Evaluation on toxic reduction and fuel economy of a gasoline direct injection- (GDI-) powered passenger car fueled with methanol–gasoline blends with various substitution ratios

Applied Energy 157 (2015) 134–143

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Evaluation on toxic reduction and fuel economy of a gasoline direct injection- (GDI-) powered passenger car fueled with methanol–gasoline blends with various substitution ratios Xin Wang a,⇑, Yunshan Ge a, Linlin Liu a, Zihang Peng a, Lijun Hao a, Hang Yin b, Yan Ding b, Junfang Wang b a b

School of Mechanical Engineering, Beijing Institute of Technology, Beijing 10081, PR China Chinese Research Academy of Environmental Science, Beijing 100012, PR China

h i g h l i g h t s
CO, HC, CO2 and PM from a GDI passenger car decreased by burning methanol gasoline.
NOx from gasohol-fueling increased with methanol addition but was lower than gasoline.
Carbonyls increased while VOCs decreased as more methanol was added.
Health risks for gasoline- and gasohol-fueled vehicles were found at the same level.
Particulate number increased with methanol addition since particles became smaller.

a r t i c l e

i n f o

Article history: Received 31 March 2015 Received in revised form 22 July 2015 Accepted 9 August 2015

Keywords: Emission Carbonyl VOCs Particulate matter GDI Methanol gasoline

a b s t r a c t Methanol exhibits better capability of lowering engine emissions and fuel cost, but a potential rise in pipe-out NOx and carbonyl emissions has increased health-related risks and stalled its application. This paper examined regulated, unregulated and particulate emissions from a 1.8 L GDI-powered passenger car running the New European Driving Cycle (NEDC). Conventional gasoline, together with three gasohol samples with 15%, 25% and 40% methanol substitution (M15, M25 and M40), was used. To evaluate health risk linked with pipe-out formaldehyde and benzene, a human exposure model was established. The results showed that compared with gasoline, burning gasohol significantly reduced pipeout CO and HC emissions. Evident decrease in NOx emission was noticed with M15 and M25 fueling, but in the case of M40, NOx emissions were similar with gasoline. Carbonyls, about 50% of which were found formaldehyde, surged sharply with increased methanol percentage in the blend, while VOCs showed an opposite propensity. Health risk assessment showed that, even under a severe exposure condition, the levels of exhaust-borne formaldehyde and benzene were not sufficient to cause acute or chronic symptoms. Cancer-related risks induced by gasoline and gasohol engine exhaust were found in the same magnitude. Burning gasohol enabled an over 30% PM removal but resulted in an increase in particulate number due to an increased number of nuclei-mode particles. At last, burning gasohol produced about 0.8–4.1% less pipe-out CO2 emission than gasoline, while saving fuel cost by 5.7–15%. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In the past decades, the booming expansion of private car markets worldwide has aggravated the issues of energy sustainability and atmospheric pollution. Therefore, the search for clean and economically-affordable alternative fuels has become a hot topic. Alcohol, which has satisfying engine performance and combustion ⇑ Corresponding author. Tel./fax: +86 10 68912035. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apenergy.2015.08.023 0306-2619/Ó 2015 Elsevier Ltd. All rights reserved.

efficiency as well as great potential to lower engine-out emissions on all spark-ignited, compression-ignited and homogenous charge compression ignition (HCCI) engines [1–3], has attracted lots of attention. In Brazil and the United States, the application of ethanol as a partial surrogate of gasoline has a history of more than 20 years. In Europe, gasoline with a 10% ethanol substitution is also being used as base fuel during type-approval emission tests. Unlike ethanol, methanol witnessed its prosperity starting from mid1970s along with the first oil crisis, but was eventually abandoned after years of trial mainly due to the availability of cheap

