LAST month, your columnist raved about the new generation of lean, clean diesel cars arriving belatedly in America (“Diesel's second coming”, April 4th 2008), but wondered where they might find their fuel. With oil pushing $120 a barrel, the diesel's fuel-sipping charms—it delivers about 30% more miles per gallon (MPG) than regular petrol—have created a shortage of diesel fuel around the world.

Soaring prices followed the shortage. Over the past year, the average price of diesel in America has risen by 117%—twice as fast as petrol. It is now 20% more expensive (60-70 cents more per gallon) than regular gas. Rising prices like these could easily negate diesel's MPG advantage.

AFP

From sweet beginnings

Half of all new cars bought in Europe are diesels; Europeans have pretty well hogged the supply of diesel fuel—from oil refineries as well as from renewable crops. In fact, much of the biodiesel produced in the United States from soybeans and corn (and subsidised by American taxpayers to the tune of $1 a gallon) winds up in Europe, where it benefits from still further subsidies. That's great for farmers in the Midwest, but offers little consolation to motorists across America.

Now comes news that another major source of diesel—South-East Asia, where it's refined from palm oil—is drying up. Refiners throughout the region are closing biodiesel plants and canceling new ones. Most are being priced out of the business by the soaring cost of palm oil, which has been rising even faster than crude.

With China and India getting richer, growers find they can make more money nowadays from selling palm oil for cooking rather than fuel. Malaysia, which has enough refinery capacity to produce more than 1m metric tons of biodiesel, turned out only 80,000 tons last year.

It's not as though conventional oil refiners can add more diesel capacity in a hurry. For one thing, the catalytic crackers used by refiners in America are optimised to produce as much petrol as possible—typically about 50% of every barrel of oil, with diesel accounting for little more than 15%.

In Europe, hydrocrackers that produce 25% petrol and 25% diesel are more common. But adapting American refineries to produce more diesel will take years—Marathon Oil is one of the few expanding its diesel capacity.

In the meantime, the motor industry is pinning its hopes on biodiesel. But growing crops for fuel instead of food is becoming politically difficult in many parts of the world. Besides, biodiesel's contribution using traditional crops will be modest at most.

It's a matter of scale. Europe has over 10m metric tons and America around 4m metric tons of biodiesel capacity. Compare that with the 490m metric tons of diesel that Europe and America consumed between them last year. To meet just America's one-third share would require all the arable land in the country (some 470m acres) be planted with soybeans for biodiesel.

Few countries have anything like enough arable land to feed themselves and grow biofuels at the same time. There has to be a better way. One answer could be synthetic biology—a set of tools for building novel functions into biological systems to solve engineering problems.

To engineer a hydrocarbon like diesel, start with a fermentable sugar or starch in the usual way. In terms of energy produced per acre of land, sugarcane is a better bet than sugar beets, corn or soybeans.

Next, forget about distillation. It may be an old and honourable way of making alcohol, but it's appallingly wasteful in energy terms. Just heating the fermented broth ready for distilling into ethanol consumes around 40% of the energy in the alcohol produced.

Instead, take a common bacterium like E. coli, which lives in the gut, or a yeast-like fungus such as Saccharomyces, which easily turns sugar into copious quantities of alcohol. Scour the world for suitable enzymes that speed up the whole process and deliver the molecular features being sought. Finally, tweak the enzymes to make them do their jobs better, and stitch their genes into the microbe being used as a fermentation factory.

Ten years ago such an endeavour would have been unimaginable. But thanks to the development of cheap and fast DNA sequencing and synthesis tools, such microbial engineering is becoming routine.

Amyris Biotechnologies, a company in Emeryville, California, that spun out of the University of California at Berkeley several years ago, has pioneered a way of custom designing microbes to produce a whole range of specialised hydrocarbons—including molecules that are exact replicas of artemisinin, an anti-malarial drug, as well as fuels like petrol, diesel and aviation spirit.

The key to Amyris's success has been a class of hydrocarbons called isoprenoids (by-products, like synthetic rubber, that result from cracking naphtha or oil). Like alcohols, isoprenoids are also found widely in plants and animals, and are actually the most common hydrocarbons produced in the human body. Enzymes that can manipulate them are therefore fairly common. That makes tailoring bugs to synthesise specific hydrocarbons a whole lot easier.

Unlike the biofuel produced by distilling witches brews of soybeans or corn, Amyris's hydrocarbons are engineered to have precisely the same molecular structure as their oil-based equivalents—and are hence one-for-one replacements for fuels on the forecourt today. As such, engines notice no difference when running on them, save for the synthetic diesel's greater consistency and its 80% reduction in emissions compared with petroleum diesel.

Last week, Amyris announced a deal with Crystalsev, one of Brazil's largest ethanol distributors, to create a renewable diesel made from sugarcane. In Brazil, where sugarcane grows like a weed, ethanol plants typically produce around nine kilowatt-hours of energy for every ton of cane crushed. The Amyris process promises to raise that figure 20-fold.

The aim is to have a demonstration plant up and running in Brazil by 2010, with large-scale commercial production starting a year or two later. Itching to replace his family's gas guzzler with one of the new generation of super-sippers, your correspondent is finally feeling more confident that there will be enough diesel fuel to go around.