EV Mode: Why I'm Lovin' it!

Just continuing the reasoning here...

The long path incorporating all of the short path elements, it cannot have a better efficiency (in your example).

Although the missing point from your reasoning is that the whole system efficiency is not fixed and some components are more efficient in some particular circumstances. For example, the ICE it not efficient in low and high RPM, the rest of the time (which is most of the time with the new 1.8L engine), it gets around 36% (then deduce some for the gears, electrical path of the PSD, etc.; I would think it is way better than the 20% you used). On the other hand, the motor is more efficient at low RPM (more power/torque).

Therefore, there are circumstances when the long path will be more efficient (like using a small amount of electric power to slowly accelerate or to keep your speed, as it avoids running the ICE in an inefficient manner).

 

Ok, I know I should probably not be a noob before finding fault. And I admit, I'm only on tank number 4 of my shiny new Prius, so experience is not forthcoming with me, but I do find a few faults in this in depth physics analysis already. Whether or not it pans out in practical use, I don't know yet. For short trips that have highway stretches before and after, this seems to work well compared to normal driving, but averaged over time, not sure yet.

1. You stated that the only power generated for the batteries is coming from gas. This isn't exactly true. There is residual charged from the regenerative brakes too. Agreed, that this still represents a large energy lose and is not "regenerative" but more akin to "recapture" instead. But efficiency is gained none the less.

2. You state that the recharging reduces mpg because of the extra energy being sent to the battery instead. On a highway drive, my ICE rarely ever kicks off, unless it's a good downhill slide. The entire time that ICE is on, it's sending energy to the battery until the battery is fully charged, then it shunts the energy (I assume). But the point is, there doesn't appear to be a time when the ICE energy is directed to only the wheels. So in reality, it looks to me that if you rely primarily on the ICE, you'll loose the added benefit of having the traction battery in the first place. If you can coast a mile or more on battery, I'm thinking that the energy will be replaced on the highway portion anyways, then you can coast another mile when you exit on battery again.

Extra batteries tied to the grid? ... hmm... sounds interesting..

Grain of salt warning... I am not an electrical or mechanical engineer.

 

This is true, but unless you just went down a high mountain, the energy you get back from regenerative braking came from the gas you used to accelerate the car first.

The arrows you see on the display don't say anything about the quantities involved. Whether you are burning gas mostly to charge the battery or whether the car is simply applying a "trickle charge", both will show arrows toward the battery. In fact, the battery acting as a big energy buffer, there will almost always be current going in or out. I was simply trying to point out that while you run on the highway, your gas consumption will be higher if you are both pushing the car forward AND charging the battery back than what it would take to only push the car forward. In both cases you will see arrows going toward the battery, but the quantities involved will be much different.

You can prove this by moving at a steady speed (lets say around 50 km/h / 30 MPH) with:
1. the battery almost fully depleted; and
2. the battery full.
In the former, you will see a higher gas consumption on the instant fuel consumption display (lower MPG for the US system), than in the latter.

I may appear as a anti-EV-mode-crusader, which I am not. But I want to avoid newbies believing EV mode is miraculous and will give them free energy, while most of the time it will rather decrease their MPG.

 

Most, maybe. All, no.

For my example, say we required 10kw of power to drive the car at about 30mph. looking at some data from ANL, the thermal efficiency of the prius engine at 10kw output is around 25%. the efficiency from 15kw and up moves to 35%.

I look at it this way. if i burn one imperial gallon at 25% efficiency i get 11.02kwh of power. if i burn a gallon at 35% efficiency i get 15.43kwh of power.
If i force that 15.43 through the inverter (10% loss 13.89kwh) into the battery pack (10% loss 12.5kwh) and back out of the battery pack (10% loss) i am left with 11.25kwh - 200wh more than the 25% short path.

so if the values are ~90% efficiency in each step, the long path is better for supplying power below 10kw than the short path. if you need less than 10kw the best thing to do would be EV a bit, then hybrid a bit (which would get short path to the wheels, and added load of charging the pack gets us to 15kw+ and into 35% territory) and load up the "long path" ready for more EV.

i guess the most efficient would be to always stick in 15kw+ territory (avoiding the long path but sticking in 35%), but this is pushing over 40mph so not always practical.

