Ameer Randolph
The throttle body is an important component of an engine's fuel injection system. It controls the airflow into the engine, while the injectors control the fuel flow. There are several types of fuel injection systems, including throttle body injection (TBI), multiport injection (MPI), and gasoline direct injection (GDI). TBI systems, which are found on older vehicles, have one or two fuel injectors mounted in the throttle body, delivering fuel into the air above the throttle plates. MPI systems, on the other hand, have one fuel injector per cylinder, delivering fuel directly into the cylinder. GDI systems are similar to MPI systems but operate at a much higher fuel pressure.
| Characteristics | Values |
|---|---|
| Type of fuel injection system | Throttle Body Injection (TBI) |
| Number of fuel injectors | One or two |
| Location of fuel injectors | Mounted in the throttle body assembly |
| Airflow control | Regulated by the throttle body |
| Fuel control | Controlled by the injectors |
| Fuel injection location | Outside of the cylinder |
| Fuel delivery | Central point air/fuel delivery atop the intake manifold |
| Fuel pressure control | Controlled by the fuel pressure regulator |
| Fuel routing | Excess fuel is routed back to the fuel tank |
| Vehicle application | Older vehicles, late 80s to early 90s |
| Performance | Not the most efficient but better than a carburetor |
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What You'll Learn
Throttle body injection (TBI)
In a direct injection car, the fuel is injected directly into the cylinder. The intake valve lets in air and fuel (if applicable) into the cylinder. In older engines, a carburettor controls the mixture, and a throttle butterfly valve controls the amount of air.
Throttle body injection is similar to direct injection and multiport fuel injection. The throttle body regulates air, and the fuel is controlled elsewhere. All engines use a throttle body to regulate airflow; otherwise, the engine would constantly be at 100% throttle and keep revving.
Some BMW, Audi, and FCA engines have continuously variable valve lift engines that don't use the throttle body to control air intake. Newer direct injection Ford engines have started doing port injection (gas on the valves) in addition to direct injection to prevent oily build-up on the valves.
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Direct injection
In direct injection, the air and fuel are mixed only inside the combustion chamber. This means that only air is sucked into the engine during the intake stroke, and the fuel is injected separately. The fuel injector acts as a spray nozzle, delivering fuel into the engine. Direct injection systems can use either a blast of air or a hydraulic system to deliver the fuel, with the latter being more common in automotive engines.
However, direct injection also has some potential drawbacks. One issue is that it can lead to a build-up of oily residue on the valves, as the fuel is not mixed with the air before entering the cylinder. To address this, some manufacturers have started combining direct injection with port injection, where fuel is injected onto the valves to prevent this build-up.
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Port injection
However, one downside of port injection is that fuel droplets can sometimes be deposited on the intake port walls, disrupting the intended fuel-air ratio. This can be mitigated by combining port injection with direct injection, as seen in Toyota's D-4S injection system. By using both types of injection, the benefits of each can be maximized while minimizing their negatives. For example, during normal driving conditions, Toyota fires both injectors to raise the density of the incoming charge and flush carbon deposits off the intake valves. During high-load conditions when maximum combustion chamber cooling is needed, direct injection handles all the fuel delivery to prevent detonation.
Ford is another major manufacturer that has adopted the dual-injection strategy. In its EcoBoost V-6 engines, Ford combines port injection with high-pressure direct injection to improve engine efficiency. Ford also uses port injection alone at idle and low rpm for smooth, quiet, and efficient engine operation.
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Carburetor vs. fuel injection
The internal combustion engine requires a way to get air and fuel into the cylinder, where it can be ignited to produce usable mechanical energy. The carburetor and fuel injection are the two primary types of fuel delivery systems used in cars, motorcycles, and planes.
The carburetor, invented in 1826 by Samuel Moey, was the first type of fuel delivery system. It uses a Venturi Tube that narrows in a section, decreasing air pressure and creating a vacuum—the Vacuum Venturi Effect. This vacuum pulls fuel into the carburetor, where the ratio of air to fuel is adjusted using two valves: the choke and the throttle. The choke decreases the amount of air and increases the flow of fuel, causing the engine to run lean, which is useful during cold starts. The throttle valve, also called the butterfly valve, regulates how much of the air-fuel mixture flows into the engine. Opening the throttle valve wider allows more of the mixture to flow in, pushing the car to accelerate.
In the 1990s, the carburetor was replaced by fuel injection systems, which offered greater efficiency and precision. Fuel injection systems use an array of fuel injectors, an oxygen sensor, and an electric fuel pump with a pressure regulator. A computer controls the amount of fuel delivered to the cylinders, resulting in better performance and fuel mileage. Fuel injection systems can be used in both diesel and petrol vehicles and come in electronic and mechanical variations. They deliver better power, fuel economy, and lower emissions.
While carburetors are simpler, cheaper, and easier to fix, fuel injection systems offer better performance and efficiency. The choice between the two comes down to specific needs and preferences. Some car enthusiasts prefer the simplicity and effectiveness of carburetors, while others favour the performance and precision of fuel injection systems.
In terms of the throttle body, it is involved in both carburetor and fuel injection systems. In fuel-injected applications, the throttle body controls airflow, while the injectors control fuel flow. In carburetor systems, the throttle body is located in the same area as the fueling components, and the throttle valve is connected to the accelerator pedal. Throttle body injection was an early version of fuel injection that resembled a carburetor and had a fuel injector inside the throttle body. However, these are no longer common, as most fuel-injected cars now have injectors in the heads.
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Airflow control
The throttle body's function goes beyond just airflow management. It also collects vital data through various sensors mounted on it. These sensors provide information such as throttle position, idle speed, and cold start warm-up, which is then used by the vehicle's engine management system to fine-tune the fuel flow rate based on multiple conditions. This ensures that the engine receives the optimal fuel-air mixture for efficient combustion.
In older engines, the carburetor was responsible for both airflow control and fuel introduction. However, with the introduction of fuel injection systems, the throttle body took over the role of airflow regulation. This separation of functions allows for more precise control of the air-fuel mixture, leading to improved engine performance and fuel efficiency.
While most engines rely on the throttle body for airflow control, there are exceptions. Some engines, such as those from BMW, Audi, and FCA, employ continuously variable valve lift technology. In these engines, the throttle body is typically held wide open, and airflow is primarily regulated by the intake valve. This design allows for more precise control of airflow and improves engine responsiveness.
Maintaining the throttle body in optimal condition is crucial for ensuring smooth engine operation. Over time, carbon deposits and dirt can accumulate, hindering the proper functioning of the butterfly valve. Regular cleaning and maintenance of the throttle body are necessary to prevent performance issues and ensure the correct airflow into the engine.
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Frequently asked questions
Throttle body injection (TBI) is a type of fuel injection system that has one or two fuel injectors mounted in a throttle body. It is an older system that was used in the late 1980s and early 1990s.
In a TBI system, the injectors deliver fuel into the air above the throttle plates. The air-fuel mixture then enters the intake manifold, where it is distributed to the engine's individual cylinders.
Some advantages of using a TBI system include:
- Quick and simple start of the car
- Reduced fuel consumption by increasing engine efficiency
- No need to manually adjust the fuel supply system
