i dont get the "thermos" that heats the ICE

[psikot]

it is said that the prius has a "thermos" that holds engine fluid at 175 degrees for 3 days. but then why does the ICE need to warm up after i park for 45 min? what does that heat actually do? i dont see the difference in start up for 5 days parked (student) and 3 hours parked. i have driven an hour on the freeway and parked for 30 min and the ICE still has to warm, why?

[qbee42]

The ICE has to warm the catalytic converter and related emissions system. The thermos keeps the ICE from having to spend energy reheating coolant that was heated just a short time ago. It's a small gain, but it does help, especially in the emissions area.

Tom

[PA Prius]

Here it is, in the left front corner.

PA P
[attachmentid=12216]

[David Beale]

The thermos seems to loose effect after about two days of sitting. Be aware it takes about 30 sec for the system to pump the coolant back into the engine. The temp starts low, rises to a high, then drops a bit as the heat is absorbed by the block and diluted by the rest of the coolant.

After one day it warms the engine coolant to about 50C in your climate. The engine still has to warm the cat. but that only takes about 30 sec.

After two days it warms the engine coolant to about 30C in your climate. The engine still has to warm the cat. and has to bring the coolant up to about 48C before shutting down, and that takes about 2 min. depending.

After three days the engine coolant is at ambient, and it seems to take about 3-5 min to warm the coolant (while warming the cat as well). If you drive it's not such a big loss of energy.

If you park and come back 30 min to 3 hrs later, the coolant is warmed to about 60C within 30 sec. but again, the engine must run to warm the cat.

I suppose you -could- insulate the cat, but I suspect it would overheat if you did. At any rate, heat loss is proportional to the temp difference across the insulation, and the cat must operate at such a high temp it isn't practical to expect to insulate it to hold the temperature it needs.

[jk450]

<div class='quotetop'>QUOTE(psikot @ Oct 26 2007, 03:48 PM) [snapback]530835[/snapback]</div>

it is said that the prius has a "thermos" that holds engine fluid at 175 degrees for 3 days. but then why does the ICE need to warm up after i park for 45 min? what does that heat actually do? i dont see the difference in start up for 5 days parked (student) and 3 hours parked. i have driven an hour on the freeway and parked for 30 min and the ICE still has to warm, why?
[/b]

1. The HV ECU wants to see if the ICE can start and run at all. Remember that this is a vehicle that can move under its own power without an engine - but not for very long. If there is an issue with the ICE or with MG1, which serves as the cranking motor for the ICE, the HV ECU needs to know about it.

2. Although the thermos supplies hot coolant to the cooling system passages in the cylinder head, which reduces the wetted area of the intake tract, it does not heat the entire engine to operating temperature. Running the engine will accomplish that.

3. The engine acts as a vacuum pump, and scavenges trapped fuel vapors from the EVAP system.

4. The air-fuel sensor and catalytic converter must be heated to their respective operating temperatures.

Insulating the cat won't help. Electrically heating it will hasten warm-up, but you'll need a lot of current for that. And the Prius needs that current to move the car...

[bwilson4web]

Hi,

<div class='quotetop'>QUOTE(psikot @ Oct 26 2007, 02:48 PM) [snapback]530835[/snapback]</div>

it is said that the prius has a "thermos" that holds engine fluid at 175 degrees for 3 days. but then why does the ICE need to warm up after i park for 45 min? what does that heat actually do? i dont see the difference in start up for 5 days parked (student) and 3 hours parked. i have driven an hour on the freeway and parked for 30 min and the ICE still has to warm, why?
[/b]

A few of in the Prius Technical Group investigated Prius warm-up last winter, Ken@Japan, some European Prius owners and we discovered some interesting characteristics written up here:
http://hiwaay.net/~bzwilson/prius/pri_temp.html

We learned that the North American 2001-03 Prius has a different warm-up profile from the Japanese and European models AND the North American 2004-current Prius. In everyone elses model, the Prius can begin limited hybrid mode as soon as the engine block reaches 40C. This means engine autostop can work and the ICE can shutdown while in motion at speeds below 42 mph. Ken did an excellent job of describing these startup phases:

