Share This article
Diesel-engine passenger vehicles are gaining popularity in the US as technology solves most of their drawbacks. Most of the time, they start and drive just like gasoline-engine cars. They travel as much 800 miles between fill-ups, which is good, because one problem lingers: the whiff of kerosene that likewise lingers on your hands after time you fill up.
A bit less than one in 100 passenger cars and SUVs sold today is a diesel. Add in pickup trucks, where the take rate is better than 10 in 100, and you have 3% market share for diesels. Small as it is, the number represents growth — roughly equal to the sales rate for hybrids, plug-in hybrids, and electric vehicles, as long as you include diesel pickups. Diesels represent about half of all sales in Europe.
What exactly is diesel technology?
Here’s what diesel technology does and why many people feel diesel is better. Diesel fuel is an oily-feeling cousin to kerosene, home heating oil, and jet fuel. Despite the fuel seeming less refined or less sophisticated than gasoline, diesel packs about 10% more BTUs per gallon than gasoline. The ruggedly built diesel engine compresses air to one-twentieth its original volume, then a trace of diesel fuel is injected into the cylinder and it spontaneously combusts without the need for a spark plug. The gasoline engine, in comparison, compresses a mixture of fuel and air to about one-tenth its original volume and then a spark plug ignites it.
A diesel-engine vehicle typically costs $ 1,000-$ 2,500 more for a car or SUV than a comparable gasoline engine. Virtually every diesel passenger vehicle uses a turbocharger. If the vehicle cost is a lot more, or less, there not be comparable equipment levels. For pickups, the cost can be $ 5,000 more for diesel engines designed to last hundreds of thousands of miles. You’re paying for more rugged engine parts built to withstand the higher compression ratios and higher combustion chambers.
The fuel injection system is more sophisticated and works under higher pressures. The simplest gasoline-engine fuel injectors work at 45-60 psi above the ambient air pressure (14.7 psi), or at 3-4 bar (atmospheres). Most second-generation diesel engines sold today using a common fuel rail technology working at 15,000 psi or 1,000 bar. Third-generation common rail diesels reach 45,000 psi or 3,000 bar, using piezoelectric injectors similar to what Epson uses in its inkjet printers. (Gasoline direct injection, where the injector nozzle sticks into the combustion chamber, works at similar pressures.)
Today’s diesels are all “clean diesels”
Most diesel cars and SUVs sold in the US are called clean diesels. That’s a term the industry composed to describe diesel cars and SUVs with emissions on par with a gasoline engine car. It’s a combination of ultra-low sulfur diesel fuel, efficient diesel engines with precise metering of fuel into the cylinders (sometimes multiple pulses per combustion cycle), particulate filters that trap soot, and advanced emissions controls including a final cleansing squirt of urea into the exhaust stream.
After the exhaust leaves the engine’s exhaust manifold and before it exits the muffler and tailpipe, it’s treated with a selective catalytic reduction or SCR process (image above) the reduces nitrogen oxides, carbon monoxide, hydrocarbons, and particular matter (soot). The industry says the SCR process can reduce NOx by up to 90%, hydrocarbon and carbon monoxide emissions by 50%-90%, and particulate matter by 30%-50%. If necessary, especially on larger commercial vehicles, there may be a separate particulate matter filter. If you see an 18-wheeler belching smoke, it’s either an old truck or something’s wrong with the emissions controls.
As part of the SCR process, automotive-grade urea is injected from a tank of 5-7 gallons. It’s also called diesel exhaust fluid, DEF, or by the trade name AdBlue. DEF causes chemical reactions: Nitrogen oxides break down into nitrogen, water, and small amounts of carbon dioxide CO2). Diesel exhaust fluid is used up slowly. Mercedes-Benz says consumption is about one liter per 1,000 kilometers or a gallon every 2,500 miles, meaning there’s at least 10,000 miles worth of driving.
Diesel engine advantages: 400-800 miles per tank
When BMW introduced the BMW 328d, it placed a cheeky billboard ad seen by New York City motorists exiting the Lincoln Tunnel into New Jersey: “Hit the Beach On 2 Tanks. (Miami Beach.)” With 45 mpg EPA-rated highway mileage and closer to 50 mpg in real life, the Gothamite headed to Miami would start looking for a service station somewhere in South Carolina. That’s the main advantage: a highway driving range of 400 to 800 miles on diesel, or one fill-up a day on long trips. Even in mixed city-highway driving, you’ll make fewer stops at the pump.
