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The Problems with Diesel

Diesel engines in vans used to be “the one you want.” They offered excellent real-world usable power, and combined it with good fuel economy and long-term reliability.

However, beginning in the mid-2000s this started to change with the introduction of stricter emissions standards. These days, diesels are strongly out of favor and being phased out by many manufacturers. This article is to discuss why.


Applicability

This is not a slight against any specific manufacturer; these problems are endemic to all modern diesel engines produced from the mid-2000s onward. Ford, Mercedes, Chevrolet, and Dodge vans all suffer from these problems. There's a few that are less bad than others, and most are pretty reliable for the first few years. But once they get over 100k miles, they all have some parts that are guaranteed to fail and are devastatingly expensive to repair.

Known Good Engines

These engines were produced before the tightened emissions regulations took effect, and are generally know for being reasonably reliable. Any engines built after these known good engines will be problematic.

  • Ford
    • 7.3L Powerstroke V-8 (1994-2003)
  • Mercedes
    • Diesel engines 2007 model year and earlier
  • Chevrolet
    • Pre-2005.5 DuraMax
  • Dodge
    • High Output Cummins 600 (model year 2004-2006)

Oxides of Nitrogen

Diesel engines are really bad emitters of NOX, a pollutant formed when combustion temperatures go over ~2000 degrees. NOX is easy to get rid of in gasoline engines by enriching the mixture a bit and possibly adding a little EGR to the intake, which helps lower the combustion temperatures by acting as a kind of “buffer”. But enriching the mixture doesn’t help diesels, so you need a lot of EGR to bring down combustion temperatures.

Which is a problem because the diesel exhaust under full throttle is extremely hot, that’s why we’re having this problem to begin with. So the manufacturers need to cool the exhaust gasses that are being injected into the engine intake with a water-to-exhaust-gas cooler.

Unfortunately, trying to build a cooler that can withstand 2200 degree gasses and 200 degree coolant right next to each other has been an engineering nightmare - they are typically made of stainless steel, but over time the coolers crack and/or plug up. This is a problem with all modern diesel engines, on everything from little VW’s to giant semi-truck Cummins ISX engines. When they crack, they result in water squirting into the engine intake which can cause internal engine damage due to the increased pressure when coolant turns to steam. Also, it’s generally a ton of work to change the EGR cooler because of its location, especially in V8 diesels. They also plug up easily because cooling exhaust causes the soot to drop out of suspension and collect everywhere.

Adding DEF/SCR helps a lot, but using too much DEF is considered a bad thing because of the hassle and expense. This is one of the things that happened after Dieselgate with the VW’s - the ‘fixed’ cars now use 3 times as much DEF.

Exhaust Soot

Diesels put out a ton of soot, this is what gives diesel exhaust its characteristic big black clouds when under heavy load. To get rid of it, manufacturers use a particulate trap that collects the soot and uses extra-injected fuel to burn it away in a process that that can go over 2800 degrees. This works okay in vehicles that are driven hard all the time, like semi-trucks (Well the chambers eventually plug up, but it takes a while).

But passenger vehicles’ light duty cycles don’t allow for automatic burning, so sometimes they have to perform forced burn cycles when the vehicle is parked. This uses a ton of extra fuel and upsets the owner, who doesn’t understand why their vehicle is running at a fast idle by itself and has superheated exhaust pouring out the back. Some early light trucks actually caught their plastic rear bumpers on fire.

These chambers also get plugged up easily, which is incredibly expensive to fix due to the part cost and where they are located.

To top it off, some early light diesels did a terrible job integrating the extra fuel delivery required - instead of adding an extra injector, they just cheaped out and had the last injector in the engine shoot extra fuel during the exhaust stroke, which caused internal engine issues (The 6.4L V-8 used in the Econoline van and Ford trucks from 2011-2015 was a particularly egregious example of this).

Fuel Injection

Modern diesels run over 30,000 psi of fuel pressure, which makes for a powerful and smooth motor but also makes for 3,000$ injectors and 2,000$ injector pumps. For example, it costs more to replace the injectors in a modern light-truck diesel than to put a WHOLE NEW ENGINE (including injectors) in most 1990s-era light pickup trucks. And they WILL go bad eventually, typically from 150,000 miles onward. This is made worse by unreliable fuel pumps, which (when they fail) will introduce a bunch of metal shavings into the fuel line which will ruin all of the injectors.

Plus new fancy variable-scroll turbos are problematic in lightly loaded applications, the vanes stick open/shut due to rust.

On top of all these myriad issues, the Marketing departments realized that diesel engines had gotten an almost godlike mythos around them among the crowd of light-truck buyers to the point where they'd easily shell out another $5,000+ on top of the top-end gas engines for a diesel. This resulted in an escalating horsepower war between all the major manufactures (especially in the light pickup truck segment), but designing a low-revving diesel to make a ton of horsepower makes materials/construction of the engine itself very very expensive to initially build.

Looking forward

Over time as manufactures have figured out what works/doesn't work, diesels have gotten slightly less bad than the first iterations of emissions-complaint engines. And for situations where they are run hard with severe duty cycles, diesel still offers a significant advantage in usable power and fuel economy. However for light-duty cycles such as seen in vans and consumer-oriented trucks, we can expect to see manufacturers realizing that diesels no longer make sense outside of perhaps the largest 1+ ton trucks. This is exemplified by Ford completely dropping the diesel option for the Transit in the US market for the 2022 model year.

rv/diesel.txt · Last modified: 2022/06/23 21:25 by princess_fluffypants