battery use

Is that a percentage of "efficiency gain" or efficiency ? If gain, what do you mean by gain, gain over a non-hybrid ?

I've heard that Regen braking only gives a 2% mileage boost. I.E. without: 50 MPG and with 51 MPG for example. So 2% may explain the thinking here.

So is it 35% or 2% and percent of what ?

And clearly this does not apply to uncongested highway driving, so I'll presume we are limiting discussion to city driving only. But number of stops and delta-V are big factors of course.

 

I'm not sure how to ask this question so it makes sense...but is there more of a load on the ICE when say the batters are 40 percent charged than when they're 60 percent, or does it work have the same amount of load for a longer time. The reason I'm asking is that if it's the same amount of load for a longer period of time, I think there are times when it would make sense to try to run on batteries only.

 

Agreed, there is no such thing as free. But gliding uses a lot less amps than stealth mode and maintains speed a lot better than 0 throttle, so it seems useful.

I guess I disagree here. Regen is only free if the choices are slow down in 5 seconds recapturing some energy through regen or slow down in 5 seconds dissipating all the energy as heat. In that case regen is a clear winner. When the choice is slow down over 30 seconds while gliding, or slow down in 5 seconds to capture as much energy through regen as possible, I think the answer is less clear. My belief is that the amount of energy saved by not burning gas for 25 seconds is greater than the amount of energy recovered through regen.

Here's a thought experiment:

Accelerating in EV mode at ~2 mph/s I believe draws about 100 A. To accelerate up to 30 mph in 15 seconds would uses about 83 Wh of energy. Slowing back down on regen generates a max of about 60A. Lets say you slow down from 30-0 in 5s under 60A regen braking. That would recover about 17Wh of energy. Lastly, the Prius seems to be generally agreed to require about 225Wh/mi to cruise.

Situation A:
Accelerate to 30mph in 15s, cruise 1 mile to red light, brake under max regen to 0. If this could all be done in EV you would burn 83Wh accelerating, 225 Wh cruising, and then recover 17Wh from regen. Net energy used 291Wh.

Situation B:
Accelerate to 30mph in 15s, cruise 3/4 mile, glide to a stop at red light. In EV mode this would burn 83Wh accelerating, 169 Wh cruising, with no energy recovered from braking. Net energy used 252Wh. Savings from gliding ~13.4%.

If we assume the ICE has to burn the energy equivalent of 3 times as much gasoline to produce the same mechanical energy output as the electric motor Situation A becomes 907Wh, while Situation B becomes 756Wh when using the ICE to accelerate and cruise. Now the savings from gliding is ~16.6%.

Now these numbers are estimated based on my research, as I don't have the benefit of a gauge to go try this out. But I believe the basic idea is sound. It fits the evidence most drivers have reported, and it passes my own personal observation. In the course of 5 minutes of city driving, lets say I typically accelerate from 0-30+ 3 times. This consumes ~250Wh accelerating and probably about the same cruising. And yet, I rarely ever see more than 1/2-1 50Wh green car on the MFD in flat city driving. By the numbers estimated above, you would expect 51Wh or one car from slowing down three times so that seems about right. I realize thats anecdotal, but its the most direct mean I have of testing this.

Rob

 

That was a great post. I don't have my new 08 P#1 yet but this really helps me think about how to operate the Prius. Sounds like the key to great mpg is to avoid the battery for extended periods of time. Probably just let the system do it's thing while I enjoy the ride.

 

The load on the ICE is higher when the battery is at 40%. All of the energy in the battery comes from the ICE; when the battery needs to be charged the ICE must work harder to do it. Someone will follow this post with a "wait, how about when the battery is charged by regenerative braking?" In that case the energy still comes from the ICE. Any energy extracted by regenerative braking was first put into the car by accelerating it up to speed. The energy to do this ultimately came from the ICE, unless you get help from an outside source.

Tom

 

If you read my previous posts, I have not suggested using the EV mode in any way. I think there might be a good EV enhanced mode, but I have not had time to look into it.

Almost all of the high-mileage advice centers around the engine, which is the least efficient energy source in the hybrid system. It makes sense to try to use the battery and braking systems to their fullest capabilities, but most of the advice here is to not use them at all. The battery and regen systems are what separate the Prius from, say, the Yaris, which has a similarly sized engine, but substantially lower gas mileage ( and a substantially lower price). Even a cursory examination of the display will show that the battery is in constant use, both charging and discharging. Charging it from the engine is far less efficient than charging it from the regen system, so it would seem that using the brakes will help in the long run. I can see no advantage to gliding to a stop. You are allowing the energy that could be reclaimed to be wasted as heat. Reclaimed energy is free to the extent that you have to pay for the equipment to do the reclaiming. Beyond that, the initial energy expenditure, whether for gasoline or other sources, is not included in the cost-benefit equation.

I think that part of the problem is that the battery is relatively small. However, unlike the gasoline that runs the engine, the battery energy can be "recycled" over and over as it is constantly charged and discharged. The kilowatt or so of battery capacity will be multiplied several times while the gallon of gas is only good once. In 50 miles of stop and go driving, how much regenerative charging will you expect to get? A gallon of gas is about 35 kwh of energy. If you can get 5 or 6 kw of regenerative charging, you will increase the available energy by 15%-20%. That is one of the primary reasons that the Prius works as well as it does.

