In the absence of proper fuel delivery, an internal combustion engine is of little value. Fuel serves as the main catalyst for combustion in any engine and must be metered to meet the demands of any operational load that is placed upon a vehicle’s powerplant. In a bid to optimize the efficiency of the modern internal combustion engine, engineers have spent decades perfecting the art of fuel delivery.
These efforts have led to the development of numerous fuel delivery systems, all of which have played a pivotal role in increasing engine reliability and performance. Today’s vehicles rely upon modern EFI systems (electronic fuel injection), to accurately distribute fuel to each cylinder, at the request of an engine’s management software.
While many realize the importance of precise fuel delivery, far fewer motorists understand the complexities of their vehicle’s electronic fuel injection system. The following will answer these questions and delve into all which define EFI technology.
The History of Fuel Injection
Since the earliest days of the internal combustion engine, man has been transfixed on the notion of bolstering performance through more efficient fuel delivery. As a result, several alternative fuel delivery systems were developed and utilized within production prior to the advent of modern EFI systems. To better understand the properties and value of EFI technology, one must first understand how such advancements came to be.
The following is a timeline of automotive fuel delivery systems throughout the decades.
Carbureted Fuel Delivery
Long before modern fuel injection came to pass, gasoline engines relied upon the use of one or more carburetors. A carburetor is perhaps best described as a vacuum-operated fuel/air metering device. Air is channeled into a carburetor through an inlet pipe that is connected to the engine’s breather.
Upon reaching the carburetor, this air encounters a throttle plate, which meters its passage. This throttle plate is controlled by a mechanical linkage, which is manipulated through the actuation of the accelerator pedal. As the accelerator pedal is depressed, the carburetor’s throttle plate opens to a greater extent, allowing increased airflow volume to enter the engine’s intake manifold. Carburetors in general are rated by volume, which is denoted by a specific CFM rating.
Fuel enters the carburetor through a supply line, where it is distributed to the unit’s reservoir, or bowl. As the carburetor’s bowl continues to fill with fuel, a buoyant float rises upward. This float is attached to a needle valve that seats within an orifice as the carburetor’s fuel level reaches its peak level. In its seated position, the needle valve prevents any additional fuel from flowing into the carburetor until necessitated through fuel consumption.
As air passes through the carburetor, a vacuum is in contact with fuel contained within the unit’s bowl. As a result, fuel is pulled through a set of additional orifices, known as “jets''. Fuel then flows through these orifices and is dispersed into the carburetor’s throat, where it mixes with the incoming air before it is distributed into the engine's intake.
For decades, carburetion served as the primary form of fuel delivery in automobiles, both foreign and domestic. However, systems of this nature proved largely inadequate in the face of mounting emission legislation that was enacted in the early 1970s. Increased industry oversight at the hands of numerous government agencies led to a number of mandates aimed at reducing smog output.
Carburetors proved to be notably inconsistent when dosing fuel under a range of circumstances, in relation to engine load and various environmental factors. Though tuning could minimize such issues to a significant degree, many manufacturers still felt that far too many variables existed with the use of carbureted fuel delivery. As a result, TBI fuel delivery gradually rose to prominence.
TBI Fuel Delivery
By the late 1970s and early 1980s, automotive manufacturers had begun searching for any possible way to enhance the combustion efficiency of their vehicle’s engines. Around the same time, the earliest forms of electronic engine management began to make an appearance. The advent of this technology led to a significant breakthrough in fuel delivery technology.
Onboard engine monitoring, even in its most primitive forms, presented the opportunity for an electronically controlled fuel system to react to operational variables in real-time, thereby adjusting delivery rates as necessary. Initial ECU controlled fuel systems were of the TBI (throttle body injection) variety, which replaced an engine’s carburetor with a specialized throttle body fitted with 1-2 fuel injectors, dependent upon manufacturer design.
Injection throttle bodies were supplied with fuel via a tank mounted fuel pump. To meet operational demands, system pressures were directly influenced by the use of an integrated fuel pressure regulator. Fuel was dosed through the system’s injectors, where it was then delivered past an open throttle plate into the engine’s intake manifold.
