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Electronic Engine Management Pt.2: Airflow Measurement – by Julian Edgar One of the fundamental parameters which an electronic engine management system must sense is the mass of air passing into the engine. If the Electronic Control Module (ECM) cannot measure airflow, then the amount of fuel that must be added cannot be determined. Use of engine revs (rpm) is insufficient, because the engine may be on the over-run – for example, when driving down a hill with a closed throttle. Even using the throttle position switch (which senses throttle plate opening) in conjunction with rpm is not sufficient to provide accurate airflow data, because actual engine load will not be indicated. Instead, airflow monitoring is carried out by a specific device designed to measure either air mass flow or air volume flow in conjunction with air The vane-type airflow meter is common in early engine management systems & is still currently fitted to some engines. The damping chamber is the curved extension in the foreground. 4  Silicon Chip temperature. Other systems look at the manifold vacuum (or boost) and calculate the airflow indirectly from this variable. Vane airflow meters The vane airflow meter is one of the oldest airflow sensors employed in engine management systems. Developed by Bosch (as almost all engine management technology has been), the vane airflow meter is common on engines made from about the mid 1970s to the present. The vane airflow sensor (Fig.1) consists of a pivoting flap, which obstructs the engine’s combustion airflow when the engine is not running. Once the engine starts, a low air pressure is experienced on the upstream side of the vane, causing the flap to open a small distance. As the throttle is opened further, the flap is deflected to greater and greater openings. To prevent the flap from overshooting its ‘true’ position, another flap is connected at right-angles to it. This secondary vane works against a closed chamber of air, thus damping the motion of the primary sensing flap. Mechanically connected to the pivoting assembly is a poten­tiometer, usually comprising a series of carbon resistor seg­ments. As the vane opens in response to airflow, the wiper arm of the potentiometer moves across the AIR STACK STEADIES SENSOR PLATE DAMPER CHAMBER COMPENSATION PLATE AIR FLOW Fig.1: a vane type airflow meter. A potentiometer connected to the pivoting vane assembly is used to vary the output voltage from the meter in response to air flow. segments, changing the resist­ance. A regulated voltage is fed to the airflow meter and so, as the vane moves in response to airflow variations, the output voltage from the meter also changes. A spiral spring with an adjustable preload is used to relate the angle of the flap to the airflow and to ensure that the flap closes when no airflow is present. A by­pass is also constructed around the measuring flap. Air movement through this bypass passage is controlled by an adjustable screw, giving control over idle mixture. A vane-type airflow meter measures just the volume of air passing through it, rather than the air’s mass. It’s the mass of the air which is important in determining the appropriate amount of fuel to be added, however. Because the temperature of the combustion air affects its density, temperature sensing is there­fore also built into the airflow meter. Temperature sensing of the airflow is carried out using a thermistor which is located within the main body of the airflow meter. Typical resistance values for this sensor are 2-3kΩ at 20°C, falling to 0.1-0.4kΩ at 60°C. In practice, vane-type airflow meters will operate well for long periods of time. The exception to this is if they experi­ence an engine backfire. This shouldn’t happen in a properly tuned engine-managed car but is a possible scenario when carrying out EFI modifications or running on LPG. A backfire will often slam the vane shut with such force that it distorts the aluminium casting, subsequent- This view shows what’s inside the base of a vane-type airflow meter. The carbon resistor segments are clearly visible (the black rectangles), while below it the spiral spring can be seen inside the tension pre-load wheel. ly causing binding when the flap is deflected by the airflow. When operating properly, the flap should move through its full travel with only light finger pressure. Hot-wire airflow meters The major disadvantages of the vane-type airflow meter are that it senses air volume instead of mass and it restricts the airflow, both because of the need to displace the moving flap and because the cross-sectional area of the flow-path is generally small to increase flow velocity. The next Bosch invention – the hotwire airflow meter – overcomes these disadvantages. Used in engines built BYPASS AIR METERED AIR SAMPLE SEAL AIR FILTER ELEMENT SEAL AIR FILTER CASE AIR INLET AIR-FLOW SENSOR (ULTRASONIC) Fig.2: basic construction of an ultrasonic airflow sensor (Mitsubishi). November 1993  5 The temperature sensor is at the front of the vane airflow meter. The rectangular flap behind it is the vane, shown here in the rest position. from about 1985 to present, it’s the most common type of airflow sensor currently used. The hot-wire (or mass sensing) airflow meter uses a Wheat­stone bridge circuit – see Fig.4. A very thin (0.07 mm) platinum