charge cooling

[AlexNeo]

hi!!

i am a bit confused about charge cooling and the resulting increase in volumetric efficiency.
in a pfi engine there is an air flow sensor upstream of the fuel injector. this sensor tells the ECM the amount of air going through. based on this input the ECM then directs the injector to inject fuel such that the A/F ratio going in is about stoichiometric. but the fuel evaporates off the intake valve and thus cools the air as a result of which the density of air increases and "more" air goes into the cylinder increasing the volumetric efficiency. but then that means that there is more air and less fuel so the trapped A/F ratio in the cylinder is not stoichiometric but slightly lean.

is there a factor in the ECM look-up tables that accounts for charge cooling?? for a particular air flow rate does the ECM inject slightly more fuel so that the A/F ratio trapped in the cylinder is stoichiometric as amount of air going in will increase after charge cooling?

do DISI engines offer a volmetric efficiency benefit over pfi and how? because if the DI engine is running in homogeneous mode the fuel evaporation will cool the air inside the cylinder but how will that increase the amount of air going into or trapped in the cylinder?is it because of the unthrottled or WOT operation for homogeneous mode?
and finally how do DISI engines offer the potential for more torque over comparable pfi engines?

thanks in advance!!

alex

 

[ivymike]

well I'm not an authoritative source for answers to these questions, but I'll take a stab at 'em.

If your air flow sensor accurately measures the mass of the air flowing through the intake, then you won't be able to get more into the cyls than you're drawing through the intake (excluding leaks, etc), regardless of what happens due to cooling.

If your flow sensor is measuring volume flow rate, then the error you're mentioning seems like it would have to be addressed.

In either case, it was my understanding that the ECU would adjust the amount of injected fuel at a given operating condition based upon O2 sensor readings, and that over time an adjustment factor is maintained to scale the "default" fuel mapping. Thus the vehicle should still run alright with slight unforeseen changes to the fuel or air flow of the system.

I don't know how DISI would increase volumetric efficiency, but it seems to me that in-cylinder evaporation of the fuel could have at least _some_ benefit, depending on when the fuel is injected, because it would increase the "compressor efficiency" by acting as an in-cylinder intercooler.

As far as increasing torque goes, I would expect that this can be accomplished in a variety of ways:

* by prolonging the burn duration (more like a diesel) so that the cyl pressure acts over a more useful portion of the cycle.

* by adjusting the injection timing, it would seem that you could avoid detonation problems and perhaps run a higher compression ratio

* I had something else in mind, but after all this typing (and listening to the radio) I can't remember what it was.

 

[franzh]

On almost all engines today, the O2 sensor is used to trim the engine fuel mixture, AFTER combustion. The idea about the intake valve changing the fuel mixture is very slim, sorry. There is little effect with gasoline in evaporative fuel mixtures unless you get into high evaporative fuels like Ethanol and Methanol with high Hydroxyl compounds.

One last thing, the back face of the intake valve is VERY hot. Look at the coking carbon buildup and you will see what I am talking about. The biggest concern is that this accumulation can restrict the airflow and absorbing the liquid gasoline, thus reducing power too.

Franz (MASE/SAE)

 

[TheBlacksmith]

I agree with the posts, there is no problem caused by fuel cooling the charge. The basic "map" of the A/F ratio is derived on a test engine and a dyno, so the programming takes into account any cooling affects. Additionally, many engines have an intake manifold temperature sensor that causes more fuel to be injected if the air is very cold and dense, in order to maintain the A/F ratio. Actually, the strotrimetric ratio is given a lambda number of 1.0 and most tuners try to get lambda between 1 and 1.05 (lean) as this is the best power, economy and emissions. If lambda is increased further, nitrogen oxide emissions increase rapidly. Finally, the feedback loop from the oxygen sensor continuously fine-tunes the A/F ratio; misguided sales people often erroneously called this self-tuning, implying that periodic tune-ups were unnecessary. This information applies to production engines and pump gasoline, where my experience lies; I understand that racing engines and exotic fuels do utilize/benefit from the charge cooling

Blacksmith


Blacksmith