Eventos Anais de eventos
CONEM 2018
X Congresso Nacional de Engenharia Mecânica
UMotor - PERFORMANCE EVALUATION OF A SMALL OVER-EXPANDED ENGINE USING 3D-CFD AND 1D ENGINE MODELS
Submission Author:
Jorge MARTINS , aaa , Portugal
Co-Authors:
Tiago Costa, Jorge MARTINS, Francisco Brito
Presenter: Jorge MARTINS
doi://10.26678/ABCM.CONEM2018.CON18-0943
Abstract
An innovative over-expansion small Internal Combustion Engine was developed in the pursuit of ultimate efficiency to power a super-mileage (Shell Eco-Marathon) car, the EconomicUM. The previous engine was based on a Honda 50cc fitted with an over-expansion cycle enabled by EIVC (early intake valve closure). A peak fuel economy of 1560 km/L was attained during a competition at the Rockingham race-track, in Britain. The following engine is a native over-expansion engine using an expansion ratio of 1.7:1 provided by a hypo-trochoid mechanism integrated within the crankshaft. With it, the expansion and exhaust strokes are 1.7 times longer than the intake and compression strokes. This enables a higher thermal efficiency, as the exhaust gases are further expanded into the cylinder before being expelled to the atmosphere. But the engine has further features that enable higher efficiencies to be achieved. As mechanical losses increase with the square of the engine speed, this engine was designed to run efficiently at low speeds, so it needs to attain fast burning rates at low rpm. Therefore, turbulence was enhanced in the form of swirl and squish, and two spark-plugs are used to increase the initial burn rate. The intake duct was specifically developed to produce swirl and the combustion chamber, although with a hemispherical design, has a shape enabling squish to be created as the piston reaches TDC. The present work shows the first theoretical performance analysis of the engine using two different, but complementary, engine models. The burning rate and in-cylinder heat transfer were evaluated using a 3D-CFD software. These results were used as an input to a proprietary 1D engine model developed by the research team reproducing the full engine flow paths and valves. This software coupling strategy allowed for an initial performance evaluation and optimization prior the testing of the engine. One of the major issues with native over-expansion engines is the higher mechanical losses resulting from the more complicated crankshaft. Although the indicated results (from the indicated cycle) of the over-expanded engine are much better than those from a conventional Otto cycled engine, the higher mechanical power required to run the hypo-trochoid mechanism will limit the improvements gained with the thermodynamic cycle improvements. However, the overall mechanical losses were minimized by the verticalization of the connecting rod during expansion allowed by the hypo-trochoid mechanism.
Keywords
Engine Modeling, Over-Expansion, Fast Combustion, High Efficiency Engine
