Excavators - the next stage

By Chris Sleight10 November 2010

Caterpillar’s Stage IIIB/Tier 4 interim 336E excavator has a visibly larger engine compartment than

Caterpillar’s Stage IIIB/Tier 4 interim 336E excavator has a visibly larger engine compartment than its predecessor to accommodate new emissions control components, although this does not hinder the o

Excavator manufacturers are grappling with the most challenging technical requirements they have ever faced ahead of next year's introduction of Stage IIIB engine emission laws in Europe and the US equivalent - Tier 4 Interim.

The new laws will come into force in several steps, based on engine power, but the first step will be significant for this sector. The start of next year will see the new laws impact on 130 kW to 560 kW diesel engines, which will impact on excavators weighing from about 25 tonnes and upwards.

The aim of the laws is to all but eliminate particulate matter (PM) emissions - the black soot that gives diesel engines such a bad environmental image in the eyes of many. There are a range of different technologies coming into play to do this, but they also have to work in tandem with measures to reduce other pollutants like nitrous oxides (NOx).

In most cases Stage IIIB/Tier 4 interim compliance will involve some sort of after treatment in the engine's exhaust system. There are different solutions to this being put forward by engine manufacturers, but in most cases there will be more components and physical bulk going into excavators, which will add cost and complexity. On the positive side, there may also be savings to be had from better fuel efficiency.

A key technical challenge will be to juggle the different requirements to reduce both NOx and PM. The formation of NOx is associated with high combustion temperatures, while PM is associated with burning fuel at a cooler temperature. The majority of engine manufacturers have opted for a solution where combustion temperatures are kept low, using technology called cooled exhaust gas recirculation (EGR).

As the name suggests, this involves feeding the exhaust gasses into a cooling system and then directing a proportion back into the combustion chamber. By doing this, NOx emissions are kept below the required limits, and this was one of the key technologies used to meet the current Stage IIIA/Tier 3 laws. But what to do with all the PM that is created now this is the focus of Stage IIIB/Interim Tier 4?

PM will be controlled in many new engines with diesel particulate filters (DPFs) - often bulky traps incorporated into the exhaust system. The large cylindrical cans contain a honeycomb-like ceramic filter that physically captures PM and prevents it escaping into the atmosphere.

Of course over time, the PM will build up, so it has to be burnt off. In some cases this will happen as part of the normal working cycle, but in some cases it will require a burst of fuel to be squirted into the ceramic matrix to incinerate the soot - what is known as active regeneration.

What this means as far as excavator design is concerned is that engine compartments will be bigger to incorporate the additional bulk of the DPF. But what equipment buyers will notice more is the price.

All this new technology will cost money, and customers will ultimately have to bear that. At the moment none of the mainstream manufacturers are prepared to say how much, but vague estimates of around +10% to +15% more on the cost of an excavator are being bandied about.

There will be some pay-back, as the new generation of engines will be a little more fuel efficient and more powerful than their predecessors. This could mean smaller engines for a given class of machine and/or lower running costs, but the savings in terms of fuel look unlikely to be enough to offset the higher purchase price for many owners.

A second, but less common technology path to Stage IIIB/Interim Tier 4 will be selective catalytic reduction (SCR), which is the opposite approach to the DPF route. In such systems, combustion temperatures will be high, so the creation of PM will be kept to a minimum, but there will be lots of NOx to deal with.

Again, this will be managed in the exhaust system, where a urea solution will be injected to reduce the NOx back down into non-polluting gasses.

The drawback to this is that excavator users will have to keep their urea tanks topped-up - another part of the maintenance routine. Availability of urea may also be an issue in some markets, and this seems to be why the majority of excavator manufacturers are going down the DPF route instead for now.

As ever, there are exceptions to the rules, and JCB is notable for its achievement of Stage IIIB/Interim Tier 4 compliance on its engines without using any form of after treatment, a feat it says it achieved by finely honing the combustion process.

Looking further ahead, DPFs may not be the final solution. 2014 will see the introduction of the Stage IV and Tier 4 (final) emissions laws, which will require a further reduction in NOx, and at the moment it is difficult to see how this will be achieved without SCR. This may be why some manufacturers - CNH for example - have gone for SCR already.

In CNH's case it has the benefit of being part of the wider Fiat group, which has already had to face these emissions issues for its on-highway trucks - a market where regulation is further ahead than in the off-highway segment.

Hybrid revolution?

Linked in with emissions laws and fuel efficiency is the emergence of hybrid technology. Such systems supplement the traditional diesel engine with an electric motor and a system to gather and store electrical energy from the machine. The general principle is that diesel engines are very inefficient at the bottom of their rev ranges, whereas electric motors are the opposite. So the idea is to build systems that use electric motors to run the machine at low revs, and to use the diesel engine at high revs.

