View Single Post
scarceller scarceller is offline
Registered User
 
scarceller's Avatar
 
Join Date: Oct 2006
Location: Southern MA
Posts: 3,645
Garage
Quote:
Originally Posted by Speedy Squirrel View Post
There are very few new gasoline vehicles running with MAF only. The range of airflow is too difficult to measure with just one sensor. This is especially true of a turbo motor, which acts like a small engine at idle, and a large one at WOT. It is common to use a MAF to control idle and part throttle only, and switch to speed/density at large throttle/boost. The speed/density measurement has become quite accurate when throttle adaption, and VE learning from the WRO2 are factored in.
No disrespect here just my thoughts on the topic of boost.
An engine does not know if it's at boost it only knows how much mass of air it has ingested. I've helped do a SuperCharged and a Turbo 3.2 running just MAF in a blow through setup with a lab grade MAF designed for this. No rising rate FPRs and no fancy MAP based ignition retard. Simply stock Motronic with MAF and a very custom chip I developed. The math for calculating injector pulse width is very simple if you have a calibrated MAF you can trust. Math looks like this:

GivenAirFlow / RPM = AirFlowPerRev (we now know how much air is ingested per rev)

A 3.2L can at a VE of 1 can ingest about 1.9grams of air in 1 rev
Here's this math approximated. 1Liter of air at 68F sea level is equal to about 1.2 grams and a 3.2L motor takes in 3.2L every 2 revs at WOT assuming a VE=1 so the 1.9grams works out like this:
3.2L / 2revs * 1.2 = 1.9 grams.

We now know that at a VE=1 this motor can ingest 1.9grams of air.
Anything below 1.9g is below VE=1 and typically the only way to get above 1.9g is with some sort of forced induction. But the motor does not know it's being forced to take in more air it simply sees boost as elevated ambient pressure. You need to think about this for a bit and then things become much simpler.

So now for fueling calculations:
Let's take an example where we assume a max flow per rev under boost to be 50% above VE=1 or a VE=1.5, under this max condition air flow per rev would be:
1.9g * 1.5 = 2.85g per rev
so we would be ingesting 2.85g/rev at a VE=1.5 now we need to match grams of fuel to that air mass like this:
Most EFI systems assume an AFR of 14.7 for this first calculation and then apply the target AFR correction later. So if we want to know how much fuel in grams is needed for 2.85g of air it's very simple calculation:
2.85g / 14.7 = 0.1939g of fuel needed to match that air mass.

Then the EFI models the injectors so it knows the relationship between injector size and grams of flow per millisecond. I could go on to explain how this is done but all you need to know is that we want to calculate a mx inj pulse width and pick an injector that achieves a pulse width of about 8ms for batch injection (per rev fueling) or 16ms (sequential). No magic in these numbers, a motor spinning at 7000RPMs is turning at about 8.6milliseconds per rev, so you MUST get all fuel injected within the 8.6ms time.

So for batch injection the inj pulse width can be from 0-8ms and if 8ms is at VE-1.5 then at VE=1.0 the PW would be 5.33ms (5.33 * 1.5 = 8ms).

So now we know that a PW > 5.33 is VE>1.0
and PW<5.33 is VE<1.0

From here we apply Fuel correction for richer than 14.7AFR and we then apply Ignition values for the given Pulse Width at a given RPM.

The main point I'm trying to make is that you can do boosted applications without
any real consideration to the fact that the engine is boosted (The MAF based EFI does not care) if you have a good air metering device you can trust then it does not matter that it's boosted, the formulas for calculating fuel and Injector PW are unchanged. Furthermore VE is directly tied to Pulse Width and it has a direct relationship for the given setup. I showed this by illustrating that the PW=5.33ms has a direct relationship to a VE=1.0

Many OEM EFI system do employ other devices such as MAP or TPS but this is mainly as a backup for a failed MAF and also for Accel Enrichment. But our Horizontally apposed motors need very little Accel-Enrichment and practically none at all above 4000RPMs. Accel Enrichment was mostly needed and very important in wet intake systems and not so much in Injected systems that inject fuel at the valve. Above 4000RPMs air flow through the runner is sufficiently high enough that not much fuel accumulates on the walls of the runner so we don't need Accel-Enrichment to recover that wall fuel.

One last point: a proper MAF can accurately measure air flow even under boost, assuming it's properly sized. The problem is that folks are trying to boost 3.2L engines and retain the stock AFM, that won't work! The stock AFM is only good for about 12,000 g/min and any boost above 4PSI will easily exceed this limit. I use a MAF with a max flow rate of 22,000 g/min it has plenty of head room for even VE as high as 1.8!
__________________
Sal
1984 911 Carrera Cab M491 (Factory Wide Body)
1975 911S Targa (SOLD)
1964 356SC (SOLD)
1987 Ford Mustang LX 5.0 Convertible

Last edited by scarceller; 11-19-2014 at 04:55 AM..
Old 11-19-2014, 04:39 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #181 (permalink)