Modern vehicles with a combustion engine have a sophisticated injection system. The injection system ensures optimum function and low fuel consumption at all engine speeds. Find out everything you need to know about the fuel injection system in cars.

Car fuel injection – function and structure

An injection system, or injection in the car, ensures that the right amount of fuel is injected into the engine. In today’s combustion engines, injection systems play a key role in the mixture composition of fuel and air. The targeted injection of petrol or diesel ensures particularly effective combustion. The injection system injects the fuel at high pressure into the intake tract or directly into the combustion chamber of the cylinders. This takes place via injection nozzles, which are supplied with fuel via an injection pump. Depending on the type of injection system, pressure is generated via a central fuel pump or, in the case of pump-nozzle systems, separately for each injection valve.

• Injection nozzle: The injection nozzles or injection valves inject the fuel into the combustion chamber or the intake manifold of the engine. At the same time, the nozzles ensure fine atomization of the fuel in the cylinder. Injection nozzles for petrol injection are usually referred to as injection valves, while diesel engines are usually referred to as injection nozzles or injectors.
• Injection pump: Modern injection pumps put the fuel under high pressure so that the required injection pressure is always present at the injection nozzles. Depending on the fuel system, the pump delivers a certain amount of fuel. A second fuel pump is used in almost every modern vehicle. This pumps the fuel from the tank to the injection system.
• Control unit: The injection system is controlled by a control unit that is connected to sensors. Factors such as the engine speed determine the fuel quantity and injection timing. In most modern cars, fuel injection is regulated by the engine control unit. However, depending on the vehicle, a special control unit may also be installed for the injection system.

Carburetor vs. injection system

Almost every modern combustion engine in cars today is designed as an injection engine. Before the injection system came into use in automotive engineering, a vehicle with a petrol engine often had a carburettor. This component has a throttle valve inside and also produces a mixture of fuel and air. However, without injection under pressure via a valve, the technology of the modern injection system is superior to classic carburettors in many respects. In contrast to the carburetor system, modern direct injection allows active control of the injection. In addition to improved cold start behavior, fuel consumption and pollutant emissions are reduced. Regular adjustment of the system is also no longer necessary.

Different systems for injection systems

The design of an injection system can vary depending on the vehicle and its age. While newer petrol engines are almost always equipped with direct petrol injection, older cars may have intake manifold injection. Depending on the engine type, the injection system may even combine both types of injection. Diesel engines are usually direct injection engines.

Petrol with direct injection

With direct injection, the fuel is injected directly into the combustion chamber. For this purpose, the engine has separate injection nozzles for each cylinder. Such a combustion engine with internal mixture formation offers many advantages. For example, the injection system with direct injection allows the required amount of fuel to be injected in several stages.

Intake manifold injection as fuel injection

In contrast to direct injection, an injection system with intake manifold injection does not inject the fuel into the combustion chambers. Instead, the ignitable mixture of fuel and air is formed outside the engine. For this purpose, one or more injection valves inject the gasoline into the intake manifold in the intake tract upstream of the intake valves.

Injection in diesel engines

The injection system of modern diesel engines is usually designed as a common rail system. A high-pressure pump brings the fuel to a high pressure level. A distributor pipe or rail serves as an accumulator and provides the diesel for injection via nozzles. The injection nozzles open separately via an electrical control unit at the appropriate time.

Defects in injection systems

An injection system can become defective for various reasons. Symptoms often include increased fuel consumption, poor starting behavior or engine stuttering. Performance and acceleration can also be reduced if parts of the injection system are defective. In addition, the exhaust behavior often deteriorates in the event of a defect.

• Clogged injection nozzles: The injection system often no longer works as intended due to defective or clogged injection valves. An injection nozzle can become blocked due to coking or contamination, for example. In addition to replacement, the nozzles can also be cleaned.
• Defective injection pump: Damage to the fuel pump or injection pump is one of the more common defects in the injection system. The pump then usually does not deliver any fuel. A high mileage or contamination in the fuel are possible reasons for defects.
• Control unit broken: The control unit or one of the sensors connected to it can also break down and be responsible for malfunctions in the injection system. In this case, it is often only possible to rectify the fault by replacing it with a new or used control unit.

Buying and replacing spare parts such as injection nozzles

Spare parts for injection systems are generally readily available. However, work on the injection system should only be carried out by expert mechanics. Fuel can sometimes escape when checking and replacing parts, and components such as injection valves are also sensitive. The design of injection system components can vary depending on the engine. Control units such as the engine control unit or a control unit for the injection system can be purchased new or used. Even for older cars, the injection control unit is often still available as a used original part. Specialist workshops also offer repairs. The devices can usually be easily replaced by loosening a connection and reconnecting the unit. Injection valves for petrol engines and injection nozzles for diesel engines are available for most engines. In addition to purchasing new components, it may be worthwhile buying a used and tested injection unit. The procedure for replacing valves and nozzles differs depending on the injection system. In most cases, the components can be easily removed and replaced.