Injector Duty Cycle (IDC) Explained

Injector Duty Cycle (IDC) is a very common term when tuning our DSM’s, or any fuel injector platform for that matter. IDC is an aftermarket value to represent the amount of time a fuel injector is energized (on, spraying fuel) during an engine cycle. The “time” is determined by the RPM.

Our 4G63’s are of course 4 stroke engines. The 4 strokes are…

  1. Intake Stroke
  2. Compression Stroke
  3. Power Stroke
  4. Exhaust Stroke

In order to complete 1 full cycle, the crank needs to rotate 2 times.

In the intake stroke, the piston goes down while the intake valves open. Because the pistons are going down, it sucks air and fuel into the cylinder through the open intake valves, like a vacuum effect.

Once the piston reaches the very bottom, it begins to return back up. In the process, the intake valves close. This is called the compression stroke. When the piston comes up, it “compresses” the air and fuel that have entered the cylinder during the intake stroke.

Once compressed, the spark plug ignites the air and fuel. When the air and fuel is ignited, it forces the piston back down. The more air and fuel, the bigger the force will be and the faster the piston will go back down. This is what generates horsepower and is called the power stroke.

One key note to keep in the back of your mind in this article is that the time available to inject fuel is from one complete power stroke cycle to the next. This means that fuel is being sprayed at the back side of a typically hot intake valve which helps cool the valve and vaporize the fuel more. When the valve does open, it sucks in all the fuel it possibly can before the intake valve closes again.

When the piston reaches the very bottom again, it returns up. As it returns up, the exhaust valves open which allows the piston to push the exhaust gasses out of the engine via the now open exhaust valves. This is the exhaust stroke.

Again, fuel (and air) is released into the engine during the intake stroke. The ECU opens the injectors for x amount of time based on RPM and the amount of air entering the intake. The more RPM the engine is turning, the less available time the injector has to send fuel into the engine between power strokes.

The ECU needs to ensure the proper amount of fuel is supplied into the engine in order to maintain the proper air/fuel ratio (AFR). The more air entering the engine, the more fuel that is needed to maintain the proper AFR. The ECU does this by determining how long the injector needs to be open in accordance with the window or total amount of time available to open the injector.

Let’s say we are at 4000RPM. RPM is measured in minutes (revolutions per minute). The amount of time the injector can be open for in its brief window between one power stroke to the next and before the intake valve opens is measured by the ECU in milliseconds (ms). The end goal is to convert information into ms.

The total time in which the injector is spraying fuel is called Injector Pulse Width. So, what the ECU is essentially doing is collecting information and determining how long the injector needs to be open. It does this by converting data beginning with the RPM. In order for us to do the same, we do so in the following sequence:

  • Convert revolution per minute (RPM) into cycles per minute (RPM / 2 revolutions per cycle = cycles per minute)
  • Convert cycles per minute into cycles per second (cycles per minute / 60 seconds = cps
  • Convert cps into seconds per cycle (1/cps)
  • Convert spc into milliseconds (1,000 x spc = ms)

At 4000RPM, it would give us 2000 cycles per minute because 1 complete cycle requires 2 crank revolutions (4000RPM / 2 = 2000 cycles).

Next, we need to convert 2000 cycles per minute into cycles per second. There are 60 seconds in a minute, so 2000 cycles / 60 sec = 33.333 cycles per second.

Now, we want this number to represent seconds per cycle (the total time available the injectors have to operate within the cycle) instead of cycles per second. To do this, we do 1 second/33.333cps. 1 / 33.333 = roughly 0.03spc. Now we need to convert the 0.03 spc into milliseconds. There are 1000ms in 1 second. So, .03spc x 1000 = 30ms.

This means the total time available for the injectors to open within the cycle at 4000RPM is 30ms. Now that we have this information, we can finally get to Injector Duty Cycle (IDC). IDC is an aftermarket value used for us as tuners that is built into our datalogger. The ECU does not care about IDC, it has no knowledge of IDC period.

Now that we know the IPW, we can calculate our IDC in a much simpler formula:

IPW = injector pulse width in milliseconds (ms)

RPM = engine speed in rotations per minute

IDC = (RPM x IPW) / (120,000)

If the injector is open for the entire 30ms, our IDC would be 100%. If it is open for 15ms of the available time of 30ms, IDC would be 50%.

When your injector has completely maxed out and your IDC shows over 100%, it just means you ran it past its operating range and the calculation presented to you shows that. The ECU calculates the IPW needed and sends that to your datalogger. Your datalogger calculates IDC by looking at the IPW and RPM (IDC = (IPW x RPM) / 1200). If the IPW is greater than the amount of total injector cycle time, you will have greater than 100% IDC.

For example, the ECU calculates that the injector needs to be open for 30ms., but now the time from one power stroke to the next is reduced from 30ms to 28ms because now the RPMs have climbed from 4000RPM to 4500RPM. Now, the injectors are being forced to stay open all the time and never closing. It is physically impossible for the IDC to be greater than 100%, although our datalogger reports otherwise because it is a calculation based on the difference between the required IPW and the amount of ms between cycles.