The Diesel Technology Forum, an industry trade group, says diesel vehicles can save owners $ 2,000-$ 6,000 in total cost of ownership over a 3- to 5-year period compared with gasoline vehicles. Our belief is that you can save money, but the total savings depend on a lot on variables that change over time, such as fuel price. In late 2014 and early 2015, diesel fuel prices spiked to as much as 90 cents a gallon more than regular and 50 cents more than premium. The price delta has dropped so much now that diesel is cheaper than premium.
Real-world fuel economy can match a hybrid as long as there’s a mix of city and highway driving. In 2008, a BMW 5 Series diesel vs. Toyota Prius hybrid fuel-economy race from London to Geneva was won by the BMW, 41.9 mpg to 40.1 mpg, despite the Bimmer carrying 500 pounds more weight. About half of American’s 175,000 gas stations also pump diesel. At highway interchanges, it’s near-universal. It’d be a small town where you couldn’t find diesel. During major storms and extended power failures, there may be odd-even gasoline rationing. Pull into a truck stop, queue up for diesel, and you may not have to wait ’til tomorrow for fuel.
Some diesel-engine vehicles actually cost less than the automaker’s comparable gas-engine vehicle. One of the finest luxury vehicles for long-distance highway cruising is the Mercedes-Benz GL. The diesel version lists for $ 1,600 less than the cheapest gas-engine GL. Diesel engines also tend to last longer because of their sturdy construction and the lubricating properties of diesel fuel (though you still need engine oil).
The nature of a diesel engine is that it delivers its torque, or power to get moving, at low rpm. So regardless of a diesel’s 0-60 times, and they can be quite good, getting from 0-20 mph often extra-quick. At speed on the highway, the occupants simply can’t tell a diesel vehicle from a gasoline vehicle. At idle, you may be able to tell if you listen hard, but it’s not really noticeable. A diesel today starts instantly unlike a generation ago, when you waited 10-30 seconds for a glow plug to warm up the cylinders on cold mornings.
While it packs more power per gallon, diesel burns less readily, so it’s safer in the handful of accidents where the car catches fire. Fire is a huge issue for military vehicles, and most have been switched from gasoline to diesel or related fuels such as jet fuel.
Diesel engine disadvantages: some, not many
The disadvantages are fewer than a decade ago, let alone a generation ago. In really cold weather, you may want an engine block heater to improve quick starting. A cold engine in cold climates clatters for a minute or two; people outside the car will notice, but sound insulation and double-insulated window glass make it marginally noticeable inside. Outside at idle, people nearby may catch a whiff of burned diesel fuel. Very rarely, there may be a trace of smoke on starting. Diesel partisans say you won’t notice these modest differences. You will, just not very much. Also, with low sulfur fuel that has been mandatory since 2007, there’s no more rotten-egg smell from idling diesel vehicles.
The tank of diesel exhaust fluid is good for thousands of miles, typically being replenished at the same time as an oil change. If the diesel exhaust fluid runs out, the car won’t run, not because it’s dangerous, but because more pollutants would escape, and that’s the way regulations are written. You’ll get hundreds of miles of advance warning, and if you let the tank go empty, the car won’t start next time. It won’t shut you down on the highway.
Despite the assurances of diesel fans, it’s possible a diesel will cost you more to run if you’re a low-mileage urban driver and if you factor in the initial prices. So much depends on the comparable price of fuel and how many miles you drive.
The biggest drawback to diesel for individuals may be refueling it yourself. You smell diesel while refueling, some of it clings to your clothes, and your hands may smell of diesel even after washing them. (Try carrying disposable latex gloves.) Diesel pumps look dirtier than adjacent gasoline pumps unless the surfaces and fueling handles are wiped down a couple times a day. (Gasoline has more solvent action.) You can almost see the continuum: oily residue on diesel pumps to unclean rest rooms to Exxon Valdez. It’s something well-maintained service stations could pay attention to. Be happy if you live in Oregon or in New Jersey, the two states than ban self-service refueling, so it’s the attendant with unclean hands, not you. While the Garden State has some of the nation’s low-priced gasoline and diesel, that’s mostly because of low state fuel taxes.
Environmentally, some parts of Europe are pushing back against the diesel. Diesel engines beat gasoline engines on CO2 emissions (carbon dioxide is a proxy for lower fuel consumption), but there are concerns over NOx and soot (particulates). Some French officials have said it was a mistake to provide favorable tax treatment on fuel for diesels; the mayor of Paris said she’d like to see diesels banned from the City of Light by 2020. Thus the race to make clean diesel even cleaner and the use of increasingly more sophisticated combustion technologies and downstream exhaust treatment.
