Stratified Charge Engine With Two-Stage Combustion Mechanism Shows 17% Reduction in Fuel Consumption Without Direct Injection
|Two-stage combustion mechanism in twin swirl combustion (1, zone containing pure air; 2, spark plug; 3, turbulizer; and 4, zone containing the fuel-rich mixture). Click to enlarge. Credit: ACS
A team of researchers from Istanbul Technical University (ITU) in Turkey has presented a 1.6-liter stratified charge gasoline engine featuring a twin swirl combustion chamber operating with a two-stage combustion mechanism and experimentally shown that it can deliver a 17% reduction in fuel consumption with a 7% increase in power compared to a conventional 1.6-liter port-injected engine.
The proposed combustion mechanism does not require high fuel injection pressures and can be applied on current production engines without significant modification and without direct injection fuel systems, according to the researchers. A paper on the work was published online in the ACS journal Energy & Fuels on 15 December.
The two-stage combustion mechanism was originally proposed by a team comprising researchers from Azerbaijan Technical University (AzTU), Warsaw Technical University (WTU), ITU, and Middle East Technical University (METU).
In conventional gasoline engines, every part of the cylinder contains a mixture having an excess air ratio (λ) of approximately 1. Stratified charge engines have frequently stoichiometric mixture (λ = 1) only near the spark plug and lean mixture in the cylinder, globally. For the special case of stratified charge engines operating with a two-stage combustion mechanism, there is a lean mixture in the cylinder globally as well; however, there is a fuel-rich mixture in the vicinity of the spark plug.
The nonhomogenous mixture in stratified charge engines is obtained usually with the modification of the piston geometry. The geometry of the intake manifold can also be modified. Because there is a lean mixture in the combustion chamber globally, stratified charge engines have a lower knock tendency than the conventional gasoline engines. Because of this fact, the compression ratio (ε) of a stratified charge engine can be higher than the compression ratio of a conventional gasoline engine; ε ≥ 12 is possible. A higher compression ratio leads to a higher efficiency. The absence of throttle losses in part load operation in combination with the ability to use higher compression ratios leads to lower fuel consumption.—Dinc et al. 2008
The proposed combustion chamber looks like a figure “8” and is separated into two zones. The spark plug mounted part of the combustion chamber contains a fuel-rich mixture with an excess air ratio of 0.6-0.8, while the other part contains pure air. The fuel is injected into the intake manifold and fed into the zone containing the fuel-rich mixture. The intake manifold is designed for the two-stage combustion mechanism, such that it increases the swirl effect and volumetric efficiency. The counter-rotating swirling motion—which occurs during the intake and compression cycles of the engine—does not allow the mixing of the two zones until ignition time. This allows stratification of the air-fuel mixture across the load range.
Because the swirl motion occurs with the start of the intake cycle, the air-fuel mixture can be prepared in the intake manifold (outside of cylinders). Therefore, current electronic injection systems or carburetor engines can be used with this method. In other words, special and expensive direct-injection systems are not required, such as in gasoline direct injection (GDI) engines, where the injection of fuel into the cylinder reduces the time available for evaporation and mixing.—Dinc et al. 2008
The two-stage combustion mechanism can also reduce emissions of criteria pollutants. Because the liquid phase of the gasoline does not contact the cold wall of the cylinders, and because the counter-rotating swirling motion reduces the contact of the flame with the piston, the stratified charge engines with the twin swirl combustion chamber produce lower hydrocarbon (HC) emissions. Incomplete combustion products (CO and H2 produced during the combustion of the rich mixture (λ = 0.6-0.8) at the first stage can be burned in the second stage of combustion with the effect of the swirl motion. The lack of oxygen in the rich mixture and low combustion temperature at the first stage of combustion do not allow NOx formation.
Cenk Dinc, Hikmet Arslan and Rafig Mehdiyev (2008) CO2 Emission Reduction Using Stratified Charge in Spark-Ignition Engines. Energy Fuels, Article ASAP doi: 10.1021/ef800349x