Your turn
Each lab is one step from the video. Press the buttons and see where the energy goes.
Each lab isolates one part of the system. The models are simplified to show the idea; real switch points vary by car.
Tap the buttons and slide the sliders.
Swap between a normal car and a hybrid while you wait. Which one keeps burning fuel?
A conventional engine burns fuel just to idle. Stop-start shuts it off and the battery runs the accessories.
Tap the two cars. Which one uses up gas while it waits?
Tap each part of the cutaway car. What does it do?
Engine, fuel tank, motor-generator, traction battery and the controller that coordinates them.
Tap a part to find out what it does.
Speed up, then brake. Compare normal brakes with regenerative braking. Where does the energy go?
Kinetic energy ½mv² has to go somewhere: into heat at the brakes, or partly into the battery through the generator.
Go fast, then stop. Which brakes fill up the battery?
Regeneration recaptures part of the energy, not all of it; the split shown is illustrative.
Slide the speed up and down, and add a hill. When is the gas engine on?
Engines are least efficient at idle and low speed, so the controller avoids running them there.
The switch points shown are illustrative; every car and driver differs.
Pull away, cruise, climb and head downhill. Watch where the energy goes, and the battery.
Watch the flows reverse: battery → motor → wheels when climbing, wheels → generator → battery going downhill.
Drive up and down the hills. When does the battery fill up?
Pick a car. Where does its energy come from? Does it need a plug?
Compare fuel source, charging and regenerative braking across a hybrid, a plug-in hybrid and an electric car.
Tap each car. Which one needs a plug?
A hybrid car has two ways to go. It has an engine that uses gas, and an electric motor that uses a battery. A little computer picks which one to use.
When a hybrid stops at a red light, its engine turns off. Shh! It is not wasting gas while it waits. When the light turns green, the motor can pull it away, nice and quiet.
A hybrid has a battery, but nobody plugs it in. So how does the battery fill up?
A moving car has lots of go-go energy. In most cars, the brakes turn that energy into heat, and it is wasted. When a hybrid slows down, its wheels spin the motor. The motor makes electricity, and the battery fills up!
A hybrid saves the most when it stops and goes a lot, like in town. All its energy still comes from gas. It just wastes less.
A hybrid car has two ways to turn its wheels: a regular gas engine and an electric motor that runs on a battery.1 A computer decides, moment by moment, whether the engine, the motor, or both should do the work.115
The clever part is where the electricity comes from. A regular hybrid is never plugged in.1 Instead, it catches energy that a normal car throws away every time it slows down, using regenerative braking, and it can also use the engine to top up the battery.2 Then it spends that electricity where a gas engine is weakest: pulling away, crawling in traffic, and sitting at a red light.3
So a hybrid doesn't run on a different fuel. It runs on gasoline, but wastes less of it.
The U.S. Department of Energy lists the key parts of a hybrid electric vehicle (HEV):1
| Part | What it does |
|---|---|
| Gas engine | Burns gasoline to drive the wheels, just like in a normal car |
| Fuel tank | Stores the gasoline. In a regular hybrid, all the car's energy starts here |
| Electric traction motor | Drives the wheels using battery power |
| Generator | Makes electricity from the turning wheels while braking. Many hybrids use one "motor-generator" for both jobs |
| Traction battery pack | Stores electricity for the motor |
| Power electronics controller | Manages the flow of electricity and controls the motor's speed and push |
| Transmission | Passes power from the engine and/or the motor to the wheels |
Hybrid batteries run at high voltage, typically somewhere between 100 and 600 volts, so they're sealed in tough cases and their cables are colored bright orange as a warning.12
A moving car carries energy of motion, called kinetic energy. The heavier the car and the faster it goes, the more it has (kinetic energy = ½ × mass × speed²).16 To stop, that energy has to go somewhere.
In a normal car, the brake pads squeeze discs on the wheels, and the energy turns into heat through friction. It's simply lost.5
A hybrid first tries a smarter brake. The moving wheels turn the electric motor, so it works as a generator, a bit like the dynamo that powers a bike light. Making electricity takes effort, so this slows the car down, and the electricity is stored in the battery.23 This happens when you brake and also when you coast.3 Later, the car uses the same electricity to drive the motor.4
The battery can also be charged by the engine.1 That isn't free energy (it still comes from gasoline), but it lets the car store power when the engine is running efficiently and use it later.
