Fuel injection is a system for mixing fuel with air in an internal combustion engine. It has become the primary system used in automotive engines, having almost completely replaced carburetors in the late 1980s.
A fuel injection system is designed and calibrated specifically for the type(s) of fuel it will handle: gasoline (petrol), Autogas (LPG, also known as propane), ethanol, methanol, methane (natural gas), hydrogen or diesel. The majority of fuel injection systems are for gasoline or diesel applications. With the advent of electronic fuel injection (EFI), the diesel and gasoline hardware has become similar. EFI's programmable firmware has permitted common hardware to be used with multiple different fuels. For gasoline engines, carburetors were the predominant method to meter fuel before the widespread use of fuel injection. However, a wide variety of injection systems have existed since the earliest usage of the internal combustion engine.
The primary functional difference between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, while a carburetor relies on low pressure created by intake air rushing through it to add the fuel to the airstream.
The fuel injector is only a nozzle and a valve: the power to inject the fuel comes from farther back in the fuel supply, from a pump or a pressure container.
The functional objectives for fuel injection systems can vary. All share the central task of supplying fuel to the combustion process, but it is a design decision how a particular system will be optimized. There are several competing objectives such as:
Certain combinations of these goals are conflicting, and it is impractical for a single engine control system to fully optimize all criteria simultaneously. In practice, automotive engineers strive to best satisfy a customer's needs competitively. The modern digital electronic fuel injection system is far more capable at optimizing these competing objectives than a carburetor.
An engine's air/fuel ratio must be accurately controlled under all operating conditions to achieve the desired engine performance, emissions, driveability, and fuel economy. Modern electronic fuel-injection systems meter fuel very accurately and precisely, and use closed loop fuel-injection quantity-control based on feedback from an oxygen sensor (or "O2 sensor"). This enables fuel-injected engines to produce less air pollution than comparable carbureted engines. Properly-designed fuel injection systems can react rapidly to changing inputs such as sudden throttle movements, and will control the amount of fuel injected to match the engine's needs across a wide range of operating conditions such as engine load, ambient air temperature, engine temperature, fuel octane level, and prevailing barometric pressure.
A multipoint fuel injection system generally delivers a more accurate and equal mass of fuel to each cylinder than can a carburetor, thus improving the cylinder-to-cylinder distribution. Exhaust emissions are cleaner because the more precise and accurate fuel metering reduces the concentration of toxic combustion byproducts leaving the engine, and because exhaust cleanup devices such as the catalytic converter can be optimized to operate more efficiently since the exhaust is of consistent and predictable composition.
Fuel injection generally increases engine fuel efficiency. With the improved cylinder-to-cylinder fuel distribution, less fuel is needed for the same power output. When cylinder-to-cylinder distribution is less than ideal, as is always the case to some degree with a carburetor or throttle body fuel injection, some cylinders receive excess fuel as a side effect of ensuring that all cylinders receive sufficient fuel. Power output is asymmetrical with respect to air/fuel ratio; burning extra fuel in the rich cylinders does not reduce power nearly as quickly as burning too little fuel in the lean cylinders. However, rich-running cylinders are undesirable from the standpoint of exhaust emissions, fuel efficiency, engine wear, and engine oil contamination. Deviations from perfect air/fuel distribution, however subtle, affect the emissions, by not letting the combustion events be at the chemically ideal (stoichiometric) air/fuel ratio. Grosser distribution problems eventually begin to reduce efficiency, and the grossest distribution issues finally affect power. Increasingly poorer air/fuel distribution affects emissions, efficiency, and power, in that order. By optimizing the homogeneity of cylinder-to-cylinder mixture distribution, all the cylinders approach their maximum power potential and the engine's overall power output improves.