X. Wang et al. / Applied Energy 157 (2015) 134–143

petroleum fuels, uneconomic overall expenses including model development and infrastructure, and tough toxic emissions. Fortunately, the development of methanol industry may bring new possibilities for methanol. In those countries with abundant coal or natural gas reserves, like China, Poland and many western Asian countries, simple and economicallyviable conversion technologies could impart remarkable price competitiveness to methanol over conventional gasoline. In China, for instance, methanol shows its priority over other alcohol colleagues partly because it can be produced with even junk-level coal. Combined with abundant coal reserves underground, retail price of fuel-grade methanol is only one third of gasoline. Moreover, recent developments in extracting bio-methane and bio-methanol from renewable materials such as lignin may further promote the sustainability of methanol as a future option. Although alcohols have strength in performance and cost, it is still unlikely to replace traditional fossil fuel in the near future due to limited scale of global arable land and industrial preparation facility [4]. Hence force, employing gasohol (gasoline–alcohol blend) may become a more practical counter-solution to sustain energy supply, and curb vehicle exhaust contamination simultaneously. Among all methanol–gasoline blends, M15 (15% methanol in volume and gasoline blend) and M85 are the most commonly used. In the early 1980s, American carmakers had debuted prototypes burning M85 [5], while currently in China, many of the attentions have been paid to so-called ‘‘low-content methanol gasoline”, namely gasohol a methanol substitution ratio no higher than 50%. With relatively low substitution ratio, low-content gasohol can be directly fed to present engine models without adjustment of engine configuration and re-calibration of central control unit. In addition, using low-content gasohol also lower the risks of rubber part corrosion as well as evaporative and pipe-out carbonyl pollution. A number of pilot works about neat methanol or gasohol fueling has been published since early 1970s. Brinkman et al. compared exhaust emissions, fuel economy and drivability of gasoline- and gasohol-powered models built from 1966 to 1974 [6]. Hilden and Parks then examined organic pollutants, including unburned hydrocarbons and aldehydes, from a single-cylinder research engine burning methanol, gasoline and methanol–water blends [7]. In addition to cycle-averaged fuel economy and pipe-out emissions, Bechtold and Pullman also compared photochemical reactivity of gasoline- and alcohol-derived emissions [8]. In a similar way, Schleyer and Koehl calculated the potential toward ozone formation of gasoline and methanol vehicles by using their specific VOC emissions [9]. Most recently, Gong et al. looked into unregulated emissions from a port fuel injection (PFI) methanol engine during cold start phase [10]. They also examined regulated emissions from a direct injection engine retrofitted for methanol fueling [11]. Wei et al. measured formaldehyde, acetaldehyde and methanol emissions from a spark-ignited engine fueled with M10 by using a pulsed discharge helium ionization detector [12]. Three-way catalyst (TWC) was found effective in eliminating approximately 85% engine-out acetaldehyde and methanol while enhanced the formation of formaldehyde in high speed conditions. A prediction model based on artificial neural network and real-world experiment results has been established and validated by Çay et al. in order to compare emission characteristics between methanol and gasoline [13]. Vancoillie et al. assessed the potential of methanol as a fuel on dedicated SI engines [14]. They also established a quasidimensional model to simulate engine performance with methanol and ethanol fueling [15]. The results revealed that a reduction of 5– 10 g/kWh raw NOx together with diesel-like high efficiency of the

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engine was attained by appropriately arranging air–fuel equivalence and EGR rate. Some researchers also promulgated new concepts of using alcohols as alternatives. Turner et al. investigated the effect of isostoichiometric ternary of methanol, ethanol and conventional gasoline [16], while Liang et al. used DME as a performance additive on both methanol- and ethanol-fueled engines [17,18]. At chassis dyno level, Zhao et al. analyzed regulated and unregulated emissions from several PFI passenger cars [19,20]. Elevated NOx emissions were noticed as methanol content increased from zero baseline to a half. Meanwhile, in context with gasoline counterparts, carbonyl compounds collected in gasohol-fueled tests were positively correlated to methanol concentration. Liang et al. provided a particular insight upon the ability of gasohol to abate particulate matter (PM) from gasoline cars [21]. In this research, M15 was proved an effective way to reduce exhaust-borne particles from both PFI- and GDI-powered vehicle. Another work done by Dai et al. devoted to pipe-out and evaporative emissions from cars burning gasohol [22]. An increment of 85% NOx and 63% evaporative THC was observed by conducting chassis-dyno tests and precise weighting in sealed chamber. Above all, although employing methanol as an alternative exhibits several advantages, public concerns over excessive toxic pollutants, such as extra NOx emissions owing to oxygenated nature of methanol, excessive evaporative THC caused by azeotropy (for low-content gasohol) and elevated concentration of pipe-out formaldehyde, all restrict future application of methanol–gasoline fuels. To better deal with the trade-off between economy and health risk, methanol substitution ratio in gasohol fuels must be accurately controlled. In this process, pipe-out NOx and formaldehyde emissions are primary constraints. Though higher blending ratio of methanol is able to maintain good price competiveness of gasohol fuels, overdosing of methanol will also increase public health risk and result in malfunction of on-board diagnostic system (OBD) [23]. In order to provide a comprehensive vision of the emissions from gasohol-fueled vehicles, regulated, unregulated and particulate emissions from an in-use passenger car burning three kinds of methanol–gasoline, namely M15, M25 and M40, were examined in this paper. Furthermore, the results were in context with gasoline counterpart to evaluate the feasibility of using methanol– gasoline as a partial alternative of conventional gasoline in the future. By using the measured carbonyl and VOC emissions and establishing a simple exposure model was established, cancerand non-cancer-related risks of gasoline and gasohol vehicle emissions were also evaluated, which had been merely reported by previous literatures.

2. Experiment setup and method In this section, experimental setup of this current research is described. Details about how the tests were conducted and procedures for raw gas sampling and analyzing had been emphasized by our previous publications. Configuration of chassis dyno and information about ELPI working principle could be found in Ref. [21]. On apparatus and methodology for the analysis of carbonyl components and volatile organic compounds (VOCs), papers [20,22] have spent particular paragraphs to discuss in-depth. Here, Eq. (1) shows the calculation method of cycle-averaged emission factors of carbonyls and VOCs.

