I have been reading for a month or so and have got some excellent ideas for driving my 07 prius. The discrepancy has been, whether or not to use electric to extend the glide phase of P and G or at other times. I started not usiing it because of the advice I read on thi site. My mileage was just staying above 50. More recently I have started using more electric with my mileage trending upward to 56.5 currently. Although my personal experience has indicated electric, I still am holding on to the advice I read on here. So am i doing something wrong? Any advice?

Cheers,
Matt
Redding, California

 

Could you be more precise?

In particular, what speed range and over what distance?

Better still, do you have a fixed, distance, segment fairly isolated from other traffic that you can drive in either mode?

What would be useful is to use your MFD to measure the mileage over that distance, on the same day and temperature, for repeated runs. In some of the runs, do 'protocol A' and the other runs 'protocol B'. Be sure to include the elapsed times for each protocol so we can calculate the average speed for each. The distance needs to be non-trivial, at least 2-3 miles:


We want to recreate a similar chart.



Thanks,
Bob Wilson

 

Sorry if I was vague. I was speaking in terms of a general driving technique. The 52.6mpg was over 461 miles. My new mpg figue is 56.5mpg over only 50 miles. The speed varies but the, I have not taken specific samples or kept data for my driving. I am refering to statemants regarding EV mode being bad for mileage as a general rule because of the regen required causing the ICE more to run thereby decreasing mpg. I was hoping that you might give me your opinion of this subject without me having to do a fullon scientific experiment. Although I very much appreciate experiments such as you reccomended, I don't think I have the ability of such precision driving required to do such experimentation.
Thank you very much,
Matt

 

Hi Matt,

Engineering tends to be a little '**** retentive' about the details so PLEASE don't be put off by my questions. But let me 'answer' the question I think you're asking.

EV mode, especially below 42 is great! In fact, it is the secret to Prius high mileage because when the engine runs, it runs in a higher power, high efficiency mode and efficiently stores the extra energy in the battery. Then in EV mode, this stored energy keeps the car rolling while the engine is off. This is not "regen", which happens when stopping the car, but energy "generation" while the car is rolling.

Let's do a simple model that assumes at 35 mph, the car needs only 10 hp to roll down the road say 10 miles:

1. Running the ICE at 10 hp over the whole 10 miles, it burns .5 gallons, 20 MPG.
2. Running the ICE at 20 hp for 5 miles, it burns .25 gallons but the extra 10 hp is saved in the battery for the rest of the 10 miles, 40 MPG.

Notice in both cases, the car speed stays at the same at 35 mph and the battery and motors are used to sustain the steady speed. Traffic tends to deal with folks holding a steady speed better than folks whose speed changes all of the time. Better still, cruise control can hold that speed you can spend more time with your eyes looking outside the cabin instead of the instruments.

Would you like some more details, say the actual numbers for our Prius?

Please don't sell yourself short. The trick is consistency so you can run the same route and speeds using two different protocols. These distances don't have to be huge, a couple of miles works great, and you've already got a nice multi-function display built into the car. The biggest thing is to find the time.

If you still have trouble working out the route, access roads that parallel an interstate are often great testing areas. They tend to be untraveled, especially late at night or early on weekends. They are often straight with few if any stops.

I love to share my test results but rejoice when others repeat the experiments. The reason is no one ever gets everything exactly the same as the original experimenter. Often there will be subtle differences that will reveal things no one has noticed before. This is how we learn.

So please, please, please, consider how you might work up an experiment. If you have some questions about protocol, ask and I and others will be quite willing to offer suggestions. This really is how we learn what works. The confidence that it gives more than makes up for the time invested.

Bob Wilson

 

I occasionally use EV mode (via pedal feathering -- no EV switch) to briefly extend a glide. For example, if I approach an expected stop (e.g., red light) that's too close to justify a short pulse of a few seconds, but too far away to glide the whole way, I'll apply a burst of EV mode. I also use EV to maintain speed for brief periods in moderate to heavy traffic if I think gliding could impede traffic. I try always to stay safe and avoid inciting road rage.

Whether to extend a glide or as the sole source of propulsion for short distances (e.g., parking lots), I try to avoid depleting the battery SOC too much. I don't want to drag it down so much the ICE lights just to charge it during subsequent segments.

