What factors determine the amount of assist (if any) the IMA_system uses when accelerating?
Manuel Santos –
CleanMPG – July 14, 2008
2008 Honda Civic Hybrid (HCH-II).
Most of the assertions I will include in this article will be mainly related to the IMA Gen4 (HCH-II) as its inputs and relative complexity has a few differences with the previous generations.
As many have noticed the electric assist routines on the HCH-II can change at times to the point of almost seeming unpredictable. For many HCH-II owners this can be as puzzling as it can be frustrating leading many to believe that some sort of manual control of assist would be best and more helpful at least from a fuel economy perspective.
But when we look at this assist issue more carefully, particularly in the context of some technical information, many patterns often evidenced by thresholds begin to emerge. I will try to identify those patterns and thresholds here in an attempt to explain the why's and how's of the assist routines.
Ambient temperature
Ambient temperature actually plays a very significant role and at times, more so than the other operational temperatures (ICE, Transmission, IPU, NiMH modules, fuel management and emissions control system). This is because it sets the stage for an off-set temperature window that affects the governance of the other system's own operational limits. Of course there are some systems that have very specific and rather inflexible operational constraints but in general their operational range is also wide enough for them not to be a primary constraint.
So to keep it simple, ambient temperature is a determinant of the amount of assist. For example very low temperatures tend to call for an increase of regen affinity. The same is often observed for very high ambient temperatures. This behavior is by design and Honda can alter it with software updates.
Engine Temperature
Engine temperature can also have an impact on the amount of assist used lower speeds depending. But the main reason why the assist affinity is often increased is not so much to cool the battery modules, improve torque or improve fuel economy but rather to smooth the power delivery curve and help eliminate ICE roughness at certain modes.
Honda has found that the application of moderate levels of assist during the engine warm-up phase can have a very positive impact on the overall levels of vibration as well as engine reliability. The lower amounts of stress due to active vibration management allowed for weight reductions and other optimizations factored into the ICE design which also lowered the manufacturing costs (every little bit, however small counts).
State of Charge
Yes. The SoC plays a very important role in increasing the assist affinity. To keep it simple, lets us just say that the NiMH chemistry has a very characteristic charge curve and that a charge level at either end of the spectrum is neither optimal nor desirable. In other words, too high or too low a charge is not something the battery management routines will allow for very long and either an aggressive assist or forced regen is often used to bring back the SoC to the optimal operational range. This optimal range generally persists in the 60%-80% range (give or take a few percentage points).
So, if we have 7 bars SoC or better then we'll have a very hard time preventing the overly aggressive assist. Likewise, if the SoC is at 4 or lower... or a forced regen pattern is underway, then preventing regen is harder to prevent. Depending on the SoC levels (and low module temperature) this regen pattern may only be somewhat avoided if the SoC reaches five bars or you switch the engine off and on again.
This behavior is by design and Honda can alter it with software updates.
NiMH Battery Module Temperature
This is one of those inputs that is often overlooked. To keep it simple: When the module temps are too high the assist is VERY hard to avoid. A module discharge has the effect of cooling the pack - Go figure
Of course, there is also the SoC level acting as a limit before the IPU finally enters the dreaded managed mode.
The reverse is also true. The BCM will prefer and call for extended periods of regen to warm up the pack in very cold days.
This behavior is by design and Honda can alter it with software updates.
Vehicle velocity
The assist affinity will decrease as the velocity increases. There are several magical speed boundaries but the most important for most of us is 37km/h (23MPH). Beyond this threshold, the assist affinity is almost halved. This means that speeds up to almost 40km/h (25mph) require nerves of steel or some kind of a trick to avoid hitting EV assist. Beyond this speed marker it becomes easier to avoid it.
I should also mention that "road grade" plays a very important role along with the speed in either enhancing or diminishing the assist affinity.
This behavior is by design and Honda can alter it with software updates.
Managed Mode or System Faults
Managed mode is a containment (or protection sequence) that the system enters when the safe operational boundaries have been breached. This can happen in extremely cold days as well as extremely hot days. During this sequence, assist and regen can be significantly altered or even completely canceled. I was told that this managed mode can also be evoked by a system fault and that the engineering group was considering a possible separation of the two scenarios... I guess it really depends on how the platform ages and what challenges the field maintenance feedback will report on.
At some point I'll be posting a couple of pictures of an assist inhibit module that I had installed on my HCH-II back in the fall of 2006. This module was quite unique in that it was one of just two of the same kind and also because it attempted to solve the assist problem.
Later, I looked for other simpler and more affordable ways to influence the assist affinity, especially inhibiting it, even after it felt at times that it was more like trying to shoot a moving target especially after a new software update.
As I found out a while later, I simply did not have enough interest from the HCH-II community in such a mod and it really made no sense for me to waste my time pursuing a simpler or more affordable solution.
Also, as yet another blow, I also found that what I did was not only the "wrong approach" but also very expensive in more ways than one. But that is entirely another story.
At the moment, my hopes for the strategic inhibiting of assist rests in the hands of the engineering teams working on the upcoming platforms. I just hope that I was convincing enough to get them to recognize the needs and wishes of the community.
Until then we'll have to utilize whatever tricks and techniques we can devise and put them to the best use possible without undermining the systems or our warranties
Anyhow, feel free to post whatever feedback, or assist inhibiting tips of your own to this thread.
Cheers
MSantos
! 
Linear Mode
