1. Introduction
Internal combustion engines are widely used in agriculture, transportation, and industry because of their high efficiency, good economy, and high reliability. However, the emission of internal combustion engines seriously affects the living environment of people [
1]. At present, emission regulations mainly restrict regulated emissions such as NOx, HC, CO, and PM. Nevertheless, in recent years, more and more attention has been paid to the unregulated emissions of internal combustion engines, such as alcohols, aldehydes, aromatics, and sulfides. Some species of the unregulated emissions, even in small concentrations in the air, have chronic toxicity. Their presence around the population has a great threat to people’s health [
2,
3].
Under the pressures of energy, environment, and health, the development and application of new alternative fuels has become a hot issue in the research of internal combustion engines. Currently, the most studied alternative fuels are alcohols (mainly including methanol and ethanol), biodiesel, and dimethyl ether. They come from a wide range of sources and are effective as oxygenated fuels to reduce particulate emissions. Qian et al. [
4] conducted the experiments on the combustion and regulated/unregulated emissions characteristics in a gasoline direct injection (GDI) engine fueled with C
3–C
5 alcohols (ethanol, n-propanol, n-butanol, and npentanol). Cheung et al. [
5] and Man et al. [
6] investigated the regulated and the unregulated emissions of biodiesel at different speed and load conditions, including the two carbonyl compounds (formaldehyde and acetaldehyde), three unsaturated hydrocarbons (1,3-butadiene, propene, and ethene), and three aromatics (benzene, toluene, and xylene). However, relevant studies have found that the use of alcohol and ether fuels intensifies the production of unregulated emissions [
5,
7]. Agarwal et al. [
8] carried out the exhaustive experiments on the effects of burning methanol- and ethanol-gasoline blends on the unregulated emissions. The results revealed that the concentrations of formic acid, iso-butane, and iso-pentane in gasohol blends are lower than that in baseline gasoline. Zhang et al. [
9] applied diesel/fumigation methanol compound combustion scheme on a four-cylinder diesel engine and found the reduction of ethyne, ethylene, and 1, 3-butadiene emissions. Nevertheless, the increase of methanol concentration in the test fuel was detrimental to higher emissions of benzene, toluene, xylene, unburned methanol, and formaldehyde.
In addition, some studies on the emissions of diesel blended with several hydrocarbon fuel surrogate components have been carried out. Tsunemoto et al. [
10] studied the exhaust emission of the paraffinic hydrocarbons (C
7–C
12) with different boiling points and aromatic hydrocarbons blended with paraffinic and olefinic hydrocarbons. Li et al. [
11] studied n-dodecane, iso-dodecane, tetralin, and decalin, and analyzed in detail the differences of unregulated emissions of aldehyde, olefins, and methane fueled with diesel blending with 10% and 20% fuel components. In order to meet the requirements of fuel economy, power performance, and emission characteristics, some new concept combustion, such as homogeneous charge compression ignition (HCCI), premixed charge compression ignition (PCCI), and partially premixed compression ignition (PPCI) have been proposed in the recent years to reduce the regulated and unregulated emissions without compromising on engine efficiency. HCCI engines are faced with such challenges as controlled auto ignition, high pressure rise rate, load extension, combustion phasing control, high emissions of CO and HC, and homogeneous charge preparation [
12,
13]. In PCCI or PPCI derived from HCCI, the end point of fuel injection and the start point of combustion are separated for preferred control over the start of combustion and better fuel-air premixed state at the beginning of combustion [
14]. References [
15,
16,
17,
18] conducted numerical simulation or experimental study on PPCI combustion.
Due to the diversity of alternative fuels and combustion modes, there are hundreds of unregulated emission species. Therefore, it is necessary to establish a systematic and efficient detection method for unregulated emissions. At present, the main analytical instruments are Fourier transform infrared (FTIR), high performance liquid chromatography (HPLC), and gas chromatography/mass spectrometry (GC-MS). Qu et al. [
19] conducted the experiments on a four-cylinder direct-injection spark-ignition (DISI) methanol engine. The measurement methods in which combine with gas chromatography and liquid chromatography were used to separate and measure regulated and unregulated emissions. Wang et al. [
20] studied the influence of ash on the emissions. The regulated gaseous emissions, and the unregulated emissions (mainly carbonyl compounds and volatile organic compounds) were measured by a gaseous analyzer, electrical low pressure impactor (ELPI), HPLC, and GC-MS, respectively. Agarwal et al. [
8] carried out exhaustive experiments fueled gasohol for characterizing the emissions. The regulated emissions and the unregulated emissions are measured by a raw exhaust emission analyzer and a FTIR emission analyzer, respectively.
