1994-03-01

Study About Burned Gas Fraction and Its Relationship with Engine Performance at Low Loads 940215

In this paper the results and main conclusions of a study concerning BGF at low loads, its relationship with valve timing and load, and its influence on engine instability and emissions, are presented.
The study covered a wide range of load and valve timings for both intake and exhaust valves. Measured values of engine performance, emissions and instability were used. BGF was estimated by a global engine code based on a full wave action model.
The good correlation found between calculated BGF with measured parameters proves the capability of wave action models to predict the influence of design modifications on engine performance at low loads.
A very interesting relationship, found among BGF, volumetric efficiency, IVO and permeability is presented. As well, the relationship between BGF and manifold pressure, with engine instability and NOx emissions are discussed and a correction of the manifold pressure which describes very well both aspects of engine performance is presented.
This study deals with the analysis of the influence of load and valve timing on Burned Gas Fraction, and of the relationship between BGF and engine performance, in a 4 valves per cylinder engine running at low loads. The high effective section offered by the 4 valves during the overlap period produces greater BGF in this kind of engine.
The study has covered a wide range of valve timing and valve overlap, and has been carried out for a 4 cylinder, 16 valve, 1.8 litre RENAULT engine running at 2.000 rpm. and for two different loads, of 3 and 0.7 bars respectively. These two points try to be representative of the low load operation.
Tests on the engine bench were carried out by RENAULT, measuring engine performance, emissions, and instability. Then BGF was calculated for exactly the same conditions as tested by a global engine code.
The study had two objectives: first to assess if BGF calculated by wave action models could be used to predict influence of design modifications, especially timing, on engine performance at low loads, and second to find the possible relationship existing among: BGF, timing, load, instability and emissions.
Very interesting works have been reported concerning this subject. At least two of them should be referenced in this paper because their scopes are similar to this [1-2] although in the present work focus has been placed more in the understanding of the involved phenomena, than in the results.
This study was part of a research project financed by the Engines Research Dept. of the Direction des Etudes of RENAULT and carried out by the Universidad Politécnica de Valencia (Spain) during 1.990. The authors wish to thank the company for their support.

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