What is Diesel Engine and How Does it Work?

What is Diesel Engine?

The diesel engine, named after Rudolf Diesel, is an internal combustion engine in which the fuel is ignited by the increased temperature of the air in the cylinder as a result of mechanical compression; thus, the diesel engine is a so-called compression-ignition engine. This is in contrast to engines that use spark plug ignition of the air-fuel mixture, such as a gasoline engine or a gas engine.

Diesel engines work by compressing only air or air plus residual combustion gases from the exhaust. Air is introduced into the chamber during the intake stroke and compressed during the compression stroke. This increases the air temperature in the cylinder so much that atomized diesel fuel that is injected into the combustion chamber ignites.

Since the fuel is injected into the air just before combustion, the distribution of the fuel is uneven; this is called a heterogeneous air-fuel mixture. The torque produced by a diesel engine is controlled by manipulating the air-fuel ratio (λ); Instead of throttling the intake air, the diesel engine relies on a change in the amount of fuel injected, and the air-fuel ratio is usually high.

The diesel engine has the highest thermal efficiency (engine efficiency) of any practical internal or external combustion engine due to its very high expansion ratio and its inherent lean burn, which allows heat dissipation through the excess air.

A small efficiency loss is also avoided compared to non-direct injection petrol engines as there is no unburned fuel during valve overlap and therefore no fuel passes directly from intake/injection to exhaust.

Low-speed diesel engines can achieve effective efficiencies of up to 55%. The combined cycle gas turbine is an internal combustion engine that is more efficient than a diesel engine, but its mass and dimensions make it unsuitable for vehicles, watercraft, or aircraft.

What is Diesel Engine and How Does it Work

How Do Diesel Vehicles Work?

Diesel vehicles are similar to gasoline vehicles because they both use internal combustion engines. One difference is that diesel engines have a compression-ignited injection system rather than the spark-ignited system used by most gasoline vehicles.

In a compression-ignited system, the diesel fuel is injected into the combustion chamber of the engine and ignited by the high temperatures achieved when the gas is compressed by the engine piston.

Unlike the emission control systems on gasoline vehicles, many diesel vehicles have additional after-treatment components that reduce particulate matter and break down dangerous nitrogen oxide (NOx) emissions into harmless nitrogen and water. Diesel is common transportation fuel, and several other fuel options use similar engine systems and components.

How does a diesel engine turn fuel into power?

Animation showing the four stages of a diesel engine: intake, compression, power, and exhaust.

Four-stroke engines:

Like a gasoline engine, a diesel engine normally works by repeating a cycle of four stages or strokes during which the piston moves up and down twice during the cycle (in other words, the crankshaft rotates twice).

  • Intake: Air is drawn into the cylinder through the open green air intake valve on the right as the piston moves down.
  • Compression: The intake valve closes, the piston moves up, compressing and heating the air mixture. Fuel (dark blue) is injected into the hot gas through the central fuel injection valve and ignites spontaneously. Unlike a petrol engine, no spark plug is required for this.
  • Power: As the air-fuel mixture ignites and burns, it pushes the piston down and drives the crankshaft, which sends power to the wheels.
  • Exhaust: The green exhaust valve on the left opens to release the exhaust gases expelled by the returning piston.

Two-stroke engines:

In a two-stroke diesel, the complete cycle happens with the piston moving up and down just once. Confusingly, there are really three phases in a two-stroke cycle:

  • Exhaust and Intake: Fresh air is blown into the side of the cylinder and pushes the old exhaust out through valves at the top.
  • Compression: The intake and exhaust valves close. The piston moves up, compressing and heating the air. When the piston reaches the top of the cylinder, fuel is injected and spontaneously ignites.
  • Power: When the air-fuel mixture ignites, it pushes the piston down and drives the crankshaft, which sends power to the wheels.

Two-stroke engines are smaller and lighter than four-stroke engines and tend to be more efficient because they generate electricity once per revolution (rather than once every two revolutions like a four-stroke engine). This means they require more cooling and lubrication and are subject to higher wear.

Types Of Diesel Engines

There are three basic size groups of diesel engines based on power—small, medium, and large.

1. Small Diesel Engine

The small engines have power-output values of less than 188 kilowatts or 252 horsepower. This is the most commonly produced diesel engine type. These engines are used in automobiles, light trucks, and some agricultural and construction applications and as small stationary electrical-power generators (such as those on pleasure craft), and as mechanical drives. They are typically direct-injection, in-line, four- or six-cylinder engines. Many are turbocharged with aftercoolers.

2. Medium Diesel Engine

Medium engines have power capacities ranging from 188 to 750 kilowatts, or 252 to 1,006 horsepower. The majority of these engines are used in heavy-duty trucks. They are usually a direct injection, in-line, six-cylinder turbocharged, and aftercooled engines. Some V-8 and V-12 engines also belong to this size group.

3. Large Diesel Engine

Large diesel engines have power ratings in excess of 750 kilowatts. These unique engines are used for marine, locomotive, and mechanical drive applications and for electrical power generation. In most cases, they are direct injection, turbocharged, and aftercooled systems. They may operate at as low as 500 revolutions per minute when reliability and durability are critical.

Applications & Uses for Diesel Engines

Diesel engines are commonly used as mechanical engines, power generators, and mobile drives. They find widespread use in locomotives, construction equipment, automobiles, and countless industrial applications. Their realm extends to almost all industries and can be observed on a daily basis if you were to look under the hood of everything you pass by.

Industrial diesel engines and diesel-powered generators have construction, marine, mining, hospital, forestry, telecommunications, underground, and agricultural applications, just to name a few. Power generation for prime or standby backup power is the major application of today’s diesel generators.  Check out our article on the various types of engines and generators and their common applications for more examples.

Advantages of a Diesel Engine

The diesel engine is much more efficient and preferable as compared with gasoline engine due to the following reasons:

  • They are more rugged and reliable
  • There is no sparking as the fuel auto-ignites. The absence of spark-plugs or spark wires lowers maintenance costs
  • Fuel cost per Kilowatt produced is thirty to fifty percent lower than that of gas engines
  • An 1800 rpm water cooled diesel unit operates for 12,000 to 30,000 hours before any major maintenance is necessary. An 1800 rpm water cooled gas unit usually operates for 6000-10,000 hours before it needs servicing
  • Diesels get great mileage. They typically deliver 25 to 30 percent better fuel economy than similarly performing gasoline engines.
  • Diesel fuel is one of the most efficient and energy dense fuels available today. Because it contains more usable energy than gasoline, it delivers better fuel economy.
  • Diesel engines are built to withstand the rigors of higher compression. Consequently, they usually last much longer than gas-powered vehicles before they require major repairs.

Disadvantages of a Diesel Engine

  • Although diesel fuel used to be cheaper than gasoline, it now often costs the same amount or more.
  • Although diesel fuel is considered to be more efficient because it converts heat into energy rather than sending the heat out the tailpipe as gas-powered vehicles do, it doesn’t result in flashy high-speed performance.
  • Diesels still need regular maintenance to keep them running. You have to change the oil and the air, oil, and fuel filters.
  • If you neglect the maintenance and the fuel injection system breaks down, you may have to pay a diesel mechanic more money to get things fixed than you would to repair a gasoline system because diesel engines are more technologically advanced.