VOLUME 23
APRIL 2009 Copyright 2009 by the American Chemical Society
Special Section on Fuels and Combustion in Engines CO2 Emission Reduction Using Stratified Charge in Spark-Ignition Engines† Cenk Dinc,* Hikmet Arslan, and Rafig Mehdiyev Mechanical Engineering Faculty, Istanbul Technical UniVersity, Istanbul, Turkey ReceiVed May 14, 2008. ReVised Manuscript ReceiVed NoVember 6, 2008
According to the proposal of the European Commission, the average CO2 emission of passenger cars will be limited to 120 g/km by the end of 2012. Decreasing the CO2 emissions is possible only with the decrease of fuel consumption, and this can be achieved most effectively by operating the engine with the stratified charge principle. With the aid of a thermodynamic internal combustion (IC) engine model, it is seen that stratified charge engines have the potential to attain a reduction in CO2 emissions up to 19%, by using higher rates of stratification and higher compression ratios. In this context, a stratified charge engine having a unique twin swirl combustion chamber operating with a two-stage combustion mechanism is presented. A 1.6 L 8v single-point injection test engine has been analyzed to compare the power output and specific fuel consumption values of a conventional gasoline engine with a stratified charge engine having a twin swirl combustion chamber. The results show that, when stratified charge is used, specific fuel consumption decreases significantly, which leads to a reduction in CO2 emission. Moreover, this combustion mechanism does not require high fuel injection pressures and can be applied easily on current production engines, without significant modification.
1. Introduction Greenhouse gases are components of the atmosphere that contribute to the greenhouse effect. Without the greenhouse effect, the mean temperature of the Earth would be about -19 °C, rather than the present mean temperature of about 15 °C.1 The greenhouse effect is a natural phenomenon and can be considered as a regulator, adjusting the temperature of the Earth. For about a 1000 years before the Industrial Revolution, the amount of greenhouse gases in the atmosphere remained † From the Conference on Fuels and Combustion in Engines. * To whom correspondence should be addressed. E-mail: dinccenk@ gmail.com. (1) Le Treut, H.; Somerville, R.; Cubasch, U.; Ding, Y.; Mauritzen, C.; Mokssit, A.; Peterson, T.; Prather, M. Historical overview of climate change. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the IntergoVernmental Panel on Climate Change; Solomon, S., Qin, D., Manning, M., Chen, Z., Marquis, M., Averyt, K. B., Tignor, M., Miller, H. L., Eds.; Cambridge University Press: Cambridge, U.K., 2007.
relatively constant. Since then, the concentration of various greenhouse gases has increased. The amount of carbon dioxide has increased by more than 30% since pre-industrial times and is still increasing at an unprecedented rate of on average 0.4% per year, mainly because of the combustion of fossil fuels.2 With the excess increase of greenhouse gases, the mean temperature of the Earth has increased by approximately 0.8 °C since the 1850s, and nearly 0.5 °C of this increase happened in the last 20 years.3 Among all of the human-sourced greenhouse gases, CO2 is the most important individual contributor to global warming. (2) Intergovernmental Panel on Climate Change (IPCC). Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the IntergoVernmental Panel on Climate Change; Houghton, J. T., Ding, Y., Griggs, D. J., Noguer, M., van der Linden, P. J., Dai, X., Maskell, K., Johnson, C. A., Eds.; Cambridge University Press: Cambridge, U.K., 2001; p 881. (3) Brohan, P.; Kennedy, J. J.; Harris, I.; Tett, S. F. B.; Jones, P. D. J. Geophys. Res. 2006, 111, D12106.
10.1021/ef800349x CCC: $40.75 2009 American Chemical Society Published on Web 12/15/2008
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there is a lean mixture in the combustion chamber globally, stratified charge engines have a lower knock tendency than the conventional gasoline engines. Because of this fact, the compression ratio (ε) of a stratified charge engine can be higher than the compression ratio of a conventional gasoline engine; ε g 12 is possible. A higher compression ratio leads to a higher efficiency. The absence of throttle losses in part load operation in combination with the ability to use higher compression ratios leads to lower fuel consumption. Improving the fuel efficiency of a vehicle leads to a decrease in CO2 emission. The fact that burning of the stratified mixture could be a very effective way to increase the fuel economy and decrease CO2 emissions in IC spark-ignition engines was already presented in the literature.7,8 It was shown that stratified charge engines have a potential to attain 20% reduction in fuel consumption.9 2. Two-Stage Combustion Mechanism Figure 1. Two-stage combustion mechanism in twin swirl combustion (1, zone containing pure air; 2, spark plug; 3, turbulizer; and 4, zone containing the fuel-rich mixture).
