Updated: Mar 18
In the last few years we have seen in the Automotive Industry a large push for downsizing concepts, What is Downsizing ?, it envisions engine sizes of around 1 - 1.4 Liter maximum thus reducing physical engine size but with the objective to maintain the same Power and Torque characteristics(outputs) as the bigger engines with the effect of reduced fuel consumption and better exhaust gas emissions (Co2 output).To further enhance the Power and Torque Output from the smaller engines, we have seen more and more manufacturers adding Turbocharger units to their engines. There is a number of other Power boosting systems currently in use or development however in this short article, we are looking at the TC (Turbocharger)Waste-gate systems and the improvements made to the Waste-gate Control to improve Boost Control and efficiency.We also take a look at the a basic Embedded Controller using Flowcode as the High level language to control the Waste-gate or Boost Pressure
This article does not cover how Turbocharges work and it is recommended that interested readers search for technical resources that covers this topic.
A few Electrical/Electronically systems in use for Downsizing and hybridization(another concept we will look at in some other article)
Boost Recuperation Machine (BRM) from BOSCH
eBooster from BorgWarner
Hyprex Pressure Wave Charger from ABB (Comprex) and Swissauto (Hyprex)
The Old and New Waste-gate Control
In the past the waste-gate actuator was controlled by a pneumatic (Vacuum/boost pressure ) which due to it mechanical accuracy is not as downsizing concept engines require. An example pneumatic Waste-gate below.
New designs use PWM Position Control Actuators and DC Motors with Position monitoring to achieve precise positioning and control of the waste-gate opening angles therefor controlling the boost pressure more precisely.
The DC Motor Waste-gate Actuator (eWG)
We take a look at the DC motor type actuator as it is the most recent developments in TC systems.
Construction and Operation of an eWG
The actuator consist of an Electrical DC Motor and a Position feed back sensor that needs to be driven and read by the External DC motor driver that determines and realizes the required shaft position, The DC drive circuits are PWM controlled.
Every Turbocharger has a Operating Map which shows its performance at various Pressure Ratio's((P2/P1) Boost Pressure/Turbo Inlet Pressure) and Turbocharger Speed(RPM) the output parameter is the Mass airflow it produces at a given Pressure ratio and Turbocharger speed. An example Turbocharger Maps below.
(With permission of SA Charging Solutions - Switzerland)
Engine Air/Pressure- Mass flow Requirement
The engine requires a certain amount of Mass flow for every engine operating point( Engine Rpm and load condition of the engine ,Torque Requirement),thus for every operating point of the engine corresponds to an operating point of the Turbocharger on it TC map. It is the work of the Waste-gate actuator to open and close the Waste-gate flap(valve) to ensure that the Turbo is working at the required Operating point demanded from the Engine.
If the Pressure P2(Boost pressure ) rises over the pressure required the Waste-gate actuator opens the Waste-gate valve, Turbo Speed decreases and in relation to the Turbo speed decrease the P2 pressure and mass flow decreases.When the P2 pressure fall below the required boost pressure the Waste-gate actuator closes the valve P2 rises and so does the Mass flow.
As you can see the more accurate and precise the Waste-gate valve is controlled the better /closer we can operate at the best operating point.
Again this is in a nutshell there are many more complex thermodynamic consideration but this is sufficient to understand why we need optimum control of the Waste-gate actuator, which is obvious not achievable with a mechanical spring controlled actuator.
Below a simple indication of actuator positions/Waste-gate valve positions
Waste-gate Actuator Control Algorithm
The Position sensor output a Voltage from 0 - 5V which translate normally as indicated by the below graph around 0.8V is closed position and 4.8V is 22-25 mm fully open position. This can vary by manufacturer of the Turbocharger and the specification for the Turbocharger must be check to get the correct voltages,between these values are the working range of the Waste-gate valve.
Block Diagram of a Embedded Controller to Drive the Waste-gate Actuator
Such and Embedded Controller consists of the following Hardware sections
Micro-controller Logic unit
Power Supply Circuits
Signal conditioning for the input and output circuits
Current Sense (Measuring the Actuator current consumption)
H Bridge Power Stage to drive the DC motor Forward and backwards(Open and closing of the Waste-gate Valve
Note:Some automotive manufacturers include these driver circuits in the Actual Engine Fuel Injection ECU(Electronic Control Unit) others use a separate Driver,
There are many after market users replacing the mechanical waste-gate with an Electrically Waste-gate Actuator and are looking for drivers for it
Simple Basic Control Algorithm
The output PWM Duty cycle is calculated from the requested opening size of the Waste-gate vale which normally comes in the form of a CAN message from the Engine controller,it can also be an analog signal that has to be converted by an ADC as in our example below, which is the Control Input Signal. As it is a closed loop Control system the second input parameter needed is the actual position of the Waste-gate actuator.The Feedback signal,this is also an analog signal that is converted by the ADC unit, these converted values are then translated into mm position and the Error signal is calculated and the Error is passed on the the PID controller that calculates the percentage of PWM Duty cycle to drive the DC motor / Waste-gate Valve to the new position or keep it at the existing position. There are limits set on the minimum and maximum values to avoid Anti wind up .The PWM signals are then applied to the H Bridge driving the DC motor in the opening or closing directions.The current measurement are use to check for a lock shaft or Waste-gate value if the current exceed a preset value the PWM is reduced or switched of and an Error Flag is set in the Software of the driver which serves as an OBD function(Onboard Diagnostics) In some units a mathematical model is placed in code that monitors the expected current consumption and if it varies by a certain amount aging of the waste.-gate valve is determined and it would report in the diagnostic that the value is not working as it should and servicing is needed.
In some algorithms a second inner loop PID is applied on the current consumption and therefore the percentage PWM is calculated from the mm position and the amount of current needed to drive the motor to the next position , This algorithm has a faster response and can be more position is more accurately controlled as it is a current control .
Flowcode Implementation of Waste-gate Driver Control Firmware
Flowcode has a very powerful DSP library which is excellent and easy to use for such application, below is the Flowcode implementation in Flowcode you will notice that all the input signal and the PID controller and the PWM signals are represented on the panel as we first simulated the system and control and then generated the code program our driver board and used it on an actual system
This is a small part of the code which highlights the use of the Flowcode PIC Controller which is part of the DSP Components in Flowcode.
It is very easy and effective using Flowcode for such Control Applications as it has a powerful DSP Component(library) ,even people with very little control experience and knowledge can implement complex controls using Flowcode with out writing complex lines of C-code or any other High level langue code .