1930
Energy & Fuels 2008, 22, 1930–1935
Optimization of an Irreversible Diesel Cycle: Experimental Results of a Ceramic Coated Indirect-Injection Supercharged Diesel Engine A. Parlak,*,† H. Yasar,‡ H. S. Soyhan,‡ and C. Deniz§ Technical Education Faculty and Department of Mechanical Engineering, Sakarya UniVersity, Esentepe 54187, Sakarya, Turkey, and Maritime Faculty, Marine Engineering, Istanbul Technical UniVersity, Tuzla, Turkey ReceiVed December 18, 2007. ReVised Manuscript ReceiVed February 22, 2008
In this work, an irreversible dual cycle analysis has been performed for enhancing the characteristics of a low heat rejection (LHR) supercharged single-cylinder indirect-injection (IDI) diesel engine. A relation which gives the maximum power (MP) and the corresponding efficiency has been derived analytically. Optimization of the diesel cycle has been performed for power and thermal efficiency with respect to the pressure ratio and temperature ratio. Optimum values of the pressure ratio and cutoff ratio of the diesel cycle, depending on the temperature ratios, have been derived analytically and compared to the results of an experimental study of the LHR engine, whose optimum performance was obtained by increasing the temperature in the combustion chamber. Effects of a ceramic coating on performance and exhaust emissions in the LHR engine have been compared to those obtained from the standard (STD) diesel engine based on the comparison of the STD and the LHR engines for identical airflow and brake mean effective pressure. Intake pressure was adjusted to give the same air consumption as the corresponding STD engine for the same brake mean effective pressure (BMEP) and engine speed to avoid a reduction in the volumetric efficiency of the LHR engine. In comparison to the STD engine, satisfactory performance was obtained with the LHR engine. Specific fuel consumption was decreased up to 4.5%, and brake efficiency was increased by 1.5%. NOx emissions were increased by 12% because of the higher flame temperature in the LHR engine.
1. Introduction
maximum power and the maximum thermal efficiency criteria have to be considered in the design.3
In recent years, many optimization studies for heat engines based on endoreversible and irreversible models have been conducted by considering fine time and finite size constraints under various heat transfer modes, mainly linear and nonlinear ones.1,2 Usually, in these studies, the power and the thermal efficiency were chosen for the optimization criteria, and the design parameters at maximum power (MP) and/or at maximum thermal efficiency (ME) were investigated. The proper optimization criteria to be chosen for the optimum design of heat engines may differ depending on their purpose and working conditions. For example, for power plants, in which fuel consumption is the main concern, the maximum thermal efficiency criterion is very important, whereas for aerospace vehicles, for which propulsion is of great importance, the maximum power criterion has significance. On the other hand, for ship propulsion systems, both fuel consumption and thrust gain may be equally important, so in such a case, both the
Much interest has been recently paid to optimization of the air standard Otto, diesel, and dual cycles.4–10 In these optimization studies, optimal design and operation parameters under MP conditions were investigated.
* Corresponding author. E-mail:
[email protected]. Phone: +90.264. 3460260. Fax: +90 0.264.3460262. † Technical Education Faculty, Sakarya University. ‡ Department of Mechanical Engineering, Sakarya University. § Istanbul Technical University. (1) Bejan, A. Entropy generation minimization: the new thermodynamics of finite-size devices and finite-time processes. Appl. Phys. ReV. 1996, 79, 1191–218. (2) Chen, L.; Wu, C.; Sun, F. J. Finite time thermodynamics optimisation or entropy generation minimization of energy systems. Non-Equlib. Thermodyn. 1999, 24, 327–59.
Insulating the combustion chamber components of low heat rejection (LHR) engines reduces the heat transfer between the gases in the cylinder and the cylinder wall and, consequently, increases the combustion temperature. The LHR engine concept is based on suppressing this heat rejection to the coolant and (3) Sahin, B.; Kodal, A.; Kaya, S. S. A. A comparative performance analysis of irreversible regenerative reheating Joule-Bryton engines under maximum power density and maximum power conditions. J. Phys. D: Appl. Phys. 1998, 31, 2125–31. (4) Sahin, B.; Kesgin, U.; Kodal, A.; Vardar, N. Performance optimisation of a new combined power cycle based on power density analysis of the dual cycle. Energy ConVers. Manage. 2002, 43 (15), 2019–31. (5) Bhattacaryya, S. Optimising an irreversible diesel cycle--fine tuning of compression ratio and cut-off ratio. Energy ConVers. Manage. 2000, 41, 847–54. (6) Akash, B. A. Effects of heat transfer on the performance of an airstandard diesel cycle. Int. Commun. Heat Mass Transfer. 2001, 28, 87–95. (7) Lin, J.; Chen, L.; Wu, C.; Sun, F. Finite-time thermodynamic performance of a dual cycle. Energy Res. 1999, 765–72. (8) Chen, L.; Lin, J.; Luo, J.; Sun, F.; Wu, C. Friction effect on the characteristic performance of Diesel engines. Int. J. Energy Res. 2002, 26 (11), 965–71. (9) Qin, X.; Chen, L.; Sun, F.; Wu, C. The universal power and efficiency characteristics for irreversible reciprocating heat engine cycles. J. Phys. (Paris) 2003, 24, 359–66. (10) Hoffman, K. H.; Watowich, S. J.; Berry, R. S. Optimal paths for thermodynamic systems: The ideal diesel cycle. J. Appl. Phys. 1985, 58 (6), 2125–2134.
10.1021/ef700765n CCC: $40.75 2008 American Chemical Society Published on Web 04/25/2008
Optimization of an IrreVersible Diesel Cycle
recovering the energy in the form of useful work.11–13 Some important advantages of the LHR concept are improved fuel economy, reduced hydrocarbon, smoke and carbon monoxide emissions, reduced noise due to the lower rate of pressure rise, and higher energy in the exhaust gases.14–16 Low cetane fuel can also be burned in LHR engines. This is enabled by the higher temperature at the time of fuel injection17–20 Within the LHR engine concept, the diesel combustion chamber is insulated using high temperature materials on the engine components, such as pistons, cylinder head, valves, cylinder liners, and exhaust ports. It is claimed in refs 9 and 21 that by reducing the lost energy and eliminating the need for a conventional cooling system, the overall performance of the engine system could be improved and potentially result in 50% volume and 30% weight reductions in the entire propulsion system. In this study, an irreversible diesel cycle analysis is performed considering maximum power and the corresponding efficiency for the purpose of investigate the effect of maximum temperature in the cylinder on performance. The comparative results of the experimental study conducted for the standard (STD) engine and LHR diesel engine are given in terms of performance and exhaust emission characteristics.
Energy & Fuels, Vol. 22, No. 3, 2008 1931
Figure 1. P-V and T-S diagrams of an irreversible diesel cycle.
