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

[smcracraft]

I don't get it.

I've been told and read that it uses the battery at speeds
below (30 (or 32 or whatever)) but when I am very slowly
accelerating, well below these numbers, from a stop,
my real-time MPG meter is not showing anywhere near
99, 50, or even 30.

If it is running from the battery, why is the MPG so low?

Stuart

[john1701a]

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

I've been told and read that it uses the battery at speeds below (30 (or 32 or whatever))[/b]

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.

[Orf]

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

[qbee42]

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. 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

[TonyPSchaefer]

I think Stuart is correct in what he thinks and in what he's been told. Let's take a few steps back and think about this.

Yes, Stuart, the Prius can operate at speeds less than 40 MPH purely on battery power. We call that Stealthing.

However, sometimes the Internal Combustion Engine (ICE) will operate just to recharge the battery or to heat itself and the catalytic converter. Sometimes you see the arrows coming from the ICE and sometimes you don't. Just yesterday morning, I thought I was stealthing along a stretch of road but I wasn't. It turns out that the ICE was running.

Now I know what you might be thinking, "Tony, your daft. If the engine were running I'd hear it, feel it, know it."
Trust me, it's sneaky that way. The engine is amazingly silent and often runs extremely smoothly.

Assuming that's the case, if the engine is running at a set RPM (we'll just assume) then the slower we roll the lower our MPG will be because we are literally traveling fewer miles for each gallon. I have been creeping along in traffic watching the display show me MPG in the teens and there's nothing I can do about it because the engine's simply cold.

That's my thought, anyway.

[PriusRos]

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.

[qbee42]

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

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?
[/b]

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

[tochatihu]

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'.

[Three60guy]

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

I don't get it.

I've been told and read that it uses the battery at speeds
below (30 (or 32 or whatever)) but when I am very slowly
accelerating, well below these numbers, from a stop,
my real-time MPG meter is not showing anywhere near
99, 50, or even 30.

If it is running from the battery, why is the MPG so low?

Stuart
[/b]

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.

[daniel]

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

I don't get it.

I've been told and read that it uses the battery at speeds
below (30 (or 32 or whatever)) but when I am very slowly
accelerating, well below these numbers, from a stop,
my real-time MPG meter is not showing anywhere near
99, 50, or even 30.

If it is running from the battery, why is the MPG so low?

Stuart
[/b]

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.