Variable valve timing on the effect of brake specific fuel consumption (BSFC) improvement: a study at part load using VeSyMA – Engines

The effects of continuously variable valve timing (VVT) are relatively well known. This blog post carries out an investigation at part load condition (3000 RPM, 3 Bar BMEP) for a three-cylinder 1 Litre SI engine with VVT to analyse the benefit of VVT on fuel consumption. All experiments are run at stochiometric AFR. Mass fraction burned at 50% (MFB50) is maintained at 8 degrees after TDC (aTDC). Throttle angle and spark timing are controlled to keep the specified load and MFB50 condition for each set of intake valve and exhaust valve timing. Figure 1 shows the BSFC improvement against intake valve opening (IVO) and exhaust valve closing (EVC). This blog post uses VeSyMA-Engines for the simulation work, which is one of the products of Claytex Ltd. The specific heat release model used in this work is predictive combustion model based which is a kinetic energy based semi-dimensional model that models thermodynamics, turbulence, ignition delay and flame entrainment, as it is used in the engine model shown in Figure 6 and Figure 7 in appendix. A more detailed introduction of the VeSyMA-Engines can be found in Figure 8 in the appendix. 

Figure 1: BSFC improvement vs IVO and EVC
Figure 1: BSFC improvement vs IVO and EVC

On the top right, late IVO, i.e. Atkinson cycle, combined with increased overlapping increases the effective compression ratio and as a result a BSFC gain of 2.7% is achieved. On the top left, early IVO reduces backflow losses and involves a higher downstream throttle pressure and lower pumping work, figure 2, 3.

Figure 2: plenum pressure vs IVO and EVC
Figure 2: plenum pressure vs IVO and EVC
Figure 3: PMEP vs IVO and EVC
Figure 3: PMEP vs IVO and EVC

The increased overlapping causes high level of internal exhaust gas recirculation (IEGR) and prolonged combustion duration, figure 4, 5.

Figure 4: residual gas fraction vs IVO and EVC
Figure 4: residual gas fraction vs IVO and EVC
Figure 5: MFB10to90 vs IVO and EVC
Figure 5: MFB10to90 vs IVO and EVC

For the two cases at the bottom left and the bottom centre in Figure 1, early exhaust valve opening reduces the expansion work, which shortens the time for the exhaust gas to expel which results in a higher mass of exhaust gas remaining in the cylinder as IEGR, figure 4. It is seen that advancing the exhaust valve opening is not beneficial for improving fuel consumption. In the middle left in figure 1, advancing the intake valve opening, without retarding the exhaust valve opening does not increase or decrease fuel consumption. However retarding the intake valve opening, without retarding the exhaust valve opening will improve BSFC, see middle right in figure 1. In the top centre of figure 1, retarding exhaust opening will improve BSFC by 1.62%.

In summary, in the part load condition analysed in this blog post, retarding the exhaust valve opening will always yield an improved BSFC. This is because the delayed exhaust opening allows for a slightly increased expansion work. Retarding intake opening will also benefit BSFC due to improved effective compression ratio, i.e. larger effective expansion to compression ratio. Advancing the exhaust valve opening will always result a higher BSFC and is not beneficial. Advancing the intake valve opening also improves BSFC. For higher load points, the valve timing effects may not apply consistently across the higher load points. That will need to be analysed separately.

Appendix

Introduction to VeSyMA-Engines

The VeSyMA-Engines library contains two types of engine model: crank angle resolved and mean value.

Figure 6: Engine-on-dyno experiments

An engine-on-dyno experiment includes the engine model, ECU, rig controller, dynamometer, cooling system and lubrication system, see Figure 6. 

Figure 7: A three cylinder naturally aspirated engine model
Figure 7: A three cylinder naturally aspirated engine model

An engine model consists of an intake, an exhaust, a timing, a camshaft, a cylinder block, a crankshaft, a friction model, a starter motor, a cooling circuit and a lubrication circuit.

Figure 8: A collection of some of the VeSyMA-Engines components
Figure 8: A collection of some of the VeSyMA-Engines components

A brief introduction of the model structures and their functionalities can be found in Figure 8. The following lists all the components in Figure 8.

  1. BMEP: calculates IMEP, PEMP, FMEP, BMEP, BSFC, brake torque and brake power.
  2. Knock detection: empirical correlation based that classifies knocks to no knock, trace knock, medium knock and strong knock.
  3. Reaction model: table based that predicts combustion species.
  4. Heat release model: it consists of a Wiebe heat release model and a predictive combustion model.
  5. Turbocharger: Stodola and Ellipse law and map based, or equation based.
  6. Intercooler.
  7. EGR circuit.
  8. FMU: Functional Mock-up Unit. A Dymola engine and/or controller model can be compiled to an FMU that can be run in other simulation environments, such as Simulink or DSpace.
  9. Detailed cooling system.
  10. Fuelling system.
  11. High pressure pump.
  12. Fuel pump and rail.
  13. Hydraulic VCT.
  14. Three-way catalytic converter: map based, or chemical kinetic based.
  15. DOC: Diesel Oxidation Catalyst, map based.
  16. DPF: Diesel Particulate Filter, map based.
  17. SCR: Selective Catalytic Reduction, map based.
  18. ASC: Ammonia Slip Catalyst, map based.
  19. Detailed intake model.
  20. Sonic restrictor.
  21. Crankshaft and piston.
  22. Piston-connecting rod model.
  23. Twin scroll turbocharger.
  24. Camshaft.
  25. Piston friction and blow by model.
  26. Intake and exhaust valve lift model.
  27. 1-dimensional engine mechanics model that uses less computational resources than multibody option.
  28. Surrogate model that enables  replication of essential variables from a master cylinder to the remaining cylinders with appropriate phasing.
  29. Injector model.
  30. Heat release model: Woschni, Hohenberg, Annand.
  31. Mean value cylinder model that simulates cycle averaged engine quantities. 

Written by: Xiaoran Han – Project Engineer

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