42 mph Threshold & Block Heaters
 

I have a couple of questions for the technical experts on the forum but feel free to provide links to technical or service manuals that may answer my questions.

I have been doing a lot of reading here trying to determine how to maximize my mpg with my ’06 Prius. I have made several modifications to my driving habits as a result and I have increased my mpg by 2-3 mpg (over 5 tanks). My first question has to do with the 42 mph threshold mentioned in many of the threads. Depending on my traffic situation, I will get orange arrows from the ICE to the drive train as early as 9 mph and as late as 33 mph. What is the difference in terms of impact on mpg when the ICE engages at these slower speeds rather than at 42 mph?

Even when I keep my speed below 40 mph for an entire trip, when I come to a stop I still feel the ICE shut down after a couple of seconds. Again, how does this differ from the actions of the ICE at and above 42 mph?

As a corollary, what happens to the ICE after extended periods above 42 mph (e.g. highway driving) and then dropping below 42 mph? I don’t feel any noticeable change in the running of the engine when I drop below 42 mph.

My second question has to do with the true value of a block heater in terms of increased mpg. How long does it take (in terms of minutes or miles) for the Prius to reach proper operating temperature? It seems to me the Prius reaches ideal operating temp within 2-5 miles of use. The reason I ask is, my initial trip in the morning covers about 30 miles and 30-35 minutes. After that, I am not in a position to utilize a block heater. Hence the only time the block heater would be used is for the first trip of the day and never after that. In addition, the Prius is in a garage overnight, where in the dead of winter (late December through early February) the outdoor low temperature rarely gets below 10 degrees (above zero). I am really trying to determine if the addition of a block heater (and its corresponding cost) will have sufficient impact (given its limited use) to pay for itself in enhanced mpg given the impact will only occur during the typical 2-5 mile warm up period at the beginning of the day.

Thanks for any feedback

UTpiper

Re: 42 mph Threshold & Block Heaters
 

With respect to the 42mph thing, you have to keep in mind that battery capacity and EV performance are both limited. It sounds like you maybe making a very common mistake amongst the HSD-driving public, which is to drive on electric power alone at every opportunity. Is that correct? If that's the case then you should be able to gain some measurable fuel efficiency by learning to use EV only when apropriate.

The reason this is bad is the fact that heavy travelling under electric-only uses more electrical energy than can be obtained from regenerative braking alone. That means that you might go a mile or three depending on conditions and then the ICE has to kick-in to replenish the charge. This is not a good thing because of the conversion losses incurred when energy takes that long, complicated route from the engine to the wheels. Some energy is lost to heat at every step along the way. The solution is to go ahead and use the ICE while accelerating and then allow it to shut off while coasting and drawing little or no current from the pack. As you get some charge back from regenerative braking you can feel free to use that electrical energy as needed without paying as much of a penalty, since it would have been wasted to friction, anyway. Just bear in mind that braking, even if regenerative, will still hurt efficiency. Momentum should be maintained whenever possible, and energy regenerated when you have no choice.

Seem counterintuitive? Many will agree. But you will find times when EV mode is the way to go, such as while waiting for a short traffic jam to open up or while cruising a short distance through the parking lot to your space of choice. Those are low-speed situations where the ICE uses too much fuel to justify spinning it up. But they also involve a limited duration so as not to draw too much power from the battery.

Driving in this manner should also maximize the health of your battery, although it seems to me that Toyota wouldn't let you drive in EV for longer if it actually hurt anything.

I hope this helps!

Re: 42 mph Threshold & Block Heaters
 

Of course, the energy replenishment problem is the primary reason why am affordable plug-in option for the Prius would be such an attractive possibility.

Re: 42 mph Threshold & Block Heaters
 

On your first question:
I think Tim offered a great answer that is hard to improve on. Needless to say, use that EV wisely. It is "expensive" energy and should be used strategically to maximize FE.

On the second question:
The block heater has plenty of value, particularly when temperatures get pretty low. In addressing your specific temperature reference of 10F I find the use of block heater essential even when the cars are inside the garage. For instance, if I do not use a block heater in a 10F morning my MPG at the end of a 15 minute trip are 15-20mpg worse. The lower the temperatures the larger the fuel economy loss is and the greater the value of a block heater. Of course in my case you have to factor in the caracteristics of my route and those are likely to be different for you.
By the way, my Prius is an 07 and I did order it with a block heater. This is pretty common for canadians to do but not so common for our US friends who must at times resort to ordering the part from Toyota Canada or other sources.

