Fuel injection system is one of the most important components of a car. This system defines fuel volume, which is necessary for engine in specific moment. We’ll introduce its operational principle in terms of some its types, existing sensors and servo units.
1. Specifics of fuel injection system operation
2. Fuel injection sensors
3. Injection servo units
1. Specifics of fuel injection system operation
There fuel carburettor system on the engines produced today. It was fully driven out by newer and advanced fuel injection system. The fuel injection is called the system of metered flow of oil fuel to engine cylinders.
It can be installed as on petrol, so on diesel engines, but the construction and operation principles will be different. Used on petrol engines, during injection, there appears a homogeneous fuel-air mixture, which is forced to ignite under plug’s spark.
As for diesel engine, here the fuel injection is done under high pressure. During this necessary fuel quantity is mixed with hot air and ignites momentarily. The volume of injected fuel and general power of engine is determined by injective pressure. So, the higher pressure, the higher power has the engine.
Nowadays there are quite a great number of various types of this system. To the main types refer: direct injection system, mono-injection, mechanical and sequential injection systems.
Operation principle of direct injection consists in delivery of oil fuel directly into cylinders with (like, for example, of diesel engines) the help of nozzles.
First this scheme was used in military aviation during the World War II and on some cars of post-war period (first was Goliath GP700). But fuel injection system of that time couldn’t gain the popularity it deserves. The reason was in expensive high pressure fuel pumps and original integral cylinder head, which were required for this.
In general, direct injection could be called the evolutionary pinnacle of fuel feed systems, but for one “but”…Such engines are very demanding to fuel quality. And with using of diluted mixture, it exhausts nitrogen oxides, which can only be removed by complicating construction.
Central injection (also called mono-injection) is a system, which came in use instead of carburetor systems in 1980. Though, their operation principles are very similar: air flows mix with oil fuel in intake manifold, but complicated and sensitive carburetor was replaced by nozzle. Of course, there was no electronics in early stages of system development, and fuel delivery was controlled by mechanical units. But, despite of all disadvantages, the injection provided more power for engine and far more fuel economy.
The use of nozzle allowed more accurate dozing of oil fuel, diffusing it into small particles. The result of mixing with air was homogeneous mixture, and at changing of car movement condition and engine working mode, its constitution altered almost momentarily. Though, there are some disadvantages: nozzle was installed in body of former carburetor, bulky sensors complicated “engine’s breath”, the incoming airflow met a serious resistance in cylinders. Theoretically, this disadvantage could easily be removed, but nothing could be done with poor oil fuel distribution. That’s why mono-injection is rear even nowadays.
Mechanical injection system appeared in the end of 1930s, when it was used in duel systems of planes. This was a fuel injection system of diesel origin, using high pressure fuel pumps and closed nozzles for every cylinder. When they were attempted to install in cars, they could not compete with carburetor mechanisms, because of their complicated construction and high cost.
First the injection system of low pressure was installed on Mercedes car in 1949 and surpassed the one of carburetor type. This fact pushed to further development of fuel injection system ideas for cars with combustion engine. From the perspective of price policy and reliability, the most successful was mechanical system "K-Jetronic" by BOSCH Company. Its production was arranged in 1951 and was wide-spread among all cars manufactured in Europe.
Sequential (multipoint) fuel injection differs from the previous one by individual nozzle, which was installed in inlet branch of every cylinder. Its task was to deliver fuel directly on induction valve, which means that air-fuel mixture was prepared before supplying it to compression chamber. Of course, in this condition the mixture is homogeneous and has an equal quality in each cylinder. As a result, engine power and fuel economy increased, the toxicity level of exhaust gas is reduced.
There were some complications on the way of fuel injection system development, but it continued upgrading. At early stage it was controlled mechanically, as the previous system. But with development of electronics it not just became more effective, but it also allowed to coordinate cooperation with other engine components. So, that’s how modern engine signalizes to driver about failures and, it necessary, shifts to limp-home mode or, with the help of security system, repair some control failures. All this is performed with the help of specific sensors, which register minimal changes in any operational part. Here are the main of them.
2. Fuel injection sensors
The fuel injection sensors are meant for register and data transfer from control device to engine control unit and vice versa. These are:
Ox sensor. Its sensory picker is located in exhaust flow. When running, working temperature reaches 360 C, the sensor starts producing its own electromotive intensity, which is directly-proportional to oxygen in exhaust gas. From a practical standpoint, when control loop is closed, the ox sensor signal constitutes a fast-changing voltage between 50 and 900 mV. The voltage changing ability is caused by constant alteration of mixture ratio near stoichiometry point, and the sensor is not adapted for generation of AC current.
Depending on fuel injection, there are two types of sensors: with pulse and constant supply of heating set. In pulse option, the sensor heating is performed by electronic control unit (ECU). Without heating, it will have a high internal resistance, which will not allow producing its own EMF. And this means that control unit will “see” only stated sustainable reference voltage. During the heating process of sensor, its internal resistance falls and the process of resistance generation starts, which ECU “finds out” a once. This is a signal ready for ECU to regulate a fuel ratio.
Mass air flow sensor is used for estimation of amount of air, coming into engine. It is a part of engine control electronic system. This device can be used together with other sensors, such as air temperature and atmospheric pressure sensors, which are used for making adjustments to its readings.
The mass air flow sensor consists of 2 platinum strings heated by electric current. One part transmits air (this way it also cools down), and the second is control element. The amount of air coming to engine is calculated with platinum sting.
