Application of HCCI Engine in Motorcycle for Emission Reduction and Energy Saving

The exhaust emission of motorcycle engine is a big problem in Asia countries due to the large number of motorcycles. This study developed a homogeneous charge compression ignition (HCCI) engine for motorcycle application with highoperating efficiency and low exhaust emissions. The development was carried out on a 150 cc spark-ignition (SI) engine with an increased compression ratio to enhance compression temperature. Dimethyl ether (DME) was selected as the main fuel for HCCI operation due to the low self-ignition temperature. Gasoline was used as an additional fuel to adjust the ignitibility of the dual fuel mixture. The dual fuel and exhaust gas recirculation (EGR) were incorporated to expand the engine operating range. Experiments of varying DME flow rates, EGR ratios, and gasoline flow rates were performed to observe HCCI operating characteristics and to identify methods for controlling HCCI combustion. And then, the engine could be operated at speeds ranging from 2000 to 4000 rpm and BMEP (Brake Mean Effective Pressure) ranging from 1.56 to 4.86 bar. These operating points were found to have lower brake specific fuel consumption and lower CO and NO emissions as compared with the original SI engine. A range-extended electric motorcycle (REEM) was built from a conventional motorcycle by using the proposed HCCI engine as a range extender. The HCCI engine was operated at 3500 rpm with very stable and high efficiency to match the REEM. The fuel economy of the proposed motorcycle is 75.62 km L, a 132% improvement compared with the original motorcycle. The control of switch from SI to HCCI will be the future work.


INTRODUCTION
Exhaust emissions of internal combustion engines are significant sources of air pollution (Yao and Tsai, 2013;Cheng, 2013).They not only create potential global warming effects, but also are implicated in human health issues.In most Asian countries, motorcycles are one of the most commonly used transportation tools (Lin et al., 2014) and contribute to air pollution more than other vehicles (Chuang et al., 2010).Previous research shows that three-way catalytic converter used in spark ignition (SI) engines could reduce most exhaust pollution, such as HC, CO and NO x , towards achieving exhaust standards (Lou et al., 2003).However, catalytic converters would not reduce carbon dioxide (CO 2 ), a major cause of global warming effect.Several researches on blends fuel or additives reached the improvements of engine efficiency and emission reduction, such as microalgae biodiesel, butanol, water, H 2 /O 2 mixture, or plasma-enhanced combustion in diesel engines (Lu et al, 2013;Lin et al, 2013;Wang et al, 2013;Shukla et al., 2014;Mwangi et al., 2015).However, they cannot be used for motorcycle engine.It may be more acceptable to address the problem in the design and manufacture of motorcycles.
The electric vehicle (EV) has the advantages of higher energy conversion efficiency and zero exhaust emission during driving.However, its disadvantages in traveling distance and higher battery cost still restrict its development currently.Hybrid electric vehicle (HEV) and range-extended electric vehicle (RE-EV) seem to be the most promising short-term solutions.The range extender (RE) consists of engine and generator, which could provide extra electric power for charging the battery, thus extending the traveling distance of RE-EV.Since a RE engine is mainly employed for driving a generator, it has the advantage in that engine operation is independent of vehicle speed and road conditions (Rizoulis et al, 2001).In order to promote fuel economy, the engine can be operated at a higher efficiency and a stable operating point in accordance with charging power requirements from a power management strategy.Exhaust emissions also can be reduced by preventing transient engine operation.
A conventional spark-ignition (SI) engine can be utilized as a RE engine; however, the pumping loss suffered from partial load will sacrifice fuel efficiency.The conventional compression-ignition engine (diesel) is operated with higher fuel efficiency due to its higher compression ratio and throttleless features.But NO x and particulate matter (PM) emissions of diesel engines are crucial problems that require an expensive after-treatment system for both NO x and PM.
A homogeneous charge compression ignition (HCCI) engine is similar to a combination of conventional SI engine and diesel engine.Its fuel-air mixture is premixed, as in a conventional SI engine, while its combustion is initiated by self-ignition, which is like diesel engine.Therefore, HCCI as a promising alternative combustion technology with high efficiency and lower NO x and PM emissions, has been widely investigated in recent years (Wang et al., 2009).However, it still faces the challenges of its operating range, the difficulty in controlling ignition timing, and too high of pressure rise rate at high loads (Mack et al., 2005;Sjöberg et al, 2007).Previous studies show that many methods can be used to facilitate HCCI operation: (1) Intake heating (Martinez-Frias et al., 2000); (2) Variable compression ratio (VCR) (Haraldsson et al., 2002); (3) Negative valve overlap (NVO) (Zhong et al., 2006); (4) Improving fuel ignitability (Yao et al., 2005).It should be noted that some of these methods are too complex and costly, even difficult, to implement for a RE engine.
Dimethyl ether (DME, CH 3 OCH 3 ) is diesel enginecompatible fuel due to its high cetane number and low autoignition temperature.Numerous investigations of DMEfuelled engines have indicated that it offers excellent promise as an alternative fuel for compression-ignition operation in the automotive sector (Arcoumanis et al., 2008).In addition, DME combustion is soot-free and has lower HC and CO emissions than that of diesel combustion (Park and Lee, 2013).Gaseous DME is a good choice as HCCI fuel because of its low boiling point and excellent ignition ability (Wang et al., 2009).However, it is still difficult to use HCCI over the full engine operation range.
Changes in operating condition of RE engine are not as dynamic as in a conventional vehicle, which increases the possibility of controlling HCCI combustion for RE application.This research, therefore, describes the development of a high-efficiency, low-emissions HCCI engine for RE application.In this development, a conventional motorcycle 150 cc SI engine is retrofitted for running HCCI.DME is selected as main fuel for HCCI operation.Experiments are then executed to observe HCCI operating characteristics and identify methods for controlling HCCI combustion.Then, a range-extended electric motorcycle (REEM) is built up of a conventional 150 cc motorcycle.And finally, the HCCI engine is used in the REEM.

