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.fuelecono...v/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.fuelecono...eg/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.
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 by hybridbear, 07 May 2013 - 11:34 AM.