Injector Pulse width and duty cycle
11-12-2005 | 12:45 PM
#1
Injector Pulse width and duty cycle So, at 7000 rpm, how long is it from the time the intake valve opens from one intake stroke to the next?
7000rpm/2 (for a 4 stroke)=3500 intake events per minute per cylender
3500/60 seconds= 58 intake events per cylender per second (almost 60hz)
1/58= 17ms
Thats only 17ms to inject enough fuel!
So, with 42lb/hr injectors,
42lb / 6lb per gallon = 7.66 Gallons per hour (100% duty cycle)
7.66 Gallons per hour / 3600 seconds per hour = .00212 Gallons per second
.00212 gallons per second / 58 events per second = 3.6686x10-5 gallons or 0.1388716 cubic centimeters per intake event (100% Duty cyle)
So at 80% duty cycle at 7000 rpm thats .111 cc's of fuel 58 times a second...
Food for thought...
7000rpm/2 (for a 4 stroke)=3500 intake events per minute per cylender
3500/60 seconds= 58 intake events per cylender per second (almost 60hz)
1/58= 17ms
Thats only 17ms to inject enough fuel!
So, with 42lb/hr injectors,
42lb / 6lb per gallon = 7.66 Gallons per hour (100% duty cycle)
7.66 Gallons per hour / 3600 seconds per hour = .00212 Gallons per second
.00212 gallons per second / 58 events per second = 3.6686x10-5 gallons or 0.1388716 cubic centimeters per intake event (100% Duty cyle)
So at 80% duty cycle at 7000 rpm thats .111 cc's of fuel 58 times a second...
Food for thought...
11-12-2005 | 12:55 PM
#2
Careful.... you're getting "too technical" for LS1Tech!
We don't ususally think about the injector duty cycle that far up... typically it is just 6000 rpms that are considered. As you point out... it is a damn short time to get all your fuel in. It certainly illustrates the need to have a fuel delivery system up tot hte task (pressure and volume)!
We don't ususally think about the injector duty cycle that far up... typically it is just 6000 rpms that are considered. As you point out... it is a damn short time to get all your fuel in. It certainly illustrates the need to have a fuel delivery system up tot hte task (pressure and volume)!
11-12-2005 | 01:09 PM
#3
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From: LT1 land...the "409" of the 90s!
Nice numbers Ryan! that would make a nice sticky 
Jim's car actually hit the highest numbers just as we dug in for the runs at 15Ms The next peak was at 5k or so(tq peak) at a little over 13 and as the run went on fell to 12 or so and stayed there. The injectors may be a little big but they are working good We have them Idling at 1.9-2.2PW at 46Psi
so Ryan how does that come out?
Jim's car actually hit the highest numbers just as we dug in for the runs at 15Ms The next peak was at 5k or so(tq peak) at a little over 13 and as the run went on fell to 12 or so and stayed there. The injectors may be a little big but they are working good
We have them Idling at 1.9-2.2PW at 46Psi so Ryan how does that come out?
11-12-2005 | 01:41 PM
#4
Ryan, first i am just trying to understand what you have presented, and don't doubt that you know what your talking about.
How does this relate to engine size and hp, as the normal calc doesn't take rpm into consideration.
For instance, normal injector calcutator:
(projected bhp x bsfc .5 n/a or .6 F/I) / (no of injetors x duty cycle 80% .8)
So using my car as an example (I use .55 for n2o):
550rwhp - 12% drivetrain lose =
(484bhp x .55) / (8 x .8) =
266.2 / 6.4 = 41.59 or 42lb injector to meet rec 80% duty cycle max
My max shift point is 6500 so,
6500 / 2 = 3250
3250 / 60 = 54 intake events per cly per sec
1/54 = ? ms, this is where I get lost. Could you write out your equations so the normal guy can just plug numbers in. This will explain what/when/why (like the first written out equation for calcing inject) Just trying to understand and add to my knowledge base. Thanks
Robert
How does this relate to engine size and hp, as the normal calc doesn't take rpm into consideration.
For instance, normal injector calcutator:
(projected bhp x bsfc .5 n/a or .6 F/I) / (no of injetors x duty cycle 80% .8)
So using my car as an example (I use .55 for n2o):
550rwhp - 12% drivetrain lose =
(484bhp x .55) / (8 x .8) =
266.2 / 6.4 = 41.59 or 42lb injector to meet rec 80% duty cycle max
My max shift point is 6500 so,
6500 / 2 = 3250
3250 / 60 = 54 intake events per cly per sec
1/54 = ? ms, this is where I get lost. Could you write out your equations so the normal guy can just plug numbers in. This will explain what/when/why (like the first written out equation for calcing inject) Just trying to understand and add to my knowledge base. Thanks
Robert
11-12-2005 | 02:12 PM
#5
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From: LT1 land...the "409" of the 90s!
