What is Throttle Body Injection Tbi
Throttle Body Injection is the lovechild of carburetion and multi-point injection.
In all seriousness, Throttle Body Injection enjoyed its heyday in the 1980s and 1990s when it was used on domestic cars and light trucks. Throttle Body Injection (TBI) is a method of gasoline delivery that is simpler than multi-point fuel injection and is more responsive than carburetion. In this respect, it is a hybrid system, incorporating the efficiency of multi-point fuel injection and the simplicity of carburetion but besting none of them.
Fuel injection works by spraying a measured dose of pressurized fuel into the air entering a cylinder. This is a technological advancement over carburetion, which uses gravity in conjunction with what is known as the "venturi effect" to introduce fuel into the incoming air. Early fuel injection systems were mechanical and prone to leakage and other maladies. These early systems made a brief appearance on American production cars in the 1950s. Electronic fuel injection systems solved the shortcomings of mechanical fuel injection systems but were prohibitively expensive when first developed. Volkswagen introduced the first successful (and affordable) electronic fuel injection system on select motors in the late sixties. Both Throttle Body Injection and multi-point injection systems are electronic. Electronic fuel injection systems use a variety of sensors to determine the amount and timing of fuel delivery. These sensors measure throttle position, air temperature, air pressure, crankshaft position, and other factors. All modern fuel injection systems use positive feedback via an oxygen sensor located in the exhaust system to determine the optimum amount of fuel to deliver (by analyzing the amount of oxygen in the exhaust gases) and uses the information from this sensor accordingly to make continuous adjustments in fuel delivery.
TBI normally uses a single large injector set in a throttle body, hence, "Throttle Body Injection." The injector is mounted above the throttle valve and the unit itself is mounted in a conventional "downdraft" position. Multi-point fuel injection systems normally use one injector per cylinder (in some cases, one injector per pair of cylinders) and are thus more complex, however, the upside is that it delivers fuel more efficiently. TBI has only one point of injection. TBI has less components (less injectors, no fuel rail) but its design is such that it is impossible to deliver a completely accurate amount of fuel to individual cylinders - there will always be a small amount of vaporized fuel in the intake manifold skewing even the best-designed systems' attempts to precisely meter fuel.
Still, TBI is more efficient than carburetion. TBI is as adaptable to environmental and operating conditions as other fuel injection systems. Carburetion's adaptability is limited to certain operating conditions, i.e., full acceleration. TBI systems can respond to changes carburetors can't, such as changes in altitude and temperature or conditions that induce engine knock. TBI systems are sometimes referred to as an "electronic carburetor" as the injector in a TBI system effectively performs the same function as the primary and secondary jets and accelerator pump in its carbureted counterpart. A person familiar with carbureted engines will notice only minor visual differences between a TBI-equipped motor and a carbureted motor, as the main component of a TBI system resembles a carburetor, right down to where it's located (mounted vertically on the intake manifold).
A TBI system improves on the efficiency of a carburetor though it is more complex and costly. Conversely, a TBI system improves on the cost and complexity of a multi-point injection system at the sacrifice of efficiency and performance. TBI, then, is an elegant compromise between cost and performance, between simplicity and adaptability.
