Discussion
It's the way the air volume is measured before it enters the engine. There is an aluminium tube in the inlet tract with a thin wire stretched across it's inside. This is the 'hot wire'. It's heated very hot and the incoming air cools the wire. More air makes it cooler still. The ecu keeps heating the wire up and calculates how much electricity was needed to heat the wire after it had been cooled. It's then able to compute how much air went into the engine. There's more to it then this but this is the gist of it.
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I've always wondered about this .. how does it know how hot the wire is?
I can understand passing a fixed current through the wire, then as it cools it's resistance increases so the current drops .. but if it increases the current to keep the wire temp constant where's the thermcouple to tell the ecu how hot the wire actually is? .. and if there's was a thermocouple how does it compensate for itself being cooled by the airstream? .. ie what's the reference for the temp?
I've NEVER found anyone who can succesfully explain the true working of a hotwire AFM to me
I can understand passing a fixed current through the wire, then as it cools it's resistance increases so the current drops .. but if it increases the current to keep the wire temp constant where's the thermcouple to tell the ecu how hot the wire actually is? .. and if there's was a thermocouple how does it compensate for itself being cooled by the airstream? .. ie what's the reference for the temp?
I've NEVER found anyone who can succesfully explain the true working of a hotwire AFM to me
trackcar said:
I've always wondered about this .. how does it know how hot the wire is?
I can understand passing a fixed current through the wire, then as it cools it's resistance increases so the current drops .. but if it increases the current to keep the wire temp constant where's the thermcouple to tell the ecu how hot the wire actually is? .. and if there's was a thermocouple how does it compensate for itself being cooled by the airstream? .. ie what's the reference for the temp?
I've NEVER found anyone who can succesfully explain the true working of a hotwire AFM to me
It works on a constant temperature principle, the hot wire is a part of a wheatstone bridge kept in balance by changing the heating current so that the temperature of the hot wire is a constant amount above incoming air temps. Each time incoming air temps change, the wire cools and resistance falls the bridge becomes unbalanced. The difference in voltage is sent to an amplifier, the output of which supplies the bridge. Thus heating up the hot wire and increasing it's resistance until balance is restored.
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cdt said:
Does this make it better or worse than a normal V8? Or is there no real difference?
It depends on what you have. On a MAF system it compensates for engine wear ie, it meters fuel according to air drawn into the engine. An air flapper system works from tables in the ecu. I would prefer a Hot wire system on a stockish engine and a mappable set up on a well tweaked engine.
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cdt said:
Does this make it better or worse than a normal V8? Or is there no real difference?
Hi Congratulations hope you enjoy the motor
I have No4 of the 350SE's built Jan 1990
Hot wire was used on the 350se's and later Wedges and most (all?) Chimps and Griff V8's.
TVR changed to Hotwire because LandRover, their base engine supplier changed. Less moving parts so in theory more relable but not sure that works in practice but no disadvantage.
Get a copy of the Heath bible for the Wedge and a Haynes RangeRover 1970 to 1992 for the engine, injection / Hot wire etc.
David
dickymint said:
One benefit of the hot wire system is that there isn't anything obstructing the inlet track unlike the large flapper used in the earlier system.
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but is the overrun any good
I've never had a flapper
I used hot film instead of hot wire. Boosted.
The hot wire or hot film meters measure the true mass of air bing consumed by the engine. Flappers measure the volume of air, and correct for temperature.
In this way the hot wire meter can cope with changes in altitude, barometric pressure, humidity, etc., better than the flapper system.
However the flapper system still produces very accurate fuelling, and is incredibly easy to maintain.
In this way the hot wire meter can cope with changes in altitude, barometric pressure, humidity, etc., better than the flapper system.
However the flapper system still produces very accurate fuelling, and is incredibly easy to maintain.
grahamw48 said:Does the ECU on the hot wire systems shut off fueling on the over run? If I ever encounter a problem with my flapper/ecu (ooe err misses) I intend to rip out the whole ecu and flapper and install a laptop programmable full engine management system with sensors but it must retain the pops and bangs that's more than half the fun!
No pop and bang ? 
That's half the fun.
Leo
leorest said:
No pop and bang ?
That's half the fun.
Does the ECU on the hot wire systems shut off fueling on the over run? If I ever encounter a problem with my flapper/ecu (ooe err misses) I intend to rip out the whole ecu and flapper and install a laptop programmable full engine management system with sensors but it must retain the pops and bangs that's more than half the fun!
Leo
Even in the 80's the Range Rover had fuel shutoff on overrun (flap system though) where the SD1 didn't bother.
There's a vacuum switch tapped into the manifold; when the depression reaches a certain value the switch causes a relay to change over, disconnecting the trigger pulses from the distributor, so the ECU thinks the engine has stopped and turns off the fuel... agriculturally simple, but it works...
leorest said:
No pop and bang ?
That's half the fun.
Does the ECU on the hot wire systems shut off fueling on the over run? If I ever encounter a problem with my flapper/ecu (ooe err misses) I intend to rip out the whole ecu and flapper and install a laptop programmable full engine management system with sensors but it must retain the pops and bangs that's more than half the fun!
Leo
With mapping ability you can provide excess fuel if you want to.
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