After that period, you're ready to drive by ICE power.

Ken@Japan

 

<div class='quotetop'>QUOTE(ken1784 @ May 16 2007, 10:14 AM) [snapback]443337[/snapback]</div>

What evidence do you have for this? Why would Toyota design the system to deliberately waste fuel?

 

i'm guessing for lower overall emissions.

Low emissions are standard...50MPG is a bonus!