Thermal barrier coated diesel engine running on biodiesel: a review

Abstract

Thermal barrier coated diesel engine, also known as low heat rejection (LHR) engine have offered the promise of reducing heat rejection to the engine coolant and increase the combustion temperature which results in increase of thermal efficiency, decrease of fuel consumption and emission rate of the engine. Biodiesel derived from the vegetable oils are a promising alternative fuel for diesel fuel. The viscosity of vegetable oil after transestrification is still higher than that of diesel fuel. The various researchers have reported that the energy of the biodiesel could be released more efficiently with the concept of LHR engine. In the case of LHR engine running on different biodiesel blends, almost all experimental studies has predicted improved performance. This paper analyses and discussed the operating conditions under which the experimental studies are carried out and the factors which affect thermal efficiency and exhaust emissions in LHR engine.

Keywords:

• Thermal barrier coating materials
• biodiesels
• diesel
• coated diesel engine
• engine performance and emissions

Introduction

Energy conservation and efficiency have always been the quest of engineers concerned with internal combustion engines. The diesel engine generally offers better fuel economy than its counterpart petrol engine. Even the diesel engine rejects about two-thirds of the heat energy of the fuel, one-third to the coolant and one-third to the exhaust, leaving only about one-third as useful power output. Theoretically, if the heat rejected could be reduced, then the thermal efficiency would be improved, at least up to the limit set by the second law of thermodynamics. Low heat rejection engines aim to do this by reducing the heat lost to the coolant (Domakonda and Puli, 2012; Soltani et al. 2005; Jaichandar and Tamilporai 2003). Thermal barrier coatings (TBCs) in diesel engines lead to advantages including higher power density, fuel efficiency and multi-fuel capacity due to higher combustion chamber temperature (Zhu and Miller, 1999; Ramaswamy et al. 2000). Using TBC can increase engine power by 8%, decrease the specific fuel consumption by 15–20% and increase the exhaust gas temperature (EGT) 200 K (Ahmaniemi et al. 2003).

The different material deposition methods are available for coating the engine combustion chamber components. There are electron beam physical vapour deposition (EBPVD), air plasma spray (APS), high-velocity oxygen fuel (HVOF), electrostatic spray-assisted vapour deposition (ESAVD) and direct vapour deposition. Among the different methods, plasma spray is the most common method of depositing thermal barrier material for diesel engine applications. It creates a splat structure with 10–20% volume fraction of voids and cracks (Cao, Vassenb, and Stoeverb 2004). The generally known principle that increased operation temperatures in energy conversion systems lead to an increase in efficiency, fuel savings and reduced emissions as particles in carbon monoxides (CO), hydrocarbons (HC), but increase in NOx emissions (Qian, Nakamura, and Berndt 1998) . The limited reserve of fossil fuels, increasing the price of crude oils and environmental concerns, biodiesel from vegetable oil is a viable alternative fuel for diesel engine. Considerable research has been conducted to investigate the properties of biodiesel and its performance in engines because of biodiesel is renewable, non-toxic and biodegradable. Non-conventional feed stocks are the possible sources for production of biodiesel at low cost (Abedin et al. 2013). The combustion characteristics of LHR diesel engines are different from standard diesel engines in four ways (Sun et al. 1994): (a) Ignition delay period shortens; (b) Diffusion burning period increases while premixed burning period decreases; (c) Total combustion duration increases; (d) Heat release rate in diffusion burning period decreases. The selection of TBC materials is restricted by some basic requirements (Cernuschi et al. 1999; Ni and Zhou 2012; Vassen et al. 2000) such as (i) high melting point, (ii) no phase transformation between room temperature and operation temperature, (iii) low thermal conductivity, (iv) chemical inertness, (v) thermal expansion match with the metallic substrate, (vi) good adherence to the metallic substrate and (vii) low sintering rate of the porous microstructure. Different materials such as silicon carbide, silicon nitride, aluminium, magnesium silicate and ceramic materials like zirconia, NiCrB, MgO–ZrO2, yttria-stabilised zirconia (YSZ), partially stabilised zirconiza (PSZ) and sintered silicon nitrate (SSN) have been used as thermal insulators for a LHR engine. So far, only a few materials have been found to basically satisfy the requirements. The literature reported that almost all type of biodiesels have been used for experimental study in conventional diesel engine but only few biodiesels were tried for experimentally in LHR engine concept. This paper describes to review on different thermal barrier coating (TBC) materials that can be used for coating the engine components and its effect on engine performance and emissions running on various type of biodiesel and to identify the best thermal coating material and biodiesel in order to justify their feasibility of implementation in diesel engine application.