Mi ¼

mabs  V mix V abs  d

ð1Þ

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Fig. 1. Configuration of the experimental system.

where Mi denotes the cycle-averaged emission factor (mg/km), d is the actual driving distance, V mix is the corrected volume of diluted exhaust (m3), mabs and V abs represent the mass of specific emission quantified in the absorbent and the total volume of sample gas flow through the absorbent. As depicted in Fig. 1, in this study, a chassis dynamometer (Ono Sokki PECD 9400), a constant volume sampling system (Horiba CVS-7400), an emission analyzer (Horiba Mexa-7200H) and a self-programmed data acquisition system were employed to run regulatory cold-start tests over New European Driving Cycle (NEDC). In order to provide insights into particulate matter emission, a classic electrical low pressure impactor (ELPI, Dekati) was introduced to illustrate number distribution. And particle mass was determined by weighting Teflon filters (PALLFLEX, 70 mm) on a precision balance (Sartorius CPA2P-F), whose accuracy could be up to 1 microgram. Weighting operations were processed in a sealed chamber (EST) which keeps a constant-temperature and constant-humidity environment at 22.0 ± 0.1 degree Celsius and 45.0 ± 0.2%RH. For quantifying carbonyl components and VOCs discharged throughout the cycle, approximately 20 liter diluted exhaust gas was suctioned out from the sample bag by using a portable sample pump (AirCheck 2000). Collected samples received a series of physical and chemical treatments and were sent into a high-performance liquid-chromatography (HPLC, Agilent 1200 Series) and a gas-chromatography mass-spectrometry (GC–MS, Agilent 6890 N) for qualitative and quantitative analysis. Supelco DNPH and Agilent TENAX adsorption tubes were respectively employed to collect carbonyl and VOCs samples. Test vehicle of this study was a Volkswagen Passat sedan with 48,882 km on odometer. Inertial weight of the chassis dyno was configured to 1650 kg and resistance on the rollers was set by executing coast-down procedures. The test vehicle is powered by a 1.8 l, in-line, four cylinder, turbocharged, direct-injection gasoline (GDI) engine. Transmission system utilizes a 7-speed dry direct shifting gearbox (DSG). Tire pressure was 2.4 bar when tests were performed. For each kind of fuel, emission tests were run 2 times. A period of adaptive driving which lasts for about 150–200 km was undertook after fuel switching to allow the control unit of engine to adapt to new fuel. All the tests were performed in March, 2014 in Beijing at 70 meter altitude, room temperature and humidity were controlled at 22.5 ± 0.5 degree Celsius and 24 ± 2%RH. Conventional gasoline and methanol–gasoline with 3 different substitution ratios of 15%, 25% and 40%, were used in this paper. Part of fundamental physical and chemical properties of methanol and gasoline were compared in Table 1, and properties of fuel blends examined in this paper were tabulated in Table 2. 3. Results and discussion In this section, regulated, unregulated emissions and particulate matter from both gasoline- and gasohol-fueled vehicle were compared. An additional fuel economy analysis along with a possible

Table 1 Properties of typical gasoline and methanol fuels [14,24]. Property

Gasoline

Methanol

Chemical formula Stoichiometric air/fuel ratio Lower heating value, MJ/kg LHV of Stoich. mixture, MJ/kg Research/motor octane number (RON/MON) Heat of vaporization, MJ/kg Laminar flame speed, m/s Adiabatic flame temperature, K Flammability limit, lambda Specific CO2 emission, g/MJ

CnH1.87n 14.6 44.0 2.83 95/85 305 0.28 2279 0.26–1.60 73.95

CH3OH 6.5 20.0 2.68 106/96 1103 0.42 2147 0.23–1.81 68.44

Table 2 Fuel properties considered in the present paper. Property

Gasoline

M15

M25

M40

Density at 293 K, g/mL Fraction of methanol, mass% C:H:O in volume Methyl tert-butyl ether (MTBE), mass% Isopropanol, mass%

0.741 0

0.747 13.08

0.754 22.33

0.762 37.53

1:1.87:0 4.17

1:2.13:0.12 3.85

1:2.32:0.21 3.36

1:2.64:0.32 2.95

0

0.46

0.71

1.31

health impact prediction on burning low-content methanol gasoline as an alternative option were also emphasized. 3.1. Regulated emissions As illustrated in Fig. 2, evident decreases of 9.4–33.2% and 9.7– 36.9% in CO and HC emissions were found when methanol concentration in the blends was increased. Given much lower stoichiometric air/fuel ratio and C/H ratio of methanol, using methanol as a partial alternative generally reduced the demand for oxygen in the combustion chambers. This behavior was beneficial to preventing in-cylinder fuel-rich zones from appearing and alleviated incomplete combustion, therefore reducing CO and HC. These efforts were especially meaningful during acceleration and highspeed phases. There is no C–C bond in the chemical structure of a methanol molecule, which is very different from other hydrocarbons comprising gasoline, this feature could also help to restrict the formation of incomplete resultants. Regarding nitrogen oxides, the addition of methanol into conventional gasoline lead to an initial dwindle of NOx when M15 was delivered, but a consistent increment of NOx was observed as constitution ratio was raised. In the case of M40 fueling, NOx concentration measured in the sample was quite close to gasoline baseline. Relatively low adiabatic flame temperature and high evaporation heat value of methanol would help gasohol lower NOx formation, while on the other hand, the addition of methanol could also drive the in-cylinder air–fuel mixtures to a slightly lean

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Fig. 2. Regulated gaseous emissions from gasoline- and gasohol-fueled vehicle.

side and significantly accelerated the production of NOx. When low-content gasohol was fueled to the engine, self-adaptive function of ECU was still able to confine lambda within an appropriate range, but this effect of self-adjustment gradually weakened as constitution ratio of methanol rose. Hence, NOx increased again as gasohol with heavier methanol contents were burnt. This result might have been opposite to many early researches because in most studies, more NOx was obtained when engine fuel switched from gasoline to gasohol. Of great possibility, some highmolecular-weight hydrocarbons, which was added into methanol gasoline to control Reid vapor pressure (RVP) and/or suppressed in-cylinder NOx formation, also has served as determinative factors to derive the result. Being different from some previous engine-level researches, all gasohol samples employed in this paper were retail–capable productions rather than simple mixtures of methanol and gasoline. More additives, such as stabilizers, combustion inhibitors, will be added into retail methanol gasoline in order to restrain stratification and azeotropy between methanol and gasoline as well as curb NOx. Though additives only shared a small percentage in mass or in volume, the way they behaved in the processes of air–fuel mixing and burning, particularly during warm-up phase of three-way catalyst (TWC), would have vital impacts on NOx and HC concentration at tailpipe.

