EMS

Converting to an Engine Management System

Internal combustion engines draw in air and fuel, pressurise the mixture, bum the mixture then dump the exhaust - suck, squeeze, bang, blow. Computerised Engine Management Systems control each of these functions, and while your foot may still control the amount of air you want to let in to your engine, it’s the computer that decides the rest. It also controls fuel quantity and spark timing. Now, before we go any further, there is one very important factor you must remember - the computer is a solid state device (no moving parts) that is far more reliable than a mechanical component. It also operates at a speed that is incomprehensible to the human brain. The computer receives data about the engine and what it’s doing and transmits responses to that data millions of times per second. It knows what the engine is doing at every degree of crank rotation.

To supply this data to the computer, sensors and output devices are integrated with the engine. Some of them are familiar to all hot rodders - temperature sensor, oil pressure sensor, manifold vacuum sensor to name a few. Some of you may remember them as "sender units" for gauges. Same thing. However, instead of just sending the information to a gauge in the dashboard, the reading is sent to the computer. Now the computer knows what the water temperature is, what the oil pressure is and what the manifold vacuum is at any point in the crankshaft’s cycle! Of course, it doesn’t stop there. There are many more sensors and actuators that make EMS the most efficient way to control and monitor an engine’s output.

Sensors are either passive or active. A passive sensor is one that produces its own signal - it is not powered from the battery or the computer, for example, temperature sensors and oil pressure sensors are passive sensors. So are knock sensors and wheel speed sensors because they produce their own voltage. They usually have only one wire coming from them, which is the signal wire to the computer. Active sensors require an external source of power or voltage from the computer before they can output a signal, for example, the oxygen sensor, the throttle position sensor and the mass air flow sensor receive a reference voltage from the computer. The signal they give back to the computer differs from the reference signal by an amount determined by the quantity of oxygen in the exhaust, the position of the throttle or the volume of air going into the engine. The connection they make with the computer is usually by three wires:

The main sensors in any engine managed system are:

The computer uses the signal from the Throttle Position Sensor as a direct reading for engine load. The TPS is needed for calculating ignition timing, fuel injection and automatic transmission functions. The TPS is a variable resistor - its resistance varies with the position of the throttle. Typically, the computer supplies a 5V DC reference, and with the throttle closed, the resistance of the TPS reduces the output signal to about 0.5 volts. As the throttle opens, the voltage increases, in this example, to a maximum of 4.5 volts. If the sensor returns the full reference voltage or no voltage at all, the computer assumes a malfunction and will ignore the signal, set a code and turn on the Check Engine Soon light. More common is a partial failure, where the signal drops out through a small range of the sensor's motion. This malfunction causes drivability problems with symptoms that don't always make you think of TPS failure, like, harsh shifting or ignition timing problems. Total TPS failure may not stop the engine, but the computer will turn on the Check Engine Soon light and probably switch into limp-back (open loop) mode.

These are pressure sensors. Pressure Sensors work on the Piezo Electric principle, whereby a force (pressure) acting upon a Piezo Electric substance (eg, quartz crystal or synthetic material) changes the electrical characteristics of the circuit that the device is a part of. Some devices generate a voltage, others vary their resistance as the pressure changes.

The Barometric (BARO) sensor is often built into the computer itself and measures the static pressure of the air around it. This is a measurement of air density. The Manifold Absolute Pressure (MAP) sensor measures atmospheric pressure in the intake manifold. Any time the engine is running, manifold pressure is less than atmospheric and increases as the throttle opens.

The data from these two sensors plus engine speed (rpm) is used to calculate the amount of air entering the engine - thus it is known as a speed/density system.

Measuring the actual engine air flow is much faster and more efficient than calculating it using the speed/density method, described above.

There are two ways to measure air flow:

Most engine management systems measure Mass Air Flow (MAF). One type of MAF sensor is a Hot Wire MAF sensor. Electrical current through a platinum wire maintains the temperature of the hot wire at approximately 120° C above ambient. The computer monitors the power required to keep the temperature at 120° C - more air entering the engine cools the wire, requiring more power to hold the temperature constant.

To keep the hot wire clean and free from deposits, the wire is momentarily heated when the ignition is switched off to bum off any dirt deposits.

Another type of MAF sensor is the Thick Film type, which uses a temperature sensing resistor rather than a hot wire. It requires no burn-off circuitry, as it is protected from deposits by the resistor’s casing. The whole unit includes much of the control and signal conditioning circuitry, and although it is slower than the hot-wire type, they both react to changing air-mass in milliseconds.

Only a small amount of the total intake air flow moves past the sensing element. The output signal of either type is usually a voltage level (say +5V), which the computer interprets as grams per second (g/s) of air flow.

The Volume Air Flow (VAF) sensor (or Vane Air Flow) measures air volume and temperature, then the computer calculates the mass. The VAF device is similar to a TPS: a vane pivots on a shaft that turns a variable resistor. Air flow through the housing pushes against the vane - the faster the air flow, the further the vane moves. The variable resistor is a part of the air metering circuit, and modifies a reference voltage to the computer. The other components of the air metering circuit is the Intake Air Temperature ( IAT) sensor, which is usually mounted in the VAF housing. Some applications incorporate the BARO sensor, and all are used to fine tune the grams per second of air flow calculation sent to the computer as an analog voltage signal.

The most accurate and sensitive air flow measurement is by a Karman-Vortex air flow sensor, used by Mitsubishi since the early 1990s. As air flows through the sensor housing, an ultra-sonic generator transmits sound waves at right angles to the flow of air. Obstructions built into the housing cause eddy currents in the flow, and an acoustic detector reads the frequency of these currents as they change with increasing air flow. This type of MAF sensor automatically accounts for differences in air temperature and density (barometric pressure), but pulsations in the air flow can sometimes cause problems. The output signal is frequency, which the computer demodulates to measure mass air flow.

These sensors report shaft speed and position to the computer. There are three different types:

The inductive sensor is a permanent magnet with a coil of wire around it, positioned close to a toothed wheel on the shaft. As each tooth passes the magnet, it disturbs the magnetic field, and current flows through the coil. The computer interprets the frequency of the sine wave as rpm. A specific position, such as TDC, can be referenced by making the gap between two of the teeth different from the rest, so that the signal is different at that location. 

The distance between the sensor and the toothed pick-up wheel is critical and often adjustable. 

The inductive sensor is simple and tolerant of harsh environments, but the other two types of sensor are usually inside the distributor or dedicated housing. The Hall effect sensor has a permanent magnet mounted next to a semi-conductor chip that has a current passing through it. A metal wheel with tabs is mounted so that the tabs pass between the magnet and the chip. As the wheel rotates, the interruption of the magnetic field changes the current flow through the chip. The resulting signal can represent either speed or position.

The other active sensor type is a photo diode, using an LED to produce light, another type of diode that detects light, and a wheel with slots or holes passing between them. The openings in the wheel represent degrees of rotation. 

The sensors described here are all an engine needs for basic fuel quantity and spark timing; air mass, crankshaft speed and camshaft position. In this next book, I will describe in full the other devices used to control fuel trim, ignition advance, valve timing, emission control and other things managed by after market computers. This will give you a head start when the actual conversion takes place, for you'll know all you need about the computer or ECM.