HCCI Engine
Due to the compact size requirement of a range-extended electric motorcycle (REEM), a 150 cc single-cylinder, aircooled SI engine is chosen as the target engine.It is a motorcycle engine with an electronic fuel injection system.Detailed engine specifications are listed in Table 1.
First of all, the target engine is retrofitted for HCCI operation without significant changes in order to lower retrofitting costs.In order to run HCCI in the target engine, the compression ratio is increased from 10.5 to 12.4 by replacing the cylinder head with smaller clearance volume.The increased compression ratio produces a higher compression temperature, which facilitates compression ignition.
Since the HCCI combustion occurs by self-ignition, the self-ignition property of fuels becomes an important factor that affects the HCCI operation.Fuel properties are listed in Table 2.The cetane number of DME is 60, which is high as compared with the normal cetane range of diesel fuel for motor vehicles (about 40 to 60).DME can self-ignite in SI engine.On the contrary, the self-ignition temperature of gasoline is high.Consequently, the combination of gasoline and DME can achieve suitable ignitability for various engine operating conditions.Hence, the DME alternative fuel and gasoline are chosen as a dual fuel for HCCI operation in this study.
The configuration of the proposed HCCI engine is shown in Fig. 1.The dual-fuel supplying system is built into the target engine.The original fuel and ignition systems are kept for starting the engine.The original gasoline injector is   also used to provide gasoline.An additional DME supplying system, which includes a DME tank, pressure regulator, filter, surge tank, and a flow control valve, is attached to the target engine with a DME supply tube installed near the intake port.After starting the engine using gasoline in SI mode and achieving a stable cylinder head temperature, the controller can switch the engine operation to HCCI mode with dual fuel by controlling the duty cycle of the DME flow control valve and injection duration of the gasoline injector.In order to measure the gasoline and DME flow rates, the gasoline flow meter, gasoline defoaming tank, and DME flow meter are attached to the dual-fuel supplying system.Both fuels were used after starting the engine.The fuel supply is adjusted for stable HCCI operation, so the percentages of these two fuels were not fixed.
The external EGR system is established on the target engine with a small EGR pump and an EGR control valve which can be controlled to change the EGR ratio and thus, controlling HCCI combustion.

