Skip to comments.Five myths about diesel engines
Posted on 06/15/2011 6:07:50 AM PDT by Red Badger
Diesel engines, long confined to trucks and ships, are garnering more interest for their fuel efficiency and reduced carbon dioxide emissions, relative to gasoline engines. Argonne mechanical engineer Steve Ciatti takes a crack at some of the more persistent myths surrounding the technology.
Myth #1: Diesel is dirty.
"We all have this image of trucks belching out dirty black smoke," Ciatti said. This smoke is particulate matter from diesel exhaust: soot and small amounts of other chemicals produced by the engine.
But EPA emissions requirements have significantly tightened, and diesel engines now have to meet the same criteria as gasoline engines. They do this by adding a Diesel Particulate Filter (DPF), which removes visible smoke. "DPFs are very effective," Ciatti said. "They remove 95-plus percent of the mass of smoke."
The smoke, trapped in a ceramic matrix, accumulates until the car's computer determines it's time to clean it out in a process called a "regeneration cycle."
While running, a small amount of extra fuel is added to the combustion chambers in the engine; the resulting heat and oxygen activate a catalyst in the DPF to burn off the accumulated soot. This renders a small fuel consumption penalty.
"Visible smoke is essentially gone, as of the 2007-2010 regulations," Ciatti said. "If you're buying a diesel car from 2007 or later, it's no dirtier than a gasoline-powered vehicle."
And in the invisible range -- diesel engines actually emit less carbon dioxide than gasoline engines do.
Myth #2: Diesel engines won't start in the winter.
"Today's technologies for cold-start are very effective," Ciatti said. "Modern diesel engines start in cold weather with very little effort."
The problem is that diesel jells at low temperatures. Below about 40°F, certain hydrocarbons in diesel turn gelatinous. "Since an engine depends on aerosolizing fuel, you don't want goopy fuel," Ciatti explained.
Often this is remedied with glow plugs, which are heated by the battery and help warm up the fuel so it can vaporize.
Low temperatures aren't a problem for gasoline engines because gasoline is much more flammable than diesel. Even at room temperature and pressure, gasoline is partly vapor. "Toss a match into a pool of gasoline, and the match will never even hit the surface of the liquid; it will ignite the layer of vapor above the pool," Ciatti said. "That's why gasoline has to be handled extremely carefully around any ignition source. Diesel isn't so volatile; if you tossed that match into a pool of diesel, it would go out."
Glow plugs and other remedies, however, effectively vaporize diesel to prepare it for combustion.
Myth #3: Diesel cars don't perform well.
Because diesel engines are still most common in trucks, many people assume that diesel-powered cars would behave like a truck behaves: slow and sluggish. "But keep in mind, that truck's likely hauling around 50 tons," Ciatti said. "In fact, to some degree, some people who drive diesels find they perform better than gasoline engines."
That's because diesel-powered engines get their best power when the engine's revolutions per minute (RPM) are low -- that is, at speeds below 65 miles per hour, where most driving takes place. Gasoline engines, in contrast, get to peak power by running the engine very high and fast; a gasoline car only reaches its peak horsepower with the accelerator pedal to the floor and the engine running at 5,000 RPM.
"Diesel car performance is far better than the perceived horsepower rating, because you're getting all that power at speeds where you actually drive the vehicle," Ciatti said. "You've got more pulling power and more acceleration at those speeds."
Myth #4: You can't find diesel at the pump.
Diesel-powered pickups and cars are popular enough that the market has taken interest; most neighborhood gas stations now have automotive diesel pumps.
"I drove a diesel car myself for 10 years. I can count on one hand the number of times I had to actually search for a pump," Ciatti said.
Myth #5: Diesel fuel is more expensive than gasoline.
Though Chicagoland diesel prices are generally higher than gasoline, in most parts of the country, diesel fuel and gasoline are priced comparably. Today, Illinois taxes diesel at higher rates than gasoline.
"Diesel fuel is not more expensive to produce than gasoline," Ciatti explained. "Its price usually has to do with the local tax structure."
Bonus: One thing you may not know about diesel!
Diesel engines actually perform better at high altitudes than gasoline engines.
Why? Gasoline engines operate at a very specific ratio of fuel and air. At high altitudes, the air is thinner--literally: there are fewer molecules of air per cubic foot. In the mountains, then, gasoline engines have to add less fuel to keep the ratio perfect, which affects performance.
