One Of Mankind's Most Powerful Tools
This material was excerpted from the book DeskTop Dynos, by Larry Atherton, published by Motion Software, Inc.
Motion Software simulations are not the first programs to simulate a part of the "real world." Mathematical models that simulate and predict the outcome of physical processes (like the combustion process within an internal-combustion engine, or the forces acting on a drag vehicle at the starting line) are not a modern invention. In fact, powerful simulation techniques were discovered hundreds of years ago! Until recently, however, many of these mathematical tools remained more a subject of theoretical discussion because nearly all models require countless mathematical operations to recreate complex physical process. Some can run on a simple PC, others require months of computer time on the fastest computers on earth performing trillions of mathematical operations per second. In this age of a PC on every desktop, performing simulations by hand is unthinkable, and for good reason: Many simulations would take longer to do by hand than the average human life span!
This close association between mathematical simulations and computers is a relatively recent development. Over the past 50 years, the computational speed of the computer has made the science of simulations a practical, almost "basic" scientific tool. But the need to develop mathematical models to help predict the outcome of otherwise expensive and time-consuming processes has been on the minds of designers for hundreds of years. In fact, it has been the dream of engine builders--for as long as there have been engines--to be able to assemble a "paper" engine, an accurate model developed through mathematical simulations. Done properly, this simulated "paper" engine could quickly evaluate new ideas, shorten development times, save money, and give a competitive edge to any engine manufacturing company.
Early Modeling Efforts
The history of the evolution of the "paper" engine is not well documented. But the first attempts date back to before Mr. Otto first applied the 4-cycle process to the internal combustion engine in 1867. From these earliest days in the development of the IC (internal combustion) engine, it has been understood that analysis would be based on one or more of the following models: fluid-dynamics (mass flow through passages and restrictions), thermodynamics (energy conversion into heat during combustion), heat-transfer (energy loss to cylinder walls, the exhaust system, etc.), and kinetics (losses in mechanical systems, i.e., friction, etc.). However, by the late 1800ís, engine developers were only just beginning to acquire a detailed knowledge of these processes, particularly as they applied to the four-cycle engine. Combined with the lack of powerful mathematical tools to analyze wave motion, describe combustion, and predict other physical phenomenon, the attempts at fundamental engine modeling were limited to very simplified models.
Zero- And Quasi-Dimensional Models
Theoreticians of the late 1800ís and early 1900ís focused on one or two of the basic four models (fluid-dynamic, thermodynamic, heat-transfer, and kinetic). Often just a thermodynamic analysis of the heat released during combustion, and the subsequent pressure rise in the cylinder, would make up the main model. This analysis used the well-understood laws of conservation of mass and energy to predict cylinder pressures and power output. These zero-dimensional methods ("zero" here means based on a single model) lacked adequate burn, kinetic, and fluid-dynamic models, and they offered only limited predictive ability. The best that can be said of these relatively "simple" models is that they could be calculated by hand, without requiring the yet-to-be-invented electronic computer, within a reasonable period of time.
Multidimensional Models: Quasi-Steady Flow
In an quest to improve the accuracy and usability of IC engine models, the next step took developers first into the world of port, runner, and manifold modeling, and then into a complex analysis of finite-amplitude waves that travel within induction and exhaust passages. Finite-amplitude wave prediction requires extremely complex mathematical models. Because of these computational difficulties, mass-flow calculations used in the first multidimensional IC engine models are based on a steady or quasi-steady flow through a series of "restrictions." These flow restrictions, representing the air cleaner, throttle, runners, valves, and exhaust system, lend themselves to a much simpler mathematical analysis.
Quasi-steady models assume that the flow of air/fuel and exhaust gasses pass through a series of separate volumes--defined by the ports, valves, and cylinders--with no accumulation of mass within any single volume. By their nature, quasi-steady models do not incorporate a means to calculate the dynamic changes in volumetric efficiency (VE) that occur as engine speed increases. Instead, these models "look up" an empirically determined VE (usually from a table) to come up with "realistic" power numbers. Quasi-steady methods are conceptually and computationally simple, and for this reason they continue to be used in several power-predicting software programs sold today (not by Motion Software). Models based on quasi-steady methods inevitably suffer from a lack of accuracy. This is most obvious when engines are tested that fall outside their internal models and look-up tables.
Quasi-steady models are not able to calculate the changing pressures and gas volumes in the ports and cylinders. Without this information it is impossible to accurately predict the effects of variations in cam timing, heat transfer to the charge, back flow into the intake system (reversion), and flow friction with the passage walls. All of these effects vary with engine speed. Some are more important at low speeds, such as charge heating, and others have a more pronounced effect at high speeds, like resistance to flow (and the consequent decrease in VE). By dividing the intake and exhaust passages into a finite series of sections, and analyzing the mass flow into and out of each section at each degree of crank rotation, a clearer picture of the pressures within the system can be obtained. This method, called Filling-And-Emptying, can accurately predict average pressures within sections of the intake and exhaust system, but cannot account for variations in pressure within these sections due to gas dynamic effects.
Despite their inability to predict gas-dynamics, filling-and-emptying techniques are substantially more accurate and information rich than quasi-steady models. These improvements are due, in most part, to the time-step analysis used to predict pressures and flow volumes. This technique requires filling-and-emptying models to perform their computations over and over, calculating pressures at each degree (typically) of crank rotation throughout the full four-cycle process. This degree-by-degree analysis accounts for the modelís sensitivity to changes in cam timing, volume in the intake and exhaust systems, and valve and port configurations. The models can even analyze transient effects that occur when the throttle is opened and closed.
