Fuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. This article focuses on fuel injection in reciprocating piston and rotary piston engines.
All diesel (compression-ignition) engines use fuel injection, and many Otto (spark-ignition) engines use fuel injection of one kind or another. Mass-produced diesel engines for passenger cars (such as the Mercedes-Benz OM 138) became available in the late 1930s and early 1940s, being the first fuel injected engines for passenger car use. In passenger car petrol engines, fuel injection was introduced in the early 1950s, and gradually gained prevalence until it had largely replaced carburetors by the early 1990s. The primary difference between carburetion and fuel injection is that fuel injection atomizes the fuel through a small nozzle under high pressure, while a carburetor relies on suction created by intake air accelerated through a Venturi tube to draw the fuel into the airstream.
The term "fuel injection" is vague and comprises various distinct systems with fundamentally different functional principles. Typically, the only thing in common all fuel injection systems have is the lack of carburetion. There are two main functional principles of mixture formation systems for internal combustion engines: internal mixture formation, and external mixture formation. A fuel injection system that uses external mixture formation is called a manifold injection system; there exist two types of manifold injection systems: multi-point injection (port injection), and single-point injection (throttle-body injection). Internal mixture formation systems can be separated into direct and indirect injection systems. There exist several different varieties of both direct and indirect injection systems; the most common internal mixture formation fuel injection system is the common-rail injection system, a direct injection system. The term electronic fuel injection refers to any fuel injection system having an engine control unit.
An ideal fuel injection system can precisely provide exactly the right amount of fuel under all engine operating conditions. This typically means a precise air-fuel-ratio (lambda) control, which allows, for instance: easy engine operation even at low engine temperatures (cold start), good adaptation to a wide range of altitudes and ambient temperatures, exactly governed engine speed (including idle and redline speeds), good fuel efficiency, and the lowest achievable exhaust emissions (because it allows emissions control devices such as a three-way catalyst to function properly).
In practice an ideal fuel injection system does not exist, but there is a huge variety of different fuel injection systems with certain advantages and disadvantages. Most of these systems were rendered obsolete by the common-rail direct injection system that is nowadays (2020) used in many passenger cars. Common-rail injection allows petrol direct injection, and is even better suited for diesel engine fuel direct injection. However, common-rail injection is a relatively complex system, which is why in some passenger cars that do not use diesel engines, a multi-point manifold injection system is used instead.
When designing a fuel injection system, a variety of factors has to be taken into consideration, including:
All fuel injection systems comprise three basic components: they have at least one fuel injector (sometimes called an injection valve), a device that creates sufficient injection pressure, and a device that meters the correct amount of fuel. These three basic components can either be separate devices (fuel injector(s), fuel distributor, fuel pump), partially combined devices (injection valve and an injection pump), or completely combined devices (unit injector). Early mechanical injection systems (except air-blast injection) typically used injection valves (with needle nozzles) in combination with a single (or more than one) relatively sophisticated helix-controlled injection pump(s) that both metered the fuel, and created the injection pressure. They were well-suited for intermittently injecting multi-point injection systems as well as all sorts of conventional direct injection systems, and chamber-injected systems. Advancements in the field of microelectronics allowed injection system manufacturers to significantly improve the accuracy of the fuel metering device. In modern engines, the fuel metering and injection valve actuation is usually done by the engine control unit. Therefore, the fuel injection pump does not have to meter the fuel or actuate the injection valves; it only needs to provide injection pressure. These modern systems are used in multi-point-injected engines, and common-rail-injected engines. Unit injection systems have made it into series production in the past, but proved to be inferior to common-rail injection.
The overview below illustrates the most common types of mixture formation systems in internal combustion engines. There are several different ways of characterising, grouping and describing fuel injection systems; the clade is based upon a differentiation between internal and external mixture formation systems.
In an engine with external mixture formation, air and fuel are mixed outside the combustion chamber, so that a premixed mixture of air and fuel is sucked into the engine. External mixture formation systems are common in petrol-fueled engines such as the Otto engine, and the Wankel engine. There exist two main external mixture formation systems in internal combustion engines: carburettors, and manifold injection. The following description focuses on the latter. Manifold injection systems can also be considered indirect injection, but this article primarily uses the term indirect injection to describe internal mixture formation systems that are not direct injection. There exist two types of manifold injection: single-point injection, and multi-point injection. They can use several different injection schemes.
Single-point injection uses one injector in a throttle body mounted similarly to a carburetor on an intake manifold. As in a carbureted induction system, the fuel is mixed with the air before the inlet of the intake manifold. Single-point injection was a relatively low-cost way for automakers to reduce exhaust emissions to comply with tightening regulations while providing better "driveability" (easy starting, smooth running, no engine stuttering) than could be obtained with a carburetor. Many of the carburetor's supporting components - such as the air filter, intake manifold, and fuel line routing - could be used with few or no changes. This postponed the redesign and tooling costs of these components. Single-point injection was used extensively on American-made passenger cars and light trucks during 1980-1995, and in some European cars in the early and mid-1990s.
