Martin Sanders, Market Development Manager for Mobile at Parker Hannifin, explains the importance of E-Steering solutions
It is around 20 years since the world witnessed the first mass-produced hybrid and electric cars roll off the production line. Since then, buses and coaches have followed suit, as more cities look to introduce cleaner and more cost-effective public transport options. As an example, the Mayor of London announced in late 2016 that the UK’s capital will soon own Europe’s largest fleet of electric buses, revealing the first two city-centre, all-electric bus routes in the process. London’s plan is for all 300 of its city centre, single-deck buses to be zero emission electric vehicles by 2020, and all 3,100 of its double-deck fleet to be hybrid by 2019.[wlm_nonmember][…]
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Where a major city like London leads, others will almost certainly follow. Paris, for example, says that by 2025 all of its 4,500 Greater Paris network buses will be clean – and four out of five will be electric. The Dutch Government has also recently said that from 2025, all new public transport buses will be zero emission vehicles. Indeed, a new report published by UITP, an international association for public transport, reveals that 19 public transport operators and authorities, covering 25 European cities, have published an e-bus strategy for 2020. According to UITP, by this date there should be more than 2,500 electric buses operating in these cities, and around 6,100 by 2025, representing 43% of the total fleet.
Trends and drivers
So what is driving this trend? Well, the main driver would appear to be emissions and the legislation related to them. It is well-documented that the air quality in an increasing number of the world’s cities is becoming markedly more hazardous to health. Diesel-powered vehicles release nitrogen oxides (NOx) and particulate matter often including hundreds of chemical elements. Buses, with their large capacity engines, are among the worst offenders. Urban pollution has been linked to increased hospital admissions for respiratory problems such as asthma, while in the US, diesel exhaust has been classified a potential human carcinogen by the Environmental Protection Agency (EPA) and the International Agency for Research on Cancer.
Moving forward, diesel engine legislation and taxation is certain to rise in line with concern for public health and the environment. With this in mind, the adoption of technologies such as Parker’s E-Steering (electro-hydraulic power steering) solution in urban transport vehicles is growing rapidly. However, even existing diesel bus manufacturers can start making progress now as the development of electro-hydraulic steering systems reduces dependency on the internal combustion engine (ICE). In short, replacing hydraulic variable displacement pumps and compressor drives with more efficient, electrically-driven systems allows ICE downsizing and emissions reductions to begin.
Cost is also an undeniable factor in the development and adoption of zero emission vehicles. While most are aware that the purchase price of an electric vehicle may be higher than its diesel counterpart, electric buses work out significantly cheaper over their typical operating lives as a result of the savings in fuel costs and maintenance. If knock-on healthcare costs are factored in, the financial argument is even more compelling.
Evolving solutions
The historical evolution of steering systems over the years is quite a story. Initial power steering systems were designed for use with a variable displacement hydraulic pump driven by a belt connected to the ICE. At the time, the size of the ICE depended on the amount of power required to propel the bus or coach, along with the additional auxiliary power needed for hydraulic and pneumatic functions. As these gave way to the power take-off (PTO) system and then the electric generator powered by the ICE, steering systems steadily became more efficient and engine sizes reduced.
The latest iteration of the power steering system is an electric generator driven by the ICE with an electro-hydraulic pump (EHP) for the steering function. This is currently the most efficient solution available as it can be implemented within standard hybrid or electric bus/trolley bus applications, ensuring that E-