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The aim of the paper is to provide information about the in-cylinder flow field optimisation in a high speed, direct injection (HSDI) four valve per cylinder diesel engine for off-highway applications. Fully transient CFD analyses of different valve profile strategies for the intake and compression strokes are at first performed, in order to evaluate the effects on both engine permeability and in-cylinder flow field evolution. Modifications are applied to each intake valve separately: gradually stretched cam profiles are imposed so that strategies range from the standard operation, i.e. the adoption of a unique cam profile for the two intake valves, up to the limit case characterized by a 40 % difference between the intake valves maximum valve lifts for three different engine conditions.

Standard design practice usually adopts steady flow tests for addressing optimisation of the intake valve-port assembly. Recently, with more user-friendly CFD tools and with increased computing power, intake stroke simulations, handling both piston and valves motion, have become practical. The purpose of this paper is to compare the design guidelines provided by the standard steady flow tests (both experimental and numerical) and the information coming from a CFD-3D intake stroke analysis. Reference is made to a four valve HSDI Diesel engine. Three swirl control strategies are investigated. It is supposed that one intake valve is kept closed, while the other one operates normally (first strategy). The second strategy consists in a 50% reduction of the lift of both valves. Finally, the third possibility is the blockage of one intake port by means of a simple butterfly valve.

Engine permeability, which is commonly known to exert a strong influence on engine performances, is usually experimentally addressed by means of the definition of a global parameter, the steady discharge coefficient. Nevertheless, the use of such a parameter to describe valve-port assembly behaviour appears sometimes to be insufficient to determine port fluidynamic behaviour, due to the simultaneous concurrency of complex mechanisms, such as mean flow distortions and boundary layer detachments. CFD simulation appears therefore to be a fundamental tool to fully understand port fluidynamic behaviour. In the present paper, two engine intake port assemblies are investigated by using the STAR-CD CFD code, showing a strongly different behaviour from the point of view of secondary detached flows generation across the valve.

The supercharging system investigated in this study is made up of a traditional turbocharger, coupled with a Roots-type positive displacement compressor. An electrically actuated clutch allows the compressor to be disengaged from the engine at high speed and under partial load steady operations (such as the ones occurring in a driving cycle). This concept of supercharging has been applied to the downsizing of a reference engine (a 2.5 litre, turbocharged, four cylinder, high speed DI Diesel engine), without penalization on the maximum brake power (110 kW) and transient response. For such a purpose, a “paper” engine has been theoretically characterized. The gross engine parameters have been optimised by means of 1-D numerical simulations, using a computational model previously validated against experiments. Performances of the reference and the downsized engine have been compared, considering both steady and transient operating conditions, full and partial load.

CFD simulations of reacting flows are fundamental investigation tools used to predict combustion behaviour and pollutants formation in modern internal combustion engines. Focusing on spark-ignited units, most of the flamelet-based combustion models adopted in current simulations use the fuel/air/residual laminar flame propagation speed as a background to predict the turbulent flame speed. This, in turn, is a fundamental requirement to model the effective burn rate. A consolidated approach in engine combustion simulations relies on the adoption of empirical correlations for laminar flame speed, which are derived from fitting of combustion experiments. However, these last are conducted at pressure and temperature ranges largely different from those encountered in engines: for this reason, correlation extrapolation at engine conditions is inevitably accepted. As a consequence, relevant differences between proposed correlations emerge even for the same fuel and conditions.

The recent interest in alternative non-fossil fuels has led researchers to evaluate several alcohol-based formulations. However, one of the main requirements for innovative fuels is to be compatible with existing units’ hardware, so that full replacement or smart flexible-fuel strategies can be smoothly adopted. n-Butanol is considered as a promising candidate to replace commercial gasoline, given its ease of production from bio-mass and its main physical and chemical properties similar to those of Gasoline. The compared behavior of n-butanol and gasoline was analyzed in an optically-accessible DISI engine in a previous paper [1]. CFD simulations explained the main outcomes of the experimental campaign in terms of combustion behavior for two operating conditions. In particular, the first-order role of the slower evaporation rate of n-butanol compared to gasoline was highlighted when the two fuels were operated under the same injection phasing.

The evaluation of the steady-flow discharge coefficient of ICE port assemble is known to be very sensitive to the capability of the turbulence sub-models in capturing the boundary layer dynamics. Despite the fact that the intrinsically unsteady phenomena related to flow separation claim for LES approach, the present paper aims to demonstrate that RANS simulation can provide reliable design-oriented results by using low-Reynolds cubic k-ε turbulence models. Different engine intake port assemblies and pressure drops have been simulated by using the CFD STAR-CD code and numerical results have been compared versus experiments in terms of both global parameters, i.e. the discharge coefficient, and local parameters, by means of static pressure measurements along the intake port just upstream of the valve seat. Computations have been performed by comparing two turbulence models: Low-Reynolds cubic k-ε and High-Reynolds cubic k-ε.