How a piston engine became a jet engine

If you put a piston engine and jet engine side-by-side they will look like different designs utilising almost unrelated engineering concepts. Yet, the jet propulsion can be viewed as a development of the piston engine. This post will cover the fascinating story of its evolution. I decided not to follow the exact chronology of the ideas presented but rather track the development of the engineering thought behind them.

Piston engines convert the energy of the hot gasses expanding inside the cylinders into mechanical energy which can be used to, for example, drive aircraft propellers. This diagram below explains the basic principle behind the process.

four-stroke-engine

Jet engines utilise the rearward thrust created by the hot gasses expanding inside the combustion chamber to propel the aircraft forward. Again, a diagram may help explain how it works.

jet_engine_diagram

Most early aircraft had propellers powered by piston engines. At some point aviation engineers realised that the exhaust gasses discharged from cylinders carry a lot of energy which could be recovered and put to some use. This has led to the widespread employment of turbo-superchargers in aircraft piston engines. Shown in the sketch below:

engine_turbo_charger_cut_away

a turbo-supercharger consists of a turbine spun by the energy of the exhaust gasses, coupled with a compressor which boosts the pressure of the air entering the cylinders. More air allows to burn more fuel per cycle and produce more power. So, instead of wastefully discharging into the atmosphere, the exhaust gasses feed some of the energy they carry back into the engine. What a great idea!

But technological progress never stops and someone came up with another clever concept – direct recuperation of the energy in the exhaust gasses. In essence, the turbine mounted on the engine exhaust fed the recovered energy directly into the propeller shaft, through a mechanical gearbox with viscous couplings. One example of direct energy recuperation is the Wright-3350 engine used in B-29 Superfortress strategic bombers.

prtgear

The crankshaft on the bottom left is powered by Energy Recovery Turbine – of which there were three, each fed by the exhaust gasses ducted from six of the engine’s eighteen cylinders. The system was reputed to boost the total power produced by the engine by 20%.

Then came the P-51 Mustang fighter utilising the Meredith Effect, theorised already in mid 1930s. Piston engines produce a lot of waste heat which must be removed from the engine to prevent overheating. In liquid-cooled engines, like the Rolse-Royce Merlin powering the Mustang, this job is done by the cooling water. Pumped through the ducts formed in the engine block it collects heat and then gives it off in the radiator. The ambient air flowing through the finned coils of the radiator takes that heat away and the cooled water is pumped back into the engine. Frederick Meredith’s idea was to direct the heated, expanding air rearwards, to produce the extra thrust to propel the aircraft.

radsta

Have another look at the pictures above. Does the Meredith Effect seem similar to another type of aircraft propulsion, already discussed in this post? What if we did away with the cylinders, pistons, superchargers, cooling systems and simply combusted the fuel inside a chamber and ejected the exhaust gasses to the rear to create thrust? Well – we have just (re)invented the jet engine, which is nothing else but a piston engine stripped of all the ancillaries like pistons etc!

For some reason I am finding the development of human thought – be it in the field of social sciences, religion or engineering – irresistibly fascinating.

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