Flex Fuel Tuning Guide

Flex Fuel Tuning Guide

Although there are a number of ways to successfully tune an engine for flex fuel, this guide is intended to show a method that allows very accurate tuning over all ethanol blends together with Long Term Learning helping with the tuning process.  The method is designed to give the best compromise to give the best possible tuning accuracy together with the least amount of time possible.

Note: Only ECU models with 4D mapping capabilities can use this guide.  At the time of writing the ELITE 1500 and 2500 models support this.

What Is Flex Fuel Tuning

Flex Fuel Tuning refers to an ECU tune that is designed to allow an engine to operate correctly over a varying blend of Ethanol and petrol.   Pure Ethanol does not contain as much energy as petrol, and has a stoichiometry value of 9.0:1 compared to petrol which is 14.7:1.  This means that more ethanol is required to maintain the same Lambda, or quite simply to run the same AFR.   There are some regions that use near to 100% Ethanol, but for the majority of users, the blend will vary from 0% Ethanol (pure petrol) through to 85% Ethanol (E85).

Besides the higher amount of Ethanol needed to maintain the same AFR, as the Ethanol percentage increases so to does the effective octane rating of the fuel, which is turn requires ignition timing correction.  To perform these corrections the ECU needs to be capable of using additional tables to make adjustments for the Ethanol percentage.  The octane change is also non-linear and typically reaches near its peak octane around the 60% Ethanol mark.  This also needs to be considered during the tuning process.

In the early days of aftermarket ECU development, it was common for people to use a switch to toggle between two maps, one for petrol and one for E85.  The issue with this is that unless the entire fuel system is drained and cleaned it is almost impossible to have exactly E0 or E85 in the tank. it invariably ends up always being a blend somewhere in between with tuning deviating from optimum.  To address this a sensor that measures the Ethanol Percentage is required, and with this, the ECU can adjust the fuel and ignition tuning to suit the blend of Ethanol present in the system. This is what is referred to as Flex Fuel Tuning.

What Is a Flex Fuel Sensor

A Flex Fuel Sensor is designed to measure the amount of Ethanol present in the fuel.  It does this by allowing the fuel to pass by a sensor that has two probes that measure the change in capacitance between them. This is read by a small microprocessor that then sends out a frequency signal to the ECU to indicate the percentage of Ethanol.  It also sends out Fuel Temperature as the pulse width of the same signal.  So you get two sensors on a single wire connection to the ECU.

The Haltech HT-011000 Flex Fuel Sensor

Wiring Of The Flex Fuel Sensor

The sensor body is labelled with the wiring information.
Haltech Wiring
Any Spare SPI or DPI
Signal Ground
Signal Ground (Black/White)
Power 12V+
12V+ Switched (Grey/Red)
Be careful to strip back the shielding to carefully reveil the inner wire.  Do not connect the outer shielding to the signal wire.

Location Of The Sensor

The Flex Fuel Sensor should be located as close to the injectors as possible.  Ideally, it should be in a location that has a continuous flow of fuel passing the sensor. 

The best location is in the high pressure feed line leading to the fuel rail. Care should be taken to ensure that the sensor itself does not restrict the fuel flow.  If this may occur a solution is to Y-piece the fuel line and run the sensor on a separate branch that runs parallel to the main fuel line before feeding back in so the sensor is not restricting flow. 

Often the sensor is mounted in the return line from the fuel pressure regulator, however, this can sometimes lead to aeration in the fuel which can cause erratic readings.  To avoid this issue there are settings present in the ECU to only sample the readings from the sensor when fuel flow through the injectors is minimal so as to ensure most fuel is returned to the fuel tank and therefore fuel flow in the return line is high. This will be covered later in the guide.

Mounting the sensor at a greater distance from the fuel rail causes delays between reading the fuel composition and the new fuel mix actually making it to combustion. This can mean that the ECU has made the corrections too early or too late and the engine may run poorly until new fuel reaches the injectors.

Configuring a Flex Fuel Sensor

With your Elite ECU live and online with the ESP software:

1. Go to the Main Setup by clicking on the Gear icon at the top of the screen

2. Go to Functions

3. If a Flex Fuel Composition/Temperature Sensor is not on the list it should be added by selecting Add/Remove Function and moving the sensor to the right-hand side of the list and then hitting OK.

4. Select the sensor and then in the Wiring tab you will need to Edit Connection and select the input wire you used.  In this example SPI-1 was used. The Edge Select should be set to RISING, the Sensor Type is HALL EFFECT, the Pullup must be ENABLED, and the Frequency Time Constant can be left on 1.

5. The Calibration-Fuel tab should be set to these values
Settings for use with Haltech HT-011000 Flex Fuel Sensor

6. The Calibration-Temperature should be set to these values.
Settings for use with Haltech HT-011000 Flex Fuel Sensor

7. If you wish to only sample the sensor under certain conditions this can be done in the Sampling Threshold - Fuel tab. In this example, the sensor will only be sampled below 1500rpm and below 10kPa of pressure.  i.e. only near idle conditions.

8. Go to the Engine section, select the Fuel Tab, and set the Fuel Type to FLEX FUEL.  Notice that Fuel Comp Correction tables are NOT enabled but a Fuel Composition Scalar table will now be available.

The Flex Fuel Sensor is now configured and you will be able to view the Flex Fuel Percentage and Fuel Temperature as available channels in your ESP Software, and the system will now react to Ethanol content. 

