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How to Calibrate an LPG-Powered Car

Victor is very passionate about anything technical, from computers to automotive technology.

To calibrate LPG-powered cars you need a basic knowledge of the electronic fuel injection (EFI) system. This is why we start this article with a discussion of EFI principles.

The Engine Management System of an LPG-Powered Car

The engine management system of an LPG-powered car consists of two electronic control units. These are:

1. The engine management system electronic control unit (EMS ECU) that comes with the car, and

2. The add-on LPG electronic control unit (LPG ECU).

These two ECUs are in a master/slave configuration in which the EMS ECU is the master, and the LPG ECU is the slave. The EMS ECU manages the entire operation of the engine, while the LPG ECU controls the amount of LPG that is fed into the engine in accordance with the instruction from the EMS ECU in the form of petrol injection signal so that the engine behaves as close as possible to its behavior when powered by gasoline or petrol. See Figure 1.

Figure 1-Master/Slave Configuration of ECUs in an LPG-powered Car

Figure 1-Master/Slave Configuration of ECUs in an LPG-powered Car

How Does the EMS ECU Operate?

The EMS ECU uses the information from the different sensors to look up the pre-programmed or basic injection time from the AFR map stored in the ECU’s memory. This injection time is the length of time that the injector is “on” to supposedly inject the right amount of fuel that makes the air/fuel ratio (AFR) equal to 14.7:1. This is the stoichiometric ratio that produces optimum engine performance.

In real situations, however, the basic injection time will not always yield an AFR of 14.7, and thus, it needs to be corrected. Correction of injection time is made possible by the oxygen (O2) sensor which monitors the oxygen content in the exhaust gas. This O2 sensor generates a voltage of up to 0.8 volt, depending on the oxygen content in the exhaust gas. The higher the O2 content, the lower the voltage.

If the mixture of air and fuel that entered the combustion chamber is lean (AFR>14.7), there will be higher oxygen content in the exhaust gas after combustion, and the O2 sensor will generate a voltage below 0.45 volt. In this case, the ECU adds a certain percentage to the basic injection time to increase the amount of fuel that will be injected in the next injection cycle to bring the AFR back to 14.7. If the fuel mixture is rich (AFR<14.7), the O2 sensor voltage will be above 0.45 volt. Then the ECU will cut a certain percentage from the basic injection time to bring the AFR back to 14.7. This continuous correction of the injection time is called fuel trim and the amount of time that is added to or subtracted from the basic injection time to get the actual injection time is the fuel trim value or simply referred to as fuel trim. This is expressed in percent of the basic injection time.

There are two types of fuel trims: short-term fuel trim and long-term fuel trim. Short-term fuel trim is the injection time correction obtained in real time, while long-term fuel trim is the average of the fuel trims over a period of time.

Assuming that all conditions required of a healthy engine are present, the engine will operate in its optimum condition as long as the ECU is able to carry out the fuel trim. Fuel trim values within -10% and +10% are normal. Fuel trims beyond -10% mean that the engine is running too rich. Beyond +10% the engine is running too lean. If the fuel trim is outside the range of -25% to +25%, the ECU will stop the fuel trim and will just use the basic injection time in the AFR map. The engine will still run in this case but not in its optimum condition and fuel efficiency is adversely affected.

How Does the LPG ECU Operate?

The LPG ECU uses the petrol injector signal to determine the amount of LPG to be injected into the engine. Because LPG and petrol have different properties, it takes a longer time for the LPG injector to be turned “on” when the engine is powered by LPG than it does to produce the same engine performance when it is powered by gasoline. So, LPG injection time is greater than the petrol injection time by a certain factor. The LPG ECU uses “coefficients” to derive the LPG injection time from the petrol injection time for different load-RPM combinations.

What Is the Objective of Calibration?

The objective of the calibration procedure then is to determine the correct coefficients for the different engine demands. This is made easier by the calibration software that comes with the LPG kit. Each calibration software has auto-calibration and fine-tuning features. In auto-calibration mode, the coefficients are determined automatically by the software. In fine-tuning mode, the coefficients are entered or modified manually by the user. In my experience, auto-calibration is not enough, and we need to do manual fine-tuning.

How Do We Know If the Coefficients We've Chosen Are Correct?

We can be confident that we have chosen the correct coefficients if by these said coefficients the fuel trim is playing somewhere between -10% and +10% and the calculated load value (CLV) is close to the typical value for your engine. Calculated load value for a given situation is a percentage of the maximum available torque of the engine at wide-open throttle (WOT).

Tools We Need in the Calibration

In this calibration procedure we need the following software:

  1. The LPG Calibration software that comes with your LPG conversion kit
  2. OBD2 Diagnostic Software

Pre-Calibration Preparation

Make sure that the engine is in good condition. No amount of LPG tuning will improve the engine’s performance if it is already problematic when powered with gasoline, unless the problem is due to the gasoline fuel delivery system of the car.

