Dinu Taraza

Simplified Elasto-Hydrodynamic Friction Model of the Cam-Tappet Contact

The paper analyses the particularities of the lubricating conditions at the contact between the cam and a flat tappet in the valve train of an internal combustion engine and develops a method for the calculation of the friction force. The existing lubrication models show the predominance of the entraining speed and oil viscosity on the thickness of the oil film entrapped between cam and tappet, predicting a very small value (less than 0.1 μm) of the oil film thickness (OFT). The oil viscosity increases exponentially with pressure in the Hertzian contact, determining non-Newtonian behavior of the oil in the contact zone. Using the model developed by Greenwood and Tripp [1 1 ] for the contact of two rough surfaces and the Eyring model [2] for the oil it is shown that non-Newtonian behavior of the oil prevails and that the OFT plays a secondary role on the friction force. The simplified friction model developed according to these assumptions is experimentally validated by measurements made on a single cylinder diesel engine. An original technique was developed to permit the measurement of the friction forces on the tappet. This technique is based on a modified design of the tappet, which is equipped with strain gauges that measure the forces and moments applied on the tappet. The processing of the measured data permits to evaluate the friction force cam-tappet and tappet-bore and, to determine the angular rotation of the tappet.

Elastodynamic transient analysis of a four-cylinder valvetrain system with camshaft flexibility

Abstract This paper presents an analysis of a line of valvetrains in a four-cylinder, four-stroke in-line diesel engine. The method highlighted in this paper predicts the vibration signature together with the prevailing contact conditions and frictional characteristics exhibited in the valvetrain system. This integrated dynamic and tribological investigation provides a practical approach that can be used during the design or the evaluation phase of automotive valvetrain systems.

Combined multi-body dynamics and experimental investigation for determination of the cam-flat tappet contact condition:

Abstract The paper investigates the tribological contact conditions between a polynomial automotive cam and a flat follower. A multi-body model of the system is presented to take into account the complex tappet motions. A set of such models is presented in order to predict the cam-tappet friction force, the tappet-tappet bore friction force and the resulting tappet spin. The tribological conditions giving rise to these kinetic actions include the elastohydrodynamic regime of lubrication in the concentrated cam-follower contact and a mix of hydrodynamic and boundary friction action in the tappet-tappet bore contact. While boundary friction dominates the tappet-tappet bore friction at low engine speeds, for high engine speeds the boundary friction dominates the closing event of the cam, and hydrodynamic and mixed lubrication dominate the opening event of the cam. The simplified friction models are experimentally validated on a single-cylinder diesel engine. The measuring techniques are based on a redesigned engine tappet equipped with four strain gauge bridges.

Chemiluminescence imaging of pre-injection reactions during engine starting

The thermal and chemical state of residual gas is known to influence the likelihood of autoignition, ignition delay and combustion phasing of the subsequent diesel engine cycle. To elucidate the role of residual gases in these processes, ultraviolet chemiluminescent reactions and their spectra are observed during the pre-injection, compression period in a dynamometer-driven, optically-accessible, diesel engine operated with a single fuel injection event. During a cold start sequence, while the engine is motored and fuel is injected without firing, the pre-injection chemiluminescence (PIC) intensity increases from cycle to cycle. This leads to a second mode of intermittent firing cycles which are observed to follow a higher intensity of PIC. In the third mode, decreased PIC intensity is measured in firing cycles that are preceded by partial misfires. In the fourth mode, firing is continuous, but with a high IMEP coefficient of variation (COV). Here, PIC intensity is found to strongly correlate with advanced combustion phasing. As firing continues, it is observed that COV, PIC intensity and the phasing correlation decrease. Upon fuel shutoff, PIC intensity decays with time. Spectral measurements confirm that reactions of low temperature combustion intermediates, including chemiluminescent formaldehyde (HCHO*) and CHO* comprise the observed PIC.

