Just My 2 bits .. for the heck of it.
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Short Version:
Under any sufficiently varying load conditions a HEV system has a net energy efficiency (and performance) benefit compared to any ICE-Only option. But Energy Efficiency alone is not the only variable people use to choose. Use the right tool for the job.
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Stop hear / skip to next post .. unless you REALLY want the basic details of why that is the case.
Quote:
Originally Posted by EVmetro
Apparently, the ICE is being used to charge the battery for the electric motor, but it is doing it at an an advantageous time. I don't know this for sure, but I do know that there is not enough regen energy available to supply enough energy to supplement the ICE enough to make up for what the ICE lacks in power.Using the ICE to supply energy for the electric motor does not make sense, but I am speculating that this is what is happening, and that it is happening at an advantageous time to minimize the penalty for doing it.
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Let the cold emotionless math decide if it has enough or not .. or what makes sense or not .. For realistic expected conditions ... Than force yourself to accept that reality .. See Bellow.
Quote:
Originally Posted by EVmetro
Is there anything unique to a hybrid other than the ability to harvest and redistribute wasted braking energy that can't be done on an ICE?
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Yes... It improves the average operating efficiency of the ICE it's attached to .. under any sufficiently varying load conditions... but feel free to do your own math for other contexts and see where your own break even points are ... See Bellow.
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Initial Notes:
I am most familiar with my 2000 Honda Insight system .. so that is the one I used in the bellow.
The specific numbers (and break even points) spit out by the math will change (of course) if you change to other ICEs , other electronics, other conditions, etc... but the same science/engineering method can be applied to allow the cold emotionless math to spit out the answer (break even points, etc) for you for any known setup of ICE , HEV , conditions , etc.
The bellow 'summary' can easily be explored point by point in as much or as little additional depth as desired... but I figured it would be best to start with just the basic 'summary'.. Post is too long already as it is.
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Step #1 Define the variables.
#A> ICE-Only vs HEV system (access cost)
A common point raised is about the 'cost' (weight, efficiency , etc.) of adding a HEV system to a ICE-Only vehicle... so we will want to better 'quantify' that cost.
#B> Efficiency benefits HEV system vs ICE-Only System.
A common point raised is the efficiency benefits that HEV system gain that ICE-Only systems do not share ... so we will want to better 'quantify' that benefit.
#C> Efficiency / Performance ICE vs Electric Motor.
An ICE's operating efficiency and it's power out are far far more effected and varied over various RPMs/Loads than electric motors are... If you can have the ICE run 100% of the time only at it's peak efficiency point it's no contest the ICE-Only wins ... but , that isn't a realistic expectation of operating conditions ... sooo the only question becomes where is the break even point... We can use known operating (performance/efficiency) curves of the systems in order to better 'quantify' this variable.
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Step #2 Quantify Numbers
(1A)
The HEV system replaces the ICE-Only need for both Alternator and Starter .. sense it does both of those jobs as well... The weight of those from a ICE-Only get deducted from the HEV system weight in order to determine the net HEV system weight 'cost' for adding that HEV system.
Starter+Alternator = ~30 Lbs
HEV system that replaces those functions = OEM about ~120 Lbs
However it is possible (as shown) with KERS and other 'high end' components .. this weight (for same HEV performance) can be reduced to as little as ~30 Lbs.
OEM 120 - 30 = 90 Lbs HEV 'cost'
Minimum that Technologically would allow 30 - 30 = 0 Lbs ( HEV 'cost')
The weight effects the rolling resistance (no significant effect to Aerodynamic losses).
OEM 90 Lbs / ~1850 Lbs GVW = ~5% additional Weight
Minimum Technology would allow 0/1850 = 0% additional Weight
The rolling resistance Force increases linearly with increases in vehicle weight... F = Crr * N
The power (from Rolling Resistance) needed to move the vehicle increases linearly with weight ... P = F * V
Therefore the 0% to 5% additional weight will increase the rolling resistance by 0% to 5%.
The total vehicle needed power for any speed (flat) will be a combination of both Aero + Rolling resistances.
For the same OEM aerodynamics and tires .. the contribution ratio varies from over 90% Rolling under 13MPH ... to as little as 25% by 65 MPH... Which means the additional weight 'cost' is variable from as high as 5% to as low as 0% of the overall vehicle needed movement energy.
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(1B)
Regenerative braking is 100% gain for the HEV .. a ICE-Only vehicle will throw 100% of that energy away.
In heavy stop and go traffic this adds up quickly to large amount of energy.
In other types of driving it adds up slower ... I've read the 'average' US commute burns up ~6% of the fuel energy to braking alone ..
Link.
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(1C)
See attached bellow efficiency curves for the ICE and IMA(Motor+Controller).
The combined IMA motor + Controller efficiency curve is over 90% or above for over about ~95% of the expected 'average' operating conditions.
I've tested the NiMH cells used ... in the window the BCM keeps them ... they are around ~90% cycle efficient ... that drops if the cells were taken into a wider SoC operating window ... but they aren't OEM.
So conservatively we know it will (in most cases) do better than 90% 90% 90% = ~73% Round Trip OEM cycle efficiency.
Again here .. there are Better motors , controllers, batteries available ... that could bring this up to ~90% Round Trip Cycle Efficiency... but that was not OEM.
This alone brings us to between 73% and 90% of that 6% 'average' wasted braking ... as much as 4.3% to 5.4% MPG gain ... just from braking ... for the 'average' US commuter.... which is already practically paid for the HEV system weight cost by itself .. and possibly already at a small 1-2% net gain for the 'average' US commuter... at higher (highway speeds) , the braking 'benefit' reduces .. but so does the MPG penalty from rolling resistance of the additional weight.
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Now looking at the ICE efficiency curve ... we see it varies from under 26% to as high as 38%.
Energy taken from the ICE at 38% going through the HEV system (73% to 90%) ... can be returned to the wheel at a net ICE to wheel of ~28% to 34% Fuel Efficiency ... Thus it is a net benefit to do so .. any time using the ICE-Only would have had a net operating efficiency bellow that net HEV round trip... The smaller 35% Lean Burn efficiency window can still net a HEV round trip from 25% to 31% ... which can still under some conditions be more efficient than forcing the ICE-Only system to run the ICE under it's worst efficiency conditions.
This is taken advantage of more so in the Prius than the Insight (but the principle still applies to HEVs in general).
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Now looking at the difference in power curves.... see Attached
We see that the HEV system produces up to about ~50% of the total torque at low RPM ... without the HEV system the ICE-Only would either:
Take much much longer (under low ICE efficiency) to accelerate .. Which makes it less total net efficient.
Or ... as is the norm ... the ICE itself would get up-sized ... to a larger heavier ICE that still has 'enough' torque in the low RPM range... If you 'up-size' the ICE itself so that the ICE-Only can do all this low end torque itself ... the ICE growth weight penalty is larger than the HEV system weight penalty ... Compared to electric motors ... ICE's power to weight ratio's (at low RPM) SUCK horribly ... thus to double that ICE torque at low end ... it cost you far more ICE weight gain.
The other 'loss' from trying to upsize the ICE itself in a traditional ICE-Only system ... is that the much larger and much power powerful (at low RPM) ICE will now be even further away from it's higher operating efficiency range when not accelerating ... like on the highway cruising ans such ... which is a penalty to the ICE-Only ... the HEV style system tries to get a ICE sized small enough that it will operate dominantly in it's actual high efficiency range ... for the majority or the vehicle's "average" loads ... not sized for it's peak loads ... and let the HEV system take up the other 'short' periods when the vehicle needs additional torque/power.