Freewheeling alternators - how much fuel do they save?
Discussion
DanielJames said:
Off topic: Anyone fancy explaining the difference between normal and free wheeling?
I could google yes but PH knows better in most cases.
A normal alternator is driven directly by the pulley, meaning there is always some resistance to the turning motion; although it varies depending on electrical system load. I could google yes but PH knows better in most cases.
A freewheeling alternator has a clutch between the pulley and the alternator itself, meaning that when the battery is charged and there is a low electrical load there is a much lower braking torque on the belt.
As mentioned above, this increases belt life and theoretically reduces parasitic engine load from the alternator but the latter is, in reality, very small anyway.
yorkshireegg said:
A normal alternator is driven directly by the pulley, meaning there is always some resistance to the turning motion; although it varies depending on electrical system load.
A freewheeling alternator has a clutch between the pulley and the alternator itself, meaning that when the battery is charged and there is a low electrical load there is a much lower braking torque on the belt.
As mentioned above, this increases belt life and theoretically reduces parasitic engine load from the alternator but the latter is, in reality, very small anyway.
Great explanation - thanks A freewheeling alternator has a clutch between the pulley and the alternator itself, meaning that when the battery is charged and there is a low electrical load there is a much lower braking torque on the belt.
As mentioned above, this increases belt life and theoretically reduces parasitic engine load from the alternator but the latter is, in reality, very small anyway.
yorkshireegg said:
A normal alternator is driven directly by the pulley, meaning there is always some resistance to the turning motion; although it varies depending on electrical system load.
A freewheeling alternator has a clutch between the pulley and the alternator itself, meaning that when the battery is charged and there is a low electrical load there is a much lower braking torque on the belt.
As mentioned above, this increases belt life and theoretically reduces parasitic engine load from the alternator but the latter is, in reality, very small anyway.
I'm not sure that's true. I thought the freewheeling one just had a ratchet in it - I didn't think there was a clutch.A freewheeling alternator has a clutch between the pulley and the alternator itself, meaning that when the battery is charged and there is a low electrical load there is a much lower braking torque on the belt.
As mentioned above, this increases belt life and theoretically reduces parasitic engine load from the alternator but the latter is, in reality, very small anyway.
When the battery is charged, the voltage regulator reduces/removes current from the field coils, so the alternator isn't having to work anyway and can just spin freely whether it's attached to the pulley or not.
I'm very ready to be corrected by someone though as that's only what I've read.
The reason manufacturers use a clutch on altenatirs is crank pulley is no longer solid one piece but its nadeip of 3 pieces inner ring bolts on to crank middle rubber dampner and finally outer piece. So if there is no load reducer on drive belt( clutch) working life of the new style crank pulley will significantly reduce and rubber dampner will be torn off the orher 2 parts. HTH.
orhan said:
The reason manufacturers use a clutch on altenatirs is crank pulley is no longer solid one piece but its nadeip of 3 pieces inner ring bolts on to crank middle rubber dampner and finally outer piece. So if there is no load reducer on drive belt( clutch) working life of the new style crank pulley will significantly reduce and rubber dampner will be torn off the orher 2 parts. HTH.
That explains it.Mr2Mike said:
It's not there primarily to save fuel. It's main purpose is to reduce reduce slippage, noise and increase belt life.
That's what I thought too. Modern high current alternators have a heavy rotating mass which still has significant momentum when the crank slows down. Therefore allowing the alternator pulley to freewheel reduces wear and the chance of the belt jumping off.
Here's the correct explanation:
1) Alternator with roller clutch built into pulley:
This is to de-couple the alternators rotational inertia from that of the crankshaft. Because the alternator is typically geared approx 3x faster than the crank, it's rotating inertia is also refferenced to the crank via this ratio. The "clutch" is a simple PASSIVE "roller bearing clutch" (google it). This means that when the crank shaft accelerates the alternator rotor is accelerated with it, but when the crank decellerates, the alternator can "freewheel" along on its own. This massively reduces the belt load and hence wear (important now cars have LONG service intervals and the FEAD (Front End Auxilary Drive) system is heavily loaded (high elec loading and A/C load etc)
This de-coupling of the alternator is important for two scenarios where the crankshaft undergoes rapid deceleration:
a) Upshifts, especially with DSG transmission etc: On a road car with wide gear ratios, the crankshaft is required to slow by several thousand rpm in as little as 100ms these days!
