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Demonstration series hydraulic hybrid transit bus yields fuel economy of 6.9 mpg, 110% better than conventional diesel, 30% better than electric hybrids

The series hydraulic transit bus. Click to enlarge.

The BUSolutions demonstrator LCO-140H (Low Cost of Ownership–1st 40-foot Hybrid) series hydraulic hybrid transit bus yields fuel economy of 6.9 mpg (34 l/100km) on the industry-standard ADB duty cycle—110% better than conventional diesel buses on the road today and 30% better than the best-in-class electric hybrid buses available today, according to the final report on the project published by the US Federal Transit Administration (FTA).

More importantly, according to the report written by a team from Altair ProductDesign, which led the project, such a bus will cost more than 20% less than a conventional diesel bus to own and operate over its life and more than 30% less than an electric hybrid.

Elements of the hydraulic hybrid powertrain. Click to enlarge.

The project originated when Automation Alley, Altair ProductDesign, and the Federal Transit Administration (FTA), in a public-private partnership, teamed up to advance a new transit bus initiative that would improve local and regional transit systems while requiring no infrastructure upgrades to operate. The goal was to develop a significantly lighter-weight, heavy-duty bus design that yields superior fuel efficiency to conventional buses at a lower lifecycle cost.

The four main areas of focus for reducing the lifecycle cost of the new bus were purchase price; fuel economy; scheduled maintenance; and unscheduled maintenance.

Since 2005, FTA issued $5.1 million in funding for the resulting BUSolutions project, with additional program support provided by the Michigan Economic Development Corporation (MEDC) and Automation Alley, Michigan’s largest technology business association. Altair, with the help of its partners and sponsors, successfully designed, fabricated, and tested the bus to validate the design and performance metrics.

Altair opted for a hydraulic hybrid powertrain primarily because of power density—the amount of energy that can be transferred during a given period of time—as well as low system entry cost and lower maintenance costs. All hybrid vehicles use regenerative braking; hydraulic hybrids capture the energy in the same braking event and store it in the form of fluid power. The advantage of the hydraulic hybrid approach, the report said, is that this method is able to store much more energy quickly, making it more suitable for heavy vehicles with high stop-go duty cycles.

Further, analysis showed that between a parallel and a series system, given the typical duty cycle of a transit bus (low average speed/high number of stops per mile), the series system would provide the optimum fuel economy. With the engine is decoupled from the drive axle, there is no longer a requirement to match engine RPM with vehicle speed. This enables the engine operation to run at the most efficient RPM to yield optimum fuel economy, the team noted.

After evaluating the offerings from three major US suppliers, Altair chose Parker Hannifin as its supplier for system components. The majority of the system components were available from Parker offerings, including the bladder accumulator, hydraulic cooler, engine mounted pump, and gearbox with pump/motors. Altair completed some initial analysis before specifying the entire system in order to understand what system capacity would yield the optimal results from the system.

Included in the analysis was the evaluation the performance differential between one 22-gallon high-pressure accumulator and two high pressure accumulators totaling 44 gallons. The analytical fuel economy results showed that the addition of a second accumulator yielded a 4–11% improvement over a single accumulator system, depending on the duty cycle. These improvements outweighed the cost and weight of the additional components and added complexity of the system and were integrated into the bus.

Because a large-capacity low-pressure reservoir (LPR) was not available to package in the space available, a custom LPR was designed and manufactured. This design required additional overhead air capacity.

Altair evaluated a swash plate pump and bent axis pump; the bent axis pump has the best efficiency of the two and was selected to be used. This is a piston pump design containing multiple axial pistons mounted at an angle to the drive shaft; this design is integrated into the Parker pump/motor offerings.

The controls development strategy was initially defined by the Parker engineering staff. In cooperative efforts, Altair and Parker engineers jointly worked together to follow the controls development process to refine systems drivability and functionality.

Early in the program, Altair selected high-strength stainless steel as the primary structure material because of its common nature in the industry, above-average corrosion resistance, and post-welded strength. As the program progressed, Altair found the availability of off-the-shelf high-strength stainless steels to be scarce, specifically in the sizes required to fabricate the optimized bus structure.

After reviewing other materials, Altair selected aluminum 6061 with a T6 temper. It is commonly used in various structural, building, marine, automotive, aerospace, and process-equipment applications. It has a typical yield strength of 40,000 psi and an ultimate tensile strength of 45,000 psi.

In designing the bus, the project team established a specific list of questions and criteria to concentrate the focus on the desired outcome. Altair wanted to use existing products and technology available but to apply them in a more efficient way that would yield better results. Because of this, the process was bounded and specific components were limited to change. For example, the internal combustion engine was labeled as an off-the-shelf product; therefore, designing a new engine was not on the table.

The following table shows a list of the design innovations made it into the final design of the LCO-140H.

Click to enlarge.


