Introduction
The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to mechanical compression; thus, the diesel engine is a compression-ignition or compression-ignition engine. This contrasts with engines using spark plug-ignition of the air-fuel mixture, such as a gasoline engine, where the ignition is caused by an electric spark. The main advantages of diesel engines are partially attributed to this type of ignition: the compression causes a high temperature which allows self-ignition of the injected diesel fuel. This eliminates the need for an ignition system and spark plugs, and with carefully shaped combustion chambers, achieves a much better specific fuel consumption than in gasoline engines. Additional advantages of diesel engines include an unthrottled intake system that supplies air at full pressure, and this improves the gas mileage.
History and Development
The diesel engine was named after its inventor, German engineer Rudolf Diesel, in 1892. His idea was to design an engine that would have even greater efficiency than Otto cycle engines. The first diesel engines had very low efficiency, but efficiency increased rapidly as designers developed electronic control, fuel injection, and advanced turbocharging. By the 1930s, diesel engine powered ships and trains were common, and they kept gaining popularity.
In the 1950s, advanced diesel two-stroke crosshead engines were used to power large oceangoing merchant ships. By the 1970s, turbocharged diesel-electric powertrains were popular for locomotives and other applications. Today, most commercial ships and half of all locomotive traction use diesel engines, and about half of the world’s new automobiles now use diesel engines. While gasoline engines remain dominant in smaller vehicles and applications requiring a power-to-weight ratio, diesel engines are more fuel efficient and generate greater torque. Advances have allowed diesel cars to meet emissions standards in many countries.
Combustion Process in Diesel Engines
The basic difference between a diesel engine and spark-ignition gasoline engine is the manner of introducing air and fuel and initiating combustion. In gasoline engines, the air-fuel mixture is compressed and ignition occurs by spark. In a diesel engine, only air is present in the combustion chamber during the initial compression stroke. Near the end of compression, fuel is injected directly into the compressed hot air in the combustion chamber. The fuel ignites spontaneously without any spark due to the combination of high temperature and high pressure. The timing of the introduction of fuel is precisely controlled by the fuel injection system as per engine load and speed.
The combustion process in diesel engines consists of the following stages:
Air is drawn into the combustion chamber during the intake stroke. A small amount of residual exhaust gases also remains in the cylinder from the previous cycle.
The piston rises on the compression stroke, compressing the trapped air to high density. Towards the end of compression stroke, fuel is injected directly into the cylinder by a fuel injection system.
The injected fuel ignites spontaneously due to the high temperature of the compressed air without any spark plug. This is a key difference from a gasoline engine.
Rapid combustion takes place with the help of excess air content, and the fuel burning proceeds at a subsonic rate from the surface of the fuel droplets inward.
The expanding gases exert pressure on the piston, providing power to rotate the crankshaft. This is the power stroke.
The exhaust gases are expelled from the cylinder during the exhaust stroke. Some residual exhaust again remains in the cylinder.
A small amount of fresh air is then taken in through the inlet port during the inlet stroke, and the cycle repeats.
Components of Diesel Engines
Here are the major components of a typical diesel engine:
Cylinder block – It is the main structure of an engine that houses the cylinders. It supports the reciprocating assembly and connecting rod.
Cylinder head – It encloses the head end of the cylinders and houses the valves, injectors, ports, etc. It is fastened to the cylinder block.
Combustion chambers – The space inside each cylinder where combustion takes place is called the combustion chamber.
Pistons – They are cylindrical structures with a crown at one end which fits tightly within the cylinder. They transmit power to the crankshaft during combustion.
Crankshaft – It transforms reciprocating motion of the pistons to rotational motion required for various applications.
Connecting rods – They connect each piston to the crankshaft to transfer motion between them.
Camshaft – It operates the intake and exhaust valves using cams and pushrods or overhead cams depending on engine design.
Fuel injection pump – It precisely times and meters fuel delivery to each cylinder via an injector.
Injectors – They inject fuel directly into the combustion chamber at the appropriate time.
