Pensioner, Well said and true! Howsomeever, there is yet another case. While your treatment holds for the mechanical equivalent of the steady state of continuum process, there remains the problem of instantaneous load. Taking your poignant example of the 70 mph cruise and the urgent need to pass the Explorer, the driver of which has just passed on a prurient digital comment. When the accelerator is pressed forward the requested acceleration ramps the engine load rapidly. Because of the rapid change in airflow (only talking throttled gasoline motors here) the cylinder pressures quickly go to the maximum for that rpm. Consequently, full torque is applied (or close) until the brick has accelerated enough for the driver to release the demand. Because of the high mass of the vehicle the ability to instantaneously drive the vehicle to a full available torque application is exaggerated (made more likely) with numerically lower final drive ratios. Taken to the extreme, this is a subset of the classic lugging phenomena - high sustained cylinder pressures at modest rpm. I think the probability of high instantaneous pressures is higher with a normally aspirated motor than a turbocharged one (necessary turbo lag), and higher still for diesel motors as compared to gasoline (SI) engines. Hence, I have felt that the lore that TDi engines break Vanagon transmissions is plausible. So to simply agree with your well put analysis, it is the shaft torque (or the shaft torque time spectrum) that breaks transmissions. To proffer some anecdotal evidence, I have broken two 3/4 gears in my Westfalia's diesel configuration and one in my 8V 1.8L Digifant configuration. With each repair, I was assured I'd never have another problem, because VW had improved the gear. And as I drove away from "The Transmission Supershop" I heard a deep belch as the owner digested my check and a cheery "Come back again, soon" from his soon to be college age daughter. Frank Grunthaner |
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