Influence of crankshaft speed on biofuel combustion in a diesel engine

At present, the use of renewable alternative fuels of non-petroleum origin has become a challenge. Primarily, this is due to the fact that oil resources are gradually being depleted, and their cost is growing proportionally every year. Switching to renewable fuels will reduce air pollution and ensure independence from crude oil. The article presented describes the results of bench tests of an air-cooled diesel engine 2F 10.5/12.0 running on methyl alcohol and methyl ether of rapeseed oil. Herewith, a dual fuel supply system was used. That allowed ensuring synchronous fuel feeding into the engine cylinder. The article also presents diagrams of indicator pressure, heat release and roughness of the combustion process of a diesel engine running on alternative fuels, depending on the change in the crankshaft speed. Along with this, positive environmental effects were also revealed in terms of the amount of toxic agents in the exhaust gases of a tractor diesel engine. Thus, there is a significant reduction in nitrogen oxides (by 47.4%) as compared to the diesel process, and the soot content in the exhaust gases has decreased by 10.42 times.

Modern diesel internal combustion engines are very high-tech, having an increased service life, reliability, high traction and power performance, as well as economic efficiency. They are widely used in the automotive industry for special-purpose vehicles, fright and passenger transport, and in agriculture. These days, it is difficult to imagine any industrial sector without the use of diesel engines. However, many scientists have been recently searching for alternative fuels . This is done in order to obtain energy independence from fossil oil resources, as well as to improve environmental security in the world . The combustion of petroleum diesel fuel releases a large number of toxic substances into the air negatively affecting vegetation, animals and nature in general. New alternative fuels must not only meet all safety requirements, but also they have to be similar in their physical and chemical properties to petroleum diesel fuel [49][50][51][52][53][54][55][56][57]. This is essential because in the future, when using new alternative fuels, it will be possible not to introduce significant design modifications to the internal combustion engine. The proposed fuels must also be renewable [58][59][60][61][62][63][64][65][66][67][68][69][70][71][72][73].
These goals can be achieved by using such fuels as methyl alcohol (methanol) and rapeseed oil methyl ether (RME). Both types of fuel are classified as "biodiesel", since methyl alcohol can be obtained from wood processing by-products, food waste and gaseous fuels, while methyl ether is produced from rapeseed oil. Owing to the presence of oxygen in the composition, these fuels emit fewer toxic agents during combustion. However, due to a low cetane number, alcohol is not capable of selfignition, in contrast to methyl ester. The solution to this problem is to install a dual fuel supply system (DFS) on a diesel engine. This system will allow simultaneous feed of the above-mentioned fuels. IOP Conf. Series: Earth and Environmental Science 548 (2020) 062065 IOP Publishing doi:10.1088/1755-1315/548/6/062065 2 The Department of Heat Engines, Vehicles and Tractors of the Vyatka state agricultural Academy conducted bench tests of a 2F 10.5/12.0 diesel engine. The peculiarity of the tests was that an additional high-pressure fuel pump was installed through a specially made spacer on the engine. Alcohol was supplied to the cylinder through the standard fuel supply system, and for ether feeding, additional holes for injectors were made in the cylinder heads. It is specified that, methyl alcohol and methyl ether were fed simultaneously at fixer angles equal to 34° to the top dead centre (TDC). These adjustments proved to be optimal, since total specific effective fuel consumption was at minimum.
During the tests, the following characteristics were monitored: fuel consumption, air consumption, crankshaft speed, and torque. In addition, an indexing sensor was installed in the head of the first cylinder, and exhaust emissions were measured y means of a gas analyzer.
Special attention was paid to the study of the combustion process of alternative fuels in the cylinder, as well as the effects of diesel operating modes on this process. For this purpose, we obtained indicator diagrams at various speed modes. Mathematical processing of these diagrams made it possible to obtain an empirical dependence of the fuel combustion process in the form of a heat release function, which shows the proportion of heat released by a specific time (integral characteristic of heat release χ) or the rate of heat release (differential characteristic of heat release dχ/dφ).
The indicator diagrams obtained as a result of bench tests are shown in figure 1. In this diagram, one can see how the indicator pressure changes depending on changes in the speed mode of the engine. It can be observed that as the speed grows, the maximum gas pressure in the diesel cylinder decreases, the entire combustion process shifting to the expansion line.
From figure 2, we can also observe that increasing crankshaft speed leads to a decline in the maximum escalation rate the "roughness" pressure.
Such decline in the maximum gas pressure in the diesel cylinder and roughness with an increase in crankshaft speed is due to special character of methanol vapors burning up. Figures 3 and 4 show the integral and differential characteristics of heat release as a function of crankshaft rotation angle at various speed modes. The analysis of these diagrams showed that in general, during methanol IOP Conf. Series: Earth and Environmental Science 548 (2020) 062065 IOP Publishing doi:10.1088/1755-1315/548/6/062065 3 combustion with the igniter fuel, the entire combustion process could be divided into two characteristic parts. At the first interval, the combustion process is distinguished by a high rate of heat release, which is due to the high rate of methanol vapors' burning up, including those, which were formed during the ignition delay period. The second section of the heat release curve has a flatter shape, and is timeextended. This form of heat release curve is primarily due to the diffusion combustion of the igniter PME and the secondary combustion of large drops of methanol that were not burnt during the initial period. Considering the heat release characteristics (figures 3 and 4), one can see that with increasing of rotation speed in the first interval, there is a decline in heat release rate, and the whole process of heat generation moves to the right towards expansion.  This is because during this period, the kinetic mechanism of methanol vapors' burning out prevails with the spread of the flame front from the igniter fuel foci. Due to the limited rate of this process at high crankshaft speed of the diesel engine, there is a delay in the fuel combustion process following the increasing volume of the combustion chamber, which is not typical for the diesel process. Therefore, it can be concluded that the use of methanol and RME as fuel in diesels with DST imposes certain specific features on the combustion process. In particular, the speed characteristic of the test engine shows that when it is running at low speeds, one can observe a high rate of fuel combustion and high gas pressure in the cylinder. With increasing crankshaft speed, due to predominance of the kinetic mechanism of methanol vapors' burning out, there is a decline in the maximum intensity of heat release and consequently, the maximum indicator pressure decreases.