It used to be that diesel fuel was reserved for heavy industry, commercial use and weird european cars. But with rising fuel prices in the '70s a lot of car manufacturers began offering diesel versions of passenger cars. Nowadays you can find a diesel version of just about any small passenger vehicle, as well as it being the fuel of choice for many 4x4s, vans and utes. Diesel is a wonderful fuel, given the fact that in terms of raw potential energy, it is vastly more powerful than petrol, and will provide more energy per unit of fuel, meaning that it is often much more economical to use. Furthermore, diesel actually has less harmful carbon-monoxide emissions than petrol, making it better for the environment in general.
How Diesel Combustion Works
Diesel combustion works a little differently to how petrol engines do. In a diesel motor, there are no spark plugs used to ignite the fuel/air mixture, and instead the diesel is injected into the combustion chamber once the air inside the cylinder has already been compressed. When the air is compressed, it heats up and so will spontaneously ignite the diesel fuel that is injected. The method used for injecting the fuel, and where it is initially fired may vary, but across all diesel motors, the basic principle remains the same: fuel+air+compression = ignition.
When you turn the key on in a diesel vehicle, there is no spark-based ignition circuit to be energised. Instead, a fuel cut-off solenoid is energised (or an electronic system activates) which allows fuel to be injected. The starter motor will then turn the engine over, which will generate the compression sufficient to heat up the air in the cylinders, and the injector pump will feed fuel to the injectors. The fuel will then be fired into either the cylinder itself, or a special pre-combustion chamber, and will ignite once it comes into contact with the compressed air. Turning the key off will simply kill the fuel flow by disengaging the fuel cut-off solenoid (or electronic injection system), and the engine will cease to fire - hence the slight run-on commonly encountered when turning a diesel engine off.
How diesel is Injected and whyAll diesel motors utilise injection, due to the method that these motors use to actually combust the fuel. In modern diesel motors, there are two different injection methods, that are further distinguished by whether they are mechanically or electronically pumped, as well as whether they utilise a common fuel rail, or individual fuel lines. These injection methods refer to where the diesel is injected into the combustion chamber.
What is Indirect Injection?
Indirect injection motors have their fuel delivered into a pre-chamber that sits before the actual cylinder and allows a bit of air to mix in with the diesel and swirl around, before that mixture is sucked into the cylinder and ignited. This setup has the advantage of requiring lower injector pressures, since the turbulence in the pre-chamber assists in the fuel delivery. It also has the side-benefit of running quieter.
What is Direct Injection?
Direct injection is, as the name suggests, injection of fuel directly into the combustion chamber. Most modern diesels are direct injection systems, as with computer control, delivery of the fuel at the exact right time can be managed far easier than with older mechanical pumps. Direct injection motors generally require less compression and with computer control can actually end up being far more efficient and reliable than their indirect counterparts.
What is Common-rail Injection?
This isn't a separate injection type per-se, rather a sub-type of direct injection. Common-rail setups utilise a reservoir of incredibly high pressure fuel, which is attached to each injector. These injectors then fire at specified time and so will always deliver the exact right amount of fuel, and this further takes the load of the injector pump since it only needs to maintain a set pressure in the rail.
Components in the fuel system
Many components are common across all vehicles, though diesel cars usually have a few additional components. All cars will have some sort of tank that holds fuel, as well as lines that deliver the fuel to the motor. Both the tank and the lines should be free from corrosion and damage, as either can inhibit fuel flow or lead to potential fire hazards. Driving the fuel delivery will be a fuel pump - whether mechanical or electric, and there is usually a filter placed in the lines between the tank and the motor. Many cars will have an in-tank fuel pump. As the name suggests, this sits inside the fuel pump and will more often than not have a filtering device attached to it as well. In-line fuel filters are usually cheap and easy to replace, and should be checked for blockage and fouling if there are any issues with fuel delivery, as a first step.
It isn't too much work to replace any of the commonly shared components, and simple tools can be used to do so - however with regards fuel lines, it is important that any custom or replacement lines be up to fuel-bearing standard and be fitted with appropriate connectors to reduce the potential for dangerous leaks. Many diesels will have special water-traps to ensure that no moisture is present in the fuel. They also usually have a high pressure pump mounted in or on the engine itself, which supplies a flow of extremely high pressure fuel to the injectors. Care must be taken when working on the injector pump and the high pressure fuel lines, to ensure that they are correctly assembled and free from damage as failure and high pressure leaks can be spectacularly catastrophic.
Common tools used when working on fuel systems
Just as with petrol fuel systems, diesel fuel systems are easily worked on with a basic set of tools, as well as a few specialty tools as and where required.
- Spanners and socket sets will be the most commonly required tools, as well a screwdriver set to remove and fasten any clamps that can't be pinched off with pliers.
- When working on fuel systems of any sort, always wear goggles and gloves to protect your skin and eyes from harmful and painful spills or splash-back.
- As with petrol systems, be sure to use a drip tray if you are doing any work that requires that you disconnect fuel lines, and make sure you keep some shop-rags handy to clean up any accidental spillage, as well as to wipe down any fuel that might run.
- A useful tool to have is a fuel line disconnect set - these come in a variety of forms, but can all be used to make the disassembly and leak prevention a breeze!
- In the case of diesel fuel systems, fuel pressure is of utmost importance, so a fuel pressure tester is invariably a good investment in order to identify potential faults in fuel delivery.
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