b) High BMEP with low cylinder count: As power and torque output increases from lower cylinder capacity and lower cylinder count, the crankshaft torsional vibration increases massively. in effect, everytime a cylinder fires the crank accelerates, and then slows again up the next compression. This kind of torsional activity can exceed 50krpm/s at high low low rpm, and would put the FEAD under huge loads, as it will be reacted into the alternators rotational inertia
2) Alternator with "smart charge" capability
Entirely seperate to the roller clutch for belt life maximisiation detailed above. The Battery management system uses a "colomb counting" strategy to determing the State Of Charge (SOC) of the lead acid battery, and only commands the alternator to charge it when is drops below a certain SOC (depending on a load of parameters, such as temp, vehicle speed/load etc etc). Because a claw pole alternator is in-efficient at light load (low output currents) it is more efficient to use the batteries charge storage capacity to buffer the alternators output as / when necessary. Further, this load can be used to move the engine operating point to a higher efficiency one (high BSFC point), and this brings significant benifits over a low average road load cycle like the EUDC emissions/fuel economy test cycle.
This smart charging requires no mechanical changes to the system, the alternator rotor is still turning all the time (at approx 3x engine rpm) but the system only energises the field windings in the rotor when required. Conventional "dumb" alternators simple have a regulator that targets a constant output voltage (voltage control mode), usually 14.7v. However, smart alternators are controlled in the current domain, and (usually) the engine ecu will command an output current, that is practically independant of output voltage, but is driven by battery SOC (modelled in the ecu control system)
1) Alternator with roller clutch built into pulley:
This is to de-couple the alternators rotational inertia from that of the crankshaft. Because the alternator is typically geared approx 3x faster than the crank, it's rotating inertia is also refferenced to the crank via this ratio. The "clutch" is a simple PASSIVE "roller bearing clutch" (google it). This means that when the crank shaft accelerates the alternator rotor is accelerated with it, but when the crank decellerates, the alternator can "freewheel" along on its own. This massively reduces the belt load and hence wear (important now cars have LONG service intervals and the FEAD (Front End Auxilary Drive) system is heavily loaded (high elec loading and A/C load etc)
This de-coupling of the alternator is important for two scenarios where the crankshaft undergoes rapid deceleration:
a) Upshifts, especially with DSG transmission etc: On a road car with wide gear ratios, the crankshaft is required to slow by several thousand rpm in as little as 100ms these days!
b) High BMEP with low cylinder count: As power and torque output increases from lower cylinder capacity and lower cylinder count, the crankshaft torsional vibration increases massively. in effect, everytime a cylinder fires the crank accelerates, and then slows again up the next compression. This kind of torsional activity can exceed 50krpm/s at high low low rpm, and would put the FEAD under huge loads, as it will be reacted into the alternators rotational inertia
2) Alternator with "smart charge" capability
Entirely seperate to the roller clutch for belt life maximisiation detailed above. The Battery management system uses a "colomb counting" strategy to determing the State Of Charge (SOC) of the lead acid battery, and only commands the alternator to charge it when is drops below a certain SOC (depending on a load of parameters, such as temp, vehicle speed/load etc etc). Because a claw pole alternator is in-efficient at light load (low output currents) it is more efficient to use the batteries charge storage capacity to buffer the alternators output as / when necessary. Further, this load can be used to move the engine operating point to a higher efficiency one (high BSFC point), and this brings significant benifits over a low average road load cycle like the EUDC emissions/fuel economy test cycle.
This smart charging requires no mechanical changes to the system, the alternator rotor is still turning all the time (at approx 3x engine rpm) but the system only energises the field windings in the rotor when required. Conventional "dumb" alternators simple have a regulator that targets a constant output voltage (voltage control mode), usually 14.7v. However, smart alternators are controlled in the current domain, and (usually) the engine ecu will command an output current, that is practically independant of output voltage, but is driven by battery SOC (modelled in the ecu control system)
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