# A Comprehensive Review of the Application Characteristics of Biodiesel Blends in Diesel Engines
## 1 Introduction  
As the limits of oil reserves are becoming more obvious and the peak of global oil production will soon pass, more and more people believe that petroleum resources can be exhausted [1]. As early as 2010, half of global oil production was used in the transportation sector. CO\({}_{2}\) released by the transportation sector accounts for more than 20% of the global emissions, of which the emissions from road vehicles unexpectedly contribute more than 70% [2]. These are important factors that have led to tight oil reserves and the growing greenhouse effect. Biodiesel has gradually played a role as a powerful substitute for petroleum fuel, resulting in increased attention and related research. Biodiesel is usually used in vehicle engines in neat form or in a blended form without any modification to the engine [3, 4]. Unlike conventional fossil diesel, the feedstocks of biodiesel are not unique. Under current conditions, raw vegetable oil (palm oil, corn oil, etc. ), animal fat (tallow, lard, etc. ), non-edible oil (algae oil, jatropha oil, etc.) and waste vegetable oil can all be used as raw materials for biodiesel [5]. Biodiesel crude oil with fatty acid glycerides as the main components needs to undergo transesterification reactions with alcohols using acidic or alkaline catalysts to generate monoalkyl esters, which are the main ingredients of biodiesel [6]. The viscosity of the biodiesel after transesterification will be greatly reduced, which facilitates its direct use as fuel in the engine [7]. Biodiesel generally has higher oxygen content, higher cetane number, higher viscosity, lower aromatic content, and almost no sulfur [8]. These special properties will affect engine performance, combustion and emission characteristics. In addition, biodiesel is non-toxic, harmless and also helps reduce carbon emissions. The carbon emissions of biodiesel produced from some crops can be partially reused by plants, which will reduce greenhouse gases [9]. It is no secret that biodiesel has a betteremission reduction effect than other fuels. Many studies have shown improvement of regulated emissions such as carbon monoxide (CO), hydrocarbon (HC), nitrogen oxide (NO\({}_{\rm x}\)) and particulate matter (PM) from diesel engines fueled with biodiesel. In most cases (except for NO\({}_{\rm x}\)), the emissions of several other gases will be reduced, even including some unregulated gases, such as volatile organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs). There are also full-cycle risk assessments of biodiesel that show few amounts of harmful gases are released before biodiesel feedstock is grown and produced [10; 11]. Most components of VOCs and PAHs have a certain toxicity and may cause harm to the natural environment and organisms, but studies focused on these exhaust products produced by engines fueled with biodiesel are still limited [12; 13]. The review of the impact of biodiesel on VOCs and PAHs emissions needs to be further summarized and supplemented. The participation of biodiesel in combustion will both directly influence the engine operating performance and indirectly affect the noise and vibration of the engine, which is an important factor related to vehicle comfort [14]. Noise refers to the sound produced by irregular vibrations, which is another major health threat after air pollution. It may has adverse effects on various human daily behaviors, and even causes physical diseases such as neurological and cardiovascular disease [15]. Therefore, competitive alternative fuels should meet both exhaust and noise pollution requirements [16]. Many researchers investigated the noise and vibration of engines fueled with different biodiesel, but related retrospective work needs to be further improved. Although the properties of biodiesel are similar to diesel, the compatibility between the fuel and fuel system will directly affect the normal operation and life of the engine.