Why does pulse and glide improve MPG?

http://priuschat.com/index.php?showtopic=1...&+glide

 

simply put,
going at a constant speed of 35 mph runs the ICE at about 1250rpm, (60mpg). most ICE's run the most effecient at 50%-75% of full power. 1250 is about 20% full power. now if you accelerate from 30-40 at about 20mpg you should be just over 2000rpms. this runs the engine closer to it's best effeciency of gas to power.

now 20mpg compared to 60mpg. first think of how far you traveled while you accellerated from 30-40mph. coast with the engine off for twice the distance without going below 30mph and you have 60mpg.

on a flat road, at that low of speed, it's easy to coast a long way in a prius. so as long as you can coast longer than twice the distance, you have better mpg. edit the calculations due to mpg you achieve when accelerating and distance it took. i know the numbers are not exactly accurate, but they're pretty close.

 

<div class='quotetop'>QUOTE(j24816 @ May 2 2006, 07:50 AM) [snapback]248470[/snapback]</div>

The coefficient of dynamic friction in this case is mostly rolling friction, which remains pretty much the same over the speed differences of pulsing and gliding. Aerodynamic drag is nonlinear, so there is some additional loss from the speed variation; however, since the speed differences are small, the additional losses will not be large, unless you are pulsing from 20 mph to 60 mph, or some other large swing.

To keep your car going down a flat road, we need some sort of power source to overcome the rolling resistance and aerodynamic drag. That, of course, is the ICE. For maximum overall efficiency, we want the ICE to run at maximum efficiency. Gasoline engines control power output by using a throttle plate, which restricts air intake to the engine. When running at less than full throttle, this restriction consumes power by increasing the pumping loss in the engine. In other words, gasoline engines are more efficient at high power settings than low power settings.

This is one of the secrets to hybrid efficiency: the hybrid can use a smaller gasoline engine more closely matched to the average load, so the engine operates at higher efficiency. The traction battery is used as a sort of electrical flywheel to store energy when there is a surplus, and to supply extra power when it is needed for acceleration. But I digress; back to pulse and glide.

Pulse and glide works to keep the ICE running in a more efficient power setting. By accelerating, you run the engine at a more wide open setting, which reduces pumping losses. When you glide, the engine stops running (depending on speed, in the Prius it may keep spinning, but at a low loss setting), so you have low or no power loss due to the ICE. The average power required to keep the car moving is still the same (actually just a bit higher), but the ICE is run in shorter, more efficient bursts, instead of running continuously at a lower efficiency level.

For you electronic types out there, this is very similar to the difference between an analog power supply and a switched power supply. The analog supply runs continuously at a partial power setting, while the switched supply turns on and off as needed.

Hope this helps,
Tom

 

<div class='quotetop'>QUOTE(qbee42 @ May 2 2006, 10:08 PM) [snapback]248487[/snapback]</div>

To follow Tom's comment, below is the best shift data during the 1,400 mile marathon.
The percent distance ICE on (pulse) was only 19%, then gliding distance was 81%.

Shift 10, Wayne Gerdes 120.5 MPG
Start 06:30 AM, End 10:30 AM, Avg Temp 73.5
Overall FE for 3 RT Segments: 51.25 Km/L or 120.5 mpg
E/T: 3:51
Distance Traveled: 150.78 Km
Avg. FE with Engine On: 9.84 Km/L or 23.1 mpg
Percent Distance ICE On: 19%
Fuel Consumed: 2.942 L

Ken@Japan