Air intake system – It draws clean filtered air into each cylinder.
Exhaust system – It carries expelled exhaust gases out of the engine.
Turbocharger – It boosts intake air density using exhaust gas energy to improve power output.
Types of Diesel Engines
There are mainly four types of diesel engines based on their speed, size, and configuration:
Two-stroke engines – They have higher power-to-weight ratio than 4-stroke but emit more emissions. The spent gas push out method does not require valves. Used in large ships.
Four-stroke engines – Most common type with separate strokes – intake, compression, power, exhaust. Lower power density than 2-stroke but cleaner. Used widely in vehicles, generators.
Medium-speed engines – These operate between 750-1500 rpm and are generally larger than slow-speed engines. Used in rail, marine applications requiring medium-power.
High-speed engines – Operate above 1500 rpm. Used mainly in trucks, buses, off-road machinery requiring better torque characteristics than gasoline engines. Features higher injection pressures.
Low-speed engines – Operate below 750 rpm. They are larger in size mainly used for marine propulsion and electricity generation requiring maximum torque at low speeds. Used in large container ships and vessels.
Advantages and Disadvantages of Diesel Engines
The diesel engine has some advantages and disadvantages compared to gasoline engines.
Advantages:
Higher fuel efficiency – Diesel engines have thermal efficiency that is 10-15% higher than gasoline engines. Fuel consumption is 20-30% less.
Generates more torque – Compression ignition provides more torque at slower speeds compared to gasoline engines. Useful for heavy vehicles.
Durability – Diesel engines are more robust with fewer moving parts. Durable and last over 1.5 times longer than gasoline engines.
Better low-end torque – Facilitates easier driving in stop-start urban traffic without gear changes.
Uses less expensive fuel – Diesel has higher energy density than gasoline. Cheaper fuel costs offset initial higher cost of diesel vehicles.
Disadvantages:
Higher initial cost – Diesel engines themselves are generally more expensive to produce than gasoline equivalents due to heavier components.
Higher noise levels – Noisier than similar gasoline engines due to direct fuel injection. Noise emission regulations have improved this.
Cold starting issues – More time and ignition source like glow plugs required to start in cold weather. Modern designs have overcome this.
Costly emissions control – Advanced emissions control systems are costlier for diesels. Regulations are tightening up.
Soot emissions – Diesel combustion produces more particulates (soot) emissions than gasoline engines. Newer catalysts have addressed it largely.
While diesel engines have better fuel efficiency and torque, advances in gasoline direct injection engines are closing the gap rapidly on these fronts. Diesel emissions control remains a challenge but advances are keeping diesel favorable against gasoline engines for commercial applications. Diesel engines will continue to play a vital role in vehicles, ships, power generation in years to come.
Applications of Diesel Engines
Diesel engines are widely used in the following applications due to their versatility:
Road vehicles – Heavy commercial vehicles like trucks, buses. Most popular in Europe. Recent surge in India and China. Mid-size SUVs now available.
Marine applications – Ships use low-speed 2-stroke marine diesels for main propulsion. Inland container ships. High-speed craft use medium-speed diesel engines.
Railway locomotives – Both passenger and freight locomotives commonly use diesel-electric or diesel-mechanical powertrains.
Port equipment – Cranes, forklifts, excavators and other cargo handling equipment use diesel engines.
Standby power – Electricity generators for emergency/backup power use diesel generators in data centers, hospitals, industries.
Construction equipment – Bulldozers, loaders, pavers use diesel engines for mobility and work functions. Off-road ability is crucial.
Agriculture equipment – Tractors, harvesters, sprayers are powered by diesel engines to endure harsh conditions.
Mining equipment – Dump trucks, drills, shovels require durable and reliable diesel engines to work in mines.
Military vehicles – Tanks, trucks, ships use diesel engines for mobility needs in defense applications.
Pumps – Irrigation, drainage, firefighting and other pumping applications employ stationary diesel engines.
Power generation – Diesel-generator sets provide off-grid
Andrew Stroud