Introduction to How Hybrid Cars Work
Have you pulled your car up to the gas pump lately and been shocked by the high price of gasoline? As the pump clicked past $20, $30, $40 or even $50, maybe you thought about trading in your car for something that gets better mileage. Or maybe you're worried that your car is contributing to the greenhouse effect.
The auto industry has the technology to address these concerns. It's the hybrid car. There are a lot of hybrid models on the market these days, and most automobile manufacturers have announced plans to manufacture their own versions.
How does a hybrid automobile work? What goes on under the hood to give you 20 or 30 more miles per gallon than the standard automobile? And does it pollute less just because it gets better gas mileage? In this article, we'll help you understand how this technology works, and we'll even give you some tips on how to drive a hybrid car for maximum efficiency.
Many people have probably owned a hybrid vehicle at some point. For example, a mo-ped (a motorized pedal bike) is a type of hybrid because it combines the power of a gasoline engine with the pedal power of its rider. In fact, hybrid vehicles are all around us. Most of the locomotives we see pulling trains are diesel-electric hybrids. Cities like Seattle have diesel-electric buses -- these can draw electric power from overhead wires or run on diesel when they are away from the wires. Giant mining trucks are often diesel-electric hybrids. Submarines are also hybrid vehicles -- some are nuclear-electric and some are diesel-electric. Any vehicle that combines two or more sources of power that can directly or indirectly provide propulsion power is a hybrid. Most hybrid cars on the road right now are gasoline-electric hybrids, although French car maker PSA Peugeot Citroen has two diesel-electric hybrid cars in the works. Since gasoline hybrids are the kind you'll find at your local car dealership, we'll focus on those in this article. Gasoline Power vs. Electric Power
The gasoline-electric hybrid car is just what it sounds like -- a cross between a gasoline-powered car and an electric car. Let's start with a few diagrams to explain the differences between a gasoline-powered car and a typical electric car.
A gas-powered car has a fuel tank, which supplies gasoline to the engine. The engine then turns a transmission, which turns the wheels.
An electric car, on the other hand, has a set of batteries that provides electricity to an electric motor. The motor turns a transmission, and the transmission turns the wheels.
The hybrid is a compromise. It attempts to significantly increase the mileage and reduce the emissions of a gas-powered car while overcoming the shortcomings of an electric car.
To be useful to you or me, a car must meet certain minimum requirements. The car should be able to:
- Drive at least 300 miles (482 km) before re-fueling
- Be refueled quickly and easily
- Keep up with the other traffic on the road
A gasoline car meets these requirements but produces a relatively large amount of pollution and generally gets poor gas mileage. An electric car, however, produces almost no pollution, but it can only go 50 to 100 miles (80 to 161 km) between charges. And the problem has been that the electric car is very slow and inconvenient to recharge.
A gasoline-electric car combines these two setups into one system that leverages both gas power and electric power.
Gasoline-electric Hybrid Structure
Gasoline-electric hybrid cars contain the following parts:
- Gasoline engine - The hybrid car has a gasoline engine much like the one you will find on most cars. However, the engine on a hybrid is smaller and uses advanced technologies to reduce emissions and increase efficiency.
- Fuel tank - The fuel tank in a hybrid is the energy storage device for the gasoline engine. Gasoline has a much higher energy density than batteries do. For example, it takes about 1,000 pounds of batteries to store as much energy as 1 gallon (7 pounds) of gasoline.
- Electric motor - The electric motor on a hybrid car is very sophisticated. Advanced electronics allow it to act as a motor as well as a generator. For example, when it needs to, it can draw energy from the batteries to accelerate the car. But acting as a generator, it can slow the car down and return energy to the batteries.
- Generator - The generator is similar to an electric motor, but it acts only to produce electrical power. It is used mostly on series hybrids.
- Batteries - The batteries in a hybrid car are the energy storage device for the electric motor. Unlike the gasoline in the fuel tank, which can only power the gasoline engine, the electric motor on a hybrid car can put energy into the batteries as well as draw energy from them.