The electrical system is charged either using the diesel engine at high revs, although that is more applicable to machines like haulers and loaders, or by capturing some form braking energy.

For most manufacturers, hybrids haven't got past the stage of press releases and prototypes, but there are a few that are commercially available. The first on the market was Komatsu's Hybrid PC200LC-8, a 20 to 22 tonne class tracked excavator that was launched in Japan in June 2008, and in China and North America last year.

The machine includes an electric swing motor, generator, and capacitor besides the usual diesel engine, and the system works by capturing energy from the braking of slewing movements. This kinetic energy is converted into electricity and stored in the capacitors, which were chosen in preference to a battery because of the speed at which they can discharge.

Komatsu says that in tests the hybrid excavator's fuel consumption was between -25% and -40% lower than for a standard PC200LC-8, depending on the application.

Another hybrid on the market is Kobelco's 8 tonne class SK80H, which was launched in Japan at the start of the year. The company says its machine, which features a nickel hydride battery for electrical storage, consumes at least -40% less fuel than a comparable conventional machine.

In fact, there were several applications where the savings were even more pronounced. Fuel consumption was -54% lower in a house demolition application, -59% lower on a waste disposal site and -60% lower on a residential site preparation project.

This is partly thanks to the use of a smaller diesel engine - the electric motor supplements the diesel unit so the machine is not under powered - as well as other more familiar fuel saving systems such as an automatic system to cut off the engine if it has been idling for more than a minute.

Kobelco makes the point that there are other advantages to hybrid power besides fuel savings. Being an 8 tonne class excavator, a lot of the jobs the SK80H will be used on will be in a built-up environment, for things like street repairs or house building. The key point here is that the electrical system is virtually silent, which is an important selling point for contractors that want good relations with nearby residents and businesses.

The use of a smaller engine in Kobelco's hybrid is also significant. The lower cut-off point for the Stage IIIB/Interim Tier 4 engine emissions laws is 37 kW, which is to say diesel engines of this power rating and lower do not have to meet any new requirements - the Stage IIIA/Tier 3 laws are as stringent as they get.

As far as excavators are concerned, this means machines of about 6 tonnes or less - mini excavators essentially - will not have to be redesigned with a new Stage IIIB/Interim Tier 4 engine in place. But with hybrid technology coming into play, and as Kobelco's machine illustrates, it will be possible to fit smaller diesel engines into bigger excavators and use the electrical system to provide the necessary additional power.

This could mean that hybrid technology becomes quickly adopted in the 6 to 10 tonne excavator bracket as a means of legitimately ducking under increasingly challenging emissions requirements. Hybrids are more expensive than traditional excavators of course, but so are Stage IIIB/Interim Tier 4 machines compared to their predecessors. But whereas Stage IIIB/Interim Tier 4 machines can only offer marginal fuel savings compared to earlier generations, the fuel savings with hybrids have already been shown to be very significant - anywhere from -25% to -60% less fuel burnt.

The equation will weigh even further in favour of hybrids as the global economy continues to recover, general consumption goes up and oil prices start to climb again.

Further in the future, when Stage IV/Tier 4 Final regulations come into force from 2014, the lower cut-off point will go up to 56 kW, which means excavators of about 11 tonnes or less will not face compliance with new laws. But again, it is conceivable that manufacturers will look to develop bigger hybrids with sub-56 kW diesel engines and electrical systems to avoid the cost of compliance and deliver better fuel economy to customers.

It does however remain to be seen how far this concept can be taken. Hybrid technology is in its infancy, and it is not yet clear which types of technology will work best in excavators and other construction machines. Even with just two hybrid excavators on the market from Kobelco and Komatsu, there are already two different technologies being used for electrical storage - batteries for Kobelco and capacitors for Komatsu

Only time will tell which type of technology will work best - hybrids after all are in their infancy. But this is also cause for optimism. As the technology develops it should get better and deliver more savings. It should also become cheaper as mass production brings economies of scale and manufacturers recover their R&D expenses.

It is also worth saying that hybrid technology should be a good fit for the construction equipment market. One of the draw-backs for on-highway vehicles like trucks and cars is that the electrical systems - often including a heavy battery - add weight and so eat into any savings in fuel consumption.

However, construction machines like excavators and loaders require a counterweight, so with good design the extra weight of hybrid components should not be a problem.

Hybrids may be new to the market, but there is every reason to think they are here to stay, particularly as laws on diesel engines get more and more stringent in both the developed and developing world.

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