A noxious whiff of diesel exhaust is unmistakable—and often the source of nostalgia. For some, it evokes memories of waiting to board the ski bus on dark winter mornings. Others equate it with the proximity of a Bradley M113 in the heat of a middle-eastern desert noon or downtown traffic in a third-world capital. But some kids growing up today will forever link diesel exhaust with the mouth-watering scent of buttered popcorn.

Diesel Hybrids Hit the Road

Whether you realize it or not, you’ve likely seen a diesel hybrid-electric vehicle, either in person or in the news. City transit buses, military ground vehicles, garbage trucks, locomotives, mining equipment, and delivery trucks have been plodding into the realm of diesel hybrids for years. Even manufacturers of 18-wheelers are beginning to turn toward hybridizing their rigs, as US federal emissions standards are scheduled to tighten in 2007.

There is a correlation between all these vehicles: size. Diesel engines run optimally at steady-speed highway driving; they also take advantage of powerful torque to excel at hauling heavy loads. Thus, their marriage to electric drive systems is a natural for gargantuan vehicles that haul heavy loads but must endure frequent stops and starts-where electric motors, and the systems that knit them to their burly cousins, shine.

Emerging Fuel Rules!

For now, the diesel fuel most commonly sold in the US is the high-sulfur variety, which will be phased out by law in 2006. So while a diesel hybrid-electric passenger car or light truck would certainly improve fuel economy, its emissions would still be abysmal. It would also likely be illegal in California and the four other states that have chosen to adopt California’s emissions standards.

Until low-sulfur diesel becomes the rule in the US. Or unless the fuel used is biodiesel-fuel that is synthesized from plants, uses no petroleum, and smells like popcorn or French fries as it burns. One compelling aspect of biodiesel is the fact that it is carbon-neutral: the soy or canola grown to create it pulls the same amount of carbon from the air that the fuel eventually returns during combustion. It can be mixed with petroleum-based diesel fuel or used in its pure state. However, some worry that agribusiness will co-opt the biodiesel market, flooding it with fuel grown using bioengineered crops that require toxic pesticide use or prevent seed saving-anathema to proponents of sustainable and organic agriculture.

You may have heard of people using straight vegetable oil (SVO), salvaged from restaurant deep fryers and filtered. Its level of viscosity tends to gum up regular diesel engines, but there are SVO conversion kits available for diesel vehicles that use small amounts of bio- or petrodiesel to warm up the engine before injecting the SVO. A hybrid-electric system could go one step further, warming up the SVO while running on batteries at lower speeds.

Show Your Green

In Europe, advanced-technology "clean" diesel passenger cars form one-third to one-half the auto market—and their share is growing. As clean diesel fuel becomes more widely available in the US, and interest in hybrid-electric technology grows, the question becomes one of affordability. By some estimates, a sedan with a diesel hybrid-electric powertrain will cost around $8,000 more than one with a comparable internal combustion engine. But then again, it may get up to 80 mpg.