Effect of intake valve lift on the mixture preparation processes of a port fuel-injected engine

• Main Author: Hindle, Mark P.
• Published: University of Brighton 2008
• Subjects: 621.43
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.492200

Modern gasoline port fuel injected (PFI) engmes have improved performance and efficiency through the use of variable valve technology. A PFI engine with continuously variable intake valve lift is now a viable option for reduced C02 emissions levels through an improvement in the cycle efficiency. This improvement is due to a reduction in the negative pumping work associated with throttled, four-stroke, engine operation. However, when operating at peak intake valve lifts of the order of 5% of conventional peak lift values, the nature of the air and fuel mixing process is fundamentally altered. The characteristics of the mixture preparation processes are not clearly understood. In order to investigate and evaluate these features, a BMW Valvetronic cylinder head was modified to allow optical access to the intake port, near-valve region and upper cylinder. This was incorporated into a Steady State Flow Rig (SSFR). The operating conditions of the steady-state flow rig were derived from data recorded in a four-cylinder, firing engine study. A sequence of well proven optical and laser diagnostic techniques were applied experimentally to describe the characteristics of the air and fuel mixing processes through the intake port and valve orifice and within the upper regions of the combustion chamber. These included Laser Light Sheet Scattering, Particle Image Velocimetry and Phase Doppler Anemometry. The experimental methods were used to establish the characteristics of the port discharge coefficient, mean and turbulent airflow and the liquid fuel spray over a range of peak intake valve lifts and flow conditions. The characteristics of the port fuel injector were initially investigated in a quiescent spray chamber. The four-hole fuel injector produced two streams of liquid fuel with an approximate cone angle of 14°. The angle between each liquid jet was approximately 20°. The spray was comprised of droplets with a mean diameter of 50Jlm and a mean axial velocity magnitude of20m/s during the main, quasi-steady, spray phase. In the steady-state flow rig measurements, five intake valve flow regimes were identified; high valve lift, the first transition phase, intermediate valve lift, the second transition phase and low valve lift. A simple analysis based upon the predicted droplet Weber number was used to provide supporting evidence to the in-situ airflow and droplet size measurements. At the highest valve lifts, the characteristics of the fuel spray were similar to that observed in the quiescent spray chamber. The fuel droplets were entrained in the free flow air regime. For valve lifts between 9mm and 3mm, the air motion in the cylinder exhibited a conventional, forward tumble, bulk motion pattern. In the intermediate range of valve lifts, between 1.5mm and 3mm, the forward tumble bulk flow motion was less evident and the discharge coefficient indicated some degree of flow separation at the valve orifice. Multiple secondary break-up mechanisms occurred at the intake valve orifice. The bulk in-cylinder air motion was consistent with a pair of counter-rotating vortex-like structures above and below the intake valve exit jet, exhibited by regions of high turbulence intensity. At the low peak valve lifts, the predicted droplet Weber number indicated catastrophic break-up of the fuel through the intake valve orifice. A narrow air and fuel spray flow was observed in the tumble plane. A 50% reduction in the mean droplet diameters was measured at an intake valve lift of OAmm compared to the full lift case. Features of the mixture preparation processes at low valve lifts included oscillating velocity measurements in the air flow that were consistent with jet flapping instabilities near the valve exit. A finely dispersed fuel spray mist was recorded in the cylinder. Both the air motion and fuel spray studies exhibited cross-flow asymmetry throughout the entire valve lift range.