Hymotion fuel economy updates

Yesterday, I accomplished something new with my Hymotion system.

I made my usual 12-mile round trip to work and back. I got home at about 3:15 pm and proceeded to charge my battery back up to full, which took 2.9 kWh in about 2 and a half hours of charging. Then, I made another trip in the evening, which included over 20 miles of highway travel with the cruise set at 53 mph. Both trips involved going down my big hill and back up. I made no particular effort to avoid the CARB warm-up or to stay in EV mode. In fact, I didn't go slowly up the hill. I just floored it and went up the hill with the gas engine guzzling away. By coincidence, my Hymotion battery didn't run out of juice until I was almost home. (I had another 30 feet to go!)

I covered a total of 40 miles and averaged 95 mpg! That's actually very good, considering the hill.

I used 1.6 full battery charges (7.7 kWh) and 0.44 gallons of gas.

This is approximately equivalent to going 27 miles on electricity alone (at about 3.5 mi/kWh) and 13 miles on gas alone (at about 30 mpg). The gas fuel economy is low because a large fraction of it was used for going up the hill.

Another even MORE approximate calculation tells me that between 55 and 60 percent of my fuel use for the day was electricity. The milage numbers above make it look more like 2/3 electricity, but you have to remember that the gasoline miles were more energy-intensive and should be weighted more heavily.

Arthur

 

170-220 mpg? That's great!

Figuring out the best strategies to optimize mileage is pretty complicated, but once you've got them figured out, it becomes an easy routine.

I can play the "stay in EV mode" game and get virtually infinite mpg. Or, I can drive more like you do but still use EV mode on about half of my uphill stretch (10-20 mph, depending on grade). That way, my battery lasts longer and I can still get well over 100 mpg. It's a good compromise and it saves me a lot of time.

I had something weird happen with my ScanGauge last week. I was driving on the highway with the cruise set at my usual 53 mph. At times, the instantaneous mpg reading was 9999, but the "current" trip reading kept going up. By the time I got off the highway, it had gone up to 280 mpg. That can't possibly be real. Later, my average data for the day seemed to look normal. I don't know what that was about.

Arthur

 

Yesterday, I tried a new experiment to determine the range (in EV mode) of my Hymotion battery on flat terrain, in a 30-mph speed zone.

In the past, I have reported that the range for my commute (including up and down a big hill) was about 14.8 miles on an average full battery charge of 4.75 kWh (as measured with a Kill-A-Watt meter. That corresponds to an EV mode fuel economy of about 3.1 miles/kWh.

So, last night, I drove into town and drove up and down the main street for almost an hour. I was in EV mode the whole time. The result was that I travelled about 23.2 miles on 4.72 kWh (as shown on my Kill-A-Watt meter this morning, after charging overnight). That is a whopping 4.9 miles/kWh!!

Needless to say, I was pleasantly surprised by the result. Granted, I never went faster than about 31 mph, but I did stop at plenty of stoplights and stop signs. So, it seems like a fairly "real-world" result.

Next, I think I'll try a test to determine my effective EV mode range at highway speeds. I plan to drive with the cruise set at 55 mph until my battery runs out. Then, I'll check my ScanGauge to see how much gas I've used. Then, I'll try driving with the battery off to see how many miles I can go on that much gas at the same speed. I figure that, if I subtract the two distances, I will have an "effective highway range" that I can compare with my city range of 23.2 miles.

Arthur

 

Last night, I did my highway test. I set the cruise at 55 mph and turned on the Hymotion switch. I went 24.37 miles before my Hymotion battery ran out. According to my ScanGauge, I used 0.145 gallons of gas while my battery was active. That's about 170 mpg.

Then, I drove far enough to use up 0.145 gal of gas with the Hymotion system off. It took 7.53 miles to use the same amount of gas, with the cruise set at the same speed. That corresponds to a fuel economy of about 52 mpg.

24.37 minus 7.53 is 16.84 miles. That is the "effective range" of my Hymotion battery on the highway. It is the range that my battery is gaining for me, compared with running the car on gas alone.

Overnight, I charged the battery back up. My Kill-A-Watt meter showed 4.67 kWh, this morning.