 

Philosophe, I retract my "boring physics aside" comment and to quote another,"Interesting stuff". Thanks!

 

Ok a few more tanks of gas under my belt and some better driving skills. So I can speak a little better on the topic, albeit still learning. So to answer the free energy concept I think I've learned a bit more to support the use for ev mode. The ICE produces a lot of energy (gas generates a LOT more energy than the drive batteries). This extra energy shunts off to the secondary electric motor for electrical generation (which recharges the drive battery). During times when the battery is very low, the ICE kicks on even at a stop (bad for MPG). But for the most part, the drive battery is recharged during normal drive. Not using the drive battery and trying to rely solely on the ICE for propulsion wastes significant energy that would otherwise be recaptured through MG2. This is why the use of EV Mode IS better for MPG. That is at least if used effectively.

So how do we use EV Mode to efficiently drive? First, don't use it in a parking lot or as primary drive on slow roads. If you just outright drain the battery, then you're going to be using ICE to recharge. But you can use EV Mode in times when the car is going to use more gas (acceleration from dead stops). Getting up to 25mpg on battery only and then kicking in on ICE saves gas when your car is it's least efficient. Let the ICE and breaks regenerate the Drive battery when your cruising.

This is why City mileage is so much better than highway mileage. In a normal car, city is significantly worse because of all the gas used to accelerate and stop and idle. The Prius leverages the battery to store and reuse that energy (albeit at a loss, it's not 100% efficient). But it's the recapture of excess energy while you're cruising that makes the difference.

Now, this technique doesn't work in heavy stop and go traffic, else the battery simply can't keep up. Learning the car and knowing it's limitations does a lot to help gas mileage.

Or at least, that's my experience and understanding so far...

Other techniques I've found very helpful for better economy?

1. Drive city not highway on small trips. Highway is good for > 20 miles.
2. Use cruise control, preferably at the speed limit, 55 seems to be a sweat spot.
3. Drafting is about a 10% increase, up to 20% if you can find one of those low clearance race car semis.
4. Take it easy on starts and stops.
5. Possible difference in different brands of Gas, still exploring this one.
6. Ev mode to get up to 25mpg if battery and start up sequence permit.
7. During initial start up when ICE is generating power only, your drive is only via the drive motor (MG1) when your car is in this state, accelerating to at least 30mph and maintaining that speed seems to get the best use of this stage of the warm up cycle.
8. Watch traffic conditions. If there's a red light or stop sign, don't burn gas getting to it. Coast.
9. Don't drive angry.

On a side note, I did get to go down a mountain, and that was hella cool in the Pruis. The regenerative breaking can slow you down quite well. I let it get up to about 90/95 or so coasting with cruise set at 70 then took the foot off the pedal. Regen breaking kicked in full charge and took me down to 70 and in the 5 minute period it took to go down the mountain, I gen'd up about 375 watts of power. Only time I've seen the battery go to full charge. Going up the mountain was a bit tougher. Right hand lane while pegging out to maintain that 65mph.

 

The OP's suggestion: use EV to accelerate slowly from 0-25 mph.
Others' opposite suggestion: use ICE only in ECO mode accelerate VERY slowly from 0-25.

My suggestion is middle of the road: use EV to accelerate slowly from 0-10 (not long enough to tigger any road-rage from the cars behind), then let ICE kick-in in PWR mode to help the MG accelerate to 45 mph fast, and I mean REALLY fast. The shorter distance you spent to accelerate, the longer distance you can Cruise Control/P&G/Coast to the next stoppage and that will recharge the battery and more than enough to compensate the mpg lost due to the crisp acceleration.

Yeah, yeah, I know. You guys' been screaming at me saying it's VERY INEFFICIENT using PWR mode. You can say what you want, but my real fuel consumption does not lie. I'm averaging 4.1L/100km (57mpg) in the summer and 4.4/54 nowadays in the cooler fall. These are all city return round trip readings so they're not artificially bumped up due to down slope road segments.