As has been noted several times, all of the energy initially come form the engine. However, maximizing gas mileage means using this energy as efficiently as possible. If you accelerate only so you can "hit the brakes" and recharge the battery, you are not getting the message. If you have to slow down or stop due to traffic or road issues, then reclaiming as much of the initial energy as possible can only help with the overall mileage increases.

Pat

 

The engine is the least efficient energy source in the hybrid system. It is also the most efficient energy source in the hybrid system. The engine is the only energy source in the hybrid system.

I think you have gotten so close to the trees that you can't see the forest. All of the energy ever used in a Prius comes from burning gas in the engine. Every other bit of energy, such as that from the battery, has previously gone through the engine. It starts out at the same efficiency as the engine, since that is where it was created (converted), and then you subtract all of the other losses.

The Prius has an electrical transmission, and as a consequence, the battery is in constant play compensating for small variations in load. This is more efficient than jiggling the speed of the engine each time the load varies a bit. The battery also allows the Prius to have decent acceleration with a small, low torque engine. In addition, the battery allows the engine to shut off when not needed and provides a place to store energy recaptured during regenerative braking. All of these things are obvious and unquestioned.

If you are simply stating that it makes sense to efficiently use the electric motors and regenerative system when they are needed, then I agree completely. That is another unarguable point. On the other hand, if you are trying to imply that the electrical system ever supplies or generates energy more efficiently than the engine, then you contradict the facts. Simple thermodynamics answers that question without any detailed knowledge of the underlying systems. That's one of the things I like about physics.

Tom

 

I realize that, its just a convenient way of estimating the mechanical energy required to move the vehicle as its pretty well characterized and very high efficiency. Its also to some degree the logical extension to the arguement that using the battery more is good.

I don't think that is true. The Yaris weighs 2300 lbs, the Prius about 2900 lbs. and both have 1.5L engines (they're basically the same engine). The Yaris has a drag Cd of 0.29, to the Prius' 0.26 but the Yaris should have a smaller cross section so wind resistance should be a wash. Given that, the Yaris should get better highway mileage than the Prius, since the electrical system is of very little use at sustained highway speeds. In reality, the Prius is rated at 46mpg highway, while the Yaris is rated at 35mpg. That puts the Prius at 24% more efficient on a consumption basis. The Prius runs an Atkinson cycle in its 1.5L, making it significantly more efficient than the same 1.5L running the Otto cycle in the Yaris. I believe that is responsible for the difference.
Prius Engine: Toyota NZ engine - Wikipedia, the free encyclopedia
Yaris Engine: Toyota NZ engine - Wikipedia, the free encyclopedia

In the city the regen system and electric motor presumably come more into play, but the efficiency gain from the engine is still there. The Yaris falls to 29mpg city, so you could reasonably expect the Prius to be 24% better than that or 38mpg. In reality the Prius is 48mpg, or 39% better. One would assume that this is the difference made by the hybrid system. I would assume that gain is a combination of regen and the auto start/stop.

So, based on the above relative to the Yaris the Prius is:
Highway: 24% more efficient (100% due to engine efficiency)
City: 39% more efficient (62% due to engine efficiency, 38% due to the hybrid system)

Now again, its all due to the hybrid system in the sense that most people would not accept the performance of the 1.5L Atkinson cycle engine without the extra low end torque provided by the electric motor. The electric motors and PSD also allow the Prius' engine to remain in its optimal efficiency zone more of the time. The Prius efficiency improvement numbers are probably even better than above, because we're not accounting for the Prius being 26% heavier and considerably roomier inside.

Well, I'm not sure what to tell you. I provided an example above where gliding saves significantly more energy than regen. If you have doubts about some of those numbers, thats a fair discussion. As I said above they seem reasonable, but are not based on my personal experiences/measurements.

My supposition is that based on the above numbers, regen can improve city efficiency by about 10-20% depending on how much of the 20% hybrid system improvement we attribute to auto stop/start. This is great, and will benefit anyone driving a Prius "normally." However, there are hypermiling techniques that can improve city efficiency by this much or more. In a regular car you can get these kind of numbers by momentum driving and shutting off the engine while coasting. The Prius makes this much easier/safer, as it automatically shuts the engine off for you w/o disabling any of your controls. Based on the trial and error experience of a lot of people on this and other lists, glide mode appears to be the most efficient mode to coast in.

Based on the number from my previous post, regen would recover ~17Wh, while gliding would eliminate ~56Wh of electric drive or ~169Wh of gasoline usage.

I think part of the problem is you vastly over estimate how much energy is generated during regen. To test this, try checking your consumption screen from time to time while driving in the city. Its very rare to get more than 1-2 little cars in each 5 minute box. When I'm driving normally, or my wife is driving, we usually get 1/2-1 car per box. Assuming that we average 30mph, the 30 minutes on the screen would equate to 15 miles. At an average of 1 car per box, and at 50Wh per car, thats a total of 300 Wh in 15 miles. That would equate to 1kWh in 50 miles, not 5-6 kWh as you were hoping for above. So if 1 little car per box = 1kWh /50 miles, to get 5-6kWh you would have to be seeing 5-6 little regen cars in every 5 minute box on the consumption screen. The best I've ever seen was coming down the ~5000 ft from Flagstaff to Phoenix, and that was 2-4 regen cars per box for about 30 minutes.

