Calculating injector duty cycles

[Sim]

I am about to calculate the duty cycle of the injectors using VAG-COM log (002). I've already checked Glenn's "study" regarding to this topic. He got bigger values than 100% which was strange for me. How could an injector be kept opened longer than 100% (all the time)?

My theory is the following:

I've calculated the duration of a full revolution. Lets say at 6000 RPM, it is 10ms. (1sec/(6000RPM/60)=0.01s=10ms).

I've taken the Inj. On Time (from the VAG-COM log 002) and divided this value with the duration of four full revolutions (4*10ms). Why four? Because our engines are four strokers and a single cylinder will fire in every fourth rev.

According to this, if the Inj On Time was 25ms, the duty cycle got 25/40 * 100 = 62.5%.

Am i correct?

 

[Sim]

looks like nobody's interested in inj. duty cycles :\

 

[slim_boy_fat]

4 stroke engine fires every 2 rpm NOT every 4!!

a stroke is either up or donw NOT a full revolution that would be 2 strokes.

 

[Sim]

Yes, you're right
But how could the duty cycle be bigger than 80% on a stock ibiza, and how could it go above 130% on a chipped one? Or how should we interpret the inj on time from logging 002?

 

[max_torque]

I would expect the std car to have a peak injector duration (probably recorded at max rpm and WOT) of about 85 to 90%, depending on ambient temps and duration for which you have been at that load. (siginificant overfuelling will occur after a period of time at high load when the cat temp model calculates the catalyst will be reaching peak temperature). It is perfectly possible to ask for more than 100% of the availible duration (which is 2 complete revolutions on a 4 stroke engine) but of course the injectors will effectively just be clipped at full on the whole time.

I suspect that the block you are logging is not the actual instantaneous injector ducty cycle, which is updated multiple times per cylinder cycle, but possibly the averaged and summed output used by the trip computer etc to calculate fuel economy.

 

[Sim]

max_torque,

I would be surprised if the injectors were on 90% duty cycle on a stock 1.8T. How could chip tuners add more (and reliable/safe) fuelling then to achieve min. 30% more power? I guess the 002 block does not repersent the actual and valid open time for the injectors (or as you write, it is already summed or summat). I will try to log other blocks (from 101-109) to see if they give different values from 002.

 

[Sim]

Gave a try to other blocks with VAGCOM (above 101), but they were the same as in 001 & 002. According to the log (which was a 2nd gear WOT), my injectors are constantly opened (inj duty cycle clipped at 100%) above ~4800 RPM. Would that make any sense?

 

[slim_boy_fat]

Gave a try to other blocks with VAGCOM (above 101), but they were the same as in 001 & 002. According to the log (which was a 2nd gear WOT), my injectors are constantly opened (inj duty cycle clipped at 100%) above ~4800 RPM. Would that make any sense?

I dont think you will be maxing out the injectors in 2nd gear, maybe try 4th..

 

[Sim]

It was 2nd gear for sure and this is what ECU reported.

I thought even chipped 1.8Ts had a bit more fuelling reserves, but it seems not. Or logging "Inj On Time" gives false results.

 

[Glenn]

I'm interested in this topic, Sim but I'm in the middle of a house move so haven't had much time. I think max is right that we're logging some sort of derived value rather than instantaneous as it doesn't change quickly enough. What bothers me is that when you run the numbers, it's not that far off.

Consider : stock 280cc/min injectors at 100% duty cycle with a BSFC of 0.5 can produce a theoretical maximum of ~213-220bhp. 90% of that is ~192-198bhp. A BSFC of 0.5 is pretty good for a forced induction car, they're usually assumed to be 0.55-0.65 and as max pointed out, you're more likely to be running rich at the top end under sustained WOT rather than lean where your BSFC could be 0.4-0.45.

Take those numbers and run at 80% (supposed to be OE "safe" maximum) and you get 170-176bhp - maybe that's why the Ibiza R is "only" rated at 180bhp? So when I see people claiming 200+bhp from a straight re-map on the Ibiza, I'm thinking "There's no f*&king way you're getting that without pushing those injectors to the limit" - something I find unacceptable. If I keep my Ibiza and go for the IHI swap, I'll be specifically requesting that the map doesn't use over 80% duty cycle, so if that means my Stage 3 only runs 280bhp or I have to fit bigger injectors to get 340bhp then so be it.

max, jump in if any of the theory I'm using is out of date - I haven't found any different calculations for more modern engine management techniques?

 

[Sim]

Glenn,

I've used your "formula" to calculate my A/F. It looks like that (see attachment), and it looks nearly perfect (i've used 282 for inj. capacity).