A fuel-injected engine often produces more power than an equivalent carbureted engine. Fuel injection alone does not necessarily increase an engine's maximum potential output, for increased airflow is needed to burn more fuel to generate more gas molecules to generate more mechanical power. The combustion process converts the fuel's chemical energy into heat energy, whether the fuel is supplied by fuel injectors or a carburetor. However, airflow is often improved with fuel injection, the components of which allow more design freedom to improve the air's path into the engine. In contrast, a carburetor's mounting options are limited because it is larger, it must be carefully oriented with respect to gravity, and it must be equidistant from each of the engine's cylinders to the maximum practicable degree. These design constraints generally compromise airflow into the engine. Furthermore, a carburetor relies on a restrictive venturi to create a local air pressure difference, which forces the fuel into the air stream. The flow loss caused by the venturi, however, is small compared to other flow losses in the induction system. In a well-designed carburetor induction system, the venturi is not a significant airflow restriction. Aside from airflow considerations, fuel injection offers a more homogeneous air/fuel mixture due to better atomization of the fuel entering the cylinders.
In cars with a manual transmission, fuel can be saved while the car is coasting in gear. This is because the car's movement is keeping the engine rotating, so there is no need to use fuel for this purpose. Control units on modern transmissions recognize this and stop all fuel flow to the engine when possible. (Automatic transmission cars rely on torque converters, not clutches, to transmit power from the engine to the gearbox, and whereas clutches can transmit power in both directions, torque converters, especially when tuned as they are in cars, are not good at transmitting power in the opposite direction.)
Frederick William Lanchester joined the Forward Gas Engine Company Birmingham, England in 1889. He carried out what were possibly the earliest experiments with fuel injection. In 1896 E.J. Pennington had detailed a crude form of fuel injection in the patent for his motorcycle (U.S. patent 574262).. Herbert Akroyd Stuart developed the first system laid out on modern lines (with a highly-accurate 'jerk pump' to meter out fuel oil at high pressure to an injector. This system was used on the hot bulb engine and was adapted and improved by Robert Bosch for use on diesel engines- Rudolf Diesel's original system using a cumbersome and less efficient 'air-blast' system using highly compressed air.
Fuel injection was in widespread commercial use in diesel engines by the mid-1920s. Because of its greater immunity to wildly changing g-forces on the engine, the concept was adapted for use in gasoline-powered aircraft during World War II, and direct injection was employed in some notable designs like the Daimler-Benz DB 603, the BMW 801, the Shvetsov ASh-82FN (M-82FN) and later versions of the Wright R-3350 used in the B-29 Superfortress.
One of the first commercial gasoline injection systems was a mechanical system developed by Bosch and introduced in 1955 on the Mercedes-Benz 300SL. This system used a normal fuel pump, to provide fuel to a mechanically driven injection pump, which had separate plungers per injector to deliver a very high injection pressure directly into the combustion chamber. A variant of this system, also by Bosch, but injecting the fuel into the port above the intake valve was later used by Porsche from 1969 until 1973 for the 911 production range in the USA and until 1975 on the Carrera RS 2.7 and RS 3.0 street models in Europe. Porsche continued using it on its racing cars into the late seventies and early eighties. Porsche racing variants such as the 911 RSR 2.7 & 3.0, 904/6, 906, 907, 908, 910, 917 (in its regular normally aspirated or 5.5 Liter/1500 HP Turbocharged form), and 935 all used Bosch or Kugelfischer built variants of injection. The Kugelfischer system was also used by the BMW 2000/2002 Tii and some versions of the Peugeot 404/504 and Lancia Flavia. Lucas also offered a mechanical system which was used by some Maserati, Aston Martin and Triumph models between ca. 1963 and 1973. The first factory electronic fuel injection, a true multi-point system, with dual 2-bbl. throttles, was optional on 1958 Chrysler products, both Hemi and wedge engines. It was jointly engineered by Chrysler and Bendix.
A system similar to the Bosch inline mechanical pump was built by SPICA for Alfa Romeo, used on the Alfa Romeo Montreal and on US market 1750 and 2000 models from 1969 until 1981. This was specifically designed to meet the US emission requirements, and allowed Alfa to meet these requirements at no loss in performance and with a reduction in fuel consumption.
