An overview of the Engine Control Unit (ECU) and its sensors for the Mitsubishi 4G93 Engine
An overview of the Engine Control Unit (ECU) and its sensors for the Mitsubishi 4G93 Engine
D.A. Sen, A.A. Zainul Abidin
Dept. of Electrical Engineering, Universiti Tenaga Nasional, Malaysia
Keywords: Engine Control Module; ECM, Engine Control Unit; Mitsubishi 4G93 Engine; Fuel Injector Duty Cycle.
Abstract:
Today’s automobile manufacturers strive to design automobiles that provide the best performance balanced against good efficiency. Efficiency has become a central issue in the design of new engines because of the need to meet tighter environmental regulations and the demand for fuel frugal automobiles by consumers. At the heart of each automobile is the engine, which serves as the automobile’s power plant. Modern engines use software loaded in the engine’s Engine Control Module (ECM) to optimize performance and efficiency of the engine. The ECM collects all sensor data, interprets and processes this data, and then sends out control signals necessary for the smooth and efficient operation of the engine. This paper will investigate how the ECM in the Mitsubishi 4G93 engine receives information from the various sensors. An overview of the sensors’ operations is presented. Finally, this paper looks at how this information is used to control the firing of the spark plugs and the amount of fuel injected by varying the duty cycle of the fuel injectors.
1.0 Introduction
The automobile powered by the Otto petrol engine was invented in Germany by Karl Benz in 1885. Benz was granted a patent dated 29 January 1886 in Mannheim for that automobile [1]. By 1913, more than a million cars and trucks were racing across America and Europe, and most of them ran on petrol or diesel [2]. The key breakthrough that led to the dominance of the internal combustion engine (ICE) was that there was compression prior to combustion. This not only increased the efficiency, but also yielded a much higher power-to-weight ratio than earlier compression-less engines such as the steam engine.
Today, the ICE is by far the most common power source for the transport sector and will remain so; at least for the foreseeable future. While alternative and renewable energy technologies are available today, their manufacturing costs, deployment issues, and required infrastructure remain stumbling blocks to competing with the ICE. Of the 750 million cars, trucks, and other vehicles now roaming the planet (and the number grows by 50 million a year) some 90 percent use oil – not because of some vast oil company conspiracy, but because, by conventional measures, oil-fueled ICEs generate more power, more efficiency, more value for the energy dollar, than any other fuel-technology pair [3]. Nearly a century of continual refinement has created a staggeringly efficient machine [4].
2.0 Background of the Internal Combustion Engine (ICE)
Almost all ICEs currently use what is called a four-stroke combustion cycle to convert petrol into motion. The four-stroke approach is also known as the Otto cycle, in honor of Nikolaus Otto, who invented it in 1867 [5
• Intake stroke ]. They are:
• Compression stroke
• Combustion stroke
• Exhaust stroke
Figure 1: An illustration of several key components in a typical four-stroke engine [6]
Figure 1 illustrates the positions and dynamics of the crankshaft, piston, intake valve, exhaust valve, and a DOHC valvetrain configuration. The arrows indicate the
direction of movement of these reciprocating components during the compression stroke.
The basic operation of a four-stroke cycle has remained essentially the same for almost 150 years. Advances in manufacturing technology, metallurgy, microprocessors, and an overall better understanding of the science behind ICE operation, has led to the development of more complex designs that continue to refine the ICE.
Modern ICEs have moved away from the cam-in-block (or OHV) systems in favor of overhead camshaft (OHC) valvetrain configurations. The carburetor, which is a device which mixes air and fuel [7] has been replaced with fuel injectors and in some cases a servo controlled butterfly valve for more precise (and flexible) control. Variable Valve Timing has been introduced by most major automobile manufacturers to improve low-end torque and high-end power. The Engine Control Module (ECM) has evolved from simple analogue circuits, to hybrid digital design with a look-up table, and finally to microprocessor based systems which can process the inputs from the engine sensors in real time [8].
3.0 Overview of the Mitsubishi 4G93 engine
The Mitsubishi 4G93 engine is a 1.8 liter double overhead camshaft (DOHC) engine used as a power plant for cars like the Mitsubishi Lancer. A double overhead camshaft (also called double overhead cam, dual overhead cam or twincam) valvetrain layout is characterized by two camshafts being located within the cylinder head, where there are separate camshafts for inlet and exhaust valves [9].
