Dual VCP Optimization at WOT & part loads for a Gasoline engine Indian GT-Suite Conference Yashaswi R Padmavathi R Saravanan Muthiah Mahindra & Mahindra Ltd. th Sep Copyright Mahindra & Mahindra Ltd. All rights reserved.
Introduction Variable Cam Phasing (VCP) or Variable Valve Timing (VVT) enables control of timing of valve events Valve opening/closing time, over lap duration, dynamic compression ratio, internal EGR can be controlled. Efficiency of the engine can be increased. Cam phasing also has a downside. The biggest downside is a substantial increase in the amount of testing required for optimization. Moving the optimization process to a virtual engine model saves a lot of development time & cost. Also, it gives us valuable insight into the engine's performance well in advance.
Impact of Variable Cam Phasing An optimized cam phasing gives an improvement in efficiency & BSFC. In a gasoline engine throttling leads to pumping losses which if reduced will lead to efficiency improvements. The engine needs high volumetric efficiency for good full load performance But part load performance needs lesser volumetric efficiency in order to reduce throttling pumping losses With cam phasing, the valve timings can be optimized so as to reduce volumetric efficiency which reduces pumping losses in turn improving BSFC
Pumping losses Comparison of P-V curves Throttling NO VCP Throttling IVCP only Throttling IVCP & EVCP IMEP =. bar IMEP =. bar IMEP =. bar IMEP=. bar IMEP=. bar IMEP=. bar PMEP= -. bar PMEP= -. bar PMEP= -. bar Residual gas= % Residual gas = % Residual gas = % Increase in residual gases, which decreses volumetric efficiency Less throttling is required with lesser volumetric efficiency hence pumping losses are reduced.
Methodology ENGINE MODELING @ WOT Engine modeling for WOT Validation of performance @ WOT OPTIMIZATION OF DVCP @ PTP Optimization throughout engine operating map Limiting residual gas % & re-optimization for trade-offs Selection of operating points for PTP ENGINE MODELING FOR PTP Modeling engine for PTP with test parameters Validation of performance @ PTP RESIDUAL GAS ESTIMATION Building a simple gas exchange model Correlation between Simulation & test Estimating RG limit for Combustion stability Selection of DVCP operating points Selection of timing lock position & VCP ranges FTP vs PTP trade-off OPTIMIZATION FOR DVCP @ WOT Optimization for WOT Selecting the VCP operating points for WOT Results & Analysis BSFC Difference Map Effect of improvement in NEDC Drive cycle
Engine Modeling Engine intake & exhaust layouts are modeled as per geometry All Pipe wall temperatures are variable (tempsolver) Combustion is modeled by Wiebe model parameterized from test data Combustion is assumed constant w.r.t VCP positions. Throttle is controlled by a PID (targeting torque) Friction is varied w.r.t. load according to CF model Engine operating points are optimized for knock limitations, exhaust gas temperature limitations, min. BSFC.
- - - - - - - -- - - - Correlation with Test data BSFC %difference map - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Air flow %difference map - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Torque - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - E n g in e S p e e d - - - - - - - - - - - - - - - - - - - - - - - - - - - Torque - - - - - - Engine Speed An engine with IVCP was tested. Simulation correlation for BSFC & air flow rates is shown here.
Residual Gas Estimation & limits A simple gas-exchange single cylinder engine is modeled Engine speed, intake pressures, exhaust pressures, intake temperatures, cylinder pressures & AF ratio is given as input from testbed data Airflow rate is correlated within % & output residual gas content is considered. At low loads, it is observed that ~% residual gas content is the limit for combustion stability
DVCP Optimization at WOT At WOT, intake & exhaust VCP are optimized for maximum volumetric efficiency & torque. An improvement in torque is seen at low engine speeds & small improvement is seen at high engine speeds. Maximum of % improvement is seen & an average improvement of % % Improvement in WOT Torque % % % % % % Engine Speed (rpm)
DVCP Optimization at Part loads (rpm bar) Exhaust VCP Position The optimization plot for rpm- bar point is shown here Min. BSFC = g/kwh IVCP = EVCP = Residual gas limit = % Higher residual gas % leads to lower throttling & hence better BSFC This trend is clearly reflected in the plot attached. o Higher residual gas has better BSFC Intake VCP Position
Exhaust VCP po osition (deg) Optimized VCP points DVCP operating points Optimum DVCP points Cam phaser limits Final DVCP points The optimum points for both Full load and Part load are shown here The cam phaser range being limited ( ), we have to select an optimum trade off range Thus, lock/reference positions for valve timings are selected. Also, it is observed that retarded IVO & EVC (retarded overlap) in part loads reduces pumping losses substantially - - - - - Intake VCP position (deg)
BSFC Difference Map Torque BSFC Difference Map (IVCP BSFC DVCP BSFC)% - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Engine Speed - - - - - - - - - - - - - - - - - - - - - - - - - - - The % difference of BSFC is shown here We can see that the benefit because of EVCP is very less at WOT & increases with decreasing load o This is explained by increasing improvement in throttling / pumping losses with decreasing loads o Average improvement of % is observed. - - - - - - - - - -
Improvement in Cycle FE Torque NEDC Cycle operating points on BSFC Difference Map - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Engine Speed - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Both engine BSFC maps when made to follow the NEDC cycle showed an FE improvement of.% Operating points of the cycle are plotted on the adjacent map NEDC Cycle
Summary This simulation activity demonstrates the optimization procedure for Variable Cam Phasing A good correlation of simulation vs test data is shown in FTP & PTP An average improvement of % is shown in the FTP due to addition of EVCP. An improvement of.% is shown in NEDC cycle fuel economy due to addition of EVCP. Although improvement in WOT performance is not much, improvement in part load consumption is significant. Effect on BSFC because of reduction in pumping losses because of residual gases & volumetric efficiency is analysed Further work in this area would include Continuous Variable Lift & Timing (CVVT) optimization