I heard these terms recently as part of the description of a new car. I know that there are intake and exhaust valves in the engine that are controlled by cams on the camshaft (if that helps you answer or not I don't know).

What do the terms "combined port" & "direct injection" mean?

4 Answers 4


Combined port & direct injection implies that the following injection systems are present:

  • port injection

    so the fuel injectors squirt fuel into the intake manifold, allowing air and fuel to mix before they pass the intake valve.

  • direct injection

    so fuel is injected directly into the cylinders, after the intake valve.

Direct injection enables greater fuel efficiency and reduced emissions. The trouble is that the intake valves have dry air flowing through them, which will eventually lead to issues with carbon buildup.

By utilizing port injection, the air-fuel mixture coats the intake valve to prevent carbon buildup formation.

By utilizing both port and direct injection, some vehicles get the best of both worlds - the efficiency of direct injection without the drawbacks associated with dry intake valves.


Correct me if I'm wrong guys.... Direct injection is the fuel injectors inject fuel directly into the cylinder using ratios based on the cpu. Combined is when they have injection points both in the cylinder and the intake manifold. Depending on the driving conditions the cpu will dictate where the fuel will be more efficiently injected. This would allow for fuel savings and better response according to how the vehicle is being driven.


In petrol engines Fuel injection traditionally used to happen(still happens in most commuter cars) before the intake valve meaning that the air fuel is mixed before entering the cylinder.

In direct Injection, the injector sprays the air fuel mixture directly into the cylinder thus you have optimum computer controlled combustion.

Direct Injection has a lot of advantages over the MPFI or port injection and its the way of the future.

  • It improves mileage through a stratified charge engine process(basically staying as close to the stochiometric ratio as possible this achieving better burn.
  • Lower emissions.
  • Prevents engine knocking/detonation.
  • Better control of the engine since before GDI it was only possible to manipulate valve timings in real time operation to change the efficiency/power characteristics of the engine, now since you can vary the fuel pressure,amount. there are a whole lot of possibilities to explore.
  • GDI engines are built sturdier compared to MPFI engines and last longer.
  • Better power output compared to mpfi. Diesels on the other hand have been using Direct Injection for quite a while now , because

The Main disadvantage of the GDI is that since the fuel is not coming in contact with the intake valve , the back side of the valve starts to have carbon buildup pretty soon.

To avoid this issue some manufactures us a combination of GDI and port injection like the new Lexus RC-F

enter image description here


The "biggy" effect of direct injection, is the "diesel" manner, that the ICE (internal combustion engine) then changes to, versus port-injection, or the old carburetor air-fuel behavior in the cylinder.

In a port-injector (same as like a carburetor), the cylinder compresses both the air and the tiny droplets of fuel, during the compression stroke, the fuel droplets typically boil to become fuel in full vapor state.

Fuel, like water, has a dramatic (for this blog lets guesstimate it at about a 1000/1) ratio of volume/pressure increase when changed from liquid to gas. That added fuel volume/pressure pushes back on the piston, during the later half of the compression stroke. That's a bad thing. It reduces "thermal efficiency", in a big way. That means more waste heat, and less mechanical engery per gallon of fuel.

So.. along comes Mr Diesel, and direct injection. Liquid fuel is directly injected into the cylinder, near the end of the compression cycle, but it has to be under extreme pressure to do this. And it requires an extreme fuel pump, and extreme fuel plumbing. The direct injected fuel, heats so fast, upon injection, at near the end of the compression cycle, it often does not need a electric spark, to set it off in spontaneous combustion.

And thus there is also LESS back pressure on the piston (this is the desired effect), during the majority of the compression cycle, and thus improved thermal efficiency.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .