How is a Diesel Engine Different From a Petrol Engine?
In this article we are going to explain the primary basics about Diesel engines and ECU operations in injections.
❓Now we can start by two important question:
- What components make an engine work?
- How does the "Injection system" operate in diesel engines?
In a petrol engine, the fuel mixes with air and is compressed by pistons then the spark plugs ignite this mixture to move the vehicle. On the other hand, in a diesel engine the air is first compressed, the fuel is injected at the top and then this makes the air hot and the fuel then ignites when it hits the hot air and then the vehicle moves forward.
How Does a Four-Stroke Diesel Engine Work?
Like a petrol engine, a diesel engine usually operates by repeating a cycle of four strokes: Intake, compression, power and exhaust. In the intake stroke, the air is drawn into the cylinder through the air inlet valve on the right as the piston moves down. In the compression stroke, the inlet valve closes, the piston moves up and compresses the air mixture by heating it up. Fuel is injected into the hot gas through the central fuel injection valve and spontaneously ignites. In the power stroke, as the air fuel mixture ignites and burns, it pushes the piston down driving the crankshaft that sends power to the wheels. In the exhaust stroke, the outlet valve on the left will be open to let out the exhaust gases pushed out by the returning piston. All of these happen during which the piston moves up and down twice.
Cylinder liner: It will serve as a place up and down the piston. The components are made of iron and aluminum alloy and located inside of the engine block by using the press method. So, it will be difficult to detach. Water jacket, located inside the engine block. It is the place where the engine cooling process will take place. Hole shaped water jackets are placed inside the cylinder block that surrounds the liner. Oil feed lines, the oil hole on the cylinder block serve to create the engine oil line from the cylinder head to the crankcase. This hole will support the engine oil circulation process to all diesel engine parts.
Pistons and connecting rod: Pistons have a function to adjust the volume inside the cylinder. In this case, when the piston moves down, the volume of the cylinder will enlarge. But when the piston moves up, the volume of the cylinder will shrink. The connecting rod serves to continue the up and down motion of the piston to flywheel. In general, there are three core parts on the piston which are attached to it. They are: Compression rings, Oil rings and piston pins.
Compression rings: These rings are elastic in function, to prevent the occurrence of air leaks during the compression stroke. The way these rings work is to close the gap between the piston wall and the main liner. Oil ring, the ring printed under the compression ring, serves to prevent engine oil from entering the combustion chamber. Piston pin, a pin located inside the piston to connect it with a connecting rod. This pin is tubular when connected to the small end. it will function like a hinge.
Crankshaft: A crankshaft is a mechanical equipment that transforms the reciprocating movement of the piston into rotary motion. A crankshaft connects with a piston through a connecting rod. The main objective of the connecting rods is to receive reciprocating motion by the pistons and deliver it to the crankshaft and rotate the flywheel which further moves the vehicle. Crankshaft is made up of cast iron and also with special iron alloy that has high strength and anti-fastness.
Some parts are attached to the crankshaft, they are: Crank pin, Crank pin is a pin that is connected to the big end on the connecting rod. Crank journal, the crank journal is a pin that serves as a shaft on the crankshaft in order to spin. Crank journaling will be attached to the cylinder block. Weight balance, this component is located opposite the crank pin. It functions as a counterweight as well as to drain the oil to the entire inside of the machine.
Timing chain assembly: The timing chain is included in a valve mechanism system. Its function is to connect the crankshaft and camshaft rotation with a certain angle. The components of this chain are located on the front of the engine. This chain will connect the sprocket gear from the crankshaft with the sprocket gear of the camshaft.
Fuel injectors spray fuel into a car's engine using electronic controlled valves, capable of opening and closing many times a second. They have a nozzle that distributes the fuel evenly for optimum combustion and efficiency. A car generally has one fuel injector per cylinder. So, if you drive a four-cylinder car, it will most likely have four fuel injectors. By injecting the fuel, the system forces it into air that has been compressed to high pressure in the combustion chamber.
How Does The "Injection System" Operate in Diesel Engines?
Differences between injection mechanisms:
Rudolf Diesel, the pioneer Diesel engine's creator invented it in 1892, but Mercedes-Benz, who made the Mercedes 260 D in 1936 was the first to employ it in an automobile. Today's high pressure common rail fuel injection systems enable the adoption of various anti-pollution systems to reduce the emissions of pollutants, thereby complying with emissions regulations imposed by the various countries around the world. This is made possible thanks to the various injection systems that the major automotive groups developed and adopted.
The Injection/ Distribution Pump System:
The entirely mechanical distribution pump served as the foundation for the development of the electronically controlled injection pump technology. Bosch by adding electromagnetic actuators and a relatively basic engine control unit with some maps that allowed it to regulate the flow adjustment slider and the advance command toward the end of the 1990s.
