Injector sizing

[Tlsalt at aol.com]

Stewart Prince wrote:

It is true that the pulse width would be longer at peak torque, but since 
peak torque always occurs at an rpm lower than peak power, the injector duty 
cycle is actually LOWER here, thus one should size the injector at peak 
power, not peak torque.

Hello Stewart:

    The reason I raised the question was that these formulas seem to ignore 
large variations in torque.  The example I used (actual production engines), 
two engines, one NA one SC,  same displacement, same hp, SC engine produces 
over 50% greater torque (350 vs 230)  The only factor in the formulas to 
account for the torque increase is the assumed BSFC. The formulas are based 
on fuel flow at peak hp, which in the SC engine's case is unexpectedly low, 
and largely leaves to chance, that the pulsewidth required at peak torque for 
the ideal AFR can be accomplished within the time window.  This may only be 
an issue with long stroke engines with high piston speeds and small valves, 
as the hp is usually higher in relation to the torque.  The fuel map for this 
engine will not resemble anything I am used to.   In any event, I am 
comfortable using the hp formula to initially size the injectors and then use 
the VE based formula to see if the resulting injector size can produced the 
required AFR at peak torque within the time frame.

Paul
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