Archive for December, 2009

Anticipatory control for hybrids

Wednesday, December 30th, 2009

Something that’s been in the back of my mind came to the fore on reading this release from Clemson University.  Also this project from NREL.

Variability of fuel economy with speed and torque is a given for a conventional vehicle.  At one extreme, idling is a complete waste of fuel (zero mpg), and high engine speeds are also inefficient; but fuel efficiency varies widely even with vehicle speed, acceleration, hill climbing, headwinds, etc. 

At a macro level, if we had perfect foresight of traffic conditions, for instance, we could choose the ‘best’ route, given constraints of desired journey time, maximum speeds, etc.

At a micro level, knowledge of neighboring vehicles’ positions, speeds and likely future positions would be helpful in navigating through traffic.  This could be tied in with traffic-following cruise controls.  These seem like the problems the Clemson team is tackling.

But the world of hybrids is more complicated.  At any point, the control system must make a choice in balancing the energy delivered electrically or chemically.  Should the ICE or the battery run the car, and when should the ICE charge the battery?  The general intent is to keep the ICE operating at its optimum speed/load points, for peak fuel efficiency, as all energy used in a hybrid is ultimately chemical .

Take a long uphill stretch as an example.  If the control system knew it was coming, it might ensure the battery was fully-charged at the bottom, and then depleted at a rate to reach its minimum charge at the crest, especially if the subsequent downhill would allow regenerative braking to recharge the battery.

Or, approaching a steep downhill, the system would know that regenerative braking was imminent, and could use up as much electricity as possible beforehand.  If it started going downhill with a charged battery, the regenerative energy would be wasted and the friction brakes would wear more than necessary.

The two ‘bad’ conditions would be those above, either facing a high-torque requirement with a depleted battery or a regenerative event with a full one.  But more than this, it should be possible to optimize the rate of battery charge/discharge if the future power requirement were known.

This cannot be too difficult for a commuting route.  The road geography is the same every day, and although traffic conditions may change, I suspect they would not be too variable.  In such a case, it would be possible for the driver to indicate ‘we are on the commute’, or the vehicle could just assume that since it was starting from home or the office at a given time, it would be following the usual route…  for infreqent trips, the route optimizer could be linked to the sat-nav system (or even influence it) to get some idea of the geography ahead.

A GPS logger would provide all the information necessary to build a profile of the commuting route – the more difficult task would be to take that data and program the hybrid controller, but it’s just a case of modeling the power train in the hybrid, and making adjustments to the hybrid control program?  We could add some settings for the desired economy/speed/performance tradeoffs, sedate-to-sporty.  I would be interested to find how much difference this would make to fuel economy.

UPS hybrid vans save 30% in fuel

Wednesday, December 30th, 2009

An interesting report here from NREL, although it’s actually an update of a program they ran in 2008.  Six of the smaller P70 UPS vans were converted by Eaton to their light truck parallel hybrid system:  complicated gearbox, 26/44 kW AC motor and 1.8 kWh Li-ion battery.  It seems the base engine stays the same, a Mercedes 904 4-cylinder diesel.

Reading between the lines, this appears to have been a purchase by UPS from Eaton (OEM was Freightliner), and NREL got to see some of the data, as there are several references to being unable to obtain all the information they wanted.  NREL monitors a number of similar projects, including FedEx and Coca Cola and Frito Lay delivery vehicles.

The bottom line is a 30% improvement in fuel economy from 10 to 13 mpg.  It’s not an apples to apples comparison, as the control group of unmodified diesels did more high-speed driving and longer routes, etc, but seems to be close enough for the 30% figure to be reliable.

The study covered almost 12 months, during which the vans covered about 20,000 miles at around 75 miles/day, which seems to be normal UPS activity.  They averaged 1.4 stop/starts per mile on the route, or 94 stop/starts per day, and 16 acceleration/braking events per mile, which is going to be a significant parameter for a hybrid vehicle.

It’s not clear from the NREL paper exactly how the diesel engines were controlled – were they shut down at a stop, like a Prius, for instance, and were they re-tuned in any way?  Normally a hybrid would be able to use a smaller ICE engine, tuned more for economy than power/acceleration, like the Prius with it’s Atkinson cycle – it’s not clear whether Eaton does this, and indeed I don’t know what would be equivalent to the Atkinson trick for a Diesel engine.

If Eaton didn’t touch the diesel engine, then the 30% gains pretty much all come from regenerative braking, which is higher than one might expect; but it might be possible for a delivery van with many stop/start events to see that degree of improvement.

No word on the weight of the vehicles.  Since the original diesel engine was retained, the transmission, electric motor, battery pack and electronics would be extra, on top of the original weight, so it has to be heavier.

There are some other results in the report which are really separate – NREL took two heavier UPS vans, one diesel, the other hybridized, and tested them on a rolling road.  This is where the NOx came out higher for the hybrid, although it beat the diesel for all other pollutants:  it’s not clear to me how one rationalizes this result… I don’t see how hybridizing an engine would make it emit more Nitrogen oxides – maybe it’s tuning again?

So a couple of unresolved questions, but an encouraging result.  30% fuel savings over a cumulative 120,000 miles should be useful.  From the financial standpoint, the report doesn’t disclose UPS’s cost of fuel or the increased capital cost of the vehicle, but it does suggest that maintenance costs weren’t any higher than for the diesel vans.

One would think that stop-start vehicles would be attractive candidates for hybridization, and if they don’t travel too many miles in the day, for full electrification.  Those stop-start cycles are really inefficient for conventional engines.  And the fleet operators like UPS do full due diligence on the financial side – they will not start to buy in quantity until the return on investment is assured.