Flow and Combustion in a Hydra Direct-Injection Diesel Engine 910177

Measurements of flow, spray, combustion and performance characteristics are reported for a Hydra direct-injection diesel, based on the Ford 2.5 L, engine and equipped with a variable-swirl port, a unit fuel injector and optical access through the liner and piston. The results provide links between the pre-combustion and combustion flow and, at the same time, between purpose-built single-cylinder optical engines and multi-cylinder production engines of nearly identical combustion chamber geometry. In particular, the spray penetration was found to depend on engine speed, rather than load, with velocities up to around 260 m/s at atmospheric pressure and temperature which are reduced by a factor of 2.5 under operating conditions and seem to be unaffected by swirl. The duration of combustion was reduced with increasing swirl and ignition delay increased linearly with engine speed. The torque was almost 40% less than in the Ford engine with increased particulate emissions as a consequence of higher frictional losses and the unrealistic flat base of the fpiston bowl.

THE ADVANTAGES OFFERED by direct- and indirect-injection diesel engines in terms of fuel economy and reduced CO₂ emissions are the driving forces behind the recent increases in the proportion of diesel cars sold in Europe. Even in the areas of acceleration, noise and particulates, where diesels are inherently inferior to gasoline engines, substantial improvements have been made as a result of developments in fuel injection and better understanding of combustion, so that present legislative standards can be satisfied without extra cost.

Emission standards, are, however, increasingly more stringent and DI diesel engines for passenger cars may have difficulties in meeting requirements in terms of NO_x and particulates. This demands more research and development in the fuel injection and combustion systems of small, high-speed DI diesel engines. Detailed information about the flow, spray and combustion in engine cylinders can be obtained by laser-based techniques which allow proper validation of computer models for predicting combustion and emissions.

The requirement for optical access into the combustion chamber can be satisfied by idealised and production single-cylinder engines which are extended by transparent acrylic cylinders, e.g. [1, 2 and 3]*, metal rings with quartz windows inserted between the head and the block, e.g. [4, 5 and 6], windows in the cylinder head, e.g. [7, 8 and 9], windows in both the liner and the piston-bowl wall [10] and even transparent sapphire cylinders [11,12] allowing full access into the combustion chamber under both motored and firing operation. The aim of these investigations is to allow parametric studies to be performed concerning the effect of engine design on flow and combustion and to provide boundary conditions and validation data for computer models. In some cases, the research engines are modified production engines and reflect closely existing or new design concepts but in most cases the differences between the research and production engine configurations and the corresponding operating conditions are such that quantitative extrapolation of the single-cylinder tests is not straightforward.

This paper presents the second part of an investigation into the flow, combustion and performance of a purpose-built Ricardo Hydra based on the Ford 2.5L high-speed DI diesel engine. In using this engine, the aim was to reproduce the geometry of a well-tested production engine, but introduce research tools such as a variable-swirl port and a unit injector for tuning the engine at various speeds and loads. The first part reported in [13] described in detail the flow characteristics in the engine cylinder during induction and compression. This second part includes the spray investigation and combustion and compares the performance of the optical engine with that of the baseline single-cylinder and the production multi-cylinder Ford engine. The engine test-bed and laser instrumentation are described in the next section followed by presentation and discussion of the results and a summary of the main conclusions.