Excavator efficiency drive

By Chris Sleight14 June 2013

Liebherr’s concept R9XX 40 tonne class hybrid features both electrical and hydraulic energy recovery

Liebherr’s concept R9XX 40 tonne class hybrid features both electrical and hydraulic energy recovery systems.

Lowering running costs is a key focus for the world’s excavator manufacturers, and one of the biggest target areas is fuel efficiency.
There is no getting away from the fact that excavators tend to be big, thirsty machines, so fuel consumption will always be an issue. Manufacturers also have to contend with the fact that there are laws in place to limit exhaust emissions in many parts of the world.

The strictest areas are Europe, the US and Japan, where the current Stage IIIB/Tier 4 Interim laws are due to transition to Stage IV/Tier 4 final next year. Emissions at Tier 4 Final levels will see exhaust gasses containing about the same level of pollutants as the ambient air in many cities. In fact some manufacturers say that in some areas, excavators with the latest engines will act as air cleaners!

Other countries including Brazil, Russia, India and China have laws in place equivalent to the older Tier 2 or Tier 3 regulations, and there is a further set of lesser developed countries with no regulations for off-highway equipment exhaust emissions – sometimes referred to as Tier 0 countries.

The emissions regulations in place in any one part of the world will have a bearing on the engine design and the on-board systems to reduce emissions. But within this framework, manufacturers have the opportunity to use their design skills and the best available technology to build machines that are cheaper to run than their competitors’.

A good example of this is JCB, which uniquely in the industry achieved the Stage IIIB emissions requirement on its own Ecomax engines without the use of exhaust aftertreatment systems such as a diesel particulate filter (DPF) or selective catalytic reduction (SCR).

This has several advantages. First, fewer on-board systems keeps the purchase cost down. Second, the fine tuning that has gone into the engine design has also improved fuel efficiency. Third, consumable costs are lower as there is no DPF to service, and standard oils can be used, rather than the more expensive low ash variants that are required for use with aftertreatment systems. Fourth, there is no need for extra consumables such as the ‘Ad Blue’ urea solution that is required for SCR.

JCB is now fitting its Ecomax engine across its mid-range excavators from 11 to 22 tonnes. The company said that it is seeing fuel savings in the region of -10% compared to previous generations. On a machine like the 22 tonne JS 220 tracked excavator working a 1,600 hour year, this could equate to savings of € 5,200 (US$ 6,750) over a three–year period, based on European diesel prices.

New technology

So even with traditional diesel engines, there are significant differences between what competing manufacturers can offer in terms of on-board technology and fuel efficiency for excavators.

But now there is a growing move towards augmenting traditional power sources with new systems to save even more fuel. The most talked-about is hybrid technology, and this is an area where Komatsu has been a clear leader.

Its first hybrid excavators were launched in 2008, and since then the company says it has sold more than 2,000 such 20-tonne class machines, logging more than 1 million hours of work. The latest version, the 21.2 tonne HB215LC-1, was on display at Bauma and the company says it offers -25% less fuel consumption and CO2 emissions than a traditional excavator.

The machine achieves this by capturing energy that would otherwise be wasted in braking slewing movements of the upper structure. Instead of a brake, a generator is used to convert this energy to electricity, which is stored in a capacitor. This is available to rotate the upper structure back again or to assist the engine in other ways, as dictated by the on-board computer.

The use of capacitors – sometimes also called super capacitors or ultra capacitors – is an interesting one, given that hybrids in the automotive industry have tended to use more traditional batteries to store energy.

One advantage with capacitors is that they can discharge their energy quicker than a battery, which is more suitable to the stop-start applications that are often found in construction. They also have weight and size advantages over batteries, and can accept and deliver high currents.

Komatsu is the only manufacturer to have an electrical hybrid excavator in production today, although other manufacturers including Kobelco, Doosan, Hitachi and Hyundai have exhibited prototype 20 tonne class hybrid excavators at various exhibitions over the last few years.

But there has not been a great flood of such machines onto the market. Part of the reason for this may lie in the fact that global excavator sales remain sluggish in the wake of the 2008 global financial crisis.

Another issue that excavator buyers have to weigh-up is how quickly buying such a machine will pay-off. Yes, fuel consumption will be lower, but the extra on-board technology means a higher purchase price.

This balance is something manufacturers are highlighting. For example speaking at Bauma, Volvo Construction Equipment president Pat Olney said any hybrid the company produced would have to provide a strong business case for its customers.

“Our focus is on fuel efficiency and viability for the customer. We continue to research hybrids and we have a lot of experience in the group through Volvo Bus,” he said.

Mr Olney added, “When we bring something to the market, we want to make sure it is viable for our customers. We think we have some good ideas, but we’re not ready to talk about them yet.”

The company exhibited a prototype hybrid wheeled loader at the 2008 ConExpo-Con/Agg exhibition, using an electric motor alongside a traditional diesel. However, it did not go on to put the machine into full production.

Hydraulic ‘hybrids’

This question of costs and benefits to the customer is what prompted Caterpillar to go down a different route in developing what it describes as a hybrid. Although the dictionary definition of hybrid would imply a machine with a combination of energy systems on-board, Caterpillar takes a broader view and defines its hybrids as anything that can recover waste energy.

The company’s 336E H is a 36 tonne class excavator featuring a hydraulic accumulator to capture waste energy. But as Caterpillar global product manager for large excavators, Ken Gray explained, there were other departures from traditional excavator design on the machine to cut fuel consumption.

