Direct Injection - the good and the bad

Dealing the with persistent carbon deposits – the unseen menace of Direct Injection fuel systems.

We often forget just how potent petrol and diesel fuels are for their intended purpose. The amount of energy that is packed into a relatively small quantity of liquid is genuinely remarkable.

Complex refining and additives have improved the quality of both fuel types but it’s worth remembering, both are still derived chiefly from crude oil, which is never wholly 'pure'.

When we use this “not quite pure” fuel in an imperfect mechanical device that is the internal combustion engine the resultant fuel burning process is equally imperfect. Apart from generating poisonous exhaust gases, the fuel leaves residues within the engine, including gums, lacquers and soot (particulates), creating headaches for owners and mechanics alike.

Chemical Help

The unfortunate consequence for early motorists was that these deposits would build to such an extent that the engine would have to be stripped and 'decoked' periodically.

As lubrication technology advanced, engine oils took on the task of cleaning these residues, by dislodging deposits left behind by the fuel, and holding them in suspension until the next service.

However, the oil cannot clean where it cannot flow, and the new challenge of keeping both the air intake and the fuel system clean emerged, along with the increasing pressure to keep the engine’s emissions at the optimum level.

Direct Injection: upsides have downsides

The mass adoption of multipoint fuel injection in the 1980s offered far greater fuel metering accuracy than was possible with carburettors. Introduced in the late 1990s, Gasoline Direct Injection (GDI) improved petrol engine efficiency even further, by moving fuel injectors from the protected confines behind the engine inlet valves (port injection) directly into the harsh environment of the combustion chambers.

This, again, provided more control over the AFR (Air-Fuel-Ratio), with the car’s computer (ECU) constantly measuring and adjusting the mix to ensure the most efficient usage of fuel and optimal burning. Unfortunately, moving the injectors into the combustion chambers introduced another problem.

In a port injection system, petrol (which is a solvent), passes over the intake valves, keeping them clean. With the fuel being injected directly into the cylinder, it no longer passes over the intake valves, effectively removing the cleaning agent from the intake tract.

If filtered air only passed through the engine's air intake, this issue would not pose a huge problem, but modern engines utilise additional technologies to meet ever-stricter exhaust emission and fuel consumption targets. Hot gases from within the engine are no longer pumped out into the atmosphere. Instead, these gasses (carrying oily vapour) are redirected into the combustion chambers for further burning.

Additionally, the crankcase ventilation system deposits a light oil mist inside the engine's inlet. Exhaust Gas Recirculation (EGR) also sends some exhaust gases back into the intake, adding soot, which sticks to the already oily surfaces of the intake valves.

It all adds up...

While GDI technology makes the engine more efficient, the exhaust gases are sootier, and these particulates stick to the air inlet's oily surfaces. This situation attracts more soot to build, which restricts both the air inlet diameter and inlet valve movement.

The engine, therefore, is being gradually but effectively suffocated. While the engine management system can make adjustments to compensate for the air intake being restricted, efficiency reduces until an Engine Fault warning light start flashing on the dashboard.

GDI inlet coking has become such a common problem that several car manufacturers started employing both direct and port injection to help keep the intake and inlet valves clean. However, most GDI engines are not so fortunate and the need for manual decoking is reappearing, where the engine requires a degree of dismantling to allow for manual cleaning and removal of combustion deposits.

Direct Injection diesel engines are also affected, although their oily sludge is relatively straightforward to remove, while the higher temperatures associated with petrol GDI engines bake the deposit into a more stubborn, rubbery substance.

Help is on its way

Fortunately, advanced chemistry continues to provide solutions to the age-old problem of engine deposits.

JLM Lubricants has formulated a gentler answer to keep engine intakes clean. When applied into the air intake of a running engine, JLM's Direct Injection Valve Cleaner spray breaks down these problematic accumulations, allowing them to pass through the engine.

For maximum effect, JLM Lubricants recommends the car is treated with its Performance Fuel System Cleaner afterwards, which combats deposits that afflict the direct injection fuel systems.