  • Design & Development of the LCO-140H Series Hydraulic Hybrid Low Floor Transit Bus. FTA Report No. 0018



That's good, but there are many thousands of buses out there with years of remaining lifetime.  Can hydraulics be retrofitted to them to achieve worthwhile savings until they are replaced?


Seems like a well planned Development/Demo.

Obviously busses and garbage trucks need very high power density for "constant" regenerative braking.

Hydraulics scale up economically and effectively for this without requiring LOTS of batteries (or HUGE caps)or big motor generators and ultra high power controllers.

But one can only wonder what a "secondary windshield" is or how fully forward seating main floor and spray on interior relate.

Kit P

What is wrong with picture, 110% better fuel economy but only 20% better life time cost? When they do not tell you the initial cost, that is an indicator that it is expensive and not like to perform well in real life situations.

Bus driver accelerate like they are coming out of the pits at Indy. I am think that sharing the cost of fuel savings with drivers would have a very low capital costs and reduce maintenance costs. Bonus checks and recognition in front of your peers is great for job satisfaction.


Electric is not always better, I was already convinced that for truck and bus an hydraulic regenerative braking was much better than an electric / hybrid system.

They now have to market it a decent price and yes retrofit the thousands of bus and truck out there that pollute and waste energy


If this technology is so good for everybody, why our glorious all effective Private Industries did not build those years ago?

Who has the right answer?

Roger Pham

Don't worry, all your concerns are answered in page 63 of the following:

In page 63, you will find that the purchasing cost of this hydraulic hybrid is $410,000 in comparison to $320,000 of conventional diesel bus. Over the operational life of the bus, $172,219 will be saved in comparison to the overall cost of a diesel bus. Still find it to be "smoke and mirror" calculations?

BTW, this vindicates what Henry Gibson has been advocating all along about hydraulic hybrid. Henry, where art thou and what's your opinion on this?


A good question - lots of things are subject to fashion and ignorance - people tend to do what they know, rather than what might be the optimal solution.

Currently, electric hybrid is the fashion - it plays well with the notion of uHybrid -> full hybrid -> phev -> BEV evolution that some people see. There is a lot of momentum behind this technology at present, and so we are seeing a lot of progress here.

This reminds me of the Intel 8086 line of processors defeating the Motorola 68000 line, even though the Motorola had a much cleaner architecture (with all 32 bit wide registers). There was so much Intel equipment out there that people made it work, even though it was a pig with the segmented architecture.

However, they may find that hydraulic hybrids suit certain usage patterns very well, and once enough people get involved, it could really take off (the way electric hybrid seems to have at present).


Im interrested to buy a ticket in that bus and enjoy myself better acceleration and breaking and less polluting diesel while doing so. They can also put some pics and writings and films in bus stations so that we can study while waiting the technology before experimenting it later on.

Roger Pham

Look forward also to riding pollution-free H2-FC Buses that will increasingly be available in more core cities around the world. H2 can be made from excess renewable energy and stored for transportation or stationary uses. Diesel-hybrid is only a transitional step until the full use of carbon-free energy.


Batteries have higher energy density, and work better for less-frequent stops and slower deceleration.  What they do not have is really high cyclic efficiency under heavy load, which is where city buses and garbage trucks spend their working day.  That's where hydraulic systems (using what's actually high-pressure pneumatic storage) shine.

ISTM that whether this could be retrofitted to older buses depends on the transmissions.  If they have a PTO connection, a hydraulic motor/pump could be added there at small expense.  A hydraulic pump/motor on the engine could operate as a belt-connected starter for idle-stop operation, allowing the engine to charge the accumulator without going through a torque converter.

Some changes to the transmission controls would be in order.  If it required a new valve body, it might not be worth it.

There are enough "ifs" here that I suspect that retrofits are not going to make much of a difference, at least in buses; garbage trucks do have PTO connections to run the hydraulics.

Roger Pham

I must hasten to add that H2-combustion-ICE for buses and trucks can also receive great benefit from this hydraulic hybrid design. Highly-advanced ICE running on H2 has efficiency close to that of FC but does not need the use of expensive noble metal and the service and maintenance infrastructure of ICE is already well developed. Meanwhile, Natural Gas is increasingly displace diesel for fueling buses due to lower emission and great saving in fuel cost. If NG dispensing infratructures is made to be also compatible with H2, then the eventual use of H2 will not require any further infratructure cost.

Roger Pham

Good points, E-P.
Frequent stop and go HDV are really the only HDV class that would really benefit from hybrid technology, due to the braking energy recuperation and avoidance of engine idling. HDV's with less frequent stops already operate around the peak efficiency point in the engine map for most of the time and won't benefit much from hybridization.

