During World War II, Germany could not keep up with producing increasingly powerful piston engines for its aircraft, unlike her American and British rivals. Germany had to think of alternatives so as not to lose the war.
An alternative came with the development of the jet engines which could be produced cheaply by the laborers. The jet engines also did not require high-octane fuels that the piston engines demanded. The combustion in a jet engine is continuous, hence, it could be run on almost anything given the appropriate ignition details. Could these “almost anything” include ethanol or diesel?
A jet engine can run on ethanol or diesel if the right modifications are applied to obtain a fuel that has good combustion property, the right density, and high energy content.
For fuel to successfully run a jet engine, it must maintain fluidity even at very low temperatures (as low as -50⁰C), attain standard density and calorific values (quantity of heat produced), be non-corrosive to the fuel pipings, and burn clearly.
Let’s consider if a jet engine can run on ethanol or diesel and find out the differences between the commonly used jet fuel, ethanol, and diesel.
Table of Contents
Diesel As An Aviation Fuel
Commonly referred to as gas oil and chiefly a mineral fuel, diesel is a middle intermediate fuel just like jet fuel. Diesel is somewhere between light and heavy fuel oils. The boiling range for diesel fuel is about 170⁰C and 390⁰C.
Diesel is suited for internal combustion engines that require accurately timed explosive ignition while jet engines work using the mechanism of continuous combustion.
Let’s consider more reasons why pure diesel cannot work efficiently in a jet engine:
- Diesel has more energy per pound than jet fuel. This makes diesel harder to combust (it needs preheating) and may require the inclusion of bigger batteries which adds to the weight of the aircraft. Furthermore, if an engine were to burn out in flight, there would be a big problem because it can’t be re-started easily on diesel.
- Diesel needs to be compressed to be burnt, which makes it burn dirty. It is a dirty fuel, found in the fractionation process close to tar and asphalt. Some of these may be found as residues in diesel fuel which goes ahead to clog the engine.
- Diesel is notable for its low freezing point at 0⁰C to -5⁰C. It freezes and gels up at low temperatures experienced at high altitudes and can cause severe damage to the engine.
Ethanol As An Aviation Fuel
There is an ongoing far-and-wide search for the replacement of petroleum. So far, ethanol has emerged as a clean and reliable energy source and a potential aviation general fuel. Ethanol is the most commonly used biofuel in the world, particularly in Brazil.
The ethanol from Brazil offers a better potential because its feedstock is from sugar. Sugar, compared to corn, is a far better source of carbon to be converted straight to ethanol because it is a much simpler carbohydrate, containing only two monomers.
There are possibilities that ethanol can be used in jet engines when mixed with Jet A-1 in the specified standard proportion.
Merits
From ongoing studies, ethanol has been found to offer some benefits to aircraft engines. These benefits may include:
- The use of ethanol gives a doubling time between engine overhauls to generate lesser vibrations.
- There is little or no carbon emission from burning ethanol thereby reducing atmospheric pollution when compared to the burning of fossil fuels.
Demerits
The use of ethanol in jet engines would come with a lot of baggage and that is why it has not yet been harnessed as a fuel for running a jet engine. Take for instance;
- The specific energy of jet fuel is 43.1MJ/kg while that of ethanol is 26.1MJ/kg. This means that ethanol is 60% less dense as jet fuel. The problem with this is that for ethanol to generate the same power as jet fuel then about twice the volume of ethanol is required. This is a dead loss in a matter sensitive industry.
- Consequently, the use of ethanol will cause a range penalty to aircraft. The heavier weight of the extra-capacity tank on the aircraft will increase fuel consumption. This means a 10000-mile range airplane would turn into a 5000-mile range aircraft.
- It would be more expensive to refine and use ethanol than jet fuel because it has to be further cracked with another fuel for it to be usable in a jet engine.
- Ethanol being an oxygenated biofuel is hygroscopic. It will always lean out of the air/fuel mix ratio which inadvertently raises the normal working temperature causing abrasion of the engine and may lead to worse problems.
- Ethanol is corrosive owing to its polar nature that attracts water. It is a violent solvent that may attack the fuel pipings, fuel bladders, fuel lines, floats leading to fuel starvation, leaks, and flooding.
Presently, ethanol, if enhanced to increase its energy/gallon/per pound, can only be used as stock to upgrade the mix but not as a major fuel for jet engines!
Jet Fuel
Jet fuel, very similar to kerosene, also known as aviation turbine fuel (ATF) or avtur is a colorless to yellow-colored mixture of different hydrocarbons. Liquid fuel types of various specifications often used in aviation are collectively termed as Jet fuel. It is used essentially for jet engine aircraft, but about twenty years ago, it has also been specially modified for diesel engines for light aircraft.
