Direct fuel injection is a technique that allows gasoline and diesel engines to run more fuel efficient. Direct injection techniques are already commonly used in modern diesel engines and are becoming more and more established for gasoline engines. Direct injection engines can lower the CO2 emission of the vehicle by about 15% with low extra costs compared to other techniques leading to similar CO2 emission reduction. However, still two thirds of all gasoline powered internal combustion engine vehicles which are sold worldwide are equipped with an internal combustion engine with indirect injection. Direct injection in diesel engines leads to small reductions in NOx and particulate matter emissions which affect local air quality. However, direct injection in gasoline engines can lead to a small increase of NOx and particulate emissions.
Direct fuel injection is a technique that allows gasoline and diesel engines to burn less fuel. Direct injection means injecting fuel, under high pressure, directly into the engine's cylinders rather than somewhere before the intake valve, which is the approach used in the majority of current gasoline engines (Leonhard, 2008). The direct injection process gives precise control over the timing and the amount of fuel which is injected. This precise control allows the motor management system to only inject relative large amounts of fuel when this is required, for example to accelerate the vehicle.
Under normal driving conditions three different combustion phases can be distinguished, each of which is characterized by the air to fuel ratio and the power demand.
- ultra lean burn mode characterized by air to fuel ratios up to 25 : 1
- stoichiometric mode characterized by an air to fuel ratio of 15:1 for gasoline powered vehicles.
- full power output characterized by fuel rich combustion mixtures.
Under low power demand conditions, such as under constant speed cruising conditions, the fuel/air mixture in direct injection engines are much leaner than in conventional engines which reduces the fuel consumption considerably.
Direct injection modifies the motor's fuel injection system, but does not require any additional changes in car design or transport infrastructure. Direct injection eninges need more robust components, because they process fuel at significantly higher pressures than indirect injection systems. The injectors must be able to withstand the higher heat and pressure of combustion inside the cylinder.
Direct injection is fully developed for diesel engines, where it has been used for a considerable time (Bosch, 2004). Sstate of the art diesel engines which employ direct injection are called common rail diesel engines.
For gasoline powered cars the technology is not yet widely implemented, although it is already being used in some vehicles. The abbreviation GDI, Gasoline Direct Injection, is commonly used.
Gasoline powered engines
In direct injection gasoline engines the fuel-air mixture is more optimal, which can lower CO and unburned hydrocarbon emissions. The newest direct injection gasoline engines can comply with the US Super Ultra Low Emission Vehicle (SULEV) emission limits for vehicles with significantly reduced emissions. Table 1.3 which compares the SULEV emission limits with Euro 5 standards which are in force in Europe today. [media:image:1]
However, due to higher cylinder heat and pressure, NOx and particulate matter emissions from direct injection gasoline vehicles can increase, the latter by about 6% (Gable, 2008). Under low power demand conditions the gasoline engine is running in lean burn mode, where the fuel to air ratio is not ideal for use of the standard three way catalytic converter to remove NOx from the exhaust gases. Lowering NOx emissions from direct injection gasoline engines requires different types of catalytic converters.
In general, diesel powered vehicles tend to have higher emissions of local pollutants, especially NOx and particle emissions. A modern common rail diesel engine has lower emissions of CO, HC and particulate matter, however, NOx emission have not been reduced much.
Compared to conventional internal combustion engines, with CO2 emissions of 185 gr/ km on average, modern engines have improved substantially. A combustion engine with direct injection has a 15% better fuel efficiency and therefore 15% lower CO2 emissions. (Sharpe & Smokers, 2009) However, still two third of all gasoline powered internal combustion engine vehicles which are sold worldwide are equipped with an IC engine with indirect injection with relatively high CO2 emissions (Leonhard, 2008). The fuel consumption of a conventional diesel engine with the common rail injection technique is similar to gasoline hybrid vehicles.
Direct injection engines are more expensive to build because their components must be more robust. A direct injection engine is about 5% more expensive than a conventional engine, which is in the range of several hundreds of dollars, and as stated above can reduce CO2 emission up to 15% (Sharpe and Smokers, 2009).
Gable, C., 2008. What is a lean-burn engine? Available at: http://alternativefuels.about.com/od/glossary/g/leanburn.htm 
Leonhard, R., 2008. De inwendige verbrandingsmotor van de toekomst- een erg laag verbruik en hoge prestaties ondanks de kleine cilinder inhoud, Bosch GmbH
Robert Bosch GmbH, 2004. The common rail diesel injection system explained. Available at: http://www.swedespeed.com/news/publish/Features/printer_272.html 
Sharpe, R. and Smokers, R.T.M., 2009. Assessment with repect to long term CO2 emission targets for passenger cars and vans. Available at: http://ec.europa.eu/environment/air/transport/co2/pdf/Report%20LT%20targets.pdf