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Different Injector Types – How Do They Work?
You have probably been driving your car around for years without giving too much thought to the principle of action behind the engine. You don’t need an engineering degree to understanding the basics of an engine. The purpose of a gas-based engine is to transform controlled fuel explosions into kinetic energy, which is further transmitted to the wheels through a gearbox. The fuel is collected through a fuel pump from a tank then it’s delivered and injected into the engine.
A critically important component within the injection mechanism is the injector. It sprays a finely tuned amount of fuel into the ignition chamber where a piston compresses it then a spark plug makes it explode and create kinetic energy. As previously mentioned, the amount of fuel sprayed by the injector is highly tuned by the ECU (electronic control unit). Any variation would prove to have a considerable impact over the vehicle performance, usually lowering it.
In order to optimize performance based on the type of fuel used, purpose of the vehicle and to adhere to the latest technological improvements, more than one type of injector has been developed. Currently there are 3 main injector types being used in automotive engine construction: top-fed, side-fed and throttle body injectors.
Throttle body injectors (TBI)
Initially introduced in airplane construction, throttle body injectors are also called single point injectors. Unlike their more modern counterparts, single point injectors are located directly inside the throttle body rather than on a rail.
They were the first upgrade of the classic carburetor system, and thanks to its design, a throttle body injection system can make use of mostly the same components as the carburetor did.
Along with their compatibility on older carburetor based throttle systems, throttle body injectors are also relatively cheap to repair or replace when compared with the other available injection systems. However there is also a drawback. Although computer-controlled, due to their emplacement and way of action, throttle body injectors are not very efficient. The fuel is dumped into the intake, creating what is known as a “wet manifold”.
Multi point fuel injection
Also known as port injection, multi point fuel injectors are designed to work as a group, with each engine cylinder having a fuel injector of its own. Injectors are placed right outside the intake valve, the fuel vapor is ensured to be delivered completely to the ignition chamber.
This allows a better tuning of air/fuel ration while in the same time it lowers chances of fuel condensing inside the manifold, as it sometimes happen with the throttle body injection systems.
Even with the high proximity placement of the injector, multi point injection systems still lack a perfect suction of the fuel inside the ignition chamber; it is however, more effective than TBI systems.
Based on the multi-port injection system, sequential injectors are also assigned one per each cylinder and attached as close as possible to the intake valve. However, unlike regular MPIs, sequential injectors are controlled individually by the ECU (electronic control unit), allowing them to open one at a time.
Sequential injection allows for fuel to be delivered solely when the intake valve opens so it directly travels inside the cylinder chamber. In case of regular MPIs, all injectors fire at the same time regardless the state of the intake valve. Sequential injectors can be timed to spray fuel in the same manner spark plugs are set to ignite it.
Fuel economy is increased with the use of sequential injectors. However, incomplete fuel consumption may arise when injectors are poorly tuned, leaving traces on the intake manifold or over the surface of the valve.
Used on more recent engine types, direct injection systems bypass intake manifolds and valves, spraying fuel directly inside the ignition chamber. Computer-controlled, direct injectors are timed to spray fuel differently according to engine type.
Diesel fuel is injected either through a high-pressure common rail connected to injectors. The other option is to use unit injectors which increase the pressure of the fuel within the injector rather than within the common rail.
Ultra lean burn is achieved when using direct gasoline injection (also known as Fuel Stratified Injection – FSI, or Gasoline Direct Injection – GDI) while also completely removing “wet manifold” issues and reducing emissions.
Although they provide more performance and better fuel efficiency, direct injection systems are also more expensive to repair or replace when damage occurs.
Side-feed and Top-feed injectors
Along with emplacement injection type, fuel injectors also differ according to the way fuel is supplied to them. Injectors can be either is supplied with fuel from the side (side-fed) or from above (top-fed). While the overall aspect may not look very different to the naked eye, there is a series of advantages and drawbacks for each type.
Side-feed injectors are fit inside the common-rail. The rail is fixed either to the engine block or the manifold. Side-feed injectors are constantly surrounded by fuel; given this, they benefit from improved cooling, thus being less prone to damage. The side-feed is the most common used injector type.
On the other end of the barricade are top-feed injectors. Unlike their side-feed counterparts, top-feed injectors are attached between the rail and the cylinder chamber. This makes them easier to replace or upgrade, as there’s usually no need to adjust or replace the common rail as in case of side-feed injectors. However, due to the fact that they are not surrounded by fuel at all times, top-feed injectors may suffer from a lower life expectancy, needing to be replaced more often, especially in high-performance vehicles.