1. Introduction
In the last 30 years, the main energy source for the field of transport has been represented by fossil fuels. They are still needed today for internal combustion engines (ICEs). Fossil fuels are not a renewable source of energy, and that is why alternative energy sources such as biofuels have been produced from various sources [
1] and blended with fossil fuels. Biofuels are an important alternative to fossil fuels as they produce fewer greenhouse gases (GHG) on a life-cycle basis and can be produced from renewable sources such as agricultural crops, forest products and food waste. They also aim to improve the quality of life by significantly reducing harmful gases. Biofuels can be used in a variety of ways, from powering vehicles to producing electricity. Because the standards of the European Union are becoming increasingly challenging in terms of the pollutant emissions of internal combustion engines [
2], emissions harmful to humans and the environment must be drastically reduced. The transesterification of plant/animal fats with methanol is the basis for producing biodiesel, an environmentally beneficial and renewable fuel that is primarily made up of high-fatty-acid methyl esters. In addition to being non-toxic, flammable, non-explosive and biodegradable, biodiesel also boasts a large supply of raw materials and supply security [
3,
4,
5].
Biodiesel production in Europe is a rapidly emerging industry, as the European Union has set ambitious targets for biofuel production in the continent. To meet these targets, Europe has invested heavily in research and development (R&D) for biodiesel production, which has resulted in the development of numerous biodiesel plants across the continent. Biodiesel is produced by combining vegetable oils and animal fats with methanol and a catalyst to produce a fuel that is substantially similar to diesel fuel. It is then blended with diesel fuel to create biodiesel blends, which can be used in any diesel engine. In addition to being a renewable source of energy, biodiesel also has several environmental benefits. It emits fewer pollutants than traditional diesel, except for nitrogen oxides. The European biodiesel industry is highly competitive, with many major players competing for market share. The largest biodiesel producers in the region are Germany, France and the Netherlands. These countries have invested heavily in R&D for biodiesel production, leading to the development of several plants across the continent. In addition, several smaller countries are beginning to enter the market, such as Austria, the Czech Republic and Denmark [
6,
7].
Despite its popularity as an alternative biofuel to diesel fuel, new alternatives to biodiesel are being developed. These alternatives do not rely on the intensive cultuvation of energy crops but rather on sustainable forest management, in which the extraction of energy does not require cutting down trees. Turpentine is such an example. It is a volatile organic compound derived from the resin of certain coniferous trees such as Pinus Pinaster, the most abundant conifer species in Portugal, and it is obtained without the need to cut down the tree. It is a powerful solvent and cleaning agent that is derived from pine trees. It has been used for centuries as a medicine, disinfectant and paint thinner. Turpentine is a clear, colourless liquid with a slight pungent odour and flavour. It is composed mainly of a mixture of terpenes and terpenoids, and its main component is alpha-pinene. It is also used in making varnishes, adhesives and inks. It can also be used to make solvents, which can be used to dissolve oils and waxes. Turpentine has several medicinal uses. Turpentine can be irritating, and it is toxic if ingested or inhaled in large quantities [
8].
In recent years, turpentine has been gaining attention as a potential biofuel. It is a renewable, sustainable, carbon-neutral fuel source. Turpentine can be produced from sustainably harvested pine trees or from the waste products of sawmills and paper mills. Turpentine can also be made from vegetable oils and other plant-based materials. Turpentine has an energy density that can surpass that of diesel fuel and can be used, to a certain degree, in diesel engines. It has a low sulphur content and generally does not produce as many pollutants as traditional fuels, except for NO
x. Turpentine can also be blended with other biofuels, such as ethanol and biodiesel, to increase the global incorporation of renewable fuels into engine fuel blends and thus reduce fossil fuel consumption and GHG emissions. This is valid for both diesel and gasoline engine fuel blends [
9,
10,
11,
12].
Mixing turpentine with diesel provides many beneficial properties that can improve the performance of diesel engines. Turpentine also has a lower viscosity than diesel, which can improve the fuel’s ability to flow through an engine’s fuel system. In addition, it can also reduce the amount of sulphur dioxide produced by diesel engines, helping to reduce air pollution and extending the life of aftertreatment systems. Being a biofuel that emits CO
2 with its combustion, it can be considered as carbon-neutral or even carbon-negative because of the CO
2 absorption by the plant during its whole lifetime [
13]. Finally, turpentine can be used to help improve the fuel economy of a diesel engine. This is related to factors such as its higher heating value, its improved combustion due to oxygen in the molecule and its lower viscosity, which facilitates injection and fuel atomisation [
14].
Although turpentine has been used for a long time in a lot of applications, its research and use as a biofuel is still not widespread. There are some notable studies on this fuel being incorporated into diesel engines [
9,
12,
15,
16,
17].
Some of the studies involving turpentine or pine oil reported a lower efficiency and higher fuel consumption. This was the case with [
12], which assessed blends of turpentine and diesel, and [
18], which assessed blends of jatropha oil and turpentine. However, there were some cases where a lower consumption was recorded during the driving cycle [
12], reporting a lower fuel consumption for pine oil blends than diesel or jatropha blends [
19]. Thus, it seems that further exploration of the impact of turpentine blends on consumption is still needed.