Multi-point injection injects fuel into the intake ports just upstream of each cylinder's intake valve, rather than at a central point within an intake manifold. Typically, multi-point injected systems use multiple fuel injectors, but some systems such as the GM central port injection use tubes with poppet valves fed by a central injector instead of multiple injectors.
Manifold injected engines can use several injection schemes: continuous, and intermittent (simultaneous, batched, sequential, and cylinder-individual).
In a continuous injection system, fuel flows at all times from the fuel injectors, but at a variable flow rate. The most common automotive continuous injection system is the Bosch K-Jetronic, introduced in 1974, and used until the mid-1990s by various car manufacturers. Intermittent injection systems can be sequential, in which injection is timed to coincide with each cylinder's intake stroke; batched, in which fuel is injected to the cylinders in groups, without precise synchronization to any particular cylinder's intake stroke; simultaneous, in which fuel is injected at the same time to all the cylinders; or cylinder-individual, in which the engine control unit can adjust the injection for each cylinder individually.
In an engine with an internal mixture formation system, air and fuel are mixed only inside the combustion chamber. Therefore, only air is sucked into the engine during the intake stroke. The injection scheme is always intermittent (either sequential or cylinder-individual). There are two different types of internal mixture formation systems: indirect injection, and direct injection.
This article describes indirect injection as an internal mixture formation system (typical of Akroyd and Diesel engines); for the external mixture formation system that is sometimes called indirect injection (typical of Otto and Wankel engines), this article uses the term manifold injection.
In an indirect injected engine, there are two combustion chambers: a main combustion chamber, and a pre-chamber (also called an ante-chamber) that is connected to the main one. The fuel is injected only into the pre-chamber (where it begins to combust), and not directly into the main combustion chamber. Therefore, this principle is called indirect injection. There exist several slightly different indirect injection systems that have similar characteristics. All Akroyd (hot-bulb) engines, and some Diesel (compression ignition) engines use indirect injection.
Direct injection means that an engine only has a single combustion chamber, and that the fuel is injected directly into this chamber. This can be done either with a blast of air (air-blast injection), or hydraulically. The latter method is far more common in automotive engines. Typically, hydraulic direct injection systems spray the fuel into the air inside the cylinder or combustion chamber, but some systems spray the fuel against the combustion chamber walls (M-System). Hydraulic direct injection can be achieved with a conventional, helix-controlled injection pump, unit injectors, or a sophisticated common-rail injection system. The latter is the most common system in modern automotive engines. Direct injection is well-suited for a huge variety of fuels, including petrol (see petrol direct injection), and diesel fuel.
In a common rail system, the fuel from the fuel tank is supplied to the common header (called the accumulator). This fuel is then sent through tubing to the injectors, which inject it into the combustion chamber. The header has a high pressure relief valve to maintain the pressure in the header and return the excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a needle valve, operated with a solenoid. When the solenoid is not activated, the spring forces the needle valve into the nozzle passage and prevents the injection of fuel into the cylinder. The solenoid lifts the needle valve from the valve seat, and fuel under pressure is sent in the engine cylinder. Third-generation common rail diesels use piezoelectric injectors for increased precision, with fuel pressures up to 300 MPa or 44,000 lbf/in2.
In 1872, George Bailey Brayton obtained a patent on an internal combustion engine that used a pneumatic fuel injection system, also invented by Brayton: the air-blast injection. In 1894, Rudolf Diesel copied Brayton's air-blast injection system for the diesel engine, but also improved it. Most notably, Diesel increased the air-blast pressure from 4-5 kp/cm2 (390-490 kPa) to 65 kp/cm2 (6,400 kPa).
The first manifold injection system was designed by Johannes Spiel at Hallesche Maschinenfabrik in 1884. In the early 1890s, Herbert Akroyd Stuart developed an indirect fuel injection system using a 'jerk pump' to meter out fuel oil at high pressure to an injector. This system was used on the Akroyd engine and was adapted and improved by Bosch and Clessie Cummins for use on diesel engines.
In 1898, Deutz AG started series production of stationary four-stroke Otto engines with manifold injection. Eight years later, Grade equipped their two-stroke engines with manifold injection, and both Léon Levavasseur's Antoinette 8V (the world's first V8 engine of any sort, patented by Levavasseur in 1902), and Wright aircraft engines were fitted with manifold injection as well. The first engine with petrol direct injection was a two-stroke aircraft engine designed by Otto Mader in 1916.