Although internal combustion (IC) engines are not the only sources for CO2 production, their contribution is significant. About 33% of carbon dioxide emissions come from the burning of fuels in internal combustion engines of cars and light trucks in the U.S.4 Transport is the main area for growth in energyrelated greenhouse gas emissions, having increased by 19.5% across the EU between 1990 and 1999, reflecting a growing demand for passenger and freight transport.5 Furthermore, there is an agreement between European car manufacturers and the European Community to reduce CO2 emissions from new passenger cars to 120 g/km by 2012, meaning a 30% In 2012, manufacturers will have to pay a penalty of 20 euros for every g/km CO2 emission exceeding 120 g/km, multiplied by the number of vehicles sold. This penalty rises to 95 euros as of 2015. Such a target creates a significant challenge to the automotive gasoline engine. When all of these facts are taken into consideration, it is obvious that CO2 emission reduction in spark-ignition engines is a challenging research area and a significant issue for global warming. Stratified charge engines have the characteristics of the two most popular forms of combustion used in IC engines. The basic motivation is to put together the best features of both forms of combustion, diesel efficiency with gasoline specific power. There exists a spark plug to initiate combustion, as in gasoline engines. Meanwhile, a nonhomogenous mixture is formed in the combustion chamber, similar to diesel engines. In conventional gasoline engines, every part of the cylinder contains a mixture having an excess air ratio (λ) of approximately 1. Stratified charge engines have frequently stoichiometric mixture (λ ) 1) only near the spark plug and lean mixture in the cylinder, globally. For the special case of stratified charge engines operating with a two-stage combustion mechanism, there is a lean mixture in the cylinder globally as well; however, there is a fuel-rich mixture in the vicinity of the spark plug. The nonhomogenous mixture in stratified charge engines is obtained usually with the modification of the piston geometry. The geometry of the intake manifold can also be modified. Because (4) Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-2004; pp 3-8. (5) Paravantis, J. A.; Georgakellos, D. A. Technol. Forecast. Soc. Change 2007, 74, 682–707. (6) Pearson, R.; Turner, J.; Kenchington, S. Concepts for improved fuel economy from gasoline engines. Seminar Proceedings of Fuel Economy and Engine Downsizing, Organised by the Combustion Engines and Fuels Group of the IMechE, London, U.K., May 13, 2004.
A stratified charge engine operated with a “two-stage combustion mechanism” is proposed in Azerbaijan Technical University (AzTU) and has been developing in cooperation with Warsaw Technical University (WTU), Istanbul Technical University (ITU), and Middle East Technical University (METU).10 The combustion chamber looks like an “8” and is separated into two zones. The geometry of the combustion chamber is given in Figure 1, as located in the cylinder head. Counter-rotating swirling motion occurs during the intake and compression cycle of the engine. The spark plug mounted part of the cobustion chamber contains a fuel-rich mixture with an excess air ratio of 0.6-0.8, while the other part contains pure air. The fuel is injected into the intake manifold and fed into the zone containing the fuel-rich mixture. The intake manifold is designed uniquely for the two-stage combustion mechanism, such that it increases the swirl effect and volumetric efficiency. Counter-rotating swirling motion does not allow the mixing of two zones until ignition time. By this way, the stratification of the air-fuel mixture can be satisfied at all loading regimes of the engine. Because the swirl motion occurs with the start of the intake cycle, the air-fuel mixture can be prepared in the intake manifold (outside of cylinders). Therefore, current electronic injection systems or carburetor engines can be used with this method. In other words, special and expensive direct-injection systems are not required, such as in gasoline direct injection (GDI) engines, where the injection of fuel into the cylinder reduces the time available for evaporation and mixing. Another advantage of the two-stage combustion mechanism, when compared to the other types of stratified charge engines, is the fact that it has the potential to reduce other exhaust gas emissions in addition to the reduction of CO2 emission. In the two-stage combustion mechanism, gasoline is injected into the intake manifold instead of cylinders; therefore, the liquid phase of gasoline does not contact the cold walls of the cylinders. Moreover, the counter-rotating swirling motion reduces the contact of the flame with the piston. Because of these facts, stratified charge engines that have a twin swirl combustion chamber produce lower hydrocarbon (HC) emissions. Furthermore, incomplete combustion products (CO and H2) produced during the combustion of the rich mixture (λ ) 0.6-0.8) at the first stage can be burned in the second stage of combustion with the effect of the swirl motion. Therefore, detonation firmness is satisfied. Satisfaction of combustion in two stages also provides an advantage to decrease NOx emissions. The lack of oxygen in the rich mixture and low combustion temperature at the first stage of combustion do not allow NOx formation. At
CO2 Emission Reduction
Energy & Fuels, Vol. 23, 2009 1783
Figure 2. Photographs of the two-stage combustion mechanism for ω ≈ 1500 s-1.
