Pulse and Glide in EV Mode

Hang on, are you're talking about you 2005 prius here? Yes moderate acceleration is efficient on the ICE, and EV glide is very efficient at low EV power levels, that's why pulse and glide works in a regular Prius. But that's NOT whats being discussed here. Please read the OP, it's about pulse and glide in a Pure EV mode.

 

Note also the Euro-PiP will have the ICE overide button (city EV or something like that), to manually force off the ICE in city centers where they are prohibited, even under heavier loads, likely at a cost to efficiency.

 

Sorry for a noob question, but what's DWL? It isn't in the glossary sticky in the Newbie section.

Is there a more complete glossary somewhere?

 

DWL = driving with load. It refers to keeping the engine under constant load, as measured by a scanguage or similar, rather than sticking to a constant speed.

DWL is like a very loose cruise control operated with the gas pedal for hypermilers, in which we lose speed on the climbs and recover it on the downs. I sometimes call it kinda steady state driving.

See the cleanmpg.com glossary and articles for more information.

 

The short answer is: Yes, in the context of hypermiling, pulse and glide in EV mode is more efficient, but only at very low speeds.

The long answer is:

Looking at the efficiency diagram of the Gen II below for example, notice that the main electric drive motor generator (MG2) has its best efficiency in the medium power range. Although not as bad as with the ICE, the efficiency at very low power output is clearly less than at medium power output. This leads me to the conclusion that the pulse and glide technique during EV mode, using an EV pulse without the ICE, would be more efficient than steady power during EV mode under certain very low speed conditions.


Combined MG2/inverter efficiency, Gen II Prius

I realize that it is generally more efficient to use the ICE pulse when possible, but there are some rare circumstances when an ICE pulse is not desirable, or it would have to be pulsed so briefly that it uses more electricity to start the ICE than would be used to propel the car directly with electricity. I also realize that the possible improvement in gas/e mileage is probably so miniscule that it can be neglected, with the possible exception of hypermilers. This is more of a theoretical question than a practical one for most people.

The battery, on the other hand, has a worse discharge/charge efficiency at higher power levels. This counteracts the good efficiency of MG2 in its optimum range, and shifts the optimum combined efficiency toward the lower midrange of motor power.

So, when EVing at very low speeds such as in parking lots or ins very slow traffic for example, it can be more efficient to do a moderate EV pulse, and then glide, rather than to EV with constant very low power (traffic permitting of course).


Also along this line of thought, under certain conditions pulse regenerative braking can be more efficient than very light steady regenerative braking.

Since the efficiency curve of MG2 during generation is somewhat similar to that of during motoring, moderate regenerbraking can be more efficient than very light regenerbraking. This has been measured and documented, for example by Attila Vass. (The new 2004 Toyota Prius : How to brake)

For example, when driving down a long, gentle slope, where very light regenerbraking would be necessary to remain within the speed limit, it might be more efficient to glide (no arrows) and periodically regenerbrake moderately, rather than to regenerbrake very lightly the whole way (traffic permitting of course).


Naturally, there are many other factors affecting total efficiency, so that it is hardly possible to calculate it exactly from just motor generator and battery efficiency. Actual driving measurements are necessary.

And once again, to be perfectly clear, this question has little practical value for most people, and I am not necessarily endorsing or encouraging pulse and glide. But it is a technically interesting question for hypermilers.
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Except that a full plug-in hybrid should allow the driver to choose among:

1. Normal blended operation: The computer does its thing as in the 2004 Prius.

2. Full EV operation: The driver tells the computer that the drive will be short enough that the ICE should NOT be used, even though it would mean reduced maximum power.

3. EV with override: Like the EV button on the 2004 Prius: Shuts off the ICE but allows the computer to override when power demand is above a threshold level. (Apparently the European version of the PiP has this.)

4. CS operation: The car operates like a 2004 prius, except that the battery SoC is maintained at a high level in anticipation of upcoming city traffic, where EV is more efficient. (I gather that the PiP does have this.)

5. Battery recharge operation: When there is slow speed driving, then highway, then slow-speed again, the driver could use EV but then instruct the computer to fully recharge the battery pack during a subsequent highway stretch, in anticipation of another slow-speed stretch in the city.

I call the PiP a mild plug-in because it does not take full advantage of the extended battery pack's potential. Just as in the old parlance, a mild hybrid does not take full advantage of the electric motor's ability to move the car without the gas engine.

