Vehicle Overspeed prediction & testing Applications Engineer Training
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This document explores the critical aspects of overspeed prediction and testing in vehicles, focusing on engine and pump dynamics during downhill operations. It discusses how engines can absorb power beyond norms, factors influencing engine and pump behavior, and the need for reliable data on braking power capacity. Emphasizing the risks associated with heavy vehicles, especially during deceleration, it provides practical testing guidelines in real downhill conditions. Solutions to mitigate overspeed risks utilizing microcontrollers and braking enhancements are also proposed.
Vehicle Overspeed prediction & testing Applications Engineer Training
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Presentation Transcript
Vehicle Overspeed prediction & testing Applications Engineer Training North America Application Engineering Mark Peterson May 2006
POWER Downhill Motor Speedup - - 2 reasons: • Pump becomes a motor, motor becomes a pump • Volumetric losses allow motor to go faster than theo., rather than slower • Engine speed goes up to and beyond NLHI (engine is absorbing power)~
Downhill Motor Speedup - - other considerations: • Newer engines have less braking power capability • Speeds up more on the same downhill grade • Know your terminology: • “Brake HP” does not equal “braking power capacity”!! • Many times engine mfrs. don’t have good data on braking power capacity • Engine speed “Flare up” is very difficult (impossible?) to predict before vehicle testing • 25% overspeed not uncommon on new engines • Engines can handle much more overspeed without failure than our pumps & motors can! • Just because the engine guys aren’t worried doesn’t mean we shouldn’t be ~
Pump Overspeed During Deceleration (flat ground) • Hydr. Motor won’t speed up, but pump may overspeed • Pump destrokes, becomes smaller, faster “motor” • Heavy vehicles when decelerating • If pump destrokes too fast, can overpower engine • Can control this with ramp rates or orifices ~
Highest Risk Vehicles • Relatively heavy machines with relatively small engines are most at risk • Especially if they transport at high speed • Very little engine braking capability + high weight + high speed • Ag Sprayers, Combines, Railway maintenance machines etc. ~
Downhill Testing - - How-To: • Test in “real” downhill conditions • May need to challenge your OEM to find a safe downhill site for testing • Apply at least max. vehicle weight (drawbar machines pulling loaded trailers, etc.) • Monitor Motor & Pump speeds • Do sustained full speed driving down steep hill • Also do deceleration down steep hill (check for engine & pump O/S) • Need safe test site - - Assume the vehicle will lose braking ability ~
Downhill Testing - - How-To: • Worst case for “block tipping” is usually just after cresting a hill • Near highest speed condition • Both system pressures near charge • Very little holddown force
What to do? • Select motor min. angle to avoid the “point of no return” • Use microcontroller based over speed control • Select ramp rates to avoid engine over speed during braking • In extreme cases, OEM can add braking capabilitiy: • Based on speed, microcontroller can: • Actuate engine exhaust brakes (“Jake brake”) • Add a hydraulic load (gear pump over orifice) • Apply vehicle service (not parking!) brakes • Theoretically - - not sure it’s been done before ~
