How the diesel engine works and its advantages

The internal combustion engine is a machine that obtains mechanical energy directly from the chemical energy consumption of fuel burned in a combustion chamber, which is an integral part of an engine.

In 1867, in Germany, the Otto engine with free piston was developed by Nicolaus August Otto (1832-1891) and Eugen Langen (1833-1895), based on the burning of a mixture of air and fuel by a gas flame inside a a cylinder. Such an engine achieved a thermal efficiency of 11%. After developments in the functional aspect aimed at greater thermal efficiency, Otto, in 1876, managed to develop the engine based on four strokes – intake, compression, expansion or power and discharge. An engine with reduced weight and volume and high thermal efficiency was obtained. This was the breakthrough that effectively founded the internal combustion engine industry, leading to current engines powered by gasoline, alcohol and natural gas (CNG).

A few years later, in 1892, Rudolf Diesel (1858-1913) developed a different engine, in which a high compression ratio was used to burn the fuel. The fuel was injected near the end of the compression phase where it was then burned by highly heated compressed air. The efficiency of this engine was increased due to the high compression ratio and expansion ratios. The current Diesel engine is designed on the same operating principle and is developed in four strokes and two strokes.

USE OF DIESEL ENGINES

In Brazil, the Diesel engine is prohibited in passenger, cargo and mixed-use vehicles, national and imported, with a transport capacity of less than 1.000kg, considering the weights of the driver, crew, passengers and cargo, with the exception of motor vehicles called jeeps, with four-wheel drive, multiple gearbox and gearbox.

Compared to other engines, the Diesel engine is 30% more economical than its gasoline counterpart and CO2 emissions are 25% lower. In addition to performance, it has greater torque. Despite costing 10% to 12% more expensive than the gasoline engine, the Diesel engine is preferred for users who use the vehicle for high mileages, as the economic return is due to the lower fuel price, in addition to the advantage greater durability.

Theoretically, this engine has constant volume combustion and uses a high compression ratio. They are generally low-speed engines, with a speed between 100rpm and 750rpm when compared to the 2.500rpm to 5.000rpm of a gasoline engine. Some types, however, work at speeds of up to 2.000 rpm. Because they use compression ratios of 14:1 or more, they are generally heavier, and this disadvantage is offset by their greater efficiency and the fact that they can be operated with cheaper fuel oils.

In the agricultural sector, the main engines used are the four-stroke Otto cycle for low-displacement engines, and two-stroke engines for portable machines such as chainsaws, brushcutters, sprayers and drillers, and the four-stroke Diesel cycle, which is the most used in conditions need for high torque in agricultural tractors, utility vehicles, trucks and large power generators.

ENGINE COMPONENTS

The essential parts of four-stroke engines are classified as stationary parts (block, crankcase and head), moving parts (piston, connecting rod, crankshaft, camshaft, valves, valve drive assembly, gears and pulleys), pumps (oil and water pump), bearings (slip and rolling) and sealing components (gaskets, rings and retainers).

The cylinders are located in the engine block. In its upper part there is the cylinder head and in the lower part, the crankcase. The block is normally made of gray cast iron, as it has great resistance to wear and compression and has a low manufacturing cost. More modern vehicle engines, which seek lightness and greater heat dissipation capacity, are already made of aluminum.

The block has internal ducts for the passage of water for cooling. In some models, the cylinders are covered with a steel and nickel alloy liner or coated with hard chrome, normally more resistant than the block, to allow a longer useful life, as the liners can be changed when they show wear.

The piston, normally made of aluminum, works in an alternating movement in the cylinder, transmitting the force of the expansion gas to the connecting rod and then to the shoulder of the crankshaft shaft, which rotates, supplying the engine's power to the other sectors. Compression rings and oil scrapers are located on the piston. Three steel rings are responsible for engine compression and can be chromed or nitrided. Two-piece oil rings are produced from either gray or nodular cast iron or steel. The first ring, which is almost at the head of the piston, has the function of containing the pressure generated by the explosion and preventing loss of pressure during compression. The second and third rings have the function of helping to retain compression like the first and creating a film of oil when it scrapes the internal walls of the cylinder. The oil ring's function is to scrape off excess oil and create a thin lubrication film so that the other rings have minimal friction, preventing wear between the rings and cylinder.

