06: GDI vs PFI, the injectors and mechanics

Just to reiterate the basics of GDI vs PFI, check the YouTube clip below, by EngineeringExplained.

 

 

Let’s continue to discuss GDI vs PFI…

In post 05 we looked at a few basic Pros and Cons when looking at GDI vs PFI. What we’re looking to discuss now is a bit more detailed.

We will be looking at fuel pressures and atomisation, mixture prep, injection angles and current limitations and how to potentially overcome them.

Fuel pressure and atomisation, a definite advantage of GDI, as the fuel pressure in a GDI system are in excess of 150 Bar (2175 Psi or 15 MPa) at the injector. In a PFI system the fuel pressure can be anywhere from 2 – 6 bar (29 – 87 Psi or 0.2 – 0.6 MPa) at the injector. The reason the location is so important is because it’s not the absolute pressure that’s important, but the differential pressure across the injector. The differential pressure in combination with the design of the spray-nozzle determines the level of atomisation and the spray-pattern, which in turn determines to how homogenous the air-fuel mixture is.

In a PFI system:

In general a higher differential pressure gives greater atomisation of the fuel, droplet size of <20 ┬Ám. This droplet size allows the fuel to be transported by the air passing through and greater mixture prep. Obviously there’s a limit to how much fuel pressure one can run, due to mechanical restriction!!!

DON’T EXCEED MANUFACTURER LIMITS OF FUEL PRESSURE, or your injector might either lose spray pattern or might not open at all!!!

Shown in the picture are a number of deviations on spray patterns. The wide/narrow cone pattern is not a universally used pattern. Opting for a patterns change is possible, but you do need to keep the potential consequences in mind, such as: bad cold start, rough/unstable idle, low efficiency/high fuel consumption.Note: A lot of aftermarket injector companies supply the injector with the “Wide cone”.

Why need these patterns? Simple. If you originally have a combination of the bent and split stream spray patterns and you switch (unknowingly) to a wide cone pattern or, even worse, the narrow cone. You end up dumping a lot of fuel on the inlet port runner or hitting the port divider, instead of spraying at the back of the inlet valve, where you want it… This will result in poor fuel delivery!

In a GDI system:

You don’t have the problem of hitting the divider or dumping the fuel on the port runner wall, because you’re injecting fuel directly in the cylinder, later in the system. You do need to consider spray patterns when moving to aftermarket injectors and you need to know your system! Whether it’s a spray-guided or a wall-guided system.

We’ve found some good reads, which are shown further down this page.

 

So much for this post.

 

Later more! Hope you enjoy!

 

VPR-Engineering

 

Fuel System Pressure Increase for Enhanced Performance of GDi Multi- Hole Injection Systems

 

A High Speed Flow Visualization Study of Fuel Spray Pattern Effect on Mixture Formation in a Low Pressure Direct Injection Gasoline Engine