# The Indicator of GDI Engine Operating Mode and Its Influence on Eco-Driving
## 2 GDI Engine Modes of Run and Their Impact on Fuel Consumption  
A spark-ignition engine is the most widely used power unit in today's passenger cars. It must be very dynamic with low emission of toxic compounds in exhaust gases and low CO\({}_{2}\) emissions, synonymous with low fuel consumption. Unfortunately, it is not possible to meet all these requirements simultaneously [13]. High dynamics and power require a rich fuel-air mixture provided to the engine cylinder. Such a supply is not favourable neither for low emissions of toxic compounds nor low fuel consumption [14]. It is also unsuitable for the effective operation of the catalytic converter. On the other hand, the supply of the spark-ignition engine with a lean mixture promotes low fuel consumption and reduces the engine's dynamics. To meet both requirements and, first of all, fulfil the stringent vehicle emissions rules, gasoline direct injection (GDI) systems for SI engines are developing [15; 16]. The specific fuel consumption for low loads of SI engines can be reduced by the supply of the engine with very lean, stratified mixtures [17]. A high level of excess air characterises the stratified mixture. One of the limits for the excess air ratio increase is caused by cycle-by-cycle variations (described by the coefficient of variation of the indicated mean effective pressure--\(\text{CO}_{\text{imp}}\)), sharply increasing for very lean mixtures [18]. A flame initiated by an ignition spark in a stratified mixture is formed in the layers with a combustible composition and spreads to leaner areas. Due to the higher air excess in the total volume of the air-fuel mixture, a more significant proportion of the fuel is completely burnt. As a result, it is possible to raise the engine thermal efficiency [19]. Stratified mixtures are formed by fuel injection to the combustion chamber just before ignition on the compression stroke [20]. In this mode of the engine, for stable combustion, it is necessary to make an appropriate mixture around the ignition plug [21]. The mechanisms of mixture creation and its combustion are complicated. In GDI engines, flash boiling of the spray occurs during throttling or low load operations [22]. The presence of flash-boiling bubbles in the nozzle can improve the end-of-injection characteristics by eliminating large drops or ligaments [23; 24]. The thermo-physical properties of the fuel, such as density, viscosity, saturation temperature, latent heat of vaporisation and surface tension, determine the atomisation and evaporation process [25]. The direct injection of fuel into the combustion chamber also improves fuel economy because the intake air is cooled by fuel evaporation [17]. Additionally, energy losses related to throttling the airflow are smaller. However, the maximum amount of fuel that can be burnt in a very lean, stratified mixture is limited by the full amount of air in the cylinder. Hence, the combustion of stratified mixtures can take place only at relatively low engine loads and limited engine speed up to 3600 rpm [26]. In addition, the lean mixture combustion makes the three-way catalytic converter less effective in reducing NO\({}_{\text{x}}\) content in the exhaust gas. This problem was solved by the application of a three-way catalyst (TWC) with lean NO\({}_{\text{x}}\) trap (LNT) [27], e.g., Pt-Ba/Al\({}_{2}\)O\({}_{3}\) LNT [28]. They can bind NO\({}_{\text{x}}\) particles when there is a higher excess of oxygen in the exhaust gas and reduce them to non-toxic particles during oxygen deficiency. Then, periodic regeneration by removing accumulated NO\({}_{\text{x}}\) is required. The regeneration process occurs when there is no free oxygen in the exhaust gas, i.e., when the engine is supplied with a stoichiometric or rich mixture. In practice, the engine supplied with a stratified mixture is temporarily supplied with a homogeneous stoichiometric mixture. The increase in the driving time while fuelling the engine with a stratified mixture extends the regeneration periods of the catalytic converter.