- Transmission - The transmission on a hybrid car performs the same basic function as the transmission on a conventional car. Some hybrids, like the Honda Insight, have conventional transmissions. Others, like the Toyota Prius, have radically different ones, which we'll talk about later.
You can combine the two power sources found in a hybrid car in different ways. One way, known as a parallel hybrid, has a fuel tank that supplies gasoline to the engine and a set of batteries that supplies power to the electric motor. Both the engine and the electric motor can turn the transmission at the same time, and the transmission then turns the wheels.
The animation below shows a typical parallel hybrid. You'll notice that the fuel tank and gas engine connect to the transmission. The batteries and electric motor also connect to the transmission independently. As a result, in a parallel hybrid, both the electric motor and the gas engine can provide propulsion power.
By contrast, in a series hybrid, the gasoline engine turns a generator, and the generator can either charge the batteries or power an electric motor that drives the transmission. Thus, the gasoline engine never directly powers the vehicle.
Take a look at the diagram of the series hybrid, starting with the fuel tank, and you'll see that all of the components form a line that eventually connects with the transmission.
The structure of a hybrid car harnesses two sources of power to increase efficiency and provide the kind of performance most of us are looking for in a vehicle. In the next section, we'll see how it accomplishes this.
The key to a hybrid car is that the gasoline engine can be much smaller than the one in a conventional car and therefore more efficient. Most cars require a relatively big engine to produce enough power to accelerate the car quickly. In a small engine, however, the efficiency can be improved by using smaller, lighter parts, by reducing the number of cylinders and by operating the engine closer to its maximum load.
There are several reasons why smaller engines are more efficient than bigger ones:
- The big engine is heavier than the small engine, so the car uses extra energy every time it accelerates or drives up a hill.
- The pistons and other internal components are heavier, requiring more energy each time they go up and down in the cylinder.
- The displacement of the cylinders is larger, so more fuel is required by each cylinder.
- Bigger engines usually have more cylinders, and each cylinder uses fuel every time the engine fires, even if the car isn't moving.
This explains why two of the same model cars with different engines can get different mileage. If both cars are driving along the freeway at the same speed, the one with the smaller engine uses less energy. Both engines have to output the same amount of power to drive the car, but the small engine uses less power to drive itself. But how can this smaller engine provide the power your car needs to keep up with the more powerful cars on the road?
Let's compare a car like the Chevy Camaro, with its big V-8 engine, to our hybrid car with its small gas engine and electric motor. The engine in the Camaro has more than enough power to handle any driving situation. The engine in the hybrid car is powerful enough to move the car along on the freeway, but when it needs to get the car moving in a hurry, or go up a steep hill, it needs help. That "help" comes from the electric motor and battery -- this system steps in to provide the necessary extra power.
The gas engine on a conventional car is sized for the peak power requirement (those few times when you floor the accelerator pedal). In fact, most drivers use the peak power of their engines less than one percent of the time. The hybrid car uses a much smaller engine, one that is sized closer to the average power requirement than to the peak power. Improving Fuel Economy
Besides a smaller, more efficient engine, today's hybrids use many other tricks to increase fuel efficiency. Some of those tricks will help any type of car get better mileage, and some only apply to a hybrid. To squeeze every last mile out of a gallon of gasoline, a hybrid car can:
The Benefits of a Hybrid Car
- Recover energy and store it in the battery - Whenever you step on the brake pedal in your car, you are removing energy from the car. The faster a car is going, the more kinetic energy it has. The brakes of a car remove this energy and dissipate it in the form of heat. A hybrid car can capture some of this energy and store it in the battery to use later. It does this by using "regenerative braking." That is, instead of just using the brakes to stop the car, the electric motor that drives the hybrid can also slow the car. In this mode, the electric motor acts as a generator and charges the batteries while the car is slowing down.
