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 and can run on normal diesel too. In this section we delve deeper into aircraft diesel fuels. As you will see there are some similarities between diesel and JET enabling the engine to use both.
Diesel is a light oil with a density of around 850 gr/l and it releases 40,9 MJ of energy per liter. It is obtained at 200 °C to 350 °C in a fractional distillation unit.
JET / 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 can be used in a normal 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 primarily 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 engines with this type of pump depend even more on proper lubrication, so these will have to take precautions too should they want to use JET for some reason.
Diesel 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. 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 when filling the tanks.
Diesel engines can use a wide range of fuel sources: peanut oil (like the original design by Rudolph Diesel), algae 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 gases.
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. 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.
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, algae, plants and even from waste cooking oil and it has good lubrication and fuel tank cleaning properties. It is biodegradable, nonhazardous and CO2 neutral.
Diesel is prone to gelling and waxing in cold weather if the right measures are not taken. Usually, the formulation of the fuel sold by roadside stations is changed when winter sets in. But when an aircraft flies at high altitude temperatures can drop below freezing easily, if it was using roadside summer diesel it would experience fuel gelling and probably an engine flame-out. Special additives are needed to keep diesel from gelling at low temperatures, or the exclusive use of JET is recommended in these operations.
Sometimes 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 thus helps to warm the fuel still in the tanks preventing gelling and waxing from occurring.
Some typical JET - Diesel properties are:
|Property||JET A/A-1||JP-5||JP-8||Diesel #1||Diesel #2|
|API Gravity @ 60 °F||44.3||41.1||45.6||43||39|
|Flash point in °C||38||62||45||38||52|
|Viscosity cSt @ 40 °C||-||1.5||1.2||1.2-2.4||1.9-4.1|
|Sulfur, %mass||0.3 max||0.4 max||0.4||0.05||0.05|
|Heat content Btu/Gallon, Net.||123608||125270||123069||130000 (typical)||129500|
Diesel weighs 7.08 lbs/US gallon or put another way: 0.85 g/ml at standard temperature (15 °C).