Hydraulic hybrid system is much heavier than electric, I suppose, that prompted them to re-design the whole bus with aluminum structure instead of steel. This will allow significant downsizing of the engine and hydraulic components. Otherwise, the power train structure will be too big and too heavy to be practical in a steel vehicle. Therefore, the 110% improvement in fuel efficiency here also reflects the lighter aluminum structure and smaller drive train components. This would make it difficult to retrofit existing buses and garbage trucks without adding too much weight and space. For details:


This just confirms what I have pointed out in the past. Hydraulic or kinetic hybrids can be more efficient than electric hybrids. It has potential to be cost-effective as well.


This could be part of the 'optimal solution' that private industries rarely choose/decide to do. Too bad because it could have been done decades ago?

Roger Pham

They really had to think outside the box on this one, and have to totally design and build a new all-aluminum chassis and body in order to keep the engine and hydraulic hybrid components small enough to fit. You can see on my referenced source above that, still, the hydraulic components are taking up all of the rear section of the bus. Hydraulic hybrid is bulkier and heavier than electric hybrid. It could have been done decades ago if petroleum was priced as high back then as now. During the decades of cheap fuel, nobody cared!


I suppose the comeback from the electric advocates would involve flywheels (Peter's "kinetic" option).  That would substitute resistance and windage losses for the viscous losses of the hydraulic system.  On the other hand, it would allow ultra-fast "topping up" of the flywheel energy using things like overhead "Busbaar" chargers at stops, allowing not just better economy but zero-emission and nearly noiseless operation for some distance afterward (for instance, through a city center).

I'd love to see analyses of these things for different use cases.


RP...our city had 400+ aluminium buses in the 1950s/1960s. Those regular aluminium city buses weighted about 8500 lbs less than the steel buses they replaced and the steel buses used to replaced the aluminium buses.

Since the aluminium used was produced locally and was in abundance and the buses were also built locally, you can guess why the city switched to 800+ imported GM steel buses in the late 1960s. Somebody probably sold out????

Recently, our very wise government (booted out 2 weeks ago) gave 4 or 5 local industries $35M to design and build aluminium electric hybrid city buses. It seems that they don't remember that (diesel) light weight aluminium city buses were mass produced and in operation in our city some 60+ years ago. Hydraulic flywheels are not considered.

Roger Pham

Thanks, HD, for the info. Though I'm surprised that these buses in the early days were made of aluminum instead of steel, which has always been several folds cheaper than aluminum and is easier for mass production. Back in these days, petrol was real cheap, perhaps 20 cents/ gallon. No one cared about fuel efficiency, so they opted for the simplicity of a regular transmission instead of a more complicated hybrid drive train, which requires sophisticated sensors and processors to optimize efficiency, and these were not available back then.


RP....the first All Aluminum Bus was built by Yellow Coach in 1936. A ban was imposed in early 1940 to reserve aluminum for war planes. Kaiser Industries build an All Aluminum articulated 63-passenger bus in 1946. Fizjohn Industries built All Aluminum bus bodies in 1947. Our Aluminum Buses were built by CanCar (under license?) in early 1950s.

Currently, many electric aluminum buses are being built in China. BYD is just one of many manufacturers using aluminum to reduce weight. Articulated aluminum e-buses with 63+ seats, to reduce the number of drivers required by 50%, will be around soon.

NB: In many cities, bus drivers total cost is about $122K/year each. Since each bus need as many as 4 to 5 drivers, a 50% reduction over the normal life of the bus (12 to 16 years) can pay for the bus 2 or 3 times.

Roger Pham

Bus drivers earn more than airline pilots? Starting pay for first officer is $30,000/year. Captain after 8-10 years will advance to just over $100,000.


RP...I referred to TOTAL pay package per driver on the road. The drivers get about $66K/year to $80K/year in basic pay but (all) other cost are almost as high. Overtime to replace absentees, at 1.5X, 2.0X and 3.0X pay rate, is very costly. They are not the only one to profit. Our Air Traffic Controllers make $100+K/year in basic pay plus another $100+K/year in well planed overtime.

Your pilots (captains etc) must be working for that R-Candidate gentleman or his good friends. Otherwise, they would be making 2X to 3X that much in basic pay and twice as much in total pay package. On the other hand, this may be the demonstration that the drive to make the middle class poorer and the 3% richer is gaining speed South of the Border. Pilots at $30K/year must be on the bread line to feed their family.

Michael McMillan

I understand the reasons to use aluminum, sort of. It sort of makes sense, but not really. You are basically building a bridge, that rolls. Use structural steel.
Ultimate Tensile Strength, psi 186,000
Yield Strength, psi 125,000

If you have any parts exposed to the elements, and are worried about corrosion, you have two very simple solutions.
1) just powder coat the whole frame.
2) hot dip galvanize the thing, like they do with lamp posts.

Aluminum is popular because it is stiff and light weight. However usually more material is needed because of it's strength. Also in high vibration situations (like a bus), it fatigues faster.

I might be wrong on this, but I think that they should have gone with the swash plate motor so that they could freewheel and accelerate smoothly ,and get regen by changing the angle of the swash plate without any funny business. The swash plate system enables more time at an engine off state.


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