Conversion of standard engine in to LHR engine

Cylinder head, cylinder liner, piston crown, exhaust and inlet valves of the diesel engine were selected for the thermal barrier coating. The suitable thermal barrier material is to be deposited on combustion chamber components using thermal spraying processes. Thermal spraying is a group of coating processes wherein a feedstock material is heated and propelled as individual particles or droplets onto a surface (Figure 1). The thermal spray gun generates the necessary heat using combustible gases or an electric arc. As the materials are heated, they are changed to a plastic or molten state which are confined and accelerated by a compressed gas stream to the substrate. The particles strike the substrate, flatten and form thin platelets (splats) that conform and adhere to the irregularities of the prepared substrate and to each other. As the sprayed particles impinge upon the surface, they cool and build up splat by splat into a laminar structure forming the thermal spray coating. The thermal spraying techniques can be divided into several processes that differ according to the thermal energy source and the respective kinetic energy of the sprayed particles (Hiemann 1996).

Figure 1. Schematic diagram of the Plasma Spray Process.

Plasma spraying method is the most popular technique to deposit thermal barrier coating in engine components due to its higher porosity (Karaoglanli et al. 2011; Salman et al. 2006). This method is suitable to apply in a surface which melts at a very high temperature. Prior to coating, the substrate surface was sand blasted to produce a surface roughness (Ra) of 4–5 μm. The sand blasted substrates were ultrasonically cleaned using anhydrous ethylene alcohol and dried in cold air. A plasma spray system consists of a power supply, gas source, gun and powder feeding mechanism. An arc is formed between an electrode and spray nozzle, which acts as a secondary electrode. A pressurised inert gas is passed between the electrodes where it is heated to a very high temperature (>16,000 °C) to form plasma jet (Bhatia 1999). The material to be deposited (feedstock) can be a powder, liquid, suspension or wire and is introduced into the plasma jet by emanating from a plasma torch. For a deposition rate of 1–5 kg/h, particle velocity reaches 200–300 m/s, porosity is reduced to 5–10% and oxide content to 1–3%. In the jet, the material is melted and propelled towards a substrate. Therefore, the molten droplets flatten, rapidly solidify and form a deposit. In this process, thickness of material is deposited on engine component in the range of 50–500 μm.

Comparison on performance and emission characteristics of different thermal barrier coated engine running on various type of biodiesel

The performance and emission characteristics of different thermal barrier coated engine running on different types biodiesel are compared to uncoated engine diesel operation have been discussed in this study. Only few of the thermal barrier coating materials has been used for biodiesel operation compared to diesel fuel (Abedin et al. 2014). This review manuscript has considered those papers for comparison. Thermal barrier coated materials, engine constraint and engine running on various type of biodiesel are listed in Table 1. Performance and emissions characteristics of those engines are discussed in the Table 2 & 3.

Table 1. Comparison on thermal barrier coated materials, engine specifications  and coated engine running on various type of biodiesel.