Cheers;

MSantos

Re: 42 mph Threshold & Block Heaters
 

Hi,
Excellent questions and in an area some of us in Prius_Technical_Stuff have been investigating. First let's address the 42 mph speed threshold for a warmed-up Prius.

Below 42 mph, the Prius will use electric mode with the ICE off if there is enough battery charge and the requested speed and slope are within limits. Otherwise, the Prius will use the ICE and if needed, put more energy in the battery. So below 42 mph, the Prius will cycle between electric vehicle and ICE modes:
http://hiwaay.net/~bzwilson/prius/pri_test_30mph.jpg

The key is the Prius needs to be warmed up and there in lies the rub. Let me share Ken's description of the NHW20 and some NHW11 warm-up stages:
Here is an example showing this in operation:
http://hiwaay.net/%7Ebzwilson/prius/pri_temp_NHW20.jpg

In the case of my NHW11 03 Prius, the rules are different:

S1: up to 40CS2: 41-70CS3: 71C and aboveS4: 71C, down to 60C and aboveThe following chart shows recent data from my NHW11:
http://hiwaay.net/%7Ebzwilson/prius/pri_temp_NHW11.jpg

Some of us in Prius_Technical_Stuff are looking at ICE thermister circuits that will allow NHW11s and possibly some NHW20s to achieve S4 operation quickly and avoid falling below the S4 mode in slow-speed, cold weather operation. However, it is a work in progress.

http://hiwaay.net/~bzwilson/prius/pri_temp.html

BTW, one of the other members is doing some experiments with pre-heating the intake air. The theory is this accelerates ICE warm-up and the less-dense air reduces pumping losses. It is an experiment in progress.

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

Thanks for the excellent and detailed posts. As I am not a hypermiler (and will likely never be due to my driving requirements) a good deal of this is over my head. However, it is quite helpful in allowing me the opportunity to see what impacts my driving "style" may have on my overall FE. Basically, what the data (and your responses) seem to indicate is that once the ICE has been engaged, either above or below 42 mph, the impact to my fuel economy is the same. The best case scenario I can achieve at that point (assuming I can’t simply glide to slower speeds) is to maintain a yellow arrow to the battery as often and as long as possible. It also seems to indicate when the ICE is engaged my best FE will occur when I remain below 40 mph or above 45 mph consistently. Hovering between 39 and 45 mph means I will run the risk of turning the ICE on and off frequently, thereby reducing my overall FE.

Also, as I reviewed the link above, it was interesting to note the key temperature thresholds of 40C (100F) and 70C (160F) where the ICE may shut down based on the demand put on the ICE. Given these hard wired thresholds, it would be nice to have a temperature gauge as part of the standard equipment in a Prius. It would certainly allow the driver more specific data in attempting to manage their FE numbers.

Thanks again for everyone’s responses.

UTpiper

Re: 42 mph Threshold & Block Heaters
 

You've got the big picture right but performance below 42 mph and especally under 39 mph has some subtle, unique to Prius/TSD/HSD drive operation:

• electric vehicle EV mode - when the ICE is off and the stored battery energy is sustaining speed; gliding; or regenerative braking. No fuel is being burned, a GOOD THING!
• ICE powered mode - when the ICE is on providing motive power AND recharging the battery. One of the subtle aspects is the additional load on the ICE moves the performance into a relatively higher engine efficiency band (aka., you get nearly the maximum energy per unit of fuel burned!) My measurements indicate that round-trip battery efficiency is ~94%, a very good thing!
• automatic transition between EV and ICE modes - the Prius will transition between these states automaticly. The 'clever' driver learns how to adjust their driving style to maximize the "EV" and minimize transitions. <GRINS>

But as far as the big picture is concerned, you've got the major elements. The rest is just practice and getting used to 22nd century, automotive technology. <GRINS>

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

When the temp is in the 30s the low coolant temp light will turn off in 1.5-2 miles if I forget to plug the block heaater in. with the heater plugged in it goes off within a block and a half. Which makes a difference in the first to 5 minute bars on the consumption screen. Instead of under 25 then just over 25 with the heater it is over 25 then almost 50.

Mike

Re: 42 mph Threshold & Block Heaters
 

Bob, Have you discussed that estimate in a thread here or anywhere else? If not it might be worth a discussion. That figure sounds a bit high to me but then again I haven't sat down and tried to do it on paper, myself. I suppose it also depends on if you are strictly talking about the battery (energy extracted from the battery divided by energy sent to it) or if you are talking about the full trip from engine to generator to battery to motor and finally back to mechanical via whichever MG drives the wheels.