16.84 divided by 4.67 is about 3.6 miles/kWh. That is my electric fuel economy on the highway.

Compare that with my electric fuel economy in the city: 4.9 miles/kWh.

It seems as if the electric motor is much less efficient at highway speeds.

Normally, the fuel economy for a Prius is only slightly greater (less than 10%) at city speeds than it is at highway speeds. The Hymotion system seems to be over 30% more efficient at city speeds than it is at highway speeds.

Arthur

 

I've been thinking about my recent "miles/kWh" tests and how they compare with my previous data for my commute with the big hill.

The recent data:
city: 4.9 mi/kWh
highway: 3.6 mi/kWh

My commute: 3.1 mi/kWh

These tests were either done without using any gas or (in the case of the highway test) the gas use was determined and subtracted to give an "effective all-electric fuel economy."

80% of my commute is at city speeds with relatively flat terrain. The remaining 20% is going up and down a long hill with an average grade of about 10% (~5.5 degrees).

For my recent city driving test, I warmed up the car on the highway (just in case it mattered) before starting the test. Let's assume that it did matter. Let's assume that the "cold" city driving in my commute gives a slightly lower all-electric fuel economy of about 4.5 mi/kWh.

That means that 80% of my commute is at 4.5 mi/kWh. 10% of my commute is going down the long hill and 10% is going up the same hill on the way home.

The downhill part actually generates more energy than it consumes. My estimate is that I am probably getting about -6 mi/kWH on the way down the hill. That's just a fancy way of saying that I would have to go down a 6-mile-long hill to generate a full kiloWatt of energy from braking. That, of course, would be impossible since I am already reaching my upper limit of the amount of regen that my Prius traction battery can receive. -6 mi/kWh is just an estimate of my average downhill fuel economy. It's negative because I am adding energy to my system. Postive numbers represent expending energy.

With that in mind, I calculate that my uphill fuel economy must be somewhere around 0.6 mi/kWh. That means that I am using 2 kWh to go up my 1.2-mile-long hill. That's about 40% of my battery capacity. I know that I need to have more than 3 blue bars when I start up the hill or I will run out of battery. So, that seems about right.

At some point, I'll have to figure out a way to measure this directly.

Some further calculations make it seem like a reasonable result:

I know that my change in elevation is about 600 feet. Some calculations and unit conversions show that this is equivalent to a 0.75-kWh increase in gravitational potential energy.

I found a reference that suggests that the efficiency of the traction motor is much lower (40-50%) when it is running at it's maximum load:
Information Bridge: DOE Scientific and Technical Information - - Document #885987
Since I go up the hill as fast as possible in EV mode, I am (by definition) running the motor at it's maximum load.

Assuming an efficiency of about 42% (a wild guess), means that it takes about 90% of my 2 kWh just to impart 0.75 kWh of gravitiational potential energy to the car (0.75/0.42). This seems like a reasonable result.

I also looked at some drag calculations to compare my city and highway driving tests. The average speed of my city test was about half of the average speed of my highway test (55 mph). Since the drag force is proportional to the square of the speed, doubling the speed will quadruple the drag force. That means that my highway test probably involved four times as much drag as my city test.

So, I take back what I said about motor efficiency. According to the reference above, low (city driving?) and medium (highway driving?) loads on the motor will both put you in the 80-90% efficiency range. Most of the difference in fuel economy between city and highway driving can probably be attributed to the quadrupled drag force.

Arthur

 

I've had the mod for 8 months.

A couple months ago, I bought a Kill-A-Watt meter and have been tracking my electricity use since then.

My data shows that I am averaging 110.6 kWh per month for charging my car. That has added $8.54 to my monthly electric bill (at a rate of $0.0772/kWh, not including set fees).

Since I am probably averaging about 3.3 mi/kWh overall (a wild guess), my miles travelled by electricity are probably about 370/month. My total miles for local driving are probably between 500 and 600 per month. Longer trips (all gas) are probably about 300 miles/month. I'm sure my total is less than 1000 miles/month.

Arthur

 

I'm trying to post a chart that shows the range of my daily MPG over the last 2 months.

If the chart doesn't show up, perhaps someone can give me some suggestions.