The fact is I get to have pretty good FE and still ENJOY driving the Prius and kicking a few butts instead of crawling along being constantly butt-kicked and screamed at for impeding traffic, and being called "gay". Not that there's any thing wrong with being gay, but since I'm not gay and being called "gay" for how I drive, not because of my sexuality is something I don't enjoy.

Did I mention I REALLY ENJOY driving the Prius? I think that to "enjoy" a car does not come solely from it's high mpg. You need to enjoy the whole driving experience.

One more thing, the warming up is THE MOST viscious FE killer. I just installed an Engine Block Heater yesterday and plugged it into a timer to pre-heat the car for 2 hours before taking off this morning. My fuel consumption went down to 4.0L/100km / 59mpg. I was thinking of the Canadian winter when I decided to install the EBH but in fact it even helped the mpg when the outside temp was still very warm at 20C/68F this morning. About 5 cents of electricity cost saved about 10 cents of gas; but I bet it will save the ICE from extra wear and tear for cold starts and long warm-ups. This should rank as the higher benefit.

 

Some of the posts above are wrong on so many points I don't know where to start. In fact I don't think it makes sense to go point by point, as it will only confuse people. Perhaps we need to drop back to basic physics and get away from perception and voodoo. Here are the basic points relating to Prius efficiency:

1) There is no free lunch. Every joule of energy in the Prius comes from burning gas. Period. There isn't any other energy source unless you have converted yours to a plug-in.

2) Energy conversion costs energy. There is always a penalty for converting energy from one form to another. Think of it as a tax or friction in the system; each conversion steals another chunk of energy for the privilege of converting to another form. There isn't any way around this.

3) Aerodynamic losses increase by the square of the speed. Driving fast wastes a lot of energy to overcome these losses.

4) Internal combustion engines, like the one in the Prius, are most efficient at medium-high loads. They don't do well idling or running flat out.

So what do we take away from this? The most efficient Prius design would have a smaller engine with a direct mechanical drive of the wheels. This Prius would only travel over flat roads at one speed, and that speed would be relatively low to avoid large aerodynamic losses. The engine would always be loaded at the optimal value at that speed.

Unfortunately, that sort of Prius would be useless in the real world. In order to make the Prius drivable but still approximate the ideal, the engineers at Toyota turned to a hybrid propulsion system. This allows for a larger engine that is used only part of the time, but is still smaller than normally needed for passing and acceleration.

The electrical drive path, and particularly the battery, are bad for efficiency, as they incur additional conversion losses. The key point is that the electrical path, as bad as it is, is still better in some situations than running the engine inefficiently. Take stop and go driving as an example. In stop and go driving, the Prius inches forward using just the battery and MG2. When the HV battery charge drops to a low threshold, the engine (ICE) comes on and recharges the battery. This cycle repeats as necessary. Obviously this is a very inefficient way to move a Prius. The conversion losses from gas to mechanical to electrical to chemical to electrical to mechanical are significant. Interestingly, though, the losses of idling the ICE would be even worse.

So here it is in a nutshell. It is very simple, but extremely important, so don't skip over this next line: The HV battery and electric motors of the Prius do not improve efficiency; they only prevent bigger losses in some phases of driving.

I'm going to say this again using a slightly different wording to make sure that everyone gets it: Since all energy in the Prius comes from burning gas in the ICE, the Prius can be no more efficient than the ICE. Every other step reduces the efficiency. The electrical drive path is fairly inefficient, but the control system in the Prius is smart enough to only use that path when direct use of the ICE would be even more inefficient.

So when does it make sense to use the electrical path? When the ICE would be idling; when the ICE would run WOT; when the ICE would have to cycle on and off; and any time virtual gearing is needed to accelerate the car, such as starting from a stop or reversing.

That's it. It's really pretty simple from the standpoint of physics.

Tom

 

Thank you Tom for such an in depth explanation. I would like to supplement some minor points for some rare situations.

