fuel injection

For gasoline engines, carburetors were the predominant method to meter fuel prior to the widespread use of electronic fuel injection (EFI). However, a wide variety of injection systems have existed since the earliest usage of the internal combustion engine.

One major distinction between carburetors and fuel injection is that fuel injection atomizes the fuel by forcibly pumping it through a small nozzle under high pressure, whereas a carburetor relies on the vacuum created by intake air rushing through it to add the fuel to the airstream.

Another notable difference is that a carburetor performs several important functions in one single component: it measures engine load, calculates the amount of fuel needed, and adds the required fuel to the airstream. With fuel injection, these functions are performed by separate subsystems and components. This means that each subsystem can be specialized and optimized for its particular role, which brings a number of important performance benefits compared to the compromise solution offered by carburetors.

The carburetor modifications and complications needed to comply with increasingly-strict exhaust emission regulations of the 1970s and 1980s gradually eroded and then reversed the simplicity, cost and packaging advantages carburetors had traditionally offered. Fuel injection appeared first on European-made cars in the late 1960s and early '70s. It was phased in through the latter '70s and '80s at an accelerating rate, with the US and Japanese 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, Europe, Japan and Australia have come equipped with electronic fuel injection (EFI).

Objectives

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:

• power output
• fuel efficiency
• emissions performance
• ability to accommodate alternative fuels
• durability
• reliability
• driveability and smooth operation
• initial cost
• maintenance cost
• diagnostic capability
• range of environmental operation

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 in a competitive manner. The modern digital EFI system is far more capable at optimizing these competing objectives than a carburetor.

Benefits

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 EFI systems meter fuel with great precision, and when used in conjunction with an Exhaust Gas Oxygen Sensor (EGO sensor), they are also very accurate. The advent of digital closed loop fuel control, based on feedback from an EGO sensor, permit EFI to significantly outperform a carburetor. The two fundamental improvements are:

1. Reduced response time to rapidly changing inputs, e.g., rapid throttle movements.
2. Deliver an accurate and equal mass of fuel to each cylinder of the engine, dramatically improving the cylinder-to-cylinder distribution of the engine.

These two features result in the following performance benefits:

• Exhaust Emissions
  • Significantly reduced "engine out" or "feedgas" emissions (the chemical products of engine combustion).
  • A reduction in the final tailpipe emissions (≈ 99.9%) resulting from the ability to accurately condition the "feedgas" in a manner that maximizes the effectiveness of the catalytic converter.

• General Engine Operation
  • Smoother function during quick throttle transitions.
  • Engine starting.
  • Extreme weather operation.
  • Reduced maintenance interval.
  • A slight increase in fuel economy.

• Power Output
  • Fuel injection often produces more power than an equivalent carbureted engine. However, fuel injection alone does not increase maximum engine output. Increased airflow is necessary to permit oxidizing more fuel, which generates more heat, which in turn generates more output. The combustion process converts the fuel's chemical energy into heat energy, whether the fuel arrived via EFI or a carburetor is not significant. Airflow is often improved with fuel injectors, which are much smaller than a carburetor. Their smaller size permits 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 approximately equal distance from each of the engine's cylinders. These design constraints generally compromise airflow into the engine.
  • A carburetor relies on a drag inducing venturi in order to create a local air pressure difference, which forces the fuel into the air stream. The flow loss caused by the venturi is small in comparison to other flow losses in the induction system. In a well-designed carbureted induction system, the venturi in and of itself is not a significant airflow restriction.
  • Fuel injection is more likely to increase efficiency than power. When cylinder-to-cylinder fuel distribution is improved (common with EFI), less fuel is required to generate the same power output. Engine efficiency is known as the BSFC, or brake specific fuel consumption. When cylinder-to-cylinder distribution is less than ideal (and it always is under one condition or another, and worse on carburetor systems), more fuel than necessary is metered to the rich cylinders in order to provide sufficient fuel to the lean cylinders. Power output is asymmetrical with respect to air/fuel ratio. In other words, burning extra fuel in the rich cylinders does not reduce power nearly as quickly as burning too little fuel in the lean cylinders. The standard fuel metering compromise is to run the rich cylinders "even richer" of the optimal air/fuel ratio, in order to provide enough fuel to the leaner cylinders. The net power output improves with all the cylinders making maximum power. An analogy is that of painting a wall. One coat of paint may not cover very well. The second coat dramatically improves the appearance of the poorly covered areas, but some extra paint is consumed on areas that were already well covered.
  • Deviations from perfect air/fuel distribution, however subtle, significantly impact emissions, by forfeiting combustion events at the chemically ideal, stoichiometric air/fuel ratio. Grosser distribution problems eventually begin to negatively impact efficiency, and the grossest distribution issues finally affect power. The hierarchy of negative functional impact with regard to increasingly poorer air/fuel distribution is: emissions, efficiency, and power.