In Table 3, results of regulated and unregulated gaseous emissions measured in this paper were put in context with literatures published before. It can be seen in Table 3 that, regulated emissions reported by this present paper were in good agreement with pilot works. For both gasoline- and gasohol-fueled vehicles, regulated emissions had been significantly reduced along with the application of TWCs. The majority of previous works had reported decreased CO emission with the introduction of gasohol, while HC and NOx emissions were maintained at a generally similar level when compared with gasoline baseline. Remarkable increases in NOx emissions of gasohol-fueled vehicles were reported by references [20,22]. This phenomenon may be attributed to the fact that both authors used methanol–gasoline blends with no additive as test fuel. Undoubtedly, the fuel the authors used should be classified into gasohol but was unable to completely reflect the performance of conventional gasohol currently available in the market. To stabilize gasohol blends and meet ever tightening emission requirements, a variety of fuel additives are being doped into methanol–gasoline mixtures. Among those, de-NOx components or sometimes combustion inhibitors have become the most important and expensive ones. With regard to pipe-out formaldehyde, the summary in Table 3 clearly showed a descending propensity with the development of emission control technologies. Mass of pipe-out formaldehyde measured on past-2006 models was no more than 30% of that of a pre-1993 model. Correspondingly, concerns over formaldehydeinduced health risk along with gasohol-fueled vehicle application shall be significantly relieved. In Fig. 3, the NEDC was divided into three bodies including the first urban driving cycle (UDC-1), the rest 3 of 4 UDC cycles (UDCother) and the extra urban driving cycle (EUDC), besides patterns of CO, HC, NOx and CO2 emissions attributed to those three phases were depicted. As shown, there was comparatively more CO produced after lighting-off of TWC when conventional gasoline was burnt. Such a phenomena proved the ability of methanol addition on preventing the appearance of fuel-rich zones in combustion chambers and curb CO formation. However, no clear correlation between HC emissions and different test phases could be addressed, probably because the formation of hydrocarbons incylinder was affected by more conditions and materials via even complex mechanisms, and these mechanisms were less sensitive to oxygenated fuels. Apart from these, a variety of fuel additives might have significant influences on HC results. For NOx, a

Table 3 Comparison of regulated and unregulated emissions results between this paper and previous literatures. Test vehicle

Test cycle

CO

HC

NOx

g/km

a b c

Crowley AW [25]

1974 Oldsmobile

FTP75

Johnson RT [26]

1975 Honda CVCC

FTP75

Bechtold R [8]

1978 Pinto

FTP75

Burns VR [27]

1992–1993 Flex-fuel

FTP75

Zhao H [20]

2006 VW Santana, Euro-3

NEDC

Turner JWG [28]

Saab 9–3 BioPower, Euro-5

NEDC

Dai PP [22]

2013 VW Jetta, Euro-4

NEDC

This paper

2013 VW Passat, Euro-4

NEDC

M5-M20 Gasoline M10 Gasoline Methyl-fuela Gasoline M85 Gasoline M15 Gasoline GEMb Gasoline M15 Gasoline M15-M40 Gasoline

7.4–22.7 12.4–51.5 4.69 5.16 1.342 3.04 0.951 0.869 0.747 0.82 0.374–0.432 0.355–0.394 0.567 0.609 0.205–0.278 0.307

Formaldehyde mg/km

0.91–1.11 0.86–0.99 0.99 0.7 0.094 0.237 0.062 0.078 0.089 0.09 0.0209–0.0503 0.0316–0.0348 0.072 0.086 0.04–0.058 0.064

0.61–1.22 0.46–1.34 1.49 1.84 0.387 0.423 0.213 0.201 0.077 0.028 0.0054–0.0097 0.0132–0.0171 0.133 0.072 0.0123–0.0171 0.0167

NGc NG NG NG 11.44 2.3 8.334 0.907 1.111 0.632 NG NG 1.954 1.069 1.442–2.358 1.096

Methyl-fuel: 75 methanol, 5% ethanol, 7.5% propanol and 12.5% iso-butanol. GEM is short for gasoline, ethanol and methanol ternary fuel, four GEM blends including G15 E85, G37 E21 M42, G40 E10 M50 and G44 E0 M56 were used. ‘‘NG” denotes ‘‘not given” in the relevant literature.

