Bosch’s Unit Injector System.
My car has a direct injection engine.
At the time I bought it, direct fuel injection technology was claimed to be state-of-the-art, offering better power and fuel economy.
Now, the latest version of my car has a modified injection system with an additional injector mounted in the intake passage of each cylinder.
In other words, it has both direct and the traditional manifold injection (port fuel injection).
Does this mean that direct injection is flawed in some way?
Before we continue, let us first define direct injection.
What is direct fuel injection?
The fuel injectors are mounted in the cylinder head, where they spray fuel into the combustion chamber.
The advantage of this is that fuel can be metered more accurately, timed more precisely and the spray directed for maximum combustion efficiency.
Engines with direct injection are indeed more fuel-efficient and produce more power than an equivalent engine with port fuel injection (manifold injection).
What are the downsides of direct injection?
At the end of combustion, there is a split second when the intake valve opens, allowing a small amount of blowback.
Over time, this causes a build-up of carbon around the stem of the intake valve.
Carbon sticks quite stubbornly to the valve and, ultimately, affects the air flow through the intake.
The usual symptom is unsteady idling but, more significantly, there will be an increase in fuel consumption plus a slight drop in engine performance.
What advantage does port injection have over direct injection? Why is it used in a direct injection engine?
To overcome the problem of carbon buildup, engineers incorporated a secondary injector in the manifold
This enables air mixed with a small quantity of fuel to flow past the valve stem, cleaning away any blowback deposits in the process.
Petrol can do this because it normally contains additives that help dislodge carbon buildup
The carbon is then carried into the combustion chamber (cylinder) where it is burned off.
How often should carbon clearing be performed on a direct injection engine?
Removing carbon on high-mileage direct-injected engines is necessary at around 80,000km.
On some engines, it may be possible to clean the intake valve stems by removing the intake manifold and clearing the carbon through the port.
However, this may only partially remove the carbon buildup.
For a complete job, it would be better to remove the cylinder heads.
If you’re not sure how to do this, ask a competent mechanic or trusted workshop to perform this process.
Excessive carbon buildup can severely impact performance and efficiency, and cause knocking and pinging, too.
