Residual Gas Control on the Combustion Engine by Means of Second Event Valve Lift. Development Valve Gear

Transcription

1 Development Valve Gear Residual Gas Control on the Combustion Engine by Means of Second Event Valve Lift The recirculation of residual combustion gas is a measure that has been successfully applied for a long time to de-throttle gasoline and diesel engines and to reduce their emissions. Researchers at the TU Kaiserslautern have examined the potentials of controlling residual gas by an additional valve lift on the exhaust side. The thermodynamic effect and therefore the development of the fuel consumption and emissions were measured on a turbocharged four-cylinder downsized gasoline engine and a six-cylinder diesel engine on a fired test stand. 18

2 Authors Prof. Dr.-Ing. Rudolf Flierl is Director of the Institute for Combustion Engines at the TU Kaisers lautern (Germany). M. Sc. Daniel Hosse is Research Assistant at the Institute for Combustion Engines at the TU Kaiserslautern (Germany). Dipl.-Wirtsch.-Ing. Arne Temp was Research Assistant at the Institute for Combustion Engines at the TU Kaiserslautern (Germany). Dipl.-Wirtsch.-Ing. Christoph Werth was Research Assistant at the Institute for Combustion Engines at the TU Kaiserslautern (Germany). Second Event Valve Lift for Gasoline and Diesel Engines Variable and fully variable valve trains are now state of the art in the combustion engine. Camshaft phasers, which offer a variable spread, are found in almost every new gasoline engine and are now also used in passenger car and truck diesel engines [1, 2]. Continuously variable valve trains are applied by many vehicle manufacturers in large naturally aspirated and turbocharged engines to reduce CO 2 emissions and have proven their worth many millions of times over long periods. The kinematics of most mechanical continuously variable valve train systems, which are presently in series production, are based on cam gears that adjust the length of the intake valve timing by the height of the valve lift, ❶. The fuel consumption and CO 2 emissions can be reduced by reducing the charge exchange work, the control of residual gas as well as by reducing the friction. In series production, exclusively mechanically continuously variable valve trains for throttle-free load control are used on the intake side today. By the continuously variable valve opening time and the resulting opening cross-section, the load is controlled, 1. With this load control method the charge exchange work is reduced, which is represented by the area of low pressure loop in the pressure volume diagram. The fuel consumption is directly proportional to the charge exchange work, ❷. This diagram shows that the influence of the charge exchange work is predominant compared with the residual gas content. The residual gas content of a throttle-free load controlled engine is almost exclusively dependent on the closing timing of the exhaust valves [3]. The quantities that can be controlled are limited by the pressure difference between the combustion chamber and the exhaust port for gasoline and diesel engines. Although the quantities can be increased by throttling, the fuel consumption then increases because of an increase in the gas ex - change work, 2. This fundamental weakness of a throttle-free load control with a continuously variable valve train can be corrected by controlling the residual gas with a variable valve installed on the exhaust side. For this purpose, the outlet valve is opened a second time during the open intake valve (so-called second event). However the charge exchange work and the friction should not be increased by this second event. In ❸ (top), an example of a second event is shown in the valve timing diagram. With the second opening of the exhaust valve, significantly larger quantities of residual gas can be recirculated. The amounts of residual gas are dependent on the valve lift and of the spread of the second event. The spread has a significant impact on the gas exchange work and therefore on the fuel consumption, as will be shown later. 1 9 Exhaust spread variation TDC Inlet spread variation 8 7 Valve lift [mm] Exhaust lift variation Inlet lift variation 2 1 ❶ Throttle-free load control Crank angle [ CA] 2I214 Volume 75 19

