A boost from fluid energy transfer
Mark Fletcher examines a new powertrain technology which, if it fulfils its promises, will have a huge impact on efficiency across the board
The drive to squeeze more efficiency out of vehicles has taken an interesting turn. With most research heading down the hybrid/electric route, it’s surprising to note that the most promising technology for the immediate future appears to be one based on hydraulics.
The two approaches with the greatest potential – one from a major automotive tier one and the other from a group of individuals in Australia – utilise hydraulic accumulator-based systems to store the energy created (and normally wasted) in braking manoeuvres and then re-supply it during acceleration. Dubbed by some as hydraulic launch assist (HLA), it may not be Formula One but it will make a significant difference to performance and efficiency. With as much as 70% of the braking energy being recovered, the overall efficiency of vehicles, under certain driving cycles, can improve by up to 50%.
Both systems use a pump, connected in series with the drive shaft, to charge a pair of hydraulic accumulators when the vehicle is undertaking a braking manoeuvre. Once the manoeuvre is complete and the vehicle pulls away, this stored energy can then be reapplied to the drive shaft providing additional energy, enhancing the vehicles acceleration characteristics without additional fuel being expended. The hydraulic system is used in combination with an electronic controller which, as well as controlling the timing of events, can also monitor and provide feedback to ensure optimum driving conditions.
Figures abound regarding the benefits of applying this technology. Although the numbers vary slightly between the companies the outcomes are very similar. They include reductions in fuel consumption, efficient recycling of normally wasted braking energy, reductions in pollution, increased stopping power, reductions in brake maintenance, due, in part, to the supplementary braking offered when charging the accumulators, and engine wear reductions.
The Australian development, from New South Wales firm Permo-Drive Technologies, uses an axial piston pump in combination with two composite accumulators mounted on the chassis rails. Developed predominantly for trucks and heavy haulage vehicles, the figures generated under test were sufficiently impressive to get the US military involved. To this end the US Army flew a C5 Galaxy out to Australia to deposit a FMTV truck to use as a test bed. When one considers that the US Army has over 246,000 tactical vehicles (the largest truck fleet in the world) and is being faced with a commitment to reduce fuel consumption by 75% by the year 2010 it is plain to see where its interest in this technology stems from.
The Australian team, after an arduous fund generating process, has been taking its development further and sees its technology being applied commercially in the next two years. It is also keen to point out the other applications for the technology, rather than just within haulage. These include: the mining industry, encompassing machinery as well as the vehicle fleets; marine applications, where fuel saving will be on offer even though the retardation benefits will not figure; agriculture is also seen as a beneficiary, especially for towing in rough conditions; and finally, the rail industry, especially where diesel is still the prime mover.
The other project, involving the automotive tier one, brings together SHEP (Stored Hydraulic Energy Propulsion), The Eaton Corporation and Ford. SHEP offers an operating principle almost identical to that provided by Permo-Drive Industries – the major differences appearing to be cosmetic rather than functional. It promises similar gains and improvement over conventional systems and cites testing by the US Environmental Protection Agency, on an inner city cycle, which resulted in a fuel efficiency improvement of 24%. Under more aggressive testing conditions, involving more frequent stops and starts, the Ford vehicle being put through its paces showed a 38% reduction in fuel usage.
The firm has also held exploratory discussions with the UK Energy Savings Trust with respect to London Taxis, Dennis Truck (Mayflower Group) refuse and transit vehicles for the European market and London Underground. Although preliminary, the discussions created very positive interest from all parties.
The involvement of Ford led to the technology being employed on a concept vehicle. The vehicle, the Tonka Concept truck, is based on Ford’s 350-series Super Duty Chassis. The Tonka has two tanks mounted on the chassis rails each about two feet long and slightly less than a foot in diameter. Made from carbon fibre, they are designed to withstand a pressure of 5,000psi and exhibit a special lining material which absorbs the substantial heat generated during the compression of the nitrogen. When discharged, the system provides an additional 600lb/ft (813Nm) of torque – almost the same as the output from the engine. The system also provides most of the stopping force. According to Eaton officials, with regard to the HLA system in general, acceleration is ‘brisk’, due to the high power density of the hydraulics, and energy transfer is virtually transparent to the driver.
Ford also took the concept a step further by installing a version of it in a Lincoln Navigator. The purpose of this was to demonstrate the ability to install a lower-powered engine in a large vehicle without sacrificing performance. The engine in question, a four litre Jaguar V8, is designed to power a 4,000lb (1,800kg) Jaguar XJR, however, in a 6,000lb (2,700kg) SUV it is decidedly under powered. The Ford Hydraulic Power Assist (HPA) system, however, offered an additional 600lb/ft (813Nm) of torque up to 2,000rpm, whereas the vehicle’s normal power plant, a 5.4litre, 32-valve V8, develops 355ft/lbs of torque at 2,750rpm. The HPA system was able to “competitively accelerate” the vehicle from a standstill.
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