Cam timing vs. injection timing/duty cycle.

[RomanV]

This is one thing that I've always wondered. lets say that you've got a 290 degree intake cam. eg. 290 degrees of the 720 degrees of a 4 stroke engine, the intake valve is open. Which is about 40% of the time. It makes sense to me, that you'd want to inject the fuel towards the back of the valves in that cylinder, during this time.
So why is it better to have injectors with longer duty cycles instead? I know that it's for better fuel atomisation or whatever, but it doesnt make sense to me. Atomising the fuel with the air is no good, if it's sitting behind a shut valve for 60% of the stroke!

[sergei]

because of inertia of injector internals plus delay of the solenoid magnetising you are limited how fast you can open injectors. With large injectors on low load it becomes very difficult to control amount of fuel, becuase say if 10ms needed for injector to be open and it takes say 5ms for it to open and another 5 ms for it to close, the injector will not spray any fuel. Well I think you get the idea.

[fivebob]

because of inertia of injector internals plus delay of the solenoid magnetising you are limited how fast you can open injectors. With large injectors on low load it becomes very difficult to control amount of fuel, becuase say if 10ms needed for injector to be open and it takes say 5ms for it to open and another 5 ms for it to close, the injector will not spray any fuel. Well I think you get the idea.

This is totally irrelevant to the problem of idling large injectors. The injector dead time is known, and is factored in, so it has no effect on the injector open time. The biggest problem with idling large injectors is supplying enough current to open them fast and controlling the injector flyback voltage to make sure the close when you want them to. If the ECU can't do this then the injector open time becomes erratic.

The only time injector dead time becomes a problem is at the other end of the scale, with a typical injector dead time of 0.8ms at 7000rpm, the injector never gets a chance to close at 90% duty cycle, so injector flutter become a problem.

To answer the original question, spraying on the back of the closed valve helps to cool it, but too much and the fuel will have poor atomistation. Which is why it's recommended to size the injectors to be at around 65% duty cycle at peak HP.

[method]

Hmm this is quite interesting.

I have my whole bank of injectors hooked up to one signal. They all go off when the ecu earths the #10 injector pin. Nothing happens when it earths the #20 pin. Ive always wondered how this would affect my cars power.
Fuel economy isn't much of a issue. It gets driven once a month or so.