How does an Air Starter work?


How does an air starter use air to start an engine?

The answer to this question lies in the type of motor an air starter is equipped with.  There are two types of motors that dominate the industry: vane motors and turbine motors.  These two types of motors work on similar principles but they use air differently and thus, in similar conditions and with similar inputs, they will often perform differently.  This article will outline the specific attributes of each motor type, discuss the differences, and recommend which type of motor to use based on the availability of volume and pressure within an air starting system.


A vane motor consists of five major components: a rotor, vanes, the motor housing, the internal cylinder, and a set of end plates (front and rear).  Upon the start cycle being initiated, air is directed into the motor housing through the inlet which then channels the air through a series of slots in the internal cylinder of the motor housing, as well as a series of small inlet holes in the end plates.  The holes in the end plates direct the air so that as it is introduced it will cause the vanes, which are inserted into slots cut into the rotor, to be displaced from their resting position within the rotor slots.  With the vanes being forced out of the rotor by the sudden introduction of air, they come into contact with the internal cylinder and cease to extend further.  Once the vanes have fully extended, the air moving through the slots in the internal cylinder of begins to build pressure behind the vanes.  As more air continues to flow into the housing the increasing pressure exerted by the air on the vanes forces them to move the rotor so that the pressure might equalize as the air seeks an outlet.  The air outlet is another series of slots cut into the internal cylinder that are intermittently covered and uncovered by the now spinning rotor and vanes.  Thus, the force of the air (i.e. pounds-per-square-inch) is converted into rotational torque and power as the vanes act as paddles and catch the air, thus turning the rotor which is used to turn the gears of the air starter. 


A key concept to remember in reference to a vane motor is that is works on the principle of positive displacement.  With the vanes being pushed against the side of the internal cylinder by the air pressure, they create pockets in which the air is trapped as it flows into the housing.  As the pressure increases behind the vane, it cannot be released until the pressure does some positive work in order to move the vane past the outlet slots.  This design controls the flow of air, delaying its ability to exhaust until it has moved the rotor.  As the rotor picks up speed, the motor demands increasing amounts of air volume as the outlet holes spend less time covered by vanes thus allowing air to escape more easily.  


A turbine motor consists of four major components: a nozzle, turbine wheel, central shaft, and a motor housing.  As air flows into the housing it is directed toward the turbine wheel through slots cut into the outside edge of the nozzle.  The purpose of these slots is to channel the air into the rotor at a specific angle so as to maximize the turbine wheel’s ability to capture the air’s potential energy.  Similar to the nozzle, the turbine wheel also has slots cut into its outside edge, however, while the purpose of the slots in the nozzle are to direct the air, the purpose of the slots in the turbine wheel are to catch the air and thus force the turbine wheel to turn. Each slot is shaped like a bucket designed to catch the air momentarily before it moves through the slot and exhausts out of the starter.  The central shaft runs through the center of the rotor and as the rotor begins to spin, the central shaft turns with the rotor and is used to translate the force of the air on the rotor to the starter gearing thus creating torque and power.


Unlike the vane motor, the turbine motor does not work on the principle of positive displacement.  In a turbine, the air is given constant access to an outlet but its potential energy is captured by directing the air flow.  The shape of the nozzle is designed to direct the air into the rotor buckets in order that the air flow will do the work of making the rotor spin.  In the initial moments of the start-cycle, while the volume of air entering the motor remains low, the impediment to the air’s ability to flow presented by the rotor is very minimal resulting in little or no work being done.  However, once the threshold for the rotor beginning to spin is met, the rotor will demand an even volume continuously to achieve required torque and power up to the starter’s maximum output.


The performance curves depicted above give broad examples of the differences in the way vane and turbine motors use air differently.  The red arrows point to curves that show that at the same PSI, the vane motor demands increasing amounts of air volume (or flow) in order to increase pinion speed and the potential to produce torque whereas the turbine motor demands the same air volume whether no matter the level of its performance.  What does this translate to in terms of practical application?  Vane motors provide an excellent option for starting in a situation where the volume of air available for starting is limited.  Looking at the Vane Motor Performance Curve, one sees that the vane starter produces a tremendous amount of torque with a relatively low volume of air in comparison to the demands of the turbine starter.  On the other hand, in a situation where the volume of air is not a concern, for instance in an application where pipeline gas might be used rather than stored air, a turbine can offer considerable advantages, especially if the engine requires a long crank cycle.  Over a longer period of time (approximately 8+ seconds), the turbine design is considerably more efficient than the vane design because the turbine starter demands an even flow of air no matter how long it runs or how much work is required of it. 

Cardinal Valley's general rules of thumb:

  • Vane Starters make a lot of torque at low volume but require ever-increasing volume to do more work
  • Turbine Starters do no work below the volume threshold, but once the threshold is reached, require even volume no matter how much work is done
  • Applications with:
    • Limited volume of starting air and a short crank cycle = Vane Starter
    • Large or unlimited volume of starting air and a longer crank cycle = Turbine Starter

When selecting a starter for a certain application, how the starter functions, i.e. how it uses air, is an important thing to consider.  Paying attention to the differences between a vane and a turbine starter can prevent a lot of hassle and unnecessary expenditure.  Although turbine technology is technically newer than vane technology, both styles have advantages over the other in certain applications. Unfortunately, the process of selecting a starter is not always as simple as figuring out how the starter uses air; obviously there are other mitigating circumstances like air cleanliness, space and fit, pressure, etc. that affect the decision-making process.