- Sometimes shut off the engine - A hybrid car does not need to rely on the gasoline engine all of the time because it has an alternate power source -- the electric motor and batteries. So the hybrid car can sometimes turn off the gasoline engine, for example when the vehicle is stopped at a red light.
- Use advanced aerodynamics to reduce drag - When you are driving on the freeway, most of the work your engine does goes into pushing the car through the air. This force is known as aerodynamic drag. This drag force can be reduced in a variety of ways. One sure way is to reduce the frontal area of the car. Think of how a big SUV has to push a much greater area through the air than a tiny sports car. Reducing disturbances around objects that stick out from the car or eliminating them altogether can also help to improve the aerodynamics. For example, covers over the wheel housings smooth the airflow and reduce drag. And sometimes, mirrors are replaced with small cameras. ?
- Use low-rolling resistance tires - The tires on most cars are optimized to give a smooth ride, minimize noise, and provide good traction in a variety of weather conditions. But they are rarely optimized for efficiency. In fact, the tires cause a surprising amount of drag while you are driving. Hybrid cars use special tires that are both stiffer and inflated to a higher pressure than conventional tires. The result is that they cause about half the drag of regular tires.
- Use lightweight materials - Reducing the overall weight of a car is one easy way to increase the mileage. A lighter vehicle uses less energy each time you accelerate or drive up a hill. Composite materials like carbon fiber or lightweight metals like aluminum and magnesium can be used to reduce weight.
You might wonder why anyone would build such a complicated machine when most people are perfectly happy with their gasoline-powered cars. The reason is twofold: to reduce tailpipe emissions and to improve mileage. These goals are actually tightly interwoven.
Let's take the example of the California emissions standards, which dictate how much of each type of pollution a car is allowed to emit in California. The amount is usually specified in grams per mile (g/mi). For example, the low emissions vehicle (LEV) standard allows 3.4 g/mi of carbon monoxide. The key thing here is that the amount of pollution allowed does not depend on the mileage your car gets. But a car that burns twice as much gas to go a mile will generate approximately twice as much pollution. That pollution will have to be removed by the emissions control equipment on the car. So decreasing the fuel consumption of the car is one of the surest ways to decrease emissions.
Carbon dioxide (CO2) is another type of pollution a car produces. The U.S. government does not regulate it, but scientists suspect that it contributes to global warming. Since it is not regulated, a car has no devices for removing CO2 from the exhaust. A car that burns twice as much gas adds twice as much CO2 to the atmosphere.
Auto makers in the United States have another strong incentive to improve mileage. They are required by law to meet Corporate Average Fuel Economy (CAFE) standards. The current standards require that the average mileage of all the new cars sold by an auto maker should be 27.5 mpg (8.55 liters per 100 km). This means that if an auto maker sells one hybrid car that gets 60 mpg (3.92 liters per 100 km), it can then sell four big, expensive luxury cars that only get 20 mpg (11.76 liters per 100 km).
You can actually take steps to drive your car in ways that increase its gas mileage. In the next section, we'll look at some tips for increasing the efficiency of your hybrid (or just gas-powered) car.
You can get the best mileage from a hybrid car by using the same kind of driving habits that give you better mileage in your gasoline-engine car:
- Drive slower - The aerodynamic drag on the car increases dramatically the faster you drive. For example, the drag force at 70 mph (113 kph) is about double that at 50 mph (81 kph). So, keeping your speed down can increase your mileage significantly.
- Maintain a constant speed - Each time you speed up the car, you use energy, some of which is wasted when you slow the car down again. By maintaining a constant speed, you will make the most efficient use of your fuel.
- Avoid abrupt stops - When you stop your car, the electric motor in the hybrid acts like a generator and takes some of the energy out of the car while slowing it down. If you give the electric motor more time to slow the vehicle, it can recover more of the energy. If you stop quickly, the brakes on the car will do most of the work of slowing the car down, and that energy will be wasted. The same reasoning applies to gasoline-powered cars: Abrupt stops waste a lot of energy.