The size of the battery is more or less irrelevant. Because the amount of regen energy in any one event is very small, you are generally using very little of the capacity you have. This has been proven two ways. First the US DOE tested two Gen1 Prius' to 160k miles, and then did end of life testing on them. They found that the battery capacity was down by ~50% from new, but that mileage had not degraded at all. Second, early on in the plugin conversion experiments people tried just adding a second battery to the Prius to double its capacity. After sorting out all the challenges of getting both battery controllers online and getting the car to use both batteries, what they found is it basically made no difference. As long as the primary source of energy is the ICE, the size of the battery is not hugely important (within reason of course). Now, for PHEVs this is clearly not true. You are offsetting gasoline burned by the ICE with electricity from the grid. In this case, the bigger your battery the more gasoline you can offset.

Rob

 

EDIT: Bah, I see that miscrms already covered many of these points. Guess I type too slow. Oh well.

As has already been pointed out, it is the only energy source for the hybrid system.

That depends on what you are trying to accomplish. If you are attempting to accomplish the maximum possible distance traveled for any given quantity of fuel, then it does not make sense.

Most of the good advice around here recognizes that it is not possible to avoid using the brakes or battery. Instead most of the good advice is to use the battery and brakes as little as possible while maintaining a safe and reasonable driving style.

The battery and regen systems are just 2 of many things that separate the Prius from the Yaris. If you are going to give advice with respect to this, I recommend that you learn a bit more about the Prius. The Atkinson Cycle engine contributes substantially to fuel savings for the Prius, as does the HSD transmission. Beyond that the Prius gets substantial fuel savings by turning the Internal combustion Engine (ICE) off when power is not needed (such as while stopped, slowing, or driving downhill). Even when the Prius is unable to stop the crankshaft of the ICE from rotating do the the speed of the vehicle (such as on the expressway), the Prius will cut fuel flow to the ICE in situations when power is not needed (such as slowing or driving downhill). Additionally the electric A/C compressor means that the passenger compartment can be kept comfortable for short periods of time with out needing to unnecessarily starting up the ICE. Furthermore, the aerodynamic shape of the Prius reduces its drag. I'm sure there are other differences (functional, aesthetic, and comfort) that contribute to the difference in cost between a Yaris and a Prius, and there are probably a few fuel saving differences I've forgotten to mention.

Except when gliding, at which time the display then falsely indicates that the battery is neither charging nor discharging.

This is where you keep getting confused. Charging from the brakes means using the ICE more than necessary to glide to a stop. Therefore the ICE has been used to create excess kinetic energy, some of which is lost to excess friction. Then the regen system must attempt to recapture some of this excess energy. It would be far more efficient to avoid using the battery at all. However, since this is not typically possible with reasonable driving style, the battery is charged whenever the brakes are necessary as well as when it is efficient and convenient to do so directly from the ICE.

Not true. If you are gliding to a stop, then you are either covering a greater distance than you would have had you used the regen braking, or you are stopping from a slower speed to start with. Greater distance increases the numerator of the mpg ratio. Slower speed means less fuel used to accelerate (or maintain speed) in the first place. Less fuel reduces the denominator of the mpg ratio. Either way you cover a greater distance per unit of fuel.

Why not? You have to pay for the fuel that is used to create the excess energy you are trying to reclaim. Why shouldn't the cost of that fuel be included in any cost equation?

I believe that I've read that increasing the capacity of the battery does not improve typically fuel efficiency in the Prius unless you also make changes to charge the battery externally (plug-in).

The energy from the gasoline is recycled over and over. It is where the battery energy is coming from in the first place. Yes recapturing energy that would otherwise be wasted allows the Prius to gain some efficiency over a vehicle that does not attempt to recapture this energy. However, it would be far more efficient not to need to recapture excess energy in the first place (ie glide to a stop). Since this is typically not possible regen is the next best thing, but gliding when possible is still better than regen.

True, if you need to eliminate kinetic energy faster than gliding will allow than thankfully the brakes attempt to reclaim as much of it as possible in the form of electricity which is then converted to a chemical potential energy to be later converted back to electricity and back to kinetic energy when needed. However, if traffic or road issues allow for a glide instead of braking, then it will be far more efficient to maintain as much of the kinetic energy as possible and avoid the conversion losses.

So

If you need/want to reduce speed,
Colliding with something is worst.
Friction braking (brake pads) is better than colliding
Regen braking is better than friction braking
Coasting (regen that occurs with foot off brake & gas) is better than regen braking
Gliding is better than coasting.

 

Let me start over:

I use pulse-and-glide and warp stealth all the time. They work really well, and I am not saying otherwise. I am not advocating the use of EV only mode.

There is only one on-board energy source: the engine. All of the energy we have to play with starts there.

The battery is going to be recharged whether it is using the engine, or the regeneration system.

The second law of thermodynamics is always trying to eat your lunch, and we are trying to keep as much for ourselves as possible.

Let me add:

If you have used the regeneration charging system to reclaim energy that would otherwise be wasted, it makes the battery an energy source in, and of, itself, and under these conditions, the battery is a much more efficient source of energy than the engine. It makes no difference where the energy originally started, once it goes through the initial extraction process (the engine) the second law has had it's way and whatever you can extract after-the-fact is free (taking into account that there are costs associated with the reclaiming process itself). You put as much of the free energy back into the battery as you can and "Bob's your Uncle". Keep in mind that p&g is also initially fueled by the engine, but doesn't seem to get the bad press that the regen systems does, probably because the mileage increases are easy to see.

