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Old 02-08-2014, 06:40 AM   #57
Former Vendor
Member#: 110029
Join Date: Mar 2006
Location: Sussex, England
2004 Citroen Picasso

Default Progressive Failsafe vs Band-gap Failsafe…

There has been a great deal discussions between the two most common failsafes algorithms. Progressive Failsafe vs Band-gap Failsafe methods. Here is our respond to the merit and pitfall of the two...

If you think the Aquamist failsafe can only detect "too little" or "too much" flow as expected from a fixed failsafe window width, think again. Our competition was preaching this to the masses, but they should have delved a bit deeper before launching their negative marketing campaign.

Simple answer, the “hi-lo” trip can detect a partial clog. If you set the "lo-trip” to 150cc/m and "hi-trip" to 500cc/m, anything outside this window will trip a failsafe. So though it seems Aquamist can only detect “too little” or “too much” flow, if one looks a little bit closer things become more clear…

Suppose the flow is dropped by 10%, (now 135c/m to 450cc/m) due to a partial clog. The failsafe will trip at the "lo side” of the window at the beginning of a spray event. Vice versa, if flow is increased by 10%, (now 165cc to 550cc/m) due to a leaky hose, the failsafe will trip at the "hi side” of the window. So who is not telling the truth?

Aquamist has tried the dynamic window method but it performed badly under a fast dynamic load change on the bench as well as in real world tests. Simple tasks such as a normal gear change will trip the failsafe at 10% error band. The failsafe continues to trip until it is set beyond 40% error band. This is caused by the turbine flow sensor over and under shooting and not keeping pace with the flow change produced by the fast acting valve. The only way to solve this problem is by applying a powerful electronic low-pass filter, but we have discovered that even doing so, the overall failsafe response time is too slow, much slower than the current setup. Consequently, we have decided not to offer this option until we can address the problem satisfactorily. At the present we prefer 10% error to 40%.

Maybe the much hyped up "dynamic-window" failsafe system only works with pump speed systems, which is probably due to the system's inability to respond quickly load changes. A few Aquamist users tried to mix and match and ended up with a very loosely-defined failsafe window and eventually returned to the original band gap method (failsafe window).

The latest HFS4 is equipped with an additional 0-5V input. It can be used in parallel with the standard failsafe. If AFR, EGT or IAT exceeds a certain set level, it will drop boost or switch maps. To us, looking ahead or capturing a problem as it happens, is a far better way, even if there are delays and over generous error bands. Take a closer look at the distribution of the captured flow curves in the advertised images, especially at the lower section of the flow range.

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