The injector isn’t spraying more fuel at 110% than it would be at 100%. When the injector is spraying fuel non-stop, the injector is considered static. Have you ever heard someone say not to run your injector past 85% IDC? This is because injectors are mathematically considered static anything beyond 90% IDC. Meaning they physically cannot supply any more fuel.

Still confused? Think of the total amount of time it takes to complete one complete injection cycle as a wall clock with one hand, that hand being a ms hand instead of a normal second hand. Starting from 12:00 would be the beginning of the injector cycle. One complete turn of the ms hand from 12:00 to 12:00 would be 100%. Let’s say the injectors are only open from 12:00 to 6:00. This would represent 50%. This means the injectors stopped firing at the halfway point which would be the beginning of the intake stroke when the piston is back at the top of the cylinder and about to go back down.

Now let’s say the IDC is 75%. The injectors stopped firing at the 9:00 position, and/or the beginning of the compression stroke when the piston is at the bottom of the cylinder and about to go back up. The injectors would stop injecting fuel until the beginning of the next power stroke. If the IDC is 100%, the injectors have now fired through all 4 stokes, from the beginning of the power stroke, to the exhaust stroke, to the intake stroke, to the compression stroke, back to the beginning of the power stroke.

As you can see, it is physically impossible for the IDC to be greater than 100%. Any IDC above 100% is just the calculation of the percentage of the injector cycle time and the IPW. Your injectors are already injecting fuel 100% of the time. They can’t physically flow more fuel thus they can’t physically flow more than 100% IDC.

There are always people I see that are hitting 100+% IDC but still have a good, clean AFR. There are a few reasons why this can happen. The simplest and probably most common is because the tune on the ECU is commanding a lot of fuel and has a goal of a rich AFR. If you are telling your ECU to send more fuel, your injectors will be injecting fuel longer, in order to achieve a richer AFR. So of course your IDC will be high.

Say you are running RC 550cc injectors. To eliminate any confusion, let’s assume your injector flows a perfect 550cc/min. If your injector is turned on and spraying that 500cc for 20ms, more fuel will enter the engine than it would if it was only open for 15ms.

If you altered your fuel maps, injector scaling, etc. to send less fuel and to run a leaner AFR, the IPW will decrease as well as your IDC. It is just as likely for your high IDC to be caused by your ECU sending too much fuel as it is for you to need larger injectors. A high IDC does not always mean you need a larger injector, it could mean you are sending too much fuel. A rich condition can easily cause your high IDC.

I’ve seen many people trying to figure out why they were running dangerously rich and then realized their IDC was way over 100%. All it was, was they were commanding way too much fuel. The ECU was doing what it was told. If you drive your car for awhile and everything seems great, the AFR is where you want it, etc., then all of a sudden over time it starts running really rich, it could just be from you turning the boost up which in turn caused the ECU to run higher parts of the fuel maps because of the higher load cells.

These higher load cells that were once uncharted waters, you now find your car is hitting these cells every time you are going WOT and are way richer cells than the areas of the map you were previously hitting on lower boost. Because you end up flowing more fuel than the additional air you gained from turning up the boost.

Plenty of things can cause you to all of a sudden hit new, higher cells, further up the map. Turning the boost up will generate more load. Higher load, newer cells. Just a significant difference in ambient air temperatures from the summer time to the winter time can cause it. The colder the air, the more dense the air is, the more power the engine will make. The more power, the higher the load. Higher the load, the further up the map you will run.

Most unaltered fuel maps are built to get richer at higher load to act as a safety blanket if something catastrophic happens that forces you to hit higher loads. For example, if you are doing a WOT pull and your wastegate line pops off. Now you are free-boosting and the turbo will boost as much as it possibly can. This clearly will force you up your maps.

As a safety blanket, a factory fuel map will have really richer fuel maps up top to prevent as much damage as possible expecting your AFR to lean out significantly when your wastegate line comes off and you end up running extremely high boost. When you run extremely high boost, a lot more air will come into the engine. The more air coming in, the more fuel you will need to try and stuff in there with it. This is also why fuel cut and/or boost cut was put into our ECU’s from the factory. This applies to your ignition maps as well.

I am simply saying that leaning out your fuel maps will reduce your IDC because now your ECU is sending less fuel. Of course, if you are already running a leaner AFR on your fuel maps and everything else in the tune is also good, high IDC could be an indication that you do need to upgrade the injectors. This can obviously be very unsafe. But always inspect things first before getting too carried away. Run some data logs and check things out.

In conclusion, this should give you a better definition of what injector duty cycle is and how it works. It should also present how IDC can be misleading. There is a reason our ECU’s do not care about IDC. IDC is only a way to present information. It is not used to alter information and determine what the ECU does. IDC is merely a tool we have in our data-loggers to look for potential issues and give us an estimated value of how much work our injectors are being commanded to do.

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