Sequential Fuel Injection

Typical Import Sequential Fuel Injection System

The Nissan electronic concentrated engine control system (ECCS) is an SFI system that has many of the same inputs and outputs as the other systems. The system also has some things not found in others, such as the dropping resistor assembly that contains a resistor connected in series with each injector. These resistors protect the injectors from sudden voltage changes and provide constant injector operation. The system uses a vane-type mass air-flow sensor. The throttle valve switch is mounted in the throttle chamber and the switch contacts are closed when the throttle is in the idle position .When the throttle is opened from the idle position, the switch contacts open. The throttle valve switch signal informs the PCM when the throttle is in the idle position. An idle control valve (ICV) solenoid and fast idle control device (FICD) solenoid are mounted on the intake manifold. When the idle speed drops below a specific rpm, the PCM energizes the ICV solenoid, and additional air is bypassed through this solenoid into the intake manifold to increase the idle speed. The FICD solenoid is energized when the A/C is on, and air flows past this solenoid into the intake manifold to maintain idle speed and compensate for the compressor load on the engine. The ICV and FICD assemblies contain an idle adjust screw to set the specified idle rpm

CENTRAL MULTI-PORT FUEL INJECTION (CMFI)

In a central port (CPI) or central multi-port fuel injection (CMFI) system, a central injector assembly is mounted in the lower half of the intake manifold. The CMFI system uses one injector to control the fuel flow to six (on six-cylinder engines) individual poppet nozzles. The CMFI injector assembly consists of a fuel metering body, pressure regulator, one fuel injector six poppet nozzles with nylon fuel tubes, and a gasket seal. The injector distributes metered fuel through a six-hole distribution gasket. The gasket seals the injector to the six lines connected to the nozzles. Each nozzle contains a check ball and extension spring that regulates fuel flow. The poppet nozzle opens when high pressure is exerted on the check ball. This action allows the nozzles to feed individual cylinders with atomized fuel.

Pressure Regulator

The pressure regulator is mounted with the central injector. Since this regulator is mounted inside the intake manifold, vacuum from the intake is supplied through an opening in the regulator cover to the regulator diaphragm. The regulator spring pushes downward on the diaphragm and closes the valve. Fuel pressure from the in-tank fuel pump pushes the diaphragm upward and opens the valve, which allows fuel to flow through this valve and the return line to the fuel tank.The pressure regulator is designed to regulate fuel pressure to 54 to 64 psi (570 to 440 kPa), which is higher than many port fuel injection systems. Higher pressure is required in the CMFI system to prevent fuel vaponzation from the extra heat encountered with the CMFI assembly, poppet nozzles, and lines mounted inside the intake manifold. The pressure regulator operates the same.

Injector Design and Operation

A pivoted armature is mounted under the injector winding in the central injector. The lower side of this armature acts as a valve that covers the six outlet ports to the nylon tubes and poppet nozzles. A supply of fuel at a constant pressure surrounds the injector armature while the ignition switch is on. Each time the PCM grounds the injector winding, the armature is lifted upward, which opens the injector ports. Under this condition, fuel is forced from the nylon tubes to the poppet nozzles. The amount of fuel delivered by the central injector is determined by the length of time that the PCM keeps the injector winding grounded. This time is referred to as pulse width. When the PCM opens the injector ground circuit, the injector spring pushes the armature downward and closes the injector ports. The injector winding has low resistance, and the PCM operates the injector with a peak-and-hold current. When the PCM grounds the injector winding, the current flow in this circuit increases rapidly to 4 amperes. When the current flow reaches this value, a current-limiting circuit in the PCM limits the current flow to 1 ampere for the remainder of the injector pulse width. The peak-and-hold function provides faster injector armature opening and closing.

