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CPI2 current draw

N546RV

Well Known Member
Working on load planning and electrical system design, and I'm planning on dual CPI2 electronic ignition. However, there seems to be a discrepancy between sources as to what current draw to plan for.

The CPI2 product page says:
Low current draw/ long spark duration. 4 cylinder coil pack and controller draw about 1.2 amps at 2500 rpm.

...but the installation manual, when discussing wiring concerns (page 5), says of the controller:

The Purple 20 ga on the 14pin main harness plug, needs a 2 to 5 amp fuse or breaker. Current draw is less than 1 amp on this wire.

...and of the coil power:

With SDS supplied coilpacks, average current draw is less than 6 amps below 5000 rpms.

Now, I do realize that, taking the "less than" wording used in the manual literally, there's no disagreement here, since 1.2 is definitively less than 7, but the manual seems to imply a higher current draw.

Long story short, I just want to verify that I'm choosing the proper values for load planning (particularly in regards to planning for various failures).

Thanks!
 
A couple of general notes here.

4 cylinder coil packs draw less current than 6 cylinder ones at a given rpm.

Current draw generally increases with increasing rpm (higher spark frequency). Pulling back the rpm in the event of an alternator failure should help extend your running time.

The CPI-2 has the ability to reduce coil charging time when a low voltage situation occurs to extend battery life. This results in lower average current draw. Coil saturation is reduced in this case which also reduces spark energy but will still fire the plugs in the relatively unchallenging environment of a naturally aspirated Lycoming engine at cruise power.

I'll try to run some bench tests tomorrow and get some more precise numbers, maybe shoot some video of both coil pack types running.

The CPI-2 controller and display don't draw much current, if memory serves me correctly, less than 0.6 amps but I will check that too so folks can get a total picture.
 
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Thanks, Ross, that sounds great. I've got to say, your activity and willingness to help here on VAF is a big reason I decided to go with the CPI2 - it's definitely appreciated by me.
 
We managed to run some tests here today so I'll try to sift the results and put up a vid and some numbers here soon.
 
Current Draw Numbers

Ok here are some numbers from the testing today:

4 Cylinder, single coil pack and controller

1000 rpm- .76 amps
1500- 1.04
2000- 1.31
2500- 2.09
3000- 1.82

Current saving mode on backup battery at 2500 rpm- .77 amps

________________________________________________________________

6 Cylinder, single coil pack and controller

1000 rpm- .78 amps
1500- 1.07
2000- 1.36
2500- 2.28
3000- 1.92

Current saving mode on backup battery at 2500 rpm- .89 amps


It's interesting to see the differences in current draw between the two at different rpms. These are two different coil brands. The 4 cylinder has integral drive transistors, the 6 cylinder uses an external transistor module. These transistors have different characteristics and the software is doing some things behind the scenes as well which explains the drop in current between 2500 and 3000 rpm on both setups.

Other factors like Magnet Position and timing value can change the current draws slightly but in this test, we kept those the same to make it apples to apples.

Note that a dual system running twin coil packs, would draw around double the current shown here.

You can see the value of the current saving strategy here in extending flight time on the backup battery.

I'll try to get the video up shortly.
 
Thanks, Ross, that sounds great. I've got to say, your activity and willingness to help here on VAF is a big reason I decided to go with the CPI2 - it's definitely appreciated by me.


I will jump on this recognition of great service!

I am installing Ross's Dual ECU with fuel injection on my -10. The support from Barry has been nothing short of outstanding. I was working through a suspect issue earlier this week and these guys overnighted a part to me in one day. Very pleasant and knowledgeable on the phone, good at helping to troubleshoot.

No regrets at all with my decision to do business with these guys. You feel you are a valuable Customer with every interaction.


Steve
 
Thank you Steve. Every customer is important to us. Customers are the ones who sign our paychecks effectively and allow us to continue improving our products which make them more functional, attractive and reliable.

I enjoy coming to work every day, talking to pilots and builders all over the world and working on our new projects, R&D and building hardware. While I put in a lot of hours, it's very satisfying to me.

Folks in Experimental aviation are the best. We see we are part of a community of friends who share a love for aviation, building and flying.

Flying the airplane you built is a very rewarding experience. We want to see folks enjoying that experience.

If we screwed up something or could improve something, let us know. We want to make it right or better.
 
Ross,

Could you expand on the effects of normal operation vs current saving mode?

Using the 4-cyl example, 2.09 amps vs 0.77 is significant. I assume the difference is entirely primary current, so the result should be a greater or lesser magnetic field. What is the result in terms of voltage rise rate and arc duration? Know if anyone has switched back and forth while running high and very lean?

We want to make it right or better.

Well said.
 
CPI-1

Ross,
Do you think us guys running two CPI-1 systems would have similar results? The thread using different backup battery types has me thinking.
Thanks,
 
Didn't want to hijack a CPI-2 thread, but since it was asked, I measured my CPI-1 at 1.69 amps at 2700RPM. This was a 6 cylinder, single ignition only.
 
Ross,

Could you expand on the effects of normal operation vs current saving mode?

