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Monday, Apr 21, 2008 10:53 AM UTC

Ask Pablo

Considering the energy used in auto manufacturing, is it better to keep my old car or buy a new hybrid?

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Topics: Environment, Energy, Science, Auto Industry,

Dear Pablo,

Is it environmentally better to keep my 1986 Mercedes-Benz W126 or buy a new hybrid?

This is a question I have gotten a lot, and one that I have wondered about myself. You see a modern-day tie-dye aficionado puttering along the highway in his VW van with black smoke spewing out the back, and you have to wonder if we wouldn’t all be better off if he traded it in for a Prius. The consensus among some environmentalists — perhaps ones who drive late-’60s Mustangs — seems to be that driving your old car creates significantly less pollution than the manufacture of a new car. I wish it were that easy.

The Argonne National Lab, a U.S. Department of Energy research center, has analyzed the material intensity and energy consumption of manufacturing vehicles and vehicle fuels. Their work is packaged in GREET models (for greenhouse gases, regulated emissions and energy use in transportation). According to the models, the average conventional internal combustion engine vehicle is made up of 61.7 percent steel, 11.1 percent iron, 6.9 percent aluminum, 1.9 percent copper/brass, 2.9 percent glass, and around 13.6 percent plastic/rubber. This information helps determine the energy required to produce a vehicle.

The energy required can be measured in British thermal units. A Btu is the amount of energy needed to raise the temperature of a pound of water by one degree Fahrenheit. According to the GREET model, it takes 100.391 million Btus to make the vehicle, batteries and fluids in an average 3,201-pound vehicle. This comes out to 31,362 Btus per pound. The obvious lesson is that, in general, heavier vehicles require more energy to make than lighter vehicles.

It’s been said that hybrids are more environmentally damaging than large SUVs because of the battery production, but this has been widely disputed. According to GREET, a hybrid electric vehicle (HEV) that weighs 2,632 pounds requires 101.726 million Btus to make, or 38,650 Btus per pound (compared to 31,362 for a conventional vehicle). As we will see, this small difference in production energy becomes negligible when you factor in the increased fuel efficiency.

Using the GREET assumptions, we can compare several vehicles: a Hummer H2, a Toyota Prius and your 1986 Mercedes-Benz W126. I will use the vehicle’s published curb weight to determine the energy used in manufacturing, based on the Btu/pound factors above, and I will use the average MPG (city and highway) to estimate fuel usage over a 160,000-mile lifespan. The energy required to manufacture the vehicles is:

  • 1986 Mercedes-Benz W126: 114.440 million Btus
  • Hummer H2: 200.717 million Btus
  • Toyota Prius: 113.322 million Btus

Now, burning gasoline generates 113,500 Btus (0.1135 million Btus) of energy per gallon. By dividing the expected lifespan of a vehicle (160,000 miles) by its average MPG, we can determine the number of gallons of gasoline used over that lifetime. We can also multiply this by the energy content of the fuel to get the total energy used. The gallons used during a 160,000-mile lifespan, and the energy contained therein, is:

  • 1986 Mercedes-Benz W126: 5,333 gallons, 605.296 million Btus
  • Hummer H2: 13,913 gallons (over $45,000 at today’s prices!), 1,579.13 million Btus
  • Toyota Prius: 2,883 gallons, 327.221 million Btus

So looking at the energy consumed in both manufacturing and burning gasoline, will you be better off keeping your old Mercedes?

Given that your car is already built, we can write off the energy used in making it. We can also write off the emissions that it has already created from burning gasoline. That means that over the next 116,000 miles, your car’s greenhouse gas emissions will essentially break even with the emissions from the production and use of a Prius. I’m guessing your 22-year-old car probably has over 200,000 miles on it. If you’re lucky, you can get another few years out of it. So if you can afford a new Prius, you are better off switching now. And think of the fewer hassles of owning a new car.

Another thing to keep in mind are the nitrogen oxides produced by cars. Nitrogen oxides are a key component of smog, which as we know contributes to asthma, heart disease and many other health and environmental concerns. In the early ’80s, the nitrogen oxide standard for cars was 1 gram per mile, or 12 kg per year, whereas cars are now required to achieve lower than 0.4 grams per mile, or 4.8 kg per year. Cars older than your Mercedes are even dirtier. A new vehicle like a Prius can reduce toxic air emissions of pre-catalytic converter cars by more than 80 percent.

Finally, perhaps the 22-year-old Mercedes is not the best comparison to make. Let’s say that you got an SUV back when they were still cool and are now considering a more efficient alternative. A Hummer H2 uses more energy (manufacture and fuel) in the first two years (24,000 miles) than the Prius will in its entire lifetime! But if you trade in your big SUV now, while it is still in working condition, what will happen to it? Someone else will buy it and drive it until it won’t go anymore. So essentially the damage is done and all you can hope to do is shift the burden to someone else. Unless you are feeling bold and have the car scrapped.

Got a question about the environment? Ask Pablo at AskPablo@Salon.com.

Got a question about the environment? Ask Pablo at AskPablo@Salon.com. More Pablo Plastic.

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