DFI and fuel dilution, redux.

[G-MAN]

In the continuing RS4 saga Terry has mentioned several times that the root cause of the excessive fuel dilution in that engine is because the DFI washes down the cylinder walls. He has said the same phenomenon occurs in diesel engines. Hmmm...

I had to drive to Columbia again today to interview some witnesses, and during the two hour drive in between thinking about the case I was thinking about this issue and some questions resulted from my cogitations. First, aren't most (if not all) modern automotive diesel engines of the pre-chamber (Ricardo) design? Meaning that the fuel is not injected directly into the combustion chamber, but rather into the pre-combustion chamber where the initial fuel explosion takes place. Thus, no fuel is sprayed directly onto the cylinder walls. But what about big truck diesel engines? These are direct injection into the main combustion chamber, right? But when does that injection take place? It's right around TDC isn't it? At TDC the cylinder walls are not exposed, so how is wash down occurring?

On these gasoline DFI engines, when is the fuel being injected? If the pulse starts just after TDC and continues during the intake stroke down to almost BDC I can see how you get cylinder wash down with that. But if the pulse starts just before TDC on the compression stroke, wouldn't that mitigate against cylinder wash down? Does anyone know when the fuel injection is actually injected on these VW/Audi DFI engines?

 

[Eddie]

Interesting comments G-Man that got me to thinking since I have a CX7 DI gas engine. The first question I thought of is: How long a time period does the fuel injection take place. The second question is: What is the relationship of the piston during the injection period? If the injection period is XXX milliseconds and the downward stroke is X inch's, how much cylinder wall is exposed? Is the gas-air mix so rich that complete combustion is unobtainable? Lots of questions that can't be answered without some really good laboratory data I think. Cheers. Ed

 

[FowVay]

Volkswagen FSI engines are designed around the spray guided principle. This allows for the controlled placement of the fuel/air mixture within the cylinder during both high load (homogenous stoichiometric) and partial load (stratified lean burn) relationships.

While there are strong similarities between a FSI engine and a TDI engine, the FSI is far more likely to suffer from cylinder wash than is the TDI simply because of piston design. Both engines will use the dished piston to provide a pre-ignition chamber. However, when comparing the critical parameters of fuel pressure, air pressure, air temperature, fuel droplet size and combustion occurrence time, I simply don't see how Mr. Dyson can relate cylinder wash between the two engines.

Think of piston position of the gasoline engine being 65º - 50º BTDC for the initial fuel pulse to be injected. Fuel burn times are up to six time longer for the gasoline engine than the diesel engine but the fuel droplet sizes are 2 - 5 times larger in the gasoline engine. This is why you have a cylinder wall wash with the FSI.. but only at the upper fraction of the cylinder.

The automotive diesel engines from VW simply don't have this problem.

Volkswagen will likely adopt a different combustion chamber style (wall guided injection pattern) to address the dilution issues with their FSI engines. I don't know if this decision has been adopted yet.