Shots Across The Bow

Hybrid cars...that's the ticket!

This week's issue of the Metro Pulse has a story on hybrid cars, the ones that run on both electric and internal combustion motors. They made an attempt at explaining the technology, but they are limited to a few hundred words, where I, you lucky readers, can go on at length.

And I will.

Regular readers know that I am against the concept of hydrogen powered, or fuel cell cars, because they don't save energy at all. In fact, as I demonstrated, they actually use more energy due to transformation losses.

So, you're probably thinking that I won't like hybrid cars either.

Wrong. Hybrid cars reduce many of the inefficiencies of the IC engine, without the ancillary costs of a fuel cell. Let's look a little deeper, and you'll see what I'm talking about.

We have to start with mileage, because that's the measure of efficiency of a car. There are several factors that affect your mileage, some designed, which you can't control, and others operational, which you can. Examples of the first include gear ratios, tire sizes, and body aerodynamics, while examples of the latter include velocity, magnitude of acceleration, magnitude of deceleration, and routine maintenance. Let's look at the engineered factors first.

Body aerodynamics and gear ratios are the two biggest factors affecting your mileage. The aerodynamics determines how hard it is to push your car through the air, and the gear ratio determines how hard the motor has to work to push the car. Aerodynamically, the smoother the air flow is around your car, the less resistance, or drag is produced, and the easier it is for your car to slide through the air. Automakers spend millions on wind tunnel tests, determining which body designs provide the smoothest air flow.

Gear ratios determine how efficiently the power of the engine is converted into thrust. The selection of a gear ratio is always a compromise between torque, speed, and economy. A very economical gear ratio will have a limited top end, and minimal torque. Conversely, high torque or high speed reduces efficiency. Car makers try to achieve the optimum balnce by providing multiple gears, which can provide low end torque and high end speed, while still giving decent fuel economy.

If you've ever driven a stick shift, you know what I mean. I drive a 4 cylinder 1.6 liter Tracker with a 5 speed stick. Depending on how I choose my shift points, my mileage can vary by .75mpg. An automatic transmission has programmed shift points; the auto maker selects those points based on his target buyer. A Corvette with automatic (sacrilege, but I'm just making a point) will shift differently than a Yugo. (Boy, will it shift differently!)

Now, each gear has a performance curve associated with it, also called a power band. Basically, this curve represents how efficiently the power from the engine is converted to thrust. For each gear ratio, there is a point where maximum power is transferred to the wheels. Once you hit this peak, any additional energy input, ie standing on the throttle, produces diminishing returns. Returning to my Tracker, if I increase my average speed on the Interstate from 70 to 75mph, my mileage decreases by about .5 mpg. (Yes, I track these numbers. I am a nerd.)

Now we've seen how different factors affect our fuel economy. So what do hybrids do to help us?

Well there are a couple of things. IC engines are most efficient when you are running in the power band, and running steady state. This is why highway mileage is significantly higher than city. It isn't sitting at lights the burns the gas; it's the starting and stopping. Think about it; as you accelerate, you spend most of that time outside of the power band, at lower than optimal efficiency. If you accelerate quickly, you're even further from the optimum, resulting in even worse efficiency.

But what about braking? You're not giving the engine any gas; how is that inefficient?

One of the factors which makes operating at steady state so efficient is that the car is acting sort of like a battery, storing energy in the form of momentum. This goes back to the first of Newton's Laws of Motion, that an object in motion tends to remain in motion. Most of the energy produced by the engine during acceleration is stored in the car's momentum. Once you've reached cruising speed, the engine only has to produce enough energy to overcome friction and drag. The problem with braking is you take all that lovely stored energy, and waste it as heat. You get no benefit out of it.

Here's where the hybrid comes in. Instead of friction braking, a hybrid uses electromagnetic braking.

I can see your eyes glazing over.

It's not that complicated. Here's how it works. A while back, some smart people found out that if you passed electricity through a wire, you generated a magnetic field. Then they found out that if you wrapped the wire in a certain way, the magnetic field would rotate. Stick a chunk of iron or a magnet in the middle of this rotating field, and you've just converted electrical energy into mechanical energy, and made the worlds first motor. Pretty cool, but it gets better.

Another smart guy wondered what would happen if you went backward, and moved a wire through a magnetic field. He was shocked to find out that it generated electricity. (Sorry, it's late and I couldn't resist...)

Now here is where is gets a little confusing. Run a wire through a magnetic field, and you generate a current through the wire. But now that you have a current running through a wire, you're generating another magnetic field, only this one is opposed to the first. This magnetic field is called a Counter electro motive force, or cemf for short.

And now we're back to the electromagnetic braking. When you hit the brakes on a hybrid, you get some conventional friction braking, but you also cut in a coil surrounding a magnet on the axle. The magnet creates a field which moves around the coil, generating electricity. This electricity is stored in the hybrids battery, for whenever it is needed next. This is just too cool, because a lot of that lovely energy that used to be wasted is saved to be used again later, greatly increasing the efficiency of the car.

Now we're getting towards the end, and if you've stayed with me this far, you're almost home. (there will be a test at the bash, so please pay close attention)

OK, so we're saving energy by electromagnetic breaking. Where does this energy get used?

Well, that depends on the hybrid your talking about. All systems use the energy to supplement the IC engine when it isn't running at optimal efficiency. Some do so during acceleration, others do it at low speeds, others a combination of both. In any case, you have a double benefit; you have an assist when IC is inefficient, and you capture a large portion of energy formerly wasted in breaking.

It truly is a win-win situation.

One last point before I go to sleep. An efficient IC engine is a clean running engine. By augmenting or replacing the IC engine when it can't run at peak efficiency, the hybrid reduces emissions even more than the improvements in economy would suggest.

So, it runs better, cleaner, and cheaper. It's a little more up front, but you'll recover that and more in fuel savings over the life of the car.

My next car will be a hybrid.

Good night.

Posted by Rich at February 25, 2003 12:34 AM | TrackBack