I could see those reasons applying to cold starts in EV, but considering I am talking about a warm car stopped for about 3 minutes ....for a redlight or store run.....I don't think anything would have time to cool down even in freezing temps. However, idling for minutes simply to charge the battery is awfully wasteful.

 

I would think the CAT temperature would PLUMMET even on warm days. With no exhaust gasses to keep it going it would seem hard for something at 1100 degrees to stay that way for long. Hmmmmmm I'm kind of surpised it stays warm as long as John says but I've never timed it or anything.

 

Heat a cast iron pan to 1,100 degrees, and see how long anyone would wait before touching it.

I remember all the stories about cat converters causing fires and such, if parked with the vehicle off, on flammable material (dead leaves?).

 

Yes... but a cast iron pan has a lot of mass compared to its surface area... whereas a catalytic converter is designed to have alot of surface area compared to its mass so it should radiate rather quickly, I guess the problem is most of that surface area is internal to the CAT and therefore sees only the hot surfaces and internal heat and your only effective cooling is the outer can of the catalytic converter... with not as much surface area.

 

The absolutely predictable and linear manner that the CAT value cools has always bothered me. It didn't seem right. However... look at the delta T.

On a hot summer day, OAT is 100'F. CAT is 1500'F. Delta is 1400'F.
On a cold winter day, OAT is 0'F. CAT is 1500'F. Delta is 1500'F.

SO the rate of cooling on a winter day compared to summer should only be about 7% faster! And if most winter days are more like 20 or 30 degrees, then the rate of cooling is only 4 or 5% faster than summer.

Does that sound correct? I'm thinking out loud.
But the rate of cooling at 75'F compared to 0'F seems too similar for me to notice any difference without getting out a stopwatch. It may be in the 5% range... too small to notice by casual observation.

Look at CHT next.
On a hot summer day OAT is 100'F. CHT is 220'F. Delta is 120'F.
On a cold winter day, OAT is 0'F. CHT is 220'F. Delta is 220'F.

The rate of cooling of the CHT should be 45% faster in winter.
This seems to jive with observations, in general terms.

 

Errr.... on my scanguage II I just tried this at lunch. I stopped... turned off my engine, turned it back on two mintues later and checked the CAT temp again. The rate of temperature lost looked more like 1-2 degree PER SECOND, not per minute at least initially.

Did I do something wrong?

 

What I remember from Thermal Dynamics a few decades ago...

So, let's see. The difference between source temp and sink temp, is factored in. But it would in Kelvin. So, you have 1088K to 310K vs 1088K to 255K, or a delta of 778K vs. 833K, or around 6.6%.

I agree with your 7%.

 

My bad... I meant 1 degree per second all along.

 

And at one degree per second from a relatively cool CAT temp of 1100 (seems like 1300-1500 is more realistic but for argument's sake we'll go with 1100)....even stopped for 5 minutes would only drop the CAT to 800 degrees which is still above the minimum CAT temp the system allows.

So Ford could easily tweak this program without risking increased emissions (or breaking emissions rules) to allow Hybrids at a stop to remain in EV even if the charge drops below parameters. I would guess this would save me a half-gallon or so per tank with all of the city driving I do. That equates to an extra 1 mpg. That's why I try to keep any eye on my SOC and shift to neutral at lights...but with 2 little kids in the car my focus isn't really on fuel economy!

 

I agree... it would be easy. However to me, being an easy (and an obvious way) to SIGNIFICANTLY increase fuel economy with no expense added to the vehicle, one would think they would have done it--and assume there must be a decent reason they didn't. Maybe 800 is above the minimum CAT temperature required by law but not hot enough to meet SULEVII levels... I don't know. Or perhaps there is some other good reason they didn't do it.

Of course we all know that happens when you assume so maybe not...