There are other benefits associated with fuel injection, such as better atomization of the fuel in the intake (constant-choke carburetors have poor atomization at low air speeds, necessitating modifications such as sequential twin-barrel designs) and better breathing due to the elimination of the carburetor's venturi.

Injection systems have evolved significantly since the mid 1980s. Current EFI systems provide an accurate and cost effective method of metering fuel. The emission and subjective performance characteristics have steadily improved with the advent of modern digital controls, 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. If this trend continues, it is conceivable that virtually all internal combustion engines, including garden equipment and snow throwers, will eventually use EFI.

It should be noted that a carburetor's fuel metering system is a less expensive alternative when strict emission regulations are not a requirement, as is the case in developing countries. EFI will undoubtedly replace carburetors in these nations too as they adopt emission regulations similar to Europe, Japan and North America.

Regulatory motivation

Throughout the 1950s and 1960s, various federal, state and local governments conducted studies into the numerous sources of air pollution. These studies ultimately attributed a significant portion of air pollution to the automobile, and concluded air pollution is not bounded by local political boundaries. At that time, the primary source of emission regulations was legislated at the local level. The ineffective scope of local regulations was gradually superseded with more strategically comprehensive state and federal regulations. By 1967 the state of California (Governor Reagan), created the California Air Resources Board, and in 1970, the U.S. Environmental Protection Agency was formed. Both agencies now create and enforce emission regulations for automobiles, as well as for many other sources.

Additionally, similar studies and regulations were simultaneously developed in Europe and Japan.

The primary source of internal combustion engine emissions is the incomplete combustion of a minuscule fraction of the total fuel consumed. This is due to having insufficient oxygen to burn all the fuel. The unburned portion of fuel is so small, the lost energy is trivial to fuel efficiency, and therefore commercially insignificant to the final customer. Auto manufacturers were eventually motivated by emission regulations to address this issue.

The modern EFI system evolved to gain deliberate control of the small fraction of unburned fuel. The ultimate combustion goal is to match each molecule(s) of fuel with a corresponding molecule(s) of oxygen so that neither has any molecules remaining after combustion - (see stoichiometry). This is a gross oversimplification of complex combustion chemistry that occurs in a difficult to manage environment. However, it accurately describes the magnitude of the fuel metering task, as well as the precision of a modern EFI system.

Basic function

The process of determining the amount of fuel, and its delivery into the engine, are known as fuel metering. Early injection systems used mechanical methods to meter fuel (non electronic, or mechanical fuel injection). Modern systems are nearly all electronic, and use an electronic solenoid (the injector) to inject the fuel. An electronic engine control unit calculates the mass of fuel to inject.

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 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. On the other hand, EFI systems require little regular maintenance; a carburetor typically requires seasonal and/or altitude adjustments.

Type of fuel

The calibration, and often the design, of a fuel injection system differs depending on the type of fuel: propane (LPG), gasoline, ethanol, methanol, methane (natural gas), hydrogen or diesel. The vast majority of fuel injection systems are for gasoline or diesel applications, and in the past, their components and designs were quite different. With the advent of "electronic" fuel injection, the diesel and gasoline hardware have grown quite similar. EFI's programmable software has permitted common hardware to be used across some of the fuels.

• Diesel Fuel
  • At one time, nearly all diesel engines used high-pressure "mechanical injection", i.e., not "electronic injection".
  • Diesels are rapidly adopting EFI, which is based on an electronic fuel injector similar in basic construction to a modern gasoline injector, although utilizing considerably higher injection pressures.
• Gasoline Fuel
  • Prior to EFI, it was extremely rare for a gasoline engine to be equipped with fuel injection. If it was, it was most likely a low-pressure mechanical system of relatively "immature" technology. These early systems were generally used on exotic performance vehicles, such as the early V8 powered Corvettes, or for racing.
  • Robert Bosch GmbH, and Bendix introduced the first electronic injection systems starting in the 1950s, and they were quite dissimilar to today's EFI. (#Evolution)
• Alternative Fuels (propane (LPG), ethanol, methanol, methane (natural gas), hydrogen)
  • The basic components of a gasoline EFI system can also be used with alternative fuels, with appropriate modification. Unique fuel metering values (the calibration contained within the software instructions) are required to accommodate each type of fuel.
  • "Flexible fuel vehicles" are vehicles that are capable of operating on both gasoline and alcohol (usually ethanol). These vehicles automatically determine the blend ratio of the two fuels present in the fuel tank and adjust the injector calculations "on the fly". Flexible fuel vehicles have a single fuel tank where a blend of both fuels can coexist.
  • "Bi-fuel" vehicles also operate on two types of fuel, but since the fuels are not functionally compatible with each other, they are stored in separate tanks, and the engine burns only one fuel at a time.

