DemystifyingDiesel Fuel systems, and how they work.

Thebasic principle of fuel injection is that the right amount of fuel must beinjected at the right time to satisfy engine horsepower demands. Since fuelrequires time to burn, the right amount of fuel must be injected at theappropriate point in the compression stroke, so that it burns completely.

Overthe years, a number of different systems have been developed to implement andcontrol the fuel injection process. These can be broadly categorised intomechanical and electronic approaches, although, as we will see, a number ofvariations exist. Nevertheless, the basic principles of diesel fuel systems areconsistent regardless of the technology used to implement them.

Indiesel engines, fuel is injected during the compression stroke before thepiston reaches top dead centre (TDC). Only the air is compressed on thecompression stroke which causes it to become hot enough to ignite the fuel.

Ina mechanical system, the fuel charge is accurately metered and pressurised bythe fuel injection pump and passed to the high-pressure injectors via pipes beforebeing sprayed into the combustion chamber, where it ignites. The resultingcombustion gasses then drive the piston down to produce the power stroke.

Theprinciple is the same in electronic systems, but the components are different. Sincemechanical systems were the first to be developed and used, it is reasonable tostart by describing them.

Mechanical fuel system basics

Amechanical fuel system contains a low-pressure circuit and a high-pressurecircuit. Fuel flows through the components in the fuel system as follows:

• Thetransfer or lift pump draws fuel from the fuel tank through the inlet port ofthe primary fuel filter, which may contain a water separator, to remove waterfrom the fuel.

• Afterflowing through the primary filter and reaching the transfer pump, the fuel ispressurised before flowing to the secondary fuel filter. The fuel flows underlow-pressure into the high-pressure fuel injection pump.

• Thetransfer pump typically is a simple diaphragm type actuated by a cam lobeacting on a lever that depresses and releases the spring-loaded diaphragm tomove the fuel. Electric transfer pumps also are used on some engines. 

Inany case, the only function of the transfer pump is to deliver an adequate flowof fuel to the injection pump.

Theinjection pump is often a rotary plunger-type unit with an individual pistonfor each cylinder of the engine. Or, it may be a single chamber pump thatmeters fuel to each cylinder using a sleeve or scroll system. In either case,the pump sends a small amount of fuel through the high-pressure fuel line toeach injector in the appropriate sequence.

Whenthe fuel pressure exceeds the injector nozzle’s pre-set pressure limitation, itopens and fuel is injected into the combustion chamber. Any excess fuel is sentout through a return line back to the fuel tank.

Becauseall of these functions are performed in what is essentially a closed loop thatgenerates positive feedback, a diesel engine requires a governor to regulatefuel delivery and keep the engine from over speeding.

Dieselengines are not self-speed-limiting because the air (oxygen) entering theengine is always the maximum amount. Therefore, the engine speed is limitedsolely by the amount of fuel injected into the engine cylinders. The engineshould always have sufficient oxygen to burn and the engine will attempt toaccelerate to meet the new fuel injection rate.

Ina mechanical system, the governor operates on the principle of balancingflyweight force against spring force. Normally, an engine is operating undersome sort of load and the governor determines what engine rpm is correct forthe applied load and operator input. The governor provides the engine with thefeedback mechanism required to change speed as needed and to maintain thedesired speed.

Obviously,there is a significant amount of technology involved in manufacturing andintegrating the elements of a mechanical fuel system into a diesel engine. But,regardless of the details, the basic principles are essentially the same. Thoseprinciples also apply to the electronic systems increasingly found on moderndiesels that have been developed in response to increasingly stringent globalemission standards.

Evolution of electronic engines

MechanicalElectronic Unit Injectors (MEUI) and Hydraulic Electronic Unit Injectors (HEUI)were among the first technologies to successfully replace mechanical componentson the high-pressure side of the fuel system. They replace the injection pump,fuel lines, and nozzles used in mechanical engines with an electronically triggeredunit injector in each cylinder.

Lowpressure fuel is delivered to each injector through a gallery in the cylinderhead, eliminating individual fuel lines. The fuel is then pressurised insideeach injector with a mechanically or hydraulically powered plunger and releasedinto the cylinder by a solenoid responding to an electronic signal from theEngine Control Module (ECM).

TheECM, which monitors multiple engine functions, eliminates the need for amechanical governor. The resulting fuel system is both simpler andsimultaneously more precise. As was the case with mechanical fuel systems, anumber of advancements have been made since MEUI and HEUI were introduced.

Themost recent step in the evolution of electronic fuel systems is common railtechnology. Instead of pressurising fuel in the injector, a dedicated fuel pumpis used to generate high fuel pressure and deliver it to a fuel manifold, orrail. From there, the high-pressure fuel flows through individual steel fuelpipes to electronically controlled injectors.

Sincethe fuel is supplied at injection pressure, the injectors themselves can bemuch simpler and more responsive. This allows fuel to be injected in precisequantities at more than one point during the combustion cycle to produce morecomplete and, therefore, efficient, combustion. This process is controlled by asophisticated ECM that monitors and responds to multiple engine, machine, andoperator inputs.

Itis important to note that in common rail fuel systems, the fuel injection pumpsupplies fuel to the manifold and injectors at pressures up to 200 MPa (29,000psi) making it critical to follow all common rail specific safety procedureswhen servicing these systems. Those procedures are in addition to the standardsafety practices that must be followed when servicing any diesel engine.

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Dieselengines are not self-speed-limiting because the air (oxygen) entering theengine is always the maximum amount. Therefore, the engine speed is limitedsolely by the amount of fuel injected into the engine cylinders.