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How does an injector work?


The article will concern the work of “brains”, which operate the engine of your car or motorbike. I will try to explain as simply as possible and in general, what and how is going on.

What do these “brains” do and what are they for? Electronics is an alternative to other systems performing the same functions. The carburetor used to deal with fuel metering, the ignition used to be operated by the mechanical or vacuum advance measure of the ignition advance. In general it can be realized not by electronics alone and it used to be just this way for quite a long time. On the cars, motorbikes, petrol-powered saws, petrol generators and other places worked and keep working the same systems that are to substitute the injector.
Why was it needed to change anything? Why should the existed, proved and rather reliable system be repealed? It is quite simple – a pursuit of economy, ecology and power. Work accuracy of the written above systems is not enough for the supply of the desired level of ecology and powerfulness, electronic systems of the engine controls began to appear long ago.

I’ll skip the principle of operation of the conventional engine, many people are familiar with the work of the engine, and those who are not – don’t lose much. In the section between the work of feed and ignition system, the engine is only a transformer of the air-fuel mixture into the mechanic energy. It might be considered as the black box, though with some peculiarities.

So, we have fuel (petrol, ethanol, propane and methane), air and the desire to get mechanic energy out of it. Complications lie only in that, in order to get the targeted characteristics, fuel and air must be mixed in the specified ratio and set on fire at the quite definite time. Moreover, with insufficient accuracy we can receive worse characteristics.

The whole point of “brains’” work comes to the fuel metering and setting the mixture in the engine cylinders on fire. These are the main functions. Apart from them there are also additional – the turbine and transmission control.

A subsystem, which controls fuel metering, is called injector. Lighting is controlled by ignition. Air comes to the engine naturally. The engine itself absorbs air, though its amount may be limited in order to reduce the engine capacity. There is no point in having maximal power all the time, actually most of the time the power is limited. In case of the turbine, the engine gets forced air, but it doesn’t change the point. When we have enough air, we can control its amount with the help of a foot bar.
How much fuel supply does the engine need and how to meter it? There is so called stoichiometric ratio, which shows, that for the one kilogram fuel consumption, certain amount of air is needed. For petrol it is at a ratio of 14,7 to 1. It’s also called AFR (Air Fuel Rate). It’s not an axiom, rather a certain optimum. The mixture can be “poorer”, containing less fuel. Such a mix burns worse, and the engine warms faster, but burns completely. The mixture may be “richer”, when there is more fuel - AFR 14 or less. At such a ratio the mix burns not in full, though the horsepower is maximal. In either way there are limits, if one’s enthusiasm runs away, the engine won’t work. Yu can’t fuel up with 20, and wait for the proportional capacity gain.
Thus to define how much fuel we need to supply to the engine, we should know how much air comes to it. Further is easier – out of the air amount at a ratio, we define the amount of petrol and it's a done deal.

Wait now, how can we define air coming absorbed by the engine? There are more ways to the wood than one. One of the following sensors is used commonly:

Mass-airflow sensor, abbreviated to MAF. This sensor measures the amount of air getting through it. According to Wikipedia “The sensor consists of two platinum hot wires heated by electric current. Air passes through one of them, cooling it, the second is control. By changing the current passing through air cooled platinum wire, the amount of entering the engine air is calculated. ". Such sensors are often installed in the civil vehicles. All in all, it's pretty simple. It seems that this is exactly what you need! That's about right.

The other type of sensors

Absolute pressure sensor, abbreviated to MAP. This sensor is connected to the intake manifold and measures the vacuum (or excessive pressure in the case of forced aspiration) in the collector. Based on the sensor value, temperature sensor and crankshaft speed, it is also possible to calculate the volume of incoming air, just what we need. In order to correct its values, the ambient air pressure must be known. For measuring the atmospheric pressure, either one more sensor is put, which measures it continuously, or the pressure is measured before engine start. In the latter case you can get in trouble if you have stated with a jerk from the seaside straight to Everest. MAP is often installed in sports cars.
One of the sensors is installed, which is required. Well now, we can roughly measure the amount of coming air.

Another required sensor is the crankshaft position sensor CPS. This sensor allows the “brains” to know the position of the crankshaft. Why do we need it? Little to know how much fuel should be supplied to the engine it must be supplied at a certain moment of time. Besides, the mixture should be ignited just-in-time. You can cope without this sensor by no means. There are several types of such sensors, but most of them are either inductive, or Hall sensors, or similar to them. In general, proximity probes are similar to those “hard-working” in your hard disk engine or in the coolers.
The next sensor together with CPS (crankshaft position sensor) gives even more information about the process in the engine, taking place at the given moment. This sensor is also known as a phase pickup. With this sensor it can be understood in which of the cylinders is the admission stroke at this moment, where do we need to supply fuel, in which of the cylinders we have the compression stroke and the time of setting fire to the mixture. On the principle of work it is similar to CPS, but often a little bit easier. In general, it is the same, but on the camshaft.

