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 miles per hour 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 miles per hour and as late as 33 miles per hour. What is the difference in terms of impact on mpg when the ICE engages at these slower speeds rather than at 42 miles per hour?

Even when I keep my speed below 40 miles per hour 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 miles per hour?

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

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

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!

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.

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

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

Below 42 miles per hour, 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 miles per hour, the Prius will cycle between electric vehicle and ICE modes:


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:


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

S1: up to 40C

ICE runs for warming up at no less than 1200 rpm.

S2: 41-70C

After exceeding 56 km/hr (~35 miles per hour), coasting allows drop to 900 rpm.

S3: 71C and above

Coasting after reaching 56 km/hr (~35 miles per hour) allows ICE shutdown below 42 miles per hour.
Allows stop transition to S4

S4: 71C, down to 60C and above

Stop allows ICE shutdown.
Coasting below 42 miles per hour allows ICE shutdown

The following chart shows recent data from my NHW11:


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

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
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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 miles per hour, 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 miles per hour or above 45 miles per hour consistently. Hovering between 39 and 45 miles per hour 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

You've got the big picture right but performance below 42 miles per hour and especally under 39 miles per hour 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

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.

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

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.

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

Operation Iraqi Oil Freedom:

Automatic, stock, project car.

My other 1500 cc car:

Automatic, stock, backup car.
Free speech, dialog and knowledge thrives without the poison of SPAM.