Effect of hydrogen addition to CNG in a biodiesel-operated dual-fuel engine

Abstract

Alternative fuels for diesel engine applications are gaining more prominence as they have numerous advantages compared to fossil fuels. They are renewable, biodegradable; provide food and energy security and foreign exchange savings. They address environmental concerns and socio-economic issues as well. Gaseous fuels such as compressed natural gas and hydrogenated compressed natural gas (HCNG) appear more attractive fuels for diesel engine applications operated in dual-fuel mode. Such dual fuel engines can replace considerable amount of liquid-injected pilot fuels by gaseous fuels besides being friendly to the environment. A small quantity of liquid fuel injected towards the end of the compression stroke initiates combustion of the inducted gas in the dual-fuel engines. The main advantage of dual-fuel engines is their lower nitrogen oxides (NO_x) and particulate emissions. Hence renewable fuels such as biodiesels and gaseous fuels can be used predominantly for transportation and power generation applications. Gaseous fuels are clean burning and are more economical as well. A suitable carburettor was designed to supply a stoichiometric mixture of air and HCNG to the modified diesel engine operated in dual-fuel mode. The biodiesel used in this study is derived from Honge oil called the Honge oil methyl ester (HOME). This paper presents the performance, combustion and exhaust emission characteristics of a single cylinder, four stroke, direct injection, stationary diesel engine operated on HOME and HCNG in dual-fuel mode. From the results it is observed that HOME–HCNG combination gave lower brake thermal efficiency (BTE) and improved emission levels when compared with diesel/HOME in single fuel operation. Lower smoke and particulate matter were obtained with dual-fuel operation. Comparative measures of BTE, peak pressure, pressure–crank angle variation, smoke opacity, hydrocarbon, carbon monoxide and NO_x emissions have been made and analysed.

Keywords:

• biodiesel
• compressed natural gas
• hythane (HCNG)
• Honge oil methyl ester
• emissions
• diesel engine

1. Introduction

Energy is one of the basic needs at the physiological level and has become an essential commodity in our lives. With growing global population, the energy demand is expected to grow significantly in future. Researchers have reported that by the year 2030, world-wide energy demand will be increased at least twice of today's level and estimated that global energy demand will be about 30% higher in 2040 than in 2010. Energy consumption across India is expected to grow at a rapid pace as it is coupled with growing population and industrialization, and it creates an insatiable deficiency for power (Source: Sterlite Technologies Limited, Maharashtra, India). In India, a large number of villages are already electrified. However, the country continues to have mismatch between demand and supply and experiences high energy and peak shortages. In order to bridge this gap, India has implemented several new policies on renewable energy during the 11th five-year plan. In this context, the need to identify an alternative to petroleum base fuel with a suitable novel engine technology that is indigenous and operating on renewable energy source has become more necessary. Renewable energy sources can supply energy for longer periods of time than fossil fuels and have many advantages.

Biodiesel being a renewable source of energy can provide a sustainable and alternative to diesel for compression ignition engines. The use of renewable energy from non-edible oils reduces the green house gas emissions and gives better food and energy security to the nation and ensures sustainable development (Banapurmath and Tewari 2010; Murugesan et al. 2009). Therefore, renewable fuels such as biofuels can be used predominantly as fuel for both transportation and power generation sectors (Atadashi et al. 2010; Basha et al. 2009; Murugesan et al. 2009).

Also clean burning characteristics of gaseous fuels such as compressed natural gas (CNG), liquefied petroleum gas (LPG) and hydrogen fuels drive more interest in using them as an alternative fuel to diesel in dual-fuel engines. Such engines can operate on compressed gaseous fuels either inducted or injected with combustion initiated by a small quantity of biodiesel. Major attention and interest is given to the utilization of different gaseous fuel in a dual-fuel engine with optimized engine parameters in terms of advanced injection timing, increased injection pressure and compression ratio.

CNG: natural gas primarily consists of methane (CH₄), the shortest and lightest hydrocarbon (HC) molecule. It is lighter than air, and so tends to dissipate. Different methods of CNG/LPG utilization in diesel engines have been reported in the literature. These methods include gas manifold induction (Yasufumi 2010; Selim 2005), port injection (Toshiaki Kitagawa et al. 2004) and direct in-cylinder gas injection (Selim 2005; Nwafor 2007).