Sl. No.	Investigators	Engine Specifications	Thermal barrier coating material	Biodiesel used
1	(Aydin, Sayin, and Aydin 2015).	3LD 510 model Lombardini CI engine	100 μm of NiCrAl as lining layer and 400 μm of the mixture that consists of 88% ZrO2, 4% MgO and 8% Al2O3	Residual frying oil and cottonseed oil
2	(Suresh, Kamath, and Banapurmath 2014)	5.2 kW constant speed engine. Rated speed: 1500 rpm	Partially stabilised zirconia (PSZ) and aluminium oxide (Al2O3)	Honne and cotton seed methyl ester
3	Prabhahar and Rajan (2013)	3.5 kW constant speed engine. Rated speed:1500 rpm	Piston crown was coated with ceramic material (TiO2) of about 500 μm thickness	Pongamia methyl ester
4	Srithar et al. (2013)	5.2 kW constant speed engine. Rated speed: 1500 rpm	The piston surface and cylinder head are coated with Aluminium Titanate	Mixture of pongamia oil ethyl esters 22.5%, and neem oil ethyl esters 2.5% by volume
5	Aydin (2013)	10 HP variable speed engine. maximum speed: 3600 rpm	Zirconium oxide (ZrO2)	Pure cottonseed oil and sunflower oil
6	Janardhan et al. (2013)	4.4 kW constant speed engine. Rated speed: 1500 rpm	PSZ (500 μm) for cylinder head and superni-90 (Nialloy) for piston and liner	Jatropha oil in crude form and biodiesel
7	Pradeep Kumar, Annamalai, and Premkartikkumar (2012)	5.2 kW constant speed engine with rated speed of 1500 rpm	400 μm thickness PSZ coating	Rice bran methyl ester
8	Modi and Gosai (2010)	7.5 kW Multi cylinder engine. Rated speed of the engine : 1500 rpm	PSZ + stabilizer agent bond coating of NiAl. Total thickness – 500 μm	palm biodiesel
9	Hazar and Ozturk (2010)	6.25 kW Lombardini 6LD400 model variable speed engine. Rated speed: 3600 rpm	Al2O3–TiO2 (250 μm) + Bond coating: NiAl (50 μm)	Corn oil Methyl ester
10	Reddy et al. (2012)	3.68 kW constant speed engine. Rated speed: 1500 rpm	PSZ (500 μm) for cylinder head and superni-90 (Ni alloy) for piston and liner	Mohr oil metyl ester
11	Mohamed Musthafa, Sivapirakasam, and Udayakumar (2012)	5.2 kW constant speed engine Rated speed of the engine : 1500 rpm	200 μm coated pure Al2O3	Rice bran and mahua methyl ester
12	Hasimoglu (2012)	66kW, 4 cylinder engine constant speed engine Rated speed: 2800 rpm	Y2O3-ZrO2 (yttria-stabilized zirconia) layer of 350 μm thickness over a NiCrAl bond coat of 150 μm	Sunflower oil methyl ester
12	Rajendra Prasath, Tamilporai, and Shabir (2010)	4.4 kW engine with rated speed 1500 rpm	PSZ coating (500 μm)	Jatropha methyl ester
14	Hazar (2011)	6.25 kW Lombardini 6LD 400 variable speed engine	250 μm thick molybdenum + NiAl (50 μm))	Cotton methyl Ester
		Rated speed: 3600 rpm
15	Mohamed Musthafa, Sivapirakasam, and Udayakumar (2011)	5.2 kW constant speed engine Rated speed: 1500 rpm	200 μm thick fly ash	Rice bran and pongamia methyl ester
16	Hazar (2009)	Lombardini 6LD 400 single cylinder 6.25 kW variable speed engine Rated speed :3600 rpm	MgO–ZrO2 (350 μm) + NiCrAl (150 μm) for cylinder head, exhaust and inlet valves For piston surface, ZrO2 was used	Canola methyl ester
17	Hasimoglu et al. (2008)	Mercedes-Benz/ OM364A: 4-cylinder 66 kW variable speed engine. Rated speed: 2800 rpm	YSZ (Y2O3ZrO2)- 350 μm + NiCrAl- 150 μm	Sunflower oil methyl ester
18	Banapurmath and Tewari (2008)	Single cylinder 5.2 kW constant speed engine. Rated speed: 1500 rpm	PSZ 400 μm)	Honge oil methyl ester

Table 2. Performance characteristics of different thermal barrier coated engine running on various type of biodiesel compared with uncoated engine diesel operation.

