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DC660 2013-03-28

Downsizing the Future of Mobile Hydraulics

In early 2014 significantly tougher emissions limits for mobile work equipment will take effect. Craig Grant an expert in the use of hydraulics in mobile applications outlines the issues involved and offers two potential solution for manufacturers.

The European Commission’s Tier 4 regulations which set strict limits on emissions from off-road diesel equipment will significantly raise the bar for manufacturers of mobile hydraulic equipment across all markets be it in construction, materials handling, municipal and agriculture.

For Tier 4 Final, the ability to meet such tight emissions would suggest that current solutions for Tier 4 interim would need to be enhanced and most likely use both technologies (EGR & SCR) as opposed to current method of one.

This represents enormous technical challenges for manufacturers. In order to meet the requirements of TIER 4 Final, most systems will have to be redesigned from the ground up.

One significant approach to meeting these requirements is to optimise the efficiency of the system. The proper arrangement of the hydrostatic drives with high pressure pumps and motors can significantly improve the efficiency of the travel drive. This, in turn, makes downsizing of the diesel engine possible.

Downsizing the diesel engine will result in identical machine performance while reducing the installed drive power. This means that downsizing the diesel and optimising the hydraulic system are inextricably linked.

A smaller engine will lower emissions and compensate for the difficulties raised by the Tier 4 Final regulations. This is only possible however if the efficiency of the hydraulics is drastically increased and the available energy can be used even more efficiently.

Downsizing allows machine performance to be maintained even while reducing the installed drive power. In an ideal scenario, downsizing results in getting below the “key” 56 kW limit for the diesel engine. The legislation for engines below 56kW will allow engine designers and machine builders to achieve particulate emission without the need for expensive after treatment, a big plus.

With downsizing, the efficiency of the hydraulics is dramatically increased and the available energy used more efficiently. The chief area for optimisation is the travel drive, however additional efficiencies can be made on the working hydraulics and further optimised by electronic control.

Rexroth’s Diesel Hydraulic Control (DHC) offers a solution which can simultaneously reduce fuel consumption by up to 20 per cent but with very little drop in machine performance. The impact of DHC can be seen by studying the impact of a load requirement on engine performance with and without DHC.

Figure 1: The first driving cycle (See downloads on the right)

In figure 1, maximum power was available with nominal speed of 2300 rpm and the DHC was not switched on. As soon as operator makes a function selection via a joystick, which in this case was the hoist, then this creates an electronic signal in the control system.

This signal from the joystick is sent with a small delay to the control valve which then energises a spool to travel thus creating a load sense signal for the pump. The load sense signal from the valve is sensed by the pump, again after a short delay.

As the pump is mechanically coupled to the engine via PTO the load requirement on the engine is an instant demand. In conclusion therefore figure 1 demonstrates that maximum power is available at nominal speed & this speed is only marginally reduced when demand is made by pump. The result is that demand is met 100 percent.

Figure 2: The second driving cycle (see downloads on the right)

In the second driving cycle, once again the DHC is not switched on but engine speed has been dropped along with associated power to 65 per cent of the original. This drop in power aids fuel consumption and emissions.

The chain of events is similar to the first driving cycle, namely the signal from the joystick bringing into action the valve, pump and then engine. However, due to reduced engine power the rotational speed of the pump is significantly reduced due to the mechanical loading. This reduces fuel consumption, but the work requirement needed from the machine is not met.

Figure 3: The third driving cycle (see downloads on the right)

In this third driving cycle, the DHC is activated. The same operating settings are again used as in cycle 2 which means the machine is running at a reduced RPM. However, on this occasion the work requirement is sent directly from joystick to the DHC and then immediately on to the diesel control system.

This ensures that the engine knows that an extra load will become operational before the mechanical load takes effect and that, crucially, the same performance will be demanded from it. The system is thus prepared for the load demand by increasing the fuel injection.

The engine can now supply the power required without any reduction in speed. In other words, the work requirement has been met even at low rotational speed because the DHC is compensating for the load characteristics of Tier4 Final diesel engine.

Figure 4: Energy Optimisation (see downloads on the right)

Operational efficiency in terms of work requirement at low rotational speed is only one advantage of the DHC system. In a world that is having to become increasingly used to high oil prices the DHC system also has significant advantages for fuel efficiency.

Figure 4 clearly demonstrates that DHC not only improves system dynamics but also helps to optimise the efficiency of the complete system with all power intensive functions activated electronically and the maximum power for every machine known in advance and mapped.

Overall, DHC has the potential to increase the total efficiency of a machine by applying the exact amount of power when required. As a result fuel consumption is reduced along with emissions and noise levels.

The result is a quieter, cleaner and more efficient machine but with little drop off in performance.

For further information on Rexroth log on to www.boschrexroth.co.uk .


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