Port vs. direct injection

The fuelling technology of a petrol engine has transformed dramatically in the last couple of decades. From the humble mechanical carburettor to single-point injection, followed by multipoint and now direct injection. The enabler for the rapid evolution is the electronic control unit (ECU) and the goal is to keep increasing efficiency and power while lowering emissions.

A MULTIPOINT INJECTION

Multipoint refers to the change from a single-point injector, where the carburettor was located feeding all the intake runners to the cylinders, to an injector per intake runner. The idea is to have the injected fuel closer to the combustion chamber and so reduce response time and provide accurate fuelling for each cylinder in the correct stoichiometric air-fuel ratio. Another advantage is the fuel has enough time to mix with the incoming air before entering the combustion chamber. (Unknown at the time, the detergent properties of petrol helped to keep the intake runners and the intake valves clean from sooty deposits.) A negative of multipoint injection is the cooling effect of the evaporating fuel (latent energy) cools only the intake port and valves, and does little in cooling the intake charge on the way to the combustion chambers.

B DIRECT INJECTION

If it is beneficial to have the fuel injection as close as possible to the combustion chamber, why not have it in the chamber itself? This is why direct injection was developed where the injector tip protrudes into the combustion chamber. Response time is now increased but the amount of time for mixing air and fuel is decreased. Therefore, injector pressures are much higher and clever “tumble and swirl” techniques are used to move the intake air swiftly around the combustion chamber to promote air-fuel mixing. The main advantage is the latent energy of the evaporating fuel now cools the charge in the combustion chamber, combating the onset of auto-ignition (knock) and allowing a much higher compression ratio that increases efficiency. This is especially true when it comes to turbocharged engines where the intake air is still hot after being compressed, even after the intercooler. Note the fuel does not pass the intake valve, which is relevant to the carbon build-up problem as the cleaning effect of petrol is lost on these surfaces.

C WHAT CAUSES CARBON BUILD-UP?

Carbon is the main element found in fossil fuel and oil. Burning either causes a sooty residue. For carbon build-up to form, deposits need to be present near metal surfaces at the correct temperature. Exhaust valves run much hotter than intake valves and burn off the carbon before layers can start to form. This is not the case on the intake side. But how does carbon get to the intake side when only clean air is digested? The next section explains the paths to the intake ports for sooty particles.