Umbra on Hybrids vs. Biodiesel

A Prius-driving reader wonders whether he should invest in an even greener vehicle -- perhaps one that runs on straight vegetable oil. Advice guru Umbra Fisk compares the two eco-cars and announces her verdict -- in Ask Umbra, on the Grist Magazine website.

Dear Umbra,

I currently drive a 2002 Toyota Prius that gets about 40 to 42 miles per gallon on the highway, which is where most of my driving takes place. However, I've recently become enamored with biodiesel vehicles, and specifically with straight vegetable oil (SVO) vehicles. I'm interested in investing in an SVO system fueled by, hopefully, recycled oil from local restaurants.

I've looked into your column archives, and in covering the question of biodiesel versus gasoline, you sided with gasoline. However, you've never grappled with the interesting question of an SVO vehicle versus a hybrid. (As there are no diesel hybrids, this is an either/or dilemma.)

So, great purveyor of ecological truths, cast your wisdom on this quandary. Which is the better environmental choice: to run a straight recycled vegetable-oil diesel engine for long trips, or to run a gasoline hybrid?



Minneapolis, Minn.

Dearest Marc,

You have stung me to the quick. I did not side with gasoline. I asserted that gasoline is better than diesel, but not better than biodiesel.

Not only will biodiesel result in lower carbon-dioxide emissions than gasoline; it is also an alternative fuel, and supporting it sends an important message about prioritizing the environment. Hybrid cars deserve our support, but they rely on gasoline, the supply of which is rapidly dwindling and the extraction of which is wrapped up in nasty world politics and pollution.

I'm glad you have a Prius, but I completely support your switch to biodiesel, and also to straight vegetable oil from restaurant fryers. Fellow readers, to briefly review: Gasoline is petroleum-based. Biodiesel is chemically altered (transesterified) vegetable oil or animal tallow. SVO is new or filtered chemically unaltered veggie oil. Diesel engines can theoretically run on any vegetable oil. In an upcoming column, I'll go into how to run your diesel on SVO, but today we will focus on Marc's question: should we bother?

Marc, scientific data with which to evaluate your SVO fantasy are hard to come by -- particularly data comparing SVO to gasoline rather than to diesel. There just aren't the miles driven on pure SVO, or the audience for the technology, to have authoritative information yet. Running your car on SVO requires retrofitting the engine with additional tanks, hoses, and heaters, so a complicating factor for research is the wide variety of diesel cars and the wide variety of conversion methods. Not to mention comparing the burning of McDonald's fryer oil run through a coffee filter against the burning of tempura fry oil filtered to five microns.

But you can feel good knowing that vegetable-oil fuels are "biomass" fuels and are considered to have a "closed carbon cycle." That means that the carbon released during their burning is balanced by the carbon absorbed during their past lives as plants. We can think of the soy and rapeseed that made the oil as the housemaids, cleaning up the mess made by our personal automobile. Phew. In the future, we may get a specific lifecycle analysis of this fuel, but at this point, given the reading I could find, I feel confident encouraging SVO, a non-petroleum, closed-carbon-cycle alternative fuel.

Down at the Environmental Defense Tailpipe Tally, your 2002 Prius, driven 12,500 miles in a year, will produce 4,990 pounds of carbon dioxide, 135 pounds of carbon monoxide, and 9.4 pounds of nitrogen oxides. In the closed-carbon-cycle competition, the Prius is shut out: fossil fuel sequesters carbon for eons and is created equally slowly, so we cannot depend on crude oil to clean up our carbon mess. If the closed cycle is like having a housemaid, the fossil-fuel cycle is like never cleaning and just waiting for the house to decompose. Which, I guess, is what we've been doing since the Industrial Revolution.


Do you have a nagging question pertaining to the environment? This email address is being protected from spambots. You need JavaScript enabled to view it. to ENN, and you might find the answer in a future "Ask Umbra."

The claims made in this column may not reflect the views of Grist Magazine or ENN. Neither the magazine nor the author guarantees that any advice contained in this column is wise or safe. Please use this column at your own risk.

Umbra Fisk is Grist Research Associate II, Hardcover and Periodicals Unit, floors 2B-4B.

Source: Grist Magazine