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How to calculator a direct injector duty cycle on V6 and V8 engine?

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Hello everyone,how to calculator a car only fitted direct injector duty cycle on V6 and V8 engine?Data log only can see injection duration can't monitoring actual duty cycle.

RPM / 2 revs per cycle = Numbers of cycles per minute.

Number of cycles per minute / 60 seconds = Number of cycles per second.

1 second / number of cycles per second = total cycle time duration available at those RPM.

Actual injector pulse width time / total cycle time duration available at those RPM = Injectors Duty cycle %

Example: 6000 RPM, actual injector pulse width is 15 ms.

6000 RPM / 2 ( Otto cycle engine) = 3000 cycles per minute

3000 cycles per minute / 60 seconds = 50 cycles per second

1 second / 50 cycles = 20 ms total time available for each cycle.

15 ms / 20 ms = 75 % duty cycle...

Does it even make sense to measure duty cycle on a direct injection engine? What would be a reasonable maximum limit? 25% (compression stroke only), 50% (intake & compression)?

Duty Cycle on a DI engine is not a percentage of the total cycle time, as there are restrictions on when the injection pulse can occur. This window of injection is typically bounded by the closure angle of the inlet valves, and the ignition angle. So if you have an inlet valve that closes at 180 BTDC and the igntion timing is at 20 BTDC, then your injection window is 160 degrees. Your DC becomes the percentage of that window that the injector is firing for.

You can sometimes start the injection before the inlet valve closes fully, using this to assist in homogenising the fuel air mixture, as long as the location of the injector is not shielded by the valves. In some cases (such as lean burn conditions) this is not wanted, as you need a rich kernal of fuel around the plug for ignition anf a lean condition in the remainder of the chamber. Most DI systems will also have a short buffer between the end of injection and firing the coil so that there is minimal fuel present in the end of the injector at ignition (protecting the injector nozzle from damage) and to ensure an even fuel mixture through the combustion chamber. DI Injection is also calculated from Start Of Injection.

So, to correctly calculate the DC for a DI engine, you need to have the Inlet valve closing angle, the SOI angle, the Injection angle (calculated from the PW and Cycle Time) and Igniton Timing. You also need to know what limits are in place in the ECU regarding buffers before the Ignition firing or valve closing.

That is very interesting, thanks for posting that Stephen. DI seems much more complex then port injection, but I'm sure it felt the same way when the industry went from carb's to EFI lol.

Interesting that DI is calculated from SOI and not EOI, seems quicker math to know what duration the buffer is before the ignition event (assuming a few ms) and work from the ignition even forward. Unless is there's an advantage to having a longer buffer? IE the injection event has more time in the cylinder it leads to better mixing?

This also leads me to believe that DI injectors are much larger cc/min then port, I am unable to find numbers as most upgrade injectors are a % above oem. I'd think theres going to be less time to inject fuel vs port injection...

Well, the DI fuel pressure is 2000+ psi (so 40 times more than port injection), so you can expect they will flow a lot more fuel mass in less time.

Most DI injectors that I have looked at the specifications of have been in the 1000cc/min range.

Stumbled upon a great visual explanation of this on ECUtek's educational section. Interestingly but not surprising that DI also has dead time lag.

David that is a good point, being reminded of that I had to google what cylinder pressures are roughly when fuel injection occurs. According to a few random articles it seems to be in the 200-600 psi range, which based on the drawing I found seems high given they are showing the injection on the intake stroke mostly. Either way it still results in a large pressure delta between the rail and cylinder.

Attached Files

It's not just the injectors that have lag, you have to take into account the time that the capacitors in the ECU take to charge as most solenoid DI injectors work in the 70V ~ 80V range, and have peak opening currents up to 20A, Piezo have an even higher voltage requirement with some in the 180V range. This is why a lot of DI injectors are wired in pairs that fire 360 degrees apart, as the ECU takes advantage of the flyback voltage from the previous firing to reduce the time it takes to charge the capacitors for the next firing. There is also a delay between the capacitor discharging and the voltage transiting the wiring loom to the injector.

Once at the injector, the solenoid coil needs time to charge and will not start lifting the pintle from the seat until the coil is almost fully charged. This is the same as the dead time in a port injector, and needs to be accomodated in the configuration of the ECU. The Piezo injectors react much quicker to the voltage inrush, this is the reason why they are used in nearly all diesel engines, as they can react quickly enough to generate 5 or more pulses in a single combustion cycle (Pilot 1, Pilot 2, Main 1, Main 2, Main 3 and Post plus an injection on the exhaust stroke for DPF light off). They do the multiple pulses to reduce noise (pilot pulses) and to give a more consistent and longer duration main burn. The post injection is usually for emissions.

The process of injecting the fuel will also increase the pressure in the combustion chamber.

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