Hydrogen is one of the most promising alternative fuels. Its clean burning characteristics and better performance drives more interest in hydrogen fuel. A distinguished feature of a hydrogen-operated engine is that it does not produce major pollutants such as HC, carbon monoxide (CO), sulphur dioxide, lead, smoke, particulate matter (PM), ozone and other carcinogenic compounds. The evolution of hydrogen engine technologies over the years has been described in Das (1990). Hydrogen has very high flame speed over a wide range of temperatures and pressures. This higher flame speed results in high rate of pressure increase in hydrogen-fuelled engines and combustion is almost instantaneous. The stratified mixture technique can be used to burn extremely lean mixture, which is less than the lower flammability limit of hydrogen (Toshiaki Kitagawa et al. 2004). Hydrogen-operated engines can have a constant and continuous flow of hydrogen without throttling as gaseous hydrogen has wide flammability. Hydrogen-operated engines can involve different induction/injection methods (López et al. 2009).

1.1 Use of natural gas–hydrogen mixtures in internal combustion engines

Utilization of hydrogen and natural gas blends in internal combustion engines was started in the early 1990s. Blending of hydrogen with CNG provides a blended gas termed as hydrogen-enriched natural gas (HCNG). HCNG combines the advantages of both hydrogen and methane. Natural gas and hydrogen engines have been studied with respect to different mixture percentages of natural gas and hydrogen. Addition of 5–30% (by volume) hydrogen in CNG improves the composition and properties of base fuel CNG (Park et al. 2013).

To meet the current emission regulation, nitrogen oxides (NO_x) are reduced with lean combustion, and an oxidation catalyst is used for reducing the total amount of hydrocarbons (THC) and CO released by heavy-duty CNG engines. However, future emission regulations, such as EURO-VI, cannot be satisfied with these traditional technologies; many studies have been conducted to achieve a further reduction of harmful emissions from CNG engines. Among these efforts for emission reduction, the addition of hydrogen is the most promising technique for improving the performance of CNG engines, while using the CNG infrastructure (Karim et al. 1996; Kukkonen and Shelef 1994). Emissions of carbon dioxide (CO₂), CO and THC that are associated with combustion of CNG are reduced by the addition of hydrogen. HCNG blends can be supplied to the engine as a mono-fuel because the phases of both hydrogen and natural gas are equal. Hydrogen has very reactive combustion characteristics, and its flammability limit is high. The production of unburned HCs can be minimized because of hydrogen's short quenching distance, and the application of a lean-burn spark ignition (SI) engine is easy because the self-ignition temperature is high and the burning speed is fast (Kukkonen and Shelef 1994; DeLuchi 1989).

In general, a HCNG engine uses the extended flammability limit in favour of hydrogen addition so that specific fuel consumption and NO_x emission can be improved. Furthermore, the strategy of using an oxidation catalyst increases THC and CO emission by lean combustion (Sierens and Rosseel 2000; Bauer and Forest 2001; Collier et al. 2005; Ma et al. 2007). The cylinder-to-cylinder variation affects the operation of multi-cylinder engines and can considerably influence HCNG engines because the fuel composition and excess air ratio differences can contribute to the cylinder-to-cylinder variations (Einewall and Johansson 2000; Misztal et al. 2009). Hydrogen-blended natural gas can be used for both SI and CI (compression ignition) engine applications.

1.1.1 Hydrogen-blended natural gas for SI engine applications

Most in-use natural gas-fuelled heavy duty engines use a spark to ignite a premixed natural gas–air charge. The concept of using hydrogen as an additive to improve the combustion in internal combustion engines was first suggested for conventional SI engine applications using gasoline fuel (Houseman and Hoehn 1974; Swain et al. 1993). Non-uniformity of the mixture in lean burn multi-cylinder engines leads to a variation in the combustion phasing for each cylinder because of the changes in burn rate; hence, the performance and emission characteristics deteriorate (Ma et al. 2008, Ma and Wang 2008; Ma et al. 2009).