Using the 4-cyl example, 2.09 amps vs 0.77 is significant. I assume the difference is entirely primary current, so the result should be a greater or lesser magnetic field. What is the result in terms of voltage rise rate and arc duration? Know if anyone has switched back and forth while running high and very lean?

In bench testing, looking at the wave form as the coil charges, peak current can reach around 10-12 amps. We typically charge the 4 cylinder coils for 3.6 ms on naturally aspirated Lycomings.

Cutting the charge duration by 25 or even 50% certainly reduces spark energy, you can hear the differences at the plug. I haven't measured how much but we know from engine testing that it will still light off the charge at reasonable AFRs and 25ish MAP. The point of current saving is strictly to get you down safely once the backup battery capacity has fallen to a pretty low level. At some point, it cannot deliver that initial high surge current and you get misfire. This would not be a useful strategy for normal running at very lean or rich mixtures or at high MAP. We don't implement the current saving until battery voltage falls below a pre-set level.

Could you save more power by charging the coils for less time in normal operation? Probably, but you'd have to experiment to know how much.

For the turbocharged Reno racers, we charge the coils longer to get closer to 100% saturation as we know duration at high power is less than 8 minutes so we are not so worried about heat distress on the coils or drive transistors. The 6 cylinder stuff has reliably ignited rich mixtures well above 90 inches MAP.
 
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Ross,
Do you think us guys running two CPI-1 systems would have similar results? The thread using different backup battery types has me thinking.
Thanks,

I'd expect current draw on a CPI would be slightly lower than the CPI-2 values as the controller draws a bit less current itself. The coil hardware and standard charging strategies are the same between the two.

The CPI doesn't have this same current saving feature as it lacks the logic and hardware to monitor the backup battery voltage.
 
In bench testing, looking at the wave form as the coil charges, peak current can reach around 10-12 amps. We typically charge the 4 cylinder coils for 3.6 ms on naturally aspirated Lycomings.

Cutting the charge duration by 25 or even 50% certainly reduces spark energy, you can hear the differences at the plug. I haven't measured how much but we know from engine testing that it will still light off the charge at reasonable AFRs and 25ish MAP. The point of current saving is strictly to get you down safely once the backup battery capacity has fallen to a pretty low level. At some point, it cannot deliver that initial high surge current and you get misfire. This would not be a useful strategy for normal running at very lean or rich mixtures or at high MAP. We don't implement the current saving until battery voltage falls below a pre-set level.

Could you save more power by charging the coils for less time in normal operation? Probably, but you'd have to experiment to know how much.

For the turbocharged Reno racers, we charge the coils longer to get closer to 100% saturation as we know duration at high power is less than 8 minutes so we are not so worried about heat distress on the coils or drive transistors. The 6 cylinder stuff has reliably ignited rich mixtures well above 90 inches MAP.

Ross,

You said that your ignition is not a capacitive discharge design (CDI) or has a step up regulator on the front end to smooth out the current draw from the battery. If that is the case then the battery will see the ignition coil charging current directly. The current into the coil will rise linearly inversely proportional to the inductance of the coil. The energy stored in the coil as a function of time is proportional to the square of the current. The current will continue to rise in the coil until one of two events occurs. 1. The magnetic flux density reaches the limit for the coil magnetic path material, the coil saturates, the inductance effectively drops to a very small number and the current rapidly rises to the limit defined by voltage divided by the coil resistance (or in circuit resistance). 2. The current rises to a value defined by the supply voltage divided by the in circuit resistance.
Case 1 may be a design possibility depending on the control on the charging time. Extending the charging time beyond the design limit will allow the coil to saturate. Based on the relationships of linear increase of current with time and stored energy being proportional to current squared it shouldn't be difficult to see that doubling the charging time will provide four times the energy. Your statement about most of the energy being stored in the first part of the charging cycle is clearly not correct. This is basic electrical engineering.
More importantly if your charging circuit is demanding large globs of current over a short time period from the battery then that creates a big problem for a battery that is well into the discharge cycle since the source impedence of most battery types rises as they run out of charge. The resultant large changes in battery terminal voltage make it really difficult for any monitoring circuit to do a reliable job of reporting battery voltage. Regarding the video. The upper trace on your oscilloscope had a lot of hit and miss pulses - what was it exactly? The current meter appeared to be indicating some mean/ average value but was clearly influenced by the current pulses. A more meaningful test case would be to put a high current variable resistor in series with the battery to simulate increase battery source resistance and get some data to show the effects of a discharged battery.
KT
 
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Just a clarification on my previous post. Energy stored in an inductor (or ignition coil) is a dynamic situation. The rate of rise in charging current is time dependent and can be considered as linear for timescales less than one time constant (given by L/R in seconds). The charging time for typical ignition system coils should be a fraction of a time constant. So if the current is going down during the later part of the charging cycle something is seriously wrong with the design. Compare the energy state of an inductor with the energy state of a vehicle moving. The energy equation is E=1/2 MV squared. The vehicle has inertia so there is time taken to accelerate up to the required velocity based on Force = Mass x Acceleration.
Capacitors can store energy (potential energy), inductors have an energy state (dynamic energy) and require time to change that state based on the circuit values of inductance and in circuit resistance.

KT
 
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