Poppet Nozzels

The poppet nozzles are snapped into openings in the lower half of the intake manifold, and the tip of each nozzle directs fuel into an intake port. Each poppet nozzle contains a valve with a check ball seat in the tip of the nozzle. A spring holds the valve and check ball seat in the closed position. When fuel pressure is applied from the central injector through the nylon lines to the poppet nozzles, this pressure forces the valve and check ball seat open against spring pressure. The poppet nozzles open when the fuel pressure exceeds 37 to 45 psi (254 to 296 kPa), and the fuel sprays from these nozzles into the intake ports. When fuel pressure drops below this value, the poppet nozzles close. Under this condition, approximately 40 psi (276 kPa), fuel pressure remains in the nylon lines and poppet nozzles. This pressure prevents fuel vaporization in the nylon lines and nozzles during hot engine operation or hot soak periods. If a leak occurs in a nylon line or other CMFI component, fuel drains from the bottom of the intake manifold through two drain holes to the center cylinder intake ports. The in-tank fuel pump, fuel filter, lines, and fuel pump circuit used with the CMFI system are similar to those used with SFI and MFI systems.

Gasoline Direct-Injection System

Direct-injection has been around for many years on diesel engines. Until recently this type of injection system has been seldom used with gasoline. With gasoline direct-injection (GDI), the gasoline is injected directly into the combustion chamber. GDI allows for very lean operation (as much as 35:1) during cruising. When the engine is operating under heavy loads, the system provides near- stoichiometric air/fuel ratios. Because the engine is able to run at such lean ratios, its fuel economy is increased by nearly 30% and the emissions levels are substantially decreased Spraying the fuel directly into the cylinder also increases volumetric efficiency and allows for higher compression ratios, without the need for higher-octane gasoline. These features result in increased horsepower and torque outputs from the engine. The fuel is highly pressurized (up to 1,500 psi [ 10,342 kPa) when it is sprayed into the cylinder. Under this pressure, the fuel arrives as a vapor. The fuel injectors are specially built to be able to close against this pressure. The fuel pump that delivers this high pressure to the fuel injectors is driven by the engine. That pump is fed by an in-tank electric fuel pump. A PCM controls the timing of the injection and ignition for each cylinder. Although GDI offers many advantages over port injection, there is concern about the NOx levels produced by the engine. Because of the very lean air/fuel ratios and the high compression ratios, combustion temperatures are very high during highway cruising. There are other concerns as well, but it is likely that there may be more GDI engines on the road in the future.

INPUT SENSORS

The ability of the fuel injection system to control the air/fuel ratio depends on its ability to properly time the injector pulses with the compression stroke of each cylinder and its ability to vary the injector “on” time according to changing engine demands. Both tasks require the use of electronic sensors that monitor the operating conditions of the engine. Many of the same sensors used with computer-controlled carburetors are used with EFI systems.

Airflow Sensors

In order to control the proportion of fuel to air in the Air/fuel charge, the fuel system must be able to measure the amount of air entering the engine. Several sensors have been developed to do just that.

VOLUME AIRFLOW SENSOR

The airflow sensor (commonly called an air flow meter or vane airflow sensor), measures airflow or air volume. The sensor consists of a spring-loaded flap, potentiometer, damping chamber, backfire protection valve, and idle by-pass channel. As air is drawn into the engine, the flap is deflected against the spring. A potentiometer attached to the flap shaft monitors the flap movement and produces a corresponding voltage signal. The strength of the signal increases as the flap opens. The signal voltage is relayed to the electronic control module and may be used to control the fuel pump. The curved shape of the airflow sensor is the damping chamber. The damping flap in this chamber is on the same shaft as the airflow sensing flap and is also about the same area. As a result, the damping flap smooths out any possible pulsation caused by the opening and closing of the intake valves. Airflow measurement can be a steady signal, closely related to airflow as controlled by the movement of the flap. The airflow sensor flap provides for backfire protection with a spring-loaded valve. If the intake manifold pressure suddenly rises because of a backfire, this valve releases the pressure and prevents damage to the system. The airflow sensor assembly includes an extra air passage for idle, bypassing the airflow sensor plate. When the throttle is closed at idle, the opening and closing of intake valves can cause pulsation in the intake manifold. Without the idle bypass, such pulsations could cause the flap to shudder, resulting in an uneven air/fuel mixture. The idle bypass smooths the flow of the idle intake air, ensuring regular signals to the electronic control.

