Propulsion Systems in Aerial Mobility

As explained in the technology section, internal combustion engines are still widely used and almost unavoidable - especially in the aviation industry.

Currently, the two most widely used engine technologies in airplanes are internal-combustion piston engines (or reciprocating engines) and gas turbines (or jet engines). [1]

As the technology has already been explained earlier, we can turn right to how the technology is specifically used in aviation. Spark ignition gasoline engines are generally used in rather small aircrafts that seat one to six passengers and fly below an altitude of 4500 meters. [2]

These engines are typically powered by AVGAS – short for aviation gasoline – which is a specific fuel for small piston-engine powered aircrafts. [3]

A special form of combustion engine is the jet engine - the standard propulsion technology for current state-of-the-art aircrafts, powering almost all commercial aircrafts. [4] Here, air is channeled through a compressor into a combustion chamber, where the air and an injected fuel are ignited. The hot gases produced are then passed through multistage turbines and create the necessary thrust to move an airplane. [5]

Jet engines are typically more reliable and weigh less than equally powerful reciprocating engines and while they burn through fuel faster, the fuel used is cheaper due to the easier refining process compared to AVGAS. [6] The fuel used for jet engines is kerosene-based and (fittingly) called jet fuel. [7]

Due to the advantages mentioned above, jet engines are currently the standard propulsion system for commercial aircrafts. For example, the Boeing 777 is equipped with one of the largest engines in the world, the GE90 by General Electric, which generates 52 tons of thrust on each side and burns through approximately 5 liters of fuel every second at full throttle. [8]

There are other types of propulsion technology which do not rely on combustion as a central process of powering vehicles. Although many promising ones have been introduced and seem to dominate the future of propulsion technology across many means of transport, the situation in aerial mobility is slightly more complicated.

Electric engines are generally more efficient and their power-to-weight ratio exceeds the one of internal combustion engines. [9]

But still, electric engines are currently not yet suitable for commercial long-distance aerial traffic: The batteries necessary to power the full weight of a long-distance aircraft are too heavy to allow for successful take-offs of these machines. The main problem of electric aircrafts seems to be energy density: While kerosene allows for roughly 12,000 Wh (Watt-hours) per kilogram, litihum-ion batteries offer only 250 Wh per kilogram - a mere 2% of kerosene’s energy density. Additionally, electrical components might need additional insulation, new cables or switchboards especially at high altitudes. Although all of this might cast a shadow on the commercial use of electric aircrafts in the near future, there are positive tendencies as well: On May 28th 2020, a Cessna Caravan 208B with a fully electric engine managed to fly for 30 minutes making it the largest electric aircraft to do so. [10]

For short-distance flights, the technological situation is different: Drones are already capable of flying with fully electric propulsion systems. The system’s specifications are dependent on the purpose of the vehicle: What is the maximum weight of motor and batteries possible? How long is the drone supposed to fly at a time? [11]

Making use of the advantages of electric engines as well as the upsides of internal combustion engines, also leaves hybrid-electric propulsion systems (HEPS) as a promising alternative. Here, the internal combustion engine (ICE) and the electric motor (EM) are either used in parallel or in series to power the vehicle making use of the electric motors higher power-to-weight ratio and the “superior energy density of hydrocarbon fuels” () at the same time (Friedrich/Robertson 2015, p.176). Overall, this type of engine is suitable for small and medium-sized aircrafts (like drones), while the energy density of batteries still poses a problem for large aircrafts. [12] [13]

Hydrogen-powered aircrafts on the other hand are yet to be refined to be commercially viable, but could be a very interesting alternative in the future as research projects by industry experts like Airbus already show. [14] [15]

Especially as a propulsion system of drones, hydrogen fuel cells offer promising advantages: no harmful pollutants emitted, based on a resource available in abundance, longer possible flight times and a quick refuelling are some of them. [16]