While there are a number of vehicle manufacturers that produce GDI engines the most common that I come into contact with are the Mitsubishi models, so for that reason I will focus on them for this article.
GDI or Gasoline Direct Injection is another step in the ongoing search for a more efficient use of the petrol that your car uses. It is simply an engine where the petrol is injected directly into the cylinder as opposed to the more common petrol injected car where petrol is injected into the air flow prior to it reaching the cylinder. Characteristics of the Mitsubishi GDI are the vertical inlet manifold, the rather odd piston design (see photo)and the design of the new "swirl" fuel injectors, named so because of the vortex type of spray pattern as fuel is injected into the combustion chamber. The airflow from the manifold into the cylinder produces a swirling effect in the cylinder which tumbles in the opposite direction than other engine designs and by injecting fuel at a very late stage of compression, into the bowl on top of the piston, a richer mixture can be concentrated around the spark plug to allow for proper ignition.
Mitsubishi GDI Swirl Injectors
This means that the GDI engine can, under certain "low load" conditions operate on an air/fuel ratio of about 40:1 rather than the more common 14:1. Under heavier loads fuel is injected once during the intake cycle which cools the incoming air and then again at the late stage of compression which results in a richer air/fuel mixture for more power.
Fuel delivery is another difference for the GDI, there is a standard type electric fuel pump immersed in the fuel tank just like most other fuel injected cars, but this pump instead of delivering fuel to the injectors, delivers it to a mechanical pump on the side of the engine which then increases the pressure from 45psi to approx 750psi and that is the pressure that is delivered to the injectors.
The unusual piston crown
on a Mitsubishi GDI piston
There is also an unusually high compression ratio with GDI cars 12.5:1 is common with Mitsubishi, this means that it is vital to use a higher octane fuel to avoid pre-ignition in the combustion chamber which can cause severe damage to the engine over time.
Mitsubishi claims that their GDI engines can burn fuel 35% to 40% more efficiently than engines with the more common MPFI (Multi Point Fuel Injection) systems and at the same time get an increase in power output by about 10%. Usually the increased compression ratio and the ultra lean mixtures would result in an increase in NOx emmisions but Mitsubishi have managed to keep these under control by an increased use of the EGR system and a better catalytic converter. It is the increased use of the EGR that is the main downside of this system, (theres always a downside) under some conditions approximately 30% of the intake air comes from the EGR system, thats dirty carbon-laden exhaust gases re-entering the induction stream and depositing themselves on the side-walls of the intake plenum, the ports and inside the cylinder head, to the point where the car has some serious driveability issues. (see Techdoc1-Induction Contamination)
The servicing requirements for GDI engines are similar to many other cars. They need a high quality oil and high octane petrol to keep the engine running as clean as possible which keeps the level of exhaust contaminants to a minimum, however from experience we have found that regardless of servicing the level of induction contamination is normally still quite high and therefore we recommend that all GDI's should have an induction cleaning treatment at every service. This contamination is just a fact of life with fuel injection and EGR/PCV systems, just like death and taxes you can not escape it and the fact that GDI's make such extreme use of the EGR system means that thay suffer all the more, however the regular servicing and induction cleaning should keep your car running smoothly and very economically as thay are designed to do. Here at Critchley Automotive we would be happy to talk with you further about your GDI's service requirements, just phone during normal working hours on 09 2981363 and ask for Darryl or Andrew. Back