When going slow, only on battery, why MPG low?

<div class='quotetop'>QUOTE(smcracraft @ Oct 19 2006, 10:07 PM) [snapback]335383[/snapback]</div>

By who? That isn't the slightest bit correct. 10kW is the threshold, which has absolutely nothing to do with speed. There's also a dependency on whether or not heat is needed.

If you accelerate slowly, you keep the draw from the battery-pack low enough to speed up on electricity alone.

If you accelerate quickly, the engine joins in. But as soon as your speed steadies, the engine will typically shut off.

Of course, it really doesn't matter... since overall MPG is what's important, not momentary spikes & lows.

 

I think Stuart might be getting mixed up with the max speed using EV mode.

 

Running on battery is not a way to get good fuel mileage. Sure, the mileage is great while you are drawing from the battery, but then the battery has to be recharged. The recharge/discharge cycle with the battery is less efficient then using the electricity directly, or better yet, getting the power right from the ICE. The battery is there to capture what would be wasted power, and to supplement the ICE when extra power is needed. The only reason to cruise around in EV mode is to avoid an engine start for a short trip, such as moving in a parking lot. Other than that, it's just fun and cool to drive in EV mode. B) If you want good fuel mileage, follow the techniques listed on this forum and let the Prius decide when to run the ICE - it does a pretty good job managing everything.

Tom

 

I think I understand what the OP is asking because I have wondered the same thing myself.

Let's say the engine is already warmed up and your battery is fully charged. If you have to stop at a light or come to a hill, accelerating to get up to speed always seems to take some fuel consumption. Why? Then, even when not accelerating, but just driving very slowly, the gas mileage will sometimes drop way down. Why would there be any fuel consumption at all during those times? I sometimes seem to get better gas mileage at, maybe 40mph, than at 15 or 25mph.

 

<div class='quotetop'>QUOTE(PriusRos @ Oct 20 2006, 03:22 PM) [snapback]335735[/snapback]</div>

As Tony says above, it all depends on whether the ICE is running. When the ICE is running, you're burning fuel, even if you are not moving. The Prius runs the ICE if and when it sees fit, and it will often run when you don't expect it, such as for cabin heat or heating the catalytic converter. It's very quiet at low speeds, so you might not even notice, but it's still burning gas. Zero speed while burning any gas is zero mpg. Moving, you'll get something better than zero, but not as good as at higher speeds.

Tom

 

When ICE is running at low vehicle speed, it is most often also operating at low rpm and power output, where it is not so efficient. This is the basis for the suggestion of 'moderate to brisk acceleration' up to (or just above) your target speed. Then lighten up on the accel pedal to get into a mode more like coasting. If you drift down several mph and then speed up again you are doing 'pulse and glide'.

 

<div class='quotetop'>QUOTE(smcracraft @ Oct 19 2006, 10:07 PM) [snapback]335383[/snapback]</div>

After reading the responses I would like to emphasize that the first 5 or 10 minutes of operation the Prius is warming up. During this period you will not get good fuel efficiencies. That is why everyone will tell you to plan your use of the Prius so you have more than short runs around town. Combining them so you have a relatively warmed up car will result in much better miles per gallon. So, if you are seeing low MPG in the first 5 or 10 minutes of your drive that is completely normal. In fact, if you live in cold climates you will see a drop of MPG during colder winter months than in the warm summer months. It is just the .....ahem.....cold facts. hehe

So combine the trip to the grocery store with the post office, gas pump and other honey do's and you will see higher numbers. Hope this helps put this all in perspective.

And Tony, I laughed when I saw you say "your daft". Haven't seen that one in awhile. (grin) Hope to see you at the Milwaukee Hybrid Group meeting Oct 28th 10am. I will be talking up my new ScanGauge II.

 

<div class='quotetop'>QUOTE(smcracraft @ Oct 19 2006, 08:07 PM) [snapback]335383[/snapback]</div>

The Prius sometimes will shut off the engine and run on pure battery power. But that is not its principal operating mode. And it can only do that for relatively short periods. Otherwise, the engine must run because the engine is the source of all its power, and the battery stores only a very small amount.

The Prius is a HYBRID gas-electric car. It was designed to use the gas engine together with the electric components in such a way as to minimize emissions and fuel usage. Therefore, most of the time the engine will be running AND the electrical system will be either recharging the battery or drawing power from it. Both together is the usual and principal mode of operation of the Prius, operating together in such a way as to be cleaner and more efficient than a conventional car.

Acceleration is most efficiently done using the engine, so while accelerating you will usually see low instantaneous mpg numbers. But as an earlier poster commented, what you really care about is overall gas usage.