My gut feeling is that 94% might be close to reasonable for the electrical pathways alone but probably not so much for the whole "round trip" from engine to driving the car. I'd be interested in your assumptions and the methodology used to get to that number.

Re: 42 mph Threshold & Block Heaters
 

Hi,
It is consistent with postings in the Prius Technical Stuff group. I've been able to replicate the data.

It is the sum of the discharging energy minus the sum of the charging energy:

Time Battery W
. . .
1577.5 2,853.3
1578.5 3,129.9
1579.6 4,253.8
1580.6 2,216.8
1581.7 1,046.2
1582.8 -5,091.8
1583.8 -6,063.2
1584.9 -5,953.5
1585.9 -5,674.2
1587.0 -5,627.8
. . .

The battery voltage and current are reported by the battery ECU to a Graham miniscanner and recorded to a text file. The data is recorded into an excel spreadsheet and the battery power, current times amps, used to calculate the discharge (+) or charge (-) power. The test distance was 5 miles at a fixed speed over a closed loop using the same entrance and exit.

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

Thanks for the explanation, that does make sense. I'm going to have to look into one of these Graham miniscanners if they aren't obscenely expensive. Or maybe CAN view would work. But what you have me thinking is that we might be able to estimate the round-trip efficiency for a regen--->battery--->propulsion cycle (kinetic to kinetic) through a controlled test. The crude methodology might be to start at a known speed (and kinetic energy) below 41mph and a known battery state of charge, allow the vehicle to coast down to a lower speed (and KE) threshold while noting the new battery SoC, then accelerate under EV and note the speed attained when the battery reaches its starting SoC. It would be desireable (more like crucial) to estimate energy losses to things like rolling resistance and aero drag in order to isolate internal system efficiency. Even better to record battery voltage and current throughout the test series to help give an idea of the magnitude of losses in either direction.

I'm just rattling this off without doing a proper experimental design so no doubt there are holes to be filled.

Re: 42 mph Threshold & Block Heaters
 

Actually I'm impressed and pleased. One of the harder things in life is to find collaborators who understand technology and how to approach these kind of problems. When I see someone with "the lights are on and someone is home," I want to shout out, "Welcome fellow engineer!"

You might want to wander over to the Yahoogroup, "Prius_Technical_Stuff" and take a look at the files and search through some of the message archives. This will 'open your eyes' and help you see some of the really interesting experimental stuff that is going on. In particualar, take a peek at what "Hobbit" has been doing.

We'll still share results and summaries here. What we learn is important and can help less technically inclined folks 'get a clue.' But the folks in that group 'play with fire' and sometimes 'the smoke escapes.' We're a friendly group of geeks.

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

Sounds like a good place to hang out and talk shop, so to speak. I'll stop over to see what you guys are up to!

Re: 42 mph Threshold & Block Heaters
 

Never good when you let the smoke out of electronics.

Done it a few times, and they never seem to work the same again.

Re: 42 mph Threshold & Block Heaters
 

The genie who makes you wish you hadn't done that. <grins>

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

They aren't obscenely expensive (IMHO), but they are not available for the third generation (2004+) Prius.

Re: 42 mph Threshold & Block Heaters
 

To simplify that first graph Bob posted, he's driving 30mph consistently - no change in speed or accelleration - yet the gas engine is turning on, and off, over and over again. While it's on it's pushing the car and any extra energy goes to the battery. Then the Prius computer shuts off the gas and drives on electric, then back to gas again. All this happens without you doing anything special - you just drive the car.

No - yellow arrow bad. Green arrow good. If you see a red arrow, you don't want a yellow one coming from the battery. Below about 15mph though, it is better to have just a yellow arrow from the battery and no red arrow (accellerating on electric-only because it's more efficient at very low speeds).

More or less. If I Pulse and Glide below 42mph I get my best mileage. If I'm going to be above 42 I do better at 60-85mph mileage-wise. But the more important factor in all this is maintaining a consistent speed whenever you can - accellerate to the speed you want and maintain it. So for example, accellerating constantly until you see a red light, then decelerating to the light, means at no point were you going a constant speed. That's going to get you bad mileage. That's also how nearly everyone in the US drives.

I originally thought this was an issue too - but it isn't. It turns out starting the gas engine on the Prius is really, really efficient. In fact, if you were travelling at 45 the Prius might be charging the battery, and at 39 it might turn the gas engine off because the battery has some extra charge - you might do very well mileage-wise driving that way.