2. Cycle Analysis The parameters have the same effect on both the air standard cycles and a real engine cycle. The maximum temperature (Tmax) in a ceramic coated diesel engine increases compared to normally cooled diesel engines. In order to determine the effect of Tmax on power and thermal efficiency, a cycle temperature ratio is defined as the ratio of the maximum temperature to the inlet temperature. An irreversible dual cycle analysis is performed considering the maximum power and the maximum thermal efficiency in order to determine the effect of the cycle temperature ratio and cutoff ratio, rc. Figure 1 shows P-V and T-S diagrams of an irreversible diesel cycle. In the diagram, the process 1–2I is an isentropic compression, while process 1–2 takes into account the irreversibility occur (11) Stone, R. Motor Vehicle fuel economy; Macmilan Education Ltd: Hong Kong, 1989. (12) Parlak, A. Comparative performance analysis of irreversible Dual and Diesel cycles under maximum power conditions. Energy ConVers. Manage. 2005, 46 (3), 351–59. (13) Parlak, A.; Sahin, B.; Yasar, H. Performance optimisation of an irreversible dual cycle with respect to pressure ratio and temperature ratioexperimental results of a ceramic coated IDI Diesel engine. Energy ConVers. Manage. 2004, 45 (7–8), 1219–232. (14) Gataowski, J. A. EValuation of a selectiVely-cooled single-cylinder 0.5-L Diesel engine; SAE paper No. 900693, Society of Automotive Engineers: Warrendale, PA, 1990. (15) Schwarz, E.; Reid, M.; Bryzik, W.; Danielson, E. Combustion and performance characteristics of a low heat rejection engine; SAE paper No. 930988, Society of Automotive Engineers: Warrendale, PA, 1993. (16) Bryzik, W.; Kamo, R. Tacom/Cummins adiabatic engine program; SAE paper No. 830314, Society of Automotive Engineers: Warrendale, PA, 1983. (17) Sun, X; Wang, W. G.; Bata, R. M.; Gao, X. Performance evaluation of low heat rejection engines. Trans. ASME J. Eng. Gas Turbines Power 1994, 116. (18) Kawamura, H.; Sekiyama, S.; Hirai, K. ObserVation of combustion process in a heat insulated engine; SAE paper No. 910462, Society of Automotive Engineers: Warrendale, PA, 1990. (19) Hay, N; Watt, P. M.; Ormerod, M. J.; Burnett, G. P.; Beesley, P. W.; French, B. A. Design study for a low heat loss version of the Dover engin. Proc. Inst. Mech. Eng. 1983, 200, 53–60. (20) Siegla, D. C.; Amman, C. A. Exploratory study of the low heat rejection Diesel for passenger car applications; SAE paper No. 840435, Society of Automotive Engineers: Warrendale, PA, 1995. (21) Alkidas, A. C. Performance and emissions achieVements with an uncooled heaVy-duty single-cylinder Diesel engine; SAE paper No. 890144, Society of Automotive Engineers: Warrendale, PA, 1989.
Figure 2. Variation of the dimensionless power with respect to the thermal efficiency of the diesel cycle, rc, for various R values (k ) 1.4, ηci ) ηei ) 0.85).
in the real compression process. The heat addition occurs in process 2–3 at constant pressure. The process 3–4I is an isentropic expansion process, while process 3–4 takes into account the irreversibility occur in the real expansion process. A constant volume heat rejection process, 4–1, completes the cycle. The working fluid in the system flows continuously so that the diesel cycle operates in a steady state. Assuming that the working fluid is an ideal gas with constant specific heat, the net cyclic power of working fluid is written in the following form: ˙ -Q ˙ ˙ )Q W D 23 41
(1)
˙ )m ˙ cV[k(T3 - T2) - (T4 - T1)] W D
(2)
where m˙ is the mass flow rate, cV is the specific heat at constant volume, and k is the ratio of specific heats (cp/cV). Let us define the cutoff ratio, rc, isentropic efficiency of compression process, ηci, and isentropic efficiency of expansion process, ηei, as follows: rc )
T3 V3 ) T2 V2
(3)
ηci )
T2′ - T1 T2 - T1
(4)
ηei )
T3 - T4 T3 - T4′
(5)
Cycle temperature ratio, R, is defined as the ratio of maximum temperature to the minimum temperature in the diesel cycle:
1932 Energy & Fuels, Vol. 22, No. 3, 2008
Parlak et al.
By rearranging, cycle temperatures are found as design parameters of the irreversible diesel cycle:
[
T2 ) T1 1 +
{
εk-1 - 1 ηci
]
(8)
T3 ) rcT2
(9)
[ ( ) ]}
rc (10) ε Substituting eqs 8-10 into eq 7 and rearranging in terms of cycle parameter, the dimensionless power and thermal efficiency can be found as follows: T4 ) Tmax 1 - ηei 1 -
(
˙ ) k(R - 1) - R(1 - η ) - k W D ei
˙ characteristics (ηci ) ηei ) Figure 3. Effects of R ratio on rc-W D 0.85).
