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Development of Secondary Source of Vacuum for Diesel Engines
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TWW is a Automotive Electro-Mechanical Engineer, Hydraulic Design Engineer with world-class expertise in electro-mechanical, mechanical, and hydraulic devices, components and systems, and automotive components
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As diesel engines became more popular in car usage, they became options in existing car lines. Most if not all of these lines had existing systems / componentry that relied upon vacuum as the source of power. These systems included HVAC controls and the like. As diesel engines do not produce vacuum at their inlet as do gasoline engines, a secondary source of vacuum was needed in order to drive the systems.
Various concepts were studied, which included a secondary nozzle-diffuser system, wherein the high gas flow of the diesel exhaust was taken advantage of by sending said flow to a nozzle which had a throat that caused the gas flow to reach sub-atmospheric pressure. At that point, a secondary nozzle was positioned to induce a flow from a channel which was hooked via plumbing to the control systems. As that point saw sub-atmospheric (vacuum), the unit created the necessary vacuum level to drive the control systems. The combined gas flow (from the control system and from the driving exhaust) was then run through a diffuser to get it back to normal pressure, which then flowed out of the vehicle tailpipe. Such a system works simply and has no possibility of durability issues if is made from a material that withstands the exhaust components. Its drawback is that it is not overly efficient, and from a Work point of view, needs a fair amount of Work input (read high driving pressure) to achieve the vacuum levels desired. It was rejected as a candidate due to the unacceptable amount of driving pressure (and hence the exhaust gas restriction placed on the diesel engine).
A more mundane system was ultimately selected, that being a vacuum pump design. This pump utilized a rolling diaphragm seal on the piston, two elastomeric check valves, and the necessary housing, pushrod, inlet tube, etc. It was a very cost effective design as there was no need for any tight manufacturing tolerances (the only machined surface was the pushrod bushing bore in the housing (press fit)).
The pump needed to have its pushrod connected to an in/out mechanism in order to drive its piston. Also, the pushrod / pushrod bushing benefitted from a good lubrication source. The driving mechanism design was then a simple cam on a rotating shaft, said shaft spun by an existing gearset in the engine. One point of novelty was that the cam was simply an offset round, which had installed on it a roller bearing with a heavier than normal outer race. The outer race then ran against the pushrod. Operation of the device caused a precession of the bearing, so that the out race had uniform wear upon it. This was a bullet proof approach, and there was no history of any bearing failure in the field.
Lastly, another novelty was the source of the lubrication mentioned above. The drive mechanism had a gear at its bottom which meshed with an existing gearset in the engine, and this gearset had a very good oil bath. The drive mechanism gear then had a simple conical shape on its upper end, which ran closely to a cast ‘wiper’ on the drive mechanism housing, which caused the engine oil at the gearset to be pushed towards the shaft upon which the gear was mounted. This lubricated the shaft lower bearing; the trick was to induce the oil to reach the other end of the shaft, its bearing, and subsequently the cam bearing and the pushrod bushing (mentioned above). A simple spiral groove was rolled on the outer surface of the drive shaft, and this acted like an auger pump, causing oil to flow upwards (vertically) as needed. Hence, lubrication was never an issue.
The pump and drive assembly was released for production and many millions were (and still are) manufactured.
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Kevin Kennedy & Associates, Inc.
Rapid Response Engineering® Solutions
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