The only time I get concerned is under the following relatively infrequent condition:

1. The car is warmed up, AND
2. I am driving at moderate speed, AND
3. I am not accelerating or driving uphill, AND
4. The battery has plenty of charge, say over 5 bars, AND
5. The instantaneous mpg is low.

That probably means it is running the engine harder than necessary and sending a lot of energy to an already well-charged battery. This is most common in cold weather when I have the heater on, but it sometimes also happens at random times. This is when I engage the EV switch to stop the unnecessary charging and draw down some of that excess charge. I then switch the EV off when the charge drops to 4 bars, OR I am going to accelerate or drive uphill.

It is a big mistake to assume that the Prius will always turn off the engine when you are driving slowly or accelerating gently. Too much depends on engine temperature, ambient temperature, battery temperature, battery state of charge, your demand for cabin heat, as well as vehicle speed, and demand for power.

Before I got my Prius I was told that it would always accelerate to around 30 mph on battery power before turning on the engine. My salesman was well-meaning, but understood almost nothing about the car.

 

<div class='quotetop'>QUOTE(daniel @ Oct 22 2006, 02:41 PM) [snapback]336476[/snapback]</div>

Good commentary! I have found the most critical of all the factors is Topography. Excellent mileage an be attained with all trips DOWNHILL. Anything else equals mixed results. If one can average 50mpg, calculated, be happy.

 

with my 03 prius, on flat land i find it really hard to cruise 50 or 60 km/h with high mpg (usually it shows between 30 and 45 mpg) but when I use cruise control mpgs 50-60 are easily obtainable! If the mpg is so low while cruising at 50 km/h (30 mph) and im giving charge to the battery... it better be a big charge because that mpg is nasty for even cruising!

heck even my previous car (7th gen civic manual) did WAY better when cruising for high mpg.

in the end, i'm impressed on what cruise control does to maintain good mileage, Ill be using it on the FLATS for sure!

 

<div class='quotetop'>QUOTE(philmcneal @ Oct 26 2006, 02:29 AM) [snapback]338467[/snapback]</div>

I have not had much success with P&G however, as you say the CC is excellent. I use it for all types of driving, except backed up freeway, then I switch to Brake. Brake can be used for slow driving with immediate slow-down benefits. This allows me stay somewhat close, discouraging "get-a-headers" plus the safety factor of quick slowdowns. With CC my mileage is always 50MPG plus. (4.7l/100KM) The CC will not work at under 25MPG, (40KPH) US model.

 

Not much to add...great posts in response!

I have noticed that sometimes in stealth, I am not getting 99.9 mpg...BUT usually that is when the ICE is still warming up, or I have just restarted and am driving after a short drive and shut down and restart.

Also, sometimes, when the ICE IS warmed up, I try to go into "glide" mode, but the car goes into electric (EV)..stealth...I can get it into glide, but it seems to go into stealth, when ICE is warmed up and SOC is near full bars....almost always I am going up and down some small inclines very slowly since this is characteristic of my neighborhood.

But then, if the car has been in EV mode (and I don't have the switch...the car seems to go very easily into EV and stay there for a stretch)...I have a steep downhill run, which regens what I lost in the EV only mode.

The car is a materpiece of engineering and seems to "know" what to do!

I don't try for glide as much as I used to...so far, my tanks of gas, all else being the similar, are around same mpg...except for last one where calculated was 4 mpg lower than the MFD...and I believe it might have to do with all the times I let the car stay on in Park while I ran in and out of a store...ICE must have come on, using gas, thereby lowering the calculated mpg.

terri

 

<div class='quotetop'>QUOTE(philmcneal @ Oct 25 2006, 11:29 PM) [snapback]338467[/snapback]</div>

One of the reasons I wanted the CAN-View (which, sadly, is not available for your '03) was just that: To see how much charge is going to the battery when the mpg is low at slow speed. And generally I find a correlation: when the mpg is lower than it should be, it's usually sending a respectable amount of charge to the battery. (Or else warming up.) When this is not the case, or the battery already has a high charge, that's when I use EV (which, sadly, is also not available for the '03).

The '04 is more efficient than the '03, but you get points for being an early adopter.

Andy: "B" mode is always less efficient. The only time you should use "B" mode is for long downhills when the battery would become fully-charged and the friction brakes would be over-used. "B" mode uses engine compression to slow the car, dissipating energy as heat through the engine, thus protecting the friction brakes.

But using the engine to dissipate energy while the battery still has room to absorb regenerative charge, is wasteful.