The Unit Injector System:
With the advancement of regulations for increasingly stringent emissions, the main automotive groups have developed increasingly high-performing systems which are capable of increasing efficiency, thus reducing consumption and above all polluting emissions. This is how the first unit injector system was born in the early 2000s, mainly used by the Volkswagen Auto Group (Volkswagen Aktiengesellschaft) on light and electronically controlled vehicles. The technical detail that stands out most in this system is that there is an individual pump for each injector in order to reach high fuel injection pressures in the thousands of bars regardless of the number of engine revolutions.
The Common Rail System:
In conjunction with the adoption of the unit injector system other automotive groups such as FIAT, have gone in another direction for developing the common rail high-pressure injection system. Although the first use in a production vehicle is attributed to the manufacturer Denso, which began marketing it in 1995 when it was equipped on the Hino Ranger truck. Given the initial development which was done by the Fiat and Elasis research and development centers, the German company BOSCH bought the design and technology to then further develop it and make it usable for mass production on light vehicles. Starting from '97 with the 2.4l JTD Fiat, up to the present day. Compared to the systems defined above, this technology, in principle, has a very simple operation and relatively low maintenance and production costs: it allows high efficiency thanks to the possibility of managing multiple fuel injections through the engine control unit during the same operating cycle, the length and moment of opening the injectors and the high injection pressure based on the specific operating condition of the engine.
The concept underlying the operation in the Diesel engine is that the air is compressed in the cylinder and consequently reaches high temperatures such that the fuel injected ignites spontaneously, as the air present in the cylinder during the compression phase has a temperature higher than its ignition temperature. Furthermore, to have an optimal mixing it is necessary to nebulize the fuel, previously pressurized by means of the injector nozzles, so as to allow combustion.
It is important to remember that current Diesel engines are equipped with supercharging, which allows the engine to work with lean carburetors, optimal for the operation of the Diesel engine. Furthermore, the greater fuel efficiency allows to keep consumption much lower than other types of fuel.
During their operation, the four-stroke engines work in accordance with the following four operating phases:
- Suction phase. It corresponds to the rotation of the crankshaft from 0 to 180°C. At this stage, air enters the cylinder via the intake valves.
- Compression stage. During this phase, the shaft rotates from 180 to 360°C. Under the influence of the piston, the air already present in the chamber is compressed 16-25 times and its temperature rises to 700–900°C.
- Combustion phase. In this phase, which corresponds to the rotation cycle 360–540°, the injection and combustion of the fuel takes place. Substances resulting from this process move the piston down.
- Discharge phase. During the rotation of the crankshaft up to 540–720°C from the initial position, the piston moves upwards and the flue gas is released.
The mass air sensor, which allows the engine control unit, after performing the appropriate calculations, to command the injectors to inject the correct dose of fuel for each cycle and thus maintain the air fuel ratio set by the manufacturer so that black smoke or high quantities of nitrogen oxides are not developed. However, this strategy was used for a short time and only for large displacement diesel vehicles. Precisely, because of its low accuracy. According to the percentage of the Accelerator Pedal pressed And the Angel speed up to the management of the various actuators, such as the high-pressure pump, the variable geometry of the turbine, the injectors or even the EGR valve.
The spontaneous combustion of the fuel is positioned at the basis of the concept of combustion by compression of the air at high temperatures inside a combustion chamber.
This high level of compression involves enormous heat and pressure inside the combustion chamber which has an incandescent resistance called a glow plug inside it, this heats the injected diesel to incandescent temperatures, so as to prepare it for self-combustion that occurs when the piston, rising towards its maximum upper point (Top Dead Centre or TDC), compresses the air and the diesel fuel, causing a controlled explosion which transforms the movement of the piston into "thrust" impressed on the crankshaft.
Today, modern diesel engines use the common rail as a fuel system, which works with a pressure of 1600/2500 bar and is capable of performing 5/7 fuel injections divided into 3 precise times: the pre-injection, the main injection and the post-injection. Now, let's understand more deeply the differences between these injections.
The pre-injection is electronically controlled by the control unit to reduce the vibration and noise of the engine in the mid-stroke of the piston, while it rises to compress the air, and before the main injection which is used by the engine to stay in motion.
The main injection is the one that starts the combustion, transforming the chemical force into mechanical force that gives a thrust to the crankshaft.
While the post-injection is controlled to cool the chamber after combustion, mainly to increase the temperature of the exhaust gases and facilitate the regeneration of the Diesel particulate filter (DPF), bringing the temperature up to 500/700 degrees, necessary for regeneration of the same.
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