This set of sensors should be enough for us to start the engine. At the very least, but it's enough to know about how much fuel should be supplied, when to do it and when set the cocktail on fire.
Let us then supply and ignite!

Operating mechanisms

Fuel is metered by spray nozzles or in other words injectors. The spray nozzle itself is just an electromechanical valve. Two wires and pipes with fuel under pressure. Energize the outputs - injector is opened, stop the transmission of power – injector is closed. For simplicity, let us first assume that the injector opens and closes instantly. Then, in order to estimate the amount of fuel passing through it is sufficient to know its static productivity. This is simply the amount of fuel that passes through the injector per minute.
Opened the injector, measured the volume of petrol, which ran out of it in a minute - we got the main parameter. Now we need for accurate dispensing to open and close the injector at a certain time. The metering turns out to be done by the exposure, when it comes to the terms of photographers. The longer the time that we open the injector, the more we fuel up the engine.
Mixture burning is performed with the same old irreplaceable spark plug that was true to the aim’s salt. And the sparking coil is in its place. Except that it is controlled by the “brains”. Ignition has not changed, but it needs CPS, without which it will go wrong.

In general it can be assumed, and is, in general terms the work of the injector. We look at the readings of sensors, measure the right amount of fuel and open the injector for the defined time. And so every clock cycle, that is, depending on the frequency - 100 times per second at a frequency of 6000rev/min of the crankshaft. Often? Not that much

Should we move on?
In real engines, everything is more complicated. Just calculate how much air enters the engine is not so simple. For adjusting the values the water temperature sensor is required – simply temperature detector, similar to the one that shows the temperature on the dashboard and the temperature detector of the incoming air. In general, slightly different from the first one, and, functionally, its “twin brother” - also simply measures the temperature, but not of the engine, but air entering the engine. Why do we need to adjust anything? The fact is that while the engine is cold, until it warms up to a certain temperature – fuel evaporates not very well, it is the steam that burns greatly. Correspondingly, we need to give more fuel to the engine in order to make it work. So we take our value for the optimum ratio, measure the temperature and adjust our value. You also need to adjust the ignition of the mixture in the cylinders - for the same reasons. Here we adjust as well.

Another not very pleasant moment – the injector, which we have accepted as an ideal - in fact, is nothing near. First, take time to open it, and then close. Accordingly, at this time, it also gives fuel, but in less quantity. Here we also make allowance. The opening and closing time itself depends on the airborne voltage. One thing is when the generator is belting out and it’s 14 Volt pressure, and when the generator has died and the battery is discharged to the indecent 10V is really something else. Injector’s opening time is changing and it needs to be corrected. Though the generator is dead, we need to keep driving and the engine should not stop working under such conditions.

The operating mechanisms for idling drag without touching the pedal were not enough for us, - the engine should not stall, and its work should be supported. To do this, there is a special actuator – IACV (idle air control valve). This is a repeater motor (sometimes just an electromagnet), which allows the special engine to "breathe" beside the throttle gate. The clever “brain” does not let the engine wither, and opens the valve when the speed gets down. But it does let the channel off, covering it, when speed increases too much.

I wonder, how hard the driver presses the accelerator pedal. For these purposes, you shouldn’t look at the pedal position, but at the position of the choke, which is controlled by the pedal. The sensor is called - TVPS - throttle valve position sensor. Technically, this is just a potentiometer, which measures the angle of throttle. What's the big idea of knowing how hard the driver presses the pedal, you may ask? It's quite simple; we need to know when to idle. When the driver is starving for adrenaline buzz and vigorously pressing on the pedal – there is no time for saving, fuel up with all your heart!

