Why does Prius run the ICE at low mpg?

<div class='quotetop'>QUOTE(Alexstarfire @ Mar 10 2007, 07:21 PM) [snapback]403435[/snapback]</div>

It's probably just too cumbersome to do effectively. It would be a quite large and probably fragile moving part that could cause big trouble if it failed.

The radiator is at least shut out of the cooling circuit by a valve until the coolant gets hot.

 

<div class='quotetop'>QUOTE(ukr2 @ Feb 14 2007, 09:16 PM) [snapback]390602[/snapback]</div>

It is not the speed but rather power. I believe the logic is quite simple.

Any power request more than 14 hp would turn on the gas ICE. Why? Atkinson cycle ICE is more efficient (31% to 33% efficiency) producing 15hp than a similar displacement Otto cycle ICE at full throttle.

At any speed, if the accelerator pedal request less than 15 hp, HSD will not burn any gas. All the power will come from the HV battery. This is why you can do stealth (<42mph) and warp stealth (>42mph).

At slow speed, 14 hp can actually accelerate Prius pretty well, especially on declines. At high speed in Warp Stealth mode, 14 hp can barely keep Prius at constant speed around 65 mph on flat surface. On the inclines / declines, Prius will loose / gain speed slowly if you stay in warp stealth. This is due to higher rolling and wind resistance. Which is also why Prius get lower MPG on the highway than in city (slow speed).

EV mode allows up to 28 hp from the HV battery. It is good to prevent the gas ICE from starting because it costs a few hp to spin up the ICE (additional gas may burn). If you stay in EV mode and use only peak 16 to 17 hp to avoid the gas ICE from starting, you will save fuel overall. If you use more than 17 hp from the battery, it is better to let the ICE start. I think!

 

I agree with what you've posted but we need to remember the limited battery capacity. This means at speeds below 42 mph, we can actually see the Prius cycle between electric and gas-recharging mode.

Going back to your original posting and what I agree with, not too long ago, Ken posted the NHW11, 03 Prius drag formula. I used it to calculate the power vs. MPH and with ICE efficiency, the theoretical MPG vs. MPH:




More to compliment your posting, I wanted to bring up the cyclical nature of low power operation and suggest the limits of battery powered operation. For more details:

http://hiwaay.net/~bzwilson/prius/pri_test.html

BTW, this lets anyone with a Prius achieve just about any MPG numbers they want. All it takes is an empty shopping center parking lot and time to burn. For NHW11s, it works better at lower temperatures if you have a thermistor hack.

Bob Wilson

 

<div class='quotetop'>QUOTE(ken1784 @ Mar 10 2007, 10:08 PM) [snapback]403587[/snapback]</div>

I came up with that number based on my commute to work. There is a downhill stretch that I am able to stay in warp stealth up to 71 mph. I now think 65 mph might be high for 14 hp on a flat surface. 60 mph or 55 mph may be closer. I guess it depends on the wind and tire pressure as well.

 

<div class='quotetop'>QUOTE(usbseawolf2000 @ Mar 11 2007, 01:25 PM) [snapback]403621[/snapback]</div>

My math shows the speed is 53.9 mph by 14 hp at flat surface and no wind.

Ken@Japan