One last note on temperature. When it's 70F or above here I find pulsing and gliding on local streets gets me fantastic mileage. When it's 40F here (lately) I actually do better on the highway because the gas engine won't stop running to keep the car warm. At least on the highway there are no stop lights and I can drive a mostly consistent speed. So to simplify, if it's warm out, driving under 42mph will probably get you your best results. If it's cold out, 65mph is your ticket.

Re: 42 mph Threshold & Block Heaters
 

OK, now I'm confused. I thought my better option (not necessarily best option) was in those cases where the ICE is running (and gliding is not an option), having a yellow arrow to the battery would produce better FE?

The idea being the the excess energy produced by the ICE is partially going back to re-charge the battery (as opposed to simply being wasted as excess consumption). In practical terms, when my ICE is running (sending an orange arrow to the wheels) I will either have a yellow arrow running from the battery to the EM or a yellow arrow to the battery. My original take from the thread is that the yellow to the battery is the preferred option in this scenario.

That's actually good news as about half to 2/3 of my driving is highway based and its been under 30F here for several weeks now.:thumbs_up I've have noticed I get much improved FE when I am running at highway speeds and P&G from 80-65 (which I have the option to do quite frequently as I am the only one in the right hand lane). It also helps due to the number of hills I have to traverse each day during my commute. I can anticipate my glides up the inclines and then use the following decline to get my speed up again without over working the ICE.

UTpiper
Likely never to be a hypermiler but desperately trying to keep my average FE above 50 mpg during the winter.

Re: 42 mph Threshold & Block Heaters
 

Sort of - I think you just have the arrow colors confused but get it just fine. If you accellerate gently a red arrow comes from the engine to the wheels, and a green arrow comes from the EM to the battery. If you see a yellow arrow between the EM and the battery, it's going to be pointing the other way - at the EM - and that means you're harming your efficiency.

Pulsing and Gliding is very effective under 42mph because you can turn the gas engine off. I'm not sure that Pulsing and Gliding is effective at high speeds, because the gas engine is running constantly. I have the most luck above 60mph by accellerating gradually to my target speed (let's say 75mph) and maintaining it. Others with more knowledge of the Prius' workings can probably better comment on the efficiency of P&G at high speeds though.

Re: 42 mph Threshold & Block Heaters
 

I don't have that much more knowledge about high-speed modes and it is something still under discussion. There are some who claim to regularly achieve a high-speed glide and that it can be used to improve high-speed MPG. I just don't do enough high-speed driving to tell.

My data indicates there are two high-speed glides:

A) partial ICE power - effectively no wheel torque, ICE running
B) battery-assisted, ICE - no wheel torque but some MG1 current to keep the ICE turning over

Although "B" has no fuel burn, it is followed by ICE powered recharging of the battery. The "A", partial ICE power mode, does not have to put a charge on the battery. What I don't have is a total energy model that indicates whether mode "A" or "B" has a lower fuel consumption over a long distance trip.

The only key I've found is avoiding running the ICE above 2,500 rpm and especially avoiding 3,500 and above. My data shows a fall-off in specific fuel consumption as the ICE rpm approaches and exceeds 3,500 rpm. This is not a function of 'glide' or 'pulse' but what energy is being requested for the current load. IMHO, the best strategy would be a cruise control that would also back-off the speed setting to keep the ICE below some given rpm:The data indicates this would provide a directly measurable cruise improvement. Notice that speed has nothing to do with it, ICE rpm only.

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

Good point Bob - the built-in cruise control on the Prius is certainly guilty of exceeding 2500rpm on hills on the highway - man that's messy.

I simply don't use Cruise Control because I'm always driving rolling hills, and instead try to maintain my foot position on the Accellerator, causing me to decelerate going uphill and accellerate downhill. At low points in the drive I might be doing 75mph, and at the crests of hills I might be doing 60mph, but I can hear the gas engine rev at a relatively constant, reasonable rate the entire time - certainly below 3500rpm, and I would guess around 2000-2500rpm.

Re: 42 mph Threshold & Block Heaters
 

Excellent! An engineer, I prefer machines handle these tasks so for me, a smarter cruise control makes sense. That way I can work on my next designs.

Bob Wilson

Re: 42 mph Threshold & Block Heaters
 

I'm wondering if the color differences we are encountering is do to the fact that I have my screen set to the "orange" color scheme rather than the default "green" color set up. I will have to go out and test this theory. Thanks for the feedback. It looks like I'm heading in the right direction.

UTpiper