whats the reason MPG is better in warm weather?

Until the emissions system has reached its operating temp the engine runs rich to generate the additional heat. It also wants to keep itself above a certain temp as well.

Nope.

If I'm not mistaken the heater is primarily driven by waste heat from the engine. I do not believe that it is a drain on the HV battery.

The AC system is run off of the HV Battery and draws on it fairly heavily if you're running the system hard. It doesn't seem to reduce gas mileage nearly as badly as running the heater on full blast.

Don't forget the winter fuel formulations. These often contain ethanol and/or anti-freezing agents that reduce the energy available in a given volume of fuel.

 

I think you think that the heater in the Prius is Electric, yes there are a pair of 12 heater elements intergated in the heater core but they are virtually useless in real life. The main heat comes from a heater core just like any conventional vehicle. On the Prius there is a rotating valve that blocks the flow of water to the heater core in the first few minutes of ICE operation so the ICE can get the most benifit from the themos flush. There is also an electric water pump in the heater flow circuit so the core recieves warm water from the ICE when it's off in stealth mode. When the ICE is running there is a fanbelt driven water pump for circulation. See attached list of operating conditions for the PTC heaters to operate at start up.

 

I believe that the previous posts are on to what I would suggest;

The ICE runs rich when cold. After reaching operating temp, in the winter, the ICE would be kicking back on more then in the summer to keep the operating temp up. Also having the heater on would draw the heat out of the engine, along with the outside temp.

Watch the tire pressure also. It drops when the outside temp goes down.

And yes there is a difference in the blends of gas between summer and winter. I believe the winter blends are formulated to ignite easier in the cold temps,but I don't know what else they put in them, .

 

Tripp,

I do not know much about the Prius thermodynamics... but I do fly airplanes... and generally when you enrich the fuel/air mixture... then the engine runs cooler (not hotter). In fact... during an inititial climb out, we have to run our aircraft engines very rich to keep the CHTs low enough to prevent damage. Once we are at altitude, we can lean the mixture even though the CHTs increase.

Why is it that the Prius would run hotter with more fuel? For those pilots on the forum... do you think that the Prius engine runs Lean of Peak (LOP)?

/Jim

 

So far I am seeing responses which indicate observations instead of reasons why the engine needs to be warm/hot. I too am truly curious why the mpg is low just due to the temperature differences between winter and summer.

I think Windstrings is asking what is the real reason why the Prius gets low mpg when the temperature is cold. Could it, for example, be tweaked to operate at high mpg at lower temperatures? What factors are at play? Would it be possible to have settings which would accomplish high mpg during winter and then switch to other settings which would accomplish high mpg during the summer. As you can see I am no engine genius. :lol:

 

The density of the air is a factor as well.

 

I'm not clear on what you are saying here. When you lean the mixture at altitude, the CHT increases and after it reaches peak, it starts dropping as you continue leaning. Later you say the Prius engine runs lean of peak.

Isn't that contradictory?

 

That makes two of us. :lol:

I'm not a gear head or an engineer but I believe that I've read something to that effect somewhere. I know that's a totally lame retort. I've looked around and not found much to back myself up except for one non-technical blurb that's not really worth anything.

Perhaps Wayne, hobbit, or some of the other hard-core types could enlighten us. I'd like to know more. Maybe I should buy the Prius tech manual so some light reading. Tech types, what's your opinion on the tech manual esp for blokes without much of a solid footing in electronics or mechanical knowlege. Not that I can't learn, but I'm starting off from a very basic understanding.

 

As the weather has gotten colder up here (Winnipeg, Canada) I've noticed a steady increase in fuel consumption. Many of the reasons have already been stated, the main one being that your engine is having to warm up/run rich when it's cold out to get up to operating temperature. To that end, I think some of this may be software related (e.g., when the snowflake symbol goes on). As far as overiding it, I don't think it would be worthwhile and you may run the risk of damaging your engine. After all, most car engines have a high and low idle for cold starts. It starts in high/rich mode and will automatically move to a low/leaner mode once the engine is up to temperature (provided that everything's working well). At least, this is what I can remember from my studies on carbeurators. Somehow I don't think fuel injection changes the process, just the implementation (computer controlled versus mechanical).

It's quite warm up here right now (-2C), so it's still possible to turn off the engine using the HVAC controls, although now that the snowflake symbol has shown itself, I find it's a little harder during the initial startup period to turn it off than before (when it was say 5C). I've been told once it hits -25C, the engine apparently doesn't shut off at all so mileage tends to drift up to 10L/100K. As for now I find that if we drive for a prolonged period of time, be it city or highway driving, the mileage improves dramatically over short jaunts and it's becomes easier for the ICE to shut itself off while driving or coming to stops.

As far as winter gas is concerned, I know the formulation does change a little - but all in all, gas is gas. Anything added to the fuel for the winter is as stated before - either for anti-freezing properties, or to prevent water buildup in one's fuel tank (which is an issue).

shaun

 

I think that things are getting a little overly complicated here. I'll try to clarify a bit, from what I've gathered from my time here. Feel free to correct me if/when I put my foot in my mouth.

