Real World "Fuel" Economy

[mustermutti]

My suggestion was that if the car could be allowed to freely accelerate from 60mph to 80mph going down hill, some of that additional kinetic energy could be recaptured through regenerative braking as the vehicle is brought back to the starting speed of 60mph. I believe that would be a slight gain compared to holding the speed at 60mph with a click or two of the paddle. In all, it is probably just picking fly s#it out of pepper.

Isn't going faster always less efficient due to increased air resistance, regardless if you're going uphill it downhill? That would match my experience from hypermiling in a Bolt, where I seemed to be able to significantly stretch my range by keeping it at ~55mph even on steep downhills. Yes there is some energy lost in the regen process, but I'd assume the energy difference required to push air out of the way for e.g. 60 vs 80mph seems significantly higher than the regen loss when keeping it at 60mph. (If it wasn't, it seems a lot of heat would have to be generated from these losses, and I've never heard of problems from overheating due to too much regen-ing.)

 

[Landshark]

Isn't going faster always less efficient due to increased air resistance, regardless if you're going uphill it downhill?

Yes, going faster “under power” will be less energy efficient than going slower.

To reiterate my proposal, allowing the car to take advantage of the force of gravity and gain speed while coasting downhill would be gaining “free” kinetic energy.

 

[mustermutti]

I'm wondering if the "free" kinetic energy is better spent converting to battery charge, or let the car coast to a faster speed (assuming the goal is to optimize for efficiency).

Imagine a Clarity with completely empty battery (and no gas) on top of a big hill. Scenario 1: Coast down in neutral and see how far it rolls (never braking, no matter how fast the car gets). Scenario 2: Use some regen braking to limit max. speed (e.g. 60mph), then use the recaptured battery energy to drive on electric power until battery is empty again.

Which scenario will give you more miles? My guess is scenario 2.

 

[leop]

Mustermutti is correct in his thinking that it is better to keep a slower speed while coasting downhill and harvesting the gravitational energy. The maximum amount of gravitational energy that can be harvested is mgh where m is the loaded mass of the Clarity PHEV, g is the gravitational acceleration constant, and h is the height of the incline. If d is the distance to be driven down the incline, the energy lost to aerodynamic and tire drag is dF where F is the total drag. Aerodynamic drag is proportional to the speed squared and the tire drag, while more complicated, is mainly quadratic in form with respect to speed. So the total drag increases as the speed increases. Thus, the energy lost to drag and not available to be harvested increases with an increase in speed.

The best technique for maximal gravitational energy harvesting with a Clarity PHEV is to use the paddles (or ACC or even the foot brakes) to control the downhill speed to the slowest constant speed consistent with good safety practices. BTW, about the only time I ever use the paddles is to control downhill speeds when taking my foot off the accelerator pedal is not sufficient. Of course, energy harvesting will not be available if the traction battery is fully charged.

LP

 

[Landshark]

Coast down in neutral and see how far it rolls (never braking, no matter how fast the car gets)

This is not a scenario that i have, or would propose. My proposal would be to crest a hill, at say, 60mph and allow the vehicle to accelerate to 80mph. Then apply brakes or paddles to lower the speed to 60mph. Rinse and repeat, until there was no more downgrade. The “free” kinetic energy would have been captured through regenerative braking, just in a different manner.

 

[Landshark]

The best technique for maximal gravitational energy harvesting with a Clarity PHEV is to use the paddles (or ACC or even the foot brakes) to control the downhill speed to the slowest constant speed consistent with good safety practices.

This makes some sense. But since we are likely to be able to capture ~50% of the kinetic energy through regenerative braking, might we not cover a greater distance by rolling to a stop?

 

[mustermutti]

Maybe the optimal tradeoff (coast vs regen, when going downhill) depends on how steep the slope is (due to the regen energy loss)? E.g. if slope is only steep enough that terminal coast velocity is ~65mph, coasting may end up more efficient overall since the energy lost to increased wind drag may end up less than the energy lost to regen inefficiencies when keeping it at 60mph. But once slope is steep enough, the opposite will be true and regen becomes more efficient.

In that case (stope sleep enough to make regen more efficient), it seems staying at the low speed consistently should be more efficient than bouncing between 60..80mph - the slower you get to the bottom, the less energy you lose to wind drag.

 

[Landshark]

- the slower you get to the bottom, the less energy you lose to wind drag.

Once I hit 60 I’ll be back on the throttle. Either way there won’t be enough energy saved to grill a bacon burger.

 

[PHEVDave]

It probably would, but other than it being a terrible idea (https://www.mercurynews.com/2014/05...-illegal-and-dumb-and-a-lot-of-drivers-do-it/) you would probably loose out on recapturing any kinetic energy, which is like, the whole point of a hybrid.

To answer your original question, my real world fuel (gas only) economy has been 42mpg indicated on the dash but 40mpg using a calculator. This was driving I-5 on mostly flat ground, with family and luggage, and keeping up with traffic which between Stockton and LA means 75-80mph.