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Fig. 4. Carbonyl and formaldehyde components from vehicle fueled with gasoline and gasohol. Fig. 3. Patterns of emissions attributed to different phases of NEDC.

majority of NOx throughout test cycle was produced within UDC-1 phase since TWC had not lighted-off. When gasohol was fueled, patterns belonging to UDC-1 increased while patterns attributed to UDC-other and EUDC decreased. That was because the addition of methanol diluted the air–fuel mixtures during warm-up phase, and provided more favorable conditions for NOx formation. It should be also mentioned here, although using gasohol to take place of gasoline could in part address emission issues, cars switching to gasohol will to various degrees encounter power derating issues, as the LHV of methanol is only 20 MJ/kg, which is no more than a half of gasoline. However, in fact, power drop could be hardly felt throughout the driving cycle because the test vehicle was always running in part load conditions. Due to the fact that the highest speed of the NEDC is 120 km/h and the strongest acceleration is only 1.04 m/s2, engine load of the test vehicle was maintained at a moderate level. Unlike in wide open throttle (WOT) conditions, when the engine was operating at part load conditions, power drop issues could be ‘‘solved” by enlarging throttle opening and fuel delivery rate, that means the man driver actually deepened the acceleration pedal to maintain engine output and follow the cycle as methanol substitution increased. Besides, the 1.8 L GDI engine adopted in this paper imparted the test vehicle a very high rated power/curb weight ratio, which alleviated or masked the power drop issues. It is anticipatable that on a fleet with lower power/weight ratio, such as an economic compact car, power drop will be more evident, in particular with M40 fueling. For the majority of countries, the top speed of NEDC, 120 km/h, is high enough considering highway limit, but for countries with unlimited highways, like Germany, gasohol is obviously unsuitable since the WOT conditions may be used much more frequently than in other countries. 3.2. Unregulated emissions 3.2.1. Carbonyls In this paper, unregulated emissions, including carbonyls and VOCs, were also gauged in order to look into possible health damage aroused by some poisonous compounds such as formaldehyde and benzene. In Fig. 4, both carbonyl and formaldehyde generated by burning the four fuel samples were illustrated. Carbonyls in that figure comprised 14 kinds of aldehydes and ketones, namely formaldehyde, acetaldehyde, acraldehyde, acetone, propionaldehyde, crotonaldehyde, butanone, methylacrolein, butyraldehyde, benzaldehyde, amyl aldehyde, toluyl aldehyde, cyclohexanone

and hexanal. The percentage occupied by formaldehyde could be up to over 40% in mass. Given toxicity, formaldehyde has been identified as the most hazardous species among these 14 carbonyls. Hence, formaldehyde emission had become the major concern of gasohol-fueled vehicle. As disclosed in Fig. 4, formaldehyde measured in the exhaust was positively correlated to methanol concentration in gasohol samples. Comparing with gasoline baseline, using M15, M25 and M40 as alternatives had emitted approximately 31.6%, 68.9% and 115.1% more formaldehyde in the tests. It should be noted that, formaldehyde has been classified into a probable carcinogen by USEPA, both short-term and long-term exposure to concentrations of formaldehyde may cause inflammation and respiratory diseases. Hence, to various degrees elevated levels of pipe-out formaldehyde induced by burning gasohol may increase the risk of public health. Although in recent years, along with the tightening of emission regulations, pipe-out carbonyls have been significantly curbed [19], higher carbonyl emissions, in particular formaldehyde, are still the biggest barrier for the implement of gasohol fuels. However, in order to better evaluate the potential damage brought by pipe-out carbonyl emissions, a conceptual estimate as follows will be somewhat helpful. Due to the fact that in most researches, pipe-out carbonyls were measured off-line and realtime carbonyl observation data was rarely available, Eq. (2) is established to derive the cycle-averaged formaldehyde concentrations in diluted exhaust.

Ci ¼

Mi  d V mix  Q i  kH

ð2Þ

where C i is the cycle-averaged concentration in diluted exhaust (ppm), Mi denotes the cycle-averaged emission factor (mg/km), d is the actual driving distance, V mix is the corrected volume of diluted exhaust (m3), Q i is the density in standard condition of formaldehyde, which is 1.395 g/L in this estimation. kH is the humidity correction factor, to simplify the calculation, this item was ignored here. Two parameters, namely C i and V mix , can be further expanded as shown in Eqs. (3) and (4).

PB  PI V mix ¼ 2:6961  V  Tp   1 Ci ¼ Ce  Cd 1  Df

ð3Þ ð4Þ

In Eq. (3), V represents the original volume of diluted exhaust (m3), P B and P i denote the atmospheric pressure and the pressure measured at the inlet of blower respectively (kPa). C e and C d are

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the concentrations of formaldehyde measured in diluted exhaust sample and ambient air (ppm). Df is defined as the dilution ratio, which can be calculated by using Eq. (5).

Df ¼

X C CO2 þ ðC THC þ C CO Þ  104

ð5Þ

where C CO2 , C THC and C CO are the concentrations in sample bags (vol % for CO2 and ppm for CO and THC). Parameter X, as shown in Eq. (6), is a fuel-dependent variant, which is calculated from specific C:H:O ratio of different fuel. In this paper, X for gasoline, M15, M25 and M40 were 13.4, 13.13, 12.95 and 12.68 respectively.

