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NREL Evaluation of Gen II BAE/Orion Series Hybrid Transit Buses Finds Lower Fuel Economy than Gen I, But Much Lower Total Cost per Mile

Comparison of the cumulative total and propulsion-related maintenance costs per mile for Gen I and Gen II BAE/Orion series hybrid transit buses. Click to enlarge.

Second-generation BAE/Orion series hybrid transit buses in service in New York City exhibited 6.8% lower fuel economy than the Gen I hybrids with similar usage and duty cycles, according to a new report by the Fleet Test and Evaluation Team at the National Renewable Energy Laboratory (NREL). The increase in fuel consumption on the Gen II hybrids is likely due to the use of the EGR-equipped engine to meet a more stringent emissions profile, according to NREL.

However, the total cost per mile for the Gen II hybrid buses is 24% lower than that for the Gen I buses. This discrepancy is driven by the 39% lower maintenance costs in operating the Gen II hybrids, according to the NREL report.

Both Gen I and Gen II BAE/Orion series hybrids use a Cummins ISB 5.9-liter diesel (270 bhp/199 kW, 660 lb-ft/895 Nm) to drive a 160 hp/119 kW generator that powers a 250 hp/186 kW continuous (320 hp/239 kW peak) drive motor. Roof-mounted sealed lead-acid Hawker XT batteries provide the energy storage.

According to BAE Systems, four subsystems on Gen II have been improved, including the engine, generator, propulsion control, and cooling and packaging. The company expects these refinements to result in improved emissions; improved power; quieter operation; and improved reliability, durability, maintainability, and performance.

The Gen II buses are certified at 2.5 g/bhp-hr NOx compared to the Gen I rating of 4.0 g/bhp-hr NOx. Gen II buses are EGR-equipped and use Johnson Matthey CRT emissions equipment; Gen I buses use Englehard DPX emissions equipment.

Among the primary conclusions NREL drew from this evaluation are:

  • With similar usage and duty cycle, the Gen II hybrids exhibited a 6.8% lower fuel economy than the Gen I hybrids. This is likely due to the MY2004 EGR-equipped engine in the Gen II hybrids.

  • The Gen I hybrid fuel economy was essentially the same over two years of evaluation up through year number three of implementation. This indicates that the lead-acid battery chemistry is capable of consistent performance, in this duty cycle application, through the projected three year lifespan.

  • The total maintenance costs for the Gen II hybrids were 39% lower than the Gen I hybrids for each respective evaluation year number 1.

  • The propulsion-related maintenance costs for the Gen II hybrids were 55% lower than the Gen I hybrids for each respective evaluation year number 1.

  • The Gen II hybrids exhibited similar reliability (as measured in MBRCs) to the Gen I hybrids. Both Gen II and Gen I hybrids exceeded NYCT’s expectations in this arena. Due to lower maintenance costs, but despite lower fuel economy, the Gen II hybrids total operating cost per mile was 24% lower than the Gen I hybrids for each respective evaluation year.

In October 2007, MTA New York City Transit and MTA Bus announced they will take delivery of 850 new Orion VII low-floor hybrid electric buses by the end of 2009. (Earlier post.) This generation of BAE/Orion hybrids will incorporate lithium-ion energy storage supplied by A123Systems. When the complete order is delivered to the city by early 2010, the buses will make the MTA's diesel-electric hybrid bus fleet the largest in the world, with nearly 1,700 hybrid buses.

DOE, through the National Renewable Energy Laboratory (NREL), has been tracking and evaluating new propulsion systems in transit buses and trucks for more than 10 years using an established and documented evaluation protocol. These DOE/NREL vehicle evaluations are a part of the Advanced Vehicle Testing Activity (AVTA), which supports DOE’s Vehicle Technologies Program.

DOE/NREL evaluated the original 10 prototype diesel-hybrid buses from Orion and BAE Systems (model Orion VI buses) operated at New York City Transit (NYCT). That evaluation was reported in July 2002 and provided results from the prototype buses from 1998 through 2001. The next report focused on 10 new compressed natural gas (CNG) and 10 next generation diesel hybrid electric (equipped with BAE Systems’ HybriDrive propulsion system) buses from an order group of 125 (Gen I) at NYCT. This was reported in November 2006.




This points out one of the most underappreciated aspects of hybrids. LOWER MAINTENANCE COSTS. It has been noted elsewhere too, but most people just don't think about that when they think about hybrids.

For example, regenerative braking saves a ton on brake wear (and money). Prius owners generally shouldn't have to replace brakes for at least 100000 miles or more, for example.

Hybrids like Toyota and Ford don't even have clutches or a typical transmission (the ECVT should last for the life of the car). BIG savings.

You never have to replace an alternator or starter motor, because most full hybrids don't have them. (that's all built into the electric motors, which are very reliable)

Because full hybrids have an electric motor to supply most of the torque, the engine gets abused much less. No super high rpm, no lugging at low rpm, and the ICE doesn't have to idle and is even shut off part of the time. This saves a lot on wear and tear.

Sure, there's a CHANCE you MIGHT have to replace (a few cells in) a battery at some point, but so far those cases are few and far between. Prius Taxis have gone 240000 miles with no problems, for example. Even if you do have some battery cost at some point, you'll have saved so much in maintenance by then.


