Study: Combination of active and passive engine-off coasting delivers optimal fuel savings with 48V hybrid systems
13 April 2016
A study by Germany-based automotive supplier HELLA found that the combination of active and passive engine-off coasting and recuperation using a 48V mild hybrid system shows the best potential to reduce CO2 emissions for all car segments, with the highest potential in the larger car segments, for both NEDC and WLTP (especially true for real world driving).
On the other hand, passive engine-off coasting alone showed no CO2 reduction potential for NEDC and some in WLTP according to the paper presented at the 2016 SAE World Congress.
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Possible 48V vehicle architecture for active and passive engine-off coasting + recuperation + boosting. Brown et al. Click to enlarge. |
Passive engine-off coasting (also called sailing) goes one step further than the traditional “decal fuel-cutoff” method of coasting by placing the transmission into “neutral - disengaged mode” and ceasing fuel injection. This eliminates engine drag resulting in a lower rate of deceleration and saves fuel. Passive engine-off coasting can be realized using 12V or 48V power systems.
However, the HELLA team noted, during passive engine-off coasting, the vehicle speed may slow down more than anticipated due to wind drag, rolling and mechanical frictions. These external forces reduce the maximum potential for energy savings due to their leading to a premature application of the propulsion system in order to maintain a constant vehicle speed.
Active engine-off coasting, which requires the higher powered 48V system with a belt-starter-generator (BSG), enhances the coasting feature by applying recuperated energy to the BSG to provide torque to spin the engine with the transmission engaged in drive in order to keep the vehicle speed constant over a longer period of time by compensating for the vehicle drag.
Generally, 15kW is the minimum peak power level required to achieve the active engine-off coasting function up to 60 km/h (37 mph) depending on vehicle size and class.
In their study, the HELLA team simulated three different vehicles representing segments B, D and E to evaluate different 48V hybrid operating modes and their effect on CO2.
B segment: 1200kg vehicle with 1.2l gasoline engine, 5-speed transmission, 8kW 48V BSG and 60Ah 12V lead acid starter battery
D segment: 1500kg vehicle, 1.7l gasoline or 2.0l diesel, 7-speed transmission, 10kW 48V BSG, and 60Ah 12V starter battery
E segment: 1800kg vehicle with 2.0l gasoline or 2.2l diesel, 7-speed transmission, 15kW 48V BSG, and 60Ah 12V lead acid starter battery
They used Li-ion battery capacities of 5Ah, 10Ah, 15Ah and 20Ah for all vehicle segments.
The E-segment vehicle benefited the most, with up to a potential of 20 g/km of CO2 savings using active and passive engine-off coasting + recuperation). CO2 was reduced by 15% as compared to a reference micro-hybrid vehicle with only a stop/start function.
CO2 savings for active engine-off coasting during NEDC was, on average, 3% to 4% better than simple recuperation or all vehicle segments. During WLTP, when boosting was added to passive engine-off coasting, there was never any improvement in CO2 and the savings due to active engine-off coasting was not much higher for the upper car segments.
Active engine-off coasting showed almost twice the potential CO2 reduction than boosting alone during NEDC.
The CO2 reduction potential of active engine-off coasting, by itself, is slightly higher within the NEDC than in the WLTP. Additional boosting always reduces the CO2 savings to a high degree within both WLTP and NEDC. Clearly the combination of active and passive engine-off coasting leads to the highest saving potential for the WLTP, concluding that combining the two coasting methods is recommended for implementation using a 48V power system approach in order to achieve the maximum CO2 reduction potential.
—Brown et al.
Resources
Brown, A., Nalbach, M., Kahnt, S., and Korner, A. (2016) “CO2 Emissions Reduction via 48V Active Engine-Off Coasting,” SAE Int. J. Alt. Power. 5(1) doi: 10.4271/2016-01-1152
My Chevy VOLT and Telsas all have automatic regen back into the battery when you let off on the "gas". I turned it off recently and the car felt out of control when it silently coasted forward.
So I think this coasting idea is dumb - better to capture regen energy back to the 48v lithium-ion battery when the driver steps off the accelerator. At steady speeds supply a trickle of energy from the 48v to an electric assist motor while keeping the engine off as long as possible.
My 2 cents..
Posted by: Juan Valdez | 13 April 2016 at 06:40 AM
Regenerating when deceleration is not called for is wasteful. The hardware is far from 100% efficient, and taking kinetic energy away just to put it back throws energy away. There are absolutely times when sailing is called for, as well as regeneration.
One of the most annoying features of my Passat TDI was the programming of the transmission to keep the torque converter locked and downshift rather aggressively rather than coasting with the throttle lifted and the selector in "D". Defeating this unwanted engine braking required shifting into neutral, and the re-engagement into 3rd gear was quite harsh. VW should have programmed it to unlock the torque converter and stay in the highest possible gear until power was called for again.
Posted by: Engineer-Poet | 13 April 2016 at 08:23 PM
I agree with Juan. If you don't drive an EV you won't get it. Gas cars even LOOK out of control from a distance once you get used to regen. The point is that true "coasting" is really an extremely rare event when you have full control of a vehicle. Constant velocity is the most efficient in an EV, and almost never involves true coasting. Of course, true coasting is quite possible in an EV.
Posted by: James McLaughlin | 14 April 2016 at 09:57 AM
I drive a PHEV, and I use regeneration heavily. It is still more efficient to sail (coast) under many conditions than to either apply or recover power. I do this by manually shifting the CVT between modes as appropriate; I spend a lot of time in "L", for maximum regen without using the friction brakes.
Posted by: Engineer-Poet | 14 April 2016 at 10:39 AM
Run an engine at its highest efficiency or not at all or at highest power if really rarely needed in an electric series hybrid. Hydraulic hybrids can handle more power and regeneration at far lower costs; too bad they can't be bought due to Bosch-Rexroth and government inaction. Artemis was told by the UK to go tilt at wind-mills and now there are two 7 Mega-Watt ones. ..HG..
Posted by: Henry Gibson | 16 April 2016 at 10:56 PM
Well if they have hired Wayne Gertes of CleanMPG.com as a 'technical consultant,' they can incorporate hypermiler control laws. The cars will be a great success 'pulse and gliding' and running at an average speed under 25 mph. Yes, this will finally prove the success of hypermiling in a commercial product.
Bob Wilson
Posted by: Bob Wilson | 19 April 2016 at 11:39 PM