Detailed function

Note: The following examples specifically apply to a modern EFI gasoline engine. Parallels to fuels other than gasoline can be made, but only conceptually.

Typical EFI components

• Injectors
• Fuel Pump
• Fuel Pressure Regulator
• ECM - Engine Control Module; includes a digital computer and circuitry to communicate with sensors and control outputs.
• Wiring Harness
• Various Sensors (Some of the sensors required are listed here.)

• Crank/Cam Position: Hall effect sensor
• Airflow: MAF sensor, sometimes this is inferred with a MAP sensor
• Exhaust Gas Oxygen: O2 Sensor, Oxygen sensor, EGO sensor, UEGO sensor

Functional description

A contemporary EFI system comprises a digital computer "engine control module" (ECM) and a number of sensors to measure the engine's operating conditions. The ECM interprets these conditions in order to calculate the amount of fuel, among numerous other tasks. The desired "fuel flow rate" depends on several conditions, with the engine's "air flow rate" being the fundamental factor.

The electronic fuel injector is normally closed and opens to flow fuel as long as an electric pulse is applied to the injector. The pulse's duration (pulsewidth) is proportional to the amount of fuel desired. The pulse is applied once per engine cycle, which permits pressurized fuel to flow from the fuel supply line, through the open injector, into the engine's air intake, usually just ahead of the intake valve.

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 ECM calculates fuel in discrete amounts. The injected fuel mass is tailored for each individual induction event. In other words, every induction event, of every cylinder, of the entire engine, is a separate fuel mass calculation, and each injector receives a unique pulsewidth 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 one of several methods, but this is beyond the scope of this topic. (See MAF sensor, or MAP sensor.)

Note: The right pedal is not the gas pedal; it is the air pedal. The throttle pedal determines the air, and in turn, the air mass determines the fuel mass. The same is true for carburetors, only carburetors were volume, not mass based devices. With some recent systems, the right pedal isn't even an "air pedal"... it has evolved to a "power demand pedal" - it isn't connected to the throttle at all, it signals the CPU how far the driver has depressed the pedal, and the CPU determines how far to open the throttle using an electric motor. This has many benefits some of which include: controlling emissions during transients, cruise control, traction control, engine start/cranking, driveline clunk, idle speed control, air conditioning load compensation, etc.

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.

To achieve stoichiometry, the air mass flow into the engine is measured and combined with the fact that the stoichiometric air/fuel ratio is 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.

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).

Note: The stoichiometric ratio changes as a function of the fuel; diesel, gasoline, ethanol, methanol, propane, methane (natural gas), or hydrogen.

Additionally, final pulsewidth 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 pulsewidth will meter 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 ECM's software.

In summary, the vehicle operator opens the engine's throttle (right pedal), atmospheric pressure forces air into the engine past sensors that indicate air mass flow. The ECM interprets these signals from the sensors, calculates the desired air/fuel ratio, and then outputs a pulsewidth providing the exact mass of fuel for optimal combustion. This process is repeated every time an intake valve opens.

The modern EFI system treats each injection as a discrete event, which when all strung together, perform one, smooth, seamless experience. An oversimplified analogy is that it is not unlike a motion picture that appears to move from a series of individual images.

Sample pulsewidth calculations

Note: These calculations are based on a 4-stroke-cycle, 5.0L, V-8, gasoline engine. The variables used are real data.

Calculate injector pulsewidth from airflow

First the CPU determines the air mass flow rate from the sensors - lb-air/min. (The various methods to determine airflow are beyond the scope of this topic. See MAF sensor, or MAP sensor.)

• (lb-air/min) × (min/rev) × (rev/4-intake-stroke) = (lb-air/intake-stroke) = (air-charge)

- min/rev is the reciprocal of engine speed (RPM) – minutes cancel.

- rev/4-intake-stroke for an 8 cylinder 4-stroke-cycle engine.

• (lb-air/intake-stroke) × (fuel/air) = (lb-fuel/intake-stroke)

- fuel/air is the desired mixture ratio, usually stoichiometric, but often different depending on operating conditions.

