Now we understand how engine sensors work we can move to the components used in an electronically controlled engine, and assess what does what, why it does it, and how it does it.
Before we move on we have to understand a few basics, and will begin with analogue and digital systems. Modern engines work with digital systems as all microprocessers do, but many of the signals are analogue signals, analogue signals can be described as any signal which can change by an infinately small amount, and can be measured by the older type multimeters with a swinging needle working across a fixed scale. Digital signal is one which works with a series of on/off switching at a fixed or variable frequency, the frequency being the number of on/off pulses which gives its frequency, or the combination of the frequency and duration or width of the pulses.
From this we can conclude that a resistor or variable resistor sensor is an analogue signal, and a pulse generator or peizo sensor is a digital signal sender. As modern electronics use digital signals we use an ADC or analogue Digital Convertor to convert these analogue sender signals to a digital signal.
The engines brain is called many things, this is usually the brain, black box, electronic control unit (ECU), or the electronic control module (ECM).
The ECU works by storing a three dimensional map in its memory, from this is can receive signals from all the sensors and process them and compare them to this stored map, it can alter various functions within an engine such as engine timing, or fuel injection period, and even do some snazzy tricks such as pulsing the fuel injectors to give small bursts of fuel.
If we look at this 3D map it would look like a terrain map on an OS map with all its other features removed, it is this map which is infinate and allows all the functions to work together, and each position represents the actual positions possible by the massive combinations of positions sent by all the sensors. This allows the ECU to precisely determine the fuel infection rate, engine timing, and duration of fuel injection period, and even compensate by adding more or less if the engine is running hotter or cooler or the driver changes throttle position. This map is called the running map.
More modern variants of the ECU have another map stored in them, this is a totally different 3D map which is the engine idling map, this works by optimising the engines running purely at tickover, and nothing else, this optimises the fuel efficiency and emissions during this state. In more modern variants the idling map allows a multi cylinder engine to close down one or more of its cylinders during idling, and rotate which ones are shut down for even wear. This map is the idling map.
Other ECU's have more than one running map, these work by allowing more performance than the normal running ECU, so why are these functions not incorporated into the original ECU? in simple terms this engine map works in conjucntion with other ECU controlled functions, these may be a suspension ECU, or a braking ECU. If their functions are not activated it will not allow the this engine map to be used. This map is usually called an optional, or enhanced map and focuses little on emissions of economy, purely on performance.
Other ECU's have another map, this is called a warm up map and only works from a cold start until the engine is warm, these are being omitted from the latest ECU's as the idling map covers this condition.
ECU's have other functions, during the initial engine start up it will perform a diagnostic check on all the systems and if a fault is found it will log it for retrieval with a fault code reader.
Adaptive driving systems are where an ECU will monitor and record the way a driver drives a vehicle, this allows the system to respond quicker to a specific drivers input within its defined parameters.
Airflow meters do exactly what they say, measure the flow or volume of air into an engine, a moving flap inside the unit responds and is moved by the volume and speed of the flowing air, and its shaft is connected to a variable resistor, these early units did little more than measure the volume and speed of incoming air.
Later variants were exactly the same but utilised a thermister in the inlet chamber, this measured the incoming air temperature of our early flap type airflow meter and allowed compensation to be made to the running from this incoming air temperature, it was better then the original systems, but not much
Later type airflow meters did away with the moving flap and variable resistor and moved to a hot wire system, basically it has a wire in the inlet which is heated to a pre-determined temperature and it stays constantly at this temperature, its basically the same as an incandescent light bulb but the filament runs much cooler and does not glow. As air passes this hot wire it cools it, the system heats it back to its operating temperature very quickly and the amount of power required to maintain its temperature is measured and converted into a signal. This system is much more accurate as it measures not only airflow volume and speed, but temperature and air density also, and this allows much finer tuning of the fuel system in response to these additional factors. This much finer and precise measurement allows emissions and fuel economy to be much more lower
Sat Sep 24, 2011 2:38 am
Yeah I did. Im not that dense... 