How To: Air Compressors

Words Dean Mellor

If you’re going to drive off-road you need an air compressor. No ifs. No buts. Even if you’re travelling on formed gravel roads, adjusting the air pressure in your tyres to suit different terrains has several benefits, including improved ride comfort, extended tyre life, reduced chance of tyre damage and improved traction. Of course, you can deflate your tyres with nothing more than a twig jammed into the tyre valve, but you’ll still need an air compressor to reinflate them.

Air compressors aren’t particularly expensive items, and they can be either permanently affixed to your vehicle and wired up to the electrical system, or can be supplied in a handy carry case with alligator clips to attach to your vehicle’s battery. If you go for the latter option, you should always carry the air compressor in your vehicle, because you never know when you’ll need it. As well as tyre inflation, an air compressor can come in handy for inflating other items such as air mattresses and kids’ toys, or for blowing out dusty air filters.

What to look for

There are lots of cheap, crappy air compressors on the market, but before you succumb to temptation, remember: a failed air compressor can leave you stranded, so you don’t want to be a scrooge when it comes to this important piece of gear.

Fortunately, there are also plenty of good quality air compressors on the market. Here’s what you should look for when choosing one, including claimed airflow capacity, maximum operating pressure, maximum ambient operating temperature, duty cycle and whether it has thermal protection. These specifications should all be listed by the air compressor manufacturer/supplier.

Airflow

The airflow relates to how fast an air compressor can inflate a tyre and is listed as litres of air per minute (l/min) at a specified air pressure. This is because an air compressor might be capable of pumping 40 litres of air per minute at 0 psi but only 30 litres of air per minute at 30psi.

To get an idea of how various air compressors compare with each other in performance, have a look at their airflow claims and see which one comes out on top.

Maximum operating pressure

When it comes to maximum operating pressure, comparing one air compressor with another is simple. If a compressor is claimed to be able to operate at 70psi then it should be able to inflate a tyre to 70psi, and most likely beyond.

Maximum ambient operating temperature

The maximum ambient operating temperature relates to a compressor’s ability to operate at a certain ambient temperature. Exceeding the stated maximum may result in the air compressor overheating, in which case it will either cut-out or suffer damage. Most good quality compressors will be rated to around 70°C, which is far greater than any extreme temperatures you’re likely to experience on your travels.

Duty Cycle

The duty cycle is simply how long a compressor can operate before it will need a rest. An air compressor’s duty cycle should be listed in minutes at a certain ambient temperature. For example, 45 minutes at 45°C means a compressor will operate for at least this duration in this ambient temperature before it clocks off for a cool down. If the duty cycle is 50 percent, the air compressor will need around 45 minutes to cool down before it can be put back into service.

It should also be noted that some air compressors have a continuous duty cycle, so they can operate indefinitely up to their maximum ambient operating temperature. Many air compressors with a continuous duty cycle will feature fan cooling, but not all of them.

Thermal Protection

A thermal protection switch is designed to shut the air compressor down if it gets too hot in order to prevent damage. The compressor manufacturer should state the temperature at which the thermal protection switch is activated.

Maximum current draw

An air compressor’s specifications will also list the maximum current draw. The higher the figure, the faster the air compressor will drain a battery. Air compressors can have a maximum current draw of anywhere between 5 amps to 30 amps. Regardless of the current draw, you should always run the vehicle’s engine when inflating tyres to ensure you don’t drain the battery.

Packaging

If you opt for a permanently affixed compressor, you’ll obviously need to choose one that will fit somewhere in your vehicle. Some people opt to fit an air compressor in the engine bay, but it can get very hot and dusty in there. A better location is somewhere inside the vehicle’s cabin (such as under a seat or in the cargo area) or, in a ute, somewhere in the tub where it’s out of the weather and out of the way.

If you decide a portable air compressor will better suit your needs, you’ll want one that’s packaged in a sturdy carry bag or a hard case that also has space to fit relevant accessories such as cables, air hoses, air filters, fittings, deflator and tyre pressure gauge.

Some air compressors have a built-in base and handle, and these are usually supplied in a sturdy zip-up carry bag. Other portable air compressors will be mounted into a plastic carry case that either resembles a fishing-tackle box or a small space-case.

Fittings and accessories

It’s important that an air compressor is supplied with good quality fittings and accessories. Check the quality of the wiring and ensure the alligator clips are of a decent size. The wiring should also be long enough that you can place the compressor on the ground and easily connect it to your vehicle’s battery.

Have a good look at the air hose to make sure it’s also a good quality item, and that it has good quality fittings at both ends. Make sure there’s enough hose length to reach all four tyres on your vehicle without having to move the compressor unit. If you tow a trailer, you’ll need an even longer hose, or add a second connectable hose to your kit.

Air tanks

Some portable air compressors certainly deliver impressive airflow, but they don’t necessarily deliver a big air volume, which is what’s needed to quickly inflate large items such as camping mattresses and beach balls, or to reseat the bead on a fully deflated 4x4 tyre. Mating an air compressor to an air tank can provide significantly increased air volume.

The compressor fills the tank with compressed air, which is then expelled in a large volume via the air hose into whatever is being inflated. The larger the capacity of the air tank, the larger the item it will be able to quickly inflate before the air volume in the tank needs to be replenished by the compressor.

Some portable air compressor kits are now sold with air tanks included.

What else?

Other handy features include an easily accessible on/off switch or a compressor that switches of automatically when disconnected from the tyre valve; a washable/replaceable air filter; and an air-pressure gauge (either a separate item, one fitted on the compressor or, preferably, one fitted to the air hose).