Ameer Randolph
Measuring the dead time on a fuel injector is important for achieving a stable, accurate idle and maintaining air-fuel ratios. The dead time refers to the lag time or latency of the injector, and it can vary depending on the voltage applied to the injectors. There are several methods for measuring dead time, including using an injector tester or a fuel rail with injectors and a fuel pump. One common method involves idling the car while slowly raising and lowering the injector electrical pulse width, and then examining the data logs to plot an XY graph. The Y-intercept on the graph represents the dead time. Another method involves measuring the injector flow rate at multiple pulse widths, plotting the results on a graph, and calculating the average gradient, which represents the flow rate. The x-intercept on this graph represents the dead time.
| Characteristics | Values |
|---|---|
| Injector dead time | Critical for a quality idle and maintaining air/fuel ratios |
| Differs from manufacturer spec if the "flyback" voltage your ECU applies to the injectors is different than during manufacturer's tests | |
| Can be measured by idling the car while datalogging and slowly raising and lowering the injector on time until the RPM drops or the car misfires | |
| Can also be measured by measuring the injector flow rate at multiple pulse widths, plotting the results on a graph, and finding the average gradient or slope | |
| Can be affected by the voltage at the injector | |
| Can be adjusted by tuning the engine and changing the injector size setting | |
| Can be calculated using the commanded injector pulse width and the number of spark events |
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What You'll Learn
Measure injector flow rate at multiple pulse widths
To measure injector flow rate at multiple pulse widths, you will need a fuel rail with injectors and a reg fuel pump, high-sensitivity scales, a bench power supply, an Arduino, two push buttons, and a MOSFET.
The first step is to trigger the injector for a solid 10 seconds using one of the buttons. This will give you the flow rate of the injector and the fuel weight per second. The second button will then fire the injector in 2000 x 5ms bursts. If there is zero dead time, both weights will match.
You can also use a calculator to determine the injector duty cycle when the engine speed and injector pulse width are known. You can also use AEMLog or MS Excel to calculate the dead time. In Excel, do a scatter plot and add a trendline. Select "linear" and then "show equation" in the options. The Y-intercept will be the dead time.
For 30 cc/min of fuel at idle, the ECU will follow the linear dotted line and see that it needs to be commanding a 0.23 ms effective PW to get that flow rate, but in reality, the injector isn't firing anymore. This is where short pulse width adder tables come in, allowing you to trace the low pulse width behavior of an injector and model its true operating curve.
When measuring the injector flow rate at multiple pulse widths, it is generally recommended to use 100Hz so that PW represents the duty cycle. For example, 8msPW = 80%. You should then do the same group of tests at all voltages you want to characterize. Plot the results Flow Vs PW on a graph, and you should get near-straight lines. The average gradient or slope is your flow rate if the injector was perfectly linear, and the PW axis intercept is the dead time.
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Plot flow rate vs pulse width on a graph
Injector pulse width analysis is a critical aspect of engine management systems, as it directly impacts the fuel injection process and, consequently, the engine's performance. The injector pulse width, measured in microseconds (µs) or milliseconds (ms), determines the duration of fuel injection into the engine's cylinders. This analysis involves identifying trends, correlations, and anomalies in the injector pulse width data, as well as comparing it to other engine parameters.
To measure the injector dead time, you will need to plot the flow rate versus pulse width on a graph. Here is a step-by-step guide:
- Measure the injector flow rate at multiple pulse widths. It is recommended to use 100Hz so that PW represents the duty cycle (for example, 8msPW = 80%).
- Perform these tests at all voltages you want to characterize.
- Plot the results of Flow Vs PW on a graph. The data points should form a line.
- The average gradient or slope of the line represents the flow rate if the injector was perfectly linear.
- The PW axis intercept is the dead time.
- You can calculate the slope from the 2 & 7 or 2 & 8ms data points, provided the plotted data fits a straight line well in this range.
- If your data is clean and has minimal noise, the Y-intercept will be visible in the equation.
- If not, select "Set Intercept" and try out different values until the trend line appears to describe the quiet part of the data.
- You can use AEMLog or MS Excel for this analysis. Excel allows you to add a linear curve fit ("trendline, linear") and view the equation.
By understanding the relationship between flow rate and pulse width, you can optimize the engine's performance, fuel efficiency, and emissions. Additionally, accurate injector dead time data is crucial for matching injectors in the non-linear low IPW operating range, ensuring consistent fuel delivery, and achieving the desired engine performance.