1.1.2 Hydrogen-blended natural gas for CI engine applications

The non-premixed combustion of hydrogen–methane blends, as found in a diesel engine operating on late-cycle direct-injected natural gas, has not been as extensively studied as the premixed case. Fundamental studies suggest that non-premixed flame stability is enhanced by higher flame speeds and improved mixing associated with hydrogen addition (Karbasi and Wierzba 1998). In partially premixed combustion, flame thickness increases with hydrogen addition (Naha and Aggarwal 2004). In industrial gas turbines and boilers, hydrogen addition increases NO formation (owing mainly to high H and OH radical concentrations), and flame stability is improved (Rortveit et al. 2002). Preliminary work on a late-cycle direct injection (DI) natural gas-fuelled engine with pilot diesel ignition suggests that NO_x emissions increase, while combustion stability improves and unburned fuel and PM emissions are reduced with hydrogen addition to the fuel (McTaggart-Cowan et al. 2006).

Effects of fuelling a heavy duty diesel engine with late cycle DI of blended hydrogen–methane fuels and diesel pilot ignition over a range of engine operating conditions have been reported (McTaggart-Cowan et al. 2009). CO₂ emissions were reported to be significantly reduced due to lower carbon–energy ratio of the fuel. The test results suggested that the proposed technology could significantly reduce both local and global pollutant emissions associated with heavy-duty transport applications while requiring minimal changes to the fuelling system. Performance and emission characteristics of a turbocharged CNG engine fueled by HCNG with high hydrogen ratio have also been reported in the literature (Ma et al. 2010).

In this context, experiments were conducted on a single-cylinder four-stroke water-cooled DI CI engine operated on dual-fuel mode with Honge oil methyl ester (HOME) and CNG/HCNG induction at optimized engine parameters, and the results were compared with both diesel operations.

2. Fuel characterization

2.1 Transesterification of Honge oil and fuel properties

Honge oil was transesterified as its viscosity is 17 times more than diesel. The factors affecting the transesterification process basically include temperature, reaction time, molar ratio and catalyst concentration. These were optimized for producing maximum biodiesel yield of HOME and were found to be 65°C for the reactants, 5:1 molar weight and 1% catalyst (NaOH) concentration, respectively (Banapurmath et al. 2008).

2.2 Fuel properties

The properties of Honge oil and HOME were determined and are summarized in Table 1. Table 2 shows the free fatty acid contribution of the Honge oil used.

Table 1 Properties of fuels used.

Sl. No.	Properties	Diesel	Honge oil	HOME
1	Chemical formula	C13H24	–	–
2	Density (kg/m^3)	840	915	880
3	Calorific value (kJ/kg)	43,000	35,800	36,010
4	Viscosity at 40°C (cSt^a)	2-5	44.85	5.5
5	Flashpoint (°C)	75	210	167
6	Cetane number	45–55	40	45
7	Carbon residue (%)	0.1	0.66	–
8	Cloud point	− 2	–	7
9	Pour point	− 5	–	4
10	Carbon residue	0.13	0.55	0.01
11	Molecular weight	181		227
12	Auto ignition temperature (°C)	260		470
13	Ash content % by mass	0.57		0.01
14	Oxidation stability	High	Low	Low
15	Sulphur content	High	No	No

a Centistokes.

Table 2 Free fatty acid contribution of the Honge oil used.

Sl. No.	Fatty acid	Fatty acid contribution
1	Palmitic	3.7–3.9
2	Stearic	2.4–8.9
3	Lignoceric	–
4	Oleic	44.5–71.5
5	Lignoleic	1.8–18.3
6	Arachidic	2.2–4.7
7	Behenic	–
8	Linolenic	–
9	Eruceic	–

Table 3 presents the properties of the gaseous fuels, namely CNG and HCNG, respectively.

Table 3 Properties of CNG and HCNG.

