Before the House Energy and Commerce Committee yesterday, Volkswagen’s U.S. chief, Michael Horn, patiently answered testy questions from lawmakers indignant over the company’s deception in its clean credibility.

“I would like to offer a sincere apology for Volkswagen’s use of a software program that served to defeat the regular emissions testing regime,” Horn said to a packed committee room.

The software fooled EPA emissions testers into thinking that Volkswagen’s diesel engines produced less emissions of nitrogen oxides than they released in the real world.

Nitric oxide and nitrogen dioxide, often referred together as “NOx,” form at high temperatures, such as those inside an engine. NOx can create particles that lead to smog or acid rain. It can also react with other compounds to form ozone or nitric acid vapor, both of which can harm breathing and have climate change impacts.

EPA regulates NOx and limits how much cars are allowed to produce. An investigation revealed that Volkswagen’s diesels emitted up to 40 times the legal limit of NOx (ClimateWire, Sep. 21).

In using the defeat device across an estimated 11 million cars around the world, Volkswagen’s engineers signaled their own defeat in the contest to make diesel a viable alternative for American consumers.

Though it holds the promise of greater fuel efficiency, and therefore less carbon emissions, getting diesel right is difficult in passenger cars, especially in the United States. The problem stems from regulations and the diesel engines themselves.

Engines that run on diesel fuel are inherently more fuel-efficient than their gasoline counterparts. Gasoline engines have only recently started hitting 50 mpg in fuel economy, and manufacturers have had to resort to making tiny, fuel-sipping econoboxes or hybrids that have an electric drivetrain glommed on to hit these targets.

Meanwhile, burly diesel sedans have been topping 50 mpg for decades, pulling duties as taxicabs and long-distance drivers in many parts of the world.

“Their efficiency is due to the fact that they run very lean,” explained Anna Stefanopoulou, a professor of mechanical engineering at the University of Michigan.

“Lean” refers to the mixture of air and fuel injected into an engine where the balance tilts in favor of air, thereby burning less fuel. “Rich” is the opposite phenomenon.

Diesel engines run lean compared to gasoline engines, which means they can produce the same amount of power with less fuel, leading to less carbon dioxide emissions. This is in part due to the compression ratio of the cylinder, how much the piston squeezes the air-fuel mixture in the engine.

Gasoline engines typically have compression ratios ranging from 8-to-1 up to 12-to-1, while diesel compression ratios range from 14-to-1 to 25-to-1. Under high compression, diesel fuel ignites spontaneously, eliminating the need for a spark plug and allowing a simpler, more durable design.

Diesel engines also generate gobs of low-end torque, which is what you need when you’re hauling a heavy trailer from a standstill.

In diesels, small is not often beautiful
So far, diesel looks good compared to gasoline, but its drawbacks start to emerge in the engine.

Inside the cylinder, the same basic ingredients blend, but different proportions and circumstances yield varying results, the same way eggs, flour, milk and baking soda can make cake, cookies or biscuits, depending on their ratios.

For combustion, the mix contains a hydrocarbon fuel and air. Diesel fuel tends to have larger, less volatile carbon molecules compared to gasoline, while air is actually four parts nitrogen and one part oxygen. Going from rich to lean, high to low temperature and early to late fuel-injection timing can all affect the performance and pollution coming from an engine.

Because diesel motors run under higher pressures and use more air relative to fuel, nitrogen plays a bigger role in the reaction, leading to more NOx in the exhaust stream. However, if there is not enough oxygen relative to the diesel fuel, the engine produces soot.

Controlling these pollutants requires additional systems outside the engine to keep them in check. Heavy-duty diesel engines often have turbo chargers that compress air before pushing it into the engine, thereby ensuring an adequate supply of oxygen.

On the exhaust side, many trucks and tractors use some of the diesel fuel to convert NOx back into nitrogen gas. Others use a urea injection system, and almost all use a filter to trap particulates.

“In a large, expensive vehicle, they have a complete secondary active system that injects fuel on the catalyst,” Stefanopoulou said. “You run richer, so there is leftover fuel in the exhaust creating the right conditions in the catalyst to absorb the NOx.”

All this extra hardware adds cost and weight, which eats into the available power to do other things like overtaking someone on the interstate.

For trucks that spend thousands of miles on the road at fixed speeds, the fuel economy savings from diesel are worth the trade-offs and the extra hardware to make them compliant with air quality regulations.

Expensive fuel and steady, long-distance driving are also arguments for diesel in areas like Europe, where they make up more than half the passenger vehicle market.

In the United States, where NOx regulations are stricter than they are in Europe and refineries favor making gasoline rather than diesel, leading to higher diesel fuel prices, automakers have a hard time convincing families to make the switch from gasoline in their daily drivers.

Volkswagen was a particularly ardent diesel promoter, with a marketing campaign featuring old women that sought to dispel notions that diesel engines were loud, smelly, dirty and underpowered on the highway as literal old wives’ tales.

VW’s dreams overtake reality
Their mainstay design, turbocharged direct injection (TDI), was marketed as a clean diesel system that delivers good fuel economy and power, claims that turned out to be too good to be true.

“The issue is you want to have as small as possible system that will do the cleanup,” Stefanopoulou said. “In this particular case, the automaker was trying to come up with an elegant solution, but it came up short.”

Volkswagen and EPA have not yet disclosed how the defeat software manipulated performance in VW engines, but it may have run cars richer on testing rigs, cutting NOx emissions and keeping regulators happy, while running lean on the road, improving fuel economy and keeping customers happy.

Nonetheless, researchers haven’t given up on making diesel work.

“There are quite a few knobs to turn on a modern diesel engine,” said Steve Busch, a senior member of the technical staff at the Sandia National Laboratories. “The most fundamental trade-off in a classic diesel engine is between NOx and particulates.”

Though the automotive industry has been harnessing combustion for more than a century, what goes on inside an engine has proved elusive (ClimateWire, Jan. 22, 2014). Both gasoline and diesel are blends of different chemicals, and each one burns or reacts differently in a cylinder, making it difficult to ensure everything occurs at the right place at the right time.

“It’s a turbulent process inside the engine,” Busch said. “Trying to design around it is very, very difficult.”

The Obama administration has argued that cutting carbon dioxide emissions reduces other pollutants from the same source, thereby leading to immediate health benefits. This is the argument behind the Clean Power Plan and is part of the argument behind higher corporate average fuel economy standards, which call for an average of 54.5 mpg for all cars by 2025.

The Volkswagen defeat device case challenges the assumption that higher fuel economy is a win in every category, showing that better mileage could lead to more pollution in some instances.

Is it possible to make a diesel engine that hits all the right notes: efficient, powerful and clean? Yes, in theory, scientists say.

Stefanopoulou said the key is to bring the combustion temperature down, where you still have a lean fuel mixture but don’t get hot enough to react with nitrogen gas, obviating expensive pre- and post-treatment systems. “It is a bit of a holy grail we are all trying to achieve,” she said.

But Volkswagen diesel enthusiasts who won’t wait for scientists to complete their quest will have to take steps to bring their wayward cars in line with environmental rules, which could be a software tweak in some models but may require retrofitting additional components on others. The cleaner cars may end up losing a bit on acceleration and fuel economy.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500