Emission components characteristics of a bi-fuel vehicle at idling condition

Abstract

Natural gas (NG) represents today a promising alternative to conventional fuels for road vehicles propulsion, since it is characterized by a relatively low cost, better geopolitical distribution than oil, and lower environmental impact. This explains the current spreading of compressed natural gas (CNG) fuelled spark ignition (SI) engine, above all in the bi-fuel version, which is able to run either with gasoline or with NG. However, the aim of the present investigation is to evaluate the emission characteristics at idling condition. The vehicle engine was converted to bi-fueling system from a gasoline engine, and operated separately either with gasoline or CNG. Two different fuel injection systems (i.e., multi-point injection (MPI)-sequential and closed-loop venturi-continuous) are used, and their influences on the formation of emissions at different operating conditions are examined. A detailed comparative analysis of the engine exhaust emissions using gasoline and CNG is made. The results indicate that the CNG shows low air index and lower emissions of carbon monoxide (CO), carbon dioxide (CO₂), and total hydrocarbon (THC) compared to gasoline.

Keywords

road vehicle engine / fuel injection systems / carbon monoxide (CO) / carbon dioxide (CO₂) / total hydrocarbon (THC) / vehicle idle conditions / air index

Introduction

As is known gaseous fuels, such as liquefied petroleum gas (LPG) and natural gas (NG), thank to their good mixing capabilities, allow complete and cleaner combustion than normal gasoline, resulting in lower pollutant emissions and particulate matter. Moreover the use of NG, mainly constituted by methane, whose molecule has the highest hydrogen/carbon ratio, leads also to lower CO₂ equivalent emissions. Some of the automobile producers already put on the market “bi-fuel” engines, which may be fed either with standard gasoline or with NG. These engines, endowed of two separate injection systems, are originally designed for gasoline operation; hence they do not fully exploit the good qualities of methane, such as its high knocking resistance [1], which would allow higher compression ratios. Moreover, when running with gasoline at medium high loads, the engine is often operated with rich mixture and low spark advance in order to prevent from dangerous knocking phenomena: this produces both high hydrocarbon and carbon monoxide emissions (also due to the low catalyst efficiency caused by the rich mixture) and high fuel consumption.

In view of the versatility of internal combustion engine (ICE), it will remain to lead the transportation sector as there is a significant restriction on the battery and fuel cell powered vehicles with respect to range and acceleration. The power to weight ratio of the ICE is much more than that of the battery powered or fuel cell operated vehicles [2]. These factors have led scientists and researchers to develop environment-friendly technologies, and to introduce more clean fuels like NG to power ICE for ensuring the safe survival of the existing engine technology. The world’s total reserve of NG was 6076 trillion standards cubic feet and on the basis of current consumption rates, it is adequate for almost 65 years [3]. There are many merits of CNG as an automotive fuel over conventional fuels [4,5]. Due to some of the favorable physio-chemical properties [6] of CNG, the gasoline run spark ignition (SI) engines can be retrofitted to CNG operation quite easily with the addition of a second fueling system.

Manufacturers are also converting gasoline engine into bi-fuel engine in order to satisfy customer demand as presently happening in Egypt. But, retrofitted natural gas vehicle (NGV) engine produces about 10%-15% less power than the same engine fuelled by gasoline [7-10]. Another main drawback is that the required fuel storage tank is heavier so that the vehicle range is compromised for avoiding very large storage tank. However, CNG has the potential to increase engine efficiency if the engine is designed for dedicated CNG operation.

The emission from vehicles equipped with spark ignition engines contributes to raising the pollutants level of atmosphere and consequently the global environment [11]. This pushes the scientific society and authorized institutions to review the current legislations to reduce the emission [12,13]. The remarkable increase in the pollutants comes from the fast growth in vehicle production [14]. Other important sources are the fuel factor, the operating condition and performance, and the engine mechanical condition. The later factor was found to be responsible for a considerable part of emission. It is believed that, the engine performance is subjected to decay due to several factors. The rate of depreciation and maintenance processes plays an important role in governing the engine performance and emission. Starting from these observations, the authors experimentally investigated on the simultaneous combustion of gasoline-natural gas mixtures in stoichiometric proportion with comburent air, so as to exploit the good qualities of both fuels to achieve cleaner and more efficient combustions: the addition of NG to the gasoline-air mixture in fact raises knocking resistance, allowing thus to run the engine with both “overall stoichiometric” mixture and more efficient spark advance even at full load, while the stoichiometric A/F ratio obviously allows to minimize pollutant emissions [15].

However, the aim of the present investigation is to evaluate the emission characteristics at idling condition. Two different fuel injection systems are used, and their influences on the formation of emissions at different operating conditions are examined. A detailed comparative analysis of the engine exhaust emissions using gasoline and CNG is made. It is observed that the CNG shows a low λ and lower emissions of CO, CO₂, THC compared to gasoline.

