Current clean diesel technology is more than capable of meeting tough air quality rules while still delivering excellent fuel performance. Biofuels could make it even cleaner.
As VW seeks to repair its damaged image, many now question whether clean diesel technology is as clean as it’s been marketed.
As a diesel technician turned instructor, I see the clean diesel solution as a viable option for sustainable transportation in the long term, even despite the current debacle. It’s still unclear why VW decided to evade emissions tests. But the technology to meet the EPA air pollution standards and still achieve relatively high fuel efficiency exists today, whether it’s from Volkswagen or other carmakers.
The current VW scandal, which I’ve seen labeled “dieselgate” now on many social media groups, involves nitrogen oxides (NOx), gases that are formed in an engine by the nitrogen and oxygen from the air. The company wrote software that was able to recognize the conditions typical of an emissions test. During testing, the controls that limit the production of these pollutants were turned on. When on the road, the controls were turned off.
Today’s clean diesel-powered passenger car represents an average 20% improvement in fuel economy over similar gasoline powered models. I personally own two TDI Jettas and previously a gasoline-powered model. I averaged 27 mpg in the gasoline car, while the TDI that I primarily drive averages 49.1 mpg – same driver, same model year, same routes.
The better fuel economy does come with a trade-off with emissions, however. Though the total amount of emissions of the average clean diesel are similar to or lower than an equivalent gasoline engine, the compounds are different. The two primary exhaust elements of a modern diesel are Particulate Matter (PM or soot), which is essentially unburned fuel, as well as nitrogen oxides (NOx) – including nitric oxide and nitrogen dioxide.
In a combustion engine, fuel mixes with air in a cylinder and combusts to drive the pistons. NOx is produced in diesel vehicles because the engines run with 25% to 500% more air relative to fuel than most gasoline engines. NOx is a byproduct of air being heated beyond 2,900 degrees Farenheit. A lean-burning diesel engine has plenty of air left in an engine cylinder after combustion. That creates the conditions for NOx to be produced from nitrogen and oxygen in the hot air remaining in the cylinder after combustion.
Particulate Matter (PM) would historically be seen as black smoke, though the majority of remaining PM emissions today are largely undetectable. PM emissions traditionally have been the biggest knock on diesels, though NOx typically carries the more severe consequences.
NOx emissions, among other things, have been identified as a cause of acid rain, photochemical smog (the smog you can see above high-traffic areas) and as contributors to emphysema and bronchitis.
Clean diesel engines today utilize two primary devices to control NOx formation: Exhaust Gas Recirculation (EGR) and Selective Catalytic Reduction (SCR).
EGR functions on the principle that previously consumed gases from within the engine are passed back into the engine. These gases take up room in engine cylinders where nitrogen and oxygen-rich air could otherwise exist and combine to form NOx. The presence of this inert gas from the engine exhaust lowers the temperature inside the engine below the 2900 degree threshold, thus minimizing NOx production.
SCR serves to cleanup any remaining NOx that EGR was unable to eliminate. SCR works by spraying a fluid known as Diesel Exhaust Fluid (DEF) into the exhaust stream to create a chemical reaction. Diesel exhaust fluid, essentially water and urea, reacts with the exhaust inside a catalyst to convert any remaining NOx into harmless nitrogen and water. SCR did not exist on Jetta and Golf models until 2015, though it was on Passat models starting in 2013.
The emissions control technology today, though quirky at times (it’s still in its relative infancy), is effective.
The unknown at this point in time is why Volkswagen Group chose to circumvent their own very effective emissions solution. One possible reason is to keep potentially dirty exhaust gas out of the engine to prevent a need for premature maintenance, as diesel exhaust can clog air intake eventually when it’s recirculated. The other reason that keeps rolling around in my mind is whether it was an attempt to reduce the consumption of diesel exhaust fuel in order to lower costs for consumers.
Preventing the exhaust gas recirculating function from working can improve the car’s performance, both in terms of fuel efficiency and acceleration. But current technology is more than capable of meeting both EPA emissions limits and delivering the high fuel efficiency Volkswagen touted before dieselgate.
In the 1980s, a diesel-powered car or light truck in the United States conjured thoughts of smoke, noise and poor reliability, leaving most of the around 400,000 diesel buyers disheartened. Fast forward to the late 1990s when Volkswagen brought the economical diesel back to the US – with a vengeance. From 1994 to 2006 alone, Volkswagen’s diesel sales in the US increased 1,500%.
The clean diesel solution has a nearly cult following. TDI owners pile miles on their vehicles, including cross-country road trips and, because of the diesel engine’s ample torque, can use the Jetta and Golf passenger cars as many use trucks.
In addition to Volkswagen and its Audi TDI models, General Motors offers a Cruze diesel, and BMW and Mercedes also sell clean diesels. All represent a compelling juxtaposition – a vehicle that’s fun to drive and economical while at the same time very utilitarian, even capable of towing a small trailer. The utility is something that many hybrids cannot compete with.
Even with the impressive fuel economy offered by clean diesel-powered vehicles, the US still has only a roughly 3% total diesel market share in the light-duty segment. In contrast, European countries overall see a 55% market share going to the diesel. The stigma of the 1980s US diesel is tough to break.
But the current scandal does not represent doom and gloom for the clean-diesel world. We are on the forefront of some absolutely amazing technology in terms of biofuels and lab-produced diesel alternatives.
The most revolutionary alternative right now is actually a solution supported by Volkswagen Group’s own Audi brand.
Audi has partnered with a company that has developed a sustainable bio-diesel produced by microbes fed only sunlight and nutrients, which Audi calls “e-diesel.”
If a fuel based on water and air, which can be run as a drop-in replacement for petro-diesel isn’t a sustainable viable option, I don’t know what is.