Often talked about, but what is Injector latency. Latency or dead time is the time delay between the initiation of a process and the occurrence of its first noticeable effect. It is the time delay from commanded to open or close to the event occurring. In the case of fuel injectors this is described in milliseconds. Fuel pressure, voltage, injector design and even fuel temperature will change the dead time.
Here's how the process typically works:
1. Electrical Signal: The engine control unit (ECU) sends an electrical signal to the fuel injector when it's time to inject fuel into the combustion chamber.
2. Injector Response: Upon receiving the signal, the fuel injector needs some time to physically open and start delivering fuel. This delay is known as opening injector latency.
3. Fuel Injection: Once the injector is open, fuel is sprayed into the combustion chamber in a controlled manner, mixing with the incoming air for combustion.
4. Injector closing response: Electrical signal ceases being sent from the ECU when the injector is programmed to close. Injector again requires time to react to the change in electrical signal. This is closing latency.
Add opening and closing latency and you have the total dead time.
Our friends at Haltech made the diagram below to visually explain what is happening.
Blue = Commanded pulse time
Yellow = Time to energise solenoid before the injector starts to open.
Red = Opening time of the injector
Orange = Closing time of the injector
You can see that the commanded amount of fuel (blue) that was injected is a smaller amount than the pulse time. We wanted fuel to be injected from the start pulse to the end pulse. What we got was visibly less.
Now, keep in mind that the Yellow, Red, and Orange sections remain the same size, no matter what the commanded pulse time is. If we halved the commanded time, we would then find that most of the time the injector is not actually open. This is why the impact of dead time is larger at idle and low speed (when small injector openings are used) and minimal affect at high speed and high load conditions.
To correct for the loss of fuel we turn the Red triangle and the Orange triangle into rectangles constructed of the same area. Add the red rectangle to the end of the pulse, and then subtract the Orange rectangle from it. We subtract the orange rectangle because this was additional fuel we received while the injector was closing and not fuel lost like in the Red section.
Now we add to the tail end the missing Yellow section which is the time to energise solenoid before the injector starts to open.
Red = Injector open time MINUS injector close time
Orange = Time to energise solenoid before the injector starts to open
We now have one large purple section of missing injector pulse time.
If we now measure the amount of time between the Old End Pulse and the New End Pulse, that is the amount of additional injector opening timing time that is needed to get the correct amount of fuel delivered to the engine at a given voltage and fuel pressure.
An example of a dead time table showing voltage and fuel pressure is below.
Entering the incorrect dead time valves either to large or to small with have a visible impact on the VE table.
This is how the VE table should look if the injector data is correct.
Dead Time Too Low
When the dead time is set too low, we need to over-compensate in the light load areas by increasing the VE numbers to get more fuel. This shows in the VE table that something is not set correctly. We must increase the dead time to bring the VE numbers back to normal. Very large VE numbers are used to correct the AFR at low-speed, low load area.
Dead Time Too High
Conversely, if the dead time is set far too high we can create situations where the VE can even reach ZERO as we try to tune the car. Obviously, the VE cannot be zero as some air is passing through the engine, we know that this is not correct and the dead time needs to be decreased.
How To Set the Dead Time If The Data Is Not Available
If no data is available, google often is your friend to find flow rate or flow rate can be measured on a flow machine.
As a general rule
With engine idle vacuum of -60kpa a VE value of around 50 is expected.
Dial in the dead time values to achieve the expected lambda value for the given engine combination. Large overlap camshaft will change the idle vacuum and lambda value targets.
Approximating both the Flow Rate and dead time.
After taking an educated guess at the flow rate and dead time so that the engine is running and idling without having touched the VE base table or using the general rule, the next step is to get a little closer to the flow rate. To do this we need to bring the engine load and rpm up to a level where the dead time has less effect, and the flow rate has a significant effect. We pretty much need the engine at full load and keep the rpm above idle but low enough that the engine is not being beaten on too hard. Aim for around 1500 to 2500 rpm for this. While under load if the Lambda is trending lean, then you need to adjust the flow rate down. If you are too rich you need to adjust the flow rate up. You need a dyno to do this.
Approximating the Dead Time
After the flow rate has been adjusted, now when you come back to idle the lambda will have changed. Now adjust the whole table up or down to bring the lambda back to your Target. After this is complete go back and retest the flow rate again, then repeat the dead time adjustment one more time.
After you have the dead time worked out, the next step is to vary the voltage. The easiest way is to disable the Alternator and allow the voltage to drop. Typically, the engine will be at around 14V with the alternator running and without it, you can let the voltage drop down to 12V. This will give you 2x point from which to take a good solid guess at the curve. In reality, if you are at 10V and still driving you probably have far bigger concerns than your fuelling being off by a couple of percent, and as such messing about with small changes from here is usually counterproductive. If it is of concern for you, then you need to find a way to let your engine run at lower voltages.
And that is dead times.