Filling-and-emptying models repeat this entire flow analysis for each rpm point at which horsepower and torque are to be predicted. As a result, computer programs that use this technique commonly "build up" engine power curves by calculating and then drawing them one power point at a time. The math needed to predict each power value involves several million calculations. Despite these oppressive computational requirements, a modern PC equipped with a math co-processor can develop an entire power curve, consisting of 10 to 15 data points, in less than a second.
Surprisingly, all of the modeling methods described thus far had been discovered by 1900, although their use beyond scientific circles was limited. This was particularly true for calculation-intensive methods. Despite the lack of wide acceptance and application of even the simple modeling systems, scientists realized that an accurate description of the IC engine would only be possible when the pressure waves that moved within the engineís internal passages could be described and predicted mathematically.
IC engine development got its biggest boost in history during World War II. The IC engine was a strategic element in the transportation of goods, as a motive force in all forms of combat vehicles, and it was the most crucial element in fighter aircraft of the day. Aerial combat during W.W.II, much like today, was fast, furious, and very dangerous. Pilots needed every advantage to defeat their opponents. If the enemy plane was more powerful and could outmaneuver, out-climb, or outrun his aircraft, he was at a serious disadvantage. And that was too often the case for many British fighters early in the war; they were simply out-horsepowered by German technology. On top of this, the process of engine development, to a large extent still dependent on empirical testing, was taking considerable time and that was costing more lives. Attention was turned in earnest toward building practical mathematical models that could be used to reliably evaluate "paper engine" concepts.
The theoretical and practical work done during the war would prove to have a substantial impact on IC engine design and modeling work for many years to come. Hundreds of mechanical components and power-optimizing techniques, such as roller cam followers, turbocharging, and nitrous-oxide injection systems, had become reliable methods of adding considerable horsepower to fighter aircraft. Many of these were later rediscovered by auto enthusiasts and racers; the most successful applications of this "new" technology often drew on the research performed between 1930 and 1945--the most frenetic years of World War II.
After the war, the emphasis on the piston-driven engine turned toward the newly discovered jet engine that would soon redefine aircraft powerplants. Despite this turn of events, the analysis of the piston engine and finite-amplitude waves continued, albeit at a substantially reduced pace. In 1964, a comprehensive numerical solution to the complex finite-amplitude wave equations was discovered (this method transformed unsolvable hyperbolic partial-differential equations into "ordinary" solvable differential equations). Called the Mesh Method, this technique was a distinctly superior to earlier methods and even extended the analysis to multicylinder models.
Within a few years, wave-dynamic solutions were fashioned into engine modeling programs and used by auto and motorcycle manufacturers throughout the world. Virtually all of these initial efforts were developed by corporate engineers working in cooperation with university research staff. The software was written for mainframe computers at a cost of hundreds of thousands to millions of dollars, and it featured the addition of accurate and comprehensive thermodynamic, combustion, kinetic, and frictional models to the Mesh-Method of analysis. For the first time, computer models existed that were powerful enough to build a "paper engine"; a practical and quintessential step in the development of any modern IC powerplant. Computer analysis using these and other advanced methods have made possible the impressive power, wide torque bands, and low emissions that are commonplace in modern IC engines.
Modern Engine Modeling Meets The PC
By 1972, an event had paved the way for an explosion in the development and widespread application of engine simulations, vehicle dynamic analysis, and numerous other mathematical models. This event, seemingly insignificant at the time, was the invention of the microprocessor; the entire "thinking core" of computer on a single chip of silicon. This innovation occurred shortly after similar reductions in the size of memory and other logic circuits that are needed to build an entire computer system.
In 1981, IBM introduced the IBM PC. While not a technical milestone, it contributed tremendous credibility to the desktop computer concept, and within a few years, the PC was a standard fixture in many businesses, schools, and laboratories. The original PC used a 4-MHz, 16-bit microprocessor (the Intel 8088) that could be equipped with a numeric coprocessor. This add-on math powerhouse sped up calculations by 20 to 100 times. A coprocessor-equipped PC could now grind through the complex math required for engine simulations that just 10 years previously required a multimillion dollar mainframe; an amazing accomplishment for an innocuous-looking box sitting on a desktop! Now, for the first time in history, incredible computing power was available to the general public.
The Future Of IC Engine Modeling
A quick visual comparison of the technology found in engine compartments of typical 1970ís versus 2000ís vehicles is clear proof that auto manufacturers are using the once-unsolvable theories of wave dynamics as just another "tool" to improve Ottoís remarkable 4-cycle design. As of today, it can be unequivocally stated that everyone involved in engine design (for subsequent manufacturing) is using computer-aided simulation technology. Top racing teams competing in NHRA, Nascar, and many other forms of racing have also begun to use advanced engine modeling tools, like Dynomation-5, along with data acquisition systems to thoroughly analyze and predict engine and chassis function. Remarkably, all of this innovation just represents the beginning. The leading edge in engine simulation is continuing to move into more efficient, complete and complex models. As these tools are refined--and made more practical by the ever-increasing power of both large and small computers--still higher levels of sophistication and accuracy will be possible.
While the engine simulation models under development today will set standards for sophistication and accuracy, you may be curious where this technology is headed? The ultimate goal will be to build virtual vehicles, including the engines, drivelines, chassis, and all the electrical and hydraulic components. The mathematical models of a complete vehicle, you might call it a "paper vehicle," would be built, tested, refined, and perfected entirely within the virtual world of the computer. These simulations, many parts of which are already in use today, provide the new car buyer thoroughly debugged models that offer an unprecedented level of reliability, performance, economy, safety, and comfort. And considering the rate at which advances are now proceeding, many of you reading this may have the unique experience of buying and driving a vehicle that, while the odometer reads nearly zero, was already "road tested" by a computer several million miles before a single bolt was tightened.
This material was
excerpted from the book
Dynos, by Larry Atherton, published by Motion
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