Environmental standards quite strictly control what our engine “exhale”. So with the best will in the world to fill up by eye – it’s impossible. You need to control the composition of exhaust gases. How can you do it? For this purpose, there is the so-called lambda sensor or oxygen sensor - the sensor that indicates whether the mixture is completely burned down, either there is fuel in the exhaust gases or free oxygen. According to the readouts of this sensor injector can adjust its behavior, either increasing or decreasing the amount of fuel supplied. It’s necessary often enough – petrol is different everywhere, and even just stored in a canister or tank - is aging. And one can compose legends about our service stations. Accordingly, the combustion modes are not invariable. Among other things the capacity of the injectors can "float". After all, you see - the calculation is based on their ongoing performance, and the injector may be clogged with the lapse of time, its performance may be reduced. The rules are strict and petrol is expensive, you still need to drive. The attentive reader has noticed that one of the sensors is enough to provide feedback. We look at the composition of exhaust gases, and if it hasn’t burned completely, we pour less. If burned clean - pour more.
Lambda sensors are of two kinds - narrowband and broadband. They are characterized by accuracy. The first show only whether our families are wealthy, the second show how wealthy they are. They even indicate the very AFR mentioned earlier in this article. Well, and the price, of course. The first cost $ 25, second - $ 200. With lambdas, too, not everything is simple - they are quite capricious, require a certain temperature to work, but it is not always possible, in some types of probes operating element is heated by a vehicle network. Yes, lambda may be not one, but it's subtlety.

One more sensor is used to analyze what is happening in the engine - knock sensor. Detonation is a process of fuel burning, which is carried explosively. In the ordinary course, the fuel simply burns out, by detonation the fuel explodes. It's bad for the engine - the same as hitting the piston with a hammer. Nobody likes to be hit with the hammer - the piston is no exception. This phenomenon is very undesirable, and this sensor is used to determine whether the mixture can detonate. It works on the principle similar to the microphone that "obey" the motor (sensor is attached to the cylinder block) and tries to filter out the noise heard from the engine and distinguish between detonation and normal operation. It’s not so easy here either. To facilitate the work of the sensor, the rough road sensor is used, which will show that our roads produce the noise, not the engine. Demand for this sensor is increasing by turbine the engines.

As a result the “brains” work in the following way:
There is a so-called fuel card - a table, where the composition of the mixture is written. The table has three dimensions - the engine crankshaft speed, engine load and the actual AFR. Just take a value from the table, laid back by an experienced friend.
Adjust this value according to temperature sensors, lambda sensor, knock sensor, throttle position change, and in accordance with all these amendments (some of them also in the plates), we calculate the necessary amount of fuel. Recalculate the amount of fuel during the opening of the injector in accordance with its performance, adjust the time according to the car-system voltage at the time of admission - open the injector for a determined time.

As you can see, there is nothing complicated or abstruse here. There are just the tables, sometimes the PID conversion, and the coefficients of influence of the various factors and in the issue – the opening time of the injector. It’s the same with the ignition; there is a map of angles, similar to the fuel map (also see the table) and also adjustment in accordance with the readings of sensors.

In the normal mode, everything works, but what should we do if one of the sensors is broken? And how should we understand it? If the temperature sensor, for example, shows that the engine is heated to 200 degrees, or that the mixture keeps detonating in spite of all adjustments? This is the reasoning of the “brains”. You should find out if the sensor is lying, do not take into account his readouts, light the «check engine» on the panel and continue keep moving. Due to this behavior the engine will work fine in case of failure of some sensors (not all of them, as you see) and will allow to go to the nearest service centre.

Most of you will take notice, that the injector is essentially quite simple device. Schematically there is nothing military – the incoming values are read by ADC (analog-to-digital encoder), simply incoming or purely binary. There are output transistors and pretty tough conditions of work. That’s far from learning about the outer space. Firmware is also not a big deal, I guess. In my opinion it’s easier than any image understanding algorithm and all that. In the course of settings nobody wants to deal with the actual firmware. In the sense that there won’t be any opening of the source code, algorithms adjusting or optimization – nothing like that. There is only software, which allows changing those fuel cards and other coefficients. The firmware is being done by the engineers at the factories or by mere mortals interested in them. Yes, yes not everyone is ready to pay small fortune for the “brains”, and somebody simply doesn’t want any control over the happenings. All that led to existence of several projects of the quite accessible “brains”. There is megasquirt — www.megamanual.com/index.html, for this hardware base has been written and still is supported by the firmware with the widened functional - msextra.com/doc/index.html. This website has the schemes of these “brains”, it might be interesting for some hardware engineers. And the programmers might find the code quite delightful. Unless I am mistaken, but it’s here msextra.com/doc/ms2extra/files/release/ms2extra_3.2.1_release.zip. There is also VEMS — www.vems.hu/wiki/, which initially was called megasquirtAVR. I have run also into such things - forum.diyefi.org/, there’s some sort of their own project FreeEMS. In my opinion, it shows, that things are not so complicated and even understandable here and there.

I hope it was interesting and sufficiently clear!
Tags: AFR, injector, MAF
Killer 7 may 2012, 18:14
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