In 1957, Chevrolet introduced a mechanical fuel injection option, made by General Motors' Rochester Products division, for its 283 V8 engine. This system directed the inducted engine air across a "spoon shaped" plunger that moved in proportion to the air volume. The plunger connected to the fuel metering system which mechanically dispensed fuel to the cylinders via distribution tubes. This system was not a "pulse" or intermittent injection, but rather a constant flow system, metering fuel to all cylinders simultaneously from a central "spider" of injection lines. The fuel meter adjusted the amount of flow according to engine speed and load, and included a fuel reservoir, which was similar to a carburetor's float chamber. With its own high-pressure fuel pump driven by a cable from the distributor to the fuel meter, the system supplied the necessary pressure for injection. However, this was "port" injection, in which the injectors are located in the intake manifold, very near the intake valve. (Direct fuel injection is a fairly recent innovation for automobile engines.) The highest performance version of the fuel injected engine was rated at from . This made it among the early production engines in history to exceed 1 hp/in³ (45.5 kW/L), after Chrysler's Hemi engine and a number of others.
During the 1960s, other mechanical injection systems such as Hilborn were occasionally used on modified American V8 engines in various racing applications such as drag racing, oval racing, and road racing. These racing-derived systems were not suitable for everyday street use, having no provisions for low speed metering or even starting (fuel had to be squirted into the injector tubes while cranking the engine in order to start it). However they were a favorite in the aforementioned competition trials in which essentially wide-open throttle operation was prevalent.
The first commercial electronic fuel injection (EFI) system was Electrojector, developed by the Bendix Corporation and was to be offered by American Motors (AMC) in 1957. A special muscle car model, the Rambler Rebel, showcased AMC's new engine. The Electrojector was an option and rated at . The Rebel Owners Manual described the design and operation of the new system. Initial press information about the Bendix system in December 1956 was followed in March 1957 by a price bulletin that pegged the option at US$395, but due to supplier difficulties, fuel-injected Rebels would only be available after June 15. This was to have been the first production EFI engine, but Electrojector's teething problems meant only pre-production cars were so equipped and none were made available to the public. The EFI system in the Rambler was a far more-advanced setup than the mechanical types then appearing on the market and the engines ran fine in warm weather, but suffered hard starting in cooler temperatures.
Chrysler offered Electrojector on the 1958 300D, the D500, and the DeSoto Adventurer, arguably the first series-production cars equipped with an EFI system, but the early electronic components were not equal to the rigors of underhood service, and were too slow to keep up with the demands of "on the fly" engine control. Most vehicles originally so equipped were field-retrofitted with 4-barrel carburetors. The Electrojector patents were subsequently sold to Bosch.
Bosch developed an electronic fuel injection system, called D-Jetronic (D for Druck, German for "pressure"), which was first used on the VW 1600TL in 1967. This was a speed/density system, using engine speed and intake manifold air density to calculate "air mass" flow rate and thus fuel requirements. The system used all analog, discrete electronics, and an electro-mechanical pressure sensor. The sensor was susceptible to vibration and dirt. This system was adopted by VW, Mercedes-Benz, Porsche, Citroën, Saab, and Volvo. Lucas licensed the system for production with Jaguar.
Bosch superseded the D-Jetronic system with the K-Jetronic and L-Jetronic systems for 1974, though some cars (such as the Volvo 164) continued using D-Jetronic for the following several years. The Cadillac Seville was introduced in 1977 with an EFI system made by Bendix and modelled very closely on Bosch's D-Jetronic. L-Jetronic first appeared on the 1974 Porsche 914, and uses a mechanical airflow meter (L for Luft, German for "air") that produces a signal that is proportional to "air volume". This approach required additional sensors to measure the barometer and temperature, to ultimately calculate "air mass". L-Jetronic was widely adopted on European cars of that period, and a few Japanese models a short time later.
