Introduction

This application note describes the methods of tuning the air fuel ratio of the engine when there is a closed loop narrow band lambda circuit.

Purpose

When the factory management system uses the closed loop narrow band lambda circuit is is difficult to maintain the air fuel ratio changes made when modifying the MAP sensor voltage. To make the changes the control of the narrow band lambda probe is necessary.

SMT8 Analog and Narrow band Wiring

Using the SMT8, you will need to tune the Fuel via the MAP sensor and the narrow band lambda circuit. To do this you would need to control the voltage from the MAP sensor and the signal from the narrow band O2 sensor. The incoming MAP sensor voltage is wired into the Analog channel on the Ana#1 in and then back to the OEM ECU on the ANA#1 out. The narrow band signal is wired into the Narrow AFR #1 in and then back to the OEM ECU on the Narrow AFR#1 out.

 

 

SMT8 Narrow Band Mapping

 

The tuning in the narrow band lambda map works as follows : if the displayed incoming AFR is 14.7 (click "Data Display", select the desired item, tick "Small Window") and you wish to make the engine richer by mapping the Analog channel. Make the change to the Ana#1 map so that the AFR now reads 14.2, you will need to offset the Narrow AFR map by entering a positive number of 0.5 into the map so that the outgoing lambda will read as 14.7, the OEM ECU should take no action to remove the extra fuel.

SMT8 Analog Setup

 

 

 

The Analog voltage has three parameters in the "Shift F1" map, they are the "An1 Upper Limit", "An1 Lower Limit" voltages and the "Analog#1 Offset %" percentage. The Upper Limit clamps the maximum voltage that would be returned to the OEM ECU, this works the same for the Lower Limit. The offset percentage will increase or decrease the entire Analog #1 map, so if the engine is too rich across the complete rev range then there would be a negative value entered to lean the engine out.

 

 

SMT8 Analog Mapping

The SMT8 has 2 Analog input channels and two separate maps. Both use a main map of 384-entry sites, which is added to two (24-row) side maps for engine and air temperature map and then multiplied by the (24-row) manifold pressure side map. The map sites are selected by the throttle position (or any other load signal) and the RPM. For this example we will only be using the Analog #1.

 

 

 

 

Testing if you have control of the fueling is simply done by entering a large number for example 50 into the green cell in the main map during idle and confirming that the engine's pitch changes. The mapping calculation is as follows :

 

A1OUT	=	AN1IN
		+- ANA1 MAP
		+- AN1 ENGT
		+- AN1 AIRT
		X AN1 AMP

 

 

 

Conclusion

Most of us spend 90% of the time while driving a car "cruising" due to speed and traffic restrictions. The SMT8 is the ideal tool to improve the cruising performance, while enhancing the power output at full throttle when overtaking.