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Another pandemic year, but still I've managed to get out for 1514 km so far. Not half bad. The Granfondo Axel Merckx is usually in the middle of July but this year it's in late September. I should have over 3000 km under the wheels by then!
The Smart fortwo MHD (mainly UK versions of the 451) starting system is unusual - it uses a system developed by Valeo called i-StARS but Smart call this system Micro Hybrid Drive where the engine can be stopped during certain vehicle conditions (to save fuel) and then instantly restarted to continue driving, The Valeo i-StARS system combines the starting and the engine alternator functions into one unit. This means that the starter is the alternator, the serpentine drive belt is made larger capacity than normal (to cope with starting the engine) and it is wider and has 6 grooves and the water pump pully has similar grooves. For this reason, the MHD alternator serpentine belt is subject to very high stresses and failure rate is very high, usually with terminal engine overheat problems (water pump stops) as drivers carry on using the vehicle despite the alternator warning light being on. Initial failure is indicated with frayed edges on the belt if you look carefully.
In essence, the MHD starting system needs 3 phase power to start the engine which is generated electronically by a small unit (called a StARS controller) mounted under the middle of the car near the petrol tank (looks like an oblong aluminium hedgehog), and connected to the alternator by a 3 phase special cable (which needs very good continuity less than 3.1milli Ohm per phase) other connections are also in place. Starting the engine uses the StARS controller with its MOSFET transistors when the battery has enough power to supply (typically 120A or more) for starting electronically switching the 12V from the battery electronically. The battery monitoring system has a important role and on MHD cars, the typically charge voltage is over 14V not 12V to cope with stop/start situations which is higher than normal cars.
In alternator mode, the controller collects current from the stator phases, and converts the alternate AC 14V signals with Metal Oxide Semiconductor Field Effect Transistors (MOSFET’s) via synchronous rectification. The so called synchronous rectification also known as active rectification replaces the conventional power diode rectifier for a much higher efficiency.
The use of MOSFET’s to achieve the rectification function conventionally performed by diodes, improves efficiency, thermal performance, power density, and reliability. Higher efficiencies can be achieved by synchronously switching the power transistors to emulate diodes,
taking advantage of the transistors’ low conduction losses. Synchronous switching means getting a transistor to turn on and off according to the polarity of the AC waveform so it acts as a rectifier. Choosing high performance MOSFET transistors reduces the forward voltage drop to a fraction of what any diode can achieve. Hence, the synchronous rectifier will have a much lower loss than a diode, improving the alternator’s efficiency. Synchronous rectification is one of the reasons for the outstanding 82% efficiency of the StARS starter alternator, which is a full 10% higher than the best conventional alternators on the market.
The same MOSFET bridge is used to control the machine in alternator and starter modes. In alternator mode the transistor bridge receives
power from the stator phases, in starter mode it is the opposite situation. In starter mode, the machine behaves as a three phase motor, it reacts to interactions between the stator and the rotor electromagnetic forces and rotates accordingly.
As in the alternator mode, in starter mode the alternator rotor coil is energized. The power unit drives the stator phases according to a hall-effect magnetic position sensor that gives at any time the rotor relative position from the stator, knowing this relative position is crucial to the machine performance, notably for the torque output. Stator and rotor electromagnetic forces interact creating a rotation. The MOSFET bridges stimulate the stator phases generating a rotating field and thus torque to the rotor. The rotor angular position and speed, as well
as the phase currents are monitored and processed to optimize the machine torque output. Temperatures are monitored to take benefit from a maximum energy conversion.
The position sensor consists in three devices (H1, H2, H3), they are each 120° shifted and require a high positioning accuracy for the machine to deliver the appropriate torque output with minimized ripple. The MOSFET half bridges are activated in multiple commutation sequences to create a 360° rotation.
●● Stator phases are excited two by two in a 360° sequence, triggering a rotating field
●● The rotor aligns to the stator magnetic field for each new sequence generating torque
●● The torque amplitude varies according to the position of the rotor, it is higher at each half bridges commutation and stator/rotor alignment
●● The machine output torque is maximized thanks to advanced control timings and communications overlapping on the MOSFET half bridges.
In StARS, the MOSFET bridge is capable of delivering a 600A starting current, generating enough torque to drive the engine immediately and at higher revs than a conventional starter. The battery needs to be the special AGM type or similar.
Serpentine belt tension is also very important in MHD cars. It is much higher than a standard car and adds additional loads to the alternator mounting points (which leads to premature failures or looseness) so needs careful adjustment and special tension pulley to control load on, load off (starting/generating).
Starter cranking power achieved using this i-StARS system in the MHD is1.2kW which is better than the 700A or more required by the traditional starter.
For many reasons, many UK version Smart fortwo MHD cars have had problems with the starting system, mainly associated with poor alternator mounting hardware (fix is available on eBay to increase mounting bolt sizes). A simple system fix is to revert to using and adding a standard 12V starter from a CDI car (the 12V starter enable output is still available from the SAM panel on connector N11-6 pin 1). I have done this myself (and fitted a starter to the aux plate above the clutch actuator but the result is that you need very a high capacity battery to deliver the 700A needed and you still need the i-StARS controller to provide battery charge voltage from the alternator during engine running. The red 'battery not charging' indicator is provided by the engine management computer as well to the instrument cluster, so adding a standard alternator will not be useful. Smart fortwo standard 12V starters have a few different versions, so some are clockwise and others anticlockwise so choose carefully.
The i-StARS on the MHD controller can be interrogated by an OBD 2 diagnostic system like Launch, and certain faults are not obvious from the codes read. When the system reports the the 3-phase line of the RSG is faulty, the code 100D is more associated with poor main battery performance. Probably that the current is not able to supply to the alternator for starting, although the resistance figures are very low for the cable, so measuring this ohmic value accurately is tricky.
The end result is that advice is to not buy an MHD Smart fortwo as they are not reliable. Stick with the standard cars! In the UK, many dealers run a mile when you ask them about starting problems on the MHD, you have been warned, and for those who want to convert their cars to MHD - dream on!!!
One thing I forgot, laser alignment is best checked for the serpentine belt on the MHD Smarts, this is crucial for reliability of the system. I use home-made laser tools for this for the crank, alternator and water pump.
My car is Smart fortwo 2014 ev,
I faced alot messages and its didnt start, the messages is 1-( charge calbe connected) 2-( hv system workshop) 3-( drive sys engin off)... Any one can help me.. am from jordan theres no dealer ship in my countrey