In 1982, Bosch introduced a sensor that directly measures the air mass flow into the engine, on their L-Jetronic system. Bosch called this LH-Jetronic (L for Luftmasse and H for Hitzdraht, German for "air mass" and "hot wire", respectively). The mass air sensor utilizes a heated platinum wire placed in the incoming air flow. The rate of the wire's cooling is proportional to the air mass flowing across the wire. Since the hot wire sensor directly measures air mass, the need for additional temperature and pressure sensors is eliminated. The LH-Jetronic system was also the first fully digital EFI system, which is now the standard approach. The advent of the digital microprocessor permitted the integration of all powertrain sub-systems into a single control module.
The ultimate combustion goal is to match each molecule of fuel with a corresponding number of molecules of oxygen so that neither has any molecules remaining after combustion in the engine and catalytic converter. Such a balanced condition is known as stoichiometry. Extensive carburetor modifications and complexities were needed to approach stoichiometric engine operation in order to comply with increasingly-strict exhaust emission regulations of the 1970s and 1980s. This increase in complexity gradually eroded and then reversed the simplicity, cost, and packaging advantages carburetors had traditionally offered.
Fuel injection appeared first on the 1955 Mercedes-Benz 300SL Gullwing, with the fuel injected directly into the combustion chambers using an engine driven pump. In the U.S., fuel injection was available in the late 1950s, such as the 1958 Chrysler products equipped with Bendix' ElectroJector, and 1957–1965 Rochester fuel injected Chevrolet Corvettes. About a decade later, more practical fuel injection systems were introduced in European-made cars. As emission regulations progressively tightened worldwide, generally led by the US state of California's especially stringent rules, automakers had to improve the precision and accuracy with which fuel was metered to the engine. Catalytic converters also became practically universal equipment.
There are three primary types of toxic emissions from an internal combustion engine: Carbon Monoxide (CO), unburnt hydrocarbons (HC), and oxides of nitrogen (NOx). CO and HC result from incomplete combustion of fuel due to insufficient oxygen in the combustion chamber. NOx, in contrast, results from excessive oxygen in the combustion chamber. The opposite causes of these pollutants makes it difficult to control all three simultaneously. Once the permissible emission levels dropped below a certain point, catalytic treatment of these three main pollutants became necessary. This required a particularly large increase in fuel metering accuracy and precision, for simultaneous catalysis of all three pollutants requires that the fuel/air mixture be held within a very narrow range of stoichiometry. The open loop fuel injection systems had already improved cylinder-to-cylinder fuel distribution and engine operation over a wide temperature range, but did not offer sufficient fuel/air mixture control to enable effective exhaust catalysis. Closed loop fuel injection systems improved the air/fuel mixture control with an exhaust gas oxygen sensor. The O2 sensor is mounted in the exhaust system upstream of the catalytic converter, and enables the engine management computer to determine and adjust the air/fuel ratio precisely and quickly.
Fuel injection was phased in through the latter '70s and '80s at an accelerating rate, with the US and German markets leading and the UK and Commonwealth markets lagging somewhat, and since the early 1990s, almost all gasoline passenger cars sold in first world markets like the United States, Canada, Europe, Japan, and Australia have come equipped with electronic fuel injection (EFI). Many motorcycles still utilize carbureted engines, though all current high-performance designs have switched to EFI.
Fuel injection systems have evolved significantly since the mid 1980s. Current systems provide an accurate, reliable and cost-effective method of metering fuel and providing maximum engine efficiency with clean exhaust emissions, which is why EFI systems have replaced carburetors in the marketplace. EFI is becoming more reliable and less expensive through widespread usage. At the same time, carburetors are becoming less available, and more expensive. Even marine applications are adopting EFI as reliability improves. Virtually all internal combustion engines, including motorcycles, off-road vehicles, and outdoor power equipment, may eventually use some form of fuel injection.
It should be noted that carburetion remains a less costly alternative where strict emission regulations and advanced vehicle diagnostic and repair infrastructure do not exist, as in developing countries. Fuel injection is gradually replacing carburetors in these nations too as they adopt emission regulations conceptually similar to those in force in Europe, Japan, Australia and North America.