Sl. No.	Investigators	Engine performance
1	Aydin, Sayin, and Aydin (2015)	BTE increases & BSFC decrease
2	Suresh, Kamath, and Banapurmath (2014)	Better performance in PSZ coating compared to Al2O3 coating
3	Prabhahar and Rajan (2013)	Improvement in brake thermal efficiency (BTE) and SFC decreased by about 10% at full load
4	Srithar et al. (2013)	BSFC is 13.9% lower and BTE is 11.9% higher than baseline engine
5	Aydin (2013)	Power and torque were increased with simultaneous decrease in fuel consumption
6	Janardhan et al. (2013)	BTE increases and SFC decreases in all injection pressure
7	Pradeep Kumar, Annamalai, and Premkartikkumar (2012)	Increase in BTE and decrease in BSFC were observed in B20
8	Modi and Gosai (2010)	BTE Increases by 1.5%at full load
9	Hazar and Ozturk (2010)	SFC decreases by4.6% and BTE increases by 5% for B20
10	Reddy et al. (2012)	SFC decreases BTE increases by 7% at190 bar injection pressure
11	Mohamed Musthafa, Sivapirakasam, and Udayakumar (2012)	An increase in engine Power (6.3–11.4%) and decrease in specific fuel consumption (0.8–10.9%).
12	Hasimoglu (2012)	Brake-specific fuel consumption (BSFC) and brake thermal efficiency were improved at part load operations
12	Rajendra Prasath, Tamilporai, and Shabir (2010)	BTE increases
14	Hazar (2011)	SFC decrease by 6%
15	Mohamed Musthafa, Sivapirakasam, and Udayakumar (2011)	BTE increases by 6.8% and SFC decreases by 16.8% for B20
16	Hazar (2009)	SFC decreases by 8.0% and BTE increases by1.7% forB35
17	Hasimoglu et al. (2008)	SFC increases by 9% and BTE- increases by 6.5%
18	Banapurmath and Tewari (2008)	BTE increases by 0.5% at 80% power output

Table 3. Emission characteristics of different thermal barrier coated engine running on various type of biodiesel compared with uncoated engine diesel operation.

Sl. No.	Investigators	Engine emission
1	Aydin, Sayin, and Aydin (2015).	Partial reductions in CO, HC and smoke emissions. But increases in NOx emission
2	Suresh, Kamath, and Banapurmath (2014)	Increased nitric oxide emissions but reduced Smoke and HC emission
3	Prabhahar and Rajan (2013)	Carbon monoxide (CO) and hydrocarbon (HC) emissions were decreased and the nitrogen oxide (NO) emission increased by 15%
4	Srithar et al. (2013)	Smoke opacity and hydrocarbon emission are lower However, nitrogen emissions are increased.
5	Aydin (2013)	CO, HC and Smoke opacity were decreased but NOx was increased
6	Janardhan et al. (2013)	Smoke decreases and at peak load NOx increases
7	Pradeep Kumar, Annamalai, and Premkartikkumar (2012)	HC and Smoke opacity were decreased but NOx was increased
8	Modi and Gosai (2010)	Smoke decreases by 45.74%. NOx increases and HC decreases
9	Hazar and Ozturk (2010)	Smoke decreases by 8.3%, NOx increases by 8.8% and HC decreases by15.0% for B20
10	Reddy et al. (2012)	Smoke decreases by 11% NOX increases at 190 bar injection pressure
11	Mohamed Musthafa, Sivapirakasam, and Udayakumar (2012).	Significant improvements in exhaust gas emissions (except NOx) and smoke opacity decreases (19.6–28.9%)
12	Hasimoglu (2012)	They have not discussed emission
12	Rajendra Prasath, Tamilporai, and Shabir (2010)	NOx is high
14	Hazar (2011)	CO decreases up to 18.0%, 8.0% decrease in smoke. NOx increase to 4.5%
15	Mohamed Musthafa, Sivapirakasam, and Udayakumar (2011)	Smoke decreases by 43.2% NOx increases by10.8% and HC decreases by 47% for B20
16	Hazar (2009)	Smoke decreases by 8.2%, NOx increases by7.3% and HC- decreases by 24% for B35
17	Hasimoglu et al. (2008)	They have not discussed emission characteristics
18	Banapurmath and Tewari (2008)	Smoke, NOx and HC decreases

