Why does Avg MPG drop so fast on start?

<div class='quotetop'>QUOTE(PriusRos @ Oct 31 2006, 11:00 AM) [snapback]341249[/snapback]</div>

The engine is cold, so it runs more in order to get it warmed up, to heat the catalytic converter to operating temp, etc., and I think colder engines also run richer, but I could be wrong on this last part. On my trips on a cold engine, the first 5 minutes on the consumption screen is usually 25 MPG, then in the 30's after that, and then 40's/50's after that, so I assume it takes about 10 minutes to warm up. I assume this "spike" of low mileage is just throwing the average off a bit, and as soon as it gets past it, it goes back to the more normal average.

 

The short answer is warming up the engine.

You can basically think of it like this: during warm up, the engine is always running, never shuts off. So when you're driving around at 25MPH, you're burning a ton more gas than you do when it's fully warmed up.

As for how long it takes for it to go back up, it's all relative - how fast you're driving, MPG/mile, so to speak... With proper instrumentation, you would be able to calculate the mpg/mile at all points and see how those add up over your commute.

 

<_< I find that after the ICE warmup, the indicated MPG changes rapidly. If you remember your High School math, this is due to the small DIVISOR (the number on the bottom of the fraction) which is the miles accumulated as you drive. It's common logic to realize that the more you drive, the slower the change in indicated MPG. The DIVIDEND is the top of the fraction, the accumulated miles driven.

Just thought that might further confuse the issues. :lol:

 

Also consider that over 41 MPH the engine is running to prevent MG1 from over spinning. Sometimes, usually on a downhill, the engine will stop providing power but will still be spinning. (Some called this hyper-stealth mode.)

This will slow down the recovery of the lost MPG. If you had a slower commute where you would run electric more the recovery would be faster.

 

It's a bit like grade point averages. Let's say you have a 3.9 average going, then you tank on a test and get a 2.0. That 2.0 is 1.9 points below your current average. To drag yourself back up you need to offset that with test grades above your current average. If we are dealing with a 4.0 scale, then 4.0 is the best you can do. Each 4.0 is only 0.1 point above your current average. It takes a lot of 0.1s to make up for one -1.9.

In the same fashion, warming up at idle is 0 mpg. Even moving you are only going to get around 25 mpg when warming up, so that is a big hit compared to 50 or 55 mpg. Obviously, the fewer miles you have driven on the current average, the bigger the percentage hit, so sometimes it will seem worse than others.

Tom

 

Remember that colder weather is going to bring an even bigger hit as your engine strugglest to reach normal operating temperature.

 

<div class='quotetop'>QUOTE(PriusRos @ Oct 31 2006, 04:32 PM) [snapback]341479[/snapback]</div>

That makes sense, when you think about it - at faster speeds, the running engine is "felt" less per mile - If you get up to 60, you might not even notice a difference in your instantaneous mpg!

As for it taking longer, well yeah - by accelerating and forcing the engine to run a little higher, you create more heat that goes towards helping the car warm up!

 

I bought my car in April, and I have a short commute (15-20 min, 5-7 miles depending on which route I take). Most of the late spring and summer, I'd see my current-tank MPG displayed on the MFD drop perhaps .1 MPG by the time I got to work, and usually recover that on my trip home.

Since we started having MUCH cooler temperatures in October, including a few mornings with frost, and afternoon temperatures in the 40's to 50's, my 'drop' on the way to work is more like .3 to .4 MPG, with recovery on my trip home. My last tank, and about 150 miles into my current tank, are showing about a 10% drop overall in MPG from around 49-50 to around 44-45. I expect it will get worse as we get more frigid weather, especially if I need to run my defroster to keep the windshield clear. Still, overall, I'm much better off than I was with my old car!

(BTW, I used to keep track of my 'old' car for about the first three years, and winter mileage was about 10% worse than summer mileage on that car. I had a longer commute, then, so I can't say this is an apples-to-apples comparison.)

 

<div class='quotetop'>QUOTE(qbee42 @ Oct 31 2006, 12:35 PM) [snapback]341430[/snapback]</div>

That's a very good analogy which I'll come back to... Another thing to realize is that you're taking a double hit during that warm up period. Not only is the Engine running in order to warm up the emissions, but the Prius doesn't want to use any ICE power during this warmup period. So in effect as the emissions are warming up the Prius is in a mode very similar to the EV-Button induced mode. Nearly all of the power to get around is coming from the battery, unless you step on the gas and the ICE revs up it's electric. So now after a minute or two it's warmed up the ICE and starts to use it for power, but you may have lost 10% of the battery charge, so now you have to burn even more gas to recharge the battery, maybe this takes another 4 to 8 minutes. So there's your poor first 5 to 10 minutes of mileage.