Ryans post was food for thought and not a way to find injector size or a balance of fuel to hp and use.
I believe it was more a of a food for thought in the life of an injector using a particualar injector size
I believe it was more a of a food for thought in the life of an injector using a particualar injector size
11-12-2005 | 02:43 PM
#6
Originally Posted by TwoFast4Lv
Ryans post was food for thought and not a way to find injector size or a balance of fuel to hp and use.
I believe it was more a of a food for thought in the life of an injector using a particualar injector size
I believe it was more a of a food for thought in the life of an injector using a particualar injector size
Robert
11-12-2005 | 09:38 PM
#7
If you take the inverse of the events per second, you get the time of each event in seconds (7000rpm, 58 events per second = 0.017 seconds per event). That is the time between subsequent intake events (again per cylender). So, at that RPM if you had an injector PW of 17 ms you would be at 100% DC.
For 6500 rpm, you would get the inverse of 54 or 0.018518 Seconds or 18.5 ms. So if you take your injector PW devided by 18.5 ms (@6500 rpm) you would get your Duty Cylce.
Ryan
For 6500 rpm, you would get the inverse of 54 or 0.018518 Seconds or 18.5 ms. So if you take your injector PW devided by 18.5 ms (@6500 rpm) you would get your Duty Cylce.
Ryan
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11-13-2005 | 12:53 PM
#8
Nice post Ryan! I see too many people post saying their stock 28s are good to 500rwhp up to 7500rpm. And while it MAY be true they can keep up, they will certainly be at or above the 80% DC, probably maxed out. The end result is they are starving the engine by running the fuel thru HOT injectors. Solenoids tend to not work right when they get too hot.....
11-13-2005 | 02:28 PM
#9
Originally Posted by Ryan K
If you take the inverse of the events per second, you get the time of each event in seconds (7000rpm, 58 events per second = 0.017 seconds per event). That is the time between subsequent intake events (again per cylender). So, at that RPM if you had an injector PW of 17 ms you would be at 100% DC.
For 6500 rpm, you would get the inverse of 54 or 0.018518 Seconds or 18.5 ms. So if you take your injector PW devided by 18.5 ms (@6500 rpm) you would get your Duty Cylce.
Ryan
For 6500 rpm, you would get the inverse of 54 or 0.018518 Seconds or 18.5 ms. So if you take your injector PW devided by 18.5 ms (@6500 rpm) you would get your Duty Cylce.
Ryan
Robert
11-13-2005 | 05:54 PM
#11
Originally Posted by Ryan K
Pretty much, it also shows the relation of Duty cycle and RPM.
Tho if you play it safe & follow the formula you don't need to be as smart as Ryan.
Last edited by WAHUSKER; 11-14-2005 at 11:26 AM.
11-14-2005 | 11:35 AM
#12
Someone told me they didn't really understand what this thread was saying so let me try to put it another way. PulseWidth is the length of time the injector is on, and DutyCycle is the % of time it is on vs the time it's off. So in English what Ryan is saying is if you have 42lb injectors, and they need to be on for 20mS (PW) to supply enough fuel for your 700hp engine, at 7000 rpm the DC is only 17mS long, so they can't possibly flow enough fuel, meaning you need bigger injectors.
And if what I said isn't what you meant Ryan, feel free to slap me upside the head.
And if what I said isn't what you meant Ryan, feel free to slap me upside the head.
Last edited by WAHUSKER; 11-14-2005 at 12:46 PM.
11-14-2005 | 12:48 PM
#13
All I was trying to say is; think about what a fuel injector has to do.
Up top there are two points, one is the maximum miliseconds for injector firing and the second is the amount of fuel per injection. The second is more of a food for thought issue.
Ryan K.
Up top there are two points, one is the maximum miliseconds for injector firing and the second is the amount of fuel per injection. The second is more of a food for thought issue.
Ryan K.
11-14-2005 | 01:10 PM
#14
Robert posted some good information for Fuel injector sizing. Those are the basic calculations for figuing out which injectors to buy. But they are based on Thumb Rules for BSFC.
In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher. And most say that Nitrous Applications are at .55 lbs/hr. Thing to remember is this is the theoretical number, not set in stone your combination will vary the actual BSFC.
So,
550 HP (Flywheel) X .50 (NA BSFC) / 8 (number of injectors) X .8 (80% DC) = 42.9 lb/hr injectors
Now this number is the Actual injector flow rate. There are other variables that affect injector flow rate also:
1. Fuel pressure at the injector; LS1's fuel pressure regulator mantains 58-60 psi at the discharge of the fuel filter (F-body) ths location does not compensate for flow losses between that point and the fuel rail. All flow has resistance, as the flow increases, so does the resistance to flow. At high flow rates the fuel pressure at the fuel rail will decrease due to the head loss. Some LS1's drop fuel pressure to 51-52 lbs due to this flow loss.
