An understanding of the physics that allows flight must be built on a foundation of understanding of the "first principles" involved. I have found recent postings about pressure and atomization a little confusing so here is a modest attempt to clarify things.

Almost all pressure gauges measure the difference in forces present on two surfaces. The force on the surface is the result of gas or liquid molecule's atomic forces colliding with that surfaces atoms' atomic forces. The resultant normal net differential force is measured directly, in the case of a bellows (altimeter) movement, or inferred by the presence of  gauge element strain in the case of diaphragm or Burdon Tube elements.

Almost all pressure gages measure strain (movement) either mechanically (levers and gears) or electrically (resistive strain gauge).

Differential gauges are divided into three classes:

1)  Absolute Pressure gauges that measure a pressure referenced to a vacuum. (Altimeter and MAP)
2)  Differential Pressure gauges that measure the pressure difference between two ports. (Cabin differential pressure)
3)  Gauge Pressure gauges that measure a pressure referenced to the local ambient pressure. (Fuel, oil and hydraulic pressure)

Differential Pressure gauges can be configured for either Absolute Pressure measurements by connecting the reference port to vacuum or configured for Gauge Pressure measurements by leaving the reference port open to the environment.

Almost all pressure gauges fall into one of these categories. There are other methods of measuring pressure or inferring pressure from density using lasers, ionization, drag, resonance and even acoustics.

Atomization is the process where large droplets of fluid are divided into smaller droplets. The smaller the droplet the larger the ratio of surface area to volume and, in the case of a fuel air mixture, the faster the droplet will vaporize.

Pressure does not cause atomization. Shear causes atomization. Shear occurs at the interface of two slipstreams of different velocity. Picture a high wind over a lake, producing whitecaps and atomizing the water into a fine mist. In the case of an intake runner, injecting fuel near the wall of the tube (just above the boundary layer) produces better atomization than if the same injector was located in the center of the tube. High performance carburetors have a secondary coaxial venturi and the fuel is injected at its periphery.

Combustion requires fuel vapor (not liquid) and atomization encourages vaporization so the more atomization the better, right?  Wrong. Vaporization requires heat, reducing the temperature and increasing the density of the fuel air charge. For best  volumetric performance the fuel should vaporize after it enters the combustion chamber but before the intake valve closes. The charge is cooled by the vaporization, increasing its density and providing more "room" for additional charge. This timing requires fuel droplets of a specific size range, not too big or small so more atomization is not better, after a point.

Does any of this matter in our aircraft engines? Not very much, really.  Our aircraft engines are big and slow and use (on injected engines) continuous injection. In high performance fuel injected racing engines, fuel injection begins at the start of the intake stroke and stops at the end. This produces a relatively homogeneous mixture of fuel and air entering the cylinder. On our aircraft fuel injected engines all the injectors are flowing all the time. The fuel accumulates in the intake tube for 75% of the time and is then gulped into the cylinder in one very rich slug followed by a very lean mixture. Fortunately, because aircraft engines operate at relatively slow speeds, there is lots of time to vaporize the fuel.  As George Braly will likely tell you, consistency from cylinder to cylinder and from cycle to cycle has a greater effect on performance and smoothness than fuel droplet size.

Wishing all of you clear and safe skies in the new year.

Brent Regan