TBI fuel systems relied upon feedback from a vehicle’s O2 sensors, throttle position sensor (TPS), and manifold absolute pressure sensor (MAP) to determine correct fueling rates under any set of operating conditions.
Many manufacturers began favoring the use of injection throttle bodies because such units could be easily mounted in place of an engine’s carburetor at a centralized location atop the intake manifold. This proved to be preferential at the time over the use of multi-port fuel injection, as the latter would require manufacturers to significantly re-engineer their available engines. Chevy, Ford, and Chrysler were among the first automakers to adopt the TBI technology in their vehicles.
Within a matter of only a few years, the introduction of TBI technology had also opened new doors for those who owned vehicles with classic car-equipped engines. Due to the relative mounting similarities between both units, a number of companies, such as Holley and Edelbrock, began offering specialty aftermarket adapter/flange kits, which made carburetor to TBI conversions possible.
More recently, these same manufacturers have begun offering their own self-tuning EFI kits, such as the Holley Sniper, Dominator, and Terminator X. These kits are of the plug and play variety, dyno-tested, and possess extensive self-learning capabilities. After basic calibration with an included handheld tuner, these Holley EFI kits go to work, powering your vehicle without any further intervention. Most of these kits are also compatible with the use of a supercharger or nitrous and bolt in place in the same manner as an engine’s OEM-grade carburetor.
Multi-Port Fuel Injection
As time carried on, automotive manufacturers again began looking to the future. Most understood that emissions standards would only continue to tighten, in turn threatening the compliance of their TBI-fueled engines. As a result, efforts to pioneer the next great advancement in fuel delivery technology were put into motion.
In a rather decisive change of pace, the vast majority of automotive manufacturers abandoned their TBI systems in favor of new cylinder-specific multi-port injection fuel delivery. Multi-port fuel injection systems utilize individual injectors for each cylinder as opposed to a single, centrally located injector, as was the case during the era of injection throttle body use.
Each of these injectors is fueled by fuel rails to which they are affixed. Each engine bank features its own dedicated fuel rail. Fuel is delivered to each rail in a pressurized form, thanks to the vehicle’s fuel pump. The exact pressure of this fuel is dictated by the system’s fuel pressure regulator.
Fuel flows to each multi-port injector, where it is held until the vehicle’s ECU energizes a particular injector’s solenoid. The timing of this injection cycle is determined by an engine's management software in correlation with engine speed and ignition timing.
Contrary to prior fuel delivery methods, multi-port fuel injection features a standalone throttle body, which is void of any actual injectors or fuel injection related components. While carbureted and TBI based fuel systems both direct fuel into the engine’s intake through the throttle body, multi-port systems introduce fuel directly into each cylinder.
The introduction of multi-port fuel injection allowed engines to achieve levels of operational efficiency far beyond what had previously been thought to be possible. Significant gains in throttle response and horsepower were some of the most notable improvements brought about by the introduction of these systems.
What is an EFI Fuel System?
EFI stands for electronic fuel injection. In the simplest sense, EFI fuel delivery describes the use of any fuel system that is controlled by an intelligent electronic interface. Systems of this nature rely upon feedback from numerous sensors to determine the correct timing of their injection cycles, thereby enhancing combustion efficiency.
Some confusion exists as to the exact fuel systems that do or do not fall under the EFI umbrella. While many use the term “EFI” to describe multi-port fuel injection, others refer to TBI systems in the same sense. By definition, both of these forms of fuel delivery could be described as EFI systems, as both rely on data transmission between sensors and computerized processors to determine injection timing and fuel trims.
Outside of the obvious difference in point of injection, the most significant difference between TBI and multi-port fuel systems is rooted in the number and variety of sensors that are utilized by a vehicle’s ECU when making system-related calculations. As time has passed, the operational characteristics of modern fuel delivery systems have become more advanced, therefore requiring additional feedback from a number of sensors in order to operate at peak capacity.
Sensors Utilized by EFI Systems
The following are several sensors that play a pivotal role in the operation of today’s modern EFI systems.