KARMAN VORTEX

Another design of airflow sensor, called a Karman Vortex sensor, works on a different operating principle. Air entering the airflow sensor assembly passes through vanes arranged around the inside of a tube. As the air flows through the vanes, it begins to swirl. The outer part of the swirling air exerts high pressure against the outside of the housing. There is a low-pressure area in the center that moves in a circular motion as the air swirls through the intake tube. Two pressure-sensing tubes near the end of the tube sense the low-pressure area as it moves around. An electronic sensor counts how many times the low-pressure area is sensed. The faster the airflow, the more times the low-pressure area is sensed. This is translated into a signal that indicates to the combustion control computer how much air is flowing into the intake manifold.

Air Temperature Sensor

Cold air is denser than warm air. Cold, dense air can burn more fuel than the same volume of warm air because it contains more oxygen. That is why airflow sensors that only measure air volume must have their readings adjusted to account for differences in air temperature. Most systems do this by using an air temperature sensor mounted in the induction system. The air sensor measures air temperature and sends an electronic signal to the control computer. The computer uses this input along with the air volume input in determining the amount of oxygen entering the engine. In some early EFI systems, the incoming air is heated to a set temperature. In these systems an air temperature sensor is used to ensure that this predetermined operating temperature is maintained.

Mass Airflow Sensor

A mass airflow sensor does the job of a volume airflow sensor and an air temperature sensor. It measures air mass. The mass of a given amount of air is calculated by multiplying its volume by its density. As explained previously, the denser the air, the more oxygen it contains. Monitoring the oxygen in a given volume of air is important, since oxygen is a prime catalyst in the combustion process. From a measurement of mass, the electronic control unit adjusts the fuel delivery for the oxygen content in a given volume of air. The accuracy of air/fuel ratios is greatly enhanced when matching fuel to air mass instead of fuel to air volume. The mass airflow sensor converts air flowing past a heated sensing element into an electronic signal. The strength of this signal is determined by the energy needed to keep the element at a constant temperature above the incoming ambient air temperature. As the volume and density (mass) of airflow across the heated element changes, the temperature of the element is affected and the current flow to the element is adjusted to maintain the desired temperature of the heating element. The varying current flow parallels the particular characteristics of the incoming air (hot, dry cold, humid, high/low pressure). The electronic control unit monitors the changes in current to determine air mass and to calculate precise fuel requirements. There are two basic types of mass airflow sensors: hot wire and hot film. In the first type, a very thin wire (about 0.2 mm thick) is used as the heated element. The element temperature is set at 100ºto 200ºc above incoming air temperature. Each time the ignition switch is turned to the off position, the wire is heated to approximately 1,000'c for 1 second to burn off any accumulated dust and contaminants. The second type uses a nickel foil sensor which is kept 75ºc above ambient air temperatures. It does not require a burn-off period and therefore is potentially longer lasting than the hot wire type.

Mainfold Absolute Pressure Sensor

some EFI systems do not use airflow or air mass to determine the base pulse of the injector(s). Instead, the base pulse is calculated on manifold absolute pressure (MAP). The MAP signal may also be used to regulate the EGR. The MAP sensor measures changes in the intake manifold pressure that result from changes in engine load and speed. The pressure measured by the MAP sensor is the difference between barometric pressure and manifold pressure. At closed throttle, the engine produces a low MAP value. A wide-open throttle produces a high value that is produced when the pressure inside the manifold is the same as pressure outside the manifold, and 100% of the outside air is being measured. This MAP output is the opposite of what is measured on a vacuum gauge. The use of this sensor also allows the control computer to adjust automatically for different altitudes. The control computer sends a voltage reference signal to the MAP sensor. As the MAP changes, the electrical resistance of the sensor also changes. The control computer can determine the manifold pressure by monitoring the sensor output voltage. A high pressure, low vacuum requires more fuel. A low pressure, high vacuum requires less fuel. Like an airflow sensor, a MAP sensor relies on an air temperature sensor to adjust its base pulse signal to match incoming air density. Many EFI systems with MAF sensors do not have MAP sensors. However there are a few engines with both of these sensors. In these cases, the MAP is used mainly as a backup if the MAF fails. When the EFI system has a MAE the computer calculates the intake air flow from the MAF and rpm inputs.

Other EFI System Sensor

In addition to airflow, air mass, or MAP readings, the control computer relies on input from a number of other system sensors. This input further adjusts the injector pulse width to match engine operating conditions. Operating conditions are communicated to the control computer by the following types of sensors.

