After reading the post ecotek cb26p and the back-and-forth arguments I decided to do a little investigating, I just came up with more information rather than a definite answer.
The first thing I would say is that getting a good burn at closed throttle has always been a problem for conventional engines thats among the reasons they have developed things like stratified-charge engines:-
Charge Stratification in a Strong Tumble SI Engine (PDF)
A Study of EGR Stratification in an Engine Cylinder (PDF)
Getting back to a normal engine (This is from the book listed below), at part-load (or part-throttle) operating conditions, it is advantages to dilute the fuel-air mixture, either with excess air or with recycled exhaust gas. This dillution improves the fuel conversion efficiency for two reasons:-
1. The expansion stroke work is increased.
2. Pumping work decreases.
If tight emisson controls are not required air can be used. If tight control of emissions is required then exhaust gas is used to keep a stoichiometric mixture for the three-way catalyst.
The amount of dilutent that an engine will tolerate depends on the engine's combustion process. Increasing excess air or recycled gas slows down the combustion process and can reduce smooth engine operation. As the load is reduced less dilutent can be added and the engine will go towards a fuel-rich mixture to obtain adequate stability.
Some of the more interesting pictures are shown below, Fig.1 shows the flow at different throttle settings.
Fig. 2 Shows the interaction of the intake jet with the wall producing a large-scale rotating flow pattern within the cylinder. The picture was obtained using a transparent model water-analog of an engine cylinder and piston.
There is some talk about swirl which is the rotation of the air mass around the cylinder axis.
Fig 3. Shows turbulent flow created past the valve. In turbuklent flows, the rates of transfer and mixing are several times greater than the rates due to molecular diffusion.
Theres also reference to a seperate paper all about turbulence in the manifold. It's hard to paraphrase the rest but the book which is Internal Combustion Engine Fundamentals is a good book with some nice pictures and at nearly 1000 pages is quite detailed.
Fig. 1
Fig. 2
Fig. 3
The first thing I would say is that getting a good burn at closed throttle has always been a problem for conventional engines thats among the reasons they have developed things like stratified-charge engines:-
Charge Stratification in a Strong Tumble SI Engine (PDF)
A Study of EGR Stratification in an Engine Cylinder (PDF)
Getting back to a normal engine (This is from the book listed below), at part-load (or part-throttle) operating conditions, it is advantages to dilute the fuel-air mixture, either with excess air or with recycled exhaust gas. This dillution improves the fuel conversion efficiency for two reasons:-
1. The expansion stroke work is increased.
2. Pumping work decreases.
If tight emisson controls are not required air can be used. If tight control of emissions is required then exhaust gas is used to keep a stoichiometric mixture for the three-way catalyst.
The amount of dilutent that an engine will tolerate depends on the engine's combustion process. Increasing excess air or recycled gas slows down the combustion process and can reduce smooth engine operation. As the load is reduced less dilutent can be added and the engine will go towards a fuel-rich mixture to obtain adequate stability.
Some of the more interesting pictures are shown below, Fig.1 shows the flow at different throttle settings.
Fig. 2 Shows the interaction of the intake jet with the wall producing a large-scale rotating flow pattern within the cylinder. The picture was obtained using a transparent model water-analog of an engine cylinder and piston.
There is some talk about swirl which is the rotation of the air mass around the cylinder axis.
Fig 3. Shows turbulent flow created past the valve. In turbuklent flows, the rates of transfer and mixing are several times greater than the rates due to molecular diffusion.
Theres also reference to a seperate paper all about turbulence in the manifold. It's hard to paraphrase the rest but the book which is Internal Combustion Engine Fundamentals is a good book with some nice pictures and at nearly 1000 pages is quite detailed.
Fig. 1
Fig. 2
Fig. 3