Sl. No.	Properties	CNG	HCNG
1	Density of Liquid at 15°C, kg/ m^3	0.77	–
3	Boiling Point, K	147 K	–
4	Lower calorific value, kJ/kg	48,000	47,170
5	Limits of flammability in air, vol. %	5–15	5 – 35
6	Auto ignition temp, K	813	825
7	Theoretical max flame temp, K	2148	2210
8	Flash point, °C	124	–
9	Octane number	130	–
10	Burning velocity, cm/sec	45	110
11	Stoichiometric A/F, kg of air/kg of fuel	17:1	–
12	Flame temperature, °C	–	1927
13	Equivalence ratio	0.7–4 0	0.5–5.4

3. Experimental set-up

Experimental investigations were conducted on a four-stroke single-cylinder DI water-cooled compression ignition engine. The experimental set-up for CNG and HCNG-operated dual-fuel engines is shown in Figure 1. Engine tests were conducted on a four-stroke single-cylinder water-cooled DI compression ignition engine with a displacement volume of 662 cc, compression ratio of 17.5:1 and developing power of 3.7 kW at 1500 rev/min. The engine was always operated at a rated speed of 1500 rev/min. The engine had a conventional fuel injection system. The injector opening pressure and the static injection timing as specified by the manufacturer were 205 bar and 23 Before Top Dead Centre (BTDC), respectively. The engine is provided with a governor and it maintains a constant engine speed at all the loads on the engine. The governor of the engine was used to control the engine speed. The engine was provided with a hemispherical combustion chamber with overhead valves operated through push rods. Cooling of the engine was accomplished by circulating water through the jackets on the engine block and cylinder head. A piezoelectric pressure transducer was mounted on the cylinder head surface to measure the cylinder pressure. Table 4 shows the specification of the engine used for the study. Exhaust gas analyser and Hartridge smoke meter were used to measure HC, CO, NO_x and smoke emissions.

Figure 1 Experimantal set-up.

Table 4 Specification of diesel engine.

Sl. No.	Parameter	Specifications
1	Type	TV1 (Kirlosker make)
2	Software used	Engine soft
3	Nozzle opening pressure	200–225 bar
4	Static injection timing	23 BTDC
5	Governor type	Mechanical centrifugal type
6	No of cylinders	Single cylinder
7	No of strokes	Four stroke
8	Fuel	H S diesel
9	Rated power	3.7 kW (5 HP) @1500 rpm
10	Maximum torque and engine speed	1500 rpm
11	Cylinder diameter (Bore)	0.0875 m
11	Stroke length	0.11 m
12	Compression ratio	17.5:1

4. Results and discussions

This section presents the results of investigations carried out on a modified dual-fuel engine. During the complete experimentation, the gas flow rate was maintained constant and engine speed was maintained at 1500 rpm. A suitable carburettor was used to ensure air–gas mixture at stoichiometric ratio. The flow rates for both CNG and HCNG were kept constant at 0.5 kg/h in order to compare their performance in dual-fuel engine. The liquid fuel of HOME being common, the properties of the inducted gases resulted in the observed behaviour. For the same power developed, more HOME than diesel is injected in either of the gaseous fuel inductions. However, the heat content of the two gases dictates the quantity of HOME injected into the engine cylinder. For HOME and diesel operation, the engine could operate smoothly up to full load. But with gaseous fuel induction, the engine could run smoothly up to 80% load (2.96 kW) operation only, and engine knocking was observed for full load operation (3.7 kW).

4.1 Performance parameters

4.1.1 Brake thermal efficiency

The variation of brake thermal efficiency (BTE) with brake power for both single- and dual-fuel combinations is shown in Figure 2. In single fuel mode of engine operation, HOME performance is lower than diesel and is mainly attributed to its poor mixture formation because of its lower volatility, higher viscosity and density. The BTEs for diesel and HOME were found to be higher than those for all dual-fuel combinations. This is due to reduced volumetric efficiency with all gaseous fuel induction and slow burning nature of the gaseous fuels. However, HOME–HCNG combination resulted in slightly higher BTE than HOME–CNG combination. This could be due to higher calorific value of HCNG and flame velocity than CNG. The presence of hydrogen allows the lean burn limit to be extended because of the fast burn rate of hydrogen. The expansion of the flammability limit influences the reduction in loss by high combustion temperature and heat transfer; hence, the thermal efficiency was improved. The fast burn rate of hydrogen causes the combustion duration to decrease, while the heat release rate and exhaust NO_x increase with an increased percentage of hydrogen (Borges et al. 1996; Cho and He 2008; Park et al. 2013).

Figure 2 Effect of brake power on BTE.

