Electronic fuel injection on spark ignition engines

Introduction

Carburettors were used until 80's on Spark Ignition (SI) engines to provide mixture of air and fuel. But their weakness were the inaccurate metering of the fuel quantity and the bad vaporization of it. Due to air pollution control and fuel efficiency, a solution had to be found. Injection system were introduced in the 80's. The first systems were mechanically driven. The pressurized injection provided a better vaporization of the fuel and thus a more homogeneous mixture which results in more efficiency. But the mechanical system had also his limitations and could not provide an accurate metering of the fuel on the whole engine working range. Electronic Fuel Injection (EFI) solved this problem by giving a more flexible metering.

This presentation will cover the design of the embedded control system used for the EFI. The history and deep going technical aspects of the carburation will not be discussed here.


Carburation basics

A SI engine works mostly with a stoichiometric mixture of air and fuel. The quantity of fuel is given as function of the quantity of air as the Air Fuel Ratio (AFR). Typically, for standard gasoline :

AFR = 14.7 / 1

The injected fuel quantity is thus proportional to the quantity of air which income in the engine's cylinder. So this mass of air can be used as parameter to calculate the requested fuel quantity.

As this relation is true for normal operating range, (e.g. : cruising) some corrections have to be made for other conditions like idling and full load or Wide Open Throttle (WOT).

The whole operating range of the engine can be determined by two parameters : Speed of the engine in Revolution Per Minute (RPM) and Manifold Absolute Pressure (MAP) which gives the load of the engine. As boundary conditions, we can retrieve the idling with low RPM and the WOT with high MAP.

The volumetric efficiency (VE) is the volume of air incoming in the engine divided by the engine's displacement. The VE quantify the filling of the engine with air and depends of the engine's state. A table of the VE can thus be made in function of the engine's speed and the MAP.

Knowing the VE, the mass of air can be found by the ideal gas law.

And so the requested fuel quantity is compute by multiplying mair by the AFR.

Deviations from the normal working conditions can then be added. These are the warm-up enrichment (choke) or the deviation from standard atmospheric pressure.

Now that the requested fuel quantity is computed, it is important to verify if the AFR is correct. That is why an Exhaust Gas Oxygen (EGO) sensor is used to do a closed loop regulation. This aspect will be discussed later.

Design Criteria

One of the most important criterion to design an embedded control system for application in automotive technology is the robustness.

The hardware has to be robust against the environment in which it will be used. In automotive applications, the temperature can go from -40°C to more than 100°C in some places of the engine compartment. Moreover, it has to resist to shocks and vibrations due to road and the dynamic of the car. This robustness also involves good quality. Most of the time, is an EFI system designed to work during the whole life of the car.

Another important aspect is the speed of the hardware. A SI engine rotate to minimum 6000 RPM. Which means 200 Hz injection sequence for a L4 engine. It is naturally quite slow comparing to the speed of an actual computer's CPU but all the operations to compute the requested fuel quantity have to be done 200 times per second which of course ask a greater clock speed.

The system also has to be robust against the failure. It must protect the engine against bad working conditions like too high speed or temperature. If something is going wrong, the system must shut down the engine and no let him accelerate for example.

The communication is also important. Most of the new cars use for example CAN-bus protocol for communication between different component. The vehicles are also equipped with On Board Diagnose system (OBD 2) which must be able to communicate with the Electronic Control Unit (ECU).

And last but not least the price must also be taken into account. A system must be as inexpensive as possible to have a chance to be used in production cars.

Hardware

The hardware can be divided in three components. The sensors, the ECU, and the actuators.

Sensors

The sensor used in this application provide all the necessary parameters to have the best view on the engine's situation and compute the requested fuel quantity. These are :

MAP sensor : Piezoresistive sensor Throttle Position Sensor (TPS) : rotational potentiometer Tachometer : this signal can be taken from the ignition coil or with a crank and a camshaft position sensor (variation in function of the car) Coolant and Air temperature sensor : NTC thermistor EGO sensor : wide band warm oxygen sensor (aka Lambda sensor)

Most of the sensor's output are varying between 0 and 5 V DC and give an analogue signal.

ECU

The most important part of the ECU is the processor. The choice of it must satisfy all the criteria mentioned above. For the example of the Megasquirt, the processor is a Motorola MC9S12C64 which is adequate for this kind of application. This processor was originally used in ABS ECU and works from -40°C to 125°C. This answers to the criterion of robustness against hostile environment. It has a 16 bits core which means accuracy in the computed values and 2K RAM which can handle more parameters (like ignition timing, Idle control, ..) 24 MHz speed which allows fuel control to 1 µs And a flash memory of 64K to keep the embedded code.

The Printed Circuit Board (PCB) have minimum 6 analogue port in for the sensors signal and 4 out for the injectors. It also has some spare ports for additional applications like ignition timing, ...

As the price must be competitive, the whole PCB cost about $85.

For OEM, the ECU does not have to be re burned or modified. So it is most of the time protected by silicon against vibrations.

Actuators

Actuators are here the injectors. They work with a piezoelectric actuator or solenoid which opens the injector under 12V voltage.

These are rated by the fuel flow they can deliver. This parameter is given in cc/Min @ full opening. The choice of injector is specific to each engine and it's application.

Once the requested fuel quantity is computed by the ECU, it can be easily converted in injector's opening time. This opening time theoretical equal to time that the injector must be feed with 12V or the pulse width. Deviations from the theory occurs due to the finite time needed to fully open the injector and the battery voltage variation. Those two facts must also be taken into account.

Injectors can be have low or high impedance. So in order to work with low impedance injectors, the current must limited. This can done by adding resistor or reducing the current electronically with a Pulse Width Modulation (PWM). If the injector is very quickly set alternatively to 12V and ground the total amount of current in the injector will decrease.

The PWM rate can thus regulate the current trough the injector.

Software

As said before OEM keep the software of EFI very secret. That is why only a few general characteristics will be given.

Because of the repetitive aspect of the system and the simplicity of the hardware, no Operating System (OS) is needed. For OEM, the software is burned on memory once for all.

The working principle is as follows :

The requested fuel quantity is computed with the VE table and the perfect gas law. Corrections are made in function of the temperature,atmospheric pressure,EGR,... The final fuel quantity is translated into a PW. The PW is translated to electric signal which is send to the injectors.

This part of the control system is a full feed forward regulation.

A closed loop regulation with EGO correction is added to the process. This regulation works only in normal working conditions (not idling or WOT). The ECU checks the value of the lambda sensor and adjust the fuel quantity. The burning process has a certain time constant. So the lambda value is not checked every time but for example every 7 ignition events. If the value returned by the EGO indicates that the mixture is rich, the fuel quantity will be lowered of one fixed incremental value and vice & versa.

The megasquirt system provide all the features above in the software but everything can be modified to suit a given application.

C programming language is used for the code and then complied to be burned on the ECU The software can be tuned by a user's interface which modify the C code before it be burned A serial port is used to read and write all the computed and measured values can be downloaded in a log file The code is open source

Conclusions

The EFI system provide a good example of how electronic can supply mechanic and how a simple embedded control system can be developed.

With the time the systems become more complex. They handle detonation control, idling,.... Like for the diesel engines the injection become also more and more accurate with a better control of the combustion.

EFI was originally the only electronic device in a car. But now, it is only a component of the system. Electronic found also place in steering, gearbox management, stability control, ... All this improvements mean a better system but can also give more failure due to the complexity.