This was only a second gear run with a peak MAF reading of 158g/s. With my highest ever MAF reading ~173 g/s MAF (which means +9.5% air, but no more fuel), the AF above 4200 is (or should be) around 12.7 which is the optimal value for peak power.

180HP engines have bigger (317cc) injectors, right?

Please enlighten my dumbness, what does BSFC stand for?

 

[Glenn]

Glenn,
I've used your "formula" to calculate my A/F. It looks like that (see attachment), and it looks nearly perfect (i've used 282 for inj. capacity).

Yeah, the shape of the curve looks about right. I think the formula is in the ball park.

This was only a second gear run with a peak MAF reading of 158g/s. With my highest ever MAF reading ~173 g/s MAF (which means +9.5% air, but no more fuel), the AF above 4200 is (or should be) around 12.7 which is the optimal value for peak power.

I remember max saying a while ago that few cars run bang on stoich at WOT as it's safer to run slightly rich than risk leaning out. Plus, I think he said that the map had to take into account different ambients as without closed loop (at WOT) there's no way to compensate apart from pulling timing and/or boost as a result of knock sensors triggering?

180HP engines have bigger (317cc) injectors, right?

I don't know about the Leons, etc but I'm pretty sure the Ibiza R is identical to the normal 156bhp 20VT apart from the map.

Please enlighten my dumbness, what does BSFC stand for?

Brake Specific Fuel Consumption - it's a measure of how much fuel is consumed to produce 1bhp for a given time period. Most of theory I've read has been on American sites and they tend to work in Imperial units so I follow suit. Hence my BSFC is quoted in lbs(of fuel)/bhp/hour. It gives a band of achieveable bhp numbers based on injector size as it varies from 0.4 (F1 engines?) to ~0.65 (Forced Induction). 0.5 is normally quoted as the average for N/A engines.

((Injector rating (in cc/min) x no. of injectors x duty cycle) / 10.5 ) / BSFC (in lbs/hr) = bhp

The 10.5 is to convert from cc/min to lbs/hour but I've also seen 10.2 used - I haven't done the proof to see which is correct.

I use this, air consumption, inlet valve size & configuration plus cubic capacity & best guess at V.E. to determine whether or not power claims are realistic - I don't trust dynos. All 4 of those methods, while only giving a rule of thumb do tend to come up with similar numbers on the engines I've looked at - 1.8T & VR6.

 

[Sim]

Glenn,

Thanks for the explanation. There's a new lesson every day .

Regarding to the 180BHP injectors, yes i was regarding to K03S equipped cars (Leons, etc)... According to an injector database which i've found on vwvortex, these cars should have 317cc injectors. Ibizas got 282cc, and theres an even smaller 241cc for some old passats, A4s.

Even the chipped 225hp S3/TT should run their injectors on full duty cycle then (360cc/min), and it looks they are up to their job as i havent read about any molten S3s/TTs yet.

360cc/282cc = ~28% bigger injectors
260hp/200hp = ~30% higher performance

.. so i persume these injectors are running under similar conditions

 

[wild willy]

Yeah, the shape of the curve looks about right. I think the formula is in the ball park.



I remember max saying a while ago that few cars run bang on stoich at WOT as it's safer to run slightly rich than risk leaning out. Plus, I think he said that the map had to take into account different ambients as without closed loop (at WOT) there's no way to compensate apart from pulling timing and/or boost as a result of knock sensors triggering?



I don't know about the Leons, etc but I'm pretty sure the Ibiza R is identical to the normal 156bhp 20VT apart from the map.



Brake Specific Fuel Consumption - it's a measure of how much fuel is consumed to produce 1bhp for a given time period. Most of theory I've read has been on American sites and they tend to work in Imperial units so I follow suit. Hence my BSFC is quoted in lbs(of fuel)/bhp/hour. It gives a band of achieveable bhp numbers based on injector size as it varies from 0.4 (F1 engines?) to ~0.65 (Forced Induction). 0.5 is normally quoted as the average for N/A engines.

((Injector rating (in cc/min) x no. of injectors x duty cycle) / 10.5 ) / BSFC (in lbs/hr) = bhp

The 10.5 is to convert from cc/min to lbs/hour but I've also seen 10.2 used - I haven't done the proof to see which is correct.

I use this, air consumption, inlet valve size & configuration plus cubic capacity & best guess at V.E. to determine whether or not power claims are realistic - I don't trust dynos. All 4 of those methods, while only giving a rule of thumb do tend to come up with similar numbers on the engines I've looked at - 1.8T & VR6.

Are these calculations still valid for calculating max bhp achievable from injector size. what BSFC should i use for a turbo car (0.5 maybe).