X ¼ 100 

x x þ 0:5y þ 3:76ðx þ 0:25y  0:5zÞ

ð6Þ

where x, y, z is the number of carbon atom, hydrogen atom and oxygen atom in the nominal chemical formula of the fuel burned here. Eventually, cycle-averaged formaldehyde concentrations in diluted exhaust were 0.05, 0.12, 0.13 and 0.18 ppm for the four fuel samples employed in this research, and the overall dilution ratio was around 20 as a dilution flow rate of 9 m3/min was used. With known diluted concentrations and dilution ratio, formaldehyde concentrations in raw exhaust can be deduced, but a nonnegligible fact is that real-time formaldehyde concentrations were significantly unevenly distributed throughout the driving cycle. Empirically, peaks measured prior to the lighting-off of threeway catalyst can be tens of times higher than cycle-averaged values. Thus, an extreme circumstance for evaluating the hazardous effects of pipe-out formaldehyde on human health could be assumed that a man stand facing the exhaust pipe and keep in touch with tenfold cycle-averaged pipe-out formaldehyde concentrations. Once raw exhaust was piped-out, it will be diluted by ambient air. To assume the case extremely, a very conservative dilution ratio of 200 was given here. Hence, the ‘‘man” will be exposing to atmospheres with 0.05–0.18 ppm formaldehyde, or 61.5–221 lg per cubic meter in mass. Such concentrations are far lower than most of thresholds summarized by USEPA [29], except for those from references [31–33]. By using mathematical models, EPA lined a cancer-related threshold of 0.8 lg per cubic meter, man inhales in such a concentration of formaldehyde will result in no more than 1/100,000 increased chance of developing cancer. As the threshold concentration is heightened to 8.0 lg per cubic meter, the morbidity of cancer also increases by a magnitude to no more than 1/10,000 chance [30]. It should be noted that, although pipe-out formaldehyde concentrations elevated with gasohol fueling, the risk levels for gasoline and gasohol are still in the same magnitude, nominally no higher than 1/10,000. With respect to non-cancer hazards, the four pipe-out formaldehyde concentrations in this paper will not cause any acute symptoms, but the extreme assumption makes the estimated concentrations evidently surmount the chronic threshold of 4 lg per cubic meter, given by the Agency for Toxic Substances and Disease Registry (ATSDR) [31]. National Institute for Occupational Safety and Health (NIOSH) also set an exposure limit for occupational safety in 1997, they suggested formaldehyde concentration should not exceed 0.02 mg per cubic meter in order to ensure the safety of workers after 8–10 h exposure [32]. Even thought, it is still uneasy to conclude that pipe-out formaldehyde emission from gasohol-fueled vehicles will have adverse effects on human health, because the aforementioned thresholds provided by EPA and ATSDR are results of persistent exposure. Given the radical assumption made in this paper and fast lighting-off of prevalent TWCs and stronger dilution in the atmosphere, it is unrealistic for a man to exposure to such high concentrations of formaldehyde in long term.

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In gasoline mode, a small part of formaldehyde was also identified in the samples, which elucidated that formaldehyde can be also formed via some interactions among light-molecular-weight hydrocarbons existing in conventional gasoline. However, for the other gasoline samples, the majority of pipe-out formaldehyde was believed to be contributed by methanol combustion. As an important intermediate, formaldehyde can be formed within the early stage of methanol combustion. The consumption of methanol begins with hydrogen abstraction. Via the reactions with H, OH and HO2 radicals, a methanol molecule initially loses a hydrogen atom and produces hydroxymethyl (CH2OH) and/or methoxy (CH3O). CH2OH and CH3O will be further consumed primarily via thermal decomposition, as R1 and R2 show [32].

CH3 O ! CH2 O þ H

ðR1Þ

CH2 OH ! CH2 O þ H

ðR2Þ

In addition to thermal decomposition, CH2OH can also react with molecular oxygen and produce a formaldehyde molecule and a HO2 radical. Relative to gasoline, methanol exhibits higher flame speed but lower adiabatic flame temperature, which were beneficial to restraining post-combustion and cooling down catalyst. Lowered temperature within the three-way catalyst may result in degraded conversion efficiency of carbonyls at stoichiometric conditions [34].

3.2.2. Volatile organic compounds In addition to carbonyls, VOCs have been another concern for gasohol-fueled vehicle. 9 species of VOCs comprising of benzene, methylbenzene, n-butyl acetate, ethyl benzene, p-xylene, mxylene, o-xylene, styrene and n-undecane were examined in this paper. Excluding n-butyl acetate, styrene and n-undecane, the rest 6 of 9 species composed BTEX, which was recognized as the principle toxic materials and was frequently employed as an indicator when assessing the contamination of VOCs. VOCs and BTEX collected from different fueling regimes were compared in Fig. 5. As mentioned in former sections, additives were essential for conventional methanol gasoline in China, while adding these high-carbonnumber hydrocarbons might sometimes bring about VOCs issue, particularly for low-content gasohol. In this study, both ensemble VOCs and BTEX were found negatively correlated with substitution ratio of methanol in gasohol. This principle has been in concert with HC results. Thanks to simple chemical structure and the absence of C–C bond of methanol molecule, it was more difficult

Fig. 5. Volatile organic compounds and BTEX from vehicle fueled with gasoline and gasohol.