Modern fuel injection schemes follow much the same setup. There is a mass airflow sensor or manifold absolute pressure sensor at the intake, typically mounted either in the air tube feeding from the air filter box to the throttle body, or mounted directly to the throttle body itself. The mass airflow sensor does exactly what its name implies; it senses the mass of the air that flows past it, giving the computer an accurate idea of how much air is entering the engine. The next component in line is the Throttle Body. The throttle body has a throttle position sensor mounted onto it, typically on the butterfly valve of the throttle body. The throttle position sensor (TPS) reports to the computer the position of the throttle butterfly valve, which the ECM uses to calculate the load upon the engine. The fuel system consists of a fuel pump (typically mounted in-tank), a fuel pressure regulator, fuel lines (composed of either high strength plastic, metal, or reinforced rubber), a fuel rail that the injectors connect to, and the fuel injector(s). There is a coolant temperature sensor that reports the engine temperature to the ECM, which the engine uses to calculate the proper fuel ratio required. In sequential fuel injection systems there is a camshaft position sensor, which the ECM uses to determine which fuel injector to fire. The last component is the oxygen sensor. After the vehicle has warmed up, it uses the signal from the oxygen sensor to perform fine tuning of the fuel trim.
The fuel injector acts as the fuel-dispensing nozzle. It injects liquid fuel directly into the engine's air stream. In almost all cases this requires an external pump. The pump and injector are only two of several components in a complete fuel injection system.
In contrast to an EFI system, a carburetor directs the induction air through a venturi, which generates a minute difference in air pressure. The minute air pressure differences both emulsify (premix fuel with air) the fuel, and then acts as the force to push the mixture from the carburetor nozzle into the induction air stream. As more air enters the engine, a greater pressure difference is generated, and more fuel is metered into the engine. A carburetor is a self-contained fuel metering system, and is cost competitive when compared to a complete EFI system.
An EFI system requires several peripheral components in addition to the injector(s), in order to duplicate all the functions of a carburetor. A point worth noting during times of fuel metering repair is that early EFI systems are prone to diagnostic ambiguity. A single carburetor replacement can accomplish what might require numerous repair attempts to identify which one of the several EFI system components is malfunctioning. Newer EFI systems since the advent of OBD II diagnostic systems, can be very easy to diagnose due to the increased ability to monitor the realtime data streams from the individual sensors. This gives the diagnosing technician realtime feedback as to the cause of the drivability concern, and can dramatically shorten the number of diagnostic steps required to ascertain the cause of failure, something which isn't as simple to do with a carburetor. On the other hand, EFI systems require little regular maintenance; a carburetor typically requires seasonal and/or altitude adjustments.
Note: These examples specifically apply to a modern EFI gasoline engine. Parallels to fuels other than gasoline can be made, but only conceptually.
The electronic fuel injector is normally closed, and opens to inject pressurized fuel as long as electricity is applied to the injector's solenoid coil. The duration of this operation, called pulse width, is proportional to the amount of fuel desired. The electric pulse may be applied in closely-controlled sequence with the valve events on each individual cylinder (in a sequential fuel injection system), or in groups of less than the total number of injectors (in a batch fire system).
Since the nature of fuel injection dispenses fuel in discrete amounts, and since the nature of the 4-stroke-cycle engine has discrete induction (air-intake) events, the ECU calculates fuel in discrete amounts. In a sequential system, the injected fuel mass is tailored for each individual induction event. Every induction event, of every cylinder, of the entire engine, is a separate fuel mass calculation, and each injector receives a unique pulse width based on that cylinder's fuel requirements.
It is necessary to know the mass of air the engine "breathes" during each induction event. This is proportional to the intake manifold's air pressure/temperature, which is proportional to throttle position. The amount of air inducted in each intake event is known as "air-charge", and this can be determined using several methods. (See MAF sensor, and MAP sensor.)
