Basic installation and adjustment guidelines for autogas systems, part III


To meet the requirements of modern engines with a fuel injection controlled by advanced electronics, gas systems must have a controller that ensures the optimum injection of gas according to the petrol controller. Thanks to innovative technologies (e.g. Q-Max, Q_box Plus), modern controllers use information read via the vehicle OBDII, while monitoring current STFT and LTFT trimmings, the engine temperature and the status of the fuel loop which is entered by the petrol ECU. Such technology enables even more precise gas dosing to the intake manifold by the gas injectors.  

The controller calculates the correct gas dosage based on a string of complex algorithms stored in and processed by its processor. This requires the monitoring of parameters related to engine operation, such as revs, petrol injection pulsing, the temperature and pressure of the injected gas. Petrol injection pulses can be then precisely calculated to gas injection pulses.

When defining gas injection time, the controller also considers the gas pressure and temperature parameters, as well as the vacuum in the intake manifold. The gas dose selected in this way is perfectly adjusted to the engine load. The controller calculates a gas dose and opens the gas injectors with pulses at the specified time, expressed in ms (milliseconds).

            What happens to the petrol injectors when the engine is fueled with gas? The signals coming from the petrol controller are emulated by the gas controller, so the petrol ECU works as normal.

            What is important in gas controller installation? It’s correct connection, of course. Many fitters are not aware of the importance of the correct connection points for certain wires. Let us analyze the post-ignition (+) wire. Some fitters ignore the importance of this wire and connect it to the first available point where (+) is present after turning the ignition on (e.g. to the ignition coil). In some cars, voltage can be present for some time after the engine has stopped. Starting the car after it has been turned off  can be problematic if the connection of the post-ignition supply is made at  this point (an example is VW Passat). The right point of connection for such a (+) wire is at the ignition switch, where we can be sure that voltage is stable, it does not disappear during cranking and it disappears right after turning the engine off.

            Each kit with the controller contains a wiring diagram for the connection of electrical cables, which should not pose any problems with connection when having some knowledge of electronics and engine operation control.

            At the beginning I mentioned Q-Max and Q-Box Plus controllers and their innovative option to connect the OBDII/EOBD system. What is OBDII/EOBD? The name is an abbreviation of On Board Diagnostic. OBDII/EOBD is understood as an onboard diagnostic system which is compliant with the specific SAE and ISO standards. With a standardized communication method, you can use a diagnostic scanner to connect to the vehicle and read the data compatible with the OBDII/EOBD. The system is able to read various parameters of engine operation and define the diagnostic procedures which are implemented by the vehicle controllers to detect failures as early as possible. The system is based on the analysis of the exhaust emission. Its main task is to monitor the level of toxic compounds in the exhaust system.  The system controls not only the elements  which are  critical for exhaust emissions, but also elements which can indirectly increase the amount of substances harmful to the environment.

When the onboard diagnostic system detects a fault, it is signaled by the activation of the MIL lamp (Check Engine) on the dashboard, and it simultaneously stores the code for the

defect and the definition in the memory. The fault can be read with a special OBDII diagnostic scanner (e.g. SXC-1011 by AC S.A).

            How do Q-Max and Q-Box Plus use the OBDII/EOBD system to determine the gas dose? While monitoring engine operation, the OBDII/EOBD system adds or deducts the trims which are overlaid on the fuel dose. These are STFT and LTFT trims. The STFT (Short Term Fuel Trim) means adjusting the fuel mixture to the stoichiometric value by extending or shortening the petrol injector opening until the moment when the Lambda probe switches from the high to low mode, and vice-versa. The introduction of the STFT by the petrol ECU is only used when the system is working in a closed loop (when the signals of both Lambda probes are taken into account). The LTFT trimming involves long-term adjustments of the fuel mixture to the stoichiometric value by extending or shortening the petrol injector opening times. Such trims are implemented in both the closed and open loop system. The Q-Max controller, which communicates with the vehicle OBDII/EOBD, monitors the STFT and LTFT trims and, based on this, calculates its own short-term and long-term trims for gas fueling. By implementing such asolution, the Q-Max gas controller is able to adjust precise fuel dosing in normal operation on gas.

            However, remember that the controller is not the be all and end all, and even the best controller available in the market will not correct errors if the system has been installed incorrectly.

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