Engine performance characteristics

An engine performance characteristic mainly consists of brake thermal efficiency (BTE) and brake-specific fuel consumption (BSFC). Both parameters could be improved in coated engine compared to uncoated engine for both diesel and biodiesel fuel. This was happened due to the reduction in heat transfer from combustion chamber wall to the cooling water. This leads to higher combustion temperature that means higher in-cylinder pressure which ensures higher mean effective pressure (Aydin 2013; Aydin, Sayin, and Aydin2015; Janardhan et al. 2013; Prabhahar and Rajan 2013; Srithar et al. 2013; Suresh, Kamath, and Banapurmath 2014). Higher in cylinder temperature also improves power and torque by 2–3% in making better vaporisation of fuel drops which results in shorter ignition delay, hence better combustion (Pradeep Kumar, Annamalai, and Premkartikkumar 2012) . Engine power increases with the increase of engine speed. The power produced by an engine is limited by the amount of air used in combustion. The faster an engine speed, the more air per unit time moves through the engine, the more energy is released for that unit of time. This was caused in coated engine for both diesel and biodiesel fuels (Mohamed Musthafa, Sivapirakasam, and Udayakumar 2011; Modi and Gosai 2010; Rajendra Prasath, Tamilporai, and Shabir 2010; Hazar 2009).The variations in SFC reflect the diversities in some of the fuel physical properties, especially calorific value and density. SFC in LHR engine was less than that of the standard engine for diesel fuel as well as for biodiesels due to higher gas and wall temperature. The complete combustion characteristics of biodiesels are weaker than diesel fuel due to its lower calorific value and higher viscosity. Therefore, the SFC values are higher for biodiesels compared to conventional diesel in both engines because more fuel is required for same output power. In the ignition delay (ID) period, the fuel atomises and mixes with air. This is a physical delay. Simultaneously, the chemical delay occurs due to slow chemical reaction. If the total ID period is longer, the more fuel will be injected into the combustion chamber. However, shorter ID period in LHR engine affects both the physical and chemical delay positively. The delay period is found to be decreased with the increase of cylinder pressure, temperature and equivalence ratio (Mohammed and Nemit-allah 2013). The increase in cylinder temperature was observed in coated engine helps to decrease in SFC compared to uncoated engine (Aydin 2013; Rajendra Prasath, Tamilporai, and Shabir 2010; Suresh, Kamath, and Banapurmath 2014). Hasimoglu (2012) have conducted experiments on a turbocharged direct injection diesel engine coated with CaZrO₃ using diesel and biodiesel(produced from sunflower oil) fuels and they reported that with the LHR diesel and STD diesel conditions the brake thermal efficiency was increased approximately 3, 4 and 6.5%, respectively, as shown Figure 2, compared to STD diesel condition. This can be explained as follows: although there is a difference between fuel’s lower heating values of approximately 14%, the engine power and torque decrease to a maximum of 4.5%. It is estimated that these circumstances increased the brake thermal efficiency in STD biodiesel condition. In LHR biodiesel and LHR diesel conditions due to the reduction of specific fuel consumption, the brake thermal efficiency was increased. Hasimoglu (2012) experimental result of the variation-specific fuel consumption against engine speed for the test a fuel is shown in Figure 3. They found that specific fuel consumption decrease approximately 4% in coated engine for both fuels. The reason was the positive effect of increased in cylinder temperature due to heat insulation.

Figure 2. Variations of brake thermal efficiency vs. engine speed (Hasimoglu 2012).

Figure 3. Variations of brake-specific fuel consumption vs. engine speed (Hasimoglu 2012).

Emission characteristics

Exhaust Gas Temperature (EGT)

The EGT is an indicator of the amount of energy released during combustion. EGT increases with the increase of engine load for all fuels in both engines. This was due to the fact that fuel burning rate increases at higher loads as a result in heat release rate increases. The increase in trend is more pronounced in LHR engine as cooling heat losses decrease significantly due to thermal barrier coating. At variable speed engine, the oxygenated nature of biodiesel will lead more complete combustion, and so higher EGT at low speeds. At higher speeds the exhaust temperature of all blends decrease than diesel fuel. From Hasimoglu (2012) graph result as shown in Figure 4, EGT of the test engine was increased with the increment of the engine load. The temperature of exhaust gas reaches a maximum for LHR Diesel during all engines. EGT of STD biodiesel was decreased because of the reduced calorific value of biodiesel. The EGT of LHR diesel increased, due to thermal insulation. For LHR biodiesel, as if the temperature increases due to insulation, this increment in EGT is limited because of lower heating value of biodiesel.

Figure 4. Variation of  an exhaust gas temperature vs. engine speed (Hasimoglu 2012).

