Optical Characterization of Methane Combustion in a Four Stroke Engine for Two Wheel Application

Abstract

In the urban area the internal combustion engines are the main source of CO2, NOx and particulate matter (PM) emissions. The reduction of these emissions is no more an option, but a necessity highlighted by the even stricter emission standards. In the last years, even more attention was paid to the alternative fuels. They allows both reducing the fuel consumption and the pollutant emissions. Regarding the gaseous fuels, methane is considered one of the most interesting in terms of engine application. It represents an immediate advantage over other hydrocarbon fuels because of the lower C/H ratio. In this paper the effect of the methane on the combustion process, the pollutant emissions and the engine performance was analyzed. The measurements were carried out in an optically accessible singlecylinder, Port Fuel Injection, four-stroke SI engine equipped with the cylinder head of a commercial 250 cc motorcycles engine and fuelled both with gasoline and methane. Optical measurements were performed to analyze the combustion process with a high spatial and temporal resolution. In particular, optical techniques based on 2D-digital imaging were used to follow the flame propagation in the combustion chamber. UV-visible spectroscopy allows detecting the chemical markers of combustion process such as the radicals OH and CH. The exhaust emissions were characterized by means of a gaseous analyzer and an opacimeter. The measurements were performed under steady state conditions, at 2000rpm at minimum and full load. Introduction In the urban area the internal combustion engines are the main source of CO2, NOx and particulate matter (PM) emissions. The reduction of these emissions is no more an option, but a necessity highlighted by the even stricter emission standards. In the last years, even more attention was paid to the alternative fuels that allow both reducing the fuel consumption and the pollutant emissions. Methane is a promising alternative fuel to petrol for internal combustion engines [1]. It represents an immediate advantage over other hydrocarbon fuels because of the lower C/H ratio. Moreover, it has a higher Lower Heating Value (LHV) and stoichiometric air/fuel ratio and a higher Research Octane Number (RON) which permits higher compression ratios, higher boost in turbocharged engines, and better knocks limited XXXIV Meeting of the Italian Section of the Combustion Institute 2 spark advances, as reduces the knock sensitivity. The major drawback of the use of methane in the spark ignition or compression ignition engines is the low flame propagation speed. The flame front propagation speed depends mainly on the turbulence and the air/fuel ratio. In particular, it increases at the increasing of the turbulence and at the decreasing of the air/fuel ratio [2]. In this paper the effect of the methane on the combustion process, the pollutant emissions and the engine performance was analyzed. The measurements were carried out in an optically accessible single-cylinder, Port Fuel Injection, fourstroke SI engine equipped with the cylinder head of a commercial 250 cc motorcycles engine. Optical measurements were performed to analyze the combustion process with a high spatial and temporal resolution. In particular, optical techniques based on 2D-digital imaging were used to follow the flame propagation in the combustion chamber. UV-visible spectroscopy allows detecting the chemical markers of combustion process such as the radicals OH and CH. The measurements were performed under steady state conditions, at 2000 rpm at minimum and full load. The engine was fuelled with commercial gasoline and methane. Experimental Apparatus Transparent Engine The experimental activity was performed in an optically accessible single-cylinder, Port Fuel Injection, four-stroke SI engine [3]. The engine bore and stroke were 72 mm and 60 mm, respectively. The geometric compression ratio was 11:1. The engine was equipped with the cylinder head of a commercial 250 cc motorcycles engine. A four-valve, pent-roof chamber engine was mounted on an elongated piston. The engine reached a maximum speed of 5000 rpm. The maximum performance is: 7.9 kW and 14.7 Nm at 5000 rpm. The head had a centrally located spark plug and a quartz pressure transducer was flush-installed in the combustion chamber to measure the combustion pressure. The in-cylinder pressure, the rate of chemical energy release and the related parameters were evaluated on an individual cycle basis and/or averaged on 400 cycles [2]. The optical engine was characterized by an elongated cylinder and a piston provided with a sapphire window which replaces the flat-bottom piston bowl. The engine is also equipped with a quartz cylinder in order to have a lateral point of view of the combustion chamber. This system enables the passage of optical signals coming from the combustion chamber. To reduce the window contamination by lubricating oil, the elongated piston arrangement was used together with self-lubricating Teflonbronze composite piston rings in the optical section. Setup for Spectroscopic Measurements During the combustion process, the light passed through the sapphire window and it was reflected toward the optical detection assembly by a 45° inclined UV-visible XXXIV Meeting of the Italian Section of the Combustion Institute 3 mirror located in bottom of the engine. Chemiluminescence signals were collected and focused on the entrance slit of a spectrograph through an UV-Visible objective. The slit was 250 μm wide open and it was located in front of the combustion chamber. Spectrograph was 15 cm focal length, f/4 luminous, and equipped with a grating of 300 g/mm, blazed at 300 nm, with a dispersion of 3.1 nm/mm. The spectral image formed on the spectrograph exit plane was matched with a gated intensified CCD camera. Data were detected with the spectrograph placed at two central wavelengths, 375 and 625 nm, respectively, and the intensifier-gate duration was set to 166.6 μs in order to have a good accuracy in the timing of the different investigated events. Chemiluminescence signals, due to radical emission species, were detected in the central and lateral locations of the combustion chamber with high spatial and temporal resolution. Engine synchronization with ICCD camera was obtained by the unit delay connected to the signal coming from the engine shaft encoder. In this way, it was possible to determine the crank angles where optical data were detected. Engine Operating Conditions All the experimental investigations were carried out at 2000 rpm. The intake air temperature was fixed at 298 K and the cooling water temperature was set at 333 K. Commercial gasoline and methane fuels were used. For all the test cases, the injection-duration (DOI) was chosen to obtain a stoichiometric equivalence ratio. Two different fuel injection strategies were tested for both fuels: minimum load (closed throttle) and full load (wide open throttle). The coefficient of lambda value variation was measured on 400 consecutive cycles. It was lower than 1.8% for all the selected conditions. The spark timing (SOS) was always fixed to operate at the maximum brake torque. More details about the operating conditions are reported in Table 1. Table 1. Engine operating conditions Test label Fuel Pinj [bar] DOI [cad] SOS [cad] Minimum load Gasoline 3.5 29.5 -29.5 Full load Gasoline 3.5 71 -71 Minimum load Methane 1.5 128.7 -378.7 Full load Methane 1.5 250.6 -500.6 Results and Discussion The measurements were performed from the Start of Spark (SOS) until exhaust valve opening. The spectroscopic measurements were binned along space direction in order to obtain three typical locations: in correspondence of the exhaust valves, the spark plug and the intake valves. The development of the combustion process was identified by means of the analysis of digital images. In particular, the flame front propagation speed, an XXXIV Meeting of the Italian Section of the Combustion Institute 4 important parameter in the study of combustion in spark-ignition engines, was evaluated. The comparison between the flame front propagation speed, for the gasoline and methane fuels at minimum and full load is reported in Figure 1. -20 -10 0 10 crank angle [degree] 0 4 8 12 16 fla m e fro nt p ro pa ga tio n sp ee d [m /s ] minimum load Gasoline