How to Size an Air Starting System: Pressure, Air Consumption and Receiver Volume
An air starter is only ever as reliable as the air behind it. Sizing that air supply comes down to four numbers: the starter's working pressure, its air consumption, the number of starts you need before recharge, and the receiver volume that delivers them. Get them right and the engine starts first time, every time; undersize the receiver and it will crank slowly, stall halfway, or fail on the attempt that matters. This guide works through all four, with a worked example you can follow.
If you already know how an air starter works, this is the next question: how much air does it need, at what pressure, and how big a receiver do you have to feed it from? Get those right and the system starts first time, every time. Get them wrong and no amount of starter quality will save you.
Why the Air Supply Matters as Much as the Starter
A pneumatic starter converts a flow of compressed air into cranking torque. While it is running it draws a large volume of air very quickly, and it only delivers full torque while the supply pressure stays above its minimum working pressure. If the receiver is too small, pressure collapses mid-crank, torque falls away, and the engine slows before it fires.
This is the most common sizing mistake: plenty of pressure on the gauge, but not enough stored volume to hold that pressure through the crank. Pressure gets the starter turning; volume keeps it turning. You have to size for both.
In our experience, most of the starting problems we are asked to look at trace back to the air system rather than the starter itself: a receiver that is too small, or sitting too low on pressure, to hold above the starter's minimum right through the crank.
The Four Numbers That Size an Air Starting System
1. Working pressure
Every air starter has a working pressure range and, crucially, a minimum pressure below which it will not deliver rated torque. Powerstart Jetstream starters operate anywhere from 3 to 30 bar, which gives real flexibility to match an existing supply. The number that matters for sizing is the minimum: the receiver has to keep the starter above it for the whole crank, not just at the first turn.
2. Air consumption
Air consumption is the volume of free air the starter uses while cranking, usually quoted in cubic metres per minute or litres per second on the datasheet. It is a large figure, and it is where the design of the starter really shows:
- Vane starters produce strong torque from a relatively low air volume, which makes them the sensible choice when the available air supply is limited.
- Turbine starters such as the Jetstream range trade a higher air demand for a superb power-to-weight ratio and smooth delivery, and they handle longer crank cycles on large engines well.
The consumption figure is per unit of time, so the air used in a single start depends on how long you crank for.
3. How many starts before recharge
A system is not sized for one start. It is sized for the number of consecutive attempts you need before the compressor has to catch up, and that number is driven by the application:
- Marine main engines follow classification society rules. Under IACS UR M61, the receivers must provide at least 12 consecutive starts for a reversible engine, or 6 for a non-reversible engine, without replenishment, usually split across at least two receivers.
- Diesel fire pumps under NFPA 20 must store enough starting energy to cover the full cranking cycle the controller would otherwise attempt on batteries.
- General industrial and standby engines are typically sized for several consecutive attempts as a working margin, more where the site is remote or unmanned.
4. Receiver volume
The receiver has to hold enough air, at usable pressure, to cover the total free air the starter will draw across those attempts. The usable air is not the whole tank: it is only the air available between the charged pressure and the starter's minimum working pressure. A receiver charged to 30 bar that feeds a starter needing 8 bar has roughly 22 bar of usable pressure swing to work with; the air below 8 bar is stranded.
Rule of thumb: usable free air is approximately the receiver volume multiplied by the usable pressure swing in bar. A 700 litre receiver with a 22 bar swing releases around 15,000 litres of free air before it drops below the starter's minimum.
A Worked Example
Say you are sizing the receiver for a turbine starter on a large standby diesel. The starter datasheet and the application give you these figures (illustrative values, use your own datasheet in practice):
| Parameter | Value |
|---|---|
| Starter air consumption | 30 m³/min of free air |
| Crank time per attempt | 5 seconds |
| Free air per attempt | 2.5 m³ |
| Consecutive attempts required | 6 |
| Total free air needed | 15 m³ (15,000 litres) |
| Receiver charge pressure | 30 bar |
| Starter minimum working pressure | 8 bar |
| Usable pressure swing | ~22 bar |
| Required receiver volume | ~680 litres (round up to 750, e.g. 2 × 375 L) |
The method is simple once the figures are in front of you. Free air per attempt is the consumption rate multiplied by the crank time. Multiply that by the number of attempts to get the total free air. Divide by the usable pressure swing to get the receiver volume, then round up and add margin for temperature, leakage and any other consumers on the same receiver.
That last point matters: if control air, an air whistle, tools or a second engine draw from the same receiver, their consumption has to be added before you size it, or given a separate vessel.
Vane or Turbine: Match the Starter to Your Air Budget
The sizing exercise often decides the starter type as much as the engine does. Where compressed air is plentiful, a turbine starter gives the best cranking performance for its size. Where air is limited or expensive to store, a vane starter's lower consumption can be the deciding factor. On large engines that need a long crank, turbine designs come into their own. It is worth running the numbers both ways before committing to a starter and a receiver together, rather than sizing one around the other after the fact.
Sizing for Critical Applications
On fire pumps, offshore platforms and remote installations, the sizing is not just about a single successful start. It is about redundancy and worst case. That usually means charging to the top of the pressure range for maximum stored energy, splitting the volume across two receivers so a single vessel or valve fault cannot disable starting, and sizing the crank margin for cold, worn or reluctant engines rather than a warm test-bench start. This is the same reliability thinking that puts non-electric starting on critical engines in the first place.
If you would rather not size it from scratch, this is exactly what we do. Give us the engine and the application and we will specify the starter and the air system together, drawing on the full pneumatic starter motor range, including the Jetstream 4 and Jetstream 5 turbine starters.
Air Starting System FAQs
How much air does an air starter use?
A large amount, quoted on the starter's datasheet as free air consumption, usually in cubic metres per minute or litres per second. Vane starters use less air for their torque; turbine starters use more but give a higher power-to-weight ratio. The air used in a single start is the consumption rate multiplied by the crank time.
How do you size an air receiver for an engine starter?
Work out the free air used per start (consumption rate multiplied by crank time), multiply by the number of consecutive starts you need, then divide by the usable pressure swing (charge pressure minus the starter's minimum working pressure). Round up and add margin for temperature, leakage and any other consumers on the same receiver.
How many starts should an air receiver provide?
It depends on the application. Marine main engines follow IACS UR M61, which requires at least 12 consecutive starts for a reversible engine or 6 for a non-reversible one. Fire pumps follow NFPA 20. General standby engines are usually sized for several consecutive attempts as a working margin.
What pressure does an air starter need?
Each air starter has a working pressure range and a minimum below which it will not deliver rated torque. Powerstart Jetstream starters operate from 3 to 30 bar. The receiver has to keep the supply above that minimum for the whole crank, not just at the first turn.
Sizing an Air Starting System?
Tell us your engine make, model and available air supply, and we will recommend the right air starter and receiver capacity.
View Air Starters