Jet fuel is a middle distillate (Gasoil) between 175⁰C and 288⁰C with a density between 0.747g/cm3 and 0.84g/cm3. And the specific energy of Jet fuel A-1 is 43.1 MJ/kg. Commonly used fuels for commercial aviation include Jet A, Jet A-1, and Jet B (which has an amplified cold-weather performance). While the military jet fuel includes JP-1, JP-2 ……. JP-10.
There are basically two kinds of jet fuels.
- Kerosene-type jet fuel. Jet A, Jet A-1, JP-5, and JP-8 are examples. Jet A-1 is the most commonly used kerosene-type jet fuel and often regarded as jet fuel because of its wide acceptance.
- Wide-cut or naphtha-type jet fuel. Examples include Jet B and JP-4.
Additionally, additives are used in jet fuels to safeguard the turbines against rust, static charging, and ice-frost.
Merits
Of all the aviation fuels, jet fuel has the perfect blend of the desired characteristics for running a jet engine. The jet fuel is both easy enough to vapourize that it does not require preheating to burn and also difficult enough to vapourize that it does not gasify under usual flight conditions.
Jet fuel is dense enough not to dissolve seals, likewise, it is light enough not to clog up dirt in the turbine. However, the outstanding characteristics of the Jet fuel must be:
- Its freezing point (around -50⁰C). This ensures that the jet fuel is not susceptible to frost in the sometimes extremely low temperature an airplane can be subjected to at its cruising altitude.
- Its higher flash point, which makes jet fuel less flammable. This makes jet fuel safe to handle and enhances flight safety.
When a hydrocarbon is ignited, kinetic energy and heat energy are both released. Distinctively, jet fuel produces very little heat but a lot of kinetic energy which generates more thrust without melting the aluminum frame of the aircraft.
In comparison to aviation gasoline (avgas) used in piston-engine aircraft, jet fuel is cheaper and has a higher energy content per unit volume (approximately 128000 Btu/gallon for Jet fuel and 114 000 Btu/gallon for avgas). This means that the same quantity of jet fuel generates more power than avgas for combustion.
Demerits
Jet fuel, like other fossil fuels, is environmentally hazardous because of its carbon emissions and spillage. Carbon emissions and oil spillage contribute drastically to ozone layer depletion, pollution of the waterways, and climate change. This has led to vehement clamors from environmentalists for the replacement of fossil fuels with biofuels in the automotive industry generally.
The exposure of humans at any level to kerosene-based hydrocarbon fuels (jet fuel) or its constituent chemicals can cause gastrointestinal disorders, cancer, neurological disorders, respiratory disorders, visual and hearing disorders.
The Use of Lead in the Various Types of Aviation Fuel
Lead is a heavy metal yet malleable and ductile. Lead also has a comparatively low melting point, inertness, oxidation, abundance in nature, and low cost granting lead an extensive usage in white paints, batteries, plumbing, radiation shield, leaded gasoline, and so on.
Tetraethyl lead (TEL) is a form of lead added to avgas to increase the fuel’s octane rating and allow a high compression ratio. This is vital because burning fuel at an octane rating lower than the minimum requirement detonates and knocks aircraft engines. Lead also prevents wear and tear caused by friction in high-performance engines.
The various grades of avgas are classified using the Motor Octane Number (MON) accompanied by an alphabet-designation to show the lead content are 100LL, 82UL, 91/96UL, avgas 115, UL102 and so on with (LL) as Low Lead (LL), (VLL) as Very Low Lead and (UL) as Unleaded.
Piston Engine Aircraft
Fuel heats up as it is compressed in a motor’s cylinder and will auto-ignite at the wrong time if it reaches its ignition point during the compression. When this happens, it causes loss of power and damage to the engine.
This necessitates the use of fuels with a higher octane rating that can handle compression well without self-igniting before the ignition point is attained. Over time, it was discovered that by adding lead to fuel, the octane rating of avgas was remarkably increased for use in a piston engine aircraft.
This significant discovery paved the way for the production of cheaper grades of fuel whilst maintaining the essential octane rating required by the piston engine.
On the other hand, Jet fuel used in jet engines of big commercial planes does not contain lead.
Downside
The metal, lead, was discovered to be a highly poisonous metal in the 19th century. It is a neurotoxin that has been revealed in scientific research to accumulate in the soft tissues and bones of humans, damaging the biological enzymes, the nervous system, the cardiovascular system, and the immune system.
This awareness has provided a high degree of motivation to get rid of lead and its compound from aviation fuels and the environment at large. The usage of lead TEL has not been outrightly banned for use in avgas, it has just been strictly moderated because no operationally safe replacement has been discovered yet. Though, with the inclusion of a special oil additive, unleaded avgas can now be burned in some piston engine aircraft.
Our article on lead in aviation fuel takes a closer look at this!
Conclusion
Going by all the aforementioned merits, jet fuel remains the perfect fuel suited for running and flying a jet engine aircraft until an eco-friendly, operationally safe and efficient substitute is found.
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