Why does Avg MPG drop so fast on start?

[PriusRos]

I guess I could figure this out if I really took the time to do the math, but maybe somebody has a simple explanation.

Let's say my consumption screen is showing an average of 45mpg for 200 miles when I turn the car off.

In the morning, all I have to do is back out of the driveway, and my average mpg immediately drops 1 or 2 points to 44.8. During the first 5 miles of driving it might drop another 3 points, especially if I have to wait at a traffic light or stop sign (which is what happens on all my daily commutes).

However, it then takes a long time of driving at 50-75 mpg to get the average back up to 45.

Why is that? How much fuel is it burning while I'm just backing out from my driveway, anyway? Is driving in reverse counted as negative miles traveled??

[Beryl Octet]

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

I guess I could figure this out if I really took the time to do the math, but maybe somebody has a simple explanation.

Let's say my consumption screen is showing an average of 45mpg for 200 miles when I turn the car off.

In the morning, all I have to do is back out of the driveway, and my average mpg immediately drops 1 or 2 points to 44.8. During the first 5 miles of driving it might drop another 3 points, especially if I have to wait at a traffic light or stop sign (which is what happens on all my daily commutes).

However, it then takes a long time of driving at 50-75 mpg to get the average back up to 45.

Why is that? How much fuel is it burning while I'm just backing out from my driveway, anyway? Is driving in reverse counted as negative miles traveled??
[/b]

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.

[eagle33199]

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.

[Charles Suitt]

<_< 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.

[jfschultz]

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.

[qbee42]

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

[Delta Flyer]

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

[PriusRos]

Thanks, everyone. Yes, I understand about the engine needing to warm up at the beginning, but it seems that the drop happens so quickly -- I mean, how much gas is it consuming during the 10 seconds it takes to back out of the driveway? -- and then it takes so long to to make up for that initial drop after warming up.

And I'm talking about a drop in the average mpg over 200-300 miles range -- not for 20 or 30 miles, where I'd understand that any small change in fuel consumption is going to show up immediately.

Also, I find that driving slowly first thing in the morning (e.g., 15-25mph neighborhoods with speed bumps and stop signs) takes longer to warm up the car and is worse on fuel consumption than being able to get up to a reasonable speed, such as over 35mph.

It's kind of the opposite of the battle of the buldge! You can put on weight just like that, but then it takes so long to lose those extra pounds!

[eagle33199]

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

Also, I find that driving slowly first thing in the morning (e.g., 15-25mph neighborhoods with speed bumps and stop signs) takes longer to warm up the car and is worse on fuel consumption than being able to get up to a reasonable speed, such as over 35mph.
[/b]

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!

[Jeannie]

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.)