I can think of no circumstance where gliding to a stop is an advantage over braking and regenerating some of the energy. Here are the two scenarios:

1. Begin gliding at 35 MPH with a stop sign in the distance. In order to glide to a stop, you will need a lot of road and take a lot of time, all the while drawing energy from the traction battery, energy that will have to be replaced.

2.Same scenario except you glide up to within a reasonable distance from the stop, apply the brakes and reclaim at least some of the energy used to get you up to speed in the first place.

In number 1, all of any remaining kinetic energy is converted to heat, while at the same time, energy is being drained from the battery. There is no gas savings over using number 2, as it includes the glide up to the stop as well. In fact, the extra time taken to glide down to below 7 MPH will add even more to the energy deficit of number 1 as the battery will continue to be discharged.

I do not currently have any good numbers on how much reclaim is possible in urban driving. It is obviously more than most people on the chat board think it is. I was able to get two "cars" in only a few miles; it was not downtown driving conditions, and I was not really working at it. My, admittedly rough, calculations show that for every three cars you would get a 1% increase in MPG, based on a 25% engine efficiency and 35 KWh of energy in a gallon of gas. The one posting with some hard numbers indicated the possibility of two cars per segment, which works out to 24 cars per hour....an 8% increase in mileage (admittedly a very rough estimate).

I want to add that the Toyota engineering documents credit the regenerative charging system with a much higher contribution to the overall Prius efficiency than the engine.

Pat

 

All good so far.

I don't think that is a fair assumption. In the limit if you don't use the battery in the first place you don't have to recharge it at all. Now realistically there is always some usage, but we can try to make it as small as possible. That is the crux of the argument I think. Using a "normal" amount of battery power and then replacing it by regen is more efficient than using a "normal" amount of battery power and then replacing it by running the ICE. However, due to <1 efficiencies both of these are less efficient than not using the battery in the first place. This assumes you can accelerate up to speed and cruise while maintaining the ICE in its peak efficiency range, which you appear to be able to do via deadbanding in the first 2-3 rpm steps.

True enough, as is so in all things.

Here's where I think the problem is. The amount of energy consumed during glide mode is very small. In my situation B above, I ignored the energy consumed during gliding, so lets calculate it. While glide mode may be up to 10 Amps, I believe typical is 2-4 Amps under 30mph based on data here:
http://priuschat.com/forums/prius-t...ta-analysis-complete-charts-2.html#post639800

So if we glide 1/4 mile in ~45 seconds starting at 30mph we would consume about 3A from the battery for 45s. This is roughly 7.5Wh. So correcting the calculation above in EV mode we use about 260Wh to accelerate, cruise, and glide stop compared to 291Wh to accelerate, cruise, and regen stop. On ICE power, its 763.5Wh for gliding, and 907Wh for regen. This still puts gliding at 15.8% more efficient.

I would agree that you can think of regen as an energy source. Once the car is up to speed, its momentum is basically potential energy that you can chose to use or throw away. Regen is one way to recoup some of that energy. Gliding is another. By using the stored kinetic enegry in the vehicle to carry it along, you are in essence providing an alternative power source to either the ICE or the Battery. From the calculation above, I believe from 30mph to 0 you can recapture about 17Wh of energy through regen. By gliding I believe you can save ~169Wh of gasoline from being burned, at the cost of ~7.5Wh of stored battery energy. That would make gliding about a 9.5X more efficient use of that stored kinetic energy than regen.

I actually believe P&G works for exactly the same reason. Its also the reason we plulse and glide, instead of pulse and coast (ie w/regen). If you can accelerate on ICE power at peak thermodynamic efficiency, and then glide with minimal drag, minimal electrical consumption and no fuel consumption then I believe you quickly come out ahead compared to cruising on ICE at somewhat less than peak efficiency. If you were to pulse and coast, the amount of drag the regen was putting on the vehicle would slow it down faster requiring the ICE to be restarted sooner. The amount of gas burned running the ICE more often would be much greater than the amount of energy being put back into the battery by regen. In essence, my situation B is just a prolonged P&G. You pulse up to speed as efficiently as possible (and without charging or discharging the battery), cruise for a little while, then glide as long as you can once you know you're going to have to stop.

First of all, let me say that situation 2 is actually what I do in practice. I drive in an urban environment, which is why I can't really P&G. I also can't glide all the way to 0 without having cars pile up behind me and/or messing up my timing through the lights (never mind that I'm not that patient). Typically I accelerate up to speed dead banding without the ICE reving at all. I then cruise while backing off the accelerator to get the best instantaneous mileage possible. Then as soon as I get a sense that I'm going to have to stop, I start gliding. If there is traffic behind me, I'll pulse a bit to keep my speed reasonable. Otherwise I'll just keep gliding, but realistically I'll be getting close to the light and have to brake by ~20 mph. Hopefully the light turns while I'm gliding and I can pulse back up to cruising speed.

This does beg the question though, if you think regen is nore efficient than gliding why would you glide at all? Your first sentance seems at odds with your examples. If you can see no case where gliding is better than regen, then why do both of your examples include gliding? Particularly as I believe the amount of regen from MG2 falls off with decreasing wheel rpms. As I recall voltage out of a generator is directly proportional to RPMs. Below a certain voltage, MG2 would not be able to charge the battery, but I believe the MG2 inverter converts the voltage up to extend its effective range. This is at the cost of amps and efficiency though.

I think your base assumption is incorrect. It appears to me that not running the ICE saves far more energy than glide consumes, and the amount of energy that can be recaptured through regen is similarly small. In this case the fact that the ICE is so inefficient is exactly what makes not running it so profitable from a net energy stand point.

