Quote:
Originally Posted by Sony
3,000psi sounds about right. The DI system has to inject fuel into the cylinder at full compression at the top of the stroke when cylinder pressures are at their highest.
|
Just wanted to point out a small detail, in concept this is how you might think DI should work but it depends on the particular system and the operating conditions. Also the fuel should never be injected at the top of the stroke otherwise there would be no time for the fuel to mix with air before ignition, I believe the direct injectors on these Toyota systems fire halfwayish up the compression stroke, but it's possible that they fire on the intake stroke too (I can't quite recall).
Diesels are different because they don't inject fuel then ignite it, the two processes happen simultaneously so the diesel injector has to be able to supply fuel at maximum cylinder pressure which is much higher than even the pressure at the end of the compression cycle.
The high pressure is not only because you need to get the fuel straight into the cylinder instead of just pump a bunch into the intake tract (which has more room), but also because the shorter amount of time the fuel has to mix with the air means you want it to vaporize more quickly.
Aside from less heat soaking into the fuel from the valve and head, another benefit is having fuel go straight to the piston, so that less ends up on the cylinder walls, the piston gets a little cooling, and more fuel is concentrated around the spark plug.