The Effects of Fuel Quality on Gasoline Direct Injection Engines

14 June 2018

Since the 1990s, direct injection for Diesel engines has been the standard for passenger car engines, building upon the technology developed in the 1960s for larger engines. However, direct injection for gasoline engine cars is a relatively new development. This is despite its origins in the fighter aircraft of WWII in which a number of aircraft, including the legendary Messerschmitt ME109, exploited the technology in order to perform negative G-force manoeuvres beyond the capabilities of their older allied adversaries.

Step forward to the mid 1990s and major car manufacturers including Audi and Mitsubishi began to adopt direct injection in petrol-fuelled vehicles, leading to widespread adoption of Gasoline Direct Injection (GDI) engine designs by other manufacturers in the ensuing decades.

As a relatively new technology, there are perhaps inevitably some longer-term quality issues starting to emerge, not least of which are deposits on intake valves, fuel injectors and the combustion chamber itself.

Fuel Injector Deposits

In a traditional engine design, the fuel / air mix is created in a carburettor or via fuel injectors into the relatively cool intake manifold (usually below 150 degrees C). At these temperatures, the fuel injectors won’t foul.

Contrast this with a modern GDI engine in which the fuel injector tip is situated in the engine cylinder / combustion chamber. The resultant temperatures are far higher, which means that fuel can break down or partially oxidise as it exits the injector tip.

As deposits build up, the injector holes are compromised with deposits building up around (rather than in) the injector holes, deflecting the fuel spray. This compromises not only performance but also fuel economy and emissions.

Fuel quality is a factor here. Heavier or aromatic hydrocarbons, together with fuels containing metal trace elements such as copper or zinc, can exacerbate the deposit formation.

Intake Valve Deposits

Modern GDI engines suffer to some extent from deposits on the rear face of the intake valves. These emanate from three primary sources: soot in the exhaust gases which recirculate back into the intake; airborne dust particles; and oxidised lubricating oil which slowly runs down the inlet valve stem before reaching the valve head.

This contrasts with traditional engine designs in which the injection of the petrol into the intake air occurs before the intake valve, ensuring that the back face of the valve is washed by the fuel, mitigating the effects of the three sources of deposits above.

Since the GDI design sees fuel injected downstream of the intake valve, the cleaning effect is lost. As deposits accrue, the back of the intake valve changes its aerodynamic profile, adversely affecting airflow which is critical to the correct air / fuel mix in the combustion chamber.

The effect is compounded by modern variable valve designs since it most compromises flow when the valve is partially open. As the fuel / air mix degrades, NOx, CO and particulate emissions rise, in addition to adverse effects on fuel economy and, ultimately, power delivery.

Combustion Chamber Deposits

The design of a GDI engine requires that all fuel is sprayed into the combustion chamber, mixing with the air therein and burning in its entirety with no residual deposits.

Any variance in the speed with which the fuel is injected or any deflection of the injection spray will result in a proportion of the fuel passing through the air and ending up on the cylinder wall, the cylinder head, top of the piston, or the piston rings. These unwanted fuel deposits can become hot enough to act as ignition sources, causing pre-ignition or even detonation with adverse effects on the engine.

Fuel Solutions

Coryton has developed a range of specialist fuels that can minimise deposits in these three areas, in addition to providing a robust test of an engine design’s resistance to them.

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