If you watched the MythBusters drive an old Mercedes diesel fueled with used cooking oil, you probably wondered why we don't do that all the time with all diesels. After all, Cummins B-series are the oldest engines still being produced today, and aren't they hardy enough to handle fry oil? The U.S. Department of Energy (DoE) has a short paper on straight vegetable oil (SVO) as a diesel substitute, and the gist is that vegetable oils are more viscous than petroleum-based diesel, are more reactive to oxygen, and exhibit long-term operational and durability issues.
That's not to say that vegetable oil has no place in your Cummins-powered Ram, but it needs transesterification to make crude biodiesel, which is then refined into stable and consistent biodiesel. Pre-heating the SVO thins it out so it won't gum up the works as much, as the DoE paper shows that SVO can actually increase engine deposits. Stable biodiesel can still have nasty effects on an engine, though, so it's usually blended with regular diesel. Common ratios are B5 (5% biodiesel) and B20 (20% biodiesel), with B100 (100% biodiesel) being used for some fleet vehicles.
Now, let's get to those biodiesel-loving (or at least accepting) Cummins offerings. Every Cummins automotive/industrial engine can take B5. Furthermore, all on-highway engines happily gulp B20, including any ISX CM570 made after January 2002, all 6.7-liter Cummins-powered Ram pickups, the ISM, ISL, ISV 5.0, ISB, and the rest of the ISX lineup made after January 2007. Most of the company's off-highway/MerCruiser diesel marine engines made after January 2007 and high horsepower off-highway/marine engines made after January 2008 can also handle B20. The QSK95 and some generator engines can use B100.
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Cummins designs its engines around the Advancing Standards Transforming Markets (ASTM) International specifications for biodiesel (ASTM D6751 for B100, ASTM D7467 for B20). Standardization is why it's a good idea to use refined biodiesel rather than just pouring leftover fry oil into your diesel car. Careful storage guidelines also prevent fungus and bacteria growth. Sulfur inhibits such contamination, but removing it from diesel to help the environment makes bacteria growth more likely, too.
Biodiesel can also obliterate hoses and gaskets unless the rubber contains the right substances, such as nylon or Teflon. Its lubricity can be used for good, though, as biodiesel loves dislodging fuel system deposits, and that high lubricity helps prevent wear and tear. Just prepare for a clogged fuel filter if you switch to biodiesel, as those deposits have to go somewhere.
In addition to designing around biodiesel's unique properties, Cummins also had to mitigate its emissions quirks. Studies generally show carbon monoxide (CO), particulate matter (PM), and hydrocarbon (HC) emissions drop on biodiesel, but nitrous oxide (NOx) usually increases. However, not every study shows this increase, and the website Journey To Forever cites a few studies that show NOx doesn't have to be a concern. One study from the U.S. National Biodiesel Board states, "biodiesel's lack of sulfur allows the use of NOx control technologies that cannot be used with conventional diesel. So, biodiesel NOx emissions can be effectively managed and efficiently eliminated as a concern of the fuel's use." Cummins' solution is a combination of exhaust aftertreatment and turbocharged air management, which practically eliminates NOx.
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Then, there's the temperature issue. Plant-based oils become viscous in frigid temperatures. However, just as there are winter and summer gasoline blends, both bio and petroleum diesel get cold flow improvers for winter. That said, Cummins points out that biodiesel has a relatively high cloud point, or the temperature at which wax crystals form, and the company suggests using additional diesel winterizer supplements to protect fuel filters, even in its biodiesel-friendly engines.
As for power, a gallon of B20 generally contains up to 2% less energy than petroleum-based diesel, but the cetane rating is higher, meaning you can get more complete and faster ignition. While diesel number two's cetane generally maxes at 45, biodiesel's cetane rating can be as high as 67.
While ethanol comes from plant material, biodiesel has multiple sources, including yellow grease, which is used restaurant oil, as well as algae, canola, soybean oil, and animal tallow, aka fat. So, if you're vegan, your diesel vehicle probably isn't. Suppose you avoid B20 or B5, well, there's a good chance that your regular diesel fuel contains at least 1% biodiesel. This isn't done to trick people into putting animal-based liquids in their vehicles, but rather to make good use of biodiesel's lubricating properties. Yes, your Cummins is an omnivore.