Experimental Setup
For the engine test, an eddy-current engine dynamometer (Borghi & SaveriSrl, FE150-S) is employed to measure the engine output brake torque and speed.The gasoline flow rate is measured by the mass burette flow detector (ONO SOKKI, FX-1110).The thermal mass flow controller (Tokyo Keiso, NM-2100) is utilized to measure the DME flow rate.The exhaust emissions for carbon monoxide (CO), hydrocarbons (HC), nitric oxide (NO), carbon dioxide (CO 2 ), oxygen (O 2 ), and air fuel ratio are measured by the emission analyzer (HORIBA, MEXA-584L).In addition, another emission analyzer (HORIBA, MEXA-584L) is installed in intake system to measure the CO 2 percentage for EGR ratio calculation.Several K-type thermocouples are installed on the engine for measuring the temperatures of ambient air, intake gas, exhaust gas, cylinder head, oil, and EGR, as shown in Fig. 1.
Since the target engine is retrofitted for HCCI operation, engine control is replaced by using a controller (Woodward, MotoHawk ECU 555-80) to control the spark timing (for SI mode), gasoline injector, DME flow control valve, EGR pump, and EGR control valve.The MotoHawk allows the user to automatically generate machine codes from Simulink blocks and to operate control hardware in real-time operation.
Temperature is the most important factor for HCCI combustion (Mack et al., 2005); thus, engine temperatures are monitored using negative temperature coefficient thermistors installed for cylinder head temperature and oil temperature.After these thermistors are connected with the input interface, Motohawk ECU for thermistor, voltage signals can be generated and read for engine control algorithm.In this study, cylinder head and oil temperature are kept above 120°C and 65°C respectively for stable HCCI operation (Wu et al., 2010).
The range-extended electric motorcycle (REEM) is built from a conventional 150 cc motorcycle.The REEM is driven by a 1.85 kW DC motor.The range extender is a combination of a 150 cc internal combustion engine and a 1.45 kW electric generator.
A hub motor of a commercialized electric motorcycle is installed onto the front wheel of the target motorcycle.Its maximum torque is 25.4 Nm at 696 rpm.The engine of the target motorcycle is modified to HCCI and used as the electric generator's power source.This generator provides electricity to drive the hub motor or charge the battery.The engine is started in spark-ignition mode and then transferred to HCCI mode.An electric throttle body is designed to replace the original throttle body.The engine does not drive the wheel.
The voltage and current output from the electric generator vary according to speed.In order to get stable voltage and current for charging the battery, a DC-DC converter is used to connect the generator and battery and its output voltage is adjusted to 52 V.The battery is YUASAREC22-12 acid battery.The voltage is 12 V and capacity is 22 Ah for one unit.Four units are connected in series to get a total voltage of 48 V.As for the motorcycle fuel economy test, the chassis dynamometer made by TECNER ENGINEERING, CORP., Model TX50 was used.The motorcycle ran on the chassis dynamometer in accordance with the ECE-40 driving pattern.

Experimental procedures
The fuel injection timing for most port-injection SI engines is set to end the injection before intake-valve opening to prevent liquid fuel from entering the cylinder directly.A previous study has shown that HC and CO emissions, and coefficient of cycle variation (COV) of indicated mean effective pressure (IMEP) could be decreased during closed valve injection (Wu et al., 2010).Therefore, the fuel injection timing of 90 o bTDC (before top dead center) of compression stroke was chosen for experiments not only to avoid the liquid fuel from entering the cylinder directly but also to maintain the homogeneity of the air-fuel mixture.
The data taken from engine tests are: engine speed, brake torque, DME flow rate, gasoline flow rate, intake air mass flow rate, CO emission, HC emission, NO emission, and EGR ratio which is calculated by the CO 2 percentages measured in intake system and exhaust system as in Eq. 1 (Yao et al, 2005).
A set of experimental data, which was taken from the target engine in HCCI operation with DME fuel at 2000 rpm with five repeated tests, was used to calculate the uncertainty of the experimental data based on the Kline and McClintock method (Holman, 1989).The results can be seen in Table 3.

Item
Uncertainty (± %) Engine Speed (rpm) 0.675 Engine Torque (Nm) 1.089 BSCO (g kW -1 h -1 ) 6.512 BSHC (g kW -1 h -1 ) 4.224 BSNO (g kW -1 h -1 ) 6.433 COV (%) 3.630 After determination of fuels, engine temperature, and fuel injection timing, the HCCI combustion is executed on the target engine.At first, the target engine is started up using SI mode with original ignition and fuel systems.When the cylinder head and oil temperatures reach 120°C and 65°C respectively, the engine is switched to HCCI mode by interrupting the ignition system and turning the throttle to wide open throttle (WOT), while fuel supply is adjusted for stable HCCI operation.Experiments are then done at various DME flow rates, engine speeds, EGR ratios, and gasoline added amount for investigating the operating characteristics of HCCI combustion.The methods for controlling HCCI combustion using dual fuel and external EGR are identified, as well.After that, a series of experiments for identifying the operating range of HCCI on the target engine is performed.