2. Voltage; All fuel injectors are not ideal. Idealy the injector would open instantly when the PCM directed it to. However, there is some dead time before the injector actually starts flowing fuel. This dead time is called offset. Voltage has an impact of the injectors offset. Higher voltage/lower offset and vice versa.
3. Manifold pressure; Manifold pressure affects flow. Basically the injector flows based on Differential pressure. The higher the DP, the more it flows. So at vacuum the injector flows more fuel. If you're running FI, any pressure in the intake reduces the flow. For me, at 10 psi the effective Fuel pressure goes from an actual 51-52 psi (fuel) to 41-42 psi Effective. Because the fuel has to overcome that manifold pressure. Now if our fuel pressure was manifold regulated ( as manifold pressure increases, the FPR raises fuel pressure) this aspect would be less of an issue.
Ok, my fingures are tired, I'll add more later.
Ryan K. Out!
In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher. And most say that Nitrous Applications are at .55 lbs/hr. Thing to remember is this is the theoretical number, not set in stone your combination will vary the actual BSFC.
So,
550 HP (Flywheel) X .50 (NA BSFC) / 8 (number of injectors) X .8 (80% DC) = 42.9 lb/hr injectors
Now this number is the Actual injector flow rate. There are other variables that affect injector flow rate also:
1. Fuel pressure at the injector; LS1's fuel pressure regulator mantains 58-60 psi at the discharge of the fuel filter (F-body) ths location does not compensate for flow losses between that point and the fuel rail. All flow has resistance, as the flow increases, so does the resistance to flow. At high flow rates the fuel pressure at the fuel rail will decrease due to the head loss. Some LS1's drop fuel pressure to 51-52 lbs due to this flow loss.
2. Voltage; All fuel injectors are not ideal. Idealy the injector would open instantly when the PCM directed it to. However, there is some dead time before the injector actually starts flowing fuel. This dead time is called offset. Voltage has an impact of the injectors offset. Higher voltage/lower offset and vice versa.
3. Manifold pressure; Manifold pressure affects flow. Basically the injector flows based on Differential pressure. The higher the DP, the more it flows. So at vacuum the injector flows more fuel. If you're running FI, any pressure in the intake reduces the flow. For me, at 10 psi the effective Fuel pressure goes from an actual 51-52 psi (fuel) to 41-42 psi Effective. Because the fuel has to overcome that manifold pressure. Now if our fuel pressure was manifold regulated ( as manifold pressure increases, the FPR raises fuel pressure) this aspect would be less of an issue.
Ok, my fingures are tired, I'll add more later.
Ryan K. Out!
11-14-2005 | 05:38 PM
#15
Originally Posted by Ryan K
Robert posted some good information for Fuel injector sizing. Those are the basic calculations for figuing out which injectors to buy. But they are based on Thumb Rules for BSFC.
In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher. And most say that Nitrous Applications are at .55 lbs/hr. Thing to remember is this is the theoretical number, not set in stone your combination will vary the actual BSFC.
So,
550 HP (Flywheel) X .50 (NA BSFC) / 8 (number of injectors) X .8 (80% DC) = 42.9 lb/hr injectors
Now this number is the Actual injector flow rate.
What I see now from what Ryan has presented is the fact that rpms play a role more so than some know. Meaning, my hp flattens out at about 5500rpm, so hp at that rpm compared to the same hp at say 7500rpm would call out for a different min injector size. The above formula isnt rpm specific, so I conclude that logic dictates theroretical is round about and actual can be had using a formula from Ryan's above info. I think we could come up with an injector sizing formula to include rpm that we could plug the #'s into?
There are other variables that affect injector flow rate also:
1. Fuel pressure at the injector; LS1's fuel pressure regulator mantains 58-60 psi at the discharge of the fuel filter (F-body) ths location does not compensate for flow losses between that point and the fuel rail. All flow has resistance, as the flow increases, so does the resistance to flow. At high flow rates the fuel pressure at the fuel rail will decrease due to the head loss. Some LS1's drop fuel pressure to 51-52 lbs due to this flow loss.
This is what I am doing to cure this problem. Moving regulator to the rails and return from there to tank. Boost referenced for you FI guys. No more pressure loss for us dry guys. (I know Ryan knows this allready)
2. Voltage; All fuel injectors are not ideal. Idealy the injector would open instantly when the PCM directed it to. However, there is some dead time before the injector actually starts flowing fuel. This dead time is called offset. Voltage has an impact of the injectors offset. Higher voltage/lower offset and vice versa.
This part of the lean spike problem for us dry guys. Isn't there a voltage booster set-up to help cure this problem?
