A diesel engine is a complex machine composed of many components with different functions, consisting of two main mechanisms and four major systems: the crankshaft and connecting rod mechanism, the valve train, the fuel supply system, the lubrication system, the cooling system, and the starting system. Because diesel engines use compression ignition, they eliminate the ignition system compared to gasoline engines. However, whether it’s a four-stroke, two-stroke, or single-cylinder diesel engine, to complete energy conversion and achieve a working cycle, it must possess these two main mechanisms and four major systems. This article from Cummins focuses on the structure, function, and working principle of each component of a diesel engine.
I. Crank-connecting rod mechanism
The crankshaft and connecting rod mechanism is the main moving part of a diesel engine that realizes the working cycle and completes energy conversion. It consists of the engine block assembly, piston and connecting rod assembly, and crankshaft and flywheel assembly. The working principle of the crankshaft and connecting rod mechanism is very simple, yet it has great practicality and efficiency. When the crank rotates, the connecting rod reciprocates, thereby driving the piston in linear motion. This type of motion has important applications in many mechanical devices. By controlling the crank speed and the length of the connecting rod, the piston stroke and speed can be adjusted to meet different working requirements.
1. Engine Block Assembly
The engine block assembly includes the cylinder block, cylinder head gasket, cylinder head, crankcase, and oil pan. These components form the basic framework of the engine, providing space for the pistons and other moving parts.
2. Piston and Connecting Rod Assembly
The piston and connecting rod assembly includes the piston, piston rings, piston pin, and connecting rod. The piston moves up and down within the cylinder, connected to the crankshaft via the connecting rod, converting the piston’s reciprocating motion into the crankshaft’s rotational motion.
3. Crankshaft and Flywheel Assembly
The crankshaft and flywheel assembly includes the crankshaft and flywheel. The crankshaft is connected to the flywheel via a torsional damper and a balance shaft. It converts the piston’s linear motion into rotational motion, which is then transmitted through the flywheel. The flywheel also stores energy, ensuring smooth engine operation.
II. Gas Distribution Mechanism
Diesel engines require an air supply to operate, and this air is distributed by the valve train. The function of the valve train is to open and close the intake and exhaust valves at set times according to the diesel engine’s firing order and operating process, allowing the combustible mixture or air to enter the cylinders and the exhaust gases to be expelled, thus achieving the scavenging process. Most valve trains are overhead valve type, generally consisting of a valve assembly, a valve train assembly, and a valve drive assembly.
1. Valve Assembly
The valve assembly includes valves, valve guides, valve springs, spring seats, retaining clips, and retaining rings. The valve assembly primarily ensures a tight fit between the valve cone and the valve seat cone, providing sufficient strength and wear resistance under high-temperature conditions.
(1) Valve. A valve consists of a valve head and a valve stem. The conical surface of the valve head is typically a 45° conical ring that fits against the conical bore ring of the valve seat, forming a sealing ring.
(2) Valve Guide. Used to guide the valve in linear motion, ensuring alignment between the valve and valve seat. To prevent valve jamming or oil leakage into the cylinder, the clearance between the valve stem and guide should be appropriate.
(3) Valve Seat. A separate seat ring made of wear-resistant material is pressed into the seat bore of the cylinder head with appropriate tightness.
(4) Valve Spring. Each valve is equipped with two springs, one inside and one outside, with opposite directions of rotation, to ensure timely automatic valve closure. (5) Valve Spring Seat and Locking Clip. The upper valve seat locking clip is locked at the tail of the valve stem, restricting the free extension of the valve spring. During installation, the gaps on both sides of the two locking clip halves should be equal and lower than the plane of the upper valve seat to ensure a secure lock.
2. Valve Transmission Assembly
The main function of the valve transmission assembly is to transmit power according to the outer contour shape of the cam, so that the valve opens and closes on time. The valve transmission assembly mainly consists of tappets, pushrods, rocker arms, rocker arm shafts, etc.
(1) Tappet. The tappet is the follower of the cam, converting the rotational motion of the cam into the linear motion of the tappet, and transmitting the thrust of the cam to the valve through the pushrod and rocker arm. The tappet has a simple structure and is lightweight. The radius of the pushrod spherical support on the tappet is slightly larger than the radius of the pushrod ball head, so as to form a wedge-shaped oil film between them to lubricate the pushrod ball head and the spherical support on the tappet.