HCCI Operating Characteristics
The alternative fuel, DME, was used for HCCI engine; however, its operating range is very small.Engine torque was limited to avoid knocking.To investigate the stable operation conditions is important purpose of this study.
The brake torque of HCCI operation with DME is very small.To get higher torque, more fuel must be added.In such case, the start of main combustion will be advanced, and cause knocking.In such instance, retarding the combustion phase is necessary for getting higher torque.The selfignition temperature of gasoline is much higher than that of DME as shown in Table 2. Therefore, gasoline can be added to increase the engine torque without knocking.
Gasoline with a research octane number (RON) 92 and DME were used in the dual fuel system.Engine brake torque at 2000 rpm was increased to 3.49 Nm.With constant DME flow rate, the engine torque was increased with increasing the amount of gasoline.The cylinder pressure and heat release rate with various engine torque at 2000 rpm are shown in Figs. 2 and 3, respectively.The heat release rate (HRR) was calculated from experimental cylinder pressure data and the heat transfer model developed by Wu et al. (2009).Two-stage DME fuel combustion in HCCI can be found from the HRR curve shown in Fig. 3. Shibata and Urushihara (2008) pointed out that this phenomenon could be observed by using low-octane number fuels for HCCI combustion.The first stage combustion provides energy to heat up the mixture during the compression stroke and has the effect of enhancing ignitibility (Bunting et al.,   In the main combustion stage, the maximum HRR increases with increasing the gasoline. The exhaust gas is a kind of diluent in air-fuel mixture and suppresses the combustion rate.Previous research (Fathi et al., 2011) indicates that auto-ignition timing is delayed and the burn duration is prolonged by applying EGR.Therefore, EGR can be used in HCCI to expand the engine operating range.In the DME and gasoline dual fuel system, at constant DME fuel rate and EGR rate, various engine torques can be achieved with different amounts of gasoline.
Cylinder pressure and heat release rate at 2000 rpm using the dual fuel of DME and gasoline in HCCI with 25% EGR are shown in Figs. 4 and 5, respectively.The engine brake torque is increased to 5.38 Nm.When the EGR rate is higher than 25%, the combustion is unstable and COV is high.

HCCI Operating Performance
The HCCI engine using DME and gasoline dual fuel with EGR was examined at various engine loads and speeds.The engine speed ranged from 2000 to 4000 rpm and the brake mean effective pressure (BMEP) ranged from 1.56 to 4.86 bar.The comparison of engine operating range between the proposed HCCI engine and original SI engine is depicted in Fig. 6.Although the operating range of HCCI engine is smaller than that of SI engine, it is good enough for REEM.The brake specific fuel consumption (BSFC) of  HCCI engine is much lower than that of original SI engine, as shown in Fig. 7.Many operating points have the BSFC less than 250 g kW -1 h -1 .It is very suitable for REEM, because the REEM engine does not need a large operating range.
The exhaust emissions of this HCCI engine and its original SI engine are depicted in Figs.8-10.The data in these figures are: brake-specific NO emission (BSNO), brake-specific CO emission (BSCO), and brake-specific HC emission (BSHC).The brake specific emissions are defined as the mass emission rates divided by the engine brake horsepower.The mass emission rates were calculated according to SAE J1088.Fig. 8 shows that the NO emission of HCCI is very small.As compared with the SI engine, it is close to zero.It could be explained that the principal source of NO x is the oxidation of nitrogen, which is temperature and oxygen concentration dependent (Wang et al., 2012).The combustion temperature of the HCCI engine is low.As a rule of thumb, when the combustion temperature is lower than 1800 K, the NO emission will be much less (Turns, 2012).The CO emission of HCCI engine is also smaller than that of SI engine due to lean combustion, as shown in Fig. 9. Carbon monoxide, which has been the focus of many environmental studies (Colbeck et al., 2011;Ma et al., 2011) is highly toxic to humans.The current study's results contribute greatly to reducing carbon monoxide in the atmosphere.However, the HC emission of HCCI engine is higher than that of SI engine, as shown in Fig. 10.This might come from the low combustion temperature.The cooling system is designed for conventional SI engine.However, the thermal load of HCCI engine is smaller than that of SI engine due to low