 

Hi JimboK. Gald to see you on board and active. Actually, and counter-intuitive as it may seem, it is best to use "EV" as much as possible near the end of a trip. This will increase you mpg substantially for 2 reasons. One is you will not be using the ICE the last minute or two of your trip. Secondly, when you power up the next time the warm-up time can be used to charge the battery. Since the ICE will run until optimal temp is reached anyway, you can use the same energy to charge the battery. If you keep the SOC high the ICE will run on startup anyway and the additional energy will be dissipated off through MG2, I believe (since the battery is near top SOC).

 

Thanks, Alden. I've considered that, but my starts are usually with a warm ICE; I use an engine block heater. With the warm mornings we've had recently, I can get into an ICE-off glide within just a couple of minutes of startup. And early in my commute route, I encounter the moderate traffic I described where I often add a little EV mode to sustain my speed.

Now, as for which technique gets better MPG, I can't say. You may be right. I do know that when I've depleted the SOC the day before, ICE-off glides and EV mode are tougher to achieve and sustain early in the morning commute.

 

I believe we also need .5 gallons at 20 hp for 5 miles.
(twice of hp) X (half of distance) = same fuel usage
The results is the same 20 MPG for that 10 miles.

Ken@Japan

 

Remember that when you start the Prius it will be using the mostly the battery until the cat warm-up occurs. If you demand power from the HSD while your SOC is low you'll use more gas since the ICE has to go at a higher RPM which is less efficient. Also the ICE has to run to bring up your SOC after cat warm-up. You will always use gas to bring-up/maintain SOC. Best thing to do, stay out of the battery, except as Jim says, for short little uses.

Wayne

 

Thank you Ken, I was hoping someone would ask about the specific values. In my example, I exaggerated the specific fuel consumption change to emphasize the differences that happen at lower power settings:The actual efficiency data, both mine and the Toyota paper SAE 2004-01-0064, shows that this efficiency relationship holds true although the actual changes are more modest, it is not a linear relationship:


At low power settings and rpms, the 1,200-1,600, we are seeing higher specific fuel consumption at the lower power levels. There is merit in looking at this rpm range at lower speeds by looking at the power needed (even without the 'background electrical power') to move the vehicle at low speeds, like 35 mph:


My 'bubble chart' combines specific fuel consumption (aka., ICE efficiency) with speed and ICE power, even if it seems a little retro:


But as the speed range increases, we find the power needed moves higher to a point where the minimum rpm is 1,600 and above. Then we are in a linear range from 1,600 through about ~2,600-2,700 rpm and then it really falls off above 3,100 rpm.

So we can actually see this cycling in this graph, from a closed loop, test track that has a little over a 10 ft, 3m, altitude change:


This analysis is specific to speeds below 42 mph, ~67 k/hr, when the ICE can cycle ON and OFF.

Your analysis is quite correct when we get into linear ICE efficiency power ranges, 1,600-2,600 rpm. My data suggests this power / ICE rpm range is above the 42 mph threshold where the ICE must always run. To the best of my knowledge, all we can accomplish in this range is to seek "energy recirculate" mode (aka, lower ICE rpms) and avoid ICE rpms above 2,600 rpm and especially above 3,100 rpm. My understanding is no one advocates P&G at speeds above 42 mph, which I think is wise.

One of the missing tests, the one that would be terribly informative, would be a P&G benchmark on a given route used to establish the start-to-end speed. Then repeat the same benchmark holding a constant speed that gives the same start-to-end speed.

This benchmark needs to use a non-trivial, flat distance of about 3 miles for each run and multiple samples averaged together. BUT the start-to-end speeds must be kept constant (aka., the same elapsed time interval) changing only the driving method. This type of comparative driving test in a Prius, a head-to-heat comparison of P&G versus constant speed, would be terribly informative.

When I get my micro-processor controlled accelerator working, I'll be able to automate both driving styles around a closed loop, a shopping center parking lot. Better still, I'll be able to rigorous document exactly the P&G protocol versus steady state. This will finally give the engineering data needed to complete this part of the performance model.

Bob Wilson