3 Development Valve Gear BSFC [g/kwh] n = 2 rpm, BMEP = 2 bar 2.-l MPI gasoline engine Throttle free load control Turbo charged ES = 79 CA Throttle free load control Turbo charged ES = 65 CA Throttle free load control Naturally aspirated ES = 79 CA 365 Throttle free load control 36 Naturally aspirated 355 ES = 65 CA PMEP [bar] ❷ Correlation of gas exchange work and consumption (test bench measurement on a 2.-l four-cylinder engine; ES: exhaust spread) Valve lift [mm] Valve lift [mm] Exhaust lift variation Exhaust lift variation Exhaust spread variation Exhaust spread variation TDC Crank angle [ CA] TDC Crank angle [ CA] Inlet spread variation Inlet spread variation ❸ Valve timing diagram of UniValve with second event without (top) and with lift holding (bottom) (schematic view) 2 Inlet lift variation Inlet lift variation Second event lift variation Second event lift variation Valve Timing System with Second Valve Lift The mechanical investigations were carried out with the so-called UniValve system from Entec [4, 5, 6] on a test cylinder head. A turbocharged four-cylinder gasoline engine and a turbocharged six-cylinder diesel engine were used for the research on the hot engine. The four-cylinder gasoline engine is fitted on the intake and exhaust side with UniValve and allows the second event exhaust valve opening with all exhaust valves, Cover Figure. The six-cylinder diesel engine is converted to built camshafts, which allow an easy replacement of the cam contours. The second valve lift is here realized directly through the installation of a double cam contour and is not variable in contrast to the system in the gasoline engine. For the second valve lift (second event), a second cam contour is added to the primary cam. The intermediate lever will then respond in each cycle with a small additional pivotal movement corresponding to the new cam contour. This generates the second valve lift, depending on the setting of the eccentric shaft, which is also in contact with the intermediate lever. Characterized in that the second valve lift is smaller than the primary valve lift, the lift of the second event can be controlled with the change of the primary lift completely to zero. The maximum timing of the second event thus results at maximum valve lift of the main lift. Reducing the second event lift reduces the amount of the primary valve lift. The timing diagram shows this behaviour if no further ac - tions are provided on the valve train, 3 (top). The diagram shows the basic correlation. To reduce the influence on the primary valve lift during the control of the second event lift, it is possible to change the design of the valve train in a way so that a lift holding is introduced for large primary valve lifts. The result is that the primary valve lift is not or only slightly reduced when the second event lift is reduced or set to zero. Only holding time and timing of the primary valve lift is decreased, 3 (bottom). This characteristic is favourable on the exhaust side, as at partial load, an ad - vantageous long control time is driven, whereas the amount of residual gas is adjustable with the second event lift. At

4 Spread [ CA] % 33 % 4 % full load without second event a short exhaust valve opening time is driven, so that no residual gas is recirculated. The realization of this valve holding occurs on the working curve at the contact between the intermediate lever and the roller finger follower. By removing material in the area of the maximum lift of the working curve, a compensation for the maximum of the cam contour is achieved. With the appropriate design of the working curve, the valve lift holding is found during the activation of the second event lift, so that only the timing of the primary lift is adjusted. The primary valve lift remains constant depending on the design or changes only to a negligible degree. During second event valve lift this part of the working curve of the intermediate lever is not used, so that there is here no lift holding, but rather advantageously results in a relatively large valve lift to the short timing of the second event. The studies with the test cylinder head show that the speed limitations are not restricted by a second valve lift, with an acceptable increase in drag torque. Acceleration tests with a maximum second valve lift of 2.5 mm result on the test cylinder head with no restrictions on the use of typical gasoline engine in speeds up to 65 rpm. In further experiments with a second event set to zero and a maximum primary valve lift, camshaft speeds of up to 4 rpm were attained. This characteristic fits well with the intended use of a second valve lift which is not typically used at 44 % 47 % 46 % 49 % Second event valve lift [mm] ❹ Amount of residual gas as a function of the timing and spread of the second event valve lift EGR rate 5 % full load and high engine speeds, but is run in part load at low and medium en - gine speeds for residual gas recirculation. Design Criteria for the Second Event in Gasoline Engines To also implement the mechanical possibilities mentioned above on a thermodynamically favourable basis, there is the question of convenient design criteria for a second event valve lift on the exhaust side. The first desire is to be able to recirculate large amounts of residual gas quickly and adjustably, without, for ex - ample, the charge exchange work deteriorating. The variables are the amount, the timing and the spread of the second valve lift. Additional functions of Uni- Valve such as cylinder deactivation and valve lift phasing are still available. It is also conceivable to drive only one exhaust valve per cylinder with second event. One-dimensional simulations show a very high residual gas recirculation potential of up to 5 % with second valve lifts of 3 mm on the gasoline engine, depending on the choice of the second event spread, ❹. For small second event lifts the influence of the spread is initially low on the residual gas amount. With significant valve lifts from 1.5 mm, the achievable residual gas increases shifted to larger spreads. This can be explained by the larger pressure ratio between combustion chamber and exhaust, which is present during a late second event lift. Furthermore, timing and spread were varied to study the in - fluence on the amount of residual gas. Timings in the range of up to 1 CA promise a high residual gas potential with not too long a valve opening time. The necessary gas exchange work, which is not necessarily decreasing with an increasing amount of residual gas, is an important consideration. A compromise between a high amount of residual gas and a low gas exchange work must be found in the design. For the entire system a small charge exchange work, ❺, should be sought, since this can still achieve a relatively high residual gas fraction in the range of 35 % of the total charge gas. From the available simulation results, it can therefore be concluded that a smaller spread in the range of 5 CA after charge exchange TDC, a valve lift of about 1.5 to 2 mm and a valve opening time of about 8 to 1 CA for the second event in gasoline engines look beneficial, as significant quantities of residual gas can be recirculated without the gas exchange work in the cycle deteriorating. With a wise timing designed second event, the charge exchange work, as 5 shows, is lower than the configuration with ordinary valve overlapping, since all four valves are open during the intake cycle. This leads finally to the desired de-throttling. Second Event in the Gasoline Engine The investigations on the gasoline engine with second event are carried out on a 1.6-l downsizing engine with monoscroll turbocharger and 4-in-1 exhaust manifold. In the original version, the engine has a roller cam follower valvetrain with dual overhead camshafts, four-valve technology and a cam phaser on the intake side. The prototype engine has a modified cylinder head to accommodate the UniValve system on the intake and exhaust side, Cover Figure. The upgrade for the use with second event is done by replacing the cam contours of the built exhaust camshaft with double cam contours. The intermediate levers on the exhaust side are replaced with a variant having a modified working curve, which supports valve holding during second event operation. With this configuration, different partload operating points were compared to 2I214 Volume 75 21