You could argue that since glide does consume some energy, you're better off making sure you get a little bit of regen to make sure you don't have to run the ICE for it. I guess thats your situation 2. However, the amount of energy we are talking about is so small that I don't think it really matters. On top of that it is almost impossible to cruise without having MG2 put some energy back into the battery so you're probably covering your glide anyway.

Actually that seems about right. Based on my apportionment above, for city driving I would have put it at ~10% regen, ~10% auto start/stop, and ~30% improved engine efficiency for a total of ~50% overall efficiency improvement. you're 8% number fits pretty well with that.

That could be. The average driver is not going to be content to accelerate the Prius at peak efficiency (0-60 in about 30s). Therefore the electric motor is going to have to kick in to try and keep the ICE from having to rev and really killing your FE. Now, if the electric motor had to run you had to use the battery. That means you have to put energy back into the battery and the most efficient way to do that is through regen. In other words if a "normal" driver drives the Prius like a "normal" car, the improvement seen relative to a "normal" car will be in large part due to the hybrid system and regen. Thats not the same as saying that the best way to drive a Prius is to drive it like a normal car. Nor is it the same as saying that a Prius is at its most efficient when using the electric motor, battery and regen.

I have a feeling we all basically agree, we're just looking at things differently. When I say I use the battery as little as possible, this is mostly to contrast those that argue you should try to use the battery more often than normal which was a popular strategy for a while (and why everyone initially thinks they want an EV button). I realize that the car will always use some electricity, but for the most part I also can't keep it from making some which mostly offsets that. There are also specific situations where I will pulse electric drive to prolong a glide and keep from restarting the ICE at a time when it would not normally run. We tend to oversimplify and talk in absolutes, particularly when trying to convey basic concepts to new drivers, but in reality all of these concepts have to be applied as they make sense. There is no one right answer that is always the best way to drive in all circumstances.

Rob

 

I've tried enough times. I don't think I have the expository skills to explain this in a way that is understandable to you. Hopefully miscrms will be up to the task. I give up, you are welcome to believe whatever you like. Hopefully anyone else who stumbles across your hypothesis will gain the knowledge they need from the other posts in this thread to avoid misunderstanding the facts.

 

Gliding is good....gliding is good....I keep saying this but no one seems to be reading it. I have never said that regen should replace gliding in the larger sense. What I am saying is that losing out on the regenerated energy available when you have to stop anyway is going to hurt the overall mileage. Gliding converts gasoline energy to heat...that is why the car ultimately slows down. Why not put at least some of that energy back into the battery and reuse it? I am sorry if some people do not understand how energy reclaim systems work, but that does not really change the physics of what is going on. Lets say your Prius has pulled you to the top of a 6000 foot pass in southern Montana, and it is 30 miles down to the river valley. Using the "regen is bad" thinking, you would glide to the bottom ( but avoid recharging the battery so as not to waste energy) since you had to use gasoline to get to the top in the first place, and gliding is always better than regenerating. I would take the "free" energy that is available and put as much of it back into the battery as possible. I would do that at every chance I got.

I have never advocated using the EV mode. The car is not designed to do that. That is not to say that there might not be circumstances where EV would be an advantage, when used in a larger operating context. I have a couple of ideas that I am going to try. Please realize that while P&G is a well established mode, it is not at all intuitive, and there may be other modes that also work well.

There is a lot of "common knowledge" on the forum that has not really been examined. I have seen references to the Atkinson engine being 50% more efficient than the Otto cycle. The real increase is about 10%. The synergy control system and regenerative braking represent about 60% of the efficiency gains of the Prius, but the thinking here is to "not use the battery at all", when it is one of the primary reasons you can get 60 MPG with the Prius, and you are "using" it constantly, even in glide mode. Probably nothing is going to beat getting 75 MPG in P&G mode, but I am always skeptical of the "don't bother me with facts" approach to anything.

Pat

 

I cannot reconcile those two statements. If you agree that gliding is a more efficient use of stored kinetic energy than regen, then I do not see how regen to a stop is ever better than glide to a stop. As mentioned before I do agree that in real world driving its not often practical to glide to 0, but that is irrelevant to the question of which is more efficient. I would agree that stopping with regen is always more efficient than stopping with friction brakes, but I would still claim that it is always less efficient than gliding. I would also claim that what you do in the last 10-15 mph doesn't matter a whole lot as kinetic energy is dwindling and regen efficiency is falling off.

Ok, lets look at the physics. Once the vehicle is up to speed, it has a certain amount of kinetic energy. There is also a drag force on the vehicle due to friction and wind resistance trying to slow the vehicle down. If the Prius uses ~225Wh/mi to cruise at ~30 mph, from that we can estimate that it takes 6.75kW of power to overcome drag. This would estimate the drag force at ~504 N. The Prius with a mass of 1325Kg moving at 30mph has a kinetic energy of 114.4kJ. Based on the Prius' mass we can also calculate the normal force of 13kN, and from that estimate the coefficient of friction at 504 N/ 13kN = 0.0388.

So in glide mode, the entire 114.4kJ of kinetic energy is as you say disappointed as heat. However, the vehicle will glide for 35 seconds covering a distance of 776 feet. This is a little shy of my 45 sec & 1/4 mile assumption. In 35s, at an average of 3A glide mode will use ~5.8Wh of battery energy.

