How do Diesel Engines Work?

Diesel fuel injection system

The fuel injection system is a vital part of the diesel engine. This system pressurizes and injects the fuel. In this way the fuel is forced into air, which has been compressed to high pressure in the combustion chamber.

The diesel fuel injection system consists of a fuel injection pump, an injection nozzle, a feed pump, a fuel filter and a high-pressure pipe. The fuel injection pump pressurizes fuel to high pressure and then sends it via the high-pressure pipe to the injection nozzle, which injects the fuel into the cylinder. The feed pump sucks the fuel from the fuel tank and the fuel filter of course filtrates the fuel. Some types of fuel tanks have a fuel sedimentor at the bottom of the filter to separate water content from the fuel.

Functions of the system

The fuel injection system has four functions: feeding the fuel, adjusting the fuel quantity, adjusting injection timing and atomising the fuel.

Feeding fuel
Pump elements such as the cylinder and the plunger are built into the injection pump body. The fuel is compressed to high pressure when the cam lifts the plunger. Then it is sent to the injector.

Adjusting fuel quantity
Adjusting the fuel quantity is performed by a combination of the pump’s adjusting mechanism and governor.

With diesel engines the intake of air is almost constant, irrespective of the rotating speed and load. If the injection quantity is changed with the engine speed and the injection timing is constant, the output and the fuel consumption changes. Since the engine output is almost proportional to the injection quantity, this is adjusted according to the opening degree of the accelerator pedal.

Adjusting injection timing
A certain period of time is required between the point when the fuel is injected, ignited and combusted and the point when the maximum combustion pressure is reached. This is due to ignition delay.

This period of time is almost constant, irrespective of the engine speed. Therefore a timer is used to adjust and change injection timing. In this way the optimum combustion is obtained to the change in engine speed.

Atomising fuel
Fuel is pressurized by the injection pump and then atomised from the injection nozzle. Accordingly, the fuel thoroughly mixes with air, thus improving ignition. The result is complete combustion. The injection nozzle is secured by the nozzle holder and is mounted on each cylinder of the engine.

Electronically controlled diesel systems

Electronically controlled pumps

There are two types of electronically controlled fuel injection pumps available: the distributor type and the common rail type. The injection method is completely different from conven-tional fuel injection pumps such as in-line pump types.

Distributor (rotary) fuel injection pump

The electronic control system of the distributor type pump consists of various sensors, an ECU (computer) and an actuator. The system detects the running condition of the engine, using the sensors while the ECU controls both fuel injection quantity and injection timing. The actua-tor controls the injection quantity and the injection timing according to the signal sent from the ECU. This ECU calculates the optimum injection quantity and injection timing for the current running condition of the engine using signals from the sensors.

Common rail fuel injection pump (supply pump)
The common rail type pump is completely different from conventional fuel injection pumps. This type has been developed to meet strict exhaust gas regulations in the 21st century. This system consists of a supply pump, common rail, electronically controlled injectors, various sensors to detect the running condition of the engine and a computer (ECU) to control these devices.

The supply pump is driven by the engine and produces high-pressure fuel. An injector is mounted on each cylinder of the engine and the high-pressure fuel from the supply pump is distributed to each injector by the common rail.

Mechanically controlled diesel systems

Mechanically controlled pumps
Mechanically controlled injections pumps are divided in two categories: the in-line type and the distributor type. In-line fuel injection pump The In-line fuel injection pump has the same number of fuel pressure mechanisms (elements) as engine cylinders. This type of pump (including the governor, timer and the feed pump on the pump’s body) is mainly used for medium and large trucks and construction machinery. The pump body is equipped with fuel pressure/feed mechanisms (elements) and injection quantity control mechanisms. Furthermore there is a camshaft to drive these mechanisms. The elements in the pump body feed fuel to each engine cylinder according to the injection order.

Distributor injection pump
The Distributor injection pump has only one fuel pressure mechanism, irrespective of the number of engine cylinders. Instead, it has a distributor designed to distribute the pressurized fuel to each cylinder according to the injection order. All the components, including the governor, timer and feed pump, are built into the pump housing. It is a small and light type of pump that can operate at high speeds which is why it is widely used for small engines.