5 Development Valve Gear EGR rate [%] CA spread 9 CA spread 25 CA spread operation without second event. Without second event, the amount of residual gas has traditionally been controlled by setting the exhaust closing time and adjusting the engine to minimum BFSC. The inner cylinder pressure sensors give information about the new development of the low pressure loop with throttlefree load control with second event, ❻. As expected, the area of the low pressure loop with second event is smaller than with pure valve overlapping without second event. The compression begins, due to the higher total filling of the combustion chamber, following the de-throttling of the additional residual gas at about.15 bar higher level. Near TDC a small recompression is indicated with second event which is due to the closing of the exhaust valve just before TDC and before the opening of the second event valve lift. Shortly after TDC the intake valves open, and the second event valve lift begins, so that it reaches a very shallow profile of the charge cycle curve during the intake cycle. The charge exchange work decreases with larger second event lifts. It should be noted that the opening time of the second exhaust valve lift is also increased. Second event with large valve lifts is limited through the residual gas compatibility limit of the engine. ❼ shows the measured fuel consumption over the angle of the eccentric shaft of the UniValve system, which controls the valve lifts. Smaller angles of the eccentric shaft mean an enlargement of the valve lift. The resulting second event lifts at 21, 22 and 23 angle of the eccentric shaft are given respectively in the upper part of the diagram. Due to lower peak combustion temperatures with increasing residual gas content, the NO x emissions decrease continuously with increasing second event lift. The fuel consumption initially decreases with increasing valve lift by de-throttling the engine. With a further increase of the residual gas supply the combustion comes closer to the residual gas compatibility limit of the engine, resulting in misfire. This explains the increasing fuel consumption to very large second event valve lifts, 7. The minimal fuel consumption with reduced NO x emissions can be Inner cylinder pressure [bar] 9 CA spread 155 CA spread Without second event Without second event Valve timing of second event [ CA] CA spread ❺ Amount of residual gas in response to spread and timing at 2 mm second event valve lift mm second event PMEP [bar] Without second event achieved with this engine at a second event of approximately 1.8 mm. The intake spread in these measurements were set to 8 CA, the exhaust spread of the primary exhaust valve lift to 1 CA. Second Event in the Diesel Engine To meet current emission standards, external exhaust gas recirculation to reduce NO x emissions has been established in diesel engines as an innermotoric measure. The problem with this residual gas system, however, is the relatively long and slow control, which does not always result in optimal amounts of residual gas in the cylinder in transient operation. A GT-Power simulation examines which residual gas amounts can be realized via a second valve lift. The residual gas is mainly dependent on the shared valve cross-section of the second event and the pressure gradient between the exhaust port and cylinder. With a maximum cross section of the valve event and a given pressure difference, a residual gas content of 5 % is possible (12 CA timing, valve lift 5 mm), ❽. With small valve lifts, the residual gas quantity is determined by the cross-section, with large lifts the pressure gradient dominates. The effectiveness of the process is improved in this residual gas control as the gas exchange work decreases with an increasing valve cross-section: On the one hand the exhaust gas back pressure is reduced by the additional shared cross Volume [cm³] n = 2 rpm BMEP = 3 bar ❻ Measured gas exchange work loop at n = 2 rpm and BMEP = 3 bar with and without second event