k-1
)
R 1 - (ηeirck - 1) ηcirc ηci (11)
Total heat supply for the diesel cycle is shown in the following: ˙ ) Q 23
[
˙ Q 23 R 1 ) k (R - 1) + rcηci ηci ˙ cVT1 m
]
(12)
Thermal efficiency can be found as
(
ηD )
) ]
R 1 - (ηeirc - 1) ηcirc ηci R 1 k (R - 1) + rcηci ηci (13)
k(R - 1) - R(1 - ηei) - k
[
[ ( )]
rck -1 R ηD ) 1 R 1 k (R - 1) + rcηci ηci R 1 - ηei 1 -
[
Figure 4. Effects of maximum temperature ratio on rc-ηD characteristics (ηci ) ηei ) 0.85).
R)
T3 Tmax ) T1 Tmin
(6)
In terms of these parameters for the cycle, the dimensionless power and the thermal efficiency is formulated as ˙ ) W D
˙ W D ˙ cVT1 m
[(
) k
) ( )]
T3 T2 T4 -1 T1 T1 T1
Figure 5. Block diagram of the experimental setup.
(7)
]
(14)
˙ ) with respect to the Variation of the dimensionless power (W D thermal efficiency (ηD) for various R values are shown in Figure 2. As seen in Figure 2, the power and the thermal efficiency increases as the maximum temperature ratio (R) increases. ˙ , and thermal The variation of the dimensionless power, W D efficiency, ηD, as a function of rc for various R values is shown in Figures 3 and 4, respectively. These figures give the optimum values for rc which maximize ηD. The optimum value of rc which maximize the dimensionless power is also obtained analytically from the solution of eqs 11 and 14. For example, (rc)mp is calculated as 2.58 for k ) 1.4, R ) 7, and ηci ) ηei ) 0.85. However, the optimum value of rc which maximizes the
Optimization of an IrreVersible Diesel Cycle
Energy & Fuels, Vol. 22, No. 3, 2008 1933
rc at maximum conditions (MP) is calculated by solving eq 16 as
Table 1. Specifications of the Test Engine engine type cylinder number stroke [mm] bore [mm] displacement [L] compression ratio engine speed [rpm] injection timing [CAD] injection pressure [bar]
Ricardo E6-MS/128/76 1 76.2 110 0.507 4-20 1000-3000 20-40 150
(rc)mp )
(
Table 2. Inlet Air Pressures of the STD and LHR Engines STD engine [bar]
LHR engine [bar]
1000 1400 1800 2200
1.100 1.200 1.300 1.400
1.115 1.214 1.316 1.417
(17)
)
˙ ) ) k(R - 1) + 1 + k - R(1 - η ) - (1 + (W D max ei ηci k)ηci1/
(
[
(ηD)mp ) k(R - 1) + 1 + (1 + k)ηci1/
Table 3. Uncertainties of Calculated and Measured Parameters
)
ηei-k/
k (R - 1) - ηci1/
1.15 1.73 1.00 1.00 1.10 1 ppm 0.1 °C
(15)
R - ηeirck-1 ) 0 ηcirc2
(16)
Rk/
(k+1)
ηei-k/
(k+1)
Rk/
(k+1)
]⁄
(k+1)
(18)
(k+1)
Rk/
(k+1)
(k+1)
+
]
1 (19) ηci
The maximum power and the thermal efficiency are obtained by generating the heat supply at the constant pressure. It can be seen from the final equation that the cutoff ratio (rc) at which the power and the thermal efficiency are maximized also increase as the maximum temperature ratio increases.
