[schmultzburger]

I finally got my (salsa) Prius last week, and I love it. I have noticed something a little odd though. I know that the gas milage is not nearly as good for the first 5 minutes or so, but what I've noticed makes me wonder how much the "accepted" reason of a cold engine is the cause.

There is a fairly large hill on my way home, and when I have driven for a distance before getting home and let my foot off the gas at the top of the hill, the car picks up about 10mph (all the while at maxed out fuel efficiency) from roughly 38 to 48. Then I can take it easy on the gas for the next straight and flat section.

However, if I am coming home from the store which is relatively close to the hill, I barely pick up any speed (maybe from 38 to 40 mph), and I end up engaging the ICE at the bottom.

It also seems harder to get into stealth when driving on flats in the first few minutes. If I use the same amount of pedal pressure that I normally would for stealth, the car slows down. In order to keep the speed up, I have to depress the pedal further, and thus engage the ICE.

Why would there be more rolling resistance when the car is cold?? It acts like it's in mode or the emergency brake is slightly engaged.

Any ideas?

 

[artman67]

Im sure sombody will post a better responce but where goes

It will always not get as good mpg when cold because it is trying to get
warm ( about 5 min is what I have seen posted )

Not sure how much of this is offset by charging the batterys while warming up

 

[clett]

Is the ICE running while you are coasting down the hill?

Tyre pressures are lower when you first set off, then heat up (quite a lot in some cases) and are then at more pressure. This is a variable that is carefully controlled in racing cars - though I doubt the effects of going from, say, 40psi cold tyres to 50psi warm tyres would be enough to explain your not speeding up down the hill when cold.

I think there are two more likely possible explanations. 1 is that when the car is cold it's taking every opportunity to fill the battery, and regen is stronger? 2 is that when cold, the computer is protecting the ICE as much as possible, putting very little strain on it for longevity purposes, but when warm can assist you down the hill? (even just a little ICE assist down the hill would speed you up from 38 to 48 ). Just some ideas....?

 

[schmultzburger]

From what I can tell, the ICE is off when rolling down hill. It seems to me that when the MPG indicator is maxed-out (99.9mpg) then the ICE is never on.

Has no one else noticed that it is significantly more difficult to maintain speed at, say, 30mph in stealth when the car is cold? I'm not saying icy cold, maybe 60 F outside. If the car is stopped at a light or traveling downhill, the ICE goes off (so it's not a matter of warming up), but in order to remain in stealth mode, the speed steadily decreases. When I try to very carefully bring the speed back up, the ICE kicks in.

OTOH, when the car is warm, I can stay in stealth for up to about 5 minutes at pretty consistent speeds even with slightly rolling roads.

I assumed it was my needing to get used to the feel of the gas pedal that was causing the problem, but the difficulty in increasing speed going downhill when cold really made it obvious something else is going on.

 

[artman67]

>OTOH, when the car is warm, I can stay in stealth for up to about 5 minutes at pretty consistent speeds even with slightly rolling roads.

How long for it to get "warm"

 

[nsfbr]

Rolling resistance is extremely dependent on tire temperature, and not only due to the internal air pressure. The actual losses are due mainly to hysteresis in the tire rubber due to flexure - the main reason radials are more efficient is because the sidewalls are less stiff and therefore less lossy.

In a vehicle like the Prius, which has the ability is coast with a relatively small amount of gear train loss, at speeds under about 35 mph, the biggest loss into the environment is going to rolling resistance. Above that aero drag will begin to dominate.

So, I'd offer that a 20% change in rolling loss from cold to hot will absolutely be noticeable, especially during a low speed coast.