There are several reasons why fuel usage is greater in the winter ("Winter Gas" among them). From personal experience, one effect seems to be the greatest. I find that, while driving in the winter, my MPG is lowest in when I have the heater on. This makes perfect sense to me: I'm drawing heat from the engine to warm the cabin, so the engine has to run more to reach its operating temperature. I don't know all of the reasons why there is a certain temperature that the engine has to operate at, but I can think of a couple (viscosity of oil, catalytic converter).

Also, aside from heating the cabin, there is a greater overall loss of heat in the winter, just because it's colder. Heat transfer is proportional to the difference in temperature. This means that heat is much more efficiently carried away from the engine when it's 20 degrees vs. when it's 70 degrees. So, therefore, the engine has to run more to stay at its operating temperature.

So, I think the best ways to improve MPG have all been mentioned here at some point:

1. Don't run the cabin heat, if you can help it.
2. Use a block heater so the engine doesn't have to work so hard when you start it up to reach operating temperature.
3. Consider blocking off part of the radiator so the heat losses to the air aren't as great.

But perhaps you are asking a more subtle question that I don't quite understand?

 

yeah so once i glanced through the wiring diagrams that DH usually brings home when he's working on something complex... and i felt like this: :blink:

suppose if i really sat down and had him explain it to me it would make sense but geez. greek to me as much as my combinatorial drug design notes were to him.

the repair manual sheets that i've seen are much more uh, easy to understand. step 1, do this. step 2, do that. step 3, check to see if the car works now, etc. but i think there's more to it than that, i just see the stuff he brings home to show me.

 

Flynz4
in the Prius the engine runs slightly rich when cold Not dramatically rich but a bit rich, but the ingnition timing goes to an ATDC setting which makes for a hotter mixture as it leaves the head. This puts more heat in the cat system and O2 sensor and the heated fuel trim sensor in the exhaust system. On and AC engine if you know about magnetos they have an impulse coupling on them to retard the timing so the engine will start. Magneto timing is set to about 20 degrees before top dead center and most engines won't start with that much timing advance, so what the coupler does is retards the timing and there are variable couplings, to some where around 5 degrees BTDC so the engine will start. Then you do your mag checks and such before TO and you run the mixture full rich, depending on altitude, to keep the cylinder heads cool then when you not extracting full power at altitude you can lean it out a bit. I never like to lean past peak, alwasy 25-50 before peak. On the Prius the fuel control system is so finely tuned that I think its set to peak. Less, no way, after peak and take a power loss and potential valve problems I doubt, I think it's peak which will still yeild good mileage and no potential for any problems. Cat's at full temp keeping things clean and the fuel trim sensor and the O2 sensor keeping it adjusted at the millisecond level.

 

I believe the reason for poor mileage in the winter is because of the cold air. The fuel spray doesn't atomize/mix as well in cold air as in warmer air, though I think this is mostly a startup issue. When the engine is warmed up, it can pass some heat to the air before the fuel is added, but that air will still be colder than on a hot day. When the cold air and fuel are ignited, they do not get as hot and there is less horsepower output as a result.

When people block their radiator, they reduce the amount that the radiator cools the engine. I'd hope the radiator's waterpump would vary flow based on temperature of the engine, but who knows. If the engine gets too hot, the oil lubricant will burn and the engine could seize.

But then again I'm not an automotive engineer, so I could be wrong

 

I hear ya sistah. Some of the stuff that I've looked at, which appears to be rather mundane, hum drum stuff to the writer and intended audience, definitely causes the :blink: effect with me. I have a rudimentary sense of what's being discussed or perhaps a dim awareness but not much above that. Guess I'll have to take come community college classes if I really want to understand what the F is going on! Perhaps a trip to Borders or Barnes & Noble is in order. I have too many bloody hobbies as it is though. I'm gonna have to quit working so that I can learn more about my car... Don't think that'll fly with the wife or our creditors.

 

first is winter fuel mixture...that reduces mileage at least 8%

2nd is car needs longer time to warm up and does not run at optimum efficiency until it does. this included emission controls, thicker lubricants, lower intake air, etc.

3rd. cold air is denser. much more air friction to overcome. this could be swapped with #2.

4th...although the heater does use additional power, its no where near the amount used by defrost. didnt have CANVIEW this summer, but willing to bet that defrost uses only slightly less than air conditioning since both use the same method to get different results

 

Popoff,

I was not stating that the Prius engine runs lean of peak. I was asking the question as a possibe explaination of why the engine would generate more heat with a richer mixture.

Generally, based on my experience as a pilot, I would consider an engine to generate more heat as it is leaned. This is in contradiction to the claim that the Prius engine enriches its mixture when cold to generate additional heat. I was *asking* (not stating) if the Prius engine was run LOP as that is the only way that I could think enrichening the mixture would generate additional heat.

I lhave since earned from Frank that the Prius also has variable ignition timing. I suspect that could also change things significantly.

/Jim

 

As I understand it, it runs rich at startup to increase the heat of the exhaust gases, as its first priority is to heat the catalytic converter to operating temperature.