About this Article
Written by: Paul Moldovan
Written on: November 1st, 2010
Tags: chemical engineering, energy & sustainability, transportation
Thumbnail by: Matthias93/Wikimedia Commons
About the Author
Paul Moldovan was a junior at the University of Southern California in Falll 2010. He majored in Electrical Engineering with an emphasis in Power Engineering.
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Volume XI Issue III > Diesel Engines: Is Old Technology Actually Green Technology?

How It Works

The diesel engine is an internal combustion engine; combustion is taking place inside an enclosed cylinder. It’s a four-stroke engine, exactly like gasoline engines. The basic principles of how a diesel engine works are illustrated in Fig. 1.
UtzOnBike/Wikimedia Commons
Figure 1: A 4-stroke engine.
A four stroke engine can be described in four separate steps or strokes. The first stroke is the intake stroke, which opens up the engine valves and allows air to enter the cylinder. The second stroke is the compression stroke, which moves the piston up inside the cylinder as the valves close and compresses the air. The combustion stroke is the third stroke. In this step fuel is injected into the top of the cylinder when the piston is near the top. Because the air in the cylinder is compressed, it becomes heated and this causes the diesel fuel to spontaneously combust and cause an explosion. During the fourth stroke of the cycle, the piston moves back down and clears the chamber of the exhaust through an exhaust valve [3].
This is a continuous, repetitive process that occurs thousands of times a minute. It is important to note that unlike a gasoline engine, there is no spark required for an explosion to take place inside the cylinder. In a gasoline powered engine, combustion is achieved by the spark plug igniting the fuel and air mixture in the cylinder. Diesel engines achieve the explosion purely through the heat caused by high pressure inside the cylinder. The fuel is also injected into the cylinder at high pressure. A high injection pressure leads to fine fuel mist entering the cylinder, which causes a more powerful explosion and a more complete burning of the fuel aggregates. The black smoke and soot that comes out of the exhaust pipe of diesel engines can be attributed to low injection pressure and an inefficient diesel engine.

Clean Diesel Technology

The development of clean diesel is actually a composite of three different evolved aspects working together to make diesel engines greener and more efficient; to achieve these goals, engineers have utilized clean fuels, cleaner engines, and emission controls.


Clean fuel is the sole aspect that isn’t completely controlled by the automotive industry. While it’s true that researchers and engineers who work in fuel industries develop cleaner fuel options, in the end it is a governmental decision to implement regulations that require the importation and use of cleaner fuels and banning fuels which cause harmful emissions. Ultra Low Sulfur Diesel (ULSD) began arriving in the United States in 2006 [4]. The new cleaner diesel fuel has 15 parts per million of sulfur whereas its predecessor had 500 parts per million of sulfur content. ULSD allows for a reduction in sulfur emissions as well as the implementation of other emission controls that would not be possible with a high sulfur concentration [5].


Cleaner diesel engines incorporate high pressure fuel injectors and stronger materials in the engine block to make the engine more efficient. The fuel injectors spray the fuel into the cylinder to cause an explosion and move the pistons. Because there is no spark to ignite the fuel, fine fuel particulates must be introduced into the cylinder, which ignite because of the high pressure [6]. Currently the newest fuel injectors being developed inject fuel into the cylinder at around 35,000 psi; the higher the injection pressure into the cylinder, the finer the fuel mist that is injected into the cylinder. This means that more fuel is burned inside the cylinder and less is expelled as exhaust, which would raise the emissions of the engine.
The new materials and forging processes available to engineers today also allow for the construction of cleaner, more efficient engines. New composite materials and a better understanding of the mechanics behind engines have allowed engineers to develop engines which transform more of the energy in the fuel into mechanical energy. This also allows for a reduction in the amount of energy that is lost as heat [6]. Such technology actually has benefited all types of engines, but it’s more noticeable on diesel engines for which technology hasn’t changed drastically since it was created.

Exhaust Control

Post-combustion filters and catalytic converters also are used in the exhaust pipe to trap particulate matter that escapes from the cylinder. The NOx and particulate matter emissions can be brought down to near the level of hybrid cars with a combination of fine filters and catalytic converter technologies. Currently, new catalytic converters are being developed that contain urea, which reacts with NOx particulates and renders them harmless, another potential future improvement in exhaust control.