Hi,

I've just purchased a new Civic Hybrid in Magnetic Pearl with Navi, and I've been catching up on some reading here at Greenhybrid.com. I've managed to get the pulse and glide to work, but I can't understand why this would save me gas. Let me explain...

When performing pulse and glide, you are pingponging energy between the battery and ICE. During the "pulse" phase, you are accelerating to speed and charging the battery. During the "glide" phase, you are decelerating slowly and discharging the battery. In this scenerio, a certain amount of energy is circulating from the ICE to the battery during the pulse, then back to the drive train during the glide. In a perfectly efficient system, this should be a wash. Just as much energy is stored into the battery by the ICE, that is later used for powering the car. However, all systems are ineffiecient. Let's say that you lose 10% of your energy when charging the battery, then another 10% when running the electric motor. That's a pretty efficient system, but you still lose 19% power when routing power through the battery vs. routing the power straight to the drive train. Doesn't it make more sense to simply run the drive train with the ICE in the first place? Every time you use the ICE to charge the battery, you are losing some power that the ICE has produced. So the pulse and glide should be the least efficient way to drive, not the most efficient. The most efficient way to drive is to leave a constant load on the ICE. This is the case when cruise control is engaged.

The advantage of a hybrid is not the fact that it has an electric motor. Every time you use the electric motor, you are losing energy. If this was not the case, then hybrids would work the way an diesel-electric engine works--the ICE charges a battery which runs the engine. The real advantage of a hybrid is that it is able to recover energy that would otherwise be lost from the system. Energy such as deceleration (braking). The most efficient hybrid configuration would actually never use the battery at all during an initial acceleration period and cruise. It would recover energy during deceleration, which could then be doled out during other acceleration periods. Creating false accelerations and decelerations via pulse and glide activates the electric system more often at the expense of effienciency.

The one time when pulse and glide seems to make sense is if you have an overabundance of recovered energy. In that case, you would want to use the potential energy of the battery to assist in normal driving as much as possible, if only to prevent the battery from "overflowing" and losing energy. In other words, you don't want to charge a full battery when decelerating. If this is happening, then pulse and glide may be a way to transfer more energy from the battery to the drive train. But it seems a better solution is just to tweak the car computer to more aggressively assist when the battery is nearly full. I suspect Honda has already done this.

So, does pulse and glide really work? I'd like to see some controlled experiments to be sure, but I suspect that it actually defeats FE in many cases. Simply driving conservatively at a constant speed will yield better results. If you are driving in a fashion that generates an overabundance of regen energy, then pulse and glide *might* allow more of that energy to be realized. But it seems there are better ways to do this.

I suspect that running on the electric motor only is mostly a psychological boost to performance. Every watt of energy in your car is produced by the ICE. If you are running off the electric engine, you are actually running less effiently than the ICE. The advantage of the electric engine is that there is a way to recover and store energy, whereas there is no way to un-combust gas.

Let me know what you think. This is all theoretical, but I'd like to know why pulse and glide works or doesn't.

--Brad

 

Pulse and Glide works in all types of cars and is always used in any mpg competition.

 

Why does it work then? Shouldn't a constant load be more efficient than a variable one?

--Brad

 

P&G works in a nutshell because it is more efficient to run an engine briefly at a higher output and then coast with the ICE off than it is to keep the ICE running at a really low output the entire time. Also P&G allows for lower average speeds, which helps mpgs.

I find it is not worth the trouble unless a hill is involved.

 

You would think so but the answer is no.

Magic

 

The idea with P&G is to not use regen/assist at all. If you allow regen during the glide then it is indeed less efficient as you describe.

 

To follow up with what Nasa explained, an effective P&G takes place with the acceleration usuing the ICE up to speed as quickly as possible while limiting assist if possible. Then for the glide elminate regen by slighty pressing the accelerator just enough to stop regen from taking place. EV mode can be accomplished if desired by pressing the accelerator slightly more. I have accomplished EV mode at 77MPH during a pulse and glide while drafting behind a truck.

 

Also, engines have a particular rpm where they run most efficiently. For small cars it's often somewhere between 3000 and 5000 rpm, so running the engine for short periods in its optimal efficiency range conserves more than running longer durations outside optimal range, even if that's a lower rpm.

I myself have experimented with P&G some but I have such a long commute that I can't tolerate much less than 65 mph average. So my pulse/glide experiments have focused on using the technique without dropping below 60 mpg, and revving back up to 75 mph. At this cycle I have not found it to be measurably more efficient than simply holding the car at a constant 70 mph. The serious P&G folks of course will coast down to speeds much lower than 60 and probably don't often go as fast as 75, so they do much better than I do.

 

Let me put this in my own words and tell me if I understand pulse and glide.
Frankly people make this so confusing and complicated using acronyms and they include aspects that, while nice to know, aren't needed and just complicate it.

First pick two speeds between which you will vary.
Let's pick 55 and 60 MPH.

1. The Pulse: While keeping the assist at only two bars, accelerate up to 60.

2. Glide: Right when you reach 60 slightly lift accelerator so no assist OR regen bars show. You will gradually loose speed.

3. When your speed goes down to 55, gently accelerate (pulse) again (with 2 bars of assist) back up to 60.

4. Repeat.

 

Your on the right track.
However, after reaching inital speed let your foot off the accelerator and reapply it just enough to stop regen. Doing this will decrease/cease fuel consumption. You don't have to use just two bars of assist. Using less saves battery life but takes longer to get to speed. Using more gets to speed faster increasing FE but can lower the battery. However, if getting to speed is done fast enough not much battery power will be used.
Either way you have the basics down. You can figure what works best for you.