Farmer's Weekly (South Africa)
Motoring
Modern direct injection offers advantages, but can lead to rapid carbon build-up. Jake Venter explores this problem, and takes a look at some solutions.
In the past I’ve warned against buying vehicles fitted with dual-mass flywheels as these have sometimes failed at well below 100 000km in certain car brands.
It now seems two more components have been inadequately tested and are on the list of expected early failures: dual-clutch transmissions and direct injection petrol engines manufactured over the past decade or so. The failures are mostly model-specific, so ask someone in the motor trade ( not a salesman) before committing to any particular model. This article will deal with the advantages and disadvantages of this system.
When fuel injectors originally replaced carburettors, the fuel was injected into the intake ports quite close to and behind the intake valve. This system is called indirect injection. The fuel spray impinges onto the back of the intake valve, which is beneficial, as we shall see.
Some years ago, engineers redesigned the system to inject fuel directly into the combustion chamber. This increases power output or decreases fuel consumption and harmful emissions, depending on the operating mode. The engine management system chooses between three combustion modes: • An ultra-lean air:fuel ratio as low as 70:1 for short periods during low-load and low-speed operation where acceleration is not required. The fuel is injected near the end of the compression stroke. • A homogeneous stoichiometric (chemically correct) 14,7:1 air:fuel ratio at higher loads and speeds. Fuel is injected during the intake stroke.
• At full power, fuel is injected during the intake stroke in a slightly rich air-fuel mixture, so the heat required to evaporate the extra fuel helps to reduce the combustion temperature.
Unfortunately, this clever system often results in a carbon build-up behind the intake valves, caused by the normal action of the positive crankcase ventilation (PCV) valve. All engines employ this valve, which is mounted in the wall of the intake manifold and connected to the crankcase wall via a hose. It enables the intake vacuum to suck vapour that collects above the oil in the sump back into the intake manifold to be burnt and then escape via the exhaust valves. The intake valve forms a restriction that slows the airflow just behind the valve enough to allow carbon particles to build up so that by 60 000km the engine displays power loss and rough idling.
Indirect injected engines do not have this problem. Some of the injected fuel arrives behind the intake valve in droplet form and this serves to keep that area clean.
a tough problem to fix
Five solutions are being tried: • Some manufacturers employ indirect injection into the cylinder-head ports during the stoichiometric phase and direct injection during the lean-burn phase. • Aftermarket ‘catch cans’ (oil separators) are fitted between the crankcase wall and PCV valve to catch most of the oil that would have ended up inside the intake manifold. It is unclear if this works. • Various fluids are claimed to remove the carbon, but I doubt whether any is effective.
• One proven, but expensive, option is to remove the cylinder head and spray the ports with glass beads. As an engine gets older, the volume of blow-by gas migrating past the rings increases, so that the cylinder ports have to be cleaned more frequently. • The only practical solution seems to be to change the engine oil more frequently than recommended.
Happily, most failures are associated with early direct injection layouts. The latest engines seem to be less prone to this error. • Jake Venter is a journalist and a retired engineer and mathematician. Email him at jacobventer77@gmail.com. Subject line: Auto engineering.