Hollow cone gasoline/ethanol sprays under cold start conditions

Licentiate thesis, 2009

To decrease the release of CO2 from fossil fuels into the atmosphere, within the transportation sector the efficiency of vehicles and their engines must be increased. Additionally renewable fuels may be used to further decrease our reliance on fossil fuels. One such fuel is ethanol which presently, in low quantities, is mixed into all gasoline presently sold in Europe. In higher quantity mix-in it is sold under the name E85 (85% ethanol, 15% gasoline) and vehicles able to run on this fuel are subject to lower tax in Sweden. The E85 vehicles do suffer from poor startability in cold conditions even though winter grade E85 contains 25% gasoline to improve starting performance. To improve the thermal efficiency of Otto-engines, direct injection can be applied. By injecting the fuel directly into the cylinder all fuel will evaporate inside the combustion chamber and thereby lower combustion chamber temperatures. This allows for higher compression ratio, bigger spark advance and/or higher boost levels. Furthermore, by stratifying the charge at low and medium load, the efficiency of the engine can be further increased due to lower throttling losses and less heat losses. By applying a stratified starting strategy the aim to start with high levels of ethanol in the fuel even at very low temperatures may be accomplished. In this thesis the performance of outward-opening, piezo-controlled direct injectors is investigated in constant pressure gas chambers using different fuels. The influence of different chambers conditions, fuel pressure and temperature as well as different injection strategies is evaluated. From these investigations, the injection pressure was found to be the most important factor influencing the structure of the spray. A spray injected at a high pressure into a moderately high pressure environment will break up into a compact fuel cloud which entrains air from outside through vortices shaped by the break-up. At the same time the mentioned vortices keep the fuel droplets and vapour from propagating outside of the cloud structure. A comparison between gasoline and ethanol gave that while the lightest compounds of gasoline evaporates quickly already at 90°C chamber air temperature, the temperature of the air needs to be increased to 200°C at constant gas density for ethanol to evaporate at a similar rate. Hence, to achieve a stratified cold start on neat ethanol under otherwise similar conditions to gasoline the compression ratio must be increased to gain more in temperature during the compression. As a final test, laser ignition was successfully applied on neat ethanol sprays under conditions similar to those valid for a -30°C arctic cold start.