COOLANT TEMPERATURE

The coolant temperature sensor signals the PCM when the engine needs cold enrichment, as it does during warm-up. This adds to the base pulse, but decreases to zero as the engine warms up.

THROTTLE POSITION

The switches on the throttle shaft signal the PCM for idle enrichment when the throttle is closed. These same throttle switches signal the PCM when the throttle is near the wide-open position to provide full load enrichment

ENGINE SPEED

The ignition system sends a tachometer signal reference pulse corresponding to engine speed to the electronic control unit. This signal advises the electronic control unit to adjust the pulse width of the injectors for engine speed. This also times the start of the injection according to the intake stroke cycle.

CRANKING ENRICHMENT

The starter circuit sends a signal for fuel enrichment during cranking operations even when the engine is warm. This is independent of any cold-start fuel enrichment demands. :

ALTITUDE COMPENSATION

As the car operates at higher altitudes the thinner air needs less fuel. Altitude compensation in a fuel injection system is accomplished by installing a sensor to monitor barometric pressure. Signals from the barometric pressure sensor are sent to the PCM to reduce the injector pulse width (or reduce the amount of fuel injected).

COASTING SHUTOFF

Coasting shutoff can be found on a number of control systems. It can improve fuel economy as well as reduce emissions of hydrocarbons and carbon monoxide. Fuel shutoff is controlled in different ways, depending on the type of transmission (manual or automatic). The PCM makes a coasting shutoff decision based on a closed throttle, as indicated by the throttle position or idle switch, or based on engine speed, as indicated by the signal from the ignition coil. When the PCM detects that power is not needed to maintain vehicle speed, the injectors are turned off until the need for power exists again.

INPUT INFORMATION SENSORS

Additional sensors are also used to provide the following information on engine conditions. This list does not attempt to cover all of the sensors that are used by all manufacturers; it contains only the most common:

1 Detonation

2 Crankshaft position

3 Camshaft position

4 Air charge temperature

5 Air conditioner operation

6 Gearshift lever position

7 Battery voltage

8 Vehicle speed

9 Oxygen in exhaust gases

10 EGR valve position

1. While some electronic control elements are being added to the basic system, continuous injection systems (CIS) meter fuel delivery mechanically, not electronically.

2. CIS injectors spray fuel constantly. They do not pulse on and off. The proper air/fuel mixture is attained by varying the amount of fuel delivered to the injectors.

2. There are three types of electronic fuel injection systems: throttle body, port injection, and central multi-port injection. In the throttle body injection system, fuel is delivered to a central point. In the port injection system, there is one injector at each cylinder. Central multi-port is a mixture of both throttle body and port injection.

3. Port injection systems use one of four firing systems: grounded single fire, grouped double fire, simultaneous double fire, or sequential fire.

4. The volume airflow sensor and mass airflow sensors determine the amount of air entering the engine. The MAP sensor measures changes in the intake manifold pressure that results from changes in engine load and speed.

5. The heart of the fuel injection system is the electronic control unit. The PCM receives signals from all the system sensors, processes them, and transmits pro- grammed electrical pulses to the fuel injectors.

6. Two types of fuel injectors are currently in use: top feed and bottom feed. Top-feed injectors are used in port injection systems. Bottom-feed injectors are used in throttle body injection systems.

7. In a speed density EFI system, the computer uses the MAP or MAF and engine rpm inputs to calculate the amount of air entering the engine. The computer then calculates the required amount of fuel to go with the air entering the engine

8. In any EFI system, the fuel pressure must be high enough to prevent fuel from boiling.

9. In an EFI system, the computer supplies the proper Air/fuel ratio by controlling injector pulse width. Most computers provide a clear flood mode if a cold engine becomes flooded. Pressing the gas pedal to the floor while cranking the cold engine activates this mode.

10. In an SFI system, each injector has an individual ground wire connected to the computer. The pressure regulator maintains the specified fuel system pressure and returns excess fuel to the fuel tank.

11. In a return-less fuel system, the pressure regulator and filter assembly is mounted with the fuel pump and gauge sending unit assembly on top of the fuel tank. This pressure regulator returns fuel directly into the fuel tank.

12. A central multi-port injection system has one central injector and a poppet nozzle in each intake port. The central injector is operated by the PCM, and the poppet nozzles are operated by fuel pressure.