I try to remind people of the fact that regenerative braking is not necessarily always contributing to efficiency. If you’re rolling down a hill in “neutral” and never exceeding your desired speed then that’s the best way to do it. Regenerative braking would be a waste in that case. I like to think of “neutral” as simply a mode where battery charge is neither lost nor gained to the acceleration or from the deceleration of the car.

 

[CIvan]

After a month of driving EV only and charging at home, I went on a trip to visit family. Total distance on the first gas tank was 772 miles. Purchasing 5.88 gallons of gas provides a Clarity calculation of 131 mpg. However, I like to use the cost of electricity to determine a more realistic mpg. So with an estimated 150 KWh (at the wall) at 9.95 cents per KWh, electricity cost was $14.925. At $2.70/gal for my first fill up, that equates to 5.53 gallons. So that results in an equivalent mpg of 722 miles / 11.41 gallons = 63.3 mpg.

My second tank was HV only with 3 adults and luggage on board. Driving at 70 mph through rolling hills at night with negligible wind, the AC on and temperatures in the upper 60’s yielded 42.4 mpg.

My third tank included several charges at relative’s homes. I don’t include that cost of electricity, because it’s free to me. That tank was 411 miles with 6.23 gallons of gas for an average of 66.0 mpg

The fourth tank was in HV mode with similar conditions as the second trip, without the other adults. That yielded 46.4 mpg (with ~400lbs less in the car).

I can confirm that coasting downhill in neutral in either EV or HV mode results in the engine being off and regen unavailable, even via braking. I would only recommend doing this on long descents if you are interested, otherwise the effort and wear and tear is not worth it.

 

[PHEVDave]

After a month of driving EV only and charging at home, I went on a trip to visit family. Total distance on the first gas tank was 772 miles. Purchasing 5.88 gallons of gas provides a Clarity calculation of 131 mpg. However, I like to use the cost of electricity to determine a more realistic mpg. So with an estimated 150 KWh (at the wall) at 9.95 cents per KWh, electricity cost was $14.925. At $2.70/gal for my first fill up, that equates to 5.53 gallons. So that results in an equivalent mpg of 722 miles / 11.41 gallons = 63.3 mpg.

My second tank was HV only with 3 adults and luggage on board. Driving at 70 mph through rolling hills at night with negligible wind, the AC on and temperatures in the upper 60’s yielded 42.4 mpg.

My third tank included several charges at relative’s homes. I don’t include that cost of electricity, because it’s free to me. That tank was 411 miles with 6.23 gallons of gas for an average of 66.0 mpg

The fourth tank was in HV mode with similar conditions as the second trip, without the other adults. That yielded 46.4 mpg (with ~400lbs less in the car).

I can confirm that coasting downhill in neutral in either EV or HV mode results in the engine being off and regen unavailable, even via braking. I would only recommend doing this on long descents if you are interested, otherwise the effort and wear and tear is not worth it.

Just wondering. What kind of ‘wear and tear’ is involved in putting the car in neutral?

 

[CIvan]

Just wondering. What kind of ‘wear and tear’ is involved in putting the car in neutral?

I don’t know for sure, but I imagine it’s like shifting an automatic transmission from drive to neutral and back into drive again at 70 mph. There would be some w & t on the shifter, but in our case that is a push button, so not so much. The shift from drive into neutral seems pretty painless too. I would be most concerned about the eCVT mechanically connecting the drive wheels spinning at 70 mph with the electric drive motor spinning at 0 rpm. I really don’t know how the electronics and mechanicals handle that event.

 

[Landshark]

However, I like to use the cost of electricity to determine a more realistic mpg. So with an estimated 150 KWh (at the wall) at 9.95 cents per KWh, electricity cost was $14.925. At $2.70/gal for my first fill up, that equates to 5.53 gallons. So that results in an equivalent mpg of 722 miles / 11.41 gallons = 63.3 mpg.

While this is an interesting exercise, estimating electricity consumption, putting a price on the electricity, or not, if it was free, converting that dollar figure to a number of gallons of gas at current prices in one specific part of the country to come up with a realistic mpg figure, is a bit of a muddy bath.

The method does allow you to calculate a cost per mile, which could prove more meaningful.

Carry on.

 

[bpratt]

I don’t know for sure, but I imagine it’s like shifting an automatic transmission from drive to neutral and back into drive again at 70 mph. There would be some w & t on the shifter, but in our case that is a push button, so not so much. The shift from drive into neutral seems pretty painless too. I would be most concerned about the eCVT mechanically connecting the drive wheels spinning at 70 mph with the electric drive motor spinning at 0 rpm. I really don’t know how the electronics and mechanicals handle that event.

Shifting from drive to neutral causes no mechanical change. The drive motor is mechanically connected to the front wheels so if the car is moving, the motor is turning. The only thing shifting to neutral does is tell the computer that controls the drive motor not to pulse the motor to cause regenerative breaking or pulse the motor to produce power.