Timed mechanical fuel injection

[Russell Hayter (AWA MicroElectronics)]

> 
> 
> Alfa Romeo used an interesting sequential mechanical injection system on 
> their USA cars from 1969-81. Many hours of my work has been spent making 
> these systems work. That isn't always easy. But it is an elegant analog 
> system which works well when properly tuned and adjusted [but no promises 
> about tomorrow]. Throttle response is very crisp, in part because of 
> individual throttles but also because of quick, high-pressure, -timed- 
> fuel delivery. Alfa had some experiences getting that event timing right 
> on the early systems. There have been some comments and questions on this 
> list concerning advantages of sequential injection. Is a description of 
> Alfa's system and its characteristics too boring, too off-the-track for 
> DIY-EFI?
> 
> 					--Carter 
> 
> 
> [I've been reading the list in part because of my interest in 
> building a motor control for the mechanical pump's rack {variable 
> displacement plunger control}].
> 
 Sure, go for it Carter. I've a '70 Porsche 911 with a bosch mechanical
injection system and dread the day I ever have to replace the thing.
It has the same rack and variable displacement plunger mechanism.
Those pumps aren't cheap I hear. So any news on part(just control the rack)
or full conversion to electronic control would be welcome.
 I'd also like to hear your(or any one else's) experiences on the effect of
injection timing on the torque curve. Normal setup for the 911 is for the
timed end of injection to occur at about 40degATDC on the intake stroke.
The begining of injection is varied by a 3-dimensional cam according to
the fuel requirements(throttle pos,rpm,air density-barometric cell). However
I discovered that the previous owner's mechanic had set it up 180deg out.
ie it was injecting on a closed intake valve during the combustion cycle.
It ran hard, accelerated well, good low rpm torque, felt noisey and harsh,
cruised roughly at low throttle settings - lot of transmission noise-like
the engine was slapping it at every combustion and the motor ran cool.
 When I changed the timing to standard the motor smoothed right out, was
running way too rich, bottom end torque felt lacking. Even after correcting
the mixture low rpm torque still didn't feel as strong as before. The motor
ran slightly hotter, cruised very smoothly and quietly without the
transmission noise. Fuel consumption was initially higher and only equalled
the 180deg case after mixture correction.
 My thoughts are that (and I could be totally wrong): 
	for the 40deg case:
	- at low rpm the corresponding low intake air
velocity results in poor vapourization of the tiny injected droplets,
giving slower combustion- longer flame duration and more cylinder wall
exposure to the flame-causing the higher operating temp. 
	for the 180deg case:
	- the droplets have the whole combustion and exhaust cycles in which
to vapourize to some extent on the hot closed intake valve before being
drawn into the engine. A lot may be lost to wetting on the relatively cool
intake walls, but are compensated for by richening the mixture. Richening will
also help throttle response to reduce leaning from rapid throttle opening.
The highly vapourized pocket of fuel burns rapidly-giving a short flame
duration and less heat loss to the engine.
	So if anyone has dyno figures on torque vs injection timing I'd love
to hear from them. Does anybody inject both on the closed valve during
combustion and during the intake cycle? 

		Russell Hayter
		Design Engineer, AWAmicroelectronics
		Email:	russellh at awam.com.au