3. Manifold pressure; Manifold pressure affects flow. Basically the injector flows based on Differential pressure. The higher the DP, the more it flows. So at vacuum the injector flows more fuel. If you're running FI, any pressure in the intake reduces the flow. For me, at 10 psi the effective Fuel pressure goes from an actual 51-52 psi (fuel) to 41-42 psi Effective. Because the fuel has to overcome that manifold pressure. Now if our fuel pressure was manifold regulated ( as manifold pressure increases, the FPR raises fuel pressure) this aspect would be less of an issue.
'97 vette return from rails and rail mounted boost referenced regulator, is one of the cures, for those reading along.
Ok, my fingures are tired, I'll add more later.
You spelled fingers wrong.
Ryan K. Out!
In most cases a naturally aspirated engine will have a B.S.F.C of .50. This means that the engine will use .50 lbs. of fuel per hour for each horsepower it produces. Turbocharged engines will want to be at .60 lbs. per hour or higher. And most say that Nitrous Applications are at .55 lbs/hr. Thing to remember is this is the theoretical number, not set in stone your combination will vary the actual BSFC.
So,
550 HP (Flywheel) X .50 (NA BSFC) / 8 (number of injectors) X .8 (80% DC) = 42.9 lb/hr injectors
Now this number is the Actual injector flow rate.
What I see now from what Ryan has presented is the fact that rpms play a role more so than some know. Meaning, my hp flattens out at about 5500rpm, so hp at that rpm compared to the same hp at say 7500rpm would call out for a different min injector size. The above formula isnt rpm specific, so I conclude that logic dictates theroretical is round about and actual can be had using a formula from Ryan's above info. I think we could come up with an injector sizing formula to include rpm that we could plug the #'s into?
There are other variables that affect injector flow rate also:
1. Fuel pressure at the injector; LS1's fuel pressure regulator mantains 58-60 psi at the discharge of the fuel filter (F-body) ths location does not compensate for flow losses between that point and the fuel rail. All flow has resistance, as the flow increases, so does the resistance to flow. At high flow rates the fuel pressure at the fuel rail will decrease due to the head loss. Some LS1's drop fuel pressure to 51-52 lbs due to this flow loss.
This is what I am doing to cure this problem. Moving regulator to the rails and return from there to tank. Boost referenced for you FI guys. No more pressure loss for us dry guys. (I know Ryan knows this allready)
2. Voltage; All fuel injectors are not ideal. Idealy the injector would open instantly when the PCM directed it to. However, there is some dead time before the injector actually starts flowing fuel. This dead time is called offset. Voltage has an impact of the injectors offset. Higher voltage/lower offset and vice versa.
This part of the lean spike problem for us dry guys. Isn't there a voltage booster set-up to help cure this problem?
3. Manifold pressure; Manifold pressure affects flow. Basically the injector flows based on Differential pressure. The higher the DP, the more it flows. So at vacuum the injector flows more fuel. If you're running FI, any pressure in the intake reduces the flow. For me, at 10 psi the effective Fuel pressure goes from an actual 51-52 psi (fuel) to 41-42 psi Effective. Because the fuel has to overcome that manifold pressure. Now if our fuel pressure was manifold regulated ( as manifold pressure increases, the FPR raises fuel pressure) this aspect would be less of an issue.
'97 vette return from rails and rail mounted boost referenced regulator, is one of the cures, for those reading along.
Ok, my fingures are tired, I'll add more later.
You spelled fingers wrong.
Ryan K. Out!
11-15-2005 | 11:42 AM
#16
So if you run bigger lines & rails, you don't lose flow at higher line pressure (from bigger fuel pumps)? Much like resistance in an electrical circuit limiting the current flow. The regulator on the fuel rail makes sense. Monitor it at the load.
Thinking about this in electrical terms, increasing the fuel pressure (like voltage) will increase flow (current) to the point the line (wire) is maxed out. A wire will burn up past that point, where the fuel line would just restrict flow (until it burst). I wonder what the stock lines are good to? Is there a formula for figuring line size vs. hp? Or even fuel pump volume?
And I thought the injector flow was based on MAF not MAP. That is a new concern all of the sudden. Will a 2 bar MAP help?
Thinking about this in electrical terms, increasing the fuel pressure (like voltage) will increase flow (current) to the point the line (wire) is maxed out. A wire will burn up past that point, where the fuel line would just restrict flow (until it burst). I wonder what the stock lines are good to? Is there a formula for figuring line size vs. hp? Or even fuel pump volume?
And I thought the injector flow was based on MAF not MAP. That is a new concern all of the sudden. Will a 2 bar MAP help?
11-15-2005 | 12:31 PM
#17
I know the stock '97 vette fuel rails and rail mounted reg is said to be good for 600+hp. You can also replace stock reg with an adjustable. Rails can be purchased from chevy. Most of the FI vette guys do this (use '97 rails) unless they are doing mondo hp, then on to bigger rails and lines.
Robert
Robert
