1. Quote:

   Pulsing and gliding your way to better fuel economy
   
   By Jeff Ronning Posted Wed Nov 21, 2007 11:46am PST
   Pulse-and-glide driving can get 15 percent better gas mileage, even in a big SUV. It's all about engine efficiency.

 

Pulse and glide advocates seldom list:

• speed range - the minimum and maximum values
• constant speed equivalent - what the vehicle achieves as a reference

This is the problem shared by the referenced article. The author claims a "15%" performance improvement but there is no reference to the minimum and maximum speed values that achieve this gain nor the reference speed. To show you what I mean, what is the performance of a given vehicle at:Without a good model, pulse and glide leaves more questions than answers. But I've done some experiments to define these terms and measure the effects with my NHW11, 2003 Prius.

25-43 P&G vs 33 miles per hour

This speed range is the minimum and maximum that my Prius cruise control uses yet supports engine-off glide. Starting at 25 miles per hour, the cruise control "resume" consistently accelerates the car back to the set speed, 43 miles per hour. Then the cruise control is set to "coast" and when the car reaches 40 miles per hour, shifted into "N" for an engine off glide. Here is a graph of the speed profiles:


So using data from the mileage display, we can plot the MPG for these runs:


So in this case, a minimum speed of 25 miles per hour and maximum of 43 miles per hour gives a mileage improvement of 11%. This not quite the 15% reported by the article at the expense of a speed difference of 18 miles per hour from 25 to 43 miles per hour. I had the advantage of a road with no other traffic so I didn't attract the attention of "road rage" warriors or the police looking for intoxicated drivers.

15 - 25 P&G vs 18 MPH

My earlier studies had determined that the maximum range speed, best fuel efficiency, for my Prius is 18 miles per hour. But trying to measure MPG at these speeds directly requires very long distance and timed runs. To shorten the time and distance, I switched to measuring the energy consumed per meter. So here are the pulse and glide and steady speed energy charts:

Steady speed:


In this case, there is a net 14% savings by holding 18 miles per hour versus the P&G equivalent speed of 19 miles per hour between 15 to 25 miles per hour. The difference in efficiency between 18 and 19 miles per hour at a steady speed is ~1%, much less than the 14% measured loss of pulse and glide:


WARM-UP PULSE AND GLIDE

Until the engine coolant reaches 70C, my Prius burns more fuel in any gear than when in "N" including "P":

So during the engine warm-up, I accelerate to neighborhood speeds, ~25-27 miles per hour, and then slip into "N" to coast down to about 18 miles per hour or so depending upon traffic. Usually there is little or no traffic on the neighborhood streets and it only takes about 1.5-2.0 miles to warm-up. But once the engine coolant reaches 70C, I revert to normal Prius efficiency driving. I'm typically seeing ~35 MPG versus ~25 MPG over ~1.5 miles of warm-up.

Conclusion

My 2003 Prius does not seem to perform exceptionally better using pulse and glide except during warm-up. Once it is warmed up, driving within a few simple, control law limits has returned reproducible, 'old' EPA and better mileage over 50,000 miles. Yes, I've done better than my signature 52 MPG and know how to reproduce results like this over any distance:


It isn't that pulse and glide has no utility but I can't find it in practical driving or to max out the fuel efficiency with my 2003 Prius. Other vehicles with different architectures may do better with pulse and glide. But sometimes, the pulse and glide advocates describe 'terrain driving' and call it pulse and glide. Regardless, I noticed there seems to be some reluctance by P&G advocates to quantify the speed ranges but great enthusiasm for claiming results. That isn't science or engineering, it is advocacy.

Bob Wilson

 

On 35 MPH or 40 MPH 4 lane roads, I do a lot of P&G in my FEH in the 25 to 42 MPH range. If there is light traffic behind me, I do 30-40 MPH. In heavy traffic, or on 2 lane roads, I skip it.

I usually net 10% better ( 44 MPG vs. 40 MPH ) compared to steady driving. I account some of it to thermal effects. The fewer number of minutes my ICE is on, the fewer minutes I am dumping heat into the radiator/environment... and by running the ICE water at 140'F with P&G vs. 188'F with constant on, there is slower heat loss to the environment by conduction/radiation... yes?

-John

 

Hi,

So is this correct:

• 25-42 PnG ~= 33 miles per hour average?

I think you are suggesting:

• 33 miles per hour -> 40 MPG
• 25-42 PnG ~= 44 MPG

Interesting, I'm see an 11% improvement with 25-43 PnG and your report is a 10% improvement. This is well within the margin of error.

I haven't done a sensitivity study to plot the sensitivity of PnG high-low speed values and the relative improvement over steady-state. However, it looks like something worth testing in the spring when 'standard days' return.

An interesting hypothesis and worth testing.

Bob Wilson

 

I think you're still in Stage 2 when you see 140F(60C).
On Prius, the thermostat starts to open at 180F(82C) and we see around 189F(87C) even on driving with P&G.
If your heater was turned ON, it's another story.

Ken@Japan

 

Ken, once the Ford gets hot, 140'F is hot enough to keep it in the best FE "stage". There is a bitterly cold phase, a moderate phase, and 140 is the "hot" phase. I've found no advantage to running the car hotter than this... unless outside air is bitterly cold. Then the "model" shifts upwards on the order of 20 degrees for each stage to occur.

 

It is very strange to me.
All Toyota hybrid engines have 80+C(176F) thermostat and they try to maintain approx. 189F(87C) even on driving with P&G.
The 140F range looks over cool area and your thermostat is maybe defective.
I would like to suggest that the FEH thermostat specification needs to be checked.

Ken@Japan

 

There is no water temperature sensor.
There is only a cylinder head temperature sensor.
The water is infered to be ~20% colder than the head temperature.

When ICE is off, cylinder head cools quickly... more so if cabin heat is on, which I think does not involve the thermostat.

The Ford thermostat is listed at 190'F.
The car never gets more than 191'F infered with ICE continuously on, which is based upon CHT of 233'F actually measured.
-John

 

It is still strange to me.
There should be OBD-II data for the engine coolant temperature.
OK, it's 190F thermostat. The Prius one is 82C(180F) specification and there is higher one which is 88C(190F) specification.
I suppose that one is used on your FEH and it starts to open at 190F and fully opened at 205F, therefore the typical coolant temperature is approximately 196F.
Again, if your heater is turned ON, it's another story.

Ken@Japan

 

Ken@Japan - But does't the Prius incorporate some sort of built in 'thermos', whereby the hot coolant is pumped into to preserve heat, then pumped back into the radiator/block when needed? If yes, does this feature work when driving between EV/ICE modes? Or does it only come into play when you stop? Or am I just full of "Hot Water"?

The FEH/MMH cools down rapidly after entering EV mode from the air passing over the radiator fins. Block the front helps considerably without a 'thermos' holding system.