Why hybrids are so efficient

[hybridbear]

We all know that our hybrids are more efficient than a gas only car, but have we really thought about why? I believe that understanding why will make all of us more efficient drivers. There are 3 main ways that our hybrids increase efficiency over a gas only car:

• Driving in EV mode
• Turning off the ICE when not needed
• Regenerative Braking

I will examine each method and how it increases the efficiency of the car.

 

Driving in EV mode

Given that all of our energy comes from gasoline, why is driving in EV mode better than driving with an ICE only? The answer can be found from examining the EPA ratings for PHEVs or BEVs. The MPGe ratings of these vehicles are often around 100 MPGe combined, with numbers as high as 120 MPGe in city driving.

 

So, what is MPGe?

Saying that the Fusion Energi gets an EPA rated 108 MPGe city means that on 33.7 kWh of electricity a Fusion Energi can go 108 miles on the city test cycle. 33.7 kWh is the amount of energy contained in a gallon of gasoline as measured in kWh. Each gallon of gas contains 125,000 BTUs, or approximately 31,000 calories if we think about it as a source of energy like we measure food. 125,000 BTUs equals 33.7 kWh of electrical energy.

 

Why are the MPGe ratings so high?

Electric motors are much more efficient than gasoline engines. According to the Dept of Energy (http://www.fueleconomy.gov/feg/atv.shtml) only about 15% of the energy in each gallon of gas is converted into kinetic energy in city driving and about 26% in highway driving. Using an electric motor to provide propulsion means that about 60% of the energy is converted into kinetic energy according to the Dept of Energy (http://www.fueleconomy.gov/feg/evtech.shtml). This is why driving in EV mode is better and why PHEVs and BEVs can drive much farther on a lesser amount of energy. The battery pack of the Nissan Leaf stores less energy than is found in a gallon of gasoline and can still carry that car almost 100 miles between charges. The C-Max Energi and Fusion Energi store 6.5 kWh of energy in the battery to drive about 20 miles; that is equivalent amount of energy to 1/5 of a gallon or 24.7 ounces of gasoline. With that tiny amount of electrical energy those cars can go farther because an electric motor is much more efficient.

 

Electric motors do not waste as much energy as heat losses compared to an ICE. They are also not affected by varying atmospheric conditions since they are not using air to initiate combustion. Since as much as 75% of the energy in a gallon of gas is converted to heat energy by the ICE this is a big advantage for the electric motor.

 

What does this have to do with a hybrid?

Our cars do not plug in to an outlet to charge and get EV only driving, but they do run for upwards of 50% of the miles in the city using only the energy stored in the battery. This means that when we are running in EV mode we are seeing much higher efficiency than if we were driving an ICE only car.

 

When designing a vehicle the ICE must be large enough to provide sufficient power for maximum acceleration conditions. This means that the ICE is much too big for everyday driving and that there is a lot of inefficiency when running an ICE with a low power demand. The hybrid turns the ICE into an electric generator by artificially increasing the power demand when driving with the ICE to a level where the ICE is more efficient. Hybrids also include a smaller ICE that is able to more often run at its peak efficiency.

 

This means that the car is most efficient in EV mode at lower power demands, and more efficient running the ICE under higher power demands. For us to maximize our EV mode driving and our fuel economy we want to use EV mode as much as possible when the power demand is low, and use the ICE when the power demand is higher.

 

Turning off the ICE when not needed

Since our cars can turn off the ICE when it is not needed we don’t waste fuel idling unnecessarily. When decelerating there is no need for the ICE to run and burn gas, yet in gas only cars it does. When stopped there is no need for the ICE to run and burn gas, yet it does in gas only cars. This doesn’t mean that we can sit and leave our cars turned on for long periods while stopped because this will use up the hybrid battery and make the ICE come on to generate electricity.

 

Regenerative Braking

Another big inefficiency with gas only cars is braking. Brake pads and rotors turn momentum into heat energy, effectively wasting that energy. The regenerative braking system in our cars seems to recover about 70-80% of the kinetic energy the car has when we begin braking. That is a huge improvement over the 0% recovered by traditional friction brakes. This helps our efficiency in city driving.