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to synthesize VOCs by methanol–methanol or methanol–gasoline interactions. Being a series of poisonous species, VOCs have very adverse impacts on human health and some extremely toxic compounds like benzene could lead to cancer. VOCs are also precursor for secondary particulate matter for they could react with NOx in the upper-air with sunshine as a catalyst, so curbing VOCs emissions at tailpipe by using gasohol as an alternative could have a variety of benefits. The simple case established to evaluate health risk of pipe-our formaldehyde was also applied to benzene here. Due to much higher cycle-averaged emission factor of benzene, concentrations of benzene in the atmosphere near the outlet of tailpipe can be several times the concentrations of formaldehyde. According to the safety thresholds provided by USEPA, man persistently exposed to engine exhaust with such concentrations could have no more than 1/10,000 increased chance to develop cancer [35]. Via decreasing the concentrations of exhaust-borne VOCs and benzene, using methanol as a partial surrogate of conventional gasoline theoretically lowers the risk of cancer, though, with little available evidence, researchers from different countries and faculties are still unable to affirm that exposure to gasoline-derived benzene or VOCs will result in the morbidity of cancer. 3.3. Particulate matter An important reason for burning gasohol was that methanol addition was able to eliminate part of pipe-out particulate matter. This advantage was especially focused by central and local governments of China as nationwide haze weather occurred quite frequently in recent years. Mileage-based mass and number of particulate matter were drawn in Fig. 6. Relative to gasoline counterpart, mass of PM emissions was reduced by 33.2–40.2% with gasohol fueling, however a slight increment of particulate number was noticed as substitution ratio of methanol was raised. This result tallied with that on a GDI engine burning gasoline and ethanol conducted by Xu et al. [36]. Due to the fact that methanol was a kind of oxygenated fuel and no C–C bond existed in its chemical structure, the route of thermal pyrolysis within fuel-rich zones forming CH radicals and polycyclic aromatic hydrocarbons (PAHs) was effectively restrained. Both CH radicals and PAHs were thought key precursors for the formation of particulate matter in chambers [35]. Besides, as indicated by Xu et al. [36], burning alcohols on GDI engines could lessen the dependence upon configurations of fuel supply system and help to lower particulate mass. Extra O and OH radicals derived by methanol addition were also beneficial to consume carbonaceous particles in-cylinder [37].

Fig. 7. Number distribution of gasoline- and gasohol-fueled vehicle.

Apart from mass caps, future emission standards also set upper limits of particulate number for direct injection models. Although methanol addition into gasoline could in part tackle with particulate mass issues, the proposal might give rise to some concerns on number. As indicated by Fig. 6, mileage-based particulate number surged 14.4%, 15.7% and 23.6% when M15, M25 and M40 were burnt respectively. Number distributions were given in Fig. 7. Particles gathered by the first 2 stages and the last 5 stages of ELPI increased with the addition of methanol. Whereas particulate number of stage 3 to stage 7 went opposite to this correlation. This behavior revealed that particles became smaller but denser as substitution ratio of methanol increased. Morphology of particles generated by burning gasoline and gasohol samples shown in Fig. 8 confirmed this hypothesis. Under a scanning electron microscope, both gasoline- and M15derived particle samples exhibited graph-like structures, and nearly no difference can be identified between these two samples. In Fig. 8a and b, small agglomerates gathered together and formed larger lumps with their sizes ranging from 1.0 to 3.0 micron. Chain structures and abundant branches can be clearly observed in the pictures. In contrast, only one or two accumulation-mode particles could be found in the center of Fig. 8c and d. The majority of particles in the visual field were in nuclei-mode with their sizes ranging from tens of nanometers to half a micron. This result was in good agreement with particle number distributions drawn in Fig. 7. It should be also mentioned that cold dilution method was adopted in this paper, so water vapor in the dilution tunnel might have some negative influences on number results, but generally diameters of these condensed water droplets were much larger than real particles, they would be collected by the last stages and had little effects on mileage-based number.

3.4. Fuel economy

Fig. 6. Particulate matter and particulate number of gasoline- and gasohol-fueled vehicle.

In addition to pipe-out emissions reduction, burning gasohol as an alternative could bring benefits on greenhouse gas (GHG) and fuel-related cost. As compared in Table 1, burning neat methanol could discharge approximately 7.5% less CO2 than gasoline by releasing the same scale of energy. In this paper, mileage-based CO2 factors for M15, M25 and M40 fueling over the NEDC were 158.26, 153.73 and 153.12 g/km, respectively. Compared with the gasoline baseline of 159.59 g/km, CO2 emission produced by burning the three gasohol samples decreased by 0.8–4.1%. Relative to conventional gasoline, methanol also has evident price advantage. In China, for example, retail price of conventional

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(a) Conventional gasoline

(b) M15

(c) M25

(d) M40

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Fig. 8. Morphology of particulate matter from gasoline and gasohol with various substitution ratios.