 

In regard to the Prius, this statement has a fundamental flaw. The problem is the lack of a variable transmission between MG2 and the drive wheels. Looking at the efficiency plot of the combined Inverter/MG2, it is easy to see that they are less efficient at low speeds, which is why one might jump to the conclusion that P&G will help for very low speeds. However, when you pulse at very low speeds with MG2, lacking a variable transmission, MG2 is forced to run at the low, inefficient speed.

With conventional gas engine P&G, the transmission allows the engine to run at an efficient speed when pulsing. Not so with MG2.

If we want to be pedantic, we can show that at very low speeds we improve efficiency by driving faster. Therefor we gain efficiency at the high end of the pulse. That part is true, but once you take that to its logical end, you realize that the longer the higher speed pulse the better the efficiency; in other words, drive faster. Using our pedantic argument, to drive 10 mph to the corner store efficiently, we drive the entire distance in one 20 mph pulse, then sit in the car for the same interval at zero. I suppose we could call that P&G, but it would be silly.

Tom

 

Hi Tom, Merry Christmas or whatever you like to celebrate!

Yes, if the efficiency of the electric drive were only dependent on the speed (rpm), then pulse and glide wouldn't help. But the efficiency also varies with load, or torque. The efficiency contour in the example above shows that under around 50 Nm, the efficiency decreases significantly.

At low speeds, the amount of torque required to maintain a steady speed on a level road is well below this level. Short pulses at the same rpm but with a higher torque of above about 50 Nm yield a better efficiency.
Fred

 

#2 has a flaw...For safety reasons, the car should respond to a sudden throttle flooring with all power possible.

#5 doesn't make sense...When in prolonged slow speed-driving, it recharges HV battery a bit, like in a normal Prius, why recharging it ahead at a cost?

I disagree, PiP takes full advantage of the extended battery pack's potential. It uses its nominal power, the largest SOC possible, and gives room for further recharge at wall socket.

 

Re: #5: If I have low SoC in my battery pack (2004 Prius) when I get on the highway, the ICE will charge the pack up to about 6 bars. The car recognizes that the low-power need of steady-state driving is outside the ICE's most efficient range, so it runs it in the efficient range and uses the excess energy to charge the battery.

With the PiP's larger battery, the same applies, but only if you are going to be back to a situation where EV is the best choice. IOW, the car's best strategy depends on whether you are heading towards a charge opportunity (then you want to make use of all your available battery energy) or heading toward slow city traffic (when you want to avoid having to run the ICE, and so would like to bank the ICE's excess energy in highway operation.)

Being able to tell the car what's coming up would enable it to run more efficiently.

Another example, which applies with the Gen II & III, but much more so with the PiP is mountain driving: If you are headed toward a long downhill you want to deplete the battery before you get there so you have more head room for regen. If you are headed for a long uphill you want to bank as much electricity as you can before you get there. The car, however, will charge the battery up to 6 bars if it's on level ground, and then you lack headroom for the long downhill. Lots of people have wished they could tell the car what's coming up so it could act accordingly. The PiP's bigger battery would make it even more beneficial to be able to tell it what the upcoming terrain is like.

I accept your point re: #2.

 

Just a thought but most of the discussion here assumes that you have charge in the battery but if the electric range is fully depleted then Pulse and Glide is going to work well until you can get to an outlet

 

I said in post #10, and I think others pointed out as well, that in CS mode, the PiP will work just like a regular Prius, including the use of P&G. But the thread title is "P&G in EV Mode" so that's what the discussion has focused on.

 

The OP makes a very reasonable point - and is guided by his instincts.

I have read through the responses and wonder if we might be being blinded by 'science' here.

The cyclists among us instinctively apply intermittent pedalling to conserve energy. Just observing nature around us and you will see examples in birds (flap and glide) and fish (twitch and glide).

I have a scientific background, but not in motor mechanics. Nature typically takes the most efficient path, and I posted some thoughts on this a few years ago.

I wonder if this will turn out to be more a matter of determining the limits of which 'intermittent energy use' is better or worse than a 'constant burn'.

 

I think the failure of your analogy is that electric motors and muscles are very different, just as electric motors and ICEs are very different. The muscles benefit from a few moments of rest. An electric motor gets no benefit from rest.

 

I don't think that's entirely true. The internal resistance of both the electric motor and battery increase with heat, so letting them cool would increase their efficiency by decreasing the amount of energy expended as heat. It's unclear to me if the benefit is noticeable however.

 

The amount of heat dissipated during the typical glide would be insignificant.