thermal efficiency is 2.27. It can be easily shown from the above equation that the optimal rc values which maximize dimensionless power and thermal efficiency increase as the isentropic efficiency decreases. It is observed in Figures 3 and 4 that cutoff ratio, rc, which maximize both the maximum power (MP) and the maximum thermal efficiency (ME) increase as the maximum temperature ratio increases. However, the cutoff ratio which maximizes the power is higher than the cutoff ratio which maximizes the efficiency (rc)mp > (rc)me. Equations which optimize the dimensionless power and the corresponding thermal efficiency can be derived analytically. The dimensionless power in eq 11 is optimized with respect to the cutoff ratio as follows: ˙ ∂W D )0 ∂rc
ηei-k/
(k+1)
k - R(1 - ηei) ηci
(k+1)
[
maximum errors [(%]
specific fuel consumption volumetric efficiency torque brake power BMEP NOx temperature
1 k+1
The dimensionless maximum power and the corresponding thermal efficiency is derived by substituting eq 17 into eqs 11 and 14 as
engine speed [rpm]
engine performance characteristics
( ) R ηciηei
3. Experimental Setup The Ricardo E6 type engine used in the present work is a singlecylinder, four stroke, water cooled indirect-injection (IDI) diesel engine. The block diagram of the experimental setup is shown in Figure 5. The engine specifications are listed in Table 1. Engine tests were conducted with variable loads at constant engine speeds of 1000, 1400, 1800, and 2200 rpm and at a constant static injection timing of 38 crank angle degree before top dead center (CAD BTDC). After the load tests were adopted for an STD diesel engine with ε ) 18.2, the same test sequence was conducted for the LHR diesel engine. At each engine speed, the fueling rate and hence the load was varied. Coating the combustion chamber components with a lower thermal conductivity and a lower thermal diffusivity leads to the combustion temperature increase. This is the main reason for the decline in volumetric efficiency of the LHR engines. The intake pressure was adjusted to give the same air consumption as those for the corresponding STD for the same brake mean effective pressure (BMEP) and the same engine speed to compensate this decrease in the volumetric efficiency and to make a comparison at
Table 4. Coating Materials and Parameters of Plasma Spray coating powder Metco210 BNS Metco 443
materials MgO-ZrO2 NiCrAl plasma gas: hydrogen gun type: 3MBN
voltage [V]
current [A]
64–70 64–70
500 500
inert gas Ar Ar inert gas: argon cooling type: water
spray distance [mm] 100–150 100–150
Table 5. Composition of the Coating Powder Used for the Test Engine Combustion Chamber Components powder MgO-ZrO2 (Metco 210 BNS) NiCrAl (Metco 443)
composition of the powders [% w] zirconium oxide magnesium oxide Al Mg Si Cr Fe Ni organic adhesive
76.0 24.0 6.0 1.0 1.0 18.0 1.0 72.0 1.0
particulate dimension [µm]
melting point [°C]
-100 + 40
2140
-125 + 45
1420
1934 Energy & Fuels, Vol. 22, No. 3, 2008
Parlak et al.
Figure 6. Operation regions of the test engine.
Figure 8. Effect of insulation on break thermal efficiency at 5.67 bar.
Figure 7. Comparison of the performance maps.
the same operation conditions with the STD engine. The inlet air pressures of the STD and LHR engines are shown in Table 2. The lubrication and cooling pumps are driven by an external AC motor. The cooling system of the test engine is a pressurized closed circuit. The water supply to test engine is controlled by a regulator valve to keep the outlet temperature at about 70 °C. Air consumption is measured with a rotometer-surge tank set. The inlet temperature was kept at about 40 °C. The fuel consumption was measured with a computer controlled fuel flow indicator. The engine is coupled to a Ward-Leonard type electric swinging field dynamometer with a 22 kW absorbing capacity for measuring the engine brake power. NOx emissions were measured by MRU95/ 3CD gas analyzer. The exhaust sample was taken from a tap located close to the exhaust outlet of the engine. The uncertainties of calculated characteristics with respect to measured parameters which are important for verifying the correctness of the test results are shown in Table 3. Atmospheric plasma spray coating method was used to coat the combustion chamber components. As for plasma gas, a mixture of Ar +5% H2 was used. The combustion chamber components (cylinder head, valves and piston) were coated with a MgO-ZrO2 layer of 0.35 mm thickness over a NiCrAl bond coat of 0.15 mm thickness. The parameters of plasma spray and the properties of coating materials are shown in Tables 4 and 5, respectively.
