InflateGate: All About Air Compressors

Part 1 of 2

So, we all know that your rig has more traction when you lower the pressure in your tires - similar to how lowering the pressure in a football gives you a better grip....kind of.  But we're not talking about deflategate here, we're talking about inflategate, and either way, you're trying to get an advantage on the trail.  So what do you do when the terrain changes, you need to repair a puncture, or get back on paved roads - you start up your air compressor.  We can't all be lucky enough to have air compressor stations at every entry and exit of the trail like Island Beach State Park, NJ.

In this two part series we will cover all of the ins and outs of air compressors.  How do they work?  What are they used for? What are the differences between available models? What's right for me? That last question is the most important, but we need to go over a lot of particulars to get there.  

 

How do Air Compressors work?

Air compressors have one job and it's pretty straight forward: to compress air.  Compressing air is nothing new, the air compressor was first invented all the way back in 1650.  A man named Otto von Guericke designed an air pump with a single piston and cylinder.  Although a similar concept to today's reciprocating compressors, it was more like a fire bellow which was used to compress air. It consisted of a single valve for intake and exhaust.  It wasn't until nearly 150 years later in 1799 when George Medhurst of England developed the first motorized air compressor.  

A lot has improved since 1799 to get us to where we are today; mounting air compressors in toolboxes and inside our overlanding rigs for use on the trail.  But how do they work? What should I be using under the hood or seat of my rig? What will allow me to run my lockers and re-inflate my 35s? Let's jump in to the design of it all and then see what's right for your specific application.

Air compressors compress air through decreasing the volume of air and thusly increasing pressure. Simply, as the amount of space air is held decreases, the pressure of that air increases.  This compression of air via one of a few different designs will determine what kind of pump you need for what you're doing.  Let's start with the basics of Air Compressor design.

 

Air Compressor Design

1. Reciprocating or "Piston" Compressors
2. Rotary Screw Compressors
3. Axial / Centrifugal Compressors

There are three different air compressor designs; reciprocating, rotary screw and rotary centrifugal.  We're going to focus mainly on reciprocating air compressors, as they're most common in 12v 4x4/offroad applications, but here is a breakdown of the other two designs and the types of compression that they create. We'll start with the types of air compressor.

Air Compressor Types

1. Positive Displacement
2. Dynamic

Positive displacement Air Compressors are mechanical devices where air is inducted in to a compression chamber and then mechanically reduced before being released.  These create the same amount of pressure in each cycle. Reciprocating and Rotary Screw Compressors are both positive displacement Air Compressors.

Dynamic Air Compressors differ because they do not physically or mechanically compress air, but reduce the volume of air by using two in-equal sized impellers.  The first impeller runs at much higher velocity and when the air is forced in to the second impeller, where the air flow is restricted and velocity decreases to cause the pressure to increase. An Axial/Centrifugal compressor is a dynamic air compressor.   A great example of a Dynamic Air Compressor is a turbo charger -they have an exhaust and intake wheel and the size of these wheels determine how much output your turbocharger puts out.

Now that we have know the types, how are air compressors designed?

Rotary Screw Compressors are in essence a screw type supercharger, think Eaton or Magnuson, where two mated helical rotors mesh together to compress air through trapping and reducing the volume air as the screws mesh together.  The benefits of this type of air compressor are that they are made to run at full load continuously.  This type of compressor is not only expensive and used on larger industrial applications (5-900 HP), but for a start-stop application like we're going to be using it for, they are inefficient.

Rotary. Out.

Axial or Centrifugal Air Compressors are dynamic air compressors.  The faster they spin, the higher pressure of air they can compress. These compressors want to spin continuously and are better for continuous use and generally commercial applications.  They are expensive, yet reliable.  Not used in 12v or smaller applications.

Centrifugal. Out.

 

Reciprocating Air Compressors are what we're going to focus on mainly as the primary design in 12v applications.  A reciprocating or "piston" type air compressor are a positive displacement compressor that mechanically increase the pressure of air by reducing the volume of air in the compression chamber.  Simplified, they are just like a single cylinder internal combustion engine.  Unlike an internal combustion engine where the stroke or compression cycle is powered by an explosion at the top of the chamber, 12v air compressors use an electric motor to rotate a crankshaft which helps to create an upward stroke of the piston.  This upward motion of the piston reduces the volume of the compression chamber and thusly increases the pressure of air before it's released. 

Sounds simple enough, right? Unfortunately that's where it starts getting more complicated.  We need to understand how air is compressed to understands the benefits and drawbacks of different products on the market.  Like with all of our automotive purchases, we want to make an informed decision so we don't have to upgrade or replace something later.  

 

Next week, for part 2 of Inflategate  we'll cover the 2nd half of our Air Compressor run-down:

• Efficiency of an Air Compressor: What affects output?
• Air Compressor Applications: What do I need?
• What are my options? What do I choose?