Top 2 reasons why City MPG is better than Highway MPG?

[vett93]

Can someone please explain why hybrid cars would get better gas mileage in city as oppose to freeway driving? In the city driving, there are a lot more stop and go traffic. I understand that if one brakes gently, most of the energy can be transformed back to the battery. However, no conversion is 100% and it takes more energy to accelerate than driving at a constant speed.

 

The only reason I can think of is that a higher speed on freeway would have more air resistance and need more energy to keep it going. But have I missed anything here? Thanks.

 

[larryh]

The main reason regular and hybrid cars get worse highway mileage is speed. Aerodynamic drag increases with the square of the speed. The resistance is four times greater at 60 mph than 30 mph. At 30 mph, tire rolling resistance and internal friction dominate. At 60 mph, aerodynamic drag dominates.

 

Hybrid cars get better highway mileage than regular cars because they have more efficient engines. Since they have both a motor and an engine, so they can use smaller, more fuel efficient engines. In addition, then can use a more fuel efficient Atkinson cycle engine. Drivers would not be happy with the performance of a small Atkinson cycle engine in a regular car, but with the assistance of an electric motor, they provide acceptable performance in a hybrid.

 

As you mentioned, hybrid cars are able to use regenerative braking to capture lost kinetic energy due to braking (up to about 80%) in city driving. Also, they tend to have lower rolling resistance tires. That is why they do so much better than gas cars in the city.

Edited May 29, 2014 at 11:08 AM by larryh

[gerdia]

Another reason as to why is the fact that in the highway the gas engine is required for more power. In the city, the engine gets to turn off more often and the lower speeds allow the electric motor to run more by itself. Shut off at lights a day heavy traffic.

Edited May 28, 2014 at 11:16 PM by gerdia

[larryh]

Hybrids also tend to have an eCVT, which allows the engine to run at its most efficient operating point. 

[Rexracer]

Normal car city:

lots of accelerations/deceleration (inefficient)

sitting idling (0mpg)

 

Normal car highway:

consistent speeds (very efficient) balanced by higher wind resistence

 

Hybrid city:

slower speeds = more electric driving (0 gas used)

regen breaking (energy gained)

engine off at stop lights (0 gas used)

 

Hybrid highway:

consistent speeds above what electric system can support over 100s of miles (more fuel used)

no regen breaking

no stopping so no gains from having the engine off

 

 

So normal cars are more fuel efficient on the freeway because of the consistent speeds. Hybrids are more efficient in the city because it takes advantages of the hybrid parts/pieces.