The influence of oxygenated fuels on transient and steady-state engine emissions
Introduction
Despite only a low fraction of engine operation, in vehicles, being at a steady-state condition [1], to date, most studies about internal combustion engines have focused on steady-state operation [2]. However the results from transient operation studies are more likely to reflect the reality. Here transient operation can be defined as any operation in which the engine speed or fuel injection change frequently; these parameters remain relatively unchanged during steady-state operation [2].
In general, combustion products under transient operation are more readily produced when compared to steady-state operation. For example, a report showed that during acceleration, the NO concentration of a diesel engine peaked around 800 ppm, while its steady-state counterpart was around 600 ppm [3]. Also shown in this study was an order of magnitude overshoot in smoke opacity during acceleration when compared to steady-state operation. Another report showed a 3.6 times increase in diesel PN during transient operation, compared to its steady-state counterpart [4].
Currently, the turbocharged diesel engine is one of the most preferred engines [5] due to its high fuel efficiency and subsequent low CO2 emission [6]. However, the transient operation of turbocharged diesel engines has been associated with slow acceleration rates, subsequently poor drivability and overshoots in combustion products [7]. This could be due to the most notable off-design parameter, turbocharger lag, which is a mismatch between the supplied air and the injected fuel [8].
The European Union issued a directive to increase the use of renewable biofuels to 10% by 2020 in order to offset fossil fuel (EU Directive 2009/28/EC). Among different biofuels introduced by industry and research groups, waste cooking biodiesel has attracted attention owing to some advantages such as global availability, its low price and close properties to diesel [9]. Also, disposal issues could be another motivation to utilise it as a fuel [10]. In terms of the effect of using waste cooking oils as a fuel on engine performance and exhaust emissions in the literature, there are some advantages and disadvantages [11]; for example, reducing the PM [11], [12], CO and HC emissions [12], [13], [14], [15], increasing the NO2 [14], NOx [11] and BSFC [12], [13], [14], and decreasing the brake power [13], [15].
The presence of oxygen in the fuel is a significant factor differentiating biofuels from conventional fossil fuels. Fuel oxygen content, which depends on the fatty acid ester profile such as carbon chain length and unsaturation level [16], is known as a major factor in reducing the emission [17], [18], [19]. Therefore, it can be concluded that in order to enhance the emission reduction a low volume of highly oxygenated fuel additives can be used.
Triacetin [C9H14O6] which is the product of the acetylation process of glycerol and acetic acid can be used as a highly oxygenated fuel additive [20]. Glycerol is a byproduct of the biodiesel transesterification process and therefore by producing more biofuels the availability of this product increases proportionally [21]. The huge quantity of glycerol may drop its price to a level that can justify the use of glycerol as a burner fuel in combustion, however there are some limitations due to its chemical and physical properties [22]. According to this, triacetin which is the production of glycerol-derived fuels could be a solution [23]. However, a limited number of studies in the literature (mostly from our research group) have focused on using triacetin as an additive [21], [24], [25], [26], [27]. It was reported that blending biofuels with this highly oxygenated fuel additive increases the kinematic viscosity and density, however, it decreases the heating value and the cetane number of the blend [20].
To-date investigation on transient engine operation has been limited due to factors such as the complexity of the experiment and the availability of automatically controlled test-beds with high-tech fast response measuring instruments. A thorough search in the literature showed only a limited number of publications studying the influence of biofuels on exhaust emissions during transient operation compared to the studies on steady-state operation [2]. Also, it was found that most transient operation studies focused on continuously transient cycles using a quasi-steady-state manner of analysis, presenting the engine performance and exhaust emissions results with mean and cumulative values over either the entire cycle or segments of the cycle. However, this method of analysis conceals the effect of individual load acceptance and engine speed changes. Hence in order to study the transient operation mechanism, it is important to evaluate the transient engine performance and emissions separately for each engine speed and load change. However, only a small portion of the publications on transient operation studied the acceleration and load acceptance [2]. For example, Rakopoulos et al. [3] studied different exhaust emissions during transient and stead-state conditions. The fuels used in this work were diesel, biodiesel and n-butanol. The results showed that during turbocharger lag NO and smoke opacity peak over their steady-state counterpart. Similarly, another study by Rakopoulos et al. [1] evaluated NO and smoke opacity during 3 different acceleration and load increase modes using a turbocharged diesel engine. This study showed the turbocharger lag as the reason of overshoots in NO and smoke opacity. In addition, the maximum global gas temperature in the cylinder during acceleration for the tested fuels were showed and used to interpret the NOx behavior. Tan et al. [28] studied the particle number emissions from a diesel engine under load increase operation mode indicating that the total particle number increases with the engine torque. The tested fuels in this study were diesel, pure Jatorpha biodiesel and two biodiesel blends with diesel fuel. The particle number emission in nucleation and accumulation modes were investigated showing that by increasing the biodiesel blend ratio the particle number in accumulation mode decreases, while the particle number in nucleation mode increases. To study more about the research done about different transient discrete modes readers can refer to Ref. [1], [4], [5], [6], [8], [28], [29], [30], [31], [32], [33], [34].