Yes sometimes there is free lunch if you drive down hill, you get to capture the potential energy into the HV battery. Ok this is only applicable to those down hill trips that are so long the battery got fully charged and cannot capture anymore energy. Wouldn't it be better to use up some of the battery charge using EV mode whenever possible (such as a slight and short flat/uphill segment) just to free up some of the battery capacity to get ready to capture the potential energy from the next down hill?

100% correct in theory.

Also 100% correct theoretically, but it really matters in freeway speeds. The difference between 70 and 80 mph would be significant while that between 35 and 45 mph would be small. So for most city driving, crawling along @ 35 mph may not be more efficient than driving at the "normal" speed of traffic, which is 45+mph. In my experiences I found that the Prius is more efficient under cruise control at around 45 mph for very slight up hill segments. Maybe the momentum of the car keeps the highest gear ratio for the ICE yielding about 47 instantaneous mpg, while CC @ below 40 mph for these slight up hill segments brings it down to 29 and below. Maybe @ 45 mph the ICE hits it's sweet spot of medium load best efficiency.

Exactly my point too. So babying the gas peddal in ECO mode not just makes you crawl like a turtle, but also makes your ICE inefficient. And by the time you crawl up to speed, it's time to brake for the next red light, and there's no time left for you to P&G or coast. On the other hand, using PWR mode to accelerate crisply takes the full advantage of the MG2's 80 horses and leaving the ICE to supplement the power in medium load - most efficient use of the ICE (I don't feel the ICE straining at all under PWR mode, as opposed to "flooring" it in ECO mode). Also you get up to speed fast leaving way more room for you to P&G or coast.

I would like to summarise the key points for driving the Prius in the most efficient way:
1) not to use EV extensively and deplete the HV battery;
2) not to use ICE only by crawling along in slow speed all the time;
3) find the sweet spot of medium ICE load for acceleration such as using PWR mode,
4) and find the appropriate times for using EV to spare the ICE from being used in inefficient situations, such as starting from 0 to 10 using EV, not 0 to 25.

 

This one is wrong. The energy recaptured by regenerative braking is not free. You already paid for this lunch when you drove up the hill in the first place. All of that nice potential energy was put there by burning gasoline in the ICE. What regenerative braking does is allow you some return on your investment. With a normal car you get none of that braking energy back. With a Prius you get to recover some of it. Either way it is not free.

Tom

 

I understood about the sacrifice of the uphill trip. My point is only for LONG down hill slopes that the battery would be fully charged way before the bottom; so using up part of the charge mid slope whenever you have a chance would free up some capacity for you to re-capture more energy later, or it'll just go wasted since the battery is already full; and you'd have to use "B" gear or even mechanical brake to slow down the car, that's even worst.

 

I work downtown San Diego, and come off the freeway downhill exiting about 10 blocks before arriving at parking lot. So the battery is all charged and the EV is good for gliding through the last few signals.

 

And how do you charge it back?

 

I readily admit to the laws of conservation of energy. However, for your argument to be true, you are making two assumptions:

1. The car converts gasoline to electrical stored energy at the exact same efficiency it converts gasoline to the kinetic energy of the car. This is not true because the engine does not just run the electric motor/generators to create electricity, it actually takes torque of the ICE and mechanically sends it to the wheels through the planetary gears in the synergy system.

2. You would also have to assume the ICE runs at identical efficiencies regardless of torque or RPM. This is also not true.

So you can't explain this away based on conservation of energy. True, gasoline is the only source of energy, but depending on how you use the Synergy system, there are very different losses at different points in the system. By not revving the ICE to really high levels during acceleration, you are probably saving a lot of energy.

The other argument is to say "The Computer Always Knows Best". But having worked around software a lot, I know that's probably not true.

 

I have experienced the same as paprius4030. Am I doing something wrong? At about 10mph, it beeps and disengages. Can anyone tell me if driving in EV mode for any duration (lets assume 1 tankful), if that really pushes the MPGs above your norm? I've tried driving in EV mode when possible, but I can't get to 25mph.