and created injection pressure. Since the 1980s, electronic systems have been used to control the metering of fuel. More recent systems use an electronic engine control unit which meters the fuel, controls the ignition timing and controls various other engine functions. Injecting of fuel The fuel injector is effectively a spray nozzle that performs the final stage in the delivery of fuel into the engine. The injector is located in the combustion chamber, inlet manifold or - less commonly - the throttle body. Fuel injectors which also control the metering are called "injection valves", while injectors which perform all three functions are called unit injectors. Direct injection systems Direct injection means that the fuel is injected into the main combustion chamber of each cylinder. The air and fuel are mixed only inside the combustion chamber. Therefore, only air is sucked into the engine during the intake stroke. The injection scheme is always intermittent (either sequential or cylinder-individual). This can be done either with a blast of air or hydraulically, with the latter method being more common in automotive engines. Typically, hydraulic direct injection systems spray fuel into the air inside the cylinder or combustion chamber. Direct injection can be achieved with a conventional helix-controlled injection pump, unit injectors, or a sophisticated common-rail injection system. The latter is the most common system in modern automotive engines. Direct injection for petrol engines During the 20th century, most petrol engines used either a carburettor or indirect fuel injection. Use of direct injection in petrol engines has become increasingly common in the 21st century. Common-rail injection systems In a common rail system, fuel from the fuel tank is supplied to a common header (called the accumulator), and then sent through tubing to the injectors, which inject it into the combustion chamber. The accumulator has a high-pressure relief valve to maintain pressure and return the excess fuel to the fuel tank. The fuel is sprayed with the help of a nozzle that is opened and closed with a solenoid-operated needle valve. Third-generation common rail diesels use piezoelectric injectors for increased precision, with fuel pressures up to . The types of common-rail systems include air-guided injection and spray-guided injection. Unit injector systems Used by diesel engines, these systems include: Pumpe-Düse Pump-rail-nozzle system Helix-controlled pump systems This injection method was previously used in many diesel engines. Types of systems include: Lanova direct injection Afterchamber injection G-System (sphere