Arthur

 

I find this extremely annoying:

If you go to Google News (Google News)and type in "Hymotion Prius," this blog article comes up at the top of the list:

REPORT: 115 Plug-in Toyota Prius test cars fail to crack 50 mpg average in year-long test

To see my response, scroll all the way to the bottom of the comments and then click "most recent." Then, scroll all the way down again.

For the past 3 weeks (15 days, since I haven't been driving on weekends), I've been keeping track of my miles, my gas use, and my electricity use. I drive in EV mode most of the time. My data does NOT include miles that I traveled AFTER my Hymotion battery ran out. On most days, I don't exceed the range of my battery anyway.

For 15 days of driving, I racked up 258.4 miles while using 1.2 gallons of gas and 71.4 kWh of electricity. The average per day is 17.2 miles on 0.08 gallons and 4.76 kWh.

That comes out to well over 200 MPG (if you ignore the electricity used). According to my best approximations, I am probably averaging about 15.6 miles from electricity (3.3 miles/kWh) and 1.6 miles from gas (20 MPG). (That includes some days when I give the battery a second, partial charge-up.) The gas MPG is low because a lot of it is useless engine idling or uphill climbing.

For cost calculations, I use $2.84/gal as the current price of gas and $0.0772/kWh as my electricity cost (ignoring base fees). With that in mind, my 15 days (258.4 mi) cost me $3.44 for gas and $5.51 for electricity. If you total that up and calculate the MPG for an equivalent number of dollars of gasoline alone (same miles), you end up with a "cost-equivalent MPG" of 82 MPG.

For my daily average of 17.2 miles, I am spending $0.60 ($0.23 for gas and $0.37 for electricity).

I estimate that less than 20% of my energy use is gasoline (in terms of the number of useful miles that that gasoline could have gotten me, if it was used normally).

Considering that I go up a 600-ft hill at least once a day (usually in EV mode, at between 10 and 20 MPH), my results are really very good.

Anyone who claims that they drive a Hymotion Prius and can only get 50 MPG can feel free to contact me for driving lessons.

Arthur

 

1) Learn all the quirks of your PHEV system. Every time it uses gas, make sure that you understand what triggered the engine to start. Then, at least you can decide whether that trigger can be prevented by changing your driving habits.

2) Develop a routine for routes that you travel often. In some cases, you can take alternate routes, depending on how much charge you have left in your battery. For short distances, you can take city streets (in EV mode) when you have lots of charge and highways when you don't. Pulse and glide is still useful in some circumstances (but is pointless for a PHEV running in EV mode). I get the car up to 55 mph on the freeway and then ease off of the accelerator to "coast" (with the engine idling in the background but with the energy display showing only battery use) through a 45 mph zone for about a 2 miles until I get to a stop sign. With a PHEV, you can do this indefinitely (on level roads, or downhill grades, or even slight uphill grades). It actually uses very little gas and uses less battery power than going slow in EV mode.

3) Drive at a moderate speed on the highway. (I use 57 mph and get over 150 mpg on the highway with my Hymotion system.)

4) You're going to have trouble with the hills. Keep in mind the mileage that a regular car would get going up the same hill. Then, it won't seem so bad that your PHEV is getting 35 mpg (or a little less) on that hill. If you can, try driving up hills when traffic is lighter. The only way to avoid using significant amounts of gas is to stay in EV mode on most of your uphill driving. Depending on the grade, you will probably need to stay between 8 mph and 20 mph to keep it in EV mode. That's why traffic is a problem. If traffic is light, you can try pulling over (but not stopping) to let other drivers pass (unless, of course, you're on a 2-lane road with no shoulder). That seems like extreme behavior, but it's the only way you're going to average over 100 mph.

5) In colder months, a lot of people have had great success with grill-blocking. It keeps the engine temperature up and allows you to stay in EV mode more of the time. I wouldn't try it in the summer, though.

6) I keep my tires inflated to about 37 psi (front and back). Some people go higher than that, but you start to lose traction that you might need to stop in an emergency.

I hope this helps.

Arthur

 

Great mileage, Chris! I've never managed to get that many miles on a single tank because I always end up going on some out-of-town trip before I get that far.

Yeah, it's definitely been cool lately. We saw a low temperature in the upper 40's on the 4th of July!

Arthur