The connecting rod and the crankshaft shaft shoulder transmit the linear movement of the piston to the circular motion of the crankshaft. The smaller end of the connecting rod works in an alternating movement with the piston and the larger part performs the rotational movement with the crankshaft. The connecting rod is normally manufactured from forged steel, with an alloy steel bushing and pin that fix it to the piston. On the opposite side, it has bushings or bearings made of lead and tin or bronze.

The cylinder head is the cylinder cover and is made of aluminum or cast iron and has the injection nozzles. The cylinder head also contains the valve drive system and injection nozzles. Current engines have four valves per cylinder – two intake valves to allow the air charge to enter the cylinder and two exhaust valves to exit the burnt gases from the cylinder. The intake valves have a larger diameter than the discharge valves and are made of alloy steel. As they work in a fuel burning environment, they are exposed to temperatures of approximately 700oC. The valves are actuated through a valve control system and rocker arms. They are normally closed by means of spring pressure. They are opened when the camshaft lobe actuates each rocker arm. The camshaft is driven by the crankshaft shaft by gears, belt or toothed chain. Two turns of the crankshaft provide one turn of the valve train.

At the bottom of the engine block is the crankcase, which closes the block with a molded steel or aluminum cover. The crankcase functions as the oil reservoir that lubricates the system. The lubricating oil is sucked in by an oil pump, driven by the crankshaft, and is directed to the moving parts of the engine through internal channels. The water pump, fan shaft and dynamo are driven by the crankshaft shaft by a system of belts and pulleys.

The fuel oil is sucked from the tank by a low pressure pump, and after passing through a series of decanters and filters, it is taken to the injection pump which has the function of sending a certain flow of fuel at high pressure to the injection nozzles, according to with the throttle position.

ENGINE OPERATION

The cylinder, usually fixed, is closed at one end and is where an intimately adjusted piston slides. The back and forth movement of the piston varies the volume of the cylinder between the upper face of the piston and the closed end of the cylinder. The lower face of the piston is connected to the crankshaft shaft by means of a connecting rod. The crankshaft transforms the reciprocating movement of the piston into circular motion. In multi-cylinder engines, the crankshaft has an eccentric part for each connecting rod so that power from each cylinder is applied to the crankshaft at the appropriate point in its rotation. Crankshaft axles have heavy flywheels and counterweights, which, due to their inertia, minimize the irregularity of the axle movement.

The four-stroke engine works in the intake, compression, expansion and discharge phases. During intake, air only enters through the intake valve, when the piston travels through the cylinder from top dead center (TDC) to bottom dead center (PMI). At this stage, only the intake valve is open. When the engine has a turbocharger system, the turbine of this system increases the amount of air in the cylinder, providing a greater amount of available oxygen and providing better burning and power when more fuel is injected.

In the second phase, the piston travels from PMI to TDC with the valves closed, the air is compressed to a small fraction of its initial volume and is heated to approximately 440oC due to this compression, with the pressure also increasing. When the piston almost reaches TDC, the fuel is injected into the combustion chamber through the injection nozzle and burns instantly due to the high air temperature in the chamber. Ignition occurs after a short delay and the pressure increases rapidly, resulting in a pressure wave.

This combustion moves the piston downwards in the third phase. The work is done by gas pressure on the piston. During the expansion phase, the temperature and pressure of the burned gas reduce. When the piston approaches the PMI, the discharge valve opens. At this stage, the chemical energy contained in the fuel is converted into mechanical energy.

The fourth stroke occurs with the piston rising and expelling the burned gases through the discharge valve. Near TDC, the discharge valve closes and the intake opens again and a new cycle is restarted with new air intake and the sequence of the other phases.

The engine and its operation

Bottom Dead Center (PMI) – This is the position that the piston reaches at the bottom of the cylinder of its stroke.

Top dead center (TDC) – This is the position that the piston reaches at the top of the cylinder of its stroke.

Compression chamber (Vc) – It is the volume contained in the cylinder at the top of the piston when it is at TDC.

Piston stroke volume (Vp) – This is the volume swept by the piston when it moves from PMI to TDC.

The engine capacity is calculated by multiplying the piston stroke volume (Vp) by the number of engine cylinders, and refers to the capacity that an engine has to absorb, in volume, in all engine cylinders.

Cylinder volume (V) – This is the total volume of the cylinder, including the piston stroke volume and the volume of the compression chamber.

Compression ratio (TC) – It is the ratio between the volume when the piston is at PMI and the volume when the piston is at TDC. Therefore, it is the ratio of the total volume of the cylinder (V) to the volume of the chamber (Vc).

By Ricardo Ferreira Garcia, Uenf