Figure 2: A cylinder head sliced in half shows two overhead camshafts—one above each of the two valves [10].
It is a 4 cylinder inline engine that is available with either a turbocharger or as a normally aspirated engine. In order to control the amount of air and fuel mixture the 4G93 uses an Engine Control Module (ECM). The engine uses multipoint injection in delivering fuel to all the cylinders. This would mean that each cylinder will have one designated injector. The engine also uses an electronic distributor-less ignition system to provide firing in the combustion chambers. The sensors connected to the ECM are:
• Hall effect crankshaft sensor
• Hall effect cam sensor
• throttle position sensor
• intake-manifold vacuum or manifold absolute pressure(MAP) sensor
• engine coolant temperature sensor
• air intake temperature sensor
• air flow sensor
• oxygen sensor
In order to realize the decisions of the ECM into mechanical or combustion form, the ECM will send its signals to two actuators which are:
• The fuel injectors
• The Spark plugs
In order for the engine to have complete combustion, the air-fuel mixture must be correct. The stoichiometric ratio for air-fuel in a petrol engine is 14.7:1. This means that, volumetrically, 14.7 times more air than fuel is needed for complete combustion. The complete combustion of air and yields carbon dioxide (CO2) and water (H2O). If there is not enough air then the combustion process will be incomplete and produce carbon monoxide, nitrous oxides and unburnt hydrocarbons as unwanted byproducts. These emissions will cause air pollution in the form of smog and acid rain.
To combat the problem of automobile emissions, many governments worldwide have made it mandatory for all new vehicles to be fitted with catalytic converters. Catalytic converters convert carbon monoxide (CO), nitrous oxides (NOx), and unburnt hydrocarbons into CO2 and H2O. However, as illustrated in Figure 3, the optimum air-fuel ratio for a 3-way catalytic converter is 14.7:1.
When catalytic converters were made mandatory, tetraethyl lead (TEL) which was used as an octane booster in leaded petrol to prevent pinging or detonation, had to be removed. This is because lead poisons the catalytic converter rendering it non-functional. This resulted in lower compression ratios, and a loss of power. However, the Mitsubishi 4G93 engine, as like many other modern engines, has been able to improve its efficiency and power to levels well above the older engines that used leaded petrol. This is thanks in part to the advancement in ECM algorithms, fuel injection technology and ignition timing.
As an open loop system, the ECM will have to calculate the amount of air entering the intake manifold before it can calculate the amount of fuel to deliver to each cylinder. To do this, the air flow, air temperature and air pressure is measured. For example, if the temperature is low, the air density will be high; therefore, more air will enter the intake manifold. However, if the pressure is high, the air
density will also be high, thus more air will enter the intake manifold.
Figure 3: The air-fuel mixture ratio “window”, within which the air-fuel ratio must remain if the three way catalytic converter is to work efficiently. (General Motors Corporation) [11]
To collect all this data the Mitsubishi 4G93 engine has many sensors. These sensors collect data and feed the information to the ECM. The ECM computes in real-time the correct amount of fuel to be injected into the cylinders and the firing times of the spark plugs.
4.0 Mitsubishi Engine Sensors
4.1 The Air Flow Sensor
In the case of the Mitsubishi 4G93 engine, the air flow is detected using a hot wire induction type sensor. The platinum wire is kept hot by current flowing through it. As air enters the manifold, this wire is cooled. The system keeps the wire hot by increasing (or decreasing) the amount of current flowing through the wire. As more air flows through the manifold, more current needs to flow through the wire to keep the temperature constant. Thus air flow is measured by sensing the amount of current flowing through the wire.
There are other ways of measuring the amount of air entering the intake manifold. However, the measuring system used will depend on the car manufacturer. Usually Mitsubishi uses the hot wire induction method. Other methods include the vane system, the air flow sensor plate, and the heated film method.
4.2 Air Intake Temperature Sensor
This temperature sensor is used to measure the temperature of the air entering the intake manifold. Generally, this is accomplished by means of a resistance temperature detector or RTD. If the temperature of the sensor falls then the resistance of the sensor will fall, conversely, if the temperature increases then the resistance will also rise.