During the earliest day of EFI technology, when TBI fuel delivery was at the height of its popularity, O2 sensors served as the primary source of feedback within most fuel systems. The data relayed by an O2 sensor allows engine management software to differentiate between “lean” and “rich” conditions, thereby determining whether fuel trims should be increased or decreased in response to situational demands.
O2 sensors are positioned within a vehicle’s exhaust system and make their standard lean-versus-rich determinations by calculating the amount of oxygen found within an engine’s outbound exhaust gasses. Because most “V” configuration engines feature O2 sensors downstream from the exhaust manifold of each head, separate oxygen content readings can be taken for both cylinder banks.
Manifold Absolute Pressure Sensor (MAP)
A manifold absolute pressure sensor monitors the air density within an engine’s intake manifold, which often fluctuates with changes in altitude. An EFI fuel system relies upon data from an engine’s MAP sensor to determine the exact amount of fuel that should be dosed at any given time. In absence of a properly operating MAP sensor, numerous drivability issues can result.
When a MAP sensor relays a high vacuum condition, such as that which is encountered when increasing in elevation along a mountain pass, fuel delivery is increased to prevent engine stalling. Likewise, fuel delivery will be reduced when coasting a sizable hill to prevent over-fueling that would typically lead to poor fuel economy.
Mass Air Flow Sensor (MAF)
A mass air flow sensor monitors the volume of the incoming air, which is introduced into an engine’s intake tract. On most vehicles, the mass air flow sensor can be found directly downstream from the engine’s air filter housing. Sensors of this variety come in two primary configurations, hot wire and vane-style. Each of these sensor configurations is quite efficient in the uptake of available data, though hot wire style mass air flow sensors are often thought to be somewhat more reliable.
An EFI system uses the data which is presented by an engine’s mass air flow sensor to determine proper dosing to maintain an optimal air/fuel ratio. If intake air volume were to increase, fuel dosing would also be elevated to prevent the onset of a lean condition. Along the same lines, a drop in air delivery would lead to a reduction in fuel dosage as a means of preventing a rich condition from developing.
Engine Coolant Temperature Sensor (ECT)
As its name would suggest, a coolant temperature sensor provides its corresponding ECU with a resisted voltage reading that is representative of the engine’s coolant temperature. An engine’s coolant temperature sensor is typically found within close proximity of the thermostat housing. On some engines, the ECT will actually be threaded into the thermostat housing itself.
EFI systems utilize data conveyed by an engine coolant temperature sensor to make minor adjustments in fuel delivery volume. More specifically, a cold engine requires additional fuel to run efficiently. A vehicle’s ECU is programmed with this logic and will automatically provide more fuel to each cylinder when the engine’s ECT presents a low temperature reading. The engine coolant temperature also controls various additional engine functions such as fan operation.
Throttle Position Sensor (TPS)
A throttle position sensor detects the percentage at which an engine’s throttle plate is opened within the throat of the throttle body, itself. As the throttle plate is opened, additional air is allowed to pass through the throttle body, and into the engine’s intake manifold. On virtually all vehicles, the throttle position sensor can be found mounted along one particular side of the throttle body.
An EFI fuel system calculates the value provided by the TPS into an equation, which determines the exact amount of fuel that will be required while an engine is under a specific load. As the throttle plate opens, an engine’s ECU calls for the introduction of additional air into each cylinder. This prevents a lean condition from presenting itself, as a significant volume of air is provided for combustion.
Engine Speed Sensors (Crank/Camshaft Position Sensors)
Most gasoline engines feature two individual engine speed sensors, the crankshaft position sensor, and the camshaft position sensor. These sensors work together to provide an engine’s ECU with real-time data surrounding an engine’s RPM and cam/crank correlation. Many engines rely primarily upon the camshaft position sensor for timing information due to its higher level of accuracy yet automatically default to the use of crankshaft position sensor data should a camshaft position sensor failure take place.
EFI systems utilize engine speed sensor data in an attempt to match fuel delivery rates to varying engine load demands. Data relayed by the throttle position sensor can be compared to engine speed sensor feedback as a means of determining whether or not current fuel trims are sufficient to meet any load placed upon the engine. Fuel delivery can then be tweaked in order to enhance engine performance.