Rajan Wilcox
Fuel injectors are controlled by a variety of systems, which have become increasingly sophisticated over time. The earliest fuel injection systems were mechanical, with the first electronic system appearing in 1957, using analogue electronics. Modern systems are now entirely electronic, with the engine control unit (ECU) determining the amount of fuel supplied to the engine by controlling the amount of time the fuel injector stays open, also known as the pulse width. This is calculated using a formula and lookup tables, with the ECU taking into account factors such as engine speed, load, air temperature, throttle position, and exhaust data. The ECU also controls the ignition timing and other engine functions, resulting in improved efficiency, power, and reduced emissions compared to older carburettor systems.
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What You'll Learn
Injector valves control metering
Fuel injection systems are used to spray pressurised fuel into the engine. The system must determine the appropriate amount of fuel to be supplied and control the fuel flow to supply this amount.
Fuel injectors that also control the metering are called injection valves, while injectors that perform all three functions are called unit injectors. Injection valves are mounted at the point of injection to connect the metering line. They precisely meter the flow volumes in a line and are securely sealed from the environment. They protect against backflow and generate a defined back pressure.
The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU (engine control unit). The engine control unit is equipped with a variety of sensors to provide the correct amount of fuel for every operating condition. For instance, the mass airflow sensor tells the ECU the mass of air entering the engine, while the oxygen sensor monitors the amount of oxygen in the exhaust so that the ECU can determine how rich or lean the fuel mixture is and make adjustments accordingly.
The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy emissions requirements for 100,000 miles, meet EPA fuel economy requirements, and protect engines against abuse. The ECU uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions.
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Injector pulse width
The ECU plays a pivotal role in managing the injector pulse width by utilising various sensors and data inputs. One of the key sensors is the mass airflow sensor, which provides the ECU with information about the mass of air entering the engine. This data is essential for calculating the required fuel injection amount. Additionally, oxygen sensors play a crucial role in monitoring the oxygen content in the exhaust, enabling the ECU to adjust the fuel mixture accordingly.
The calculation of the injector pulse width involves a complex algorithm that takes into account numerous factors. The ECU uses lookup tables to determine the base pulse width, which is influenced by engine speed (RPM) and load. By cross-referencing the engine speed and load values, the ECU can determine the base pulse width from the lookup table. This base pulse width serves as a starting point for further adjustments.
Other parameters, such as coolant temperature and oxygen level, are also considered in the calculation. These factors are multiplied by their respective coefficients (Factor A and Factor B) obtained from the lookup tables. These coefficients represent the impact of each parameter on the injector pulse width. By incorporating these factors, the ECU can fine-tune the pulse width to match the specific operating conditions of the engine.
The injector pulse width calculation is a dynamic process, as the ECU continuously monitors the engine's performance and adjusts the pulse width accordingly. This real-time adjustment capability ensures that the engine receives the precise amount of fuel required, optimising combustion efficiency, power output, and emissions. The injector pulse width plays a critical role in maintaining the delicate balance of the fuel-air mixture, ultimately influencing the overall performance and efficiency of the engine.
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Mechanical vs electronic control
Fuel injection systems have two main types of control: mechanical and electronic. Mechanical injection systems use a rotary or inline pump to provide individual fuel bursts, one for each cylinder. This is normally controlled through a mechanical linkage to the timing belt or gears, which open the injectors and control the fuel pressure. The mechanical linkage means the fuel delivery is fixed, and the ability to match engine power delivery to the application requirement is limited. Mechanical systems were used in the 1960s and 1970s by many manufacturers for their higher-performance sports cars and sports saloons.
The first electronic fuel injection (EFI) system was introduced in 1957 with the American Bendix Electrojector system, which used analogue electronics for the control system. However, reliability problems meant that the system was not offered. The first cars known to use an EFI system were the 1958 Chrysler 300D, DeSoto Adventurer, Dodge D-500 and Plymouth Fury. The Bosch Motronic multi-point fuel injection system was the first mass-produced system to use digital electronics.
In an electronic system, the injector is held closed by a spring and opened by an electromagnet built into the injector body. The electronic control unit (ECU) determines how long the injector stays open by monitoring the engine's speed and load using a number of sensors. This allows for very accurate amounts of fuel to be injected, and multiple injections can be made into the cylinders during the compression cycle, building up fuel vapour to control the engine's speed and power output efficiently. Electronic control systems bring improved fuel performance, better machine optimisation, and enhanced power delivery compared to mechanical systems.