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Calculate the average gradient
Measuring the dead time of a fuel injector is important for getting a stable, accurate idle and maintaining accurate air/fuel ratios when there are accessory loads on the engine. To calculate the average gradient or slope, which represents the flow rate of the injector, you can follow these steps:
- Measure the injector flow rate at multiple pulse widths. It is recommended to use 100Hz so that the pulse width (PW) represents the duty cycle. For example, 8msPW = 80%.
- Perform these tests at various voltages to characterise the injector's behaviour across a range of conditions.
- Plot the results of Flow vs PW on a graph. The data points should typically form near-straight lines.
- Calculate the average gradient or slope of these lines. This average gradient represents the flow rate of the injector if it were perfectly linear.
- The point where the line intercepts the PW axis is the dead time of the injector.
You can calculate the slope using common trigonometry formulas or by dividing the change in the y-values by the change in the x-values. This process helps determine the steepness or degree of inclination of the line formed by your data points.
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Idle car while data logging
To measure the dead time on a fuel injector, you must first idle your car while data logging. This is because getting the injector dead time correct is important for getting a stable, accurate idle and accurate AFRs while engine braking.
To do this, you must have a reasonably stable idle at a fairly fixed RPM, and your battery voltage must remain constant throughout. You can then log MAP, AFR, battery volts, and injector electrical pulse width while idling.
It is recommended to do a full tune using stock/OEM injectors with OEM injector dead times. If you are tuning a boosted car, for example, that the stock/OEM injectors cannot provide enough fuel for, just tune the idle and part-throttle areas of the fuel and ignition maps fully. You should also turn off all accessories (headlights, heater, stereo, etc.).
You can then slowly raise and lower the injector on time until the RPM starts to drop significantly or the car starts to misfire. You can then examine the data logs and do an XY plot. Plot MAP divider AFR on the X-axis, then injector on-time on the Y-axis. The data points should form a line. If you project this line to the Y-axis, the intercept is the dead time.
You can also measure the injector flow rate at multiple pulse widths. Plot the results Flow Vs PW on a graph, and you should get near-straight lines. The average gradient or slope is your flow rate if the injector was perfectly linear, and the PW axis intercept is the dead time.
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Log MAP, AFR, battery volts, and injector electrical pulse width
Injector dead time refers to the time it takes for a fuel injector to open and close. This can be measured by testing the injector flow rate at multiple pulse widths and plotting the results on a graph. The pulse width axis intercept will indicate the dead time.
Logging the Manifold Absolute Pressure (MAP) and Air-Fuel Ratio (AFR) can help identify any issues with the fuel injector. For example, if the MAP and AFR start to oscillate, it could be due to the MAT correction adding fuel and changing the pulse width as the car heats up. Logging the battery voltage is also important, as changes in voltage can affect the injector dead time and AFR. For instance, a drop in battery voltage from electrical devices being turned on may cause the AFR to climb and the engine to run rough.
Additionally, logging the injector electrical pulse width can provide valuable information about the injector's performance. The pulse width represents the duty cycle, and by testing at various pulse widths and voltages, you can create a dead time table. This table will allow you to adjust the injector time and pulse number for accurate fuel delivery.
It is important to note that different ECUs may have varying definitions of dead time, so ensure you are using the correct parameters for your specific ECU.
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Frequently asked questions
Accurate injector dead time settings are critical for a quality idle and maintaining air/fuel ratios when there are accessory loads on the engine.
You will need a fuel rail with injectors, a fuel pump, high-sensitivity scales, a bench power supply, an Arduino, two push buttons, and a MOSFET. The Arduino code should be simple, with two buttons: one will trigger the injector for 10 seconds, and the other will fire the injector in 2000 x 5ms bursts. If there is zero dead time, both weights will match. Calculate the difference in fuel delivered to find the injector dead time.
Idle the car while datalogging and ensure that the RPM is stable and the battery voltage is constant. Log MAP, AFR, battery voltage, and injector electrical pulse width. Slowly raise and lower the injector on time until the RPM drops or the car misfires. Examine the datalogs and do an XY plot, plotting MAP divider AFR on the X-axis and injector on-time on the Y-axis. The intercept on the Y-axis is the dead time.
Adjust the value in the box for the voltage you are seeing in your tuning software until you reach the desired air/fuel ratio. If the engine is running rich, decrease the dead time value; if it is running lean, increase the dead time value. Repeat this process until you reach the desired air/fuel ratio.