4. Results and Discussion Emission and operating regions are divided into four parts. These are low speed-low load (1), low speed-high load (2), high speed-low load (3), and high speed-high load (4) as shown in Figure 6. It can be clearly seen in Figure 7 that specific fuel consumption (SFC) is lower compared to the STD engine for the same BMEP and the same engine speed for all the selected operating conditions of the LHR engine. The first and second regions are
Figure 9. Effect of insulation on brake power at the full load condition.
Figure 10. Effect of insulation on brake torque at full load conditions.
the parts where SFC was lower compared to the STD engine. In the second region, minimum specific fuel consumption in LHR engine was 260 g/(kW h) while it was 270 g/(kW h) for the STD engine. It is shown that minimum SFC was found in the second region for the LHR engine while it was the fourth region in the STD engine. As a result of the ceramic coating, brake efficiency was increased up to 1.5% at 5.67 bar as shown in Figure 8. The power and torque of the LHR engine were increased up to 1.6% and 1.5%, respectively, compared to the STD engine as shown in Figures 9 and 10. The exhaust temperature of the LHR engine was increased up to 25% depending on the load compared to that of the STD engine. The reduction in heat rejection resulted in an increase in exhaust energy. The rates of increase at the first and second
Optimization of an IrreVersible Diesel Cycle
Energy & Fuels, Vol. 22, No. 3, 2008 1935
Cut-off ratio values of the cycle can be considered as an equivalent of injection timing of a real engine. In a real diesel engine, when the amount of fuel injected into the combustion chamber per cycle is kept constant, the maximum combustion temperature is increased as a result of thermal barrier coating. Injection timing of the LHR diesel engine must be retarded in order to optimize the power and the thermal efficiency at modified conditions.
Figure 11. Effect of insulation on the exhaust gas temperature.
In the experimental study, based on the comparison of the STD engine and the LHR engine for equal airflow and brake mean effective pressure, the following conclusions were reached: • The specific fuel consumption of the LHR engine decreased up to 4.5% at 6.80 bar and 1800 rpm. • The exhaust gas energy of the LHR engine increased from 2 to 25% depending on load and engine speed. • NOx emissions of the LHR engine were 12% higher than the STD engine.
Nomenclature BMEP ) brake mean effective pressure (bar) CAD ) crank angle degree cp ) specific heat at constant pressure (kJ/(kg K)) cV ) specific heat at constant volume (kJ/(kg K)) IDI ) indirect injection LHR ) low heat rejection k ) ratio of specific heats ME ) maximum thermal efficiency MP ) maximum power m ˙ ) mass flow rate (kg/s) NOx ) nitrogen oxide (g/(kW h)) rpm ) revolution per minute STD ) standard SFC ) specific fuel consumption (g/kW h) Figure 12. Comparison of the NOx emission maps.
operating regions are more visible compared to the other regions for the LHR engine as shown in Figure 11. Maximum decrease in specific fuel consumption was 4.5% at 6.80 bar and 1800 rpm. However, NOx emissions were 12% higher than that of the STD engine for the same BMEP and engine speed as shown in Figure 12. Higher NOx emissions are attributed partly to the shorter ignition delay compared to the STD engine. NOx emissions of the LHR engine were lower (6%) at high speed high load conditions (6.80 bar and 2200 rpm) compared to the STD engine. This is a result of increased combustion temperature in LHR engine. 5. Conclusion In this study, the relations which give the maximum power and the corresponding efficiency have been derived analytically.
˙ ) dimensionless power W T ) temperature (K, °C) TDC ) top dead center R ) maximum temperature ratio ε ) compression ratio rc ) cutoff ratio ηD ) thermal efficiency ηci ) isentropic efficiency of compression process ηei ) isentropic efficiency of expansion process Subscripts D ) diesel cycle max ) maximum p ) constant pressure V ) constant volume e ) exhaust EF700765N
