Audi logo2

We hear a relatively constant stream of information about alternative fuel vehicles in the media, but the future of alternative fuel is here, and it’s available today. It’s clean diesel, and its advantages over gasoline become obvious as soon as you’re behind the wheel.

Many Americans drive cars and trucks that are powered by spark-ignition gasoline engines, in which a high-voltage spark plug ignites a fuel-air mixture. In a diesel engine, the heat created by compression itself is used to ignite the fuel.

Diesel engines offer the most thermal efficiency of any internal combustion engine, thanks to their high compression ratios. A traditional gasoline engine without direct injection might offer a compression ratio of 9.5:1 or 10.5:1. The Audi TDI® clean diesel engine in the A8 L for instance offers a compression ratio of 16.3:1, allowing for higher efficiency and power output with less engine displacement.1

Diesel engines are also known as “lean-burn” engines, utilizing as little fuel as possible to provide ignition. Gasoline engines without direct injection are “rich-burn” engines, utilizing more fuel to accomplish the same task as a diesel engine.

Theoretically, this is great, but diesel always had higher compression ratios, right? So why weren’t diesel-powered cars more popular during the 1970s oil embargoes?

The truth is, they were. But like technologies such as fuel injection, cylinder deactivation and evaporative recovery systems, they were well ahead of their time. Until powerful computers could make these systems work better, all of these mechanical systems suffered.

Diesel of old vs. newer Ultra-low-sulfur diesel
Diesel fuel and gasoline both start from the same base product: crude oil. What makes diesel different from gasoline, jet fuel and home heating oil is how it’s distilled and refined.

At the point of distillation, diesel fuel ends up with a higher boiling point than gasoline. By increasing the boiling point, the amount of energy included in a diesel molecule is increased. Measured in BTUs, a gallon of diesel fuel contains about 15 percent more energy than a gallon of gasoline.

The refining process is also a win for diesel because it requires less refinement — and thus, less expended energy — than gasoline. During the refinement process, the object is to reduce the amount of sulfur as much as possible — up to 97 percent less sulfur than diesel fuel that existed before 2006. Since 2006, all 50 states require the use of ultra-low-sulfur diesel (ULSD). The complete line of Audi TDI® clean diesels — the A6 and A7 sedans, the Q5 crossover and Q7 SUV and the luxurious A8 L flagship — are all powered by this fuel.

The entire point of ULSD is that it allows much more sophisticated emissions control technologies to assist clean diesel exhaust, to help eliminate what many people remember about diesel in the early days: soot.

Today, diesel starts cleaner, and it ends cleaner, too. The reason is that Audi utilizes technologies such as close-coupled oxidation catalysts, coated particle filters and active gas after-treatment systems to detect soot levels and emissions in the exhaust and help trap them before they ever leave the exhaust pipe.

Audi TDI® clean diesel — utilizing ULSD and the advanced engine technologies that capitalize on the fuel’s benefits — has led the way to helping reduce emissions not just compared to older diesels, but compared to gasoline engines, as well.


1 2014 Audi A8 L TDI® clean diesel: 36 hwy/24 city/28 combined mpg. EPA estimates. Your mileage will vary and depends on several factors including your driving habits and vehicle condition.