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Peak power is at high rpm in piston engines, but peak efficient power is at low rpm (generally between 1000 and 2000 rpm). The reason car makers are stuffing s
by theluketaylor 8y ago
Peak power is at high rpm in piston engines, but peak efficient power is at low rpm (generally between 1000 and 2000 rpm).
The reason car makers are stuffing so many gears into transmissions now is to keep the engine at peak efficiency far more often (to meet emissions and fuel economy targets).
Trying to hit top speed needs power (mostly due to wind resistance going up by its own square law) not efficiency. So the transmission will use the tallest non-overdrive gear (6th) for top speed and the overdrive gears (in this case 7th and 8th) for efficient cruising.
- alexanderwept 8y ago>but peak efficient power is at low rpm (generally between 1000 and 2000 rpm). I've yet to find a car engine that has an efficiency peak between 1k and 2k rpm. The Skyactiv Mazda engine gets closest [0]. But most engines seem to have peak BSFC efficiency below their torque peak. [1] [0]: https://www.researchgate.net/figure/Base-20L-Mazda-SkyActiv-131-CR-engine-maps-of-BSFC-left-and-BTE-right-from_fig12_301242888 https://www.researchgate.net/figure/Base-20L-Mazda-SkyActiv-... [1]: https://www.google.com/search?q=bsfc+maps&safe=strict&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjX6reH86zfAhWCiFQKHSjhCrAQ_AUIECgD&biw=1263&bih=742&dpr=1.1#imgrc=maUaj004j3VAZM https://www.google.com/search?q=bsfc+maps&safe=strict&source...:
- hinkley 8y agoSo I'm partly stuck in 5 gear thinking. The gears are a lot closer together. That said, Porsche's data sheet for the current 911 says peak power is at 6400 rpm, so the ratio in rpms between 'go fast' and 'cruise' is in fact a lot wider than on an older, cheaper vehicle, where cruise might be 1800 while you pass at something around 4400-5000 rpms.