This paper studies the influence of oxygenated fuels on engine performance and emissions during transient and steady-state operations using a range of fuels with 0–14.23% oxygen content. The oxygenated fuels were based on waste cooking biodiesel (primary fuel) with triacetin (highly oxygenated additive). A custom test was designed for this study to investigate the exhaust emissions during steady-state, load acceptance and acceleration operation modes. In addition, to evaluate the exhaust emissions during a whole transient cycle, a legislative cycle (NRTC), which contains numerous discrete transient modes, was utilised. The results can also correlate against the discrete transient modes in custom test. A comprehensive search in the literature could not find any investigation studying the influence of triacetin as a fuel additive with waste cooking biodiesel on diesel engine exhaust emissions under transient engine operation.
Section snippets
Engine specification and test setup
The engine used in this study was a fully instrumented, 6-cylinder turbocharged, aftercooled, common rail compression ignition engine, as described in Table 1. The engine was coupled to an electronically controlled water brake dynamometer to control the engine load during steady-state and transient operation. In addition, this engine research facility has the ability to program different test cycles that can be run automatically.
Fig. 1 illustrates the schematic of the experimental setup.
Result and discussion
This section studies the influence of oxygenated fuels on exhaust emissions during turbocharger lag, acceleration, load increase, steady-state and a transient cycle. In the analyses, in addition to the effect of fuel properties, the transient engine performance and exhaust emissions mechanism is also discussed. The method of analysis in this study is to investigate different engine performance and exhaust emissions over the custom test and the NRTC transient cycle. Then, it studies the
Conclusion
This paper studied the effect of oxygenated fuels on transient and steady-state engine performance and emissions. The engine used in this study was a fully instrumented, 6-cylinder, common rail turbocharged compression ignition engine. This study used a range of fuel oxygen content (0–14.23%), based on waste cooking biodiesel (primary fuel) with triacetin (highly oxygenated additive). A custom test was designed for this study to investigate the engine operation characteristics and exhaust
Acknowledgements
This research was supported by the Australian Research Council’s Linkage Projects funding scheme (project number LP110200158). The author would also like to acknowledge Mr. Andrew Elder from DynoLog Dynamometer Pty Ltd and Mr. Noel Hartnett for their laboratory assistance, Dr. Michael Cholette and Dr. Meisam Babaie for their guidance, Peak3 Pty Ltd for assistance with measuring instruments, Eco Tech Biodiesel for the fuel supply.
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2023, Sustainable Energy Technologies and AssessmentsTurning biodiesel glycerol into oxygenated fuel additives and their effects on the behavior of internal combustion engines: A comprehensive systematic review
2022, Renewable and Sustainable Energy ReviewsCitation Excerpt :The higher Cetane number of some glycerol derivatives could also speed up ignition in the combustion chamber, thus shortening ignition delay and decreasing NOx formation [162,172]. Contrary to the abovementioned investigations, several studies reported an increase in NOx emissions when glycerol-derived oxygenated additives were included in diesel-biodiesel blends [144,145,147,149–152]. The oxygen content of some glycerol derivatives elevated the maximum local temperature during the combustion process, stimulating NOx formation [147,167,168,170].