Injector Driver Module

[Greg Hermann]

>IF   I read your analysis correctly....you forgot that is a 4 cycle engine...so
>the on time is only ~40%  on a real time basis....NO?   GENE
>
>Jose Rodriguez wrote:
>>
>>   I am designing an injector driver module. I would like your input, to
>>see if my calculations are correct, particularly in my assumption that
>>all eight injectors could be on at the same time, for up to 85% duty
>>cycle. Thanks in advance for your answers!
>>
>>   The current specs I have are:
>>
>>   Eight injectors.
>>   Injector coil resistance: 1.5 ohms (it could also be 2.3 ohms)
>>   Injector current: 4A peak, 1A hold.
>>   Duty cycle: up to 80..85%
>>   Operating ambient temp: -40C..125C
>>   Operating voltage: 4.5V to 16V.
>>
>>   I did some calculations, to estimate how much power would I need to
>>dissipate, worst case, ASSUMING ALL EIGHT INJECTORS COULD BE ON, FOR 85%
>>OF THE TIME (is this assumption correct?)
>>
>>   First, I set Vbatt=14V. In order to have 4A circulating, the driver's
>>output voltage would be (14-4*1.5)=8V. Injector's coil voltage = (14-8) =
>>6V.
>>   Instantaneous power at the driver: (8V)*(4A) = 32W. Instantaneous
>>power thru injector's coil: (6V)*(4A) = 24W.
>>   The 4A will only circulate for a brief period of time, so average
>>power over one cycle will be much less.
>>
>>   One amp, sustaining, automatically means (14*1)=14W have to be
>>dissipated between the injector driver and the injector coil (100% duty
>>cycle).
>>   In order to have the 1A sustaining, the driver's output voltage would
>>be (14-1*1.5)=12.5V. Injector's voltage =(14-12.5) = 1.5V.
>>   Inst. power at driver: (12.5V)*(1A) = 12.5W. Inst power at injector:
>>(1.5V)*(1A) = 1.5W. (total=14W, as predicted).
>>
>>   Thus, since the Duty Cycle can be up to 85%, and disregarding the
>>initial 4A power peak, and the switching power losses, then the average
>>power for each injector driver is (12.5W)*(0.85) = 10.625W. The average
>>power for each injector coil is (1.5W)*(0.85)=1.275W.
>>
>>   If all 8 injectors could be on 85% of the time, then the maximum
>>average power at Vbatt=14V (again, disregarding some power losses) would
>>be (8)*(10.625W) = 85W at the injector driver side. Obviously, at 16V the
>>power dissipation would be higher (98.6W!).
>>
>>   As you can imagine, dissipating 98.6W+ into an under-the-hood ambient
>>of 125C, with no expected air flow, is a formidable task. The heatsink's
>>surface temperature should not be allowed to go above 150C; that would
>>mean the thermal resistance from the heatsink to ambient would be
>>(150-125)/98.6 = 0.25C/W. I can not see any feasible heatsink doing that,
>>without the need for forced cooling.
>
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>
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        Depending on how universally applicable you want your driver board
to be, you might want to consider what would happen to the driver power
dissipation if it were to be applied to the same injectors at the same peak
and hold current levels, but powered from a (nominal) 24 v. electrical
system.
        You may also want to consider that there is much unexplored
potential for better BSFC in using two sets of staged injectors, neither of
which exceeds about a 25% to 30% duty cycle (the length of the intake
stroke on a four cycle engine). The primary set needs to provide enough
flow for about 50% more than cruising power (at a 25% duty cycle); the
second enough flow to get the rest of the way to maximum power (at the same
25% duty cycle). With this kind of s set-up, the primaries would be plenty
small enough to give truly repeatable flow at idle.
        I have been giving very serious consideration to building an engine
with this kind of a set-up, with the additional twiddle of putting the
primary injectors in something like a BG "venturi fuel injection" (four
barrel) body. Each (of the four) primary injectors would then be called
upon to perform TWO (up to) 25% duty cycle squirts (net 50% duty cycle for
each injector) every two engine revolutions. (Fire the injector closest to
the runner drawing mixture at the time.) The purpose of these gymnastics is
intended to be to take advantage of the graealy inproved fuel atomization
available at light loads from a set-up like the BG venturi body.
        I would suggest that anyone who doubts the positive impact of
better fuel atomization on BSFC (Brake Specific Fuel Consumption, or units
of fuel consumed in order to produce a unit of work--it relates directly to
fuel mileage) take a look at dyno BSFC numbers on an engine run with Weber
DCOE or IDA type carbs vs. the same engine run with different carbs or port
injectors at the same power levels. The idea of getting to a shorter duty
cycle on the secondary (port) injectors is pointed in the same direction:
Don't vaporize (quite a distinct process from "atomize") the fuel by
cooling parts in the intake port (that's what the cooling system is for).
Get (most of) the atomized droplets of liquid fuel into the cylinder where
their (heat of) vaporization can absorb some of the heat of compression
during the compression stroke, thus improving net output and BSFC and
allowing a higher compression ratio (for still more power and still better
BSFC) and/or better fuel tolerance.
        I didn't mean for this to turn into a lecture or speach. I'm new
here :). So far I find myself wishing I had even a third of the knowlege
I've seen demonstrated about the electronic side of things, and so I'm here
primarily to learn. However, meanwhile, if I may contribute a little
perspective as to what we're all trying to get the electronics to do for
us, I'm willing to try to contribute that if you all are willing to listen.
                                       Glad to meetcha, Greg