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Staged Injection | Discussion and How to Setup on Modular ECUs

December 15, 2017

In this article, we’re going to discuss staged injection, what it is, and how to set it up on a Modular ECU.

Staged injection is where you have multiple injectors per combustion chamber. The reason for doing this is that a single injector, sized so that there is sufficient fuel flow at the full power condition, will struggle to deliver the small amount of fuel required in the overrun and possibly the idle condition.

The only engines I’ve seen these on from factory are the Mazda rotaries and some motorbike engines, but in addition to these it’s a common addition to high performance piston engines.



The first thing to remember is that the ECU has a complete fuel model. So what the ECU needs to know is how much fuel to try to deliver using the primary injector, and how much to deliver on secondary injectors or any subsequent stages.

Let’s pick a 2 stage system as an example. The ECU first calculates the amount of fuel that’s needed for each rotor or cylinder, the same as on a single stage engine. But with a 2 stage injection engine, the ECU needs to work out how much fuel to attempt to deliver with each injector stage.

It does this using the secondary staged injection map. This value is a percentage, which represents the percentage of fuel which is to be delivered by the second stage. Eg if this is 0, the ECU will deliver all the fuel using the primary stage. If the value in the table is 50%, that means that the fuel will be split equally between the two stages. Note that this does not mean that the primary and secondary injectors will have equal duration, unless they are the same type of injector. And if the value is 100%, all of the fuel will be delivered by the secondary injector and the primary injectors will not be fired at all. So as well as a traditional staged injection engine, this system allows you to do things like have injectors up above the throttle bodies which are active at high RPM to improve vaporization.

ECU calculations when MAP is zero

ECU Calculation when MAP equals to 50%

ECU Calculation when MAP equals to 100%


You can probably appreciate that this system is open to mistakes, by asking an injector to deliver more than it can physically flow. There are practical limits and we’ll discuss those now.

You can select a minimum off-time for each injector stage. For nicely behaved, linear performance, this should be the amount of time it takes for the injector to fully close, so that when you begin the next pulse, it releases the same amount of fuel as if the delay was much longer. If you make this off-time too short, then the injector does not fully close, which means that it takes less time for the injector to open, and therefore the amount of fuel delivered is higher than the amount calculated. We recommend 2ms for this figure, based on our testing of many types of injectors. This gives you the limit of the duty cycle of that stage, which you can calculate quite easily – for example at 6000 RPM on a rotary or 2-stroke, a 2ms off-time means a maximum injector pulse of 8ms which therefore corresponds to an 80% duty cycle.


If the ECU can not deliver the percentages selected in the staged injection map, then it reverts to a default strategy which is used on the Select ECUs, which is to deliver as much fuel as possible on the primary injectors, and then any left over will be delivered on the next stage, and if there’s any after that it will be delivered on the 3rd stage and so on, all the time while respecting the minimum off-time (which was a fixed value, not a variable, in the Select ECUs). It stands to reason that if you just set the whole staged injection map to zero, it will follow this strategy all the time, and that’s how we set it in the base maps for rotary engines. The only reason not to do this is if you believe the engine can or might make more power by increasing the relative amount of fuel delivered by the higher stages, for example if this creates more charge-cooling vaporization or promotes better mixing.

The Modular ECUs currently support up to 4 injector stages, and on a higher stage system, the behavior is just an extension of the 2-stage. On a 4-stage system, there are 3 staged injection maps – 2nd stage, 3rd stage and 4th stage – and the balance (100 percent minus the sum of these three) is to be delivered by the primary injectors. Therefore if you leave all these maps at zero, you again have the default automatic behavior.


So that the ECU can convert the fuel volume into the injector pulse duration, the ECU needs to know the fuel flow rate versus fuel pressure and also the dead time or offset. Because in the general case you will have different sized injectors between the different stages, these are all separate settings for each stage.

The wiring and mapping of the injectors is the last point we will discuss. The mapping is fixed so that the order of the injector outputs is all the first stage first, then all the second stage, then all the third stage, then all the fourth stage. For example if you have a 2 rotor, 3 staged engine, we would have:

Injector output 1 = rotor 1, primary
Injector output 2 = rotor 2, primary
Injector output 3 = rotor 1, secondary
Injector output 4 = rotor 2, secondary
Injector output 5 = rotor 1, third stage
Injector output 6 = rotor 2, third stage




We chose to do it in this order, rather than pull all cylinder / rotor 1 together and then all cylinder / rotor 2 together, so that you can add stages easily without having to change all the existing wiring, just adding the injector wires for the new stage. We find it’s more common for people to add stages than to change the number of rotors or cylinders.

Thanks and happy learning!