Wednesday, March 23, 2011

The Scangauge has arrived!

Awesome mpg in a regular commute (no, I didn't hold up traffic to do this...), 71.8 mpg over 12.1 miles in slow, one-lane traffic over a half hour or so with Scangauge. Usually only get 60-65 mpg on this trip without extra instrumentation. You also see in the background my current mpg for the entire tank on the Prius' display.

The Scangauge, as many Prius regulars know, is a nifty little device you hook up to your OBDII port, the port usually found under the steering column above the gas pedal in many cars that mechanics will use to communicate with a car's computer. For non-Prius regulars, it basically is a little screen that displays various real-time operating conditions in whatever car you decide to use it, including instantaneous miles per gallon, as well as providing you with things like average speed, top speed, and overall mileage for the current trip, day, previous day, and tank.

I was pretty reticent to get it, mostly because of its $160 pricetag, and for someone already getting near 60 mpg, I couldn't imagine how it would help improve my fuel economy enough to pay for itself. It's not even a device that gives you better mileage, it's just a display of various info that you have to put into action.

Luckily I had saved up a boatload of gift certificates, and was getting increasingly curious about the inner workings of the Prius, so I sprang for it, and just got it today.

I headed out for a test run, hooked up the device, and set a couple parameters out of the dozens offered that seemed most useful for my non-highway driving or which piqued my interest for odd reasons:

-Engine revolutions per minute - that's right, a Prius does not have a standard tachometer, ugh (RPM)
-Engine coolant temperature (FWT)
-Traction battery current (BTA - added as XGauge)
-Percentage of gas pedal depression (Gps - added as XGauge)
-Average trip mpg (AVG)

You may wonder why I had average trip mpg on when the Prius can do that itself. Firstly, the Prius typically overestimates by 5% or so. Secondly, to test that very issue out, I haven't been resetting it for each trip on this tank of gas, but rather letting the overall mpg build over several hundred miles to compare the end result with the hand-calculated mileage I'll get when I fill up next week.

Later on in the day I drove to work, and discovered a couple things.

Revolutions per minute

The first change to my driving was rate of acceleration. From a stop I had been accelerating what felt like a nice, fuel-efficient conservative pace, but which was in fact far too slow and laborious. When there was a clear road, I would accelerate to about 15 mph using electric only per a comment of Wayne Gerdes at, then engage the gas engine, but had been accelerating from that point with RPM a couple hundred too low. Today I tried accelerating to speed between 1400-1600 rpm depending on the terrain, and trying to keep it right around 1500 most of the time, and as you saw above, had a wonderful series of five-minute bars with gliding.

Here is an older mileage pic over basically the same route. Compare to above.

Just visually it's obvious that accelerating around 1500 rpm is much more efficient for pulse and glide than what I was doing before. For non-instrumented fellow Prius owners, note that typically this acceleration had the instant mpg showing about 85%-100% of the current mph value. Occasionally I was deadbanding (no arrows to or from battery on energy screen) during these pulses, but when possible I would ease up to get some flow into the battery.

Coolant temperature

The Prius has several stages of operation, explained in this famous posting by Hobbit. To sum up for non-link readers, you can measure which stage the engine is in by monitoring the coolant temperature as it heats up. One of the key temperatures to reach is 163.4°F, or 158°F, depending what you read, and at that moment, assuming a couple other parameters are in place, you have use of full hybrid operation, including the essential option of turning the engine off for a glide, or using electric vehicle (EV) mode if desired.

One of the huge annoyances, especially in the winter, is running into a big downhill in the first few minutes of driving. If the engine hasn't warmed up to 158°F yet, you're going to burn gas while going downhill in your Prius, whether you like it or not. The magic of having the coolant temps right in front of you is being able to approximate how soon you'll arrive at full hybrid mode, and perhaps use the engine a bit more if needed for temp maintenance. You then know the exact second when you can switch to its most efficient state of operation, the glide. Today my warmup phase was a good percentage more efficient than in the past with this knowledge, and I also managed, on my required short trips between clients of about 3 miles, 3 miles, and 4 miles, to get 60 mpg, 50 mpg, and 50 mpg, respectively. How many 3-mile trips do you usually get 60 mpg on?!? Only possible with the perfect timing afforded by the Scangauge.

Traction battery current

There have been plenty of debates on the use of EV, and its potential deleterious effects on mileage. For non-Prius owners, you might be surprised to learn that EV mode is actually rarely desirable, since much of the energy from the battery comes from the gas engine - you're burning gas and converting energy back and forth to charge the battery, so technically it's more efficient to avoid conversion losses and just go for straight pulse or straight glide. The thing is, from observations today...

Sitting in park: 0.9 to 1.0 amps
Use of headlights: +1.2 amps to current current (muwaha)
Neutral glide: 1-2 amps
Foot off the pedal creep from zero mph: 3 amps
Glide in drive: 5-6 amps
Warp stealth with little accelerator demand: 10 amps
Accelerating lightly from a stop = 20-30 amps
Accelerating after first turning on the car (exclusive battery use while engine warms up): 50 amps or more(!!)

So, back to the statements that EV will harm fuel a couple situations where I had to go 20-25 mph and wanted to avoid the inefficient ICE at that point, I could just or almost maintain speed with 7-10 amps. EV with a light touch on the pedal is an almost inconsequential increase in battery use, in fact, even less than warp stealth use. It can be used even somewhat extensively over a small distance with less depletion on the battery than accelerating from 0-15 mph in EV, for example. Even going to 15 amps, knowing that pulsing can provide a similar value back into the battery, doesn't seem so egregious anymore.

I will add a caveat, and this is what others had warned about. Even a slight increase in gas pedal depression will bring you right up to 25+ amps, and this is what should likely be avoided for long periods. Yet another little detail impossible to perceive without the Scangauge or other instrumentation.

For preliminary conclusions as to its effectiveness, I'd say the Scangauge won my respect right off the bat, and I easily got 10% better mileage than the already high mileage I was getting. While it might take me a year or two to recoup the money in improved gas mileage for my high mileage habits, someone driving 12,000 miles a year in a 20 mpg vehicle that jumps to 25 mpg will recoup the money in a couple months, and continue to easily save hundreds of dollars a year after that.

1 comment:

  1. Hi Mike, good article. Is there an article or document on using the ScanGauge with a Prius. My Prius is a Gen 3, year 2011. If not, would you consider writing one? What are the key "must know" facts to get the most help from the ScanGauge? Thanks.