Considerations Of Auxiliary Port Driven Air Compressor Systems

February 16, 2017

Many industrial engines come equipped with an auxiliary port (pump drive) to power accessory equipment. You will often find a hydraulic pump or a standard piston-type air compressor mounted to this port, but that isn't the only option. We've had many inquiries asking if there is a better compressor option to meet heavy duty cycle and air flow requirements for specific jobs, including spray foam, pump priming, air gouging, and dust collection. Rotary screw air compressors can meet that need.

VMAC's OEM division has direct drive, port-mounted rotary screw compressors available for unique applications, but there are several factors to consider before choosing to mount a rotary screw compressor to that auxiliary port.


Factors including the HP required per CFM, the thermal efficiency of the compressor, and other loads on the engine determine the engine size you'll need. The engine manufacturer publishes the available power at the auxiliary port. If you run additional equipment from the same engine, you’ll also have power consumption from those items. Do power and RPM curves of the engine match each piece of equipment you are using? A more in-depth look at these factors can be found in our article on power requirements for industrial engine powered air compressors.

Torque Pulses

The firing order, angle and number of cylinders in an engine cause torque pulses. The high compression ratio of a diesel-powered engine also amplifies the magnitude of these pulses. Lovejoy Inc. has an informative article on diesel vibration that goes into much more detail. Using materials designed to handle these pulses is a consideration when designing port driven components.

Port Limitations

Manufacturers publish specifications (ft-lb torque) and limits for the power and load that the port can support. However, the initial specification may be adjusted (service factor) depending on the type of equipment driven. A smooth, low-load blower puts less stress on the engine than a heavier-loaded rotary screw compressor. A piston compressor or crusher with load spikes puts a greater demand on the engine. Inside the port, the gear train is designed to transmit power and handle the load of the equipment attached. Working with the engine and equipment manufacturers to ensure they match is important.

Output vs. Engine RPM

Rotary screw compressors deliver air (CFM) in ratio to the speed of the rotors. The more rotor rpm, the more air delivered. Compressor gearing can help deliver more air at lower engine speeds. When coupling a compressor to the auxiliary engine port, the port gearing also affects the speed at which the engine is required to run. Some ports have 1:1 gearing, while others are designed as a speed increaser. Knowing the air requirements of the job and the engine speed, port gear ratio, compressor gearing, and compressor displacement will determine which compressor is required.

Direction Of Rotation

Engine ports may turn clockwise or counterclockwise depending on the engine manufacturer, location of the engine, and ordering options. Rotary screw compressors are designed to turn in one direction, once again dependent upon gearing and the manufacturer. A rotary screw compressor running in the wrong direction will quickly result in mechanical failure of the compressor.

crankcase-257x300Space Constraints & Clearances

Engine components, mounts, casting features and the location of the engine port will determine how much space is available to mount a component to the engine port. The size and shape of that component is also critical. Some components have mounting flanges that can be rotated to increase the options for mounting. Ultimately, mounting the component to the engine, either in CAD or in reality, will determine fitment.

Port & Input Shaft Types

Most auxiliary engine ports are manufactured to an SAE standard. SAE A, SAE B, and SAE C ports are all common in North America. In Europe, DIN ports are standard. Input shafts generally have a few options: tapered shafts, keyway shafts and spline shafts are common. The auxiliary component must match both the port and shaft type for it to fit.

Overhung Load

Port specifications include how much load the port will support without needing a brace to mount the auxiliary component. If using a gear on the compressor to interface with the gear train in the port, an overhung load can cause issues. Longer components are also likely to vibrate.

Unsupported radial and axial overhung loads may cause failures to your equipment. The Gates Corporation defines overhung load as a force exerted perpendicular to a shaft beyond the outermost bearing. When that force exceeds the maximum rated capacity for the equipment, shafts and bearings become overloaded and wear out at a higher-than-normal rate. Although the article references belt driven components, the theory also applies to port drives.


Another consideration is the cooling requirements of your compressor and engine. We’ve written an article on managing heat rejection requirements.

Air Intake

Mounting a compressor tight to the engine can optimize the available space, however, it can also introduce the challenge of ensuring your intake air is cool. Routing your intake hose away from hot components, like the exhaust system, and drawing in air from the outside of an enclosure are considerations for your port driven compressor system.

Operating Temperatures

Operating temperatures play a part in any compressor choice. The location of the compressor and the location of the air intake can affect performance. Read more about how inlet temperature affects air flow here.

There are various ways of driving a compressor from your industrial engine, and choosing the auxiliary port might be one of the easiest for mounting the compressor. If the power requirements of the engine and port meet the air compressor and airflow needs, then a rotary screw air compressor might be a great option for you. Remember to work with the manufacturer of the engine and the compressor to confirm the match of the components being considered.