“What’s going on in there?” is my classic question pertaining to changes in combustion caused by variation of one more variables in the combustion and injection system. Even for simple changes, it would not be uncommon to have multiple answers from various engineers or combustion experts relative to the various schemas running in our brains and or various computer models used to support our thinking. Even the simplest change, such as the change in smoke related to a change in injection pressure can solicit explanations of droplet size, air entrainment, spray penetration and turbulence generation and combinations thereof.
When considering the tradeoffs between NOx, BSFC, Smoke, HC and cylinder pressure influenced friction, and after-treatment options, the questions and schemas become very complex when we consider the vast number of injection parameters timing, multiple injections, post injection, injection pressure, hole size and number, hole taper, hole rounding, injection rate shape in combination with innumerable combustion bowl dimensions describing diameter, depth, lips, and turbulence generating details as well as basic parameters of boost, EGR, and compression ratio, and charge temperature, swirl, squish and turbulence. Further complicating the schemas is the various trade-offs that exist between selected parameters and operating conditions of speed and load relative to parameters that are fixed and not variable and must be compensated by variable parameters available. The interaction of the turbocharger, responding with more boost or airflow, with poor efficient combustion further complicates the answer.
Combustion and spray modeling provides some insights and the quality of the models has improved significantly over the years. Despite the improvements and the increased use of modeling in diesel combustion development, I don’t believe it has significantly affected the combustion systems in engines today. Virtually all systems have started with a basic system that is known to work and incremental changes and experimentation extensive calibration work to optimize the system. The technical paper claiming the combustion development through computer models, when looked at closely, often show the work was supplemented by extensive single or multi-cylinder engine work with several bowl shapes and an extensive array of injector nozzle variations of hole size, spray hole number, spray angle and nozzle protrusion used to tune and validate the model with the “optimal” system being one the configurations found through experimentation. I credit a discussion I had once with Rich Winsor (John Deere – retired), that perhaps the state of modeling is more “suggestive” than “predictive”.
I am encouraged by the progress in diesel combustion and engine modeling and will discuss my views in a separate section in the site. However, a great concern I have in the industry is that engineering managements are increasingly more disconnected with the reality of the complexity of diesel combustion and assume all the answers should be quickly realized through modeling! As a result they limit the time, budget, hardware and resources engineers need to good job in optimizing and exploring new and creative solutions!
The schemas I utilized have evolved through my career through experimentation and observation in both by own work and that of others. Starting my career at Detroit Diesel, with significant involvement with unit injectors with a relatively high number of holes, high injection pressure and large shallow bowl combustion systems, the path of the evolution of my combustion schema(s) was unquestionably influenced by my starting point. Working in R&D I had the good fortune of being able to explore a wide variety of combustion and fuel system variations. Most of my work revolved around experimentation on the Series 71 2-stroke combustion and injection system an AVL 520 single cylinder research engine converted from and inline pump to an experimental common rail injection system in the later 1970’s. I also benefitted from the work going on around me on various other Detroit Diesel engines, Series 92, 53, and 149 as well as the 8.2 and eventually the Series 60. Constant debate with engineers around me, Richard Winsor, Leland Haines, Arjun Tuteja, John Pearce, and my manager Dick Hames, helped to validate or invalidate the theories. This debate extended to other competitive engines, fuel systems and combustion systems.
The sections, in the sub bullets under combustion reflect the things I have observed and the things I have concluded shaping my diesel combustion schema. Your comments are welcome!