Nitrogen oxides (NO_x) emission

NO_x forms by chain reactions involving O₂ and N₂ with the presence of sufficient temperature. The oxygen concentration and surrounding temperature are the key influence factors for NO_x emission. The kinetics of NO_x formation is governed by Zeldovich kinetics and the availability of oxygen (Hazar 2011; Palash et al. 2013). The biodiesel and biodiesel/diesel blend fuels produced higher NO_x for all engine speeds as expected. There were some reasons for this behaviour: (1) Regarding the adiabatic flame temperature, some authors state that it is slightly higher for biodiesel because of its oxygenated nature which help for more complete combustion and so higher temperature and NO_x emission (Banapurmath and Tewari 2008; Hasimoglu 2012; Prabhahar and Rajan 2013). (2) Regarding the reduction in soot formation with biodiesel. Radiation from soot produced in the flame zone is a major source of heat transfer away from the flame, and can lower bulk flame temperatures by 25–125 K, depending on the amount of soot produced at the engine operating conditions (Hazar 2009; Mohamed Musthafa, Sivapirakasam, and Udayakumar 2011). (3) Biodiesel typically contains more double-bonded molecules than petroleum derived diesel. These double-bonded molecules have a slightly higher adiabatic flame temperature, which leads to the increase in NO_x production for biodiesel(Lin, Wu, and Chang 2007; Sahoo and Das 2009). NO_x emissions measured by Huseyin Aydin (2013) from the gas analysing of the test engine for all test fuels are presented in Figure 5. NO_x emissions are mainly produced at considerably high-temperature combustion. The prolongation of the high-temperature regime results in NO_x emissions accumulation. Thus, total NO_x emissions increase. As can be seen in Figure 5, the lowest NO_x emissions were obtained for D2 usage in uncoated engine operation. When engine is insulated, the combustion temperature increases and results in increased NO_x emissions. Therefore, D2 usage in the coated engine operation resulted in considerably higher NO_x emissions than normal D2 operation. When vegetable based fuels are used in the test engine, the higher oxygen content is considered to improve combustion and results in higher combustion temperature and thus higher NO_x emissions as well.

Figure 5. Variations of NOx emission at different engine speed (Aydin 2013).

Hydrocarbon (HC) emission

Hydrocarbon emission from LHR engines is more likely to be decreased due to shorter quenching distance and leaner flammability limit associated with LHR engine combustion. High in-cylinder gas temperature and wall temperature in LHR engine assist the oxidation reactions to proceed close to completion. HC emission in LHR engine is very much low for pure diesel and biodiesels compared to uncoated engine. This happened due to the increase of after-combustion temperature as a consequence of cooling heat loss reduction in LHR engine. Neat biodiesel significantly reduces HC emission compared to blends and pure diesel fuel in both LHR and standard engine. The reasons are: biodiesels contain about 10% higher oxygen molecule than diesel; quenching distance decreases and lean flammability limit increases in LHR engine compared to uncoated engine (Prabhahar and Rajan 2013). Most of the investigations show reduction in HC level. From Aydin (2013) results, HC emissions for all test fuels can be seen in Figure 6. It can clearly be observed from Figure 6 that HC emissions were considerably decreased when using vegetable oil blends fuel in diesel engine. The higher oxygen contented in the vegetable oil-diesel blend fuels take part in combustion and make the combustion environment enriched with oxygen. Hence, the surplus oxygen content helps to achieve more complete combustion thus results in decreased in-complete combustion products such as HC and CO emissions. When both coated and uncoated diesel engine experiments were compared, average HC emissions were found slightly lower for coated diesel engine operation.

Figure 6. Variations of HC emission at different engine speed (Aydin 2013).

Carbon monoxide (CO) emission

CO emission occurs due to the incomplete combustion of fuel and depends on many engine parameters, mostly in-cylinder temperature and equivalence ratio (Hazar 2011; Karabektas 2009). It might be expected that LHR engines would produce less CO, for reasons similar to those for HC emission. The reduced level of pre-mixed combustion in LHR engine decreases initial production of CO and later (i.e. during diffusion combustion) high temperature accelerates CO oxidation. At lower engine speeds, due to poor combustion, CO emission is high at lower speed and decreases at medium speeds in both LHR engine and standard engine for all fuels. Due to ceramic coatings the post compression temperature increases which decreases CO emission in LHR engine. As higher oxygen content makes better combustion, the higher biodiesel blended fuel decreases CO emission than diesel fuel. When both coated and uncoated diesel engine experiments were compared with each other, average CO emissions were found slightly lower for coated diesel engine operation even though the main purpose of insulation of the engine was to improve the performance and usability of vegetable oils in diesel engines. (Banapurmath and Tewari 2008; Hasimoglu 2012; Hazar 2011; Modi and Gosai 2010). It can clearly be observed from Figure 7 of Aydin (2013) results.

