A New Flame Jet Concept to Improve the Inflammation of Lean Burn Mixtures in SI Engines

Engines with gasoline direct injection promise an increase in efficiency mainly due to the overall lean mixture and reduced pumping losses at part load. But the near stoichiometric combustion of the stratified mixture with high combustion temperature leads to high NO_x emissions. The need for expensive lean NO_x catalysts in combination with complex operation strategies may reduce the advantages in efficiency significantly.

The Bowl-Prechamber-Ignition (BPI) concept with flame jet ignition was developed to ignite premixed lean mixtures in DISI engines. The mainly homogeneous lean mixture leads to low combustion temperatures and subsequently to low NO_x emissions. By additional EGR a further reduction of the combustion temperature is achievable. The BPI concept is realized by a prechamber spark plug and a piston bowl. The main feature of the concept is its dual injection strategy. A preinjection in the inlet stroke leads to a homogeneous lean mixture with an air-fuel ratio of λ = 1.4 to λ = 1.7. During the compression stroke a second direct injection with a small amount of fuel (about 3 % of the total fuel mass) is directed towards the piston bowl. The enriched air fuel mixture in the piston bowl is transported by the piston motion towards the prechamber spark plug. Due to the pressure difference between main combustion chamber and prechamber the mixture is transported with a highly turbulent flow into the prechamber. After reliable ignition of the enriched mixture in the prechamber, flame jets penetrate into the main combustion chamber and ignite the lean mixture.

Numerical and experimental investigations were carried out in a modified 3-valve single cylinder engine for part load operation. The in-cylinder flow including the mixture process in the main combustion chamber and in the prechamber was investigated by CFD simulation, so that the local mixture composition could be predicted. With extended test runs and measurements the functionality of the BPI concept has been proved. For the quantification of the mixture enrichment in the prechamber spark plug ion current measurement has been found as an appropriate measurement tool [11][12]. At part load operation in BPI mode significant reductions in fuel consumption and NOx emission have been achieved compared to stoichiometric operation. Further investigations at full load have been carried out on a single cylinder engine and a 4-cylinder production engine to analyse the influence of prechamber spark plugs on one hand and the influence of lean operation on the other hand on the engine process. Homogeneous full load operation with the prechamber spark plug has shown a reduction in knock sensitivity. Due to significantly reduced cyclic fluctuations the maximum knock amplitudes at the knock limit was reduced.