Mx00/M84 Idle Control

Introduction
This Technical Note provides a detailed explanation of various Idle Speed Control strategies that can be implemented using MoTeC Gold Box ECUs. Idle speed control is an essential function to maintain the engine's idle RPM within a desired range, and it can be achieved by using either a 2-wire, 3-wire or 4-wire idle speed valve. These types of valves operate by adjusting the Duty Cycle, which refers to the ratio of the signal being switched off to on, effectively controlling the valve's position. Additionally, stepper motor valves are supported in the M400, M600, and M800 ECUs, offering another method of idle speed control.

Auxiliary Outputs
When it comes to controlling these valves, 2-wire valves require a single Auxiliary output to be driven, whereas 3-wire valves need two Auxiliary outputs for proper operation. 

In both scenarios, one Auxiliary output must be configured specifically for Idle Speed Control, which corresponds to function 2 in the ECU setup. For 3-wire valves, the second Auxiliary output should be assigned as an Idle Slave, identified as function 7. 

In the case of 4-wire stepper motors connected to an M400, M600, or M800 ECU, the system utilises Auxiliary outputs 5, 6, 7, and 8. Configuring output 5 for Stepper Idle Speed Control will automatically allocate the other outputs to this function, simplifying the setup process.
 

Furthermore, the M400, M600, and M800 ECUs are capable of managing idle speed through a Drive by Wire (DBW) throttle system. This Drive by Wire function is configured on Auxiliary output 1, which in turn automatically assigns Auxiliary output 2. The idle control parameters are then adjusted through the settings for Auxiliary output 2. For accurate wiring and basic setup details, it is recommended to consult the specific valve drawings provided by MoTeC.

Setup Parameters
It is important to tailor the setup parameters to match the specific installation requirements. Each type of idle control has associated help documentation that offers detailed information and suggested starting values to guide the tuning process.
 

Proportional Gain
The proportional gain parameter is responsible for the immediate response to any discrepancy, or 'Error', between the actual engine RPM and the target idle RPM. It controls the majority of the system's response when the engine is operating away from its intended idle speed. The proportional response is calculated by multiplying the proportional gain (the ‘P’ parameter) by the Error value, which is the difference between the current RPM and the aim RPM. However, proportional gain alone cannot completely eliminate the error because as the error decreases, the proportional response also diminishes.
 

Integral Gain
Integral gain is a more gradually changing factor that works to reduce the error over a longer period. The integral component continues to increase as long as the error is not zero. For example, the idle control valve often requires a certain duty cycle to maintain its position. When the error is close to zero, the proportional and derivative components have minimal effect, but the integral component will maintain the valve’s position steadily.
 

Derivative Gain
Derivative gain provides a damping effect on the control system, improving its response time. It is based on the rate at which the error value changes, meaning that the derivative component becomes larger during sudden changes in error compared to gradual changes. This helps to stabilise the system and prevent overshooting the target idle speed.
 

Dead Band
The dead band represents a tolerance zone around the target RPM where the ECU will not attempt to correct the error. Since physical systems can never perfectly match the aim value, the error will never be exactly zero. The dead band prevents the controller from continuously trying to fine-tune the engine speed unnecessarily. For instance, if the engine speed varies by plus or minus 50 RPM around the idle target, this is usually acceptable and will not cause noticeable effects. Setting this range as the dead band means the ECU will cease idle control adjustments once the RPM is within this tolerance.
 

Anti-Stall Gain
The anti-stall gain is an additional gain applied to the proportional gain when the idle RPM falls 300 RPM below the target idle speed. This feature increases both the magnitude and speed of the ECU’s response under these conditions, helping to prevent the engine from stalling.
 

Air Conditioning and Power Steering Compensation
When the Air Conditioning (AC) or Power Steering (PS) systems are active, the ECU compensates for the extra load on the engine by adjusting the idle control accordingly. This compensation can be achieved by adding a fixed duty cycle to the valve, moving the stepper motor by a specified number of steps, or increasing the throttle angle in Drive by Wire setups. To enable this functionality, digital inputs for the AC and PS status flags must be properly configured within the ECU.

Maximum Step Rate, Maximum Position, Maximum Duty, and Integration Limit
These parameters impose limits on how aggressively the idle control system can react to errors. For stepper motors, the maximum step rate defines the fastest rate at which the motor can be stepped. For Drive by Wire throttles, there are limits on the maximum throttle angle and maximum duty cycle to control how quickly the throttle can be adjusted. The integration limit restricts the maximum duty cycle that the integral gain can add, preventing the integral component from becoming excessively large if the error never reaches the dead band. These parameters should be set based on the characteristics of the specific device used for idle control.
 

Frequency
The frequency parameter defines the operating frequency of the valve control signal. It is essential to set this frequency according to the specifications and characteristics of the valve being used to ensure proper operation.
 

Minimum and Maximum Duty Cycle
The minimum and maximum duty cycle parameters define the range within which the valve operates effectively. These settings ensure that the valve has sufficient time to fully open at the minimum duty cycle and fully close at the maximum duty cycle. These values are determined by the physical properties of the valve.
 

Idle Tuning
Mechanical Setup
For optimal idle control, the load sensing should be configured to use Manifold Pressure. If throttle-based load sensing is used, the MAP sensor must be connected to the manifold to provide accurate readings. This setup ensures that fuel delivery is correctly adjusted as the control valve opens and closes. A linear MAP compensation table should be established, with -100% fuel trim at 0 kPa and 0% trim at 100 kPa. For boosted engines, this table should be extended linearly for pressures above 100 kPa. Additionally, the fuel pressure regulator should be connected to the manifold to maintain consistent fuel pressure.
 

The throttle stop should be adjusted so that the hot idle RPM is approximately 100 RPM below the desired idle RPM. This provides a baseline for the idle control system to work effectively.
It is important to note that idle speed control will not compensate for erratic idle behaviour. Firstly, fuel and ignition tables should be tuned to achieve stable idle conditions. The ignition advance table should be relatively flat in the idle region to support idle stability. If the engine is in poor condition, the idle speed control may struggle to maintain a steady RPM. It is also essential to ensure there are no air leaks in the intake system.
 

Idle control is not intended to fix mechanical issues that cause poor idling but is designed to maintain a steady RPM when additional loads, such as power steering or air conditioning, are applied or removed.
 

Parameters
Please note that Idle Speed Control is only active when the throttle position is less than 2.0% and the engine RPM is within 500 RPM of the desired idle speed. Additionally, the idle speed is increased when the engine is cold, reaching 1.5 times the aim idle speed at 0 degrees Celsius and gradually reducing to the aim idle speed as the engine temperature rises to 60 degrees Celsius.
 

The first parameter to adjust should be the Proportional Gain, as it has the most significant impact on the idle control system’s response. The gain should be set as high as possible without causing the engine to hunt or oscillate.
 

As previously mentioned, proportional gain alone cannot completely eliminate the error, so some integral gain is necessary to provide a baseline duty cycle or valve position. Integral gain drives the system towards the aim idle RPM over the long term, but excessive integral gain can cause overshoot and hunting.
Derivative gain can be applied to reduce overshooting of the aim idle RPM. This parameter helps improve the response time of the idle control system. However, some setups may not require any derivative gain at all.