Figure 3. Photographs of the two-stage combustion mechanism for ω ≈ 700 s-1. Table 1. Reduction of CO2 Emission for Full Load Operation inputs full load (n ) 6000 rpm)
ε
conventional
10
stratified charge stratified charge stratified charge
11 12 13
outputs
λ
Ne (kW)
be (g/kWh)
relative CO2 reduction (kg/h)
1.00
59.6
287
0 (reference)
1.39 1.45 1.50
59.6 59.6 59.6
263 251 243
8.7% 12.6% 15.5%
Table 2. Reduction of CO2 Emission for Partial Load Operation inputs partial load (n ) 2000 rpm)
ε
conventional
9 10 11.5 13
stratified charge stratified charge stratified charge
Figure 4. Schematic of the experimental setup.
the second stage, because incomplete combustion products (CO and H2) have been burned quickly, nitrogen cannot find enough time to oxidize and NOx formation decreases. Because of the fact that this study focuses on CO2 emissions, experimental studies related to NOx, CO, and HC emissions will not be presented. Detailed experimental results for these exhaust gas emissions can be found in refs 10 and 11. Two types of combustion mechanisms can be observed in Figures 2 and 3, according to the changing swirl velocity (ω), ω ≈ 1500 and 700 s-1, respectively. The swirl velocity is determined experimentally, using high-speed photography methods in WTU.10 For the case of ω ≈ 1500 s-1, after the rich
outputs
λ
Ne (kW)
be (g/kWh)
relative CO2 reduction (kg/h)
1.00
17.9
264
0 (reference)
1.70 1.80 1.90
17.9 17.9 17.9
239 224 212
9.8% 14.8% 19.2%
mixture starts to burn, the flame front follows the created rotational path and primarily completes its combustion in this region. Because the density of the rich mixture is greater than the density of the burning products, the flame front is directed toward the center of this region by centrifugal force after quarter revolution, and then combustion seems to be restarted and occurs (7) Mehdiyev, R. I.; Karpov, V. P.; Wojcicki, S.; Wolanski, P. Arch. Termodyn. Spalania 1979, 9 (4), 562–588. (8) Cai, P.; Kaneko, T.; Yoshihashi, T.; Obara, T.; Ohyagi, S. Experiments on combustion of injected fuel in a constant volume chamber. In the 17th International Colloquium on the Dynamics of Explosions and Reactive Systems (ICDERS), Heidelberg, Germany, 1999. (9) Takagi, Y. In the 27th International Symposium on Combustion, The Combustion Institute, Pittsburgh, PA, 1998; pp 2055-2068.
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Table 3. Uncertainty of Measured and Calculated Engine Characteristics measured or calculated parameter
source of accuracy or uncertainty
measurement device
accuracy or uncertainty (%)
engine torque
eddy current dynamometer
calibration error reading error total uncertainty
(0.5 (2.5 (2.6
engine speed
tachogenerator
engine speed fluctuation
(1
fuel consumption
chronometer electronic balance
reading error reading error total uncertainty
(1 (1 (1.4
BMEP (RSS of engine torque and engine speed)
total uncertainty
(2.8
BSFC (RSS of engine torque, engine speed, and fuel consumption)
total uncertainty
(3.1
Table 4. Engine Specifications excess piston compression air ratio cylinder stroke diameter ratio (λ) volume (L) (mm) (mm) conventional engine stratified charge engine
9.2
1.0
1.6
86.9
76.5
12.0
1.4
1.6
86.9
76.5
Table 5. Comparison of the Experiments with the Theoretical Calculations for Full Load inputs full load (n ) 6000 rpm) ηV
ε
theoretical
experimental
relative CO2 Ne be reduction Ne be λ (kW) (g/kWh) (kg/h) (kW) (g/kWh)
conventional 0.75 9.2 1.0 stratified charge 0.95 12.0 1.4
57 62
302 249
0 (reference) 9%
58 62
300 250
similar to a ring with increasing radius. A slow ring radius increase is explained with a high value of centrifugal force because it tries to take away incomplete combustion products from the central region and forces them for diffusion toward the combustion zone. When the radius of the combustion ring arrives at the wall of the combustion chamber, the first step of the combustion is completed, while incomplete combustion products are passing through to the air mixture zone. Because these processes are following each other sequentially, burning time becomes longer and the combustion is finished in approximately 24 ms. Because of the fact that most of the burning time has been spent near the cold combustion chamber walls, the increase of HC emissions becomes inevitable. The second type of combustion, which is the case for ω ≈ 700 s-1, occurs under the effect of lower centrifugal forces, such that the flame front can move toward both regions of the combustion chamber and the burning processes become parallel
Figure 5. Power and specific fuel consumption changes versus engine speed of a 1.6 8v SPI engine at full throttle loading.