 

Driving conditions for peak efficiency

Based on the above information we see that for peak efficiency we want to accelerate using the ICE, then back off once reaching cruising speed and use the electric motor to maintain our cruising speed. Since the electric motor’s efficiency advantage over the ICE is highest at lower power demands we see the highest results at lower speeds in the city. At 25 MPH the power drawn from the battery to maintain that speed is minimal. I’ve cruised for miles without using the ICE when cruising at 25 MPH without stopping. This is the logic behind the hypermiling technique known as DWB (driving without brakes). The DWB technique involves trying to drive in a manner than you brake and stop as little as possible. While braking is much more efficient in our cars than in a gas only vehicle, it’s still better to brake as little as possible since an electric motor only converts 60% of the electrical energy stored in the battery into kinetic energy and regenerative braking only converts 70% of the kinetic energy back into electrical energy. For example, 1 kWh of electrical energy pulled from the battery would only get you 0.6 kWh of kinetic energy and the 0.6 kWh of kinetic energy would be converted into 0.42 kWh of electrical energy back into the battery through regenerative braking. It is much better to use that 1 kWh to drive 5+ miles without stopping than it is to stop and only recover 0.42 kWh of electrical energy.

 

There are certain routes I have found where I can consistently get better than 60 MPG because I don’t have to stop very often and I can maintain a constant rate of speed. Such routes are the pinnacle of efficiency for a hybrid. I imagine that on those same routes I could get 75 MPG out of a Prius and over 130 MPGe in a C-Max Energi or Fusion Energi.

Edited May 7, 2013 by hybridbear

[gadgetguy]

Wow that's an awesome post and very informative! Thanks for taking the time to break that down like that..

[milleron]

Neat essay. It contains some facts and theories I'd never even considered. My only question is a trivial one: the part about 1/5 gallon = 3.08 ounces. 3.08 ounces might be about 1/5 of a pint, but a fifth of a gallon is closer to 25 ounces, isn't it?

[hybridbear]

So basically, if you can find a route that minimizes stops and keeps the speed below highway speeds you will see greater efficiency. This is why Tom sees such high MPG numbers on his commute, he has described it as driving through the country with minimal stops and speeds low enough to maximize the efficiency of the electric motor.

 

Something I forgot to include above is that the MPGe ratings for PHEVs and BEVs is always lower on the highway. This is because the electric motor is not as efficient under higher loads when fighting to overcome wind resistance at high speeds. The efficiency gap narrows at high speeds between what the ICE can do and what the electric motor can do with an equal amount of energy. The ICE is terribly inefficient at city speeds because it is always using more gasoline than is really needed when the load is light

 

Neat essay. It contains some facts and theories I'd never even considered. My only question is a trivial one: the part about 1/5 gallon = 3.08 ounces. 3.08 ounces might be about 1/5 of a pint, but a fifth of a gallon is closer to 25 ounces, isn't it?

Oops!! Fixed now, sorry. I multiplied by 16 ounces in a pound instead of the ounces in a gallon! Lol

[rjent]

Absolutely Outstanding. Great writeup! :worship:

[GrySql]

Hybridbear, that was excellent and would make a good read for any hybrid owner.

Understanding the engineering of these hybrids can make all of us better drivers and can take some of the mystique out of why driving the FFH is different than other cars.

Once again you've provided us with a clearly outlined and easily understood post on a very difficult subject, thus making our experience a better one, bravo!

 

Edited May 7, 2013 by GrySql

[corncobs]

Awesome write up and I agree it's something for every new or potential hybrid owner to read!

 

Thanks hybridbear ?

[B25Nut]

As everyone has said, this is outstanding Hybridbear. It is now a permanent document on my computer. I'm waiting for Prof. Lolder to give you a grade on it.

[acdii]

Damn, and I thought I had too much time on my hands! LOL :) Good write up.

