It is essential that we understand the basics of how a diesel engine works, it is relatively simple, but, gives us the information to base our decisions upon.
Diesel engines are sometimes called compression ignition engines, this is because as the piston rises it compresses the air above it, this is compressed to a degree where it generates heat, this heat is hot enough to ignite finely atomised diesel fuel. When it reaches a certain point, the diesel is sprayed into the engine as a finely atomised spray, it burns and in doing so, expands; this expansion pushes the piston down, this giving us motion. Basic diesel engines do not have spark plugs or electrical circuits to ignite the fuel as petrol engined cars do, this simplicity is what makes the diesel engine robust and reliable, add to this that the combustion is more efficient, and it is the engine of choice for durability and reliability. As long as a basic diesel engine has diesel and air, it will continue to run; until either the fuel or air supplies are cut off, it is not dependant upon electrical power, other than for starting in motor vehicle applications.
Two main types of fuel injection systems are used, these are the basic non common rail type, and the more modern common rail type, many people will have heard of them, but may not know how they work.
Basic non common rail types have been used since fuel injection was first fitted to diesel engines, it is basic, simple, and has been refined over the years to be ultra reliable and reasonably efficient, this type is fitted to the Fourtrak. Systems consist of a fuel tank, a fuel filter, an injection pump, and a set of injectors, this applies to all vehicles with basic injection, although some minor differences may apply to their layout.
Fuel is either pumped or sucked from the fuel tank, it passes through a fuel filter which removes debris, and separates out any water, and goes to the injection pump, this generates the high pressure required by the injectors to spray fuel into the engine in an atomised form. Controls made by the injection pump are by applying throttle to allow more fuel to the injectors, or by temperature control on some systems, which open the throttle slightly when the engine is cold, this is noticeable as when the engine is cold, it has a slightly higher tickover speed. This increased tickover warms the engine more quickly, once the engine is warm, this tickover speed reduces to its normal tickover speed.
No other form of control is available on this type of system, it is identifiable by looking at the injection pump, it will have one fuel injector per cylinder, and one pipe per injector coming from the injection pump, Fourtrak’s are four cylinder engines, so four fuel injectors; four injection pipes coming from the injection pump. Fuel is supplied by the injection pump to the injector, it allows high pressure fuel up the injection pipe, inside the injector is a spring and a seal, as the pressure rises, it pushes against the spring. At a predetermined pressure the spring is compressed by the pressure, to a point where it allows fuel to enter a single, or multiple ports, bypassing the spring assembly, it then travels to the injector nozzle or pintle, this pintle gives the fine spray pattern to the diesel as it is injected into the engine; passing diesel, being an oil, lubricates the injection components.
Common rail fuel injection systems by comparison, are far more complex; these are fitted to most modern vehicle engines, they are electronically controlled by an ECU, and have a range of sensors which provide information to the ECU. These systems depend upon a range of electronics to run, but are more efficient as they have a wide range of adjustments, they can also be interlinked with other electronic systems on the vehicle such as anti lock braking, or traction control systems.
Common rail systems have a fuel tank, fuel filter, and an injection pump just as the basic systems do, but the fuel injectors sit in the engine, the common rail sits across the top of them and is bolted to the engine, this holds the fuel injectors in place. No individual injector pipes, just one pipe from the injector pump to the common rail, this one rail is pressurised to a far higher pressure than a basic system, and supplies all the injectors.
System components differ from basic systems by having an ECU or electronic control unit, this is basically a programmed computer which receives information from a number of sensors, this, processes the information, adjusts settings, and provides the optimum required quantity of fuel to the engine, at exactly the right time.
Sensors are as follows:
Crankshaft position sensor – This tells the ECU exactly where the engine is on its 720 degree cycle. (CPS)
Mass air flow sensor – this measures the quantity of air entering the engine, most even measure the air density of the air entering the engine. (MAF)
Throttle position sensor – this measures the position of the throttle pedal. (TPS)
Coolant temperature sensor – this measures the engines coolant temperature. (CTS)
Knock sensor – this actually listens to the engine and determines whether the engine is under load, no load, and the actual combustion. (KS)
Many other sensors may be fitted depending upon the engines complexity or specification, combine the huge amounts of wiring required by this system, we can see why they are not as reliable as the basic system fitted to the Fourtrak. Sensors supply information to the ECU, this compares this to a three dimensional map stored inside it, this then alters the parameters, the injectors are opened at a time and position by wires controlling a solenoid. They can be opened earlier or later than the fixed system on basic systems, and for a long; or shorter time duration, depending whether the engine is accelerating, towing a heavy load, or climbing a steep hill and is in danger of stalling. Injectors may even be pulsed to supply many short bursts of fuel when no throttle is applied, or a short, then long burst of fuel; in simple terms, they are fully adjustable.
Irrespective of which type of fuel injection system is fitted, they are all manufactured to very tight tolerances, and are especially vulnerable to water or contamination by solids such as dirt, and can be affected by higher viscosity fuels.
Now we know the differences between the systems, we may understand the implications highlighted in the following chapters.