4.2 Emission parameters

4.2.1 Smoke opacity

The variations of smoke opacity for diesel, HOME, HOME–CNG and HOME–HCNG operation with respect to various loads are presented in Figure 3. Smoke opacity for HOME engine operation is higher than that for diesel. It is mainly due to its heavier molecular structure and higher viscosity which makes atomization difficult, and this leads to higher smoke emission. The major advantage of dual-fuel engine is that the smoke emissions are lower than single fuel operation. The smoke opacity for diesel and HOME is found to be higher than that for all the dual-fuel combinations. This may be due to less carbon content and clean burning characteristics of gaseous fuels. The higher burning velocity and flame temperature of HCNG lead to more better burning than CNG during the dual-fuel operation.

Figure 3 Variation of smoke opacity with brake power.

4.2.2 HC emissions

The variations of HC emission levels for diesel, HOME, and HOME–CNG and HOME–HCNG operation with respect to various loads are presented in Figure 4. HC emissions with HOME were higher compared to diesel. Relatively poor atomization and lower volatility of HOME compared to diesel is responsible for this trend. The lower BTE associated with HOME operation could also be responsible for this behaviour.

Figure 4 Variation of HC with brake power.

Dual-fuel combinations showed lower HC emissions compared to HOME operation. HOME being common the properties of the two gases inducted results in the behaviour shown and accordingly HOME–HCNG operation results in lower HC emissions compared to HOME–CNG operation. The H₂ content gives a strong reduction of unburned HC emission results in more complete combustion (Simio et al. 2013). In addition HCNG engine increases the H/C ratio of the fuel, which drastically reduces the carbon based emissions. The presence of hydrogen in CNG (HCNG) has higher flame velocity and flame temperature results in better combustion compared to CNG.

4.2.3 CO emissions

The variations of CO emission levels for diesel, HOME, HOME–CNG and HOME–HCNG operation with respect to various loads are presented in Figure 5. Higher CO emission levels were observed for HOME–CNG combination compared to diesel and HOME–HCNG operation. This could be attributed to incomplete combustion of HOME due to poor atomization and improper mixing of fuels compared to diesel, and HOME–HCNG operation. However Lower CO emissions levels were observed for dual-fuel operation with HCNG compared to CNG. It could be due to better mixing of fuel with air leads to complete combustion which help in reducing the CO emissions. Moreover, the combustion temperatures are higher with HCNG fuel and the engine runs hotter thereby facilitating better combustion. The dual-fuel operation yield higher CO emissions at low load conditions. It could be due to most of the fuel left is unburnt and leading to poor combustion. At higher loads the CO emission are also observed to be higher compared to liquid fuel operation because of decreased combustion temperature and lower BTEs associated with.

Figure 5 Variation of CO with brake power.

4.2.4 NO_x emissions

The variations of NO_x emission levels for diesel, HOME, HOME–CNG and HOME–HCNG operation with respect to various loads are presented in Figure 6. The NO_x for diesel and HOME operation were found to be higher than that for HOME–CNG/HCNG operation. This is mainly attributed to longer combustion duration, lower peak pressure and lower heat release rate observed for neat dual-fuel operation. Compared with pure HOME–HCNG operation, it is observed that the HOME–CNG operation resulted in lower NO_x emissions. The higher viscosity and lower volatility of HOME, leads to poor mixing of fuel combinations and the improper spray pattern associated during dual-fuel operation results in lower heat release rate. Compared with pure HOME–CNG operation, it has been concluded that the presence of hydrogen increases the NO_x emissions while reducing the HC emissions. It is observed that NO_x emissions for the HOME–HCNG operation were greater than the emissions of pure HOME–CNG operation. This is because of the elevated flame temperature due to hydrogen. However, the NO_x emissions of HOME–HCNG are still considered relatively low compared with other diesel and HOME operations (Figure 6).

Figure 6 Variation of NO_x emission with brake power.