How To Get The Best Tuning Result

For the purposes of demonstration, the following section is written for a generic V8 with a WB2 Wideband kit installed.  This is to show how the tuning can be done over a banked engine with 2x wideband sensors. Also, the tables have been simplified purely for size.

Step 1 - Do the Petrol Tune First

After configuring the Flex Sensor, perform your normal tune when the fuel has the least possible Ethanol in the system.  Preferably this is done before any Ethanol has been added to the system previously.  This is to ensure that we have an accurate baseline from which all additional tuning for increased Ethanol content is corrected.  This includes:
  1. Set the Target AFR Table to the correct values for petrol
  2. Injector Flow and Dead Time data are as accurate as they can be
  3. Boost Control is set to an appropriate value for the fuel grade
  4. Tune all Fuel and Ignition Base and Correction tables 
This needs to be a complete tune and you are happy with how it runs on Petrol before proceeding.

Step 2 - Perform the E85 Tune

Now that the petrol tuning is complete we can move on to the tuning for Ethanol.  To do this accurately the fuel needs to be thoroughly drained before adding E85.  In some cases, a small amount of E85 is added and cycled through the system before being drained again and replaced with fresh E85.  The purpose is to have as close to the maximum Ethanol percentage in the engine when we next start tuning again.

The important part from here is to expand the relevant tables that we will need to adjust to have an additional axis.  Tables that were 3D like the Fuel, Target Lambda, and Ignition Base table now need to be 4D.  Tables like the Boost or Fuel Prime Pulse that were 2D now need to be 3D.  This is to allow us to make changes to the tuning for ethanol that are not going to affect the Petrol tuning in any way. When adding the additional axis the only cell values that will be present at this time are for 0% and for the percentage that is currently present in the fuel. For this example, we will use 85%.

Keep in mind that there is already a background calculation that uses the Fuel Composition Scalar table to control the blending of the change in stoic of the fuel.  But if there are differences due to unforeseen errors, like unknown injector data, the system allows for tuning around these issues.

Table Expansion

For this example, we will use the Target Lambda table.  Tables can be expanded by selecting the Table first, and then the Table Axis Setup button at the top of the screen.

In this example, we are expanding the Target Lambda table from 3D to 4D.  This is done by enabling Axis 3, selecting the Flex Fuel Sensor, and adding values for 0 and 85. Then hit OK. Notice we are not adding any additional Ethanol percentages at this point.  Only 0 and 85.

We now have a 4D table that has a slider bar beneath it. This ball can be moved to the desired table that needs to be tuned. Currently, in this image, it is set for 0% Ethanol.

By sliding the ball over to 85 we now can adjust the Target Lambda for E85. This does not affect the values used for 0% Ethanol.  In this example, the Target was made slightly leaner.

By following this example it will allow you to directly enter in different tuning values for the two different amounts of ethanol.   Effectively you have dual maps, for now.
The following tables should also be expanded to have the additional axis point for Flex Fuel 85%.  
  1. Bank 1 Long Term Fuel Trim.  Bank 2 automatically mirrors the axis values from Bank 1
  2. Fuel Base
  3. Fuel Prime Pulse
  4. Fuel Cranking
  5. Ignition Base
  6. Boost Control Closed Loop Base Duty Cycle
  7. Boost Control Closed Loop Target Pressure 

Tuning For E85

After the relevant tables have all been expanded now you should move the slider to 85 and perform a full tune again to suit E85 fuel.  Always be wary of accidently selecting the wrong slider position. To be sure you have the correct table selected you can press the Space Bar while in any table and it will jump to the current cell that is being used by the ECU.

Sometimes the additional volume of exhaust gas as the byproduct of using high concentrations of Ethanol requires boost control duty changes to maintain the same Boost Level. This is done by adjusting the Boost Control Closed Loop Base Duty Cycle table to suit the new duty requirements for a particular Ethanol concentration.

Step 3 - Learning The Tuning Between 0% and 85% Ethanol

After the extreme ends of the Ethanol percentage spectrum are tuned we now need to set up the system to allow the areas in between to be tuned.  Because the extremes are already tuned, any additional sections we add will be interpolated so we are most of the way there already.  If we added them earlier when we expanded the tables all values other than for 85% will have the same values as for 0%.

You do not need to go crazy with excessive amounts of ethanol percentage sites, in fact, only a few more are all that is needed.   Thanks to the Elite ECU having an excellent Long Term Learning system, we can set up the system to learn the small differences in tuning as the driver varies the Ethanol percentage over time.  You should not need to do this for most tables, more so for the Fuel Base and the Long Term Trim tables.  It can also be done for the Ignition Base if you find the Scalar table is a little off, but you can also tune the Scalar tables if need be.

To do this, we simply add more sites to the Flex Fuel Sensor axis.  Typically at values at around 25 and 50 to give good coverage.

Fuel Base table with additional Ethanol percentages

Long Term Fuel Trim table with additional Ethanol percentages

If there are ignition corrections that need to be applied they can mostly be handled by the Scalar table, however, if more precise tuning is required and a Knock Sensor is present the Knock Control Long Term Trim table can also be configured to 4D in the same way as for Fuel Long Term Learning and the Ignition Base table also expanded the same way.

Now that the tables have been tuned and expanded to cover different blends, the Long Term Learning can be used to fill in the fine details.  This method also saves time for the tuner in not having to test every possible blend of Ethanol.

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