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Why Diesel Deserves Another Look

Why Diesel Deserves Another Look

By Jeff Youngs, February 24, 2012

How diesel fuel earned back its respect
It maybe time to give diesel-powered vehicles another chance. The lacklusterperformance of the 1980s-era diesels-soot-spewing, noisy andpowerless-deserves to be relegated to a distant memory, as a whole newgeneration of technically advanced diesels arrives in showrooms. Thanksto new fuel injection, sophisticated electronic engine controls,turbochargers, and improved emission controls, cold start issues,sluggishness, and black smoke of the past has been replaced bycare-free operation, power and fuel efficiency.

A diesel engine operates in a very similar manner to its gasolinecounterpart except for one major difference. Instead of using a sparkplug to initiate combustion, a diesel engine uses high pressure. To bemore specific, both diesel and gasoline engines convert liquid fuelinto energy by exploding it in a controlled manner in a process knownas "combustion."

In each case, the fuel is mixed with air and pressurized inside thecylinder. While the gasoline engine's fuel/air mixture is ignited withan electric spark plug, the diesel engine pressurizes its fuel/airmixture so high-three times that of its gasoline counterpart-that thefuel ignites by itself. The unique combustion process is 30 to 35% morefuel efficient than a similar-sized gasoline engine, says the U. S.Department of Energy.

Severalautomakers offer the same model in both gasoline and dieselvariants-presenting dramatic insight to the diesel advantage. Volkswagen offers its Jettacompact sedan with the choice of either a 2.0-liter gasoline or a2.0-liter turbocharged diesel engine. The gasoline engine earns 24 mpgcity and 32 mpg highway, while the more powerful diesel model earns 30mpg city and 42 mpg highway in EPA testing.

The advantage is obvious in small cars, but it translates to largervehicles as well. BMW sellsits X5 sportutility vehicle with either a 3.0-liter gasoline or a 3.0-literturbocharged diesel powerplant. The gasoline engine earns 15 mpg cityand 21 mpg highway, while the more powerful diesel engine earns 19 mpgcity and 26 mpg highway-a substantial improvement.


Diesel-poweredvehicles sold in the United States operate on a new ultra-low sulfurdiesel (ULSFD) fuel. The reduced sulfur content means the enginereleases fewer emissions, and it is also cleaner for the environmentthan the early-generation diesel fuels. Most diesel engines are alsocapable-with minor modifications-of running on "biodiesel." This typeof fuel is vegetable- or animal fat-based (not petroleum-based),meaning it can be made from crops such as soybeans, peanuts, or evenalgae, while preserving natural resources.

The diesel-powered engine offers many other benefits in addition to itsimproved efficiency and flexibility. The elimination of a spark plugmeans that diesels aren't required to have a high-voltage electricalignition system-often resulting in better overall reliability.Furthermore, to compensate for the higher internal pressures, a dieselengine is typically constructed more durably than its gasolinecounterpart. Combine that with the natural lubrication properties ofdiesel fuel, and it generally pays off with increased longevity.Lastly, unlike highly flammable gasoline, diesel fuel is safer tohandle. It has an oily consistency-it will not explode nor does itrelease flammable gasses when stored.

Diesel engines are good, but still not perfect. They emit moreparticulates and smog-forming nitrogen oxides (NOx) thantheir gasoline counterparts, and diesel fuel isn't as readily availableas gasoline in most markets. Engineers have addressed the emissionsconcern (diesels must meet the same stringent pollutant standards asgasoline vehicles) with scrubbers in the exhaust system that filter outparticulates and "urea injection" to chemically alter the hot gasses.

Theconsumable urea injection canister, commonly referred to by its tradename "AdBlue," is stored in a reservoir tank near the trunk. Theinjection reacts in the exhaust to break the harmful nitric oxides intoenvironmentally compatible nitrogen and water vapor. The process is soeffective that urea-injected diesel engines are able to meet stringent50-state EPA Tier 2 Bin 5 compliance.

As of today, only a handful of automakers are offering diesel-poweredpassenger car and SUV alternatives-and all of them are from Europeanmanufacturers. The poorly-engineered diesel offerings from the 1980sleft a bad taste with American consumers, while buyers in the rest ofthe world welcomed the subsequent generations of diesel-poweredcompacts. With most of the problems now in the past, diesel-poweredvehicles are finally a viable alternative to gasoline-a fact thatdomestic automakers, and consumers, will soon embrace.

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