Another early use of petrol direct injection was on the Hesselman engine invented by Swedish engineer Jonas Hesselman in 1925. Hesselman engines use the stratified charge principle; fuel is injected towards the end of the compression stroke, then ignited with a spark plug. They can run on a huge variety of fuels.
The invention of the pre-combustion chamber injection by Prosper l'Orange helped Diesel engine manufacturers to overcome the problems of air-blast injection, and allowed designing small engines for automotive use from the 1920s onwards. In 1924, MAN presented the first direct-injected Diesel engine for lorries.
Direct petrol injection was used in notable World War II aero-engines such as the Junkers Jumo 210, the Daimler-Benz DB 601, the BMW 801, the Shvetsov ASh-82FN (M-82FN). German direct injection petrol engines used injection systems developed by Bosch, Deckel, Junkers and l'Orange from their diesel injection systems. Later versions of the Rolls-Royce Merlin and Wright R-3350 used single point injection, at the time called "Pressure Carburettor". Due to the wartime relationship between Germany and Japan, Mitsubishi also had two radial aircraft engines using petrol direct injection, the Mitsubishi Kinsei and the Mitsubishi Kasei.
The first automotive direct injection system used to run on petrol was developed by Bosch, and was introduced by Goliath for their Goliath GP700, and Gutbrod for their Superior in 1952. This was basically a specially lubricated high-pressure diesel direct-injection pump of the type that is governed by the vacuum behind an intake throttle valve. The 1954 Mercedes-Benz W196 Formula 1 racing car engine used Bosch direct injection derived from wartime aircraft engines. Following this racetrack success, the 1955 Mercedes-Benz 300SL, became the first passenger car with a four-stroke Otto engine that used direct injection. Later, more mainstream applications of fuel injection favored the less-expensive manifold injection.
Throughout the 1950s, several manufacturers introduced their manifold injection systems for Otto engines, including General Motors' Rochester Products Division, Bosch, and Lucas Industries. During the 1960s, additional manifold injection systems such as the Hilborn, Kugelfischer, and SPICA systems were introduced.
The first commercial electronicially controlled manifold injection system was the Electrojector developed by Bendix and was offered by American Motors Corporation (AMC) in 1957. Initial problems with the Electrojector meant only pre-production cars had it installed so very few cars were sold and none were made available to the public. The EFI system in the Rambler worked well in warm weather, but was difficult to start in cooler temperatures.
Chrysler offered Electrojector on the 1958 Chrysler 300D, DeSoto Adventurer, Dodge D-500, and Plymouth Fury, arguably the first series-production cars equipped with an EFI system. The Electrojector patents were subsequently sold to Bosch, who developed the Electrojector into the Bosch D-Jetronic. The D in D-Jetronic stands for Druckfühlergesteuert, German for "pressure-sensor controlled"). The D-Jetronic was first used on the VW 1600TL/E 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.
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 L-Jetronic uses a mechanical airflow meter (L for Luft, German for "air") that produces a signal that is proportional to volume flow rate. This approach required additional sensors to measure the atmospheric pressure and temperature, to calculate mass flow rate. L-Jetronic was widely adopted on European cars of that period, and a few Japanese models a short time later.
The first digital engine management system (engine control unit) was the Bosch Motronic introduced in 1979. In 1980, Motorola (now NXP Semiconductors) introduced their digital ECU EEC-III. The EEC-III a single-point injection system.
Manifold injection was phased in through the latter 1970s and 80s at an accelerating rate, with the German, French, and U.S. markets leading and the UK and Commonwealth markets lagging somewhat. Since the early 1990s, almost all petrol passenger cars sold in first world markets are equipped with electronic manifold injection. The carburetor remains in use in developing countries where vehicle emissions are unregulated and diagnostic and repair infrastructure is sparse. Fuel injection systems are 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.
In 1995, Mitsubishi presented the first common-rail petrol direct injection system for passenger cars. It was introduced in 1997. Subsequently, common-rail direct injection was also introduced in passenger car diesel engines, with the Fiat 1.9 JTD being the first mass market engine. In the early 2000s, several car manufacturers attempted to use stratified charge concepts in their direct injection petrol engines to reduce fuel consumption. However, the fuel savings proved to be almost unnoticeable and disproportionate to the increased complexity of the exhaust gas treatment systems. Therefore, almost all car manufacturers have switched to a conventional homogeneous mixture in their direct injected petrol engines since the mid 2010s. In the early 2020s, some car manufacturers have still been using manifold injection, especially in economy cars, but also some high performance cars. Ever since 1997, car manufacturers have been using common-rail direct injection for their diesel engines. Only Volkswagen used the Pumpe-Düse system throughout the early 2000s, but they have also been using common-rail direct injection since 2010.