Green Diesel Engines

Modern diesel engines power much of the world’s equipment and, most notably, are prime movers commonly available today. Among personal and commercial vehicles, the diesel engines hold a significant market share worldwide. And their share and popularity are both increasing.

Diesel engine is a compression ignition engine of a 2- or 4-stroke type. From the p − V diagrams (Fig. 2.1), it can be seen that the duration of the whole diesel cycle is 360°CA for the two-stroke engine and 720°CA for the four-stroke engine. The whole cycle consists of the following phases: intake of air, compression of air, fuel injection, mixture formation, ignition, combustion, expansion, and exhaust. The intake phase begins with the intake valve opening and lasts till the intake valve closing. After that the intake air is compressed to a level corresponding to compression ratios from 14:1 to 25:1 (Bauer 1999) or even more. The compression ratio ε is a geometrical quantity, defined as

Diesel engine characteristics depend significantly on the engine type. But, even for a given engine type, the engine characteristics can still be varied in a wide range in dependence on engine management, exhaust gas after treatment, and usage of alternative fuels (Fino et al. 2003; Gray and Frost 1998; Maiboom et al. 2008; Peng et al. 2008; Stanislaus et al. 2010; Twigg 2007) (Fig. 3.1). Engine management and alternative fuels usage offer a possibility to reduce the formation of harmful emissions. On the other hand, exhaust gas after treatment techniques enable a reduction of harmful emissions already produced by the engine.

In recent years, the interest to use biodiesel as a substitute for mineral diesel has been increasing steadily. Biodiesel is a renewable fuel, consisting of various fatty acid methyl esters with the exact composition depending on the feedstock. This is a distinctly different composition than the hydrocarbon content of mineral diesel. In spite of that, biodiesel has many properties very close to those of mineral diesel. Consequently, the required biodiesel-related modifications of the diesel engine are typically rather minor. On the other hand, because of its different chemical character, biodiesel has several properties, which differ from those of mineral diesel just enough to offer an opportunity to reduce harmful emissions without worsening other economy and engine performances. It should be noted, however, that biodiesel properties may depend heavily on its raw materials.

A replacement of mineral diesel by biodiesel causes variations in injection process characteristics. Most notably, biodiesel usage influences the injection pressure, injection timing, and injection rate. Clearly, these effects depend to a great extent on biodiesel properties (raw material, production technology, ingredients, etc.), operating regime, and ambient temperature. This means that one has to deal with many highly interdependent influencing parameters. Consequently, the determination of these effects is a sophisticated process that is typically done only partially (e.g., only for one injection system type, for one biodiesel type, etc.) in individual research laboratories. Once such results are known, it is very difficult to estimate how these results can be used in slightly modified circumstances (e.g., when a somewhat different fuel is used). This is why one can often make only some general observations or guidelines. For example, many investigators show that in engines equipped with mechanically controlled fuel injection systems (MCFIS) the cyclic fuel delivery, pressure wave propagation time, average injection rate, and maximum pressure during injection are significantly affected when neat biodiesel replaces mineral diesel (Kegl 2006a; Luján et al. 2009a). In general, the injection pressure and injection rate are higher and the needle opens earlier for neat biodiesel (Caresana 2011; Kegl 2006a, b). For electronically controlled fuel injection systems (ECFIS) the situation is somewhat similar in the sense that various biodiesel types may result in various effects. Although in electronically controlled systems these effects are less exposed than in mechanical systems, numerous tests have to be performed in order to evaluate the influence of biodiesels for various injection pressure levels or injection time strategies (Boudy and Seers 2009; Luján et al. 2009a; Ye and Boehman 2010).

The spatial and temporal distribution of a fuel spray in a diesel engine has a determining effect on noise and exhaust emissions, fuel consumption, and engine performance (Battistoni and Grimaldi 2012; Kuti et al. 2013; Park et al. 2008). The most important fuel spray characteristics are spray penetration, spray angle, and Sauter mean diameter. These characteristics depend to a great extent on the injector type, fuel composition, and engine operating regime (Kousoulidou et al. 2012; Lin and Lin 2011; Mancauruso et al. 2011).

Biodiesel usage influences directly the injection and combustion processes and consequently also the engine performance, ecology, and economy characteristics. This influence is determined by the chemical and physical properties of biodiesel and depends on many parameters related to the engine, operating conditions, and so on.

The ultimate green diesel engine—the one with zero harmful emissions, with theoretically minimal fuel consumption, and running on environment-friendly renewable fuels, is something that will always remain out of reach. However, reaching the exact theoretical limits is not as important as is the question: how close to these limits can we push a modern diesel engine. By looking back into the last decade, one can only be astonished what a tremendous progress was achieved by diesel engine developers and producers. Fuel consumption decreased substantially, exhausts became much cleaner, and the engines became lighter and smaller due to downsizing in piston displacement volume and number of cylinders. From that point of view one can be very optimistic for the future. In spite of that, one should be aware of several hard nuts still waiting to be cracked in the coming years. Two of the perhaps mostly exposed are briefly addressed in the following.