Reasons Why Volkswagen Declared Defeat in Diesel Cars

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.