6 BSFC [g/kwh] Min. BSFC without SE, BMEP = 4 bar Min. BSFC without SE, BMEP = 6 bar BMEP = 4 bar BMEP = 5 bar 264 BMEP = 6 bar section and the piston has to do less exhaust gas exchange work. On the other hand, the intake port pressure increases, since a higher pressure prevails in the exhaust port and thus the pressure level is raised. With the opening time of the second event, the charge exchange work can be further reduced. The lowest charge exchange work is achieved at a beginning opening of 37 CA of the second event for all valve lifts. In order to minimize gas exchange work, the residual gas recirculation is first controlled with the valve, as with a pressure difference, where the gas exchange work becomes increasingly deteriorated. With an earlier opening, the residual gas amount in the cylinder also in - creases. The reason is the pressure difference between exhaust port and combustion chamber. This determines the flow velocity in the valve area and is dominated mainly by the pressure wave in the exhaust port. The pressure wave is determined by the number of cylinders and the geometry of the exhaust port and is therefore engine-specific. The timing of the peak pressure changes only marginally with a variation of the second opening of the exhaust valve. Therefore, the maximum content of the residual gas is achieved when the maximum shared valve cross section coincides with the maximum pressure in the exhaust port. In this case, the pressure maximum is located at 43 CA. At this time the cross section is at maximum when the valve opens at 37 CA. In further experiments, the potential of the second event with a valve lift of 3 mm at one exhaust valve is compared with an external exhaust gas recirculation. The gas charge exchange work falls to.15 bar due to the significantly lower exhaust back pressure and a de-throttling in the intake stroke. This reduction has a positive effect on the fuel consumption. The BFSC is significantly lower than with external EGR with the same NO x emissions. Only from NO x emissions below 2 g/kwh does the consumption increase in comparison with an external EGR, ❾. There are two reasons for this behaviour: firstly, the exhaust gas back pressure must be raised with the VTG vanes within the measurement series to generate the necessary pressure gradient between the exhaust port and the cylinder. The overall process can be improved at this point by throttling the intake air. Secondly, the ignition and the combustion are strongly influenced. Due to the hot residual gases, the ignition delay of the pilot and main injection is shortened, and so more energy is released earlier. The combustion ends at the same time, so that the overall combustion efficiency is slightly reduced. Therefore, the injection strategy needs to be adapted. Overall, the proportion of the premixed combustion de creases by the earlier ignition and it increases the particulate emissions compared to the external EGR. By reducing the injection amount of the pilot injection, it is possible to increase the ignition delay of the 2.3 mm SE 1.8 mm SE 1.3 mm SE Angle of excentric shaft [ ] n = 2 rpm IS = 8 CA ES = 1 CA ❼ BFSC depending on the selected second event valve lift at intermediate pressures of 4, 5 and 6 bar (SE: Second Event; IS: intake spread; ES: exhaust spread) EGR rate [%] Maximum valve opening main injection, and thus to reduce the particulate emissions. The minimal particulate emissions are achieved without pre-injection, but this is unsuitable for use due to the strong pressure rise after ignition, for comfort reasons. The exhaust temperature management, thus raising the temperatures in special operating modes is quite essential for modern exhaust gas aftertreatment systems. Due to the significantly hotter residual gases, the temperature level in the exhaust system is significantly increased and can be used as warm-up or for regeneration phases to improve the efficiency of exhaust after Pressure gradient [bar] 5 mm second event 2 mm second event mm second event ❽ Amount of residual gas as a function of the lift of the second event and the pressure gradient 2I214 Volume 75 23