(2) Pushrod. The pushrod is positioned between the tappet and the rocker arm, transmitting the motion and force from the tappet to the rocker arm. In a camshaft-down valve train, the pushrod is a slender rod, and due to the large force it transmits, it is extremely prone to bending. Therefore, the pushrod is required to have good longitudinal stability and high rigidity.
(3) Rocker Arm. The function of the rocker arm is to change the direction of the motion and force transmitted from the pushrod and camshaft, transmitting it to the valve to open it. The rocker arm is a double-arm lever with the rocker arm shaft as the fulcrum, and the two arms are of unequal length. The shorter arm end is machined with a threaded hole for screwing in the valve clearance adjustment screw. The longer arm end is machined into an arc surface, which is the working surface for pushing the valve.
(4) Rocker Arm Shaft. Fixed to the cylinder head, it supports the rocker arm, and its cavity also serves as an oil passage.
III. Fuel Supply System
The function of the fuel supply system is to prepare a certain amount and concentration of air-fuel mixture according to the requirements of the diesel engine, supply it into the cylinder, and discharge the exhaust gas after combustion from the cylinder into the atmosphere; the function of the fuel supply system of the diesel generator is to supply diesel and air into the cylinder separately, form a mixture in the combustion chamber and burn it, and finally discharge the exhaust gas after combustion.
1. Fuel Tank
A container for storing fuel, supplying fuel to the fuel system. The fuel tank is generally located near the diesel engine and connected to the fuel filter via fuel lines.
2. Fuel Filter
Filters the fuel, removing impurities and particulate matter. Maintains fuel purity and reduces damage to the injectors and fuel pump.
3. Fuel Pump
Draws fuel from the fuel tank and pressurizes it to supply fuel to the injectors. A fuel pump generally consists of drive gears, plungers, and a plunger pump.
4. Injector
Atomizes fuel into tiny particles and injects it into the cylinders. An injector generally consists of the injector nozzle, injector needle valve, and nozzle orifice.
IV. Lubrication System
Diesel engines operate based on the relative motion of their parts, which inevitably generates friction and wear. Dry friction produces a large amount of heat, raising the temperature of parts, accelerating wear, causing surface scratches, adhesion, and even seizing. Therefore, a diesel engine cannot function without lubrication; poor lubrication will lead to malfunctions and shorten its lifespan. Statistics show that 50% to 60% of diesel engine malfunctions in use are caused by improper lubrication. Therefore, diesel engines require a good lubrication system that continuously supplies lubricant (oil or grease) to all friction surfaces to reduce friction, minimize wear, and protect the surfaces. In addition to lubrication, lubricating oil also dissipates heat, removes contaminants from friction surfaces, provides short-term rust prevention, and seals the cylinder and piston assembly.
1. Oil Pump
A rotary oil pump consists of an inner rotor, an outer rotor, and a pump housing. When the pump is working, the inner rotor drives the outer housing to rotate in the same direction. The inner rotor has four convex teeth, and the outer rotor has five concave teeth; they can be considered as a pair of internal meshing drives differing by only one tooth, with a speed ratio of 5:4. Regardless of the rotor’s rotation angle, there is always contact between the teeth of the inner and outer rotors, creating five cavities. The cavity on the oil inlet side, due to the rotor disengaging, increases in volume, creating a vacuum (i.e., the pressure inside the cavity is lower than the external pressure), drawing oil into it. The radial clearance of the outer rotor is approximately 0.06–0.15 mm. Excessive clearance reduces the pumping efficiency, resulting in insufficient oil supply. When wear increases the axial clearance to 0.15 mm and the radial clearance to 0.22 mm, the lubricating oil volume and oil pressure will significantly decrease, affecting normal lubrication. In this case, the axial clearance of the rotor can be adjusted by adjusting the shims between the pump body and the pump cover. 1. Replace the inner and outer rotors when the meshing clearance exceeds the wear limit.
2. Oil Filter
Oil filters can be divided into two main categories according to their filtration method: filtration type and centrifugal type. Filtration type filters can be further classified according to their structure into screen type, scraper type, paper filter element type, wire-wound type, and composite type. Tractor diesel engines widely use single-stage paper filters. The filter element is a cylindrical pleated paper filter element. The filter paper is chemically treated and has numerous micropores, with both ends bonded and sealed to end caps. Oil is filtered when it enters the inner cavity through the micropores of the filter paper and then flows into the main oil passage. The oil pressure in the main oil passage can be adjusted using the pressure regulating valve on the filter; this pressure is generally 0.2–0.4 MPa. During the filtration process, if the filter element is too dirty or the viscosity of the lubricating oil increases during a cold start, the resistance to oil passing through the filter element increases. If the oil level in the oil passages is insufficient, the oil will open the bypass valve (safety valve) and enter the oil passages directly without filtration. The opening pressure of the bypass valve is generally 0.1~0.2 MPa.