The three elemental ingredients for combustion are fuel, air and ignition. However, complete combustion can only occur if the air and fuel is present in the exact stoichiometric ratio, which allows all the carbon and hydrogen from the fuel to combine with all the oxygen in the air, with no undesirable polluting leftovers. Oxygen sensors monitor the amount of oxygen in the exhaust, and the ECU uses this information to adjust the air-to-fuel ratio in real-time.
To achieve stoichiometry, the air mass flow into the engine is measured and multiplied by the stoichiometric air/fuel ratio 14.64:1 (by weight) for gasoline. The required fuel mass that must be injected into the engine is then translated to the required pulse width for the fuel injector. The stoichiometric ratio changes as a function of the fuel; diesel, gasoline, ethanol, methanol, propane, methane (natural gas), or hydrogen.
Deviations from stoichiometry are required during non-standard operating conditions such as heavy load, or cold operation, in which case, the mixture ratio can range from 10:1 to 18:1 (for gasoline).
Pulse width is inversely related to pressure difference across the injector inlet and outlet. For example, if the fuel line pressure increases (injector inlet), or the manifold pressure decreases (injector outlet), a smaller pulse width will admit the same fuel. Fuel injectors are available in various sizes and spray characteristics as well. Compensation for these and many other factors are programmed into the ECU's software.
Note: These calculations are based on a 4-stroke-cycle, 5.0L, V-8, gasoline engine. The variables used are real data.
Injector pulsewidth typically ranges from 4 ms/engine-cycle at idle, to 35 ms per engine-cycle at wide-open throttle. The pulsewidth accuracy is approximately 0.01 ms; injectors are very precise devices.
The fuel consumption rate is 68 times greater at maximum engine output than at idle. This dynamic range of fuel flow is typical of a naturally aspirated passenger car engine. The dynamic range is greater on a supercharged or turbocharged engine. It is interesting to note that 15 gallons of gasoline will be consumed in 37 minutes if maximum output is sustained. On the other hand, this engine could continuously idle for almost 42 hours on the same 15 gallons.
In piston aircraft engines, continuous-flow fuel injection is the most common type. In contrast to automotive fuel injection systems, aircraft continuous flow fuel injection is all mechanical, requiring no electricity to operate. Two common types exist: the Bendix RSA system, and the TCM system. The Bendix system is a direct descendant of the pressure carburetor. However, instead of having a discharge valve in the barrel, it uses a flow divider mounted on top of the engine, which controls the discharge rate and evenly distributes the fuel to stainless steel injection lines which go to the intake ports of each cylinder. The TCM system is even more simple. It has no venturi, no pressure chambers, no diaphragms, and no discharge valve. It simply uses a butterfly valve for the air which is linked by a mechanical linkage to a rotary valve for the fuel. Inside the control unit is another restriction which is used to control the fuel mixture. From the control unit, fuel flows to the flow divider, then through the lines to the injectors.
All modern EFI systems utilize sequential MPFI. Some Toyotas and other Japanese cars from the 1970s to the early 1990s used an application of Bosch's multipoint L-Jetronic system manufactured under license by DENSO.
Many diesel engines feature direct injection (DI). The injection nozzle is placed inside the combustion chamber and the piston incorporates a depression (often toroidal) where initial combustion takes place. Direct injection diesel engines are generally more efficient and cleaner than indirect injection engines. Some recent gasoline engines utilize direct injection as well. This is the next step in evolution from multi-port fuel injection and offers another magnitude of emission control by eliminating the "wet" portion of the induction system. By virtue of better dispersion and homogeneity of the directly injected fuel, the cylinder and piston are cooled, thereby permitting higher compression ratios and more aggressive ignition timing, with resultant enhanced output. More precise management of the fuel injection event also enables better control of emissions. Finally, the homogenity of the fuel mixture allows for leaner air/fuel ratios, which together with more precise ignition timing can improve fuel economy. Some direct-injection systems incorporate piezo electronic injectors. With their extremely fast response time, multiple injection events can occur during each power stroke of the engine.
DFI costs more than indirect injection systems; the injectors are exposed to more heat and pressure, so more costly materials and higher-precision electronic management systems are required.