Trucks and smaller vehicles with diesel engines run on diesel fuel which looks more or less like kerosene - less flammable than gasoline, low viscosity liquid. However larger diesel engines like this 120 thousand horsepower marine engine have more or less the same design but use fuel oil which looks very different from diesel fuel - much higher viscosity and I'd guess igniting fuel oil at room temperature is a challenge.

How does it happen that engines of the same design use two different fuels? If one of them is superior to the other then why won't they all stick to that superior fuel?

  • $\begingroup$ Same reason some people buy Kellogg's Frosted Flakes and other people buy ACME Brand Frosty Corn Cereal. $\endgroup$
    – Air
    Mar 14, 2015 at 21:10

5 Answers 5


Have you looked at the size of one of those maritime diesel engines? They are larger than your car and need to deliver a lot of power to move and power the ship. That takes a lot of fuel so it's cheaper to burn more cheaper fuel even if it is of inferior quality. The bigger size also lets it use wider fuel lines so the viscosity is less of an issue. You also need to heat fuel oil to be able to pump it around.

Cars use diesel because emission regulations and fuel economy. The narrow regulations of diesel fuel lets the manufacturers design the engine to burn just that fuel as efficiently and as cleanly as possible. There is also a size constraint on car engines. So more viscous fuel would be an issue and take up precious space for wider fuel channels and heating elements to keep the oil liquid.

Cost per ton per mile just works out better to use diesel for trucks and cars while fuel oil works out better for the large ships.

  • $\begingroup$ Good catch on the emissions implications. Ships often have to switch to cleaner-burning fuels as they approach a port of call to comply with that nation or region's regulations. See "Demonstration of fuel switching on oceangoing vessels in the Gulf of Mexico" by Browning et al. for one published example. $\endgroup$
    – Air
    Mar 16, 2015 at 16:39
  • $\begingroup$ Don't forget the ability to pump in cold weather, and fuel duty (in some countries it is unlawful to use fuel on the road unless duty is paid, which limits the grades you can buy.) Filling stations sell a less viscous grade of diesel road fuel in winter. $\endgroup$
    – dcorking
    Mar 17, 2015 at 13:52
  • 1
    $\begingroup$ I think the first two sentences of your second paragraph are wrong. Cars used diesel well before there were any emission regulations, and while early diesels cared about efficiency, there was no "narrow regulation". $\endgroup$ Nov 18, 2019 at 9:16
  • 1
    $\begingroup$ Fuel oil is the most dirty you could create of crude oil. During distillation, it is not going anywhere as vapor. It's literally the dirt that is left after distillation, except the part that is not even liquid when hot. Regarding pollution, It does not get worse. And it does not get cheaper. $\endgroup$ Nov 18, 2019 at 10:58

Simple economics. Marine engines consume enormous amounts of fuel, so in order to reduce operating costs, they use the cheapest, least desirable sludge that the oil refineries can produce.

  • $\begingroup$ Then why would smaller diesel engines not use fuel oil too? With half million miles of lifetime for a single engine that would be a serious saving too. $\endgroup$
    – sharptooth
    Mar 16, 2015 at 7:11
  • $\begingroup$ @sharptooth: Users of smaller engines need a fuel that's easier to handle and has more tightly-controlled characteristics, and they can afford to pay the higher price. $\endgroup$
    – Dave Tweed
    Mar 16, 2015 at 11:41
  • $\begingroup$ @sharptooth Just a comment from an economic standpoint: Crude oil is refined into 4 basic categories: light distillates (including gasoline), medium distillates (kerosene, "diesel"), heavy distillates (fuel oil), and superheavy distillates (waxes, asphalt, lubricants). This is unchangeable, based on the laws of chemistry and the actual stuff that exists in crude oil. Now, from an economic standpoint, if we only used one of these, it's price would go way up because of huge increase in demand. So we Humans try to use all four of them in a more or less balanced way. $\endgroup$
    – DrZ214
    Nov 2, 2016 at 22:19
  • 1
    $\begingroup$ @sharptooth diesel engines will run on pretty much any fuel, so long as the injectors can pump it into the cylinders. In countries where there is a high rate of fuel tax, running standard diesel car engines on cooking oil is a well known (and illegal!) way to save fuel costs. "Igniting the fuel at room temperature" is irrelevant, because the air in the cylinder is heated when it is compressed - and even diesel car engines have additional heaters for cold starting. $\endgroup$
    – alephzero
    Jan 20, 2017 at 21:01
  • 3
    $\begingroup$ @DrZ214: That split can definitely be changed; it's called cracking. Heck, the Germans made diesel out of coal in WW2. $\endgroup$
    – MSalters
    Nov 18, 2019 at 7:50

A diesel engine for a car needs a fuel which is liquid even in winter. This fuel should contain a very small amount of sulfur to limit air pollution.

The marine bunker oil is not liquid at room temperature, it has to be heated to about 50 °C before pumping out of the tank and to about 130 to 140 °C before injecting it into the cylinders. It contains a lot of sulfur, a lot of harbours do not allow it to be used inside the harbour because of the air pollution. The ship needs an extra fuel tank with diesel oil for the harbour and for the cold start of the engine. The bunker oil has to be pre heated after a cold start before it could be used.

  • $\begingroup$ Couldn't emissions control equipment (including scrubbers) take care of the pollution caused by bunker fuel? $\endgroup$
    – El Ectric
    Jun 22, 2020 at 3:47
  • $\begingroup$ Sulpher reduction regulation is international, not just in harbors, since 2020-2021. Some ships added scrubbers: some ships buy the more expensive low-sulpher fuel. Demand for low-sulpher fuel has increased price, which has had the effect of making scrubbers economical for some ships. $\endgroup$
    – david
    Jun 9 at 1:59

Diesel engines can run on a wide variety of fuels, as long as the fuel provides some lubrication.


Residual fuel oil is less useful because it is so viscous that it has to be heated with a special heating system before use and it may contain relatively high amounts of pollutants, particularly sulfur, which forms sulfur dioxide upon combustion. However, its undesirable properties make it very cheap. In fact, it is the cheapest liquid fuel available. Since it requires heating before use, residual fuel oil cannot be used in road vehicles, boats or small ships, as the heating equipment takes up valuable space and makes the vehicle heavier. Heating the oil is also a delicate procedure, which is impractical on small, fast moving vehicles. However, power plants and large ships are able to use residual fuel oil.


Diesel and heater oil are the same thing , the tiny difference is the additive package ( a fraction of 1 % of the total). Diesel may have a better additive to prevent wax from crystallizing and plugging lines in cold weather , for example. From time to time in various locations there may be different limits on sulfur ; depending on the refinery this may or may not result in different S levels in the products. Heavy marine fuel is a different from diesel because marine engines have heaters to reduce the viscosity of heavy oils like "bunker C".


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