Common Aviation Fuels, Diesel fuel

Experimental aircraft commonly use engines which consume AVgas (Lycoming / Continental / Franklin and equivalent types) or engines running Mogas (Rotax, Subaru, Jabiru etc). Some engines are capable of running either fuel, although with restrictions.

Purpose built diesel aircraft engines are designed to use JET fuel and can run on normal diesel fuel too. In this section we delve deeper into aircraft diesel fuels. As you will see there are some similarities between diesel and JET fuel.

Diesel Fuel Types

Diesel fuel is a light oil with density of around 850 gr/L and releases 40,9 MJ of energy per liter fuel. It is obtained at 200°C to 350°C in a fractional distillation unit.

JET fuel / kerosene is derived from the same source at 150°C to 275°C has 5% less energy. The basic properties compare so much that either fuel can be used in a diesel engine. Main difference is that the lubrication properties of diesel are much better and as the fuel is sometimes used as lubricant for the high pressure fuel pump, running only on JET fuel can ruin this pump if precautions are not taken. Mixing JET with about 2 to 5 % biodiesel will aid in increasing lubricity for the high pressure fuel pump.

Diesel fuel is denser, heavier, contains more energy than gasoline. It also is used to lubricate the fuel pump. But not in aircraft diesel engine designs as JET fuel does not have the lubrication properties as normal diesel fuel. Either engine oil is used to lube the pump or an additive (use a good synthetic two stroke oil in 1 : 100 ratio, 2 - 5% biodiesel, Stanadyne also has some very good additives for fuel) is added to the fuel. Diesel engines can use a wide range of fuel: peanut oil (like the original design by Rudolph Diesel), algea oil, coconut and those renewable types also known as biodiesel. By using these fuels (biomass) the engine is CO₂ neutral, meaning that you will not add to the CO₂ in the atmosphere. Biodiesel also reduces PM (particulate matter) in the exhaust gasses.

Compared to a gasoline engine, an aircraft with a diesel engine of the same power will have more range, endurance, uses less fuel for the same performance and burning less fuel means less pollution in the environment and this is even more so when biodiesel is used as fuel. Diesel engines are more reliable, simpler in design, have less components thus having more hours to TBO, in short: it is the ideal 'new' engine for the future of general aviation.

Biodiesel

Aircraft diesel engines should be able to run on biodiesel too. Confirm this with the engine manufacturer. Biodiesel can be obtained from palm trees, coconuts, algea, plants and even from waste cooking oil and it has good lubrication and fuel tank cleaning properties. It is biodegradable, nonhazardous and CO₂ neutral.

Cold weather

Diesel fuels are prone to gelling and waxing in cold weather if the right measures are not taken. Usually, fuel sold by roadside fuel stations is changed when winter sets in. But when an aircraft flies at high altitude temperatures can drop below freezing easily, if that aircraft was using roadside summer diesel it would experience fuel gelling and probably an engine failure. Special additives are needed to keep diesel fuel from gelling at low temperatures, or the exclusive use of JET fuel is recommended in these operations.

Sometimes fuel heat exchangers are used to make sure that no ice can develop in the fuel system causing engines surges during critical phases of the flight. And as the high pressure fuel pump intakes more fuel than the engine uses, the return fuel is warm/hot and helping to warm the fuel in the tanks preventing gelling and waxing.

Fuel properties

Some typical JET - Diesel fuel properties are:

Diesel fuel density

Diesel fuel weighs 7.08 Lbs/US gallon or put another way: 0.85 g/mL at standard temperature (15°C).

Detailed information on diesel fuels in this document from Chevron (opens in new window):

• Diesel Fuels (pdf)