How do hybrids get such great gas mileage?

Answer 1

If one asks where the increased mileage comes from, the temptation is to say "from the battery".

But there is a big problem with that.

Namely, where does the battery get it's energy from?

If you answer that only by saying "from the engine", you have a problem.

The engine gets it's energy from gasoline, and gasoline has a finite amount of energy. Portions of that finite amount can be used for propelling the car, while other portions can be used for charging the battery, but no portion does both. If it did, the total amount of energy being used would be greater than the amount of energy in the gasoline.

If the energy used were greater than the energy available, hybrid cars would be perpetual motion machines, violating the law of conservation of energy.

So where does the energy come from?

It all comes from reducing energy loss. Some of this reduction comes from disconnecting the drive train, when cruising on a flat surface or a downward slope, which keeps the engine from sapping the energy of momentum. Some of it also comes from turning the engine off when the drive train has been disconnected.

Where do the majority of the energy savings come from, though?

As you know, a car in motion has a huge amount of kinetic energy. That's why a car that's "in neutral" doesn't just come to a stop when it hits something. It tries to "barrel through" what it hits, converting it's kinetic energy (or momentum) into damage to both the hitter and the hittee. That's the least desirable way to slow the car down (but if a TV villain had sabotaged your car, making it your only alternative, hitting a tree is better than flying off a cliff).

In a normal car, to slow the car down, you either do so by braking, whereby the friction of the brakes converts that energy to heat, or you do so by "engine braking", which uses low gear to convert the kinetic energy to a racing of the engine.

Compared to never having to brake at all, drivers using either of those two methods of slowing a non-hybrid car down inevitably reduce the car's potential maximum MPG by up to 50%. In other words, the energy is just wasted.

While there is no way to put the energy of brake friction back into the gasoline, car makers learned to do the equivalent of engine braking using a DC motor as an electric generator, instead of the car's engine, to capture that energy instead of throwing it away.. This puts a great deal of the energy otherwise lost to braking into the hybrid's battery.

But a non-hybrid car can't do that. In an ordinary car, a heavy foot can reduce the maximum potential MPG by half. This is especially evident in slow traffic, when drivers closely approach (aka "tailgate) the car ahead of them, instead of trying to maintain a safety cushion of several car lengths. The heavy footed driver tries to maintain only a short distance from the car ahead. Maintaining such a short distance, the following is inevitable:

• Call yourself car 1, the car ahead of you car 2, and the car ahead of him car 3.
• Car 1 is barely maintaining a car length from car 2, who is maintaining the same from car 3.
• When car 3 brakes, car 2 must brake hard to keep from hitting car 3.
• Seeing car 2's brake lights, car 1 must brake hard to keep from hitting car 2.
• Car 1 comes to a stop.
• Up ahead, car 3 (who had been momentarily distracted while stopped) notices that the traffic ahead of him is moving. He accelerates to catch up to the car ahead.
• Then, car 2 accelerates to catch up with car 3.
• Seeing that car 2 is accelerating, car 1 accelerates to match car 2, momentarily relieved that traffic isn't stop and go any more. Then, since all the traffic ahead of car 3 is only averaging 10 mph, and he's tailgating that traffic, as soon as car 3 sees a brake light, he brakes too, and comes to a stop.
• Car 2, seeing car 3 brake and come to a stop, must do the same to avoid hitting car 3.
• Car 1 now must brake hard and come to a stop to avoid hitting car 2. Thus, car 1 (like car 2 and car 3) is continually alternating between

  A) accelerating to catch up to a car ahead (that is soon going to come to a stop) and

  B) having caught up to the car ahead, braking to avoid colliding with it (and coming to a stop).

• Meanwhile, yours truly is keeping several car lengths behind car 1, and has plenty of room to come to a stop if necessary, but rarely needs to because as he gets closer to car 1, car 1 has started accelerating (from a stop) again. We're both maintaining the same average speed, but I'm relaxed, while car 1 is tearing his hair out. Not only that, but the car who is tailgating ME does so at a constant speed and does not do the accelerate/brake seesaw. The same goes for many of the cars behind him. Not only am I *wasting much less gasoline* than the people in front of me, so are the cars behind me!
• Thus, good driving can help a non-hybrid come closer to the mpg of a hybrid, but can't bring it all the way there, since there is no way of recycling the braking energy.

Why do so many drivers insist on not maintaining a good cushion in stop and go traffic? It's not going to get them where they are going any faster. Instead it both makes the traffic BE stop and go, and increases the chance of collision! Some aggressive drivers tailgate deliberately to prevent other drivers from changing lanes into the space in front of them, perhaps because it will make them be later to wherever they are going by the amount of time it takes for a car to go ten feet. For some drivers, the reason may be herd mentality. Others do it, so they must too. What reasons others may have, I don't know.

When drivers follow the stop and go herd in stop and go traffic, it's difficult not to mentally picture the gasoline being burned to produce the acceleration they need to match the momentary speed of the gas waster in front of them, and then a couple of seconds later see all that newly introduced kinetic energy being turned into heat by sudden flooring of the brakes to avoid the collisions that would otherwise arise from being too close to the car ahead.

The above is meant to illustrate the following: Drivers who maintain an adequate distance and match the overall average speed almost never have to brake, and thus can get highway mileage, rather than city mileage, in stop and go traffic. In a car that can also intelligently disconnect (and/or shut off) the engine, good driving can even let one approach hybrid MPG's.

This also explains why hybrids can get better mileage in the city driving than on the highway: braking more often can charge the battery more frequently unless the driver has a heavy foot on the brake pedal, in which case the brakes themselves have to be engaged, which wastes energy. Hybrid drivers actually get a bit of a reward for bad driving, unless they have lead feet!

The simple act of driving defensively by keeping a good distance not only improves gas mileage, but in a hybrid car, can even make it possible for one to never need to replace brake pads. With regenerative braking, the brake pads get much less wear than they would otherwise (but because regenerative braking cannot quickly bring a car to a stop, when a hybrid driver hits the brakes hard, the car must use the actual brakes instead of regenerative braking). So, while hybrids do have advantages, they can't fully substitute for leaden feet.