I sometimes worry about the wear and tear of the engine on my Camry Hybrid starting and stopping all the time.
Any comments?

Brian P

Don't worry about it, it's designed to handle it.


Yep, it's OK. An electric motor spins up your camry ICE to idling speed, and then fuel gets added. That's a much gentler way to to start an engine than in a traditional car.


Think about this:

As the electric motor brings your engine up to idle speeds the oil pump and water pump are running at normal speed BEFORE any stresses (heat, pressure, forces) are created by fuel ignition in the combustion chambers.

Jim G.

For a series hybrid bus, the generator has a simpler job than the Prius engine. It just wants to run at a fairly constant RPM, optimized for electrical generation.


"Roof-mounted sealed lead-acid Hawker XT batteries provide the energy storage."

The roof seems like a high place for heavy batteries. I have seen diesel CNG tanks up there, so maybe not.


so it got down to -50 here in saskatoon saskatchewan here today, if i were to get a hybrid would it start easier, because despite being plugged in my car barely and i mean BARELY made it, i'm sure i just pulled of 5000km of it's life on that one start. if they did start better on insanely cold weather that would be another great reason for me to get one



When you say "plugged in" are you talking about an engine block heater or a battery trickle charger or both? I haven't seen it colder than -25F here in Colorado and the combination of the two devices has let me start my diesel pickup easily at any temperature but maybe I'm just spoiled by the "mild" climate. My batteries and my truck are now six years old and keeping a trickle charger on the batteries and keeping the acid topped up with distilled water appears to be giving me very good battery life (despite the alternator dying by degrees just after the warranty expired and the batteries getting abused by erratic charging for a time).


You also have to wonder about battery performance at very low temperatures. We all have heard stories about car batteries not producing enough cranking amps in cold weather. A large 200+ volt battery bank in a hybrid car may be affected by the cold, but it still could have enough to turn over the engine.


When it's that cold, the hybrids run the ICE pretty much constant, so battery "performance" doesn't matter much. In fact, when its real cold, the battery gets TOO charged up, since calling for the heater to run prevents all electric driving and even the idle shut-off (depending on the amount of heat you call for and the car's operating temp). The battery provides assistance at an appropriate level to keep the charge level DOWN to it's optimum range. This is differnet than the rest of the year, where the charge depletes and the ICE kicks in to get it back up.


For those wondering how the Prius uses it's batteries to start:

There is a small 12v lead-acid battery in the trunk along with the large NiMH high voltage battery.

The 12v battery is used to "boot" the car up. When the car is off, the NiMH battery is disconnected.

To start the car, the 12v battery is used to bring up the computer systems and switch on the NiMH battery. Then the NiMH battery can be used to power the generator to start the engine if required. Once the NiMH ain battery is connected, it is then used to charge the 12v battery as necessary as your alternator would typically do.

Because the 12v battery in the Prius is relatively small (compared to the typical 12v battery in a car which needs to be strong enough to turn over the engine in cold weather). It normally does not need to do much more than power up the CPU and power the electronics when the car is off 12v battery.

This means that it isn't hard to discharge the 12v battery leaving you with a Prius that won't "start", even though the NiMH has plenty of battery capacity to do it. Especially in cold weather.

Luckily, you can jump start the Prius just like any other car using a 12v positive terminal under the hood and a chassis ground. And since the 12v system in the Prius doesn't need much current, it doesn't take much to get things going again.

Don't forget that once you've drained a 12v lead-acid battery to dead, the capacity will be significantly reduced so the chance of it dying again will go up - unless you can get a good, smart battery charger like a "Battery Tender" on it to help restore it to near-full capacity.


With regard to the ICE being on to supply heat: I was under the impression that the Prius has an electric heater just for that reason--so you can maintain a comfortable cabin temp even when the ICE is off. Same for the air-conditioner which is not driven by the engine belts as in a typical car. Am I totally wrong on that?


With regard to the ICE being on to supply heat: I was under the impression that the Prius has an electric heater just for that reason--so you can maintain a comfortable cabin temp even when the ICE is off. Same for the air-conditioner which is not driven by the engine belts as in a typical car. Am I totally wrong on that?


The Prius has an electric A/C compressor, not sure about the heater.

Roger Pham

The Prius' cabin heat comes from engine coolant. In real cold weather, the engine must remain idling to supply heat when cabin heating is required when the car is not moving, thus adversely affecting mpg in cold weather. However, when the cabin fan is turned off signifying that cabin heat is not needed,the engine shuts itself off as usual when the car is not moving.


Correct. However, when it's below 0 degrees F. as it has been here lately, it takes about 15 minutes for the car to warm up enough to allow the engine to start shutting off at lights. And it will do this only if the heat is off or at 1-2 clicks. 3-6 will bring the engine back on.


NREL report

"compared the performance of the second-generation Orion VII buses to first-generation Orion VII buses and found that the newer versions cost an average of 75 cents a mile to maintain, compared to $1.23 a mile for the earlier versions"

"Total operating costs, including fuel and maintenance costs, for the second-generation buses were $1.41 a mile, versus $1.85 a mile for the first-generation buses."

The average price of a diesel bus is $350,000, BAE said. Hybrid buses are priced around $500,000.

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