Layout of experiment

The experiment is conducted on a specified vehicle to avoid vehicle-to-vehicle variation stemmed from different designs, operating conditions and maintenance processes. According to the previously mentioned plan of research, the idling condition is submitted to study to clear the governing factors. In idling mode, the ignition system as well as the mixture quality is examined according to the following sequence. The spark plugs gap is stretched from 0.3 mm up to 1.9 mm. Ignition timing is varied from 1 to 19 degree before TDC. Idling mixture valve is adjusted at different positions in order to control the excess air factor. Each of the previous factors is varied separately at the constancy of the other parameters. The experiments are conducted via an infrared gas analyzer and an r/min pickup transducer. The emissions and engine rotational speed are recorded. The measurements are taken at steady state condition. Figure 1 shows the vehicle engine emission gas analyzer, while Fig. 2 shows the exhaust probe inside the exhaust tailpipe.

Vehicle used

The vehicle used in the present study is selected to represent statistically the average of manufacturing year for most vehicles in Egypt during the period of study. The vehicle is Hyundai-star. The engine is spark ignited four strokes, four cylinder in-line and water cooled. It is transversally mounted with front wheel drive technique. The engine is equipped with manually operated gear box mounted transversally and shares the same oil sump of the engine. The gear box offers four forward speeds and single reversal speed. The technical data for the vehicle engine are represented in Table 1.

Instrumentation

The measurements program is focused on evaluating the emission characteristics at different operating conditions. For this reason, measuring the concentration of exhaust constituents as well as the engine speed is essential. For this purpose, a portable version of infrared gas analyzer is used. The fuel injection system used is tabulated in Table 2. The gas analyzer is equipped with a gas sampling probe to collect the exhaust gas from the muffler (Fig. 2). The gas is then filtered and dried before entering the analyzer (Fig. 1). A magnetic inductive pickup transducer is used also to measure the engine speed in r/min. It is clipped to any of spark plugs cable in order to capture the spark signal. The sparking rate is then considered as linear proportion to the engine speed.

Test procedures

Studying the emission characteristics from the vehicle’s engine requires intensive measurements program at different operating condition. The selected vehicle is equipped with previously mentioned measuring instruments. The gas analyzer and its accessories are mounted in the rear seat of the passenger cabinet. Rechargeable power supply and printer are the most important attachment to the analyzer. A gas sampling probe with a length of 3 m is inserted inside the muffler, where its other terminal is connected to the gas analyzer through the window of the rear door. The magnetic inductive transducer is clipped also to the spark plug cable to measure the engine rotational speed. Before starting the measurements, the following precautions are taken into account:

1) The engine is warm enough before starting the measurement and runs steadily at standard idling configuration.

2) All electric accessories like electric fan, lights and radio-cassette are switched off.

Two persons are required to conduct the experiment. The first person is the engine driver who performs the test program with certain sequence and is responsible for running the vehicle engine steadily for enough periods required to obtain steady measurements. The second person is the instrument operator who is responsible for reviewing the test procedure with the driver and observes the output readings. When the signals become steady, the output readings are recorded and the next step of the vehicle engine speed is performed. The λ is considered to represent the fuel consumption, and is defined as

(1)\lambda ={\displaystyle \frac{(\mathrm{A}\mathrm{i}\mathrm{r}/\mathrm{F}\mathrm{u}\mathrm{e}\mathrm{l})\mathrm{O}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{n}\mathrm{g}}{(\mathrm{A}\mathrm{i}\mathrm{r}/\mathrm{F}\mathrm{u}\mathrm{e}\mathrm{l})\mathrm{S}\mathrm{t}\mathrm{o}\mathrm{i}\mathrm{c}\mathrm{h}\mathrm{i}\mathrm{o}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{i}\mathrm{c}}}.

Results and discussion

The vehicle engine is kept in its original configuration to be used as a bi-fuel engine. The CNG systems used for bi-fuel converted vehicles are adopted to be seamlessly switchable between gasoline and CNG, thus providing minimum exhaust emissions to deliver CNG into the engine for clean and efficient operation.

CO

The relationship between the vehicle engine CO and vehicle engine speed when the vehicle was operated by gasoline MPI system is illustrated in Fig. 3, where the level of CO is decreased as the vehicle engine speed is increased. The relationship between the vehicle engine CO and vehicle engine speed when the vehicle was operated by CNG MP-injection system is displayed in Fig. 4, where the level of CO is decreased as the vehicle engine speed is increased. The relationship between the vehicle engine CO and vehicle engine speed when the vehicle was operated by CNG closed loop (venturi) injection system is demonstrated in Fig. 5, where the level of CO is decreased as the vehicle engine speed is increased.

The influence of fuel type on CO and the injection type is exhibited in Figs. 6 and 7 respectively. The vehicle engine operated by CNG produces lower level of CO than that operated by gasoline (Fig. 6), where the injection system is multi-point (MP). In Fig. 7, the influence of injection system type on the CO level is presented, which indicates that it is better to use closed-loop (venturi) injection system than MPI injection system when CNG is used as a fuel.