Now lets say we regen brake with an effective coefficient of friction of 0.388, and a generous average regeneration efficiency of 60%. Of the original 114.4kJ of kinetic energy 10 % is now lost to friction, and 40% of the remaining energy is lost as heat in the regen system. This would put 61.8kJ or 17.2 Wh back in the battery. This is very close to my original 17 Wh estimate. In reality it will be lower as the last ~23% of the energy must be dissipated as heat by the friction brakes below 8 mph, so lets say its actually 13.2Wh. We had ignored this fact in the original estimate. The car now stops in 3.5 seconds over a distance of 77.6 ft.

So, we are 13.2 Wh richer for having used regen, but we are 698.4 ft short of the traffic light. So, we need to wait and apply the brakes 698.4 ft later. Overcoming the friction forces to maintain 30 mph over this period consumes an additional 29.8 Wh. We can argue that 13.2Wh of this can be electric as we are about to get that much back from regen. But the remaining 16.6Wh must be generated by the ICE. For glide mode we were negative by 5.8Wh, so the delta is 10.8 Wh.

Using the same ICE efficiency of 33%, stopping with regen will consume an additional 32.4Wh of gasoline compared to gliding. As far as I can see, any amount of regen you use will result in a lowering off overall efficiency.

I'm sorry but this an artificial condition at best. What we are discussing is what is the most efficient way to drive for the driving conditions encountered by the average person every day. An extension of a theory to its limits is not valid if that limit introduces non-linearities as this one clearly does. Without the limits of wind resistance and safe driving speed you would be much better off gliding down the hill because as shown above, any regen you recoup will be less than the energy required to travel the distance you could have coasted at the bottom of the hill. If you do take terminal velocity and safe handleing speed into account, then this case has zero relevance to normal everyday city driving. If you do commute over a 6000' pass every day, then you are on your own to figure out the most efficient way to do that.

Very true. P&G is a very old technique developed on traditional ICE vehicles that happens to work very well on the Prius. If you have an alternate proposition and some math or real world data to demonstrate it I'm sure lots of people will be very interested to look at it. So far we haven't seen either.

Also some truth to that, but your example is a poor one. If we say that a traditional ICE is about 20% efficient while an Atkinson cycle is 33% efficient, that 13% increase in efficiency results in a 37% reduction in fuel consumption. Looking at it the other way, the Otto uses 58% more fuel than the Atkinson, so that 50% claim is not too far off. On page 10 of the following document, Argonne National Labs found the Gen 1 Prius engine to have a peak efficiency of ~37%, and an average of 33-35%. Not sure what a typical Otto cycle is, ~20% is what I've seen/heard.
http://www.transportation.anl.gov/pdfs/HV/2.pdf

I think people are more than happy to be bothered with facts, you just haven't provided any. There is a good working theory based on observation, a good scientific/mathematic rational for it, and lots of people that seem to see the same results. If you want to challenge the status quo, you need something more than an idea. Lets see some numbers & data!

Rob

 

If anything it looks like I'm overestimating drag. This page puts the Prius at 153Wh/mi at 40 mph.
Prius Palm Mileage Simulator

With this value steady state power requirement becomes 4.5kW, drag force becomes 335.8 N and coefficient of friction becomes 0.0258. The stopping distance is now 1167 ft, in 52.9 seconds. Much closer to original 1/4 mile, 45 second estimate. The glide now consumes 8.8 Wh.

The distance deficit that the regen case must now overcome is 1089 ft., which at the revised 150 Wh/ mi is 30.9 Wh. 30.9 - 13.2 - 8.8 = an energy shortfall of 8.9 Wh in the regen case, or 26.7 Wh of gasoline saved by using glide mode over regen.

Rob

 

Okay, I see the problem. It comes down to this misconception:

A rolling car is overcoming friction, whether gliding to a stop or braking to a stop. The rolling and aerodynamic frictional losses are the same either way, assuming the same speed profile. In actuality, the aerodynamic losses are higher with a braking stop, since the car stays at higher speeds for a longer time, but we can ignore that for the moment since the difference is small and works in favor of gliding. Let's look at two cases. We assume that slowing begins at the same point for each case, and in both cases there is a stop sign ahead at the same distance.

Case 1 - Braking to a stop: The driver presses on the brake pedal, causing regenerative braking to occur. Some of the kinetic energy of the car is recaptured and stored in the battery, some is lost due to losses in the regenerative system. A smaller amount of energy is lost to rolling resistance and aerodynamic drag. The car comes to a stop at the sign.

Case 2 - Gliding: The driver removes power from the drive system. The car begins to slow down due to rolling friction and aerodynamic drag. No energy is recaptured. The driver is ticked for running a stop sign. Why? Because the car still contains considerable kinetic energy when it gets to the stop sign. In other words, the car doesn't glide to a stop in the same distance from the same speed.

Here is the kicker: to glide to a stop from a given speed, you have to start a lot earlier. Using the brakes reduces the distance by extracting energy. That's why we have brakes. In the case of friction brakes all of the energy is wasted as heat. With regenerative brakes you at least get some of it back. Either way you have suffered rolling and aerodynamic losses, which also happen to be pretty much the same losses you get gliding. With gliding, you start sooner and feed that excess kinetic energy into the unavoidable rolling and aerodynamic losses. You don't get to skip those losses when you brake, you simply add additional losses into the equation. When you use the brakes, you pay for gliding and braking. That's why it's more efficient to avoid braking if possible.