gasoline basically ranged from $0.96 to $0.99 per liter, while the average price methanol was only around $354.8 per ton, namely $0.28 per liter (collected in Apr, 2015). Considering the gap in LHV, burning M15 instead of conventional was estimated to save about 5.7% fuel cost. If gasohol with higher methanol substitution was used, such as M40, the price gap between conventional gasoline and gasohol will be further extended to nearly 15%. In the past decade, spot price of WTI crude oil reached its peak of $145.31 per barrel on 3 Jul, 2008, and then the price fell down to the bottom by the end of 2008. From 2011 to 2014, the price was basically controlled at the level of $100 per barrel, but owing to increased output in the U.S. and decreased demanding in Asia, spot price of WTI crude oil was more than halved this year and down to $43.39 on 17 Mar, 2015. Currently, the price was around $50 per barrel. Given the fact that, there is no available data of international methanol price, price of methanol is anticipated by reviewing natural gas and coal prices, the most important raw materials for methanol production. Spot price of natural gas at Henry Hub has reached $15.39 and $13.31 per million British thermal unit (Btu) on 13 Dec, 2005 and 2 Jul, 2008 respectively. Almost at the same time, futures price of Central Appalachian coal attained the historical plateau of $135.61 per short ton on 30 Jun, 2008. Recently, both prices of natural gas and coal have been down to a quite low level, nominally around $3 per Btu and below $50 per short ton. It is easy to define two price ratios, crude oil/natural gas (C/N) and crude oil/coal (C/C). From 2006 to the end of 2008, the C/N basically ranged from 6 to 12, but this indicator once surged to

30 within the 2010–2014 period. At present, the C/N is around 20. On the other hand, the C/C ratio was generally maintained around 1 before 2011 although it has been descended to nearly 0.7 when oil price ushered in its bottom in 2008. After 2011, the C/N climbed to about 1.5 and recently went back to 1.1 as oil price has sunk sharply. Ratios clearly demonstrated that, even though the price of crude oil has dropped more than 50% in the international market, using methanol as a surrogate has actually become a more economic option than 6 years ago. Particularly for natural gas-derived methanol, considerable profit margin could be anticipated since the price of natural gas is still on the down side. Consequently, it is expected that burning methanol as a partial alternative to gasoline could at least retain a fuel-cost saving of 5–15% in the next few years. As stated in previous sections, burning low-content gasohol did not require modifications of vehicle originally designed to burn pure gasoline, so there is no additional development cost and purchasing expense of the car, unlike flex-fuel vehicles. However, higher methanol substitution ratio may result in malfunction of oxygen sensor and activate malfunction indication light (MIL) on the dashboard. In many countries with inspection and maintenance (I/M) program, cars or trucks with MILs on shall immediately fail the program, and the drivers will be asked to repair their vehicle until the MIL shuts down. Sometimes, emptying fuel tank and refilling with gasoline can be enough, but long-term usage of over-oxygenated fuel may induce permanent damage to oxygen sensor and increases service burden for the drivers.

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4. Conclusion and recommendation

References

In this paper, regulated, unregulated and particulate emissions from an in-use passenger car burning three kinds of methanol– gasoline, namely M15, M25 and M40, were examined by running chassis-level tests. The results were also put in context with gasoline baseline. In order to better understand the potential adverse effects of pipe-out formaldehyde, a simple model emulating man exposing to severe concentrations of pipe-out formaldehyde was established to evaluate possible acute, chronic and cancer-related risks. As demonstrated by the results, the following conclusions can be made here. Regulated – With the rising of methanol substitution ratio in gasohol samples, CO and THC emissions decreased by 9.4–33.2% and 9.7–36.9% respectively, while NOx emissions increased. Even though, in this paper, NOx emissions derived from the three gasohol samples were found basically no higher than gasoline baseline. Similar levels of pipe-out NOx emissions were measured in the tests with gasoline and M40 fueling. Unregulated – Carbonyls, majority of which was identified formaldehyde, increased sharply with methanol substitution ratio. However, VOCs exhibited an absolutely opposite inclination with enriched methanol in the blend. By establishing a simple case, possible health damage brought about by methanol-derived unregulated pollutants was evaluated. Compared with security thresholds summarized by USEPA, even in severe exposure conditions, concentrations of exhaust-borne formaldehyde and benzene were unable to cause any acute or chronic symptoms. Though elevated concentrations of pipe-out formaldehyde caused by the addition of methanol into fuel will increase the change of developing cancer, cancer-related risks for both gasoline and gasohol are still in the same magnitude, nominally no higher than 1/10,000. With respect to benzene-related risk, using methanol–gasoline as a surrogate will be beneficial to lower the morbidity of cancer. Particulate matter – Compared with gasoline, burning gasohol samples was able to remove 33.2–40.2% tailpipe PM emissions. However, an increase in particulate number with the addition of methanol was simultaneously observed, since larger part of particles were in nuclei mode. Fuel economy – With 15–40% gasoline being substituted by methanol, tailpipe CO2 emission decreased by 0.8–4.1%. Regarding current fuel price, fuel-cost saving by using gasohol as an alternative was estimated around 5.7–15%. Alternative acceptability – Based on the assessments of pipeout emissions, health-related risks, fuel economy and other aspects, M15 and M25 were graded as more practical choices due to well-balanced regulated, unregulated and particulate emissions, controllable health-related risks, little impacts on the original engine calibration and a small benefit of fuel economy. On the contrary, in spite of the lowest fuel-cost and carbonaceous emissions, M40 was eventually phased out as over-high NOx contamination propensity and potential to break down engine parts. However, maximum RVP of gasohol fuel must be strictly capped to control evaporative emissions once M15 or M25 was used, particularly for those tropical and subtropical countries. Drivability of gasohol-fueled vehicle shall be also taken into account, if the car is often running under high engine load conditions, then gasohol may be not an appropriate option.

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Acknowledgement This work was financially supported by National Natural Science Foundation of China (Grant No. 51476012).

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