Figure 7. Variations of CO emission at different engine speed (Aydin 2013).

Smoke emission

LHR engines would produce less smoke and particulates than standard engines for reasons such as high-temperature gas and high-temperature combustion chamber wall. Factors such as short ignition delay, poor air–fuel mixing are also responsible for the formation of smoke and particulates. Biodiesels and their blends have favourable effect on smoke emissions due to enhanced soot oxidation, which was made possible by both the high combustion temperature and the intense turbulence created by the reversed squish (Hasimoglu et al. 2011). As the cylinder wall and gas temperature increase due to thermal insulation, the smoke density decreases by a large amount in LHR engine. The trend is more pronounced in case of biodiesel fuels and their blends. Smoke opacity levels of diesel fuel and vegetable fuels application in the coated diesel engine for different engine speeds are presented by Aydin (2013) in Figure 8.Smoke levels obtained in the exhaust were decreased with increase of engine speed for all test fuels. It can be attributed to the increased air jets, swirl and turbulence movements which increase more homogeneous charge and thus resulting in more complete combustion. The highest smoke opacity levels were ever obtained for diesel fuel in uncoated engine. The oxygen inherently contained in the biodiesel blends which remove any possible local oxygen deficiencies are considered to have helped in the concern of decreasing smoke level. At high engine speeds, the combustion must be completed in a shorter period than that of low speed. So, it can be said that the reduction in the ignition delay period assists to reduce the smoke further.

Figure 8. Variations of smoke opacity at different engine speed (Aydin 2013).

Conclusion

In this study, the usability of various biodiesel as fuel in a thermally insulated diesel engine coated with different material was reviewed. Comparisons were made on the different thermal barrier material-coated engine fuelled by various biodiesel with normal uncoated diesel engine operation using diesel fuel. Almost all authors have reported that an improved engine performance in terms of decreased specific fuel consumption and increased brake thermal efficiency by combustion chamber insulation hence, heat transfer through the walls is reduced which results in higher in cylinder temperature. CO and HC emissions were considerably reduced for all mentioned biodiesel fuels usage in the coated engine. They have mentioned common reasons as the surplus oxygen content of biodiesel blend fuels helps to achieve a more complete combustion thus results in decreased in-complete combustion products such as HC and CO emissions. However, NOx emissions were increased probably due to the increased combustion temperature. Smoke emissions were lower than those obtained from diesel fuel in both coated and uncoated diesel engine.

The critical and comprehensive studies have been carried out to justify the future possibilities of the LHR engine to be used in wider application from the view point of best thermal barrier materials, coating techniques and biodiesel. The following conclusions are drawn from the detailed literature review.

1. Partially stabilised zirconia (PSZ) is identified as the best thermal barrier coating material for LHR concept among the mentioned materials in the table1, because of PSZ coating exhibited low thermal diffusivity, thermal conductivity, chemical stability and strong adherence to the substrate than the other thermal barrier materials have mentioned in the studies.

2. Atmospheric plasma thermal spray method is extensively used to make thermal barrier coating and to achieve the required thickness of coating by the selection of the optimum combination of input variables (current, powder feed rate, stand-off distance, number of passes, etc).

3. Cotton seed oil, corn oil, jatropha and pongamia methyl ester are found as the best biodiesel fuel among the mentioned biodiesel in the table1, because of their superior physiochemical properties, viz. viscosity and calorific value are close to diesel fuel and they are in non-edible oil.

R&D activities required to promote the further development are

• The higher combustion chamber temperature of coated engine deteriorates the properties of lubricating oil. Hence, the main direction of the research in adiabatic engines is in finding of alternate lubricating oils should be capable of retaining viscosity at the higher temperatures encountered in the engine.

• The thicknesses of the coating must be optimised to minimise the stresses under service conditions.

• The objectives of improved thermal efficiency, improved fuel economy and reduced emissions are attainable, but much more investigations under proper operating constraints with improved engine design are required to explore the full potential of low heat rejection engines

Disclosure statement

No potential conflict of interest was reported by the author.

Notes on contributor

M Mohamed Musthafa is a senior assistant professor in School of Mechanical Engineering, SASTRA University. His area of interests are Alternate fuels for Internal combustion engine, fuel combustion, Heat transfer and solar.