• (lb-fuel/intake-stroke) × (1/injector-size) = (pulsewidth/intake-stroke)

- injector-size is the flow capacity of the injector, which in this example is 24-lbs/hour if the fuel pressure across the injector is 40 psi.

Combining the above three terms . . .

• (lbs-air/min) × (min/rev) × (rev/4-intake-stroke) × (fuel/air) × (1/injector-size) = (pulsewidth/intake-stroke)

Substituting real variables for the 5.0L engine at idle.

• (0.55 lb-air/min) × (min/700 rev) × (rev/4-intake-stroke) × (1/14.64) × (h/24-lb) × (3,600,000 ms/h) = (2.0 ms/intake-stroke)

Substituting real variables for the 5.0 L engine at maximum power.

• (28 lb-air/min) × (min/5500 rev) × (rev/4-intake-stroke) × (1/11.00) × (h/24-lb) × (3,600,000 ms/h) = (17.3 ms/intake-stroke)

Injector pulsewidth typically ranges from 2 ms/engine-cycle at idle, to 20 ms/engine-cycle at wide-open throttle. The pulsewidth accuracy is approximately 0.01 ms; injectors are very precise devices.

Calculate fuel-flow rate from pulsewidth

• (Fuel flow rate) ≈ (pulsewidth) × (engine speed) × (number of fuel injectors)

Looking at it another way:

• (Fuel flow rate) ≈ (throttle position) × (rpm) × (cylinders)

Looking at it another way:

• (Fuel flow rate) ≈ (air-charge) × (fuel/air) × (rpm) × (cylinders)

Substituting real variables for the 5.0 L engine at idle.

• (Fuel flow rate) = (2.0 ms/intake-stroke) × (hour/3,600,000 ms) × (24 lb-fuel/hour) × (4-intake-stroke/rev) × (700 rev/min) × (60 min/h) = (2.24 lb/h)

Substituting real variables for the 5.0L engine at maximum power.

• (Fuel flow rate) = (17.3 ms/intake-stroke) × (hour/3,600,000-ms) × (24 lb/h fuel) × (4-intake-stroke/rev) × (5500-rev/min) × (60-min/hour) = (152 lb/h)

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.

Various injection schemes

Throttle body injection

Throttle-body injection (called TBI by General Motors and CFI by Ford) was introduced in the mid 1980s as a transition technology toward individual port injection. The TBI system injects fuel at the throttle body (the same location where a carburetor introduced fuel). The induction mixture passes through the intake runners like a carburetor system. The justification for the TBI/CFI phase was low cost. Many of the carburetor's supporting components could be reused such as the air cleaner, intake manifold and fuel line routing. This postponed the redesign and tooling costs of these components. Most of these components were later redesigned for the next phase of fuel injection's evolution, which is individual port injection, commonly known as EFI. TBI was used briefly on passenger cars during the mid '80s, and by GM on heavy duty trucks all the way through OBD-I (ending in 1995).

Continuous injection

Bosch's K-Jetronic (or CIS, though that abbreviation also refers to the Constant Idle Speed subsystem used in many K-Jetronic installations) was introduced in 1974. In this system, fuel sprays constantly from the injectors, rather than being pulsed in time with the engine's intake strokes. Gasoline is pumped from the fuel tank to a large control valve called a fuel distributor, which separates the single fuel supply pipe from the tank into smaller pipes, one for each injector. The fuel distributor is mounted atop a control vane through which all intake air must pass, and the system works by varying fuel volume supplied to the injectors based on the angle of the air vane, which in turn is determined by the volume flowrate of air past the vane. The injectors are simple spring-loaded check valves with nozzles; once fuel system pressure becomes high enough to overcome the counterspring, the injectors begin spraying. K-Jetronic was used for many years between 1974 and the mid 1990s by Mercedes Benz, Volkswagen, Porche, Audi, Saab, and Volvo. There was also a variant of the system called KE-Jetronic that used an oxygen sensor to fine-tune the mixture. Some Toyota models and other Japanese cars from the 1970's to the early 1990's used a licensed version of the L-Jetronic system made by DENSO.

Central port injection (CPI)

General Motors developed an "in-between" technique called "central port injection" (CPI) or "central port fuel injection" (CPFI). It uses tubes from a central injector to spray fuel at each intake port rather than the central throttle-body. However, fuel is continuously injected to all ports simultaneously, which is less than optimal.

Multi-point fuel injection

Multi-point fuel injection injects fuel into individual cylinders, rather than at a central point within an intake manifold. MPFI 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 synchronisation to any particular cylinder's intake stroke, or Simultaneous, in which fuel is injected at the same time to all the cylinders.