3. Oil Pan
The oil pan is a container for storing lubricating oil, usually located at the bottom of the diesel engine. The oil pan has a certain volume to ensure the normal circulation of lubricating oil during diesel engine operation. An oil level gauge is also installed in the oil pan to monitor the lubricating oil level and replenish it in time.
4. Oil Cooler
The oil cooler is an accessory component in the lubrication system. Its main function is to cool the lubricating oil, lower its temperature, and prevent overheating.
Diesel engine lubrication system structure and principle
Analytical diagram of oil filter parts
V. Cooling System
The function of the cooling system is to dissipate the heat absorbed by heated parts in a timely manner, ensuring that the diesel engine operates at the most suitable temperature. The cooling system of a water-cooled diesel engine typically consists of a coolant jacket, water pump, fan, water tank, thermostat, etc.
1. Cooling Water Pump
The water pump is the core component of the diesel engine cooling system. Its main function is to circulate water through an impeller, creating a continuous cycle to effectively dissipate heat from the diesel engine. The water pump consists of an impeller, water seal, and bearings; their quality directly affects the stability of the diesel engine.
2. Radiator
The radiator is where the coolant flows. Its double-layered metal fins, with alternating bends, ensure sufficient contact for better heat dissipation. Its function is to transfer heat dissipated from the diesel engine to the surrounding environment, lowering the temperature and maintaining the diesel engine within its normal operating temperature range.
3. Thermostat
The thermostat is a control device that regulates the coolant temperature. It automatically adjusts the water temperature based on the diesel engine’s operating temperature. It is typically located at the outlet of the radiator system.
4. Radiator Fan
The fan is an important component of the diesel engine cooling system. Its main function is to dissipate wind resistance generated by the vehicle’s movement and accelerate airflow. Airflow over the radiator helps it dissipate heat more quickly, maintaining the diesel engine’s normal operating temperature.
Schematic diagram of diesel engine coolant circulation path
Diesel engine coolant circulation flow chart
VI. Starting System
To transition a diesel engine from a stationary state to a working state, an external force must first rotate the crankshaft, causing the pistons to reciprocate. The combustible mixture in the cylinders burns and expands, doing work and pushing the pistons downwards, thus rotating the crankshaft. Only then can the diesel engine start automatically, and its working cycle begin automatically. Therefore, the entire process from the crankshaft starting to rotate under external force to the diesel engine automatically idling is called starting the diesel engine. The device required to complete the starting process is called the diesel engine starting system. The electric starting system mainly consists of a starter motor, battery, generator, and main controller.
1. Battery
When starting the diesel engine, the battery provides the necessary power to the starter motor, ensuring a smooth start.
2. Starter Motor
The starter motor converts electrical energy into mechanical energy. Through control devices such as electromagnetic switches, starter relays, and ignition switches, it drives the engine flywheel to rotate, thus starting the engine.
3. Rechargeable Generator
The rechargeable generator is a DC generator with a rotor containing permanent magnets and coils. When the rotor rotates, the magnetic field between the permanent magnets and coils changes, generating an induced electromotive force, ultimately converting mechanical energy into electrical energy.
4. Main Controller
The main controller is the command device that controls the diesel generator’s starting, speed regulation, braking, and reversing by changing the wiring of the main circuit or control circuit and altering the resistance values in the circuit according to a predetermined sequence. It consists of a program counter, instruction register, instruction decoder, timing generator, and operation controller. It is the “decision-making body” that issues commands, coordinating and directing the operation of the entire computer system and controlling the overall operation of the generator set.
Schematic diagram of the internal structure of a diesel engine starter
Schematic diagram of diesel engine starter solenoid switch
In summary:
A diesel engine is an internal combustion engine that uses a compressed fuel-air mixture to ignite. Compared to gasoline engines, diesel engines are more efficient, economical, and durable. In short, a diesel engine has a complex structure, composed of multiple components. Each component has its own characteristics and functions, forming the overall operating system of the diesel engine. Understanding the structure of a diesel engine is crucial for its proper operation, inspection, and maintenance.
