Fine-Tuning an AEB-Manufactured LPG System

AEB-manufactured LPG systems include the following brand names – AEB, Bigas, Emer, King, OMVL, Romano, Tartarini, Zavoli and possibly others too. This article assumes that the installation is complete and that you have the correct software on your PC and have made the connection to the LPG ECU, completed the system configuration and run auto-calibration successfully. It also assumes that the vehicle has both petrol and LPG fuel supplies to the engine.

Before you start changing any settings, save a copy of the system configuration so that, if you make the tuning worse, you can restore the system to the way it was before you started. This does not cover physical changes to the system such as rail pressure, so make a note of the Press.Gas reading when running on gas.

This document does not cover direct injection engines.

Make sure that the engine is at normal operating temperature before tuning. From the Main Menu select Vehicle Configuration, then Modify Carb. Un-tick the box marked Adaptivity.

From the Main Menu select Vehicle Configuration, then Lambda.

If your engine has a V or a Boxer configuration it will most probably have 2 separate banks of cylinders with 1 or 2 lambda probes per bank. Check that your LPG system is configured with 2 banks. The same applies to engines with 5 or more cylinders. Some 4-cylinder engines are run as a single bank and some as 2 banks. On a 4-cylinder engine, set the number of banks to 2.

Turn off the ignition and wait for the lights on the change-over switch to go out, then re-start the engine. This procedure is referred to below as RESTART.

For a 4-cylinder engine, compare the 2 Tinj.Petrol readings at the bottom of the screen. If they are the same then the engine is being run as a single bank, so switch the setting back to 1 bank and RESTART. If the LPG system setting has been switched from 1 to 2 banks, re-run auto-calibration and save the resulting configuration.

If the engine is being run as 2 banks, when running on gas, compare the two Tinj.Petrol timings. They will be different, but if one bank is consistently more or less than the other, you should adjust the Fuel Trim Bank 2 until the timings are about the same. If Tinj.Petrol for bank 2 is, say 10% higher than bank 1, add 10 to the Fuel Trim Bank 2 figure and vice versa. The trim figure can be negative. Repeat the above until the two Tinj.Petrol values average around the same.

From the Main Menu select Vehicle Configuration, then Map. The columns relate to revs and the rows to petrol injector timings. The numbers in the cells are parameters for the fuelling algorithm and vary from 0 to 255. You will see a ball hovering somewhere in the top-left corner of the map when the engine is idling. As the load on the engine increases, so do the petrol injector timings and the ball drops down the screen. As the revs increase, the ball moves to the right. When making changes to the map it is best to apply a change to a block of cells so that a smooth transition is maintained between different parts of the map.

To change the values in the map, highlight the area to be changed by pointing to the cell in one corner, pressing the left mouse button and dragging the pointer to the opposite corner. Press Enter and a pop-up box allows you to change values in one of 3 ways. Percentage is the best option to use, so click that mode and enter a number (either positive or negative, between 1 and 50) in the box, then click OK.

The best way to tune your engine is under normal driving conditions, concentrating on the way the car is normally driven and the speed at which it is normally driven. Tuning requires that the car is driven at a given load and given revs, running on one fuel and then the other. It therefore means that you need a fairly clear road or smooth-running traffic. You will also need a driver who can hold a steady throttle. This technique covers the part of the power band that is most often used. The remaining parts can be viewed as either over-run or full(ish) throttle and will be dealt with later.

Drive at a constant speed at the lower end of the normal driving speed with the engine running on petrol. Make a note of the Tinj.Petrol value. Switch to gas and drive at the same speed and load. Make a note of the Tinj.Petrol value. Ideally these should be the same. If the Tinj.Petrol value when running on gas figure is higher, add to the numbers in the block of cells around the ball. Calculate the percentage difference between the 2 values and add the same percentage to the block of cells. If the Tinj.Petrol value when running on gas figure is lower, subtract the percentage from the numbers in the block of cells. Repeat this test at the same speed and load until Tinj.Petrol is about the same value for the 2 fuels.

Repeat the above procedure for different loads and speeds.

The next area to tune is the overrun part of the map. Overrun occurs when you take your foot off the accelerator and allow the car to slow down. Initially, most petrol ECUs will cut off the fuel to the engine by not firing the injectors. So, the ball will move to the top row of the map. As the revs drop, a point is reached when the ECU restarts the fuel supply. At this point, the lambda reading should be around 0.2 volts (if you have a 0-1 volt probe). If the reading is higher, reduce the numbers in the top row of the map until you have the correct reading. At tick-over, the ball should be in the second row of the map. If it is in the top row, you may need to adjust the tuning in the tick-over area of the map.

The final part of tuning is the most difficult – brisk to hard acceleration. When driving briskly, a point is reached when the petrol ECU stops looking at the lambda probe reading and calculates fuelling based on air-flow, throttle-position, revs and possibly other parameters. This is called open-loop fuelling because the feedback loop from the lambda probe is not used. The purpose of this mode of operation is to avoid lean running when the engine is under stress. Lean running increases the temperature of the engine and can result in component burn-out (valves, pistons). Since there is no automatic correction of the fuelling, it is important to ensure that the LPG system supplies enough fuel to avoid a lean mixture, but not so much that it is wasted. If you have an LPG ECU that has an OBD connection, then you can tell when the petrol ECU is in open-loop mode from the fact that the fuel trims freeze. On systems where there is no OBD connection, there is no way of checking when open-loop mode starts, all you can do is drive briskly (over ¾ throttle) and check that the lambda reading never falls below half for more than a second.

Drive the vehicle in open-loop mode and monitor the lambda reading. For a 0-1 volt probe, the reading should always be around 0.8 volts. This will ensure that the mixture is adequately rich, without wasting fuel. If the reading is below 0.8 volts, increase the numbers in the operational area of the map until 0.8 volts is obtained. If the reading is over 0.85 volts, reduce the numbers in the operational area of the map until 0.8 volts is obtained.

Once you are satisfied with the tuning, from the Main Menu select Vehicle Configuration, then Modify Carb. Tick the box marked Adaptivity.

Save a copy of the system configuration. The name of the configuration should include the registration number of the vehicle, the Press.Gas value (when running on gas, e.g p137 if Press.Gas is 1.37 bar), and the odometer reading.

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