Do you save money with diesel?
The formula to calculate break-even includes several variables you may be guessing at. Calculate the number of miles you’ll drive the vehicle per year. Calculate the price of diesel versus gasoline and guesstimate it forward for the life of your vehicle. Calculate the mpg you get with gasoline and with diesel. Calculate whether to add 5%-10% to the EPA highway rating for diesel because, real world, they seem to fare better than the test numbers. Decide whether to compare diesel versus gasoline versions of the same model based on horsepower or mpg or acceleration. (Also see if one or the other has more equipment standard that shouldn’t be part of a total cost of ownership comparison.)
As of mid-May 2015, fuel costs were $ 2.66 for regular gasoline, $ 3.06 for premium, $ 2.85 for diesel (nationwide average). Six months before, diesel was more expensive than premium. The average 2007 to present has been $ 3.11 a gallon for regular (all formulations, all areas of the US), $ 3.38 for premium, $ 3.44 for ultra-low sulfur diesel, according to Department of Energy data.
Consider the Volkswagen Jetta (main photo), the best-selling diesel in America: Nearly a quarter of the 160,000 Jettas sold here last year were diesels. The 2015 Jetta TDI gets 31 mpg city, 46 mpg highway, 36 mpg combined based on the EPA’s projected 55-45 city-highway mix of miles. A sporty Jetta with a gasoline engine using premium fuel gets 23/33/23 and an economy-minded Jetta with smaller engine running on regular fuel gets 25/37/30. Let’s assume the car is driven 20,000 miles a year, a third more than most cars travel, and the mix is 25% city, 75%. Assume we use the 2007-15 average fuel prices. Here’s what we get:
Jetta diesel, 42 mpg average, 473 gallons @ $ 3.44, $ 1,628 per year for 5,000 city, 15,000 highway miles
Jetta gas, premium fuel, 31 mpg, 656 gallons @ $ 3.38, $ 2,216 per year, $ 588 more than diesel
Jetta gas, regular fuel, 34 mpg, 588 gallons @ $ 3.11, $ 1,829, $ 201 more than diesel
Choose current (mid-May 2015) fuel prices and the premium-fuel cost disadvantage would climb to $ 657, while the regular fuel disadvantage would increase slightly to $ 216. Become a traveling salesman, drive 40,000 miles this year, and the dollars saved with diesel would be as much as $ 1,500. All of these would earn you back any premium on the Jetta diesel in a couple years. In this case, the Jetta diesel would be cheaper to own.
Should you buy diesel?
The ideal buyer of a diesel car or SUV should drive 20,000 miles a year or more, the majority of them highway miles. Diesels are at their best, most efficient state running at highway speeds. (They’re also great at idling; it doesn’t take much fuel to keep the engine ticking over at 800 rpm.) When the automaker’s equivalent gas engine car uses premium fuel, it’s easier to show a cost advantage for diesel. Premium is closer in price to diesel, lowering the time it takes to make back the upfront cost. (VW is somewhat unique in offering regular-gasoline, premium-gasoline, and diesel versions of the same models.)
On mostly highway trips, as we mentioned above, you’ll be able to drive 400 to 800 miles on a tank of fuel. Your bladder doesn’t hold out for that many miles, but that’s beside the point. Refueling takes an extra 10 minutes beyond time needed for the bio break. Even in a mix of city and suburban driving, you might go 10 days instead of a week between fill-ups, and that’s fewer times you have to touch the grimier diesel pump handles.
As of 2014, VW held down positions 1-2-3 in diesel sales among cars and SUVs. The top 1o was the Volkswagen Jetta Diesel (about 40,000 sales), Volkswagen Passat Diesel (inset photo), Volkswagen Golf Diesel, BMW X5 Diesel, Audi Q5 Diesel, Chevrolet Cruze Diesel, Mercedes-Benz GL-Class Diesel (photo above), Porsche Cayenne Diesel, and Audi Q7 Diesel. That’s six SUVs and four sedans. The one other U.S. car or SUV sold with a diesel engine was the Jeep Grand Cherokee Diesel. Japanese automakers have so far shied away from diesel cars and SUVs. Mazda announced a Mazda’s SkyActiv-D diesel Mazda6 for the US, but it has yet to arrive. Mazda officials said they need to improve the cars performance.
This entry passed through the Full-Text RSS service – if this is your content and you’re reading it on someone else’s site, please read the FAQ at fivefilters.org/content-only/faq.php#publishers.
ExtremeTech » News & Updates For Hardcore Tech Fans | ExtremeTech