Through the 1980's and 90's In trying to keep up with emissions and fuel efficiency laws, the fuel system used in modern cars has changed a lot. The 1990 Subaru Justy was the last car sold in the U.S. to have a carburetor;
Fuel injection itself for petrol engines has been around since the 1950s, firstly mechanical and later electronic fuel injection was used widely on European cars starting in the early 1980's. Now, all cars sold have fuel injection systems. Most European systems are based upon the Bosche injection systems.
For most of the existence of the internal combustion engine, the carburetor has been the device that supplied fuel to the engine; and on many other machines, such as lawnmowers and chainsaws, it still is. But as the car evolved, the carburetor got more and more complicated, trying to handle all of the operating requirements. For instance, to handle some of these tasks, carburetors had five different circuits:
- Main Circuit - provides enough fuel for fuel-efficient steady speed cruising
- Idle Circuit - provides fuel to keep the engine idling
- Accelerator Pump - provides an extra burst of fuel when the accelerator pedal is first depressed
- Power Enrichment Circuit - provides extra fuel when the engine is under heavy load
- Choke - provides extra fuel when the engine is cold
(If you think Fuel injection is complicated then obviously you have never had to work on a set of 40 DCOE Weber carbs.)
In order to meet stricter emissions requirements, catalytic converters were introduced. Very careful control of the air-to-fuel ratio is required for the catalytic converter to be effective. Oxygen sensors monitor the amount of oxygen in the exhaust, and the engine control unit (ECU) uses this information to adjust the air-to-fuel ratio in real-time. This is called closed loop control; it was not feasible to achieve this control with carburetors. There was a brief period of electrically controlled carburetors before fuel injection systems took over, but these electrical carbs were even more complicated than purely mechanical ones.
At first, carburetors were replaced with throttle body fuel injection systems (also known as single point or central fuel injection systems) that incorporated electrically controlled fuel-injector valves into the throttle body. These were almost a bolt-in replacement for the carburetor, so the automakers didn't have to make any drastic changes to their engine designs.
Gradually, as new engines were designed, throttle body fuel injection was replaced by multi-port fuel injection (also known as port, multi-point or sequential fuel injection). These systems have a fuel injector for each cylinder, usually located so that they spray right at the intake valve. These systems provide more accurate fuel metering and quicker response.
The Accelerator in your car is connected to the throttle valve; this is the
valve that regulates how much air enters the engine. So the Accelerator is really
air pedal.
 Partially open throttle valve |
When you step on the gas pedal, the throttle valve opens up more, letting in more air. The engine control unit (ECU, the computer that controls all of the electronic components on your engine) "sees" the throttle valve open, and increases the fuel rate in anticipation of more air entering the engine. It is important to increase the fuel rate as soon as the throttle valve opens; otherwise, when the Pedal is first pressed, there may be a hesitation as some air reaches the cylinders without enough fuel in it.
Sensors monitor the mass of air entering the engine, as well as the amount of oxygen in the exhaust. The ECU uses this information to fine tune the fuel delivery so that the air-to-fuel ratio is just right.
A fuel injector is nothing but an electronically controlled valve. It is supplied with pressurized fuel by the fuel-pump in your car, and it is capable of opening and closing many times per second.
If your car runs to 6000 revolution per minute, then in a four stroke the
injector will need to function 3000 times per minute, 50 per second
 Inside a fuel injector |
When the injector is energized, an electro-magnet moves a plunger that opens the valve, allowing the pressurized fuel to squirt out through a tiny nozzle. The nozzle is designed to atomize the fuel to make as fine a mist as possible so that it can burn easily.
 A fuel injector Atomizing fuel |
The amount of fuel supplied to the engine is determined by the amount of time the fuel injector stays open. This is called the pulse width, and it is controlled by the engine control unit.
 Fuel injectors mounted in the intake manifold |
The injectors are mounted in the intake manifold so that they spray fuel directly at the intake valves. A pipe called the fuel rail supplies pressurized fuel to all of the injectors.
 Injectors the fuel rail is the pipe on the far right. |
In order to provide the correct amount of fuel for every operating condition, the engine control unit (ECU) has to monitor a huge number of input sensors. Here are just a few:
There are two main types of control for multi-port systems. The fuel injectors can all open at the same time; or each one can open just before the intake valve for its cylinder opens (this is called sequential multi-port fuel injection).
The advantage of sequential fuel injection is that if the driver makes a sudden change, the system can respond more quickly because from the time the change is made, it only has to wait only until the next intake valve opens, instead of for the next complete revolution of the engine.
BMW's Modern Straight Six, not a Carb in sight
The algorithms that control the engine are quite complicated. The software has to allow the car to satisfy emissions requirements , fuel economy requirements and protect engines against abuse. And there are dozens of other requirements to meet as well.
The engine control unit uses a formula and a large number of lookup tables to determine the pulse width for given operating conditions. The equation will be a series of many factors multiplied by each other. Many of these factors will come from lookup tables. We'll go through a simplified calculation of the fuel injector pulse width. In this example, our equation will only have three factors, whereas a real control system might have a hundred or more.
In order to calculate the pulse width, the ECU first looks up the base pulse width in a lookup table. This is a function of engine speed (RPM) and load (which can be calculated from manifold absolute pressure). Let's say the engine speed is 2000 RPM and load is 4. We find the number at the intersection of 2000 and 4, which is 8 milliseconds.
| RPM | Load | ||||
| 1 | 2 | 3 | 4 | 5 | |
| 1000 | 1 | 2 | 3 | 4 | 5 |
| 2000 | 2 | 4 | 6 | 8 | 10 |
| 3000 | 3 | 6 | 9 | 12 | 15 |
| 4000 | 4 | 8 | 12 | 16 | 20 |
In the next examples, A and B are parameters that come from sensors. Let's say that A is coolant temperature and B is oxygen level. If coolant temperature equals 100 and oxygen level equals 3, the lookup tables tell us that Factor A = 0.8 and Factor B = 1.0.
| A | Factor A | B | Factor B | |
| 0 | 1.2 | 0 | 1.0 | |
| 25 | 1.1 | 1 | 1.0 | |
| 50 | 1.0 | 2 | 1.0 | |
| 75 | 0.9 | 3 | 1.0 | |
| 100 | 0.8 | 4 | 0.75 |
So, since we know that base pulse width is a function of load and RPM, and that pulse width = (base pulse width) x (factor A) x (factor B), the overall pulse width in our example equals:
From this example, you can see how the control system makes adjustments. With parameter B as the level of oxygen in the exhaust, the lookup table for B is the point at which there is (according to engine designers) too much oxygen in the exhaust; and accordingly, the ECU cuts back on the fuel.
Real control systems may have more than 100 parameters, each with its own lookup table. Some of the parameters even change over time in order to compensate for changes in the performance of engine components like the catalytic converter. And depending on the engine speed, the ECU may have to do these calculations over a hundred times per second.
Performance chips are made by aftermarket companies, and are used to boost engine power. There is a chip in the ECU that holds all of the lookup tables; the performance chip replaces this chip. The tables in the performance chip will contain values that result in higher fuel rates during certain driving conditions. For instance, they may supply more fuel at full throttle at every engine speed. They may also change the spark timing (there are lookup tables for that, too). Since the performance-chip makers are not as concerned with issues like reliability, mileage and emissions controls as the Original car makers are, they use more aggressive settings in the fuel maps of their performance chips.