Tom

 

Let me start with the Atkinson engine. It may have a theoretical peak efficiency of 33% in the lab, but it is 25% (or less) in the Prius. A lot of the time it is operating at 10% or less due to the specific engine mode it is in. There are plenty of references available, and the increase in efficiency over an Otto cycle is 10%. The increase cited above from 20% to 33% is not 13%, it is 65% and is not correct.

How does pulse and glide work? Why does it work? It uses the engine to accelerate (a very inefficient mode) over and over, but results in an increase in mileage. According to most of the information here, that won't work. It works because once the energy has been extracted by the engine, there remains a myriad of different ways to use it. P&g is a very efficient way of using the car's low internal friction losses, and the synergy drive-battery combination to minimize losses and maximize mileage. Regenerative braking does the same thing. Once the gas has been burned, the whole cycle starts again.

It does not really matter what the exact amount of energy available for reclaim. What matters is how reclaim compares to the only other method of charging the battery- the engine. The efficiencies are pretty easy to calculate:

The regen system is very efficient, and extracts over 90% of the energy
available; an NiMH battery can typically absorb about 66% of the
charging energy put into it; the Atkinson engine operates at 25%
efficiency

regen-batt-eff= (.9)(.66)= about 60%
engine-batt-eff= (.25)(.66)= about 16%

This four to one ratio agrees closely with the Toyota analysis.

If you watch the scan gauge (or the display), it is apparent that the control system wants to keep the battery at 60% or above, and will charge it even during low efficiency driving modes. The highest mileages will be available when the battery is consistently in the green or as close as possible to that state. There are two ways to charge the battery, one of which is four times more efficient than the other.

Exactly how does regeneration compare to gliding? I have not found any references on PriusChat to field experiments to determine how the two modes differ. I did the following over the weekend:

I drove a ten minute course using p&g (from 40 to 25 mph). I was able to
get a bit over 70 mpg, but also accrued a 10% deficit in the battery
(over the ten minutes)

I then drove the same route using pulse and coast; I would pulse to 40
and then coast (using regen charging) back down to 25 mph. I was
able to get 48 mpg ( which was higher than I expected) and three "green
cars" per five minute segment

In order to compare the two results, we have to normalize them to a standard mpg number. We have to subtract mpg's for the energy used from the battery by p&g, and to add mpg's for the charging energy developed by p&c:

If you use one gallon of gas in p&g ( at 32.5 mph), you will have gone
71 miles in about 2 hours and 10 minutes. At 5% per 5 minutes, this
represents a total battery deficit of 130%, which will have to be "paid"
back by the engine. The battery holds about 1.5 kwh, so 130% is
1.95 kwh. The engine needs to produce 4 wh to put 1 wh into the
battery, so the total energy cost is 7.8 kwh. A portion of this has to
be subtracted from the 71 mpg posted on the display. You have to use
a portion since there is some charging during the pulse process,
depending on the state-of-charge of the battery. If you use 25%, then
the 71 mpg has to be reduced by [(1.95)/(35)][100], where 35 is the
khw energy content of a gallon of gasoline:

[(1.95)/(35)](100)= about 5

The actual mileage is 71-5, or 66 mpg

If you use one gallon in p&c, you will have gone 48 miles in 1.5 hours,
and accrued 54 green cars. Each car is a net 50 wh into the battery, so
the total is 2.7 kwh.: 48 + [(2.7)/(35)](100)= 56 mpg. This is better
mileage than I get from a typical tank of gas.

Everyone is free to draw their own conclusions. I expected more than a 10% difference.

Pat

 

This is just plain wrong. The engine is more efficient during mild acceleration, not less efficient. That's the whole point of P&G. It has nothing to do with the HSD, the battery, or any other high tech part of the Prius. P&G works because all cars, the Prius included, have engines larger than needed for moving across the flat at a steady speed. Cars have oversized engines to allow for passing, hill climbing, and merging into traffic. The Prius, by nature of its HSD, has a less oversized engine than most cars, but it still is bigger than needed for cruising. Gasoline engines are inefficient when lightly loaded. Accelerating loads the engine and allows it to more efficiently deliver energy to the car, where it is stored as kinetic energy. The engine is then taken off line and the kinetic energy of the car is used to overcome frictional losses, until the car's speed becomes too low. At this point the cycle is repeated. All of the gain from P&G comes from running the engine at a more efficient loading, which is done by accelerating.

The Prius has an extra trick, which is storing energy in the battery. Energy storage in the battery is less efficient than energy stored as kinetic energy, but in many cases the overall loss associated with battery storage is still less then the inefficincies from running the eninge at light loading. In these cases the Prius will run the engine harder and use the excess energy to charge the battery.

Regenerative braking is simply bonus energy that would otherwise be burned off as heat. The process of producing energy and then collecting it through regeneration is extreamly inefficient, but since you are dealing with waste energy, anything better than an efficiency of zero is a bonus.

Only because the Prius will use as much electric power as possible to pull down the SOC. Average mileage does not improve with all green bars. Instantaneous mileage improves because electrical energy is not counted in the mpg calculation. You already paid for that electricity in reduced mileage, or you will pay for it with reduced mileage later. Cycling the battery SOC up and down will reduce your average mileage. Instantaneous mileage is a red herring.

After all of the hand waving and smoke and mirrors, it still comes down to an efficient gasoline engine. The main reason that the Prius gets good mileage is that it uses a small, efficient gas engine. The HSD allows the Prius to funtion normally with this small low torque engine. Regenerative braking is a plus, but it's not the major factor. Shutting down the engine at stops can be done without the hybrid system.

