Engine Control Unit (ECU)

In document Oxford Octane Formula Student Report (Page 131-139)

7. Electrical and Control System (Yuntao Zhu)

7.2 Engine Control Unit (ECU)

7.2.1 Introduction

An engine control unit (ECU) is a type of electronic control unit that controls a series of actuators on an internal combustion engine to ensure optimal engine performance. This is done by reading values from different sensors placed in the engine, comparing the values to the default setting values in the performance map, then adjusting the engine actuators accordingly.

7.2.2 ECU

The ECU should fit the engine very well, and it was decided to select an ECU from the manufacturer. The engine we used is Honda CBR 600rr, it is a 4-cylinder engine and it is using a Dual Stage Fuel Injection technique. [1] In a racing car, at high engine speed, the time for the fuel injection is very little and only 4 fuel injectors are not enough to satisfy the fuel demand. The Dual Stage Fuel injection technique can solve this problem by placing a second set of injectors as far from the intake valve as the length of intake track would permit, and they can inject fuel at higher rpm to meet the fuel demand. [2] Thus, the ECU should be able to control up to 8 injectors with

Table 7.2.1(1) ECU comparison [3] [4] [5]

The above table shows a comparison of 3 ECUs that were found in the market, they all satisfy the requirements stated in previous paragraph, and the Omex710 is selected to be the our ECU with following reasons. It has the smallest size the lightest weight in these three ECUs, and it can control up to 12 cylinders and up to 12 injectors with controls for staged injectors, which fits the requirements from the engine. What’s more, the Omex710 is programmable so it can be set to fit the engine better, and it can provide cooling control for the radiator fan and cooling pump as well.

Last but not the least, the sensors required for the ECU can also be purchased from the same manufacturer to make sure the sensors are functional to the ECU. [3]

Figure 7.2.1(1) Omex710 product image [3]

Figure 7.2.1(2) Omex710 function diagram

7.2.3 Control of Air/Fuel ratio

The most important function of ECU is to control the fuel injection to the engine to satisfy the fuel demand based on the instructions from the driver. The instructions are in the form of electrical signal coming from different sensors. If the throttle position sensor is showing the throttle pedal is pressed further down, the mass flow sensor will measure the amount of additional air being sucked into the engine and the ECU will inject a fixed quantity of fuel into the engine. If the engine coolant

temperature sensor is showing the engine has not warmed up yet, more fuel will be injected causing the engine to run slightly ‘rich’ until the engine warms up. [6]

Air-fuel ratio AFR =mass of air mass of fuel

‘Rich’ means the AFR is small, the mass of fuel is much more than the mass of air. If exactly enough air is provided to completely burn all of the fuel, the ratio is known as stoichiometric mixture. And for pure octane the stoichiometric mixture ratio is approximately 14.7:1.

λ= AFR

AFR at stoichiometric

In naturally aspirated engine powered by octane, maximum power is frequently reached at AFRs ranging from 12.5:1 to 13.3:1 or λ of 0.85 to 0.9. [7] The λ value is monitored by the lambda sensor, and the ECU maintains the λ to be slightly ‘rich’ for an optimal engine performance but non-optimal fuel consumption, as shown in the figure below.

Figure 7.2.3(1) Power versus lambda diagram [8]

7.2.4 Ignition timing control

In a spark ignition internal combustion engine, the engine requires a spark to initiate the combustion in the combustion chamber. And it is important to calculate the timing of the spark in advanced since the fuel can not burn the instant spark fires completely. The combustion gases take a period of time to expand, and the rotational speed of the engine can increase or decrease the time that an expansion should occur. [9] The ECU can adjust the exact timing of the spark to provide better power and economy, and this is done by reading data from the crankshaft position sensor and knock sensor, and then calculate the corresponding ignition timing. If the knock sensor detects a knock, that means the spark occurs too early in the compression stroke, so it will delay the timing of the spark to prevent this. [6]

7.2.5 Cooling control

The Omex710 can control up to two cooling fans, which is done by monitoring the temperature of the coolant by the temperature sensor. [3] If the temperature goes above a set point, the fan is turned on, and it is turned back off when the temperature drops below that point. The temperature can also be controlled by controlling the coolant pump, by changing the speed of the coolant flow to maintain the optimum temperature.