7 Development Valve Gear BSFC [g/kwh] Second event External EGR Ignition) combustion processes. But also for further de-throttling of the engine and emission control with a conventional combustion process, engine developers are given more degrees of freedom in hand to achieve further fuel savings and comply with stricter emission limits. With UniValve and second event, studies on engines with continuously variable compression ratio and HCCI combustion process are planned treatment systems [7]. The emission gas temperature can be raised up to 6 K with second event, ❿. By utilizing exhaust gas back-holding with a camshaft phaser a temperature raise in the range of only 3 K is possible [8]. Potentials and Prospect High amounts of residual gases up to 5 % have been achieved on the fired gasoline engine. In the prototype engine used, the residual gas compatibility increased significantly, which can be explained with additional charge motion in the combustion chamber. The NO x emissions sink continuously with increasing valve lift, as expected. The fuel consumption is not rising in this case, but is reduced slightly. In diesel engines with a second event at the exhaust side, the same NO x emissions are achievable as with an external cooled EGR, but with a lower charge exchange work. Thus, the disadvantage of the long control distance of the external EGR can be solved with the second event. A cycle-fast and optimal amount of residual gas can also be accurately controlled in transient operation. Due to the significantly higher temperatures in the cylinder and the associated change of the combustion process, the particulate emissions rise. This problem can be largely neutralized by adapting the injection strategy. The high temperature level of the exhaust gas can be used to shorten the NO x [g/kwh] ❾ BFSC and NO x emission with 3 mm second event valve lift and external EGR only 24 n = 2 rpm BMEP = 3 bar EGR variable warm-up of the catalysts. The continuously variable valve train allows through demand and continuous variation of valve lift and valve timing to adjust each load point for optimal fuel consumption. With the extension with adjustable second event valve lift on the exhaust side, great and hot residual gas amounts can be controlled quickly and continuously in an effective way. Thus, the range of application of such a valve train is further increased, where a cycle-accurate and fast control of the residual gas is needed. This is necessary for example in HCCI (Homogeneous Charge Compression Ignition) and CAI (Controlled Auto Exhaust gas temperature [ C] References [1] Böckenhoff, E.; Herrmann, H.-O.: Der Ladungswechsel bei der neuen Generation von Daimler Trucks Nutzfahrzeugmotoren. Conference Ladungswechsel im Verbrennungsmotor, Stuttgart, 212 [2] Neußer, H.-J.; Kahrstedt, J.; Jelden, H.; Dorenkampf, R.: Die EU6-Motoren des Modularen Dieselbaukasten von Volkswagen innovative motornahe Abgasreinigung für weitere NO x - und CO 2 -Minderung. International Vienna Motor Symposium, 213 [3] Paulov, M.: Analyse eines mechanisch vollvariablen Ventiltriebs an aufgeladenen Ottomotoren mit Saugrohr- und Direkteinspritzung. TU Kaiserslautern, dissertation, 212 [4] Flierl, R.; Lauer, F.; Breuer, M.; Hannibal, W.: Cylinder Deactivation with Mechanically Fully Variable Valve Train. In: SAE International Journal of Engines 5 (212), No. 2 [5] Flierl, R.; Lauer, F.: Mechanisch vollvariabler Ventiltrieb und Zylinderabschaltung. In: MTZ 74 (213), No. 4, pp [6] Schmitt, S.: Potenziale durch Ventiltriebsvariabilität auf der Auslassseite am drosselfrei betriebenen Ottomotor mit einstufiger Turboaufladung. TU Kaiserslautern, dissertation, 212 [7] Honardar, S.; Deppenkemper, K.; Nijs, M.; Pischinger, S.: Rohemissionsvorteile und verbessertes Lightoff-/Regenerationsverhalten mithilfe von Ventiltriebsvariabilitäten am Pkw-Dieselmotor. Conference Ladungswechsel im Verbrennungsmotor, Stuttgart, 212 [8] Temp, A.: Auslassvariabilitäten am Dieselmotor. TU Kaiserslautern, previously unreleased dissertation, 214 Exhaust gas temperature second event Exhaust gas temperature external EGR Filter smoke number second event Filter smoke number second event with reduced pilot injection Filter smoke number external EGR 3, NO x [g/kwh] ❿ Particle emissions and exhaust gas temperature after turbine in dependence of NO x n = 2 rpm BMEP = 3 bar EGR variable 5, 4,5 4, 3,5 3, 2,5 2, 1,5 1,,5 Filter smoke number [-]

8 2I214 Volume 75 25