Hybrid Car

How hybrid cars work

Hybrid cars are constantly in the news right now, and for a good reason – they get great gas mileage. But have you ever thought about where the great mileage comes from? Last week I received a question from a reader that gets at the essence of hybrid car technology. The question was, “how can a hybrid car be any more efficient than a normal car, given that the engine has to charge the batteries?” In other words, since the engine has to burn gas to charge the battery anyway, why doesn’t the mileage of a hybrid car work out to be the same as a normal car?

To understand the full meaning of this question, it is helpful to explore the technology of hybrid cars for one minute. We can use the Toyota Prius as an example. The Prius has both a small gasoline engine and an electric motor. There is also a battery pack for the motor, a special transmission that lets the engine and the motor work together, and a computer that controls the whole thing.

When you are sitting still in a Prius, for example at a traffic light, the gasoline engine does not need to run at all. By eliminating the idling, you automatically save some gas. If you are traveling slowly in a Prius, for example inching forward in bumper-to-bumper traffic, the electric motor can do the driving and the engine does not need to run. Once you get above 15 MPH, the gas engine kicks in. Also, if you drive for more than a few minutes in bumper-to-bumper traffic, the engine will kick in. In that case the engine is spinning a generator that recharges the batteries.

That last part – the “recharging” part – lies at the heart of the reader’s question. If the engine has to run anyway to charge the batteries, then where is the advantage? It’s not like the electric motor is moving you for free. And since an engine driving a generator to charge batteries is going to add some waste to the process (both the generator and batteries will lose some power through friction and heat), it seems like the hybrid should get worse mileage than a normal car.

Here is how the Hybrid gains an advantage. First of all, a hybrid car does not ever need to idle. That in itself safes some gas, especially in city driving, because idling is a total waste. The batteries in the Prius can power the air conditioner and the radio when the car is sitting still. Then you have to understand that bumper-to-bumper driving is a huge gas-waster for a normal engine. When you rev the engine up to travel at 5 miles per hour and then stop again, you create the worst possible scenario for a gasoline engine. There is no better way to waste gasoline.

When you drive a hybrid car in bumper-to-bumper traffic and it is time to recharge the batteries, the engine can run at a consistent, optimal speed to spin the generator. There is no revving and stopping. So the engine is always being used as efficiently as possible. That makes a hybrid car like the Prius much more efficient in city driving.

So there are two things automatically working in favor of a hybrid car: 1) no idling, and 2) no more revving and stopping, which means highly efficient engine operation at all times. Now you add to that the fact that the gasoline engine in a hybrid car can be very small and light compared to a normal car engine. That makes the engine more efficient all the way around. Then you add to that the fact that most hybrid cars like the Prius are very aerodynamic. They also have special tires, etc. to make them more efficient. Many hybrid cars use regenerative braking to capture the car’s energy during braking and store it in the batteries. Finally, there is the car’s computer and display. The display tells the driver when he or she is wasting gas (for example, by accelerating too fast), and this helps the driver do a better job.

When you add all of these things together, you get a car that has much better mileage than a normal car. This is why a Hybrid car like a Prius is able to save so much gasoline.

Hybrid Cars