Direct injection

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. See also High-pressure Direct Injection (HDi) .

Some recent petrol 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. See also: Gasoline Direct Injection

Evolution

Pre-emission era

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.

Fuel injection has been used commercially in diesel engines since the mid 1920s. The concept was adapted for use in petrol-powered aircraft during World War II, and direct injection was employed in some notable designs like the Daimler-Benz DB 603 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.

In 1957, Chevrolet introduced a mechanical fuel injection option, made by General Motors' Rochester division, for its 283 V8 engine. This system directed the inducted engine air across a "spoon shaped" plunger, which 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 engine produced 283 hp (211 kW) from 283 in³ (4.6 L), making it one of the first production engines in history to exceed 1 hp/in³ (45.5 kW/L), after Chrysler's Hemi engine and a number of others. In another approach, Mercedes' used six individual plungers to feed fuel to each of the six cylinders.

During the 1960's, 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.

One of the first electronic fuel injection system was Electrojector, developed by the Bendix Corporation and introduced on the 1958 DeSoto Adventurer, arguably the first production (throttle-body) EFI system. The patents were subsequently sold to Bosch.

Bosch developed an electronic fuel injection system, called D-Jetronic (D for Druck, the German word 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 L-Jetronic and K-Jetronic systems for 1974, though some cars (such as the Volvo 164) continued using D-Jetronic for the following several years, and General Motors installed a very close copy of D-Jetronic on Cadillacs starting in 1977. L-Jetronic first appeared on the 1974 Porsche 914, and uses a mechanical airflow meter (L for Luft, German for air) which produces a signal that is porportional to "air volume". This approach required additional sensors to measure the barometer and temperature, to utlitmately calculate "air mass". L-Jetronic was widely adopted on European cars of that period, and a few Japanese models a short time later.

Post emission era

In 1975, California's emissions regulations (the most stringent in the world) required manufacturers to dramatically reduce tailpipe emissions. The only feasible technology of that era that enabled auto manufacturers to meet the new regulations was the catalytic converter. GM had only recently invented the automotive exhaust catalyst, and automakers rushed the new technology into production. A catalyst promotes a reaction without itself becoming consumed in the reaction. In this case, an oxidation catalyst was designed into the vehicle's exhaust system to promote reactions of the exhaust constituents in the presence of heat. When hot products of combustion, such as unburned hydrocarbons and carbon monoxide, are exposed to the catalyst material (platinum and/or palladium), the exhaust compounds are nearly all oxidized into water and carbon dioxide.

Stricter legislation to further limit a family of compounds called oxides of nitrogen occurred in 1980. This required a reduction catalyst (rhodium) to reduce the various nitrogen oxides into free nitrogen and oxygen. The addition of a "reducing" catalyst, along with the oxidation catalyst, is an approach called a "3-way" catalyst system. The "3" comes from the ability to dramatically reduce all three families of regulated compounds addressed in the EPA "Clean Air Act."

The reduction catalyst is placed upstream of the oxidation catalyst, usually in the same housing. The reduction process liberates oxygen from the NOx compounds, and this oxygen is then used in the downstream catalyst to oxidize unburned hydrocarbons and carbon monoxide.

In order to take maximum advantage of a 3-way catalyst, excellent air/fuel ratio control is essential. EFI systems improved fuel control in two major stages.

• Open loop EFI systems improved cylinder-to-cylinder fuel distribution, but generally had poorer air/fuel ratio control than a carburetor due to manufacturing tolerance issues.

• Closed loop EFI systems improved the air/fuel ratio control with an Exhaust Gas Oxygen Sensor (EGO sensor). The EGO sensor is mounted in the exhaust system upstream of the catalyst. It detects excess oxygen in the exhaust stream. Oxygen, or the lack of it, indicates whether the air/fuel is lean or rich of the stoichiometric ratio. The EGO sensor is also known as a Lambda-Sond sensor or O₂ sensor.

Combining all three features,

• cylinder-to-cylinder fuel distribution
• closed loop air/fuel ratio control
• 3-way catalytic converter

current exhaust emissions are now less than 0.1% of their pre-regulated level.

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 Luft, or air, and H for Heiße-leitung, or hot-wire). 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 "all 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. Full exploitation of the digital revolution has further improved EFI air/fuel ratio control, as well as many other automotive control systems unrelated to the engine.

See also

• High Diesel Injection (HDi).

External links

• History of the D Jetronic system
• Fuel Injection theory
• How Fuel Injection Systems Work
• The role of spray technology in fuel injection