Since its creation in the late 1800s, the Diesel engine has utilized fuel injection in one form or another to deliver power and fuel efficiency to some of the world’s biggest (and smallest) vehicles.
How exactly? You’re about to find out.
Unlike a standard gasoline engine that uses spark plugs to initiate combustion, Diesel engines use compression. Highly-compressed air mixes with pressurized and atomized fuel, causing the fuel and air to ignite and combust in the cylinder. By using combustion ignition (CI), diesel engines burn less fuel and create more energy with each stroke than their spark ignition (SI) counterparts.
The injectors work by spraying a calibrated amount of fuel either directly into the cylinder or into a separate area first and then into the cylinder to create ignition. In most vehicles today, fuel injection timing and calibration are electronically controlled, allowing them to meet increasingly stringent government emission standards.
Two Types of Fuel Injection
Generally, fuel-injection engines fall into two categories: direct and indirect. Indirect injection pumps the fuel through a small, usually spherical, chamber, known as the pre-combustion chamber, where it mixes with high-speed swirling air. This chamber often has something called a glow plug that allows the fuel/air mixture to heat quickly on a cold start. The explosion begins in this smaller area and then spreads into the combustion chamber to power the engine.
Manufacturers used indirect fuel injection for many years in smaller diesel automobiles mainly because this system allowed vehicles to utilize diesel power in compact engines and was cheaper to produce than a direct-injected engine. However, the extra chamber also meant more lost heat and the need for increased amounts of fuel, bringing down overall fuel efficiency.
Direct fuel injection, on the other hand, delivers fuel straight into the cylinder’s combustion chamber, where it meets with the compressed air and ignites. Earlier versions of direct-injection diesel engines were usually limited to heavy commercial vehicles. Great strides in technology, however, have rendered indirect fuel injection almost obsolete in most of today’s diesel-operated commercial and personal use vehicles. That’s because direct injection engines aren’t big, bulky, and loud anymore.
Different Ways Fuel Injection Works in Your Vehicle
Your diesel car or truck will generally use one of these three primary delivery systems to get the fuel from your pump to your injectors: Common Rail, Distributor (Rotary), and Inline Unit. Each of these different diesel fuel injection methods has both advantages and disadvantages.
1. High-Pressure Common Rail
This particular fuel supply assembly represents the most popular delivery system used in modern passenger and commercial diesel engines today. A separate, high-pressure pump feeds the fuel into a rail, where it remains at a constant level until it’s released into each injector situated along the mechanism. Injectors then spray the fuel directly into each cylinder, powering the engine. The vehicle’s Electronic Control Unit (ECU) precisely governs fuel supply, rail pressure, and injection timing to produce maximum engine efficiency.
Advantages to common rail systems include higher fuel economy, lower emissions, higher torque and power output, and significantly less engine noise. But all that comes with a high price tag, as electronically-controlled components tend to be more expensive and require more complicated repairs. Additionally, common rail injectors are more susceptible to impurities and clogging and thus demand better fuel quality and more frequent filter changes.
2. Distributor (Rotary) Injection
Unlike the common rail, the distributor or rotary injection system can either be electronic or mechanically driven. Instead of delivering the fuel all at once, each injector is filled and activated in sequential order.
The mechanical distributor has a single plunger that rotates off a cam disk to push pressurized fuel through a separate line for each injector. The plunger pumps high-pressure fuel into the injectors in specific firing order, as determined by a governing device.
All of the mechanical distributor’s components are located in the pump housing, including the governor and the feed pump, which makes it compact, light, and an excellent fit for smaller high-speed engines. On the flip side, mechanical distributor injection systems are not as precise as their newer electronic siblings. This translates into lower fuel efficiency and decreased power if the timing or fuel modulation is even a little bit off or if engine conditions change (e.g., due to weather changes, load variations, etc.).
Like common rail, the ECU monitors the engine via sensors and controls the activation of an electronic rotary injection system. The control unit works in conjunction with an actuator to deliver precise amounts of fuel to each injector at exactly the right time, according to your engine needs.
The benefits, of course, include the ability to react quickly to changing engine conditions by keeping fuel pressure levels constant and fuel delivery and timing exact. However, as with all electronically-driven injection systems, repairs can often be complicated and expensive, involving more than just replacing or remachining a particular diesel fuel injection part.
3. Inline Unit
Unit injectors, often called pump nozzles, are unique in that the pump and injector comprise one component. As with the rotary assembly, the diesel camshaft determines fuel quantity and delivery timing, and injectors fire in sequence. An inline system, as the name suggests, aligns the injector units linearly instead of in a circular fashion. Fuel moves through your engine at a lower pressure until it reaches the injector, where it is highly pressurized and released directly into the cylinder. Unit injection systems can also be both mechanical and electronic.
One of the benefits of a unit assembly is that it uses less fuel at the start, which substantially reduces soot and other emissions. You’ll usually find inline and rotary injection systems in off-road and industrial applications, whereas common rail is becoming increasingly more prevalent in on-road diesel vehicles.
As you can see, fuel injection systems vary widely from engine to engine. It’s essential to know what kind of fuel injection set-up your vehicle uses so you can properly maintain it and have a clear picture of what modifications it can and cannot handle. When you’re ready to repair, replace, or rebuild, your local diesel parts supplier can point you in the right direction.
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The Diesel engine, which was developed in the late 1800s, has used fuel injection in one way or another to provide power and fuel economy to vehicles in the world. Diesel engines that use combustion ignition produce more energy per stroke and burn less fuel. Today's majority of automobiles have electronically controlled fuel injection timing and calibration, enabling them to comply with ever-tougher regulatory pollution rules.