Those are the simple facts.

Tom

 

Engines are most efficient when placed under load.

A moderate amount of acceleration is more efficient then a high amount of acceleration due to pumping losses, and that extra "squirt" of gas to make the engine spin up faster.

Lets talk about the main point of a car. Distance traveled over amount of resources expended.

Lets make a few assumptions: both drivers accelerate at the same speed (pulses to the same maximum speed. The de-acceleration rate is exactly half of the rate of acceleration.

Lets also say that the point which both drivers reaches the maximum speed is exactly at 2/6 point.

The most efficient driver would go into glide mode exactly at the 2/6 point and glide to the 6/6 point.

The less efficient driver would keep using the engine until the he reaches 3/6 or 4/6. Based on his rate of de-acceleration, he is forced to regeneratively brake from 5/6 to 6/6 because the speed is greater then what is required.

The extra time that the engine is on for driver #2, means more gas is used ALTHOUGH he may have recovered 20% of that extra gas used into electrical energy. This electrical energy, however, does not make up for the 80% of the extra gas that is lost to conversion losses.

This is still however better then the least-efficient driver:

The guy uses his engine all the way to the 11/12 point and then slams on the brakes, losing 100% of the "extra" energy to heat.


Hows this example?

 

You could be right about the Prius engine, I'd like to see your references. The ANL data provided was measured in the car (GEN1 Prius) on a dyno with a torque sensor between the engine and the THS. I know very little about ICEs in general. I've just read that the Atkinson cycle is generally more efficient than both the otto and diesel cycles, and know the diesel fans are always talking about 20% vs. 30% kind of differences for efficiencies. The ANL data seemed to match up pretty well with that, but could just be coincidence.

The real world data I had looked at seemed to go along with that too:

2008 Toyota Yaris:
Weight: 1040 kg
Cd: 0.29
Frontal Area: 2.433 m^2
Highway Mileage (MT): 36 mpg (2.78 gal/100 mi)

2008 Toyota Prius:
Weight: 1330 kg
Cd: 0.26
Front Area: 2.570 m^2
Highway Mileage (MT): 45 mpg (2.22 gal/100 mi)

From the Prius simulator page linked above, at 60mph the Prius requires 215.5Wh/mi to overcome drag/resistance, 47.6% of which is related to wind resistance. So we can break the resistance force into two components, a friction force that is proportional to weight, and an air resistance force that is proportional to CdA. We can calculate the power needed to overcome drag/resistance as 12.93 kW. Since both of our quantities are linear we shouldn't need to break that down to the forces or coefficients involved. The power requirement to overcome friction is 6.77kW, while the requirement to over come air resistance is 6.16 kW.

The Yaris is 21.8% lighter than the Prius, and has a 5.6% higher CdA. So friction power required would drop to 5.29kW, while air resistance power requirement goes up to 6.50kW. So total power required from the Yaris to cruise at 60mph should be 11.79kW. On the highway the Yaris uses 25.2% more fuel to do 8.8% less work. I believe that makes the Prius 37.3% more efficient than the Yaris. The result is interesting, as I believe this is essentially the same engine, one Otto Cycle, one Atkinson Cycle. Most all effects of the hybrid system should be minimal at highway speeds, except the "CVT". So you have to account for differences in rolling resistance, operating point and driveline efficiency, but its still a pretty impressive difference.

Examples that are more supportive of your argument are the Corolla at ~22% efficiency improvement, and the Hybrid Camry over the Carmy 4 cyl at about 10-12% on the highway.

Hmmm, that doesn't sounds right. If you look at the above numbers for power required to cruise at 30mph (4.5kW) and 60mph (12.9 kW) and then look at where they lie on the engines efficiency curve:



It seems pretty clear that the engine is much more efficient accelerating moderately than cruising, particularly at lower speeds.

This makes a lot of sense to me based on observation. I don't get to P&G much at all due to traffic, so I usually try to find the most efficient cruising speed I can which often ends up being around 43 mph. I would normally expect slower to be better, but it seems like you need to get enough resistance built up on the car to load the engine up into the higher efficiency zones.

Sounds reasonable.

I think you just demonstrated that gliding is more efficient than regen

I think its important to note again, that the less efficient the ICE is, the bigger the payoff for gliding vs. regen braking. The key to gliding is that it reduces ICE usage by about 25%, for a single accelerate, cruise, stop sort of event. 25% less ICE means 25% less fuel use. The amount of battery consumed by gliding is very small compared to the benefit of not running the ICE, which seems like considerably more savings than you can get from using regen.

For what its worth I can achieve 60-65mpg on my round trip commute in city traffic in a dense urban environment (light every 1/4-1/2 mile, so-so timing, some small hills/grades) by accelerating moderately in the deadband, cruising at ~43 mph with as little throttle as possible, bleeding a little speed on up hills and gaining a little on down hills, and then gliding as far as possible before a situation where I'll have to stop. Wish I could throw some pulse and glide in there!

It would be interesting to see if the efficiency was slightly improved if you could get the pedal just at the point where current = 0A. Hard to say if the little less ICE to make up the battery consumption of glide mode would be worth the extra drag of the generator. My guess is the difference would be little none, and that it is not a mode that most people could hit w/o a meter. I think thats one of the beauties of glide mode, anybody can get there w/o instrumentation.

Rob