Pre-Calibration Preparation for Bi-fuel Cars (can run either on gasoline or LPG)

  1. Run the engine and wait until it reaches its normal operating temperature.
  2. Using the OBD diagnostic software, check if the engine is ‘Ready for Emission Testing’. See Figure 2.
  3. If the engine is ready for emission testing, open the dashboard window of the diagnostic software and note down the calculated load value (CLV) at idle when the aircon is off and when the aircon is on. Then proceed to the tuning procedure.
  4. If not ready for emission testing, make the necessary steps to recondition the engine then go back to step 1.

Note: We can proceed with the tuning of our car even if it is not ready for emission testing when running on gasoline, but we cannot expect the engine to be in top condition.

Figure 2- Ready for Emission Testing

Figure 2- Ready for Emission Testing

Pre-calibration Preparation for LPG-only Cars

  1. Be sure you have a typical CLV for your car, both with aircon on and with aircon off. The CLV can be obtained by checking with the OBD2 diagnostic software a known good-running-condition car of the same make and model as yours.
  2. Proceed to the calibration procedure.

Calibration Procedure

  1. Turn on the engine and run it at idle speed until it reaches its normal operating temperature.
  2. While waiting for the temperature to normalize, run the LPG software and the OBD2 software. Adjust the LPG window and the OBD2 window sizes so that the important parameters are displayed as shown in Figure 3.
Figure 3- LPGCalibration Software and OBD2 Diagnostic Spftware in One Screen

Figure 3- LPGCalibration Software and OBD2 Diagnostic Spftware in One Screen

3. Select the display option of the LPG software. Make sure that the gas pressure is 1.0 to 1.3 bars. Adjust the LPG reducer or vaporizer when necessary. Also check if the size of the LPG injector nozzle is correct. Replace nozzle when necessary. See Figure 4.

Figure 4- Gas Pressure and Injector Nozzle Size

Figure 4- Gas Pressure and Injector Nozzle Size

4. This procedure is for bi-fuel cars only. If your car is LPG-only, go to procedure no. 5. Select the LPG calibration window, start the autocalibration and then follow the instructions as prompted. In this procedure, the LPG ECU is automatically setting the coefficients. Then go to procedure no. 6.

5. This procedure is for LPG-only cars. Check if the fuel trim indicators are functioning. If not, the engine is either running too rich or too lean that the ECU suspends its fuel trim operation. If this is so, determine which side the engine is into (rich or lean) so that we can make the corresponding adjustment on the LPG Map. This is done by switching to the ‘Data Logging’ window of the OBD2 software and checking the waveform of the O2 sensor voltage. If the voltage is close to 0.8 volt, the fuel mixture is too rich. If the voltage is close to 0.2 volt, the mixture is too lean. See Figure 5.

Figure 5- O2 Voltage Waveforms for Normal, Too Rich, and Too Lean Fuel Mixtures

Figure 5- O2 Voltage Waveforms for Normal, Too Rich, and Too Lean Fuel Mixtures

Switch to the MAP window of the LPG software and change the coefficient or multiplier until the oxygen sensor voltage waveform is back to normal (by then the fuel trim scale of the OBD software must be functioning already).

6. Switch to the ‘Dashboard’ window of the OBD software while the LPG software is on ‘MAP’ window.

7. Coefficient adjustment at low engine load at idle and at different RPMs. While the car is parked, the shift lever in neutral position, and the aircon is off, adjust the coefficient at idle, making sure that the fast trim is close to zero and engine load is close to the CLV determined earlier. Do the same procedure for three other engine speeds like 1,500 RPM, 2,000 RPM, and 3,000 RPM.

8. Coefficient adjustment at increased engine load at idle and at different RPMs. Adjust the coefficients for different RPMs with the aircon and headlights “on” while observing the fuel trim and engine load.

9. Coefficient adjustment at heavy engine load at idle and at different RPMs.

9.1. Make sure that the parking brake is fully engaged, and the tires are blocked at both sides of the wheels.

9.2. Heavy engine load at idle. Step on the clutch pedal fully to the floor, then place the shift lever to 3rd gear. Do not release the clutch yet. While at idle, slowly release the clutch pedal until the engine load has increased, but not too much as to cause stalling of the engine. Hold on to this pedal position to keep the engine load steady, then adjust the coefficient. Shift to neutral and then release the clutch pedal.

9.3. Heavy engine load at higher RPM. Step on the clutch pedal fully to the floor, then place the shift lever to 3rd gear. Do not release the clutch yet. Rev up the engine to 1,500 RPM. Keep the load and RPM steady, and then adjust the coefficient. Do the same for higher RPMs. Shift to neutral and then release the accelerator and the clutch pedals completely.

10. Now that we have the coefficients for various engine loads at various RPMs, as shown in Figure 6, we can then extrapolate coefficients for the other T.inj-RPM combinations on the map. Make sure that there is no big abrupt change in coefficient to ensure smooth engine operation.

Figure 6-Coefficients for Different Load-RPM Combinations

Figure 6-Coefficients for Different Load-RPM Combinations

11. Save your calibration data. We might need this in the future.

Calibration is done.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.