Figure 7.2.5(1) Cooling Control

7.2.6 Electronic throttle control (ETC)

The electronic throttle control is an automobile technology which electronically connects the accelerator pedal (driver) to the throttle (engine), it uses the information from the throttle position sensor (TPS), accelerator pedal position sensor (APPS), and a variety of other sensors to determine how to adjust the throttle position.

7.2.6(1) Electronic Throttle Control System [10]

As shown in the figure (1), the ETC is a closed loop control system. The reference signal is the accelerator pedal position, which indicates where the driver truly wants the throttle to be. The reference minus the current throttle position to create an error signal, and this error signal indicates how the throttle position should be changed. The throttle position is changed by controlling the throttle control motor to open or close the butterfly valve, as shown in the figure (2).

7.2.6(2) Throttle control schematic diagram [11] 7.2.6(3) Toyota ETC schematic diagram [11]

According to the 2015 Formula SAE Rules, at least two separate sensors have to be used as TPSs, and if an implausibility occurs between the values of the two TPSs, the power to the electronic throttle must be immediately shut down completely. The implausibility is defined as a

deviation of more than 10% throttle position between sensors. If a TPS failure occurs, the power to the electronic throttle must be completely shut down within 50ms. Therefore, the throttle control technique used by the Toyota is applied in the car to follow the rules. As shown in the figure (3), a two separate sensor setup is applied to both throttle position detection and accelerator pedal position detection to improve safety. If the system detects a malfunction, such as the two TPSs are not matching to each other, or the two APPSs are not matching each other, or the throttle position being out of range of the correct operation, a fail-safe mode will be immediately employed by the ECU. The ECU immediately shut down the electronic throttle, and the specific data code will be stored in the memory of the ECU to help the failure check, and the warning lights in the vehicle such as check engine light will be illuminated to warn the driver.

Also, in the situation when the car is braking hard, (for example bigger than 0.8g deceleration without locking the wheels), if the TPSs shows that the throttle is greater than 10% open, the power to the electronic throttle and fuel pump will be completely shut down by the ECU and this result in the electronic throttle closing to the idle position.

7.2.7 Sensors

If the ECU can be treated as the ‘brain’ of the control system, then the sensors act as the ‘ears and eyes’. The sensors convert the physical states of the vehicle into electrical signals, which can be understood and interpreted by the ECU, and then adjusting the system accordingly. Here, some crucial sensor mechanisms will be briefly discussed.

7.2.7.1 Position sensor

The position sensor is required for the accelerator pedal position detection and throttle position detection. Although the structures of accelerator pedal and throttle are different, a potentiometer that works as voltage divider can be used to measure the position of both accelerator pedal and throttle. The wiper arm shown in the figure (1) is connected to the accelerator pedal to act as a APPS and connected to the butterfly valve spindle to act as a TPS. As the butterfly valve opens or closes, or the rotation of the accelerator pedal, the wiper arm is rotated and causes the output voltage of the voltage divider changes.

Figure 7.2.7.1(1) Voltage divider diagram, [10]

Vout= R 2

R 1+R 2

∗Vref

So the ECU can calculate the relative position of the accelerator pedal and throttle by calculating the ratio of Vout to Vref.

7.2.7.2 Air mass flow sensor

Figure 7.2.7.2(1) Air mass flow sensor [12]

As shown in the figure (1), a heated wire is suspended in the engine’s air stream, with either a constant voltage over the wire or a constant current through the wire. When the air flows past the wire, the wire is cooled. So the resistance of the wire decreases and allowing more current to pass through the wire. Then the wire is heated up again until it reaches the previous temperature

(equilibrium). And the air mass flow is proportional to the increased current, and this signal is sent to the ECU. [12]

7.2.7.3 The Oxygen (lambda) sensor

The lambda sensor can measure the Air to Fuel ratio, so it can be used to monitor whether the engine

is

working efficiently and cleanly. The lambda sensor is based on a solid-state electrochemical fuel cell called the Nernst Cell. Its two electrodes provide an output voltage corresponding to the quantity of oxygen in the exhaust relative to that in the reference air.

Figure 7.2.7.3(1) The wideband zirconia sensor [13]

In document Oxford Octane Formula Student Report (Page 131-139)