“We have historically designed excavators around a hydraulic pump speed, but we designed this machine around a 1,500 rpm engine speed, which is the lowest fuel consumption point. You compensate for that with a larger displacement pump to get all the hydraulic power you need, and you electronically control that to make sure you don’t lose performance and response.

“Then we have the ACS adaptive control valve, which is there to optimise the flow that is available. We need to put hydraulic oil where we need it, when we need it in exactly the right quantity. That means there is no wasted flow going to a circuit that doesn’t need it.
“The hybrid element, the save and re-use element, captures swing brake energy and puts it into a hydraulic accumulator.”

In explaining the decision to use a hydraulics to capture waste energy, rather than more common electrical systems, Mr Gray acknowledged Caterpillar had gone against the grain. “The prevailing politics in our industry is electric. It follows the automotive industry and that’s how people define it in their minds,” he said.

However, he continued, “With electrical systems, you basically have a big alternator on top of the swing drive. You run it as a generator to charge a capacitor when you’re braking and as a motor when you swing. You’re converting from mechanical to electrical energy, storing it and then converting it back. You’re going to get losses.

“Caterpillar has economy of scale in hydraulics. I can do a hydraulic system for one fifth of the cost of an electrical one and I can get the same performance. We built three generations of electric hybrids, but we didn’t sell them because we didn’t believe they could lower customer costs.”

And expanding on those cost factors he said, “In most places the 336E H costs about +9% more than the standard machine. I would estimate customers will save about 25% of their fuel costs and it’s a pretty simple equation to work out how quickly the payback will come – x months or x hours. And from a servicing point of view, I also like the fact that we’re keeping it all hydraulic.

“The electric excavator would be about +30% more expensive than a standard machine.”

Another unusual factor is that Caterpillar has chosen a 36 tonne model as its first hybrid, rather than the 20 tonne excavators its competitors have gone for. The argument for the 20 tonne bracket is that it is by far the largest segment of the global excavator market. So why did Caterpillar go for such a large machine as its first foray into hybrids?

“The more hours a customer puts on the machine, the shorter the payback period. Everyone who buys a 336 is going to put at least 1,500 hours on it a year on it. More commonly it would be 2,000 hours, and often much more than that. The payback period was always going to be shorter for a production machine like this than a 20 tonne machine. I know there are 20 tonne machines in production applications, but it would be a smaller proportion of that class.

“What customers were telling us was that they payback period had to be three years or less. We targeted 18 months, and the hour usage pattern of a 336 fitted that well,” said Mr Gray.

According to Mr Gray, various elements of the technology on the 336E H will start to be seen on other machines, based on what will lower costs for customers and provide a quick pay-back. In fact in the future, he believes this type of technology will be standard.

“I probably won’t even call the next generation ‘hybrid’. I think the technology is going to be pervasive. For a 75 or 90 tonne machine it’s a no-brainer. You just put it in.”

While Caterpillar has designed its own systems for the 336E H – systems we can expect to see on some of its other excavator models in the near future – components suppliers to the industry are starting to offer off-the-shelf packages that include similar fuel-saving elements.

Bosch Rexroth’s new Virtual Bleed Off (VBO) system for example, is designed to provide just the right amount of hydraulic flow to a hydraulic circuit. Not only does this save fuel, but the company said it also makes for more comfortable operation and faster response.

Doosan has already adopted VBO for its DX340LC-3 and DX380LC-3 Stage IIIB compliant crawler excavators, where it is branded as D-Ecopower technology. A +26% improvement in productivity and up to 12% in fuel savings is being claimed for the electronic pressure-controlled pump and closed centre hydraulic system, depending on the mode selected.

Similarly, the MHL 350 Hybrid Fuchs wheeled materials handler on show at Bauma was the fruit of a partnership between Fuchs’ parent company Terex and Deutz, which now offers off the shelf hybrid systems.

A compact Deutz TCD 6.1 diesel engine was turned into a hybrid using a Bosch electric motor and the two drive units can work in series or parallel. Capacitors store electrical energy and as well as capturing waste energy, the machine has other fuel saving features such as a start-stop function that shuts down the engine when the excavator is idle.

Re-starting is automatic, initiated by the motor generator, which also supports the overall system by providing a power boost when the drive system is operating in the peak load range.

Hybrid future?

Although it is early days, hybrid technology looks to have a place in the excavator designs of the future. There are competing technologies, and the market will decide which is best. It may be that different technologies work better in different applications.

As a final note on the hybrid question, Liebherr exhibited an interesting concept machine at Bauma incorporating numerous systems into a 40 tonne package. The R 9XX concept crawler features both electrical and hydraulic systems to capture and store waste energy, and Liebherr said these make a 160 kW diesel engine viable, whereas a standard a 40 tonne machine would normally have an engine well over 200 kW.

As well as capturing slew brake energy, Liebherr has devised a system to grab waste energy used to lower the boom and stick.

Hybrids will become more attractive options if fuel prices go
up because fuel efficiency will become a bigger part of the equation when it comes to working out the whole-life cost of a machine, and Liebherr’s concept R 9XX demonstrates that there are more areas on an excavator where waste energy can be caught and re-used.

But as hybrids become more and more available, it is always going to be a wise move to weigh up the increase in purchase cost against the fuel savings that can be achieved to work out how soon the pay-back will come (if at all).

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