REEM Vehicle Test Results
Fuel economy was tested on the chassis dynamometer following Taiwan driving cycle regulation, which combines a driving pattern of six cycles of ECE-40.
To make sure the state of charge (SOC) of the battery after fuel economy testing is the same as the initial state, the initial and final voltage of the battery were measured.The generator does not charge the battery during the first two ECE-40 cycles.After that, the engine and generator operating conditions were adjusted to ensure that the generator charges the battery during the last four ECE-40 cycles.In such situation, the final battery voltage is very close to the initial voltage.
The HCCI engine cannot start from a cold engine, so the gasoline SI mode is used during starting, and then transferred to dual fuel HCCI mode.The process of transition from SI to HCCI is depicted in Fig. 11.A PWM circuit was designed for electric generator to control the engine operated at 3500 rpm, which can run HCCI very stable.After cylinder head temperature achieves 120°C, DME fuel is added, gasoline fuel is reduced, and the throttle is fully opened.The transition timing is indicated by the red dotted line shown in Fig. 11.
The engine operating condition of HCCI is very simple in the REEM.Range extender conditions have made the study even simpler than normal HCCI study since control is less important for this application and HCCI combustion will be less challenging.The generator starts to charge the battery from the beginning of third ECE-40 cycle.At the end of driving cycle test, the battery voltage is kept at its initial voltage.The fuel consumption of DME and gasoline is measured and converted to equivalent gasoline according to the low heating value.The fuel economy of this REEM is 75.62 km L -1 , which is 32.61 km L -1 of the original motorcycle.So the fuel economy of REEM is improved by 132%.The good fuel economy might be caused from the low fuel consumption of the proposed HCCI engine.Although, the exhaust emissions cannot be measured in vehicle test, the NO and CO emissions must be very low as shown in Figs. 8 and 9.

CONCLUSIONS
A homogeneous charge compression ignition (HCCI) engine is designed for motorcycle to reduce the exhaust emissions and fuel consumption.The target engine is a 150 cc spark ignition engine with increased compression ratio.DME is selected as main fuel for HCCI operation and gasoline is selected as an additional fuel.The ignition and combustion characteristics are improved by using dual fuel and EGR.Since the gasoline added in the dual fuel system could retard the combustion timing, higher combustion pressure upon compression stroke can be constrained, which helps to increase the torque output.EGR introduction are good methods for further increasing the engine operating range due to the lower combustion rate.
Consequently, the operating range is expanded as the engine speed ranges from 2000 to 4000 rpm and the engine BMEP ranges from 1.56 to 4.86 bar.Most of the operating points reach very low fuel consumption as BSFC is smaller than 250 g kW -1 h -1 .The NO and CO emissions are reduced tremendously.However, the HC emission of HCCI engine is higher than that of SI engine.
A range-extended electric motorcycle (REEM) is built out of a conventional motorcycle.A hub motor from a commercialized electric motorcycle is installed to the front wheel of the target motorcycle.The proposed HCCI engine is used in this REEM, which engine operating condition is very simple.The engine is operated at very low BSFC point, so the fuel economy of the proposed REEM is improved by 132% as compared with the original motorcycle.The emission reduction and energy saving characteristics of HCCI engine found in this study is similar to other researches.The key point is to develop the HCCI for a small scale SI engine and matched to the proposed REEM.The control of switch from SI to HCCI will be the future work.

Fig. 2 .
Fig. 2. Cylinder pressure at 2000 rpm using dual fuel of DME and gasoline in HCCI.

Fig. 3 .
Fig. 3. HRR at 2000 rpm using dual fuel of DME and gasoline in HCCI.

Fig. 4 .
Fig. 4. Cylinder pressure at 2000 rpm using dual fuel of DME and gasoline in HCCI with 25% EGR.

Fig. 6 .
Fig. 6.Comparison of engine operating range between the proposed HCCI engine and original SI engine.

Fig. 7 .
Fig. 7. Comparison of BSFC between the proposed HCCI engine and original SI engine.

Fig. 8 .
Fig. 8.Comparison of BSNO between the proposed HCCI engine and original SI engine.

Fig. 9 .
Fig. 9. Comparison of BSCO between the proposed HCCI engine and original SI engine.

Fig. 10 .
Fig. 10.Comparison of BSHC between the proposed HCCI engine and original SI engine.

Fig. 11 .
Fig. 11.The transition process from SI mode to HCCI mode.

Table 1 .
Specifications of the target engine.
a Valve timing is defined at 1 mm of valve lift.
a Self-ignition temperatures are obtained from Material Safety Data Sheet (MSDS).

Table 3 .
Uncertainty in experimental results of SI target engine.