Both mechanical and electronic systems can be used to control continuous and timed injection. In a continuous injection system, fuel flows at all times from the fuel injectors, but at a variable flow rate. In a timed injection system, the fuel is delivered in bursts to coincide with the induction stroke of the cylinder. The earliest systems were mechanically controlled, and the fuel flow is controlled by a mechanical regulator assembly.
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Continuous vs timed injection
Fuel injection systems can be categorised into two types: continuous injection and timed or intermittent injection. Continuous injection systems, also known as modern continuous-flow fuel injection systems, feed fuel into the inlet port of the engine at all times during its operation. The amount of fuel sprayed is then adjusted by a mechanical or electrical control unit. The most common automotive continuous injection system is the Bosch K-Jetronic system, which was introduced in 1974 and used until the mid-1990s. This system was a mechanical injection system, using a plunger actuated by the intake manifold pressure to control the fuel flow to the injectors.
On the other hand, timed injection systems deliver fuel in bursts to coincide with the induction stroke of the cylinder. The earliest systems were mechanically controlled and are often called petrol injection (PI). In these systems, the fuel flow is controlled by a mechanical regulator assembly. Timed injection systems can be further categorised into sequential, batched, simultaneous, and cylinder-individual systems. In sequential systems, injection is timed to coincide with each cylinder's intake stroke. In batched systems, fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder's intake stroke. In simultaneous systems, fuel is injected at the same time to all the cylinders. Lastly, in cylinder-individual systems, the engine control unit can adjust the injection for each cylinder individually.
The amount of fuel supplied to the engine in both continuous and timed injection systems is determined by the amount of time the fuel injector stays open, known as the pulse width. This is controlled by the engine control unit (ECU), which uses various sensors and lookup tables to determine the appropriate pulse width for a given set of operating conditions. The ECU takes into account factors such as engine speed, load, coolant temperature, and oxygen level to calculate the pulse width.
While continuous injection systems provide a constant flow of fuel, timed injection systems deliver fuel in precise bursts. Timed injection systems offer the advantage of quick response to sudden changes by the driver, as they only need to wait until the next intake valve opens, rather than for the next complete revolution of the engine. Additionally, timed injection systems, particularly those with cylinder-individual configurations, can ensure that each cylinder receives a precisely balanced fuel-air mixture, enhancing combustion efficiency and power output while reducing emissions.
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Sensors and lookup tables
The Engine Control Unit (ECU) is at the heart of these systems, receiving data from various sensors and utilising lookup tables to make real-time adjustments to the fuel injection process. The ECU's primary function is to maintain the ideal air-to-fuel ratio, known as the stoichiometric value, which is approximately 14.68:1. This ratio ensures the complete combustion of the fuel-air mixture.
One of the key sensors in the system is the mass airflow sensor, which measures the mass of air entering the engine. This information is crucial for the ECU to determine the appropriate amount of fuel required. Additionally, oxygen sensors play a vital role in monitoring the amount of oxygen in the exhaust. By analysing the oxygen levels, the ECU can adjust the fuel mixture accordingly, ensuring it is neither too rich nor too lean.
The ECU also relies on lookup tables, which are essentially pre-defined values and formulas that guide its decisions. For instance, the ECU uses lookup tables to determine the pulse width, which dictates how long the fuel injectors remain open. By cross-referencing engine speed (RPM) and load, the ECU can identify the appropriate pulse width from the lookup table. This process ensures that the fuel injectors deliver the right amount of fuel for a given operating condition.
Furthermore, performance chips, often used to boost engine power, replace the standard chip in the ECU that holds the lookup tables. These performance chips contain more aggressive settings, resulting in higher fuel rates during certain driving conditions, such as full throttle. They may also adjust spark timing, utilising lookup tables to fine-tune the engine's performance.
In summary, sensors and lookup tables work in tandem with the ECU to optimise the fuel injection process. By continuously monitoring and adjusting various parameters, such as airflow, oxygen levels, engine speed, and load, the ECU ensures that the engine receives the precise amount of fuel required for optimal performance, fuel efficiency, and compliance with emissions regulations.
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Frequently asked questions
Fuel injection systems are operated by spraying pressurised fuel into the engine. The system must determine the appropriate amount of fuel to be supplied and control the fuel flow to supply this amount.
The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the ECU.
The fuel injectors can either all open at the same time or each one can open just before the intake valve for its cylinder opens (this is called sequential multi-port fuel injection).
