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A study by Cao (2007) demonstrated that a cold engine block, which can cause incomplete combustion, can significantly affect emissions. Additionally, the higher viscosity of the engine lubricant due to its low temperature increases friction losses and decreases thermal efficiency (Roberts et al., 2014). The friction losses during cold-start increase up to 2.5 times, compared to when the engine is warmed up (Will and Boretti, 2011). To overcome high friction losses, to maintain brake power output, more fuel must be injected into the cylinder during cold-start, regardless of the ambient temperature (Khair and Jääskeläinen, 2013; Will and Boretti, 2011). A study showed a 13.5% increase in fuel consumption during cold-start compared to hot-start (Samhaber et al., 2001).

Emissions during cold-start constitute a significant proportion of an engine’s total emissions

(Reiter and Kockelman, 2016; Sakunthalai et al., 2014). Nam (2008) estimated that particulate matter (PM) emissions during the cold-start phase of the LA92 Unified Driving Cycle, Phase 1, can consist of up to 30% of total PM emission from that cycle. This is despite Phase 1 only being a 12% and 21% proportion of the entire cycle distance and duration, respectively. They also showed that PM emission from Phase 1 is 7.5 times higher than Phase 3, which is a hot- start phase with the same driving schedule as Phase 1. A report showed that PM and hydrocarbons (HC) were several times higher during the first three minutes of cold-start than during the time the engine was warmed up; more than 40% of total emissions were related to those first three minutes (Bielaczyc et al., 2001). Lee et al. (2012) used the FTP test and demonstrated that nitrogen oxides (NOx), CO and HC emitted from a cold engine, in comparison to a warmed up engine, can be up to two, three and four times higher, respectively.

A significant proportion of daily driving starts and finishes while the engine temperature is still below its regular operating level (Reiter and Kockelman, 2016). A study of the driving patterns

of 55 French cars, which included 1000 trips (71 000 km representing 1260 hours), under real conditions showed that one third of the journeys were completed before the engine coolant and lubricant exceeded 70°C (André, 1991). It should be noted, however, that the cold-start time and distance depend on emission species (André and Joumard, 2005; Reiter and Kockelman, 2016). Andre and Joumard (2005) studied and modeled excess emission during cold-start based on a survey of 39 European laboratories. The data they used were obtained from 1766 vehicles and 35 941 measurements. They estimated an average distance of 5.2 km for the cold-start distance, the average distance at which emissions (CO, CO2, HC and NOx) stabilised, at an ambient temperature of 20 °C. It should be mentioned that most of these reports are based on conventional petro-diesel.

The use of biodiesel instead of diesel has some advantages in reducing air pollution, such as lower PM (Rahman et al., 2014a), particle number (PN) (Nabi et al., 2016), CO (Ahmed et al., 2014; Ruhul et al., 2016), HC (Rahman et al., 2014b; Sanjid et al., 2016; Sanjid et al., 2014) and CO(Zare et al., 2016) emissions. Despite this, they can potentially lead to more toxic emissions (Hedayat et al., 2016; Stevanovic et al., 2013). Among the different types of biofuel, waste cooking biodiesel has the potential to be used as an alternative fuel due to advantages such as global availability, close properties to diesel and low price (Kulkarni and Dalai, 2006). Doroda et al. (2003) studied the effect of waste olive oil, as a fuel, on engine emissions. They reported that CO and COemissions decreased by up to 58.9% and 8.6%, respectively. While, NOemission showed an increasing trend. Another recent study demonstrated that using waste cooking biodiesel instead of diesel can decrease PM, PN, CO (at higher loads), HC, and COemissions; however NOx emissions showed an increasing trend (Zare et al., 2016).

Different fuel properties have been used to interpret the changes in exhaust emissions, such as fuel oxygen content (Rahman et al., 2014a). The presence of long-chain alkyl esters, which have two oxygen atoms per molecule, is an influential factor that distinguishes biofuels from

conventional fossil fuels. However, the oxygen content in biofuels is related to the fatty acid profile such as carbon chain length and unsaturation level (Pham et al., 2014). Since the presence of oxygen in fuel can reduce emissions, a low volume of a highly-oxygenated fuel additive can significantly reduce emissions (Zare et al., 2016).

Triacetin [C9H14O6]—a triester of glycerol acetic acid—could be introduced as an additive to biodiesel (Casas et al., 2010). Since glycerol is a byproduct of the biodiesel transesterification process, its production increases proportionally by producing more biodiesel. Consequently, this can reduce the price of this feedstock. However, there are some limitations to using glycerol directly as a fuel due to its physical and chemical properties (Gupta and Kumar, 2012). The solution could be triacetin, a glycerol-derived product, which is produced by the acetylation process of glycerol and acetic acid (Lapuerta et al., 2015). Mixing this highly- oxygenated additive with biodiesel increases fuel oxygen content, viscosity and the density of the blend, while the heating value and cetane number of the blend decreases (Casas et al., 2010). There is limited published research on triacetin as a fuel in engines (Hedayat et al., 2016; Nabi et al., 2016; Nabi et al., 2015; Zare et al., 2015; Zare et al., 2016). A recent study by Zare et al. (2016) showed that, by increasing the share of triacetin in the blend, the PM and PN emissions decrease while NOx and HC emissions increase.

Due to the increasing share of biofuels in the fuel market, the use of these oxygenated fuels utilising triacetin as an additive should be evaluated under cold-start conditions. This quantification is essential to evaluate the possible use of these alternative fuels in the market. To date, most studies have only focused on hot-start engine operation.

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