Previous studies have been focused on the emission characteristics and regulated pollutants. In recent years, unregulated emissions have received increasing attention, but are mainly concentrated in biodiesel or alcohol-diesel blends. Most of the contents are focused on specific components (formaldehyde, acetaldehyde, 1,3-butadiene, and benzene) or further on the effects of load, rotational speed, injection timing, and ignition on these components [
6,
21,
22]. There is lack of information on the unregulated emissions of diesel-light hydrocarbon fuels as applied to a diesel engine and the measurements of species are not entirely complete. Hence the aim of this study is to identify the unregulated emissions of a marine diesel engine fueled with diesel and light hydrocarbon blends in PPCI at full load, to further compare the results with those obtained in conventional diesel self-ignition combustion.
2. Experimental Setup and Method
The experiments were carried out on the basis of a six-cylinder prototype marine diesel engine manufactured by a Chinese automaker with the purpose of power generation. The prototype engine was refitted to realize PPCI where large amounts of LHC was injected into intake manifold by adopting PFI injector and then premixed LHC-air mixture was ignited by a small quantity of diesel directly injected (DI) into cylinder, and the schematic layout of the experimental setup was presented in
Figure 1. In this study, the engine speed and torque were controlled automatically by using an electrical dynamometer (AVL504/4.6 SL). A piezoelectric pressure sensor (Kistler 6052C) along with a combustion analyzer (Dewetron M0391E) was adopted to acquire in-cylinder combustion pressure and corresponding crank angle. A shunt pipe was installed on the exhaust gases pipe to collect the exhaust gas in PPCI and conventional diesel self-ignition combustion, respectively. Under the action of gas sampling pump, exhaust gas was cooled down in the shunt pipe first, and then flowed through the activated carbon. The exhaust particulate impurities and macromolecular substances were absorbed by activated carbon. Finally, the filtered exhaust entered a florence flask with absorption liquid. The absorbent liquid was n-hexane with purity up to 99.99%, which could absorb unregulated gaseous emissions from the exhaust.
Table 1 lists the main technical parameters of the test diesel engine.
Commercial Euro IV diesel fuel and light hydrocarbon (mainly C
5, C
6) were adopted in the experiments. In this study, the recommended PPCI scheme was that 85% light hydrocarbon was premixed with air through port fuel injection (PFI), and 15% diesel was directly injected into the cylinder. The physical properties of commercial Euro IV diesel fuel and light hydrocarbon are listed in
Table 2.
GC-MS is a comprehensive analytical technique, which combines the separation ability of gas chromatography with the identification ability of mass spectrometry. In this paper, an Agilent 6890N/5975 gas chromatography/mass spectrometry was adopted. Under each operating condition, 0.5 mg activated carbon granules were taken to make samples. With the CDS Pyrolyzer 5150, samples could be heated rapidly and accurately from room temperature to 1000 °C. Samples placed in pyrolysis tube were pyrolyzed in helium atmosphere. The chemical components, both in particulate and gaseous phase, play a significant role in exhaust gas of engine. While a mass spectrum can contain numerous information representing the highly complex nature of the exhaust gas sample, it also presents considerable challenges for the analysis and interpretation of the sample data. A challenge is the identification and separation of peaks with similar but not identical masses. In this study, specific spectral fitting techniques were needed to resolve the overlapping peaks at the same integer mass, and corresponding squares fit was made to the spectrum by using Squirrel/PIKA software [
23]. The pyrolysis products were separated by gas chromatography (GC), and the common pyrolysis products were independently acquired using the same GC-MS conditions and saved as references. The detailed composition of unregulated emissions was then determined through a comparative analysis between the reference standards and the samples of unregulated emissions. The unique interfacing design permits a direct pyrolysis path to the GC inlet or rapid sample heating transfer to the trap without interrupting the pneumatics of the GC-MS Detector for separation and detection. In addition, for the absorbent liquids with exhaust components, 1 μL of liquid was taken respectively. The samples could be directly injected from the gas phase inlet, and the relative proportion of each compound in the sample was determined via integration of individual peak regions arising in the GC.