Using a voltage divider circuit, a constant voltage will be injected into the circuit. The voltage at the RTD, which is connected in series with a constant resistor, is then measured. When the temperature is high, the resistance will be high; therefore the voltage across the RTD will also be high. This will register as a high temperature in the ECM.
4.3 Manifold Absolute Pressure Gauge
The pressure in the manifold is measured using a manifold absolute pressure gauge (MAP gauge). The MAP gauge will compare the pressure in the intake manifold to the pressure of a specific vacuum. This provides a more accurate measurement than the vacuum gauge because the vacuum in a MAP gauge is fixed, whereas a vacuum gauge compares intake manifold pressure to atmospheric pressure, which varies.
The vacuum in a MAP gauge is separated from the intake manifold pressure by a flexible diaphragm. The diaphragm is connected to a strain gauge which will convert the pressure to a voltage signal, which is then transmitted to the ECM as a varying voltage signal.
4.4 Coolant Temperature Sensor
The coolant temperature sensor functions like the air intake temperature sensor. The reading from this sensor will be used by the ECM to increase the amount of supplied fuel when the coolant temperature is low – effectively taking the place of the traditional choke. As the engine heats up, the ECM automatically reduces the amount of fuel injected by the fuel injectors.
This sensor is also used by the ECM to switch on an electric fan that is used to cool the radiator when the coolant temperature is too high. The radiator fan’s cut-in and cut-out points are predefined in the ECM to maintain optimal engine temperature.
4.5 Throttle Position Sensor
The throttle position sensor is used to control the idle speed of the engine. It is a rotary type sensor that uses a wiper blade and resistance coil to form a simple voltage divider. This configuration provides a voltage signal to the ECM.
Air-fuel Mixture Ratio
Conversion Efficiency %
100
80
60
40
20
0
13:1
14:1
14.7:1
15:1
16:1
NOx
CO
HC
Best operating area for 3-way catalyst
Rich Lean
If the throttle is wide open the sensor will send a 5V signal while if the valve is completely shut the sensor will send a 0V signal. The idle speed is controlled by a screw located on the throttle body. The screw controls the set point of the throttle position sensor voltage.
4.6 The Oxygen Sensor
The oxygen sensor will be used in order to measure the amount of oxygen that is left after the combustion process. The oxygen sensor is about the size of a spark plug and will produce a small voltage when it is exposed to oxygen. The voltage that the oxygen sensor produces is between 0.15V to 1.3V. This voltage is sent to the ECM to determine weather the amount of fuel is to be increased or decreased. The air-fuel ratio is correct when the oxygen sensor reads 0.45V.
If the voltage drops below 0.45V, there is too much oxygen and more fuel is needed. However, if the voltage rises above 0.45V, the oxygen content is too low and the amount of fuel will have to be decreased. The oxygen sensor is also used to determine if the engine is working in open loop or closed loop because the oxygen sensor can only operate well at a temperature of between 200°C and 800°C. Below 200°C, the ECM will work in the open loop mode, without feedback on the content of oxygen in the exhaust gases.
4.7 The Hall Effect Crank Sensor
The Hall Effect crank shaft sensor is used in order to calculate the speed of the engine and, at the same time, calculate the position of each cylinder. The transducer will send a voltage signal to the ECM via the ignition coil module depending on the magnetic field detected.
Using a stationary magnet, a transducer to detect magnetic fields, and three vanes that are connected to the crankshaft, the Hall Effect crank sensor can determine the speed of the engine and the piston position. This is done by having the vanes move in between the stationary magnet and the transducer. The vanes will cause the magnetic field (produced by stationary magnet) that is sensed by the transducer to have an interruption thus causing a change in the voltage sent to the ECM. When there is an interruption, that is called an off signal and when there is no interruption that is called an on signal. How frequent the on and off signals happen will be used by the ECM to calculate the engine speed. Another signal from the camshaft sensor will be used with the crank sensor in order for the ECM to calculate the piston position.
4.8 The Hall Effect Camshaft Sensor
Like the crank shaft sensor the camshaft sensor is also Hall Effect in nature. However rather than having vanes on the camshaft, a permanent magnet is mounted on the cam shaft gear. The camshaft is used to open and close valves in the engine. The firing must occur a few degrees before Top Dead Center (TDC) just after the compression stroke is over. This sensor will be the indicator for the ECM to know if the piston has reached that point or not.
