Research Engines for Optical Diagnostics

Hannu Jääskeläinen

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• Introduction
• Sandia Optical Engine
• Diagnostic Techniques

Introduction

Modern imaging and laser technologies form the basis of many diagnostic techniques used to better understand the in-cylinder process governing engine performance and emissions in modern internal combustion engines. In order to perform many of these diagnostic techniques, a suitable apparatus that has optical access and that simulates the process from real engines is required. While much useful information can be acquired from a relatively simple closed combustion bomb-type apparatus equipped with a window, the most useful information can sometimes only be acquired from an operating engine.

The challenge with using these techniques on engines is providing suitable optical access. Optical access to some engine designs, such as older L-head designs that have their valves mounted in the cylinder block, can be readily achieved through the cylinder head. However, these engine designs are relatively archaic and the information obtained would be of little use to understanding processes in modern engines. For modern engines, the most common approach to obtaining optical access is through the piston—an elongated piston is used with a window in the crown that reciprocates over a stationary mirror. This approach was pioneered in the 1960’s by Fred Bowditch at General Motors and the piston design is often referred to as a “Bowditch Piston” [2165].

Extensive engine modifications are necessary to provide the optical access. Such extensive modifications to an engine means that it is no longer identical to the production engine it was derived from. There may also be limitations on engine operation and the engine may no longer be operated in exactly the same way as a production engine. While these differences are important, proper design can help ensure that the impact on the processes of interest is minimal. Some of the measures that may be required to accommodate optical access include:

• The engine may need to be operated in a skip-fired mode (e.g., fire the injector once every 8^th engine cycle) to reduce the required frequency of window cleaning and the risk of window failure due to thermal and mechanical stresses. Skip-firing reduces the amount of heat rejected to the coolant, so an electrical heater under closed-loop control may be required to maintain the coolant temperature. Skip-firing also reduces the concentration of residuals trapped within the cylinder from the previous fired cycle (i.e., scavenging is enhanced because there are a number of motored cycles between two consecutive fired cycles).
• The bottom of the piston bowl (top of the piston-crown window) may need to be flat rather than contoured. In addition, the compression rings may need to be positioned lower on the optical piston to allow for the placement of windows in the piston bowl-rim and to prevent the rings from riding over windows in the cylinder wall. These modifications would result in an increased clearance volume and hence a decreased compression ratio relative to the production engine (see the example in Table 1).
• The temperature and pressure of the intake mixture may need to be increased so that in-cylinder conditions are representative of those a production engine over the given crank-angle range of interest (e.g., motored top-dead-center (TDC) conditions).

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