4.3 Combustion analysis

The combustion in a diesel engine differs when gaseous fuels are used and it depends on the engine operating conditions, engine design, fuel properties and air–fuel mixture quality. Different combustion characteristics are discussed as follows:

4.3.1 Ignition delay

The variations of ignition delay for diesel, HOME, HOME–CNG and HOME–HCNG operations with respect to various loads are presented in Figure 7. The ignition delay is calculated based on the static injection timing using pressure crank angle history for 100 cycles. Ignition delay for liquid-injected fuels was found to be lower than that for dual-fuel operation with different fuel combinations. Quantity of fuel taking part in mixing of fuel with air is responsible for this trend. The diesel particles were mixed efficiently with air and the reduced physical and chemical delay ensure maximum part of fuel burning in the uncontrolled combustion phase. Hence it results in lower ignition delay than other fuel combinations. However, HOME resulted in slightly higher ignition delay than diesel fuel because it takes more time in mixing with air. For biodiesel fuels, the premixed combustion heat release rate is found to be lower with more fuel burning in the diffusion phase and the combustion starts later than diesel fuel. (Banapurmath et al. 2008).

Compared with single fuel operation, HOME–CNG and HOME–HCNG operation resulted in higher ignition delay. It may be due to slow mixing rate with gaseous fuel operation and they have higher octane number and self-ignition temperature. Burning velocity of HCNG being comparatively higher than CNG is responsible for this observed trend (Figure 7).

Figure 7 Variation of ignition delay with brake power.

4.3.2 Combustion duration

Combustion duration for diesel, HOME, HOME–CNG and HOME–HCNG with respect to various loads is presented in Figure 8. The combustion duration was calculated based on the duration between the start of combustion and 90% cumulative heat release. The combustion duration increases with increase in the power output with all the fuels. This is due to increase in the quantity of fuel injected. Higher combustion duration was observed with dual-fuel combinations than with single-fuel operation. Improper air–fuel mixing and longer time for mixing result in incomplete combustion with increased diffusion combustion phase. HOME–HCNG dual-fuel operation shows improvement in heat release rate compared with HOME–CNG operation with increased premixed combustion phase. Higher flame velocity, higher calorific value and fast burning rate of hydrogen in CNG (HCNG) cause the combustion duration to decrease while the heat release rate and exhaust of NO_x increase with hydrogen addition (Figure 8).

Figure 8 Variation of combustion duration with brake power.

4.3.3 Peak pressure

Figure 9 shows in-cylinder pressure versus crank angle for diesel, HOME, HOME–CNG and HOME–HCNG operation at 80% load. Peak pressure depends on the combustion rate and on how much fuel is taking part in rapid combustion period. The uncontrolled combustion phase is governed by the ignition delay period and by the mixture preparation. Higher cylinder pressure was obtained for diesel operation due to its rapid mixing of diesel particles with air and higher heat release rate during the rapid combustion phase. However, lower pressure was observed for HOME operation because of poor mixing of air with HOME particles due to its higher viscosity and lower volatility.

Combined effect of poor mixture preparation, longer ignition delay, lower calorific value and adiabatic flame temperature and slow burning nature of the HOME–CNG/HCNG resulted in lower peak pressure and maximum rate of pressure rise than those of single fuel operation. However, higher flame velocity, calorific value and slightly increased ignition delay of HCNG during dual-fuel operation lead to increased combustion during rapid combustion phase. Hence it results in higher peak pressure and maximum rate of pressure rise. Accordingly higher peak pressure and heat release rate were observed for HOME–HCNG operation due to the higher flame velocity and fast burning of hydrogen content in the presence of CNG (Figure 9).

Figure 9 Variation of peak pressure with brake power.

5. Conclusions

In this paper, an attempt has been made to study the behaviour of some gaseous fuels such as CNG and HCNG in diesel engines on dual-fuel mode. From the study the following conclusions were made.

• Single fuel operation resulted in higher BTE than dual-fuel operation.

• HOME–CNG/HCNG operation can partially substitute fossil fuels.

• The soot and NO_x emission levels were found to be lower for dual-fuel operation than for neat liquid fuels operation.

• HCNG resulted in increased performance. Addition of hydrogen to CNG as a fuel in dual-fuel engines resulted in significant improvement on performance; HCNG makes it possible to run the engine on a leaner mixture, resulting in lower emissions of HC and CO and higher NO_x emissions.

• Use of HCNG in CI engines on dual-fuel mode helps to meet the Euro-V norms which can be enforced in the near future in India.