The authors of [
20] reported that pollutant emissions such as HC, CO and smoke decreased by 65%, 30% and 70%, respectively, compared to diesel at a high engine load. However, at maximum load, pine oil had up to 25% higher NO
x production than the reference fuel.
Torque and power are parameters that are important in assessing vehicle performance. Often, authors either report results on turpentine blends for a full engine load [
21] or as a function of several engine loads. However, not only is a full engine load an engine setting that is rarely used in regular driving but also many studies do not even highlight the torque/power corresponding to a given engine load. This makes it difficult to assess the usefulness of the results for real driving scenarios.
From the abovementioned literature review, it is apparent that this fuel is still insufficiently explored. The recent advances in biotechnologies for increasing turpentine yields have raise the potential economic interest of this fuel, especially in a country such as Portugal, in which the pine industry has such a strong economical role. Additionally, the existing studies on turpentine fuel do not present the results in a way that is practical for assessing vehicle performance under engine loads that are typical of road driving and highway driving. Under these circumstances, it seems important to carefully test this fuel that is still insufficiently studied now that its economical relevance might increase.
The present study performs a series of tests using mixtures of turpentine with diesel in different proportions in a light-duty four-cylinder 1.6 L direct-injection engine.
The mixtures tested were 5T95D (turpentine 5% and diesel 95%), 10T90D, 15T85D, 20T80D and 30T70D. The performance of these mixtures was compared under four combinations of speed and load that are typical of different driving scenarios: 1700 RPM and 2250 RPM, roughly corresponding to a speed of around 90 km/h and 120 km/h for a light duty car, respectively, and two different loads. The variation in the degree of turbocharging was avoided by fixing it at 0.5 bar. The performance characteristics (torque power, consumption and efficiency) and pollutant emissions were analysed and compared.
4. Conclusions
In the present work, incorporations of turpentine into regular diesel of between 5 and 30% were assessed in a direct-injection diesel engine used in light-duty vehicles. The objective was to assess whether it would be possible to maintain or even improve the performance and emissions parameters of the engine. One of the main advantages of this incorporation being feasible would be a direct cut in fossil greenhouse gas emissions.
The comparisons were made in a slightly different way compared to the existing literature. Namely, comparisons for conditions typical of road vehicle driving were made, with the comparison of the different fuel blends being carried out for fixed values of the engine load.
For the conditions tested, the following conclusions may be drawn:
The differences obtained in the performance parameters such as torque, power and efficiency were not big, so the discussion of these results is necessarily limited. Nevertheless, a qualitative assessment may be conducted.
The incorporation of turpentine into regular diesel fuel was beneficial for the brake thermal efficiency for most of the turpentine incorporations tested. Namely, improvements of up to 5.5% were recorded at high loads and low speeds for the fuel with highest turpentine incorporation (30%). The brake torque always increased when incorporating turpentine into diesel fuel. Increases of nearly 8% and 7% were obtained for the fuels with 15% and 30% incorporations of turpentine. The improvement in the brake power was even better, with 9% being obtained with a 15% turpentine incorporation at the low-speed setting, while a 5% improvement was achieved for the high-speed setting with a 30% incorporation of turpentine.
Regarding pollutant emissions, the results were mixed. On the one hand, the level of unburned hydrocarbons (HC) was almost always higher with the turpentine–diesel mixtures than with pure diesel, although the specific emissions (per unit torque or power produced) were not as high because the torque was also higher. On the other hand, this study found that the particulate-matter-induced opacity, commonly referred to as smoke, displayed either higher or lower values with the turpentine incorporation when compared to diesel. However, a sharp drop in the opacity occurred in the case of low speeds and high loads for all the turpentine blends. This might be related to the positive effect of the slight oxygen content of turpentine on the combustion. Also, the highest incorporation of turpentine (30%) had a positive effect on the smoke reduction in almost all cases.
Unfortunately, the level of nitrogen oxides (NOx) produced registered a considerable increase in relation to the concentration of turpentine, which was probably due to not only the extra oxygen in the composition but also due to the improved performance of the engine. In fact, it is known that an increase in combustion performance is generally accompanied by a rise in NOx emissions.
As an overall evaluation of the results obtained, it seems reasonable to assume that incorporations of turpentine of up to 30% could represent a good opportunity for a proportional reduction in fossil fuel consumption and the associated CO2 emissions due to the neutral or even negative contribution of turpentine coming from sustainable pine forest management (pines are not cut down for turpentine extraction). This substitution could be achieved with minimal modifications to engines, and even a slight increase in torque, power and efficiency can be obtained in most driving conditions, with potentially lower smoke emissions at high engine loads and low speeds.
The higher emissions of HC and NOx seem to be an objective disadvantage of turpentine incorporation. However, these results were obtained without implementing advanced aftertreatment. A full analysis of the impact on the viability of turpentine due to these emissions would need further assessment, which was out of the scope of the present study.
Turpentine as a fuel might come closer to becoming economically competitive with the advent of the latest-generation biotechnology-enhanced pines. And in the case of Portugal, Pinus Pinaster is among the most-explored tree species, and it could be explored for turpentine production without losing its economic value for the wood and paper paste markets in which it is currently explored commercially [
38].