|Pursuing Excellence Through Trial and Error
The start of development meant, of course, that the team would begin facing the first of many anticipated challenges. There was often fear in Kajitani's mind, as well. "I thought we might not be able to achieve it because the goal was too high," he said, recalling the many travails of project development.
It was quite difficult, for example, to balance the valve-timing lift against the load placed on the timing belt, which would increase at high engine speeds due to the spring and other factors. Although it was a problem needing a solution in order to achieve the target output, such an answer would not be easy to find. To make matters worse, they found that the low-speed single-valve timing system was not applicable because it had been patented by another company. After examining numerous countermeasures through laborious trial and error, the team decided to change the entire specification surrounding the valve operating system. Subsequently, they introduced the combination valve-timing system after reviewing the valve diameter, lift and port shape, and identifying settings to ensure sufficient output. Further, they created a lightweight driven pulley using high-density, high-strength sintered alloy, and modified its shape for reduced thickness. This resulted in a 10 percent lower moment of inertia. Through these efforts the team satisfied the requirement for timing-belt load while achieving its output objective.
Output across the full rev range was increased by widening the diameter of the intake valve from the conventional DOHC engine from 30 mm to 33 mm. Also, the team adopted valve timing and lift settings that were comparable to Honda racing engines in order to enhance volumetric efficiency. The improved output resulting from that technique actually served to improve performance at high speeds. Additionally, measures were taken to reduce intake resistance. At last, the goal was reached, with a full 160 horsepower at 7,600 rpm and a redline of 8,000.
Low-speed torque, an initial project objective, was obtained by changing the low-speed cam's setting from the traditional 35 degrees to 20/30 degrees ABDC (after bottom dead-center). This permitted the intake valve to close early, drastically improving the engine's volumetric efficiency. Since the engine now had higher efficiency at low speeds of operation, a broader torque band could be realized.
The implementation of new materials was certainly a factor in the successful application of these technologies. For example, since the VTEC engine's three cam followers must be positioned in a single bore, the camshaft offers relatively limited cam width. Therefore, the shaft must be designed to withstand high surface pressures. To achieve this, the team developed a new camshaft of cast steel. The shaft was made of new high-carbon, high-chrome cast steel alloy, which was given a combination of heat and surface treatments. As a result the unit's extreme rigidity increased its critical surface pressure by as much as 40 percent.
The exhaust valve, too, employed a newly developed material made of a nickel-based, extremely heat-resistant steel combined with molybdenum, titanium, and tungsten. Accordingly, its heat resistance was increased by 30 percent. Moreover, the larger diameter of the valve's umbrella section and its reduced stem thickness produced a drop in weight of nearly 20 percent. These ideas and effort gradually shaped a reliable VTEC engine.
|<< previous||5 of 7||next >>|