How to Make Petrol Engines Cleaner

Particulates are the bane of direct fuel injection engines, but increasing pressure and other innovations can solve this. Importantly, the new high-pressure injection systems make petrol engines even cleaner and able to meet emissions protocols such as the WLTP.

Gasoline direct injection (also known as gasoline direct injection or GDI) is rapidly becoming the best friend of car manufacturers. It provides better fuel economy and lower CO2 emissions while improving low-speed torque but, like most things, it is not a perfect system.

GDI engines produce more particulates by number and mass than a conventional port-injection engine. One way to solve this is with high-pressure fuel injection systems. Current GDI systems operate at 200 bar of pressure, but one of the first innovations from Delphi Technologies involves a 350-bar system.

Delphi, Magneti Marelli and Bosch are also working on future 500-bar systems. First, however: what do GDI engines have that makes them produce more particulates than port-injected ones?

https://www.youtube.com/watch?v=uxZAcYx1AV4

The fuel is identical, so what is the difference?

The traditional method injects a measured mist of fuel into each inlet port as the pistons draw air in on the induction stroke. The fuel rushes through the intake manifold runners, evaporating into the airflow as it travels, and reaches the inside of the cylinder as a charming, evenly mixed (homogeneous) charge ready for ignition.

GDI engines, on the other hand, draw in only fresh air and inject the fuel directly into the combustion chamber as the air is compressed inside the cylinder, just before combustion occurs. The inlet ports are designed to swirl the air into the cylinder so the fuel arrives in the middle of a small storm that mixes the air and fuel. However, there is still much less time for the fuel droplets to evaporate than during the long journey both take in a port-injected engine, and incomplete evaporation of the fuel droplets equates to more particulates.

Increasing fuel injection pressure to reduce fuel droplet size achieves better evaporation in the shorter time allowed within a GDI engine. Delphi claims its first industry-first 350-bar system reduces particulate size and quantity by up to 70% and that its next 500-bar system surpasses the first by an additional 50%. But doing so is not as easy as it sounds: it is not just a matter of turning up the tap.

To begin with, the higher the pressure, the greater the energy required to produce it, and that consumes a small proportion of engine power and has a slight impact on fuel economy. It can also generate more noise and places much greater stress on all the injection components. The new, more powerful injectors must measure the fuel more precisely, as well as cope with the additional pressure and detailed changes to the fuel rail that feeds the injectors.

A solution for reducing CO2 emissions in petrol engines?

Delphi’s new injector pump design provides the additional pressure without increasing noise and while minimising power consumption. It also avoids increasing the load on the drive mechanism and subsequent costly modifications to the engine itself. Delphi’s 350-bar system is the first step, and more, both 350 and 500 bar, should appear from other major suppliers in the coming years.

Particulate filters such as the one seen in the main image of Porsche are already being fitted to new cars to reduce particulate emissions leaving the engine by 90%. The filters are coupled and mounted very close to the engine so that they light off (heat up and start working) quickly. They also function as three-way catalytic converters already fitted to petrol-engined cars to clean up nitrogen oxides (NOx), carbon monoxide (CO) and hydrocarbons (HC).