I need to capture some data and graph this effect with a cold start in stock mode, the same trip out of the neighborhood when starting out warm, a cold start with EV-mode to bypass the warmup, and a cold start in PHEV mode to bypass the use of gas altogether. It should show that the electric power used in the stock and stock-ev mode are nearly the same, only difference is that the ICE doesn't warm up in ev mode. The pre warmed up mode should show an entirely different useage of the electric side of the powertrain. In PHEV mode it would also look the same as a cold start or ev-mode, but the battery wouldn't go empty for about 10 miles or so.

Now back to the GPA story, the 0.0 to 4.0 GPA is equivalent to 0 to ~60 MPG in the Prius, that is to say that it's much easier for 0 or 25 mpg to pull down your average than it is for 60 or 55 or 50 mpg to pull it back up. Now enter the 10 or 30 mile PHEV Prius with a maximum mpg of ohh, infinity for 10-30 miles which is a whole lot better than 50, 60, 100, or even 200 mpg. This is like being able to score extra credit on a quiz that's equivilant to a 16.0 GP or better. Of course the power is coming from an outlet in your garage, but you can choose for yourself where that comes from in most cases. The great thing is that it has the potential to make a big difference regarding that smelly stuff we have to buy at a gas station that got pumped out of some well and is going to release some 18-22lbs of CO2 per each gallon of which we burn, so best to go as many miles as possible on each gallon, even if you have to cheat or take extra credits...

L8r
Ryan

 

I have also found out that if I wait a minute or so for the engine to warm in the mornings and then go ahead and start driving at a normal rate of speed rather than trying to take it easy while the engine warms, that my loss of mpg is actually less.

An explanation above makes sense to go ahead and use the engine while it is warming rather than taking it easy. Driving a little faster and putting a load on the engine should help it warm faster.

I am also learning quickly from others that the faster that I get up to speed from a stop, the better mpg I get (or the less loss of mpg if you want to look at it that way).

 

Don't wait for the engine to warm up. That wastes fuel for no gain. If you can, just take it easy for the first few minutes, otherwise you tend to use a lot of battery power, which isn't the best for mileage.

Tom

 

5-10 minutes depending on how fast you are going. i wont even bother to answer your last question.

read my response to your mileage post

 

true true. if you are averaging 45 mpg over 200 miles, that implies you have burned 4.44 gallons of gas. so you start up, run a few blocks, essentially not increasing the distance traveled but burning about a tenth of a gallon, now your mileage has dropped to 44 mpg... so as you can see, it does not take much. granted you will not burn a tenth right away, but in actuality, it VERY normal to burn that much gas in the first 5 minutes in cold weather. granted, by your location, you are not getting that cold weather yet (but then again, the weather has been unusual)

but, are you grill blocking? do you run your defrost right away and constantly or do you turn it on and off as needed? if not, double the warmup penalty.

as far as driving fast, slow, or idling to get the car warm, if your trip is long enough, it will make nearly no difference at all. if your trip is say, less than 10-15 miles, it might matter.

on my commute, i dont get up early enough to futz around. my commute is 25 mph for the first 3 blocks then 40 mph. i simply drive the speed limit and you should too.

my commute is 7 miles. if you have any questions about my performance, check out my "short tripping" thread. i am in the process of posting stats for several different scenarios

 

Everyone has their own preference. Some think (myself included) that it's best to let the car get to stage 1b before driving, which takes about a minute (57 sec.). During that time, the ICE is warming the catalytic converter and the car is powered by battery. Does it really make a big difference? I doubt it.

 

As you say, it doesn't make a lot of difference. I suspect much of the variation comes from the type of driving. For example, where I live the first minute of driving is at very low speeds. I back out of the garage, turn around, stop at the end of the driveway, stop in one short block, and then go 25 out of the village. It would be a lot different if I had to pull into fast moving traffic.

Tom

 

i agree that it makes little difference how you start if gentle driving is needed for first few minutes. now i used to live on a major highway with a speed limit of 50 mph. for those situations, warmup is definitely needed.

 

I agree. I leave my driveway and immediatly drive on a highway for a few miles. Of course, on those days I'm running late, I forgo the warmup time.

 

If you control your pedal and have a scangauge monitoring amps from the battery, you can actually accelerate a bit while the engine is warming up - without drawing from the battery... but this energy is barely enough to go to 10mph...