"But a diesel engine runs fuel-lean; you don't have to keep the ratio perfect," Ciatti said. Diesel engines have turbochargers, which are pumps driven by exhaust gas. They add more air to the combustion chamber, and more air means more fuel can be added. At altitude, it can pull in more air and more fuel, and thus gets more power than gasoline engines can. Turbochargers don't use extra energy; they run off thermodynamically "free" energy that would be lost as exhaust if not used.
"Drive a diesel at altitude and you'll see other cars struggling while you zip past," Ciatti said. "The effect is very noticeable."
Provided by Argonne National Laboratory
Sorry for the late post, Mine is the ‘02 and gets the best mileage of the later models. (Averages 21 mpg) And it has the one ton frame.
It tows at better than 15 mpg and still does not leak oil anywhere and I go 8,000 miles between changes and never have to add even a quart in that time. It has been by far the best vehicle I have ever owned. The body is also extremely tight and quiet to this day. (Full Leather interior and LT mag wheels with memory seats.)
Provides quiet injection without knock because of the ability to release such small amounts of fuel over a longer portion time during the end of the intake stroke and the combustion(or Power)stroke. And fuel is able to be metered more precisely for what is actually needed.
The reason many powerstroke fords smoke so bad is that most on the road still have solenoid controlled by computer injectors or mechanical. And the 6.0 ltr powerstroke are plagued with o ring failures and misdiagnosed rail pressure and injector problems. 7.3 LTR Powerstroke is the most reliable. 6.4 LTR I don't know. I'll take a Dodge Cummins.
It depends. The starting price for a gas Jetta is $16,495 and the TDI is $22,995. The base model Jetta gets 33 MPG on the highway, and the TDI shows to get 42 (23 and 30, respectively, in the city). So, there is a 7-9 MPG difference, let's keep with the high side of 9. To make up the difference in purchase price, given the average $0.40/gallon difference between diesel and gas today, you'd have to drive 470,000 miles to make up the $6,500 difference in price. Granted, prices will change over time, and diesel and gasoline may get closer in price, or diesel may actually go back down, but I wouldn't bet on it.
“To make up the difference in purchase price, given the average $0.40/gallon difference between diesel and gas today, you’d have to drive 470,000 miles to make up the $6,500 difference in price.”
I recommend getting a “slightly used” diesel instead of a new gas version. The engine will likely last much longer, so that’s actually a reasonable tradeoff. Also since we’d expect at least one engine replacement for the gas version over 400,000 miles, that would probably narrow things quite a bit. Still, who keeps their vehicles that long these days besides me? heh
“Granted, prices will change over time, and diesel and gasoline may get closer in price, or diesel may actually go back down, but I wouldn’t bet on it.”
It’s actually insane for diesel to be priced the way it is at the moment, but since the government is involved I suppose that’s no surprise... High diesel has an impact on a wide range of prices, certainly to include food.
We’ll have to see over the long run.
About diesels and altitude, in the 1980’s a had a brand new Datsun diesel kingcab pickup truck that I drove to the top of Pikes Peak in Colorado. I thought I wasn’t going to make it to the top and there was a 10 foot long plume of soot coming out of the exhaust. Maybe there was no altitude sensor or something like that on it.
lower oxygen level at that altitude caused incomplete combustion, so more soot.........
When air density is lower in high altitudes the turbine simply turns faster because of less resistance in pushing air that is less dense . It will compensate up to he point that it is capable of. In other words,lightly turbo or non turbo (normally aspirated) engines will still smoke at high altitude.
I had a 92 non turbo 7.3 ford diesel in a F 350 and would make service calls up to Paradise (Mt Rainier WA)and it would really leave nice black cloud. I really felt bad when I passed bicycle riders on the way up. Poor Bastards!
,,, you're right. I'm in New Zealand. I just sold a Benz 108CDi turbo diesel Vito van that I'd done 347,000km in. It was a sootmeister. I've replaced it with a turbo diesel Ford Focus wagon (1,8 litre) and it flies and is a lot cleaner. It's German assembled, with cruise control. The South African assembled model doesn't have cruise control. I don't think I could go back to petrol - there's just so much more to a tank with diesel.
I’m working on my third hundred thousand miles on my 97 VW Passat. I still get nearly 45 mpg. I’ve never had to replace the turbo. I can afford any car I want, I take frequent 1000 mile trips. I love the milage. When they come out with something better I will buy it.
Diesel fuel DOES cost more than gasoline, #5 is not a myth.
I bought my F250 midyear 2003, and for the first 6 months or so diesel was cheaper than gasoline, then is changed, and for the past 7 years fuel has been more expensive than gas.
Not per gallon, per mile.....
Ah, missed that important part.