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Unread 14-12-2004, 13:28
Unsung FIRST Hero
Al Skierkiewicz Al Skierkiewicz is offline
Broadcast Eng/Chief Robot Inspector
AKA: Big Al WFFA 2005
FRC #0111 (WildStang)
Team Role: Engineer
 
Join Date: Jun 2001
Rookie Year: 1996
Location: Wheeling, IL
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Re: Shifting Efficiently

Quote:
Originally Posted by JessR

3. The optimum efficiency for a given motor is at lowest rpm that still gives good combustion efficiency. Crossover of the friction loss & combustion efficiency curves. Motor's with good torque at low speed, good combustion effieciency and low thermal losses win.

...Any FIRST engineers out there from the motor city with some more detail? Hey speak out FORD, GM, Daimler guys!
I checked with a knowledgeable person on this subject and I will try and sum up what I think I heard. Jess is right on the subjects so far. My friend also clued me into modern drivetrain design and efficiencies and suggested that the lower the engine speed and the higher the gear tend to be the most efficient but are vastly affected by the control computers cutting in to modify operation (i.e. trans and engine). For the most part the design shift point takes into account all of the variables mentioned above with the addition of the designers choice for reserve power, i.e. being able to speed up without having to downshift. ( I am going to make a vast departure here and make an assumption.) I am guessing that in high gear at say 60-70 MPH is likely the engine RPM at it's most efficient point. For most cars I have driven (other than a Mazda RX 4) that seems to be in the 3000-4000 range the same as Jess pointed out above. My Honda and Toyota four bangers in the 80's shifted lower than that (about 2500-3000) but didn't have a lot in the way of engine mod and emmission control.
Now my dad ( a mechanic) always taught me to feel and listen to the engine, it's trying to talk to you. If you shift too soon, the engine balks (a lot of power vibration) until you speed up (more RPM). Shift too late and the engine is screaming at you to shift. At the right point, the engine is happy, the trans is happy and you are happy.

On my RX 4, 4000 RPM=70 MPH, red line at 7000, 10 MPH/500RPM you do the math. I never found a point where power fell off. Oh WOW!
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Good Luck All. Learn something new, everyday!
Al
WB9UVJ
www.wildstang.org
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Storming the Tower since 1996.

Last edited by Al Skierkiewicz : 14-12-2004 at 13:33.