CO₂

In Fig. 8, the relationship between the vehicle engine CO₂ and vehicle engine speed when the vehicle was operated by gasoline MP-injection system is shown. The level of CO₂ is decreased till the operating speed reaches 2000 rpm and then decreased as the vehicle engine speed is increased. In Fig. 9, the relationship between the vehicle engine CO₂ and vehicle engine speed when the vehicle was operated by CNG MP-injection system is shown. The level of CO₂ is decreased till the operating speed reaches 2400 r/min and then decreased as the vehicle engine speed is increased. In Fig. 10, the relationship between the vehicle engine CO₂ and vehicle engine speed when the vehicle was operated by CNG closed loop (venturi) injection system is shown. The level of (CO₂) is decreased till the operating speed reaches 3000 r/min and then decreased as the vehicle engine speed is increased.

Figures 11 and 12 depict the influence of fuel type on CO₂ and the injection type respectively. The vehicle engine operated by CNG produces lower level of CO₂ than that operated by gasoline (Fig. 11), where the injection system is multi-point (MP). In Fig. 12, the influence of injection system type on CO₂ level is presented, which indicates that it is better to use closed-loop (venturi) injection system than MPI injection system when CNG is used as a fuel.

THC

Figure 13 depicts the relationship between the vehicle engine THC and vehicle engine speed when the vehicle was operated by gasoline MP-Injection system, where the level of THC is decreased as the vehicle engine speed is increased. Figure 14 depicts the relationship between the vehicle engine THC and vehicle engine speed when the vehicle was operated by CNG MPI system, where the level of THC is decreased as the vehicle engine speed is increased. Figure 15 depicts the relationship between the vehicle engine THC and vehicle engine speed when the vehicle was operated by CNG closed loop (venturi) injection system, where the level of THC is decreased as the vehicle engine speed is increased.

In Figs. 16 and 17, the influence of fuel type on THC and the injection type is respectively. The vehicle engine operated by CNG produces a higher level of THC than that operated by gasoline (Fig. 16), where the injection system is multi-point (MP). In Fig. 17, the influence of injection system type on THC level is presented, which indicates that it is better to use closed-loop (venturi) injection system than MPI system when CNG is used as a fuel. The reason behind this increase is attributed to the fact that it is difficult for CNG operated vehicles to oxidize the unburned hydrocarbons in the exhaust gases, where the oxidization of hydrocarbons is one of the functions of the three-way catalyzer. The exhaust hydrocarbons of a gas-operated vehicle have a significantly different composition from those of a gasoline-operated vehicle.

Air index

The λ for multi-point injection (MPI gasoline-sequential), multi-point injection (MPI gasoline-sequential) and closed-loop injection-venturi CNG systems are presented in Figs. 18 to 20, where the lowest values are found to be at 3000 r/min (gasoline), 2000 r/min (MPI CNG) and 2500 r/min (CNG Venturi) respectively. The ranges are 1.4-0.85 at 3000 r/min for gasoline, 1.02-1.007 at 2000 r/min for MPI CNG and (1.4-1.07) at 2500 r/min. This indicates that less MPI CNG is required for a given air charge. Furthermore, CNG has a greater lower heating value (LHV) indicating more available energy for each unit of mass. The LHV value for CNG is approximately 46736 kJ/kg, while the LHV for gasoline is approximately 43961 kJ/kg.

In Fig. 21, a comparison is given, showing the difference between the λ for CNG and gasoline fuelled multi-point injection (MPI) engine. It can be observed that the reduction in λ produced at the lower vehicle engine speed of 2000 r/min is nearly the same as that produced at the higher vehicle engine speed of 3500 rpm. Figure 22 depicts a comparison between two different injection systems, i.e., MPI and closed-loop venturi from the point of view of evaluating the λ and both of them used CNG as a fuel and normally used in this vehicle version. It can be seen that the air indexes measured for closed-loop venturi system are lower than those measured for MPI system.

Conclusions

1) A Hyundai-star vehicle was instrumented and operated with gasoline and NG and two injection system (multi-point and closed-loop venturi). Over a range of vehicle and engine speeds existing at idle state conditions, the exhaust gas emission components are presented.

2) In idle state, it is observed that the current bi-fuel (gasoline-CNG) tested has the potential to meet the standards with either of the two fuels. When working with CNG, it produces less CO₂ (CO₂%) emission component than the gasoline one. Furthermore, it is found that the CO (CO%) produced from the vehicle operated by CNG is much lower (reaching almost zero) than that produced by gasoline.

3) In idle-state tests, a significant difference is observed between the λ for CNG and gasoline fuelled MPI engine. The comparison between the two different injection systems, i.e., MPI and closed-loop venturi from the point of view of evaluating the λ. where the air indexes measured for closed-loop venturi system are lower than those measured for MPI system.

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