Highway mpg lower?

I assume one factor, if not the major factor, is that wind resistance goes up exponentially as you drive faster. Therefore, to cover the same distance, the slower you go the less energy is burned for that trip in overcoming wind.

For internal combistion engines this was not as impressive as they were inefficient at low speeds, or had gearboxes, so there was a countervailing reason to drive faster. For electric motors, the efficiency is high at all speeds, so the wind resistance issue is revealed as the primary remaining issue.

That said, I seem to get more in the mid-40s mpg on highway in hv mode. Not sure if something else is going on.

 

If you have a head wind that will also reduce mpg as you know. But putting the car in eco mode can help slightly. I tend to go into hv charge when traveling down steep grades if possible. This allows for a bit more ev mode. But 38 mpg is not too bad for 75 mph in a 4000 pound clarity running mostly on ice. Something i have noticed that will also hurt the mpg is the collision mitigation doing moderate braking when i'm 200 ft behind a slower moving vehicle. Then when i change lanes it rapidly accerates to reach the set speed. This is not very smart. I wish i could set my cms to only be active when an emminent collision is detected. I really takes away from the cars efficency in traffic. Also can be dangerous when it brakes unexpectedly. This system gets a thumbs down from me.

 

What is the state of charge on your battery? A depleted battery seems to reduce HV mileage. Otherwise, yes, the faster you drive, the worse the EV range and the worse the HV mileage due to wind resistance.

 

Thanks! Your experience provides another data point consistent with HV mileage being worse with a depleted battery. Except for the 100 mph portion. I don't think a full charge would have helped there

 

Probably all the regen and the range you can use in EV in a stop and go VS freeway
My commute is 90% freeway and I set my ACC to 85, the car expected EV mileage is between 42 to 45 miles and it is pretty much the case without traffic. With traffic the regen can usually extend that range much longer as I can see that sometimes my EV range will catch up / exceed the remaining miles show on the gps.

My wife's Prius prime canot do that with regen... So this car is really good at capaturing regen....probably because it weight a lot more too

 

Many of us avoid zero battery during HV on trips by starting (local) in EV, then switching to HV on the highway (above 50%) and staying there until within EV range of destination, and then switching back to EV for arrival & charge.

 

I haven't noticed that HV mileage is worse with a depleted battery or not, averaged about 40 mpg between 75 - 80 mph over 2000 miles either case. I will try more, but don't think it really matters.

 

The transmission is a one speed, the faster you go the more gas or battery it burns.

Even when the battery is depleted, it’s never truly dead. Let the car manage hybrid mode, do not switch to HV mode. The HV mode is designed to save battery life, but not after the battery is run down.

 

The one-speed transmission, when the engine is actually connected to the gears that propel the car (Honda calls this behind-the-scenes mode that drivers can't select "Engine drive mode"), occurs only when the Clarity determines that mode would be the most efficient way to propel the car on the highway. Still, it's correct that the faster you go, the more gas or battery it burns.

Although it hasn't been proven that the Clarity can achieve better gas mileage in HV mode with a charge in the battery to back up the engine, it's certainly more pleasant to drive when the car can select between the engine and the traction motor as it sees fit. If the journey includes significant inclines, many recommend strategically maintaining a charge in the battery to assist the power from the engine when tackling those inclines. HV Charge Mode gives the driver a way to restore the battery's charge before encountering such obstacles.

 

Right. It should be perfectly normal for vehicles to use less energy when driving at less than highway speeds, for exactly the reason you cite. That exponential rate of increase in drag (and therefore energy consumption) at higher speed, due to air resistance, really bites!

The only reason gasmobiles have lower MPG in stop-and-go traffic is because of the gasoline engine's very low energy efficiency at low RPMs, and the fact that they keep idling when stopped at a stop sign or stop light. The improved tech in a PHEV stops the engine when the car stops, and regen helps mitigate the loss of energy from constant acceleration and deceleration in stop-and-go traffic. Both of those improvements over gasmobiles help cars achieve the energy efficiency that they ought to have at low speeds.

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Most hybrid vehicles will get better mileage in city driving than in highway driving due to the energy recovery from the stop and go nature of city driving. This is the whole point of a hybrid. With only an internal combustion engine, once you have expended the energy in the gasoline by burning it, that energy is gone. In a hybrid, you have the opportunity to recover some of that energy during braking or downhill portions of the trip. Since you can't put the energy back into the gas tank as gasoline, the energy is converted to electricity and stored in a battery.