A Generic, Transient Model of a Turbocharged, Multi-Cylinder, Common-Rail Diesel Engine

The paper presents a generic model of a turbocharged, common rail diesel engine developed on a SIMULINK platform. The model is developed for the main task of predicting transient behavior of the turbocharged engine, especially acceleration under load. In order to reduce computer time, the model is zero-dimensional as far as combustion is concerned, but split injection is considered to simulate the main characteristic of modern common rail diesel engines. A simplified generic model of the turbocharger is developed such as to determine the effects of different design parameters on the acceleration of the engine. A detailed dynamic model of the engine is considered to allow estimating the torsional behavior of the crankshaft and evaluate the impact of torsional vibrations. The model is validated on a four-cylinder, 2.5 liter common-rail diesel engine and is further used to identify the fueling strategy and the design parameters that could be improved in order to obtain the best possible engine acceleration under different load conditions.Copyright

Cylinder Pressure Reconstruction From Crankshaft Speed Measurement in a Four-Stroke Single Cylinder Diesel Engine

In a single cylinder engine, the speed fluctuation during steady state operation of the engine is influenced only by the cylinder pressure variation, the engine friction and the dynamics of the crankshaft. This dependency is used to explore the capacity of the lumped mass model of the crankshaft to correctly represent its dynamics. Based on this model, the paper establishes the relationship between the cylinder pressure variation and the crankshaft speed fluctuation for steady state operation of the single cylinder diesel engine. Correlations are determined between the harmonic components of the tangential gas-pressure and the harmonic components of the angular speed of the free end of the crankshaft. These correlations are used to predict the angular speed variation of the crankshaft, when the cylinder pressure variation is known, or to reconstruct the cylinder pressure when the crankshaft speed fluctuation is known. The reverse calculation of the pressure variation from the measured crankshaft speed is strongly influenced by the elastic characteristics of the crankshaft. If the stiffness of the crankshaft is not accurately determined, the results are significantly distorted.© 2005 ASME

Two Dimensional Effects in the Lubrication of the Cam-Tappet Contact

The cam-tappet contact experiences very high loads and the lubrication conditions are Elasto-Hydrodynamic (EHD). In EHD lubrication the oil film thickness (OFT) is mainly determined by the entrainment speed of the oil and the oil viscosity. Under the high load present in the cam-tappet contact the OFT reaches very small values in the order of magnitude of a fraction of one micrometer and asperity contacts cannot be avoided. In this situation the friction force has two components, a boundary and a viscous one, and non-Newtonian behavior of the oil prevails. The friction force is determined by the rheological properties of the oil and the geometry of the contact (combined radius of curvature, cam width and combined asperity of the rubbing surfaces). For valve trains with bucket type tappets, the mechanism is design to rotate the tappet during engine operation. In this situation, both the entrainment speed and the radius of curvature are varying along the contact line determining variations in the OFT and, consequently, variations in the contribution of the boundary friction component. Based on a simulation model that considers the tappet spin under the action of the friction force between cam and tappet, and the friction force between tappet and its bore, the entrainment speeds and their directions were calculated along the contact line. It was found that the boundary friction component reaches a maximum value at the inner edge of the cam, at the beginning of the valve-closing event. Because the boundary component of the friction force is mainly responsible for wear, an increased wear of this region should occur. Visual inspection of a used camshaft seems to confirm this finding.Copyright

A CFD Study of the Effect of HCHO Addition on Autoignition and Combustion

Experimental and theoretical research [1] targeted towards the effect of formaldehyde on combustion has identified its OH-scavenging role and recent data suggests it plays an important role in combustion instability observed during engine cold-starting. This effect was further studied using a CFD approach and the kinetic inhibiting effect of formaldehyde on the combustion process has been found to be enhanced by thermal/diffusion effects.Copyright

Transient Analysis of a Line of Valves With Camshaft Flexibility and Valve-Spring Surge Effects

The paper presents analysis of a line of inlet valves in a 4-cylinder, 4-stroke diesel engine. The combined dynamics and tribological investigation provides important data that can be used in the design or design evaluation of automotive valvetrain systems, inlcluding vibration signature of the system, as well as prevailing contact conditions, including friction and lubrication data.© 2003 ASME