Diesel Engines

Hannu Jääskeläinen, Magdi K. Khair

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Abstract: The diesel engine, invented in the late 19th century by Dr. Rudolf Diesel, is the most energy efficient powerplant among all type of internal combustion engines known today. This high efficiency translates to good fuel economy and low greenhouse gas emissions. Other diesel features that have not been matched by competing energy conversion machines include durability, reliability, and fuel safety. The downsides of diesels include noise, low specific power output, NOx and PM emissions, and high cost.

What Is the Diesel Engine?

Most modern diesel engines use the conventional cylinder and piston arrangement operated with a slider crank mechanism common to other internal combustion engines such as the gasoline engine. Considering this basic mechanism, there is very little difference between the basic structure of diesel and gasoline engines.

Conceptually, diesel engines operate by compressing air to high pressure/temperature and then injecting a small amount of fuel into this hot compressed air. The high temperature causes the small amount of highly atomized injected fuel to evaporate. Mixing with the hot surrounding air in the combustion chamber, the evaporated fuel reaches its auto-ignition temperature and burns to release the energy that is stored in that fuel [391].

The definition of the diesel engine has evolved over the years. For example, in the early 20th century, a distinction was made between a “true Diesel Engine” and one that shared some aspects of the diesel cycle but did not encompass all aspects considered to be part of the diesel cycle as then envisioned. One early definition of a “true Diesel Engine” is one having the following features [2959]:

  1. Compression sufficient to produce the temperature requisite for spontaneous combustion of the fuel.
  2. Injection of fuel by a blast of compressed air.
  3. A maximum cycle pressure (attained during combustion) not greatly exceeding the compression pressure, i.e., absence of pronounced explosive effect.

While the first point of the above characteristics is consistent with the modern diesel engine, the last two are not. Over the course of the 1920s and 1930s, the other two characteristics lost their significance.

Solid-type fuel injection started to emerge around 1910, but it wasn’t until the end of the 1920s that it started to rapidly gain acceptance. It is interesting to note that Diesel himself chose air blast injection more out of necessity than choice. Diesel envisioned a solid-injection type of fuel system rather than an air-blast system.

Diesel was quite strict about adhering to constant pressure combustion, item 3. This, however, was only possible in the large relatively slow speed Diesel engines that were common prior to the 1920s. In the smaller high speed engines that emerged in the 1920s, practical considerations meant that combustion was closer to a constant volume process as in the Otto cycle rather the constant pressure as in the Diesel cycle.

A brief overview of the early history of the diesel engine is discussed elsewhere.