[MXGOLF]

Thanks for the write up. Good information. I kind of new that but the numbers help justify hybrids. Except mine of course. LOL

[storksb]

hybridbear, thanks for the post. Very informative. Should help me get better mpg.

[majorleeslow]

Cheers hybridbear wow that's an interesting way to look at it. Does anyone know how the 2 motors are connected to the ICE and cvt? basically is there a flow chart of the power transmission?

[hermans]

Damn, and I thought I had too much time on my hands! LOL :) Good write up.

You both do.

[kuzzi]

Cheers hybridbear wow that's an interesting way to look at it. Does anyone know how the 2 motors are connected to the ICE and cvt? basically is there a flow chart of the power transmission?

See here http://fordfusionhybridforum.com/topic/6775-what-is-an-ecvt-how-does-it-work-here-is-the-answer/?hl=ecvt

[hermans]

The Little Rascals had the first "hybrid". It was a boat that was powered by ducks swimming after popcorn on a string. It doesn't get any simpler than that.

[acdii]

You both do.

Not as much as I used to.

[acdii]

The Little Rascals had the first "hybrid". It was a boat that was powered by ducks swimming after popcorn on a string. It doesn't get any simpler than that.

Now if THAT doesnt show your age! .... :dogwalk:

[lolder]

Here's a graphic of a gen I Prius ( 2001-2003 ). The principle of all eCVT hybrids is the same. The architechture and speeds vary. Put a check in the driving mode box, put it in D and take a spin

http://www.wind.sannet.ne.jp/m_matsu/prius/ThsSimu/index_i18n.html?Language=en?Country=US

Edited August 22, 2013 by lolder

[hermans]

Now if THAT doesnt show your age! .... :dogwalk:

That episode aired in 1938....10 years before I was spit on a rock and hatched by the sun.

[Twinkle]

I have read this article a couple times because it is so informative. I live in congested Orange County Ca and I sure wish I had some country roads so I could use the EV more. I have to use HWY to get to work. Speeds here are 70- 80 mph. I have noticed high mpg when there is traffic because then I am in EV mode

[kuzzi]

I have read this article a couple times because it is so informative. I live in congested Orange County Ca and I sure wish I had some country roads so I could use the EV more. I have to use HWY to get to work. Speeds here are 70- 80 mph. I have noticed high mpg when there is traffic because then I am in EV mode

Same here. Makes me hate traffic jams less when there's I can creep above 50 mpg when there is one.

[Gregavi]

My concern is that by paying so much attention to all these new (for me) driving techniques, I will become distracted and not be as aware of what's going on while driving. My wife has already commented on this. She said she was so intrigued with the leaves, that she almost rear-ended someone. Hopefully these driving techniques will become more second nature and I can get back to paying attention to the road, which is where it should be.

[B25Nut]

My concern is that by paying so much attention to all these new (for me) driving techniques, I will become distracted and not be as aware of what's going on while driving. My wife has already commented on this. She said she was so intrigued with the leaves, that she almost rear-ended someone. Hopefully these driving techniques will become more second nature and I can get back to paying attention to the road, which is where it should be.

I think you should turn the leaves off and switch to another option for right screen. I feel Engage is the best option to start with for the left screen since a quick glance gives you a lot of information. It is what I've stayed with.

Edited September 15, 2013 by B25Nut

[corncobs]

I think you should turn the leaves off and switch to another option for right screen. I feel Engage is the best option to start with for the left screen since a quick glance gives you lot of information. It is what I've staid with.

100% agreed the leafs are a nice gimmic but don't help to drive the car anymore efficiently. IIRC they didn't even survive the drive home from the dealer. ;-)

Edited September 15, 2013 by corncobs

[DeeCee]

I think you should turn the leaves off and switch to another option for right screen. I feel Engage is the best option to start with for the left screen since a quick glance gives you a lot of information. It is what I've stayed with.

Agree fully too. The leaves are a worthless gimic and can be a distraction.