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How WW II Aero-engines Were Developed to Extract More Power

The performance of a fighter was inseparably linked to engine power. The more power, the better the performance.

Engine power comes from the fuel. Gasoline is burned in the presence of oxygen. The more fuel is burned, the more energy available to the engine. If there is a lack of oxygen, it is not possible to burn all the fuel. So, the limiting factor is the oxygen present in the burning chamber. Air provides oxygen.

When an aero-engine is designed, the power it develops is proportional to the size of the burning chamber and this in turn is a function of the area of the cylinder (A) times the stroke (L). This product (A x L) is volume, a.k.a. ‘displaced volume’. When you multiply this volume by the number of cylinders in the engine, you obtain the ‘total displaced volume’ of the engine. The higher the displaced volume, the greater the power (everything else being equal).

Also, the faster the revolutions per minute of the engine the more power it will develop. In a four-cycle engine there is a power stroke (burning of fuel) every two cycles, so to keep the proper dimensions we use rps/2 (revs per second divided by 2).

Once a good engine was designed and available in quantity, it was not possible to increase the number of cylinders or the size of the pistons. It was also difficult to increase the rpm because of many limiting mechanical factors (i.e., exhaust valve closing, material strength, etc.).

As a result, the development of an engine became a quest of increasing the air that could be packed inside the pistons and this meant increasing the air pressure in the intake manifold. This was accomplished by using more powerful superchargers (driven by the crankshaft), turbochargers (driven by the exhaust gases), or by injecting oxygen-rich materials (nitrous oxide). Because the increased pressure meant greatly increased temperatures, the charge was prone to burn during compression (before the spark) reducing power. It was essential to decrease this temperature by using intercoolers or injecting water. It was also necessary to use fuels that were less prone to premature combustion (higher octane).

This is expressed as Power (in Watts) = PLANK/2.

The units of Pressure, Stroke, Area, revolutions and number of cylinders (adimensional) can be seen in the photo.


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