Not true of all hybrids, but of those where it isn't, the mileage ratings are very close for city and highway, closer than for vehicles with only an internal combustion engine.

I would suggest that the arguments being put forward about drag/wind resistance at highway speed are completely irrelevant. Those factors would be in play for any car, regardless of the type of propulsion system. In an internal combustion engine vehicle, stop and go (read "city") driving is so much less efficient than highway driving because you are constantly wasting energy by stopping. You expend a certain amount of energy to get the vehicle up to a certain speed and then waste that energy by applying the brakes to stop the car. In a hybrid, you don't waste the kinetic energy of the vehicle when you stop, you convert it back to potential energy in the form of an increased charge on the battery.

 

What you are describing is not the collision mitigation braking system (CMBS), it is the adaptive cruise control (ACC) system. The collision mitigating braking system will first warn you of an impending collision by flashing the "BRAKE" warning on the dash, followed by actually applying the brakes to prevent a collision if not done by the driver. The adaptive cruise control system will adjust the speed of the vehicle to prevent getting any closer to a preceding vehicle than whatever setting is selected, then returning to the speed set in the system prior to encountering a slower moving vehicle once clear of that obstacle.

By the way, you can turn off the adaptive cruise control feature and make it just a regular cruise control where it will hold your speed. No slowing and accelerating as you describe, just an old fashioned cruise control.

 

You're exaggerating how much energy a car with regenerative braking can recover. At best it's about 35% of the wasted energy.

Most of the excessive energy loss in a gasmobile in stop-and-go driving comes from idling and the fact that ICEngines are very inefficient when run at low RPMs. The latter is why the range extender in PHEVs is typically run unthrottled... wide open, at high RPMs, for maximum fuel efficiency.

Regenerative braking certainly does help with energy efficiency in PHEVs and BEVs, but it doesn't help that much compared to how much energy is lost by constantly accelerating and decelerating.

Not mere opinion here, but facts.

It's true that wind resistance causes drag to increase exponentially at higher speeds (and becomes quite significant above ~35 MPH) in all cars, not just PHEVs or just gasmobiles. But the question in the OP was why highway MPG is lower than city MPG for PHEVs such as the Clarity.

The most significant reason why that is, is increased drag from wind resistance at highway speed. It is said that at 55 MPH, a car spends fully half of its energy (and fuel) fighting wind resistance. At higher speeds, it's even worse. That's why MPG drops off so quickly at speeds higher than 55 MPH.

 

It's that combined with lower wind resistance. A hyper-miler trying to squeeze the most miles out of a tank or charge is going to drive a consistent ~30 MPH to get that, with no stop and goes, and no hills. Like this guy....

 

Pushmi-Pullyu, you state in the first part of this post that what you have said isn't opinion, but facts. Can you give citations for your facts, please? Because what you are calling facts directly contradict my understanding of these things. I'm not saying you're wrong, but your facts don't fit my understanding and observations.

For instance, you state that internal combustion engines are very inefficient at low RPM. Okay, so if that is true, why don't all cars use a single or maybe two speed transmission so that the engine will run at max RPM all the time? Add to that, cars that use to have three or four speed transmissions now have as many as 10 gears and when running at highway speeds they are only turning 1200 to 1500 RPM when the redline might be as high as 8000 RPM?

In the second part of your post, it appears to me that you contradict yourself. You assert that the reason that a Clarity gets worse gas mileage in highway driving is primarily due to wind resistance. If that were true, why isn't it also true for "gasmobiles"? The laws of physics apply equally, so if wind resistance is the culprit in giving lower mileage in a hybrid, why not in an standard vehicle? There may be an example out there with which I am not familiar, but I don't know of any gasmobile that gets better mileage in city driving than it does on the highway.

Until I am enlightened, I stand by my previous statements. A hybrid vehicle is less efficient at highway driving because there are less opportunities to recover energy by braking. It is this energy recovery from braking in stop and go driving that makes hybrids so much more efficient at city driving than highway driving, especially when compared to any vehicle of similar weight.

I look forward to continuing the discussion.

 

New user here trying to get a real-world sense of EV vs. HV on the clarity.

My one-way commute is about 20 miles and is 90% on a highway with speeds of 75 MPH to 85 MPH.

Given the clarity's 40+ EV range, can I generally get the round-trip done just in EV, even with the high speeds ?
I may or may not get a charging spot at work depending on how crowded it is on a particular day.

I ask because, in the Sonata PHEV review, it was mentioned that the gas engine kicks in above 75 MPH.

Also, how bad is the "angry bees" noise in highway speeds ?

Thanks in advance !