Consider the journey a gallon of ultra-low sulfur diesel fuel takes before it's burned and sent through an Audi A8 L TDI® clean diesel exhaust system. It's an odyssey that starts in the ground, and ends up as emissions from the exhaust. The goal is extremely high efficiency, and reduced emissions and every step of the process concentrates on these tasks.1
The journey begins with the fuel
Gasoline and diesel fuel start out from the very same product: crude oil. Density — literally, the weight of all the energy potential packed into a gallon of diesel fuel — is what makes diesel more efficient that gasoline. Diesel fuel's volumetric energy density — measured in British Thermal Units (BTUs) — is around 147,000 BTUs per gallon. The same volume of gasoline only contains 125,000 BTUs. Greater energy density = greater efficiency.
Efficiency through direct injection
Once the fuel arrives in the 23.8-gallon fuel tank in the Audi A8 L TDI® clean diesel, the next step in the journey is from the tank, through the high pressure fuel system, into the engine's six combustion chambers.
Direct Injection is the first step in helping maximize efficiency. At the top of each combustion chamber, Audi uses a piezo injector with eight holes in each nozzle just 0.12 mm in diameter, each opening not much bigger than a human hair.
Each injector supplies tiny bursts of diesel fuel into the combustion chamber at pressures up to 29,000 psi, at the right time, at the right place, and in the exact amount.
As a bonus, the piezo injectors help lower the combustion noise often associated with diesel engines, helping make every Audi TDI® clean diesel much more quiet and refined, compared to yesterday’s diesel-powered vehicles.
Efficiency in combustion
How fuel is compressed in the combustion chambers is another key to the efficiency of a TDI® clean diesel. The average gas engine compresses at a ratio between 8:1 and 12:1 depending whether the engine is naturally aspirated or uses power adders in the form of forced induction. A TDI® clean diesel crushes it with a 16.8:1 compression ratio, helping lead to greater efficiency and power.
Efficient power extraction
Turbocharging is a form of forced air induction, and simply means forcing more air into the combustion chambers. The operating RPM range of a TDI® clean diesel also helps contribute to its efficiency. A gasoline engine might build its maximum power at 5,500 rpm, spinning faster to reach its performance potential, thus burning more fuel. The A8 L TDI® clean diesel builds its maximum torque — the power you feel when accelerating, say, between 50 and 65 mph2 — at just 1,750 RPM.
Every step of every gallon of ultra-low sulfur diesel fuel in an Audi TDI® clean diesel is aimed at efficiency. The combination of efficiency3 and power is what makes the 2014 TDI models of these vehicles — the A6 and A7 sedans, the Q5 crossover , Q7 SUV and the A8 L luxury flagship sedan — such compelling, engaging vehicles to drive. It's your chance now. Visit your Audi dealer and experience them for yourself.
1 2014 Audi A8 L TDI® clean diesel: 36 hwy/24 city/28 combined mpg EPA estimates. Range of 666 combined miles based on 28 mpg combined EPA estimates and a 23.8 gallon fuel tank. Your mileage will vary and depends on several factors including your driving habits and vehicle condition.
2 Always obey speed and traffic laws.
3 See www.fueleconomy.gov for EPA estiamtes. Your mileage will vary and depends on several factors including your driving habits and vehicle condition.