<blockquote> * Stage-1: initial 30 seconds, very rich to heat-up catalytic converters and up to 40C, ICE is always ON, Stage 1 take about 80 seconds. Ken@Japan,"We see it is 56 or 57 seconds and it is called Stage-1a on NHW-20. During the period, we see retarded ignition timing. It is after the TDC or top dead center, or minus number of BTDC, to make the exhaust hot. Then, NHW-20 Prius goes to Stage-1b until the coolant becomes 40C."
* Stage-2: This does not exist in the HC equipped, North American, NHW11 Prius. In NHW20, 2004-2007 Prius, from 40C to 70C, ICE will be OFF when you are not accelerating or using the heater. The NHW20 behaves the same in Stage-4.
* Stage-3(aka idling check mode): 70C and above, but you have not been exceeding 60km/h or 35 mph, ICE won't go OFF. To transition to full hybrid mode, Stage-4, the car must be stopped long enough for ICE autostop, 7-10 seconds.
* Stage-4(full hybrid mode): 70C and above, and after several seconds idling check mode in Stage-3. Now, you are in real hybrid mode, and enjoy coasting. </blockquote>

Sad to say, but the North American, NHW11, 2001-03 Prius doesn't recognize the 40C limit but has to reach 70C before any of the hybrid modes work. This extends the non-hybrid mode to over 5 minutes and is a major performance impact:


North American NHW11 warm-up:


As described in the report URL, Clive in the UK proposed using a resistor to adjust the engine thermistor to read a warmer temperature. The team soon discovered that a diode resistor works much better and allowed us to trick the NHW11 ICE into thinking it was warmer than actual. Testing revealed that the ICE needs to be at 40C before biasing the thermistor to 70C. The makes a substantial reduction in time to full hybrid operation:



Subsequent testing revealed this not only substantially reduced the time to hybrid operation but it also avoided a problem of 'cool down' when stationary in a strong, cold wind. If parked in ready in a strong, cold wind, the engine can cool off enough, reaching 60C, that the ICE has to come on just to warm-up the engine.

A Toyota SAE paper covering the differences between the NHW11 and NHW20 pointed out that the addition of the thermos allowed them to drop the special, hydrocarbon converter on the NHW11. My NHW11 has an extra converter with a valve that traps the hydrocarbon emissions until the catalytic converters are warm enough. These hydrocarbon emissions occur when gasoline is not fully burned when the engine first starts up. So how does this relate to the thermos bottle?

The thermos bottle heat helps to warm-up the intake manifold and fuel injectors that in turn preheat the fuel passing through before the rest of the engine is warmed up. This improves the vaporization and reduces the "hydrocarbon emissions." With fewer hydrocarbon emissions, the NHW20 does not require the extra HC converter on the exhaust pipe.

So, does that help?

Bob Wilson

[jk450]

<div class='quotetop'>QUOTE(bwilson4web @ Oct 26 2007, 06:32 PM) [snapback]530907[/snapback]</div>

The thermos bottle heat helps to warm-up the intake manifold and fuel injectors before the rest of the engine is warmed up. This improves the vaporization and reduces the "hydrocarbon emissions." With fewer hydrocarbon emissions, the NHW20 does not require the extra HC converter on the exhaust pipe.
[/b]

Just a bit of clarification: the fuel injectors themselves do not need to be warmed up. The intake manifold walls, however, do benefit from a bit of preheat.

Vaporized fuel will condense on the inside of a cold manifold tract, and the air/fuel ratio must to be richened to compensate for this, which would require a richer fuel mixture. This will produce more carbon monoxide exiting the engine (which a quickly warming cat can take care of pretty well) as well as more hydrocarbons (which a cat can't handle well until it has warmed up all the way).

The advantage of the NHW20 system over the NHW11 system is that it reduces emissions AND saves fuel. The NHW11 HCAC system simply reduced emissions. Sharp drivers with both versions of the Prius will notice that the NHW20 has markedly better fuel economy in the first five minutes of cold start-up operation.

That thermos, by the way (about $650 with pump), also provided a bit of revenue to collision centers that repair the NHW20. It's often one of the first things to go in a front-end collision.