With the slew of launches, the electric cars are beginning to generate serious buzz if not the sales. There are many ways in which electric cars differ from fuel cars: handling, maximum speeds, acceleration and efficiency. Yes, the electric cars are more efficient when you drive them in city which is the opposite of fuel-based cars that return the best mileage on highways. Why so?
Why do petrol-diesel cars return better mileage on Highways than Cities
The petrol-diesel cars deliver the best mileage when they operate at the ‘Best RPM’. Car gearboxes are generally designed to get you the best fuel consumption at 55 to 80 kph as the car hits those speeds at pretty low RPMs—say around 1,800.
So when you are driving on a highway, you can hit that RPM/speed and extract the best mileage.
The situation in city is quite different. Steady speeds are impossible which means you are constantly shifting gears trying to stay close to best RPM but its not possible all the time. In fact, as you are moving through gears, the engine is often running at worst RPM.
The other problem for petrol-diesel cars is that when you need to accelerate – the engine needs more fuel to increase the speed – burning fuel to generate “kinetic energy” to give a push the car. When you apply the brakes, you’re throwing away kinetic energy and turning it into waste [heat] in the brake pads and disks. Even when you’re stopped, the fuel engine is probably still consuming gasoline just idling.
So that’s why gasoline engines do so badly in city driving.
Why do electric cars return better mileage in Cities than Highways
A rather surprising thing happens at times with electric cars. If you are driving the car from the highway into the city, you might find that range actually goes up. Petrol in the tank never goes up, does it?
So, what is happening here. Once you are in the city, you are decelerating frequently. And every time that happens, regenerative braking kicks in.
What is regenerative braking?
When a car is moving, it has a lot of kinetic energy. The moment you apply the brakes, to slow a vehicle, all of that kinetic energy has to go somewhere [Remember, energy is neither created or destroyed, it merely changes form]. In internal combustion engine cars, brakes are friction-based and therefore, kinetic energy of the vehicle get converted into wasted heat. All of that energy is simply lost for good.
Regenerative braking uses an electric car’s motor as a generator to convert much of the kinetic energy lost when decelerating back into stored energy in the car’s battery. Then, the next time the car accelerates, it uses much of the energy previously stored from regenerative braking instead of tapping in further to its own energy reserves.
It is important to realize that on its own, regenerative braking isn’t a magical range booster for electric vehicles. It doesn’t make electric vehicles more efficient per se, it just makes them less inefficient.
The other bit is that electric cars have only one gear and therefore their efficiency is not as closely related to speed as fuel-based cars.
In a nutshell, the difference in efficiency behaviour arises out of how each type of car reacts to city’s stop-go traffic: fuel-based cars waste energy and therefore see reduction in mileage. Electric cars conserve energy and see improvement in range.