braking energy → generator → battery → motor → wheels
A bonus: because the motor does much of the slowing, hybrid brakes generally last longer than those on conventional cars.12
Gas engines are surprisingly wasteful. In a regular gasoline car, only about 12–30% of the energy in the fuel actually moves the car down the road. Most of the rest is lost in the engine, mainly as heat.5 Engines are least efficient at low speeds,3 and an idling engine gets zero miles per gallon.14 Hybrids use three tricks to avoid those bad moments:3
Thanks to all this, about 24–38% of a hybrid's fuel energy moves the car, compared with 12–30% for a conventional gas car.45
Full hybrids have bigger batteries and stronger motors, and can drive on electricity alone for short distances at low speeds. Mild hybrids use a smaller battery and motor to help the engine and switch it off at stops, but they can't drive on electricity alone.2
In a parallel hybrid, the most common design, both the engine and the motor are connected to the wheels and can drive them directly. In a series hybrid, only the motor turns the wheels, and the engine just makes electricity. That design is more common in plug-in hybrids.28
Hybrids save the most fuel in stop-and-go city driving, where there's lots of braking to recapture and lots of idling to avoid. On a long, steady highway drive there's little braking and no idling, so the Department of Energy says hybrids offer little advantage over conventional vehicles there.4 On the highway the gas engine does most of the work, with the motor helping out when you speed up or climb.15
Weather matters too. In cold weather (20°F compared with 77°F), a hybrid's city fuel economy can drop about 30–34%, partly because a cold battery can't take in as much braking energy.14
| Hybrid (HEV) | Plug-in hybrid (PHEV) | Electric (EV) | |
|---|---|---|---|
| Energy source | Gasoline only | Electricity from the grid + gasoline | Electricity only |
| Can you plug it in? | No | Yes (and it still works if you don't) | Yes, and it must be |
| Gas engine? | Yes | Yes | No |
| Battery | Small | Larger | Large |
| Electric-only driving | Short distances at low speed (full hybrids) | About 15 to 60-plus miles in current models | Always; typical range about 150–300 miles |
| Regenerative braking? | Yes | Yes | Yes |
Sources: AFDC and fueleconomy.gov.2691011
A plug-in hybrid usually runs on electricity until its battery is nearly empty, then switches to the engine automatically.7 If you never plug it in, you get about the same fuel economy as a similar regular hybrid, so charging it regularly is how you get the most out of it.6 Plug-in hybrids use roughly 30–60% less petroleum than conventional cars.8
An all-electric car has no engine, no fuel tank and no tailpipe. It must be plugged in to charge.9
| Advantages | Disadvantages |
|---|---|
| Better fuel economy without losing performance2 | Often cost more to buy than similar gas cars2 |
| No need to find a charger1 | Smaller savings on steady highway drives4 |
| Brakes generally last longer12 | Fuel economy drops more than a gas car's in the cold14 |
| Quiet at low speeds and at stops15 | A battery replacement outside warranty can be expensive12 |
Hybrid batteries are generally designed to last the lifetime of the car.12 Many hybrids have used nickel-metal hydride batteries, while most plug-in hybrids and electric cars use lithium-ion.13
A hybrid has a gas engine and an electric motor with a battery. When the car slows down, the motor works as a generator and stores energy in the battery that normal brakes would waste as heat. The car then uses that electricity to help pull away, to drive slowly on electricity, and to let the engine switch off when it's stopped.
No. A regular hybrid can't be plugged in at all. Its battery is charged by regenerative braking and by the gas engine. Only plug-in hybrids and fully electric cars have a charging port.
Yes. It charges whenever you brake or coast (regenerative braking), and the engine can also top up the battery by spinning the generator. All of that energy started out as gasoline, though: the trick is that the hybrid wastes less of it.
It depends on the car and how hard you press the pedal. In general, full hybrids can drive on electricity alone at low speeds and for short distances, and the engine takes over or joins in when you need more power, such as on the highway, when accelerating hard, or going uphill. The car's computer decides.
In the city. Stop-and-go driving has lots of braking to recapture and lots of idling to avoid. On a steady highway there's little braking and no idling, so the U.S. Department of Energy says hybrids offer little advantage over conventional cars there.
A hybrid runs on gas only and uses a small battery to recycle braking energy. A plug-in hybrid has a bigger battery you can charge from the wall, so it can drive roughly 15 to 60-plus miles on electricity before switching to gas. An electric car has no gas engine and must be plugged in.
Hybrids often cost more to buy than similar gas cars, though fuel savings can win some of that back. Their savings are smaller on long highway drives, and cold weather can cut a hybrid's city fuel economy by about 30% or more. Battery replacement outside warranty can be expensive, although the batteries are generally designed to last the life of the car.