Hybrid cars: what is their real advantage over common cars?

We all hear that we should use hybrid cars, but why is that? I mean, what do you really get from using this type of car? We all know it’s about less fuel consumption and reduced pollution, but how do you get these?

A hybrid car (let’s say Toyota Prius, since it’s the poster car for the industry) is equipped with a thermic engine that uses petrol and an electric engine, powered by power cells. The car uses the electric engine while in cities (aprox. 50km autonomy), with almost no sound at all and and zero pollution.

When it is stopped (like on traffic lights) the engine can be turned on and off by pushing the, obviously, On/Off button. This means 5-10% less consumption.

Outside of the cities, the car uses the petrol engine which also refills the power cells.

If you take a good look at the hybrid cars, you will notice that the actual zero pollution happens when the car uses the electric engine, while the fuel consumption is less because of the stops, where you can turn the car off. But it should be obvious that the car doesn’t actually uses less petrol when it uses the petrol engine. It’s the same engine you would get on a common car.

So, if you take into consideration that the energy transformation efficiency for using the electrical engine (change thermal energy to mechanic energy then to electric energy then to refilling power cells which, finally, power the engine that moves the car) is lower than the efficiency of transforming thermal energy into mechanic energy to move the car, it can be observed that the hybrid car has, when it’s used both in and out of the city, a higher fuel/power consumption then a common car.

The main advantages of the hybrid car remain the on/off button, which translates in fuel savings at stops and the lack of noise and pollution in cities, when used on electric engine.

While no pollution is an important advantage, you should know that this comes to a price: around $40.000 for a Toyota Prius, almost double than a common car in the same class. Unfortunately, not all of us can and will make that “sacrifice” in order to preserve the planet.

Later edit: One of our readers (thanks, Wes) came to complete this article with some more information on the subject, more detailed and informed. Read all the comment below:

First of all, hybrid cars do not use their internal combustion engines (ICE’s) “outside the city” and their electric motors “inside the city.” It’s a complex interplay of the two most of the time, handled by an on-board computer that determines the most efficient mix.

Second, you don’t save energy at stops by pushing the “on/off button.” The on-board computer knows you’re stopped and shuts the ICE off for you. When you press the gas, it starts it back up (while getting you rolling using the electric motor).

Third, the efficiency gains don’t just come from shutting down the ICE at stops. They also come from regenerative braking (the car will use its own momentum during braking to spin the motors–rather than the other way around–turning them into generators which charge the batteries a little) and from the relative torque characteristics of the ICE vs. the electric motor.

That last part deserves a little more explanation. In a typical non-hybrid, ICE-powered car, you need an engine that’s powerful enough to get your car moving in all situations. Unfortunately, ICE’s are not very efficient at low speeds (in fact, they have a fairly narrow RPM range of efficient operation, that’s partly why you get more miles per gallon while driving fast down the highway than you do driving slow in the city). So in order to get moving from a stand-still, or accelerate at low speeds, you need several different gear ratios in your transmission and a bigger, less fuel efficient, more-polluting engine.

Electric motors, in contrast, have a much flatter torque curve. Their efficiency at low speeds is more similar to their efficiency at high speeds. This translates into the ability to generate lots of low-RPM torque to get you moving from a stand still or accelerating at low speeds.

The on-board computer is programmed to understand this and it uses the electric motor when it’s most efficient (lower speeds) and the ICE when it’s most efficient (higher speeds, or when it’s simply needed to charge the batteries a little) but often mixing the two (especially in Toyota’s “hybrid synergy drive”–which the example above, the Prius, uses in more recent model years). This also has the nice side effect of letting you get away with a smaller, lighter, more fuel efficient, less polluting ICE.

All of these factors combine to make hybrids more fuel efficient and less polluting than an ICE-powered car of the same size and weight.