wire is formed into a triangular shape and is suspended within the combustion airflow. The platinum wire forms one arm of the bridge and is maintained at a constant temperature. As the mass of air passing the wire increases, the wire is cooled and its resistance drops. The heating current now imme­ diately increases in response to the bridge becoming unbalanced and returns the wire to its original temperature, thus restoring the balance. The greater the heating current required, the great­er the voltage drop across a resistor which is in series with the platinum wire. The voltage drop across this resistor is therefore related to the rate of airflow into the engine. Very quick response – in the region of milliseconds – is gained using this system. Because resonant pulsing is a potential problem in the airflow meas- urement of reciprocating engines, this very fast reaction time is important. A platinum-film resistor is used for temperature compensation, with quick reaction from this device also needed for accuracy. To make sure that the platinum wire remains clean, it is heated to red-hot for one second each time the engine is switched off. This action burns off any dirt or other contamination which may have settled on the wire. A potentiometer is placed within the bridge circuit to allow idle mixture variations to be set. In some applications, the platinum wire is replaced with a hot-film resistor. Hot wire airflow meters should last for ever under normal operating conditions. Physical interaction with the platinum wire will cause damage and so screens are placed at each end of the meter by the manufacturer. A massive backfire will also destroy the meter. I’ve seen one totally wrecked with a huge nitrous-oxide and turbo induced explosion! Karman Vortex meters Used solely in Mitsubishi vehicles, the Karman Vortex air­ flow meter (Fig.2) is also one of the few engine management devices not invented by Bosch! In this type of airflow meter, vortices are generated in the air as it flows past vortex generators. The frequency of these vortices is related to the volume of air passing through the meter. Ultrasonic waves are used to measure the frequency of the generated vortices. These are propagated at right angles to the airflow and are detected by an ultrasonic receiver located on the other side of the tube. Various receivers, amplifiers and pulse shapers are then used to give an output signal which is interpreted by the ECM. For performance applications (on turbo Mitsubishis, for example) the meter can be replaced by a rewritten software pro­ gram within the ECM. This can be done because the airflow meter is utilised by the ECM only at low throttle angles. MAP sensor Manifold Absolute Pressure (MAP) sensing is used in place of an airflow meter is some systems & has the advantage of not causing any restriction to intake airflow. This photo shows a Holden MAP sensor. 6  Silicon Chip A manifold absolute pressure (MAP) sensor can also be used to derive airflow. When the throttle valve is near shut with the engine running, a high negative pressure is present in the manifold (or plenum chamber as it The MAP sensor & its associated assembly is usually mounted on the firewall. The tube connected to the sensor goes to the plenum chamber to sense manifold pressure, while the small cham­ber is for damping pressure pulses. more usually is in an EFI car). As the throttle opening increases, the pressure approaches atmospheric and, in a turbo car, the manifold pressure can then go on to become positive. Thus, the manifold pressure will have a direct relationship with the combustion airflow. MAP sensors work in one of two different ways: (1) either as a variable capacitor with the plates being moved closer to­gether under greater air pressures, or (2) as a strain gauge which forms part of a Wheatstone bridge. While MAP-sensing tends to be used more on simple engine management systems (like single point injection systems), all of the programmable aftermarket injection systems (Autronic, Motec, etc) also use this approach to airflow sensing. Top racing cars – like the current Group A Touring Cars – are therefore using MAP sensing in conjunction with throttle opening and rpm to sense load. One convincing argument for MAP sensing is that, when the throttle is quickly opened, the ECM can start supplying more fuel and/or different ignition advance before the engine rpm (and therefore airflow) starts to rise. In other words, ECM reaction to quick changes can be faster. Because the MAP sensor derives its pressure sensing from a small-bore tube connecting it to the plenum VORTEX STABILISER PLATE FILTER TRANSMITTER VORTICES VORTEX POLE AIR RECEIVER MODULATOR TO ECU Fig.3: an ultrasonic airflow meter works by measuring the frequency of the vortices generated as the air flows past a vortex pole. Fig.4: external view of a hot wire airflow meter. chamber, sensing airflow in this indirect manner causes no restriction on intake airflow. A mixture of hardware and software is now available which allows the replacement of restrictive vane airflow meters with a MAP sensor. This is especially useful in high performance, naturally aspirated engines. That’s all for this month. In Pt.3 of this series, we will take a look at how an engine management system can be modified by changing the software in SC the main memory chip. November 1993  7