to each other in these zones. Therefore, burning is completed in half the normal burning time (12 ms). Moreover, two flame fronts formed in the two zones force incomplete rich mixture products toward the center of the combustion chamber. Then, the burning process is completed at the center of the combustion chamber, which is the hottest region. In this way, complete combustion is satisfied, combustion efficiency increases, and therefore, HC emissions decrease significantly. As a result, this two-stage combustion mechanism (ω ≈ 700 s-1) has an advantage over the other type of combustion mechanism (ω ≈ 1500 s-1), and during the design of the twin swirl combustion chamber, this fact should be taken into consideration. 3. Theoretical Calculations The CO2 emissions of conventional and stratified charge engines are compared using a thermodynamic IC engine model. In this model, several assumptions are made: Conversion of heat into mechanical energy is accomplished in a closed space by one and the same constant amount of working medium. The heat input to the medium is at a constant pressure and constant volume. The compression and expansion processes are adiabatic; therefore, there is no heat exchange with the surrounding environment. The heat capacity of the working medium is assumed to be constant. In the theoretical cycles, heat losses that take place because of friction and radiation are neglected. Details of the thermodynamic IC engine model are given in the references.12,13 The CO2 emissions are calculated as kg/h emissions for 1.6 L engine volume, 76.5 mm cylinder diameter, and 86.9 mm stroke. The CO2 emission for the conventional gasoline engine is taken as a reference. The CO2 emission reductions obtained with the stratified charge engines are given relative to this reference value. The simulation results are given in Tables 1 and 2, for full load and partial load operation, respectively. The values for compression ratios and excess air ratios are chosen such that the power output calculated for each stratified charge engine simulation is equal to the power output calculated for the conventional gasoline engine. It is seen that a 15.5% reduction in the CO2 emission is possible with the use of stratified charge for full load operation. Note that the reduction percentage of CO2 emission is very close to the reduction percentage of the specific fuel consumption for different simulations. For the partial load case, CO2 emission reduction can go up to 19.2%. The reduction of CO2 emission in partial loads is especially critical because of the fact that engines are usually operated in this mode, especially in urban traffic conditions. The reduction of CO2 emissions is mainly due to three factors: (1) leaner fuel-air mixtures and stratified combustion, (2) higher volumetric efficiency because of the reduction of pumping losses and modified intake manifold design, and (3) the improved thermal efficiency because of the usage of
CO2 Emission Reduction
higher compression ratios. This reduction can be increased by using higher compression ratios and higher stratification rates (excess air ratios). In practice, the use of high compression ratios is limited by the occurrence of knock, and the use of very lean mixtures can lead to inappropriate conditions for the flame propagation. 4. Experiments and Validations A 1.6 L 8v single-point injection (SPI) test engine has been analyzed at full load to compare the power output and specific fuel consumption values of a conventional gasoline engine with a stratified charge engine having a twin swirl combustion chamber.14 The experimental setup is given in detail in Figure 4. Measurement devices and uncertainty data of the measured or calculated parameters are given in Table 3. Engine specifications are given in Table 4. The results of the experiments are given in Figure 5, which shows the changes of power and specific fuel consumption values versus engine speed when conventional and twin swirl combustion chambers have been used. Table 5 compares the results of these experiments to the previously mentioned thermodynamic IC engine model, for full load. The theoretical calculations are very close to the experimental results (with an error