Basing on the information from mass air flow sensor, the ECU calculates amount of fuel necessary for sustaining stoichiometric proportion of air and fuel in set engine operation modes. Besides, ECU uses the received information to determine performance point of engine. Nowadays there are several different types of mass air flow sensors: ultrasonic, vane (mechanical), thermoanemometric etc.
Water temperature sensor (WTS). It resembles thermistor, or resistive component, in which resistance alters depending on thermal index. The thermistor is located in sensor and shows minus coefficient of temperature resistance (with heating, the resistance power reduces).
As most of sensors, this device doesn’t guarantee accurate results. So, dependence of temperature sensor resistance on water temperature sensor index is approximate. Well, despite the fact that the device mentioned above is almost unbreakable, sometimes it “makes serious mistakes”.
Throttle valve position sensor. It is mounted on throttling junction pipe and connected to valve axle. It is made as potentiometer with 3 ends: one end is supplied with positive power (5V), another is connected with mass. The third end supplies the output signal to controller.
By pressing a pedal, the throttle valve turns, and sensor’s output voltage alters. If the throttle valve is in closed condition, then the resistance is lower than 0.7V. When the valve opens up, the voltage increases, and in fully open condition it must be higher than 4V.
Depending on angle of throttle valve opening, controller corrects fuel supply, while controlling sensor voltage.
Considering that controller identifies voltage of the device and takes it for value zero, it doesn’t require adjustment. Some drivers think that this is the most unreliable car component and requires periodic replacement. It sounds good, but it is not that easy to perform the replacement without specific tool of high quality.
Crankshaft position sensor (CPS). It is meant for transmitting signal of crankshaft rotation and position to controller. This signal is series of frequent electrical impulses, generated by sensor during crankshaft rotation. Basing on received data, controller manages nozzles and ignition system. The CPS is located on oil cooler bonnet.
The crankshaft position sensor is the most important among all sensors mentioned here, because if it fails, car won’t work at all.
Speed sensor. The operation principle of this device is based on Hall effect. It transfers potential pulses with frequency directly-proportional to traction wheels rotation. On the assumption of conjoint connecters of harness pad, all speed sensors have some differences.
Basing on outcoming signals of speed sensor, control system can identify cut-off threshold of fuel supply and establish electronic speed limit.
Camshaft position sensor is a device meant for camshaft angle and transferring data to vehicle’s ECU. Afterwards, basing on received data, the controller can manage the ignition system and fuel supply to every cylinder.
Knock sensor is used for search of knock shocks in engine. Constructionally, this is a piezoceramic plate located in engine block.
Nowadays there are 2 types of knock sensors: resonance and broadband. In resonance models element filtering of signal spectrum is performed inside the device and depends directly on its construction. That’s why different types of engines use different models of knock sensors with different resonant frequencies.
The broadband knock sensor has even features in combustion knock noise range. And the signal filtering is performed by ECU. Nowadays resonance sensors are not installed in standard car models.
Absolute pressure gage provides tracing of changes in atmospheric pressure, which are gained as a result of bar pressure alteration and/or changing of ground clearance index.
The bar pressure can be altered during ignition process, before engine starts rotating. Due to ECU, there is a possibility of bar pressure data update with operating engine, when throttle valve is fully open at low engine rpm.
Also, with using of absolute pressure gage, there is a chance to measure pressure alterations in induction pipe. The alterations are caused by changing engine loads and crankshaft rotation speed. The absolute pressure sensor transforms them into output signal, which has a specific voltage. When throttle is closed, the signal has a low voltage and vice versa: when the throttle valve is open, the signal has high voltage. The high voltage forms due to matching of atmospheric pressure with pressure inside induction pipe when throttle valve is fully closed.
Electronic control unit (ECU). Though, this is no sensor, but considering that it is directly relevant to work of devices mentioned above, we decided to add it to the list. The ECU is “brain” of fuel injection system, which constantly processes data received from various sensors and, in reliance to which, controls output circuits (of electronic ignition system, nozzles, idle air control valve and various electromagnetic switches). It is equipped with built-in diagnostic system, which detects system failures and warns driver about them with indicator light “CHECK ENGINE”. Moreover, its memory keeps diagnostic codes, which point to specific areas of failure, which makes repairs much easier.
The ECU consists of 3 types of memory: programmed read only memory (PROM), random access memory (RAM) and electrically alterable read-only memory (EAROM).
3. Injection servo units
The servo units of injection are: nozzles, fuel pumps, ignition modules, idle air control valve, radiator fan, fuel consumption signal and adsorber. Let’s observe each of them.
Nozzle. It fulfills function of magnetic valve with restricted performance. It is used for injection of specific fuel amount meant for specific operational mode.
Fuel pump is used for delivering fuel to fuel rail, in which pressure is sustained by air-free mechanical pressure regulator. In some systems it can be combined with fuel pump.
Ignition module is an electronic device meant for sparking process control. In latest models of device its low-voltage elements are specified in ECU. To gain a high voltage used two channel remote ignition coil, or the coils located in spark plug.
Idle air control valve. Its purpose is to maintain target rpm under no-load conditions. It is produced as repeater motor, which controls air bypass channel in throttle valve body. This provides engine with necessary airflow, especially when throttle valve is closed.
Radiator fan. As you can see from its name, this component does not allow overheating. It is controlled by ECU, which receives signals from water temperature sensors.
Fuel consumption signal comes on trip computer by a ratio of 1600 impulses on 1 calculated litre of consumed fuel. Of course, this data is approximate and the calculation is based on time spent on nozzle opening.
Adsorber exists as element of closed-link chain during petrol fumes recirculation. Euro-2 standards eliminate a risk of contact of fuel tank ventilation with atmosphere, and petrol fumes must be adsorbed and during blowdown must be burnt completely.