The myth of pulse and glide

The first step is to understand how the HSI correlates ICE power. I've started with inertial measurements, the power needed to accelerate. Similar data could be collected using a hill climb. But I'm less interested in mapping HSI to ICE power than instrumentation to measure ICE torque.

Until we get some better instrumentation, we can't realistically address these questions:

This earlier data came from a Graham miniscanner, which sampled the data ~200 ms. apart. It was better than nothing but still many things happen in much smaller time intervals and I'm looking at direct instrumentation.

If I am able to measure MG1 current accurately and fast enough, I should be able to scale it for ICE torque with the ICE rpm and injector timing to come up with a relative BSFC meter. Unlike the static charts derived from Graham miniscanner data, this will be direct and we should be able to combine it with other vehicle data including temperature and even fuel quality. For those interested in driving the HSI, it would be a trivial task.

The earlier data was collected for the 1.6L engine and we now have a 1.8L engine with cooled exhaust gas recirculation, exhaust heat recovery, and a host of technical improvements. I suspect we are looking at a very different beast than the 1.6L. Think of our earlier studies as laying the ground work for what we need to look at in the 1.8L. <grins>

Bob Wilson

 

I would think the actual average MPG on the MFD would be inaccurate for opposite reasons as someone previously mentioned. If you pulse at 20 MPG, and glide at 100 MPG (even with the required 5 times longer glide as previously mentioned), you have averaged 60 MPG, right? But my question is, does the computer use the 100 MPG that is displayed during the glide to come up with the average? If so, these calculations are incorrect in real-world FE because in the prius, when gliding, you're not actually getting 100 MPG, you're getting infinite MPG, since the motor is not even consuming any fuel. Any thoughts?

 

No, it would not necessarily average to 60 MPG. The actual average is a product of the miles covered at each given mileage. They are not necessarily the same.

The computer does not use the 99 MPG limit in its calculations. The computer totals the miles driven and the gallons burned. Instantaneous mileage is not used to derive average mileage.

Tom

 

The primary problem is that the 20 and 100 mile/gal numbers were the OP's PDOMAs. One could PDOYA any two rates of fuel consumption desired to achieve the desired result.

To his credit, he tried to analyze the process and posted his assumptions.

 

Excellent, that makes sense.

 

The first paragraph of the blog

convinced me to read no further.

 

Why? 48 mi/gal correct.:rockon:

I think hos 50mi/gal comment was just an indirect attempt to show what happens when you don't do the math correctly.

 

Oops, right you are xs650. I read that he thought he should get 50 mpg. I'll keep reading ...

 

Ok, so can someone post the equation to figure MPG? I'm really having a hard time with this one. I understand mathematical concepts, but seem to be stumped on this one. As far as how you guys are coming up with what your pulse's and glides would need to be to achieve a gain. When I can manage most of my driving at 50-70 MPG on a good majority of my route, and even 99.9 for a few miles of the 12 mile drive to work, how the HELL do I still only manage to get an average of 42 by the time I get there?! Yes, it's a very hilly drive, and most hills drop me to <20MPG, but I then regain it by getting 99.9 on the way down the hill.

 

andrewrohn: MPG = miles / gallons

hope this helps

It seems easy, but the lower MPG portions bring down the overall MPG a lot more than some would expect. For example, 5 miles at 20 MPG and 10 miles at 60 MPG comes out to a total of 36 MPG over 15 miles.

10 miles/60 MPG = 0.1667 gallons
5 miles/20 MPG = .25 gallons
15 miles/0.4167 gallons = 36 MPG

In fact, that's essentially what the first post in this thread was about: the lower MPG pulse dragging down the overall MPG. The problem with the original "myth of pulse and glide" analysis is that he did not account for the fact that the glide is supposed to consume roughly no fuel in a car like the Prius where the ICE can turn off while the car is gliding. So a pulse at ~20 MPG (high consumption for a pulse in the 40 MPH range, IME) with a glide over the same distance (really short for a good glide, IME) should yield roughly 40 MPG. However, you can see from his chart that he doesn't think 40 MPG is achievable because he assumes additional fuel consumption during the glide.

So, in your case, if you went up a hill at 20 MPG and went down it at 100 MPG (equal distance up and down), you got 33 MPG total. If you did a glide (no fuel consumption at all) on the way down, that would be 40 MPG, which is obviously much better. Unfortunately, the instantaneous MPG readout alone will not tell you the difference between these two conditions.

 

MPG = Total distance traveled / total fuel used.

If you climb a hill at <20 mpg, then turn around and coast down using no fuel (9999 mpg on a ScanGauge), the round trip will be <40 mpg.

This is yet another reason why 'Gallons per 100 miles' would be more sensible than MPG..

 

The OP is using knowledge of a FFH in his examples, so this post is probably in the wrong Forum. It belongs in "Other Cars"

Furthermore, the numbers used in the "myth of pulse and glide" have absolutely nothing to do with the realities of pulse and glide. The OP specifically chose unrealistic numbers in order to create an unrealistic result.

What the OP is actually describing is the tendency for those who don't understand math to try to average averages. This occurs in many places and not just MPG values. It could just as easily have been called the myth of improving your GPA with an A, or the myth of increasing average speed by driving faster, or the myth of improving average service time by working faster.