The ECM will use the information gained from the sensors above in order to activate the actuators to generate power for the engine. The two type of actuators that are connected to the ECM are the fuel injectors and the spark plugs.
5.0 Mitsubishi Engine Actuators
5.1 The Spark Plug
The 4G93 is a four cylinder engine. Each cylinder has one spark plug to ignite the air-fuel mixture. The ECM will first obtain the piston position from the camshaft and the crankshaft position sensors. These readings will be sent to the ECM through the ignition module. A decision of which cylinder has reached TDC will be made by the ECM using the readings obtained from the ignition module. The cylinder that has reached TDC will then be given a signal through an ignition circuit. In our case the 4G93 has 2 ignition coils. The ignition coil has 2 windings: a primary and a secondary winding. The signal from the ignition module will go into the primary winding and induce a voltage in the secondary winding, which steps the voltage up to 25kV. This will cause the spark plug connected to the secondary winding to ignite.
5.2 The Fuel Injectors
The 4G93 uses solenoid operated fuel injectors. When the ignition key is turned on, there will be a voltage present at the solenoid. The fuel pump in the gas tank will also start pressurizing the fuel lines at this time. A mechanical fuel regulator will allow some of the fuel to enter the fuel rail and some to return back to the fuel tank. During operation, the ECM will provide a ground signal when it wants the solenoid to open. When the solenoid is open, the pressurized fuel from the fuel rails will spray out the fuel injector nozzle and enter the combustion chamber.
The driver of the car will press the accelerator that will open a butterfly valve which allows air to enter the intake manifold via the air cleaner or air filter. This air will then enter the intake manifold. The ECM will use the readings from the following sensors to determine the duty cycle of the fuel injectors:
• The air flow sensor
• Air intake temperature sensor
• Manifold absolute pressure gauge
• Coolant temperature sensor
• Throttle position sensor
• The oxygen sensor
Figure 4 illustrates the various different fuel injector firing profiles. These profiles are built into the ECM to allow the
ECM to cater for the different driving condition. Transition between any two profiles is fully transparent to the driver and occurs when the ECM determines that a specific profile is invalid. A new profile is then selected based on the input from the sensors at the time. All this is calculated by the ECM in real time.
Figure 4: The wider the pulse width, the longer the injector is open and the greater the amount of fuel that sprays out (Ford Motor Company) [12].
6.0 Conclusion
The Mitsubishi 4G93 engine is indeed a complex and well tuned power plant that is able to deliver the best performance in an economical manner. It is able to use both open loop and closed loop control depending on driving conditions. The development and integration of the ECM into the automobile has greatly enhanced the reliability, performance, and efficiency of the ICE. Together with better aerodynamics and low rolling-resistance tyre technology, the ECM has contributed to raising the average mileage of vehicles from 15mpg in 1975 to 21mpg in 2004 [13].
References
[1] http://en.wikipedia.org/wiki/Automobile, 19 June 2006.
[2] S.H. Schurr, Energy in the American Economy, The Johns Hopkins Press. Baltimore, MD., 1960, pg 116.
[3] P. Roberts, The End of Oil, Bloomsbury Publishing, London, 2005, pg 68.
[4] P. Roberts, The End of Oil, Bloomsbury Publishing, London, 2005, pg 89.
[5] http://en.wikipedia.org/wiki/Internal_combustion _engine, 19 June 2006.
[6] http://en.wikipedia.org/wiki/Image:Four_stroke_ cycle_compression.jpg, 20 June 2006.
[7] http://en.wikipedia.org/wiki/Carburetors, 20 June 2006.
[8] http://en.wikipedia.org/wiki/Engine_Control _Unit, 14 June 2006.
[9] http://en.wikipedia.org/wiki/Dohc, 15 June 2006
[10] http://en.wikipedia.org/wiki/Image:DOHC-Zylinderkopf-Schnitt.jpg, 15 June 2006.
[11] W.H. Crouse & D.L. Anglin, Automotive Mechanics 10th Edition, McGraw-Hill, 1993, pg 226.
[12] W.H. Crouse & D.L. Anglin, Automotive Mechanics 10th Edition, McGraw-Hill, 1993, pg 227.
[13] C. Lave, A New CAFÉ, University of California – Berkeley, 2001, pg 2.
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