- Diesel fuel injection
- High pressure components
- Diesel fuel
- Diesel fuel characteristics
- Cetane number
Diesel fuel injection
All diesel engines draw air only, past the intake valve into the cylinder. A high-pressure fuel-injection system injects fuel into the cylinder. The amount of fuel injected is varied to suit the load on the engine, and to control engine speed. Intake air volume does not change.
In a basic diesel fuel system, a fuel tank holds the diesel fuel. A lift pump takes fuel from the tank. It keeps the injection pump full of fuel. A sedimenter removes any water, and larger particles in the fuel. A fuel filter removes minute particles. An injection pump delivers fuel under very high pressure to the injectors, along injector pipes. It must send the correct amount of fuel, and it must send it at the correct time in the engine cycle. An injector, at each cylinder, sprays fuel into each combustion chamber. Leak-off pipes take fuel used for cooling, and for lubrication, from the injection pump and injectors back to the tank. They also help to remove air from the system. A governor controls engine speed. And a control lever on the governor is connected to the accelerator pedal.
The basic system is divided into 2 sections. This is the low-pressure side.
The low-pressure side cleans the fuel and delivers it to the high-pressure side, or fuel injection system. Dirt and water will damage a diesel fuel injection system. The highly polished components need a very efficient filtration system to ensure all traces of dirt and water are removed.
The highly-polished finish is achieved by lapping 2 components together to form a matched set. Matched components must not be interchanged after lapping is completed.
The high-pressure side of fuel injection system must raise the pressure of the fuel high enough to open an injector. This allows the fuel to be forced into the combustion chamber at the correct time.
High pressure components
A light-automotive diesel engine typically has a higher compression ratio than a comparable gasoline engine. This high compression ratio heats the air in the combustion chamber to a temperature high enough to ignite the fuel when it is injected. High injection pressures are needed to overcome the compression and combustion pressures in the combustion chamber, and break up the fuel into small particles.
Because of these high pressures, the injector pump and the injector are made from highly polished, and accurately-sized components.
The injector pump can be an in-line type, and it is driven by the engine.
Or it can be a rotary type, also driven by the engine.
The quantity of air taken in on the intake stroke is not controlled by the driver. The driver controls how much fuel is delivered to the engine.
A characteristic of all diesel engines is that at a fixed fuel setting, the amount of fuel delivered to the engine will increase as engine speed, and pump speed increases.
This is called the rising characteristic of the fuel injection system, and unless it is controlled, over-speeding of the engine will result.
Also, because combustion pressures are greater in a diesel engine than in a comparable gasoline engine, the components are stronger and heavier. The higher mass of these reciprocating and rotating components needs to be controlled, or damaging forces could be generated.
To achieve this control, all diesel engines use a governor to control how much fuel is delivered from the injection pump, to the injectors, and into the engine.
There are several types of governors. Some operate mechanically, some are pneumatic, and some are hydraulic.
Diesel engines need assistance to make cold starting easier. Most diesel fuel injection systems inject extra fuel when starting, to ensure sufficient fuel will vaporize and burn in the combustion chamber.
Indirect injection engines may also use heater plugs, or glow plugs, which usually screw directly into the combustion chamber. They only heat the air, and do not begin the combustion.
Diesel fuel
Like gasoline, diesel is a compound of hydrogen and carbon, extracted from crude oil.
There are different grades of diesel fuel for diesel engines. What is commonly sold in a service station is highly refined, and is suitable for use in high-speed diesel engines, including those in light automotive use.
The cetane rating of a diesel fuel defines how easily the fuel will ignite when it is injected into the cylinder.
The lower a fuel’s cetane rating, the longer it takes to reach ignition point.
Using a fuel with too low a cetane rating will increase the amount of diesel knock in an engine.
When diesel fuel is injected into the cylinder, it does not ignite instantly. It takes time for the heat of the compressed air in the cylinder to heat the fuel sufficiently for it to ignite. This period of time from the start of injection, to the start of combustion is called the delay period. During this delay period, fuel continues to be injected into the cylinder.
When the fuel is heated sufficiently, it erupts into flame. Combustion occurs. The sudden pressure rise sends a shock wave through the combustion chamber, that can be heard outside the engine. This is the sound called diesel knock.
Diesel knock can also be caused by poor atomization of the fuel, which can take too long to reach combustion temperature.
The higher the cetane rating of the fuel, the easier a cold engine will be to start. The engine will produce less smoke and odors, and there will be fewer deposits in the combustion chamber.
Diesel engines are also required to operate in low temperatures. During low temperatures, the fuel becomes thicker. If the temperature is too low, paraffins in the fuel begin to solidify, and form waxes. These waxes can block filters, causing fuel starvation, and low power output.
To help prevent this, filters are fitted close to the engine, and sometimes heaters are used.
Diesel fuel also acts as a lubricant for the fuel system components - provided it is free of water, and abrasive particles.
Diesel fuel characteristics
The main factors that affect the performance of diesel fuel are:
- the Cetane rating of the fuel,
- the fuel’s viscosity,
- its cloud point,
- and the extent to which the fuel is contaminated.
The Cetane rating is a comparative measure of the speed with which diesel fuel ignites under compression. The higher the number, the faster the fuel ignites. If the fuel does not burn rapidly after the piston has reached
TDC (Top Dead Center) then it may not burn completely during the power stroke, in which case it will not deliver the maximum possible power, and unburned fuel may then escape into the atmosphere as black exhaust smoke.
Diesel fuel should have a minimum Cetane rating of 40 for direct injection diesel engines, and 35 for indirect injection diesel engines. Fuels with lower Cetane ratings contribute to harder starting, ignition delay, power loss and decreased fuel economy. Cetane improver additives can improve ignition and reduce white smoke during cold weather startups.
Diesel fuel’s viscosity value measures its resistance to flow. Diesel fuel with viscosity which is either too high or too low can cause serious damage to the engine’s injection system.
The ‘Cloud Point’ is the temperature at which fuel turns cloudy to look at. When the fuel temperature drops to the fuel's cloud point, paraffin waxes that occur naturally in diesel fuel crystalize and cling together, making the fuel appear cloudy. This is known as “waxing” and, if not prevented, can clog filters and stop fuel flow to the engine. Clouding can be combated by using fuels with a lower cloud point, providing heat to the tanks, or including a cloud point improver to the fuel. This improver separates the clinging wax particles so they can pass through the fuel filters. Some oil companies produce special winter grade fuels for cold weather operation.
Diesel fuel is vulnerable to contamination, particularly from water in the tank, and from various types of sediment. As the fuel is fed into the system from the bottom of the tank, it is easy for contaminants to enter the system with the fuel. As the fuel in the tank is used, air from the atmosphere enters the tank. Water condenses on the walks of the tank and runs down into the fuel. The water is heavier than the fuel and usually ends up in the bottom of the tank along with sediment such as rust, or scale, or weld slag. Water in the fuel can cause injector seizures and engine failure. It also accelerates component wear. Water traps or separators are very important in diesel fuel systems. Dirt and other debris can clog fuel filters and form deposits, resulting in reduced power and excessive fuel system wear.
Cetane number
Cetane number or CN is to diesel fuel what octane rating is to gasoline. It is a measure of the fuel's combustion quality.
Cetane is an alkane molecule that ignites very easily under compression, so it was assigned a cetane number of 100. All other hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. The cetane number therefore measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions. Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components.
A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.
To measure cetane number properly is rather difficult, as it requires burning the fuel in a special, hard-to-find, diesel engine called a Cooperative Fuel Research (CFR) engine, under standard test conditions. For most practical purposes, people control quality using the Cetane Index, which is a calculated number based on the density and distillation range of the oil.
There are various versions of this, depending on whether you use Metric or Imperial units, and how many distillation points are used. These days most oil companies use the '4-point method'.
There is very little actual cetane in diesel fuel. Diesel engines run just fine with a CN between 45 to 50. There is no performance or emission advantage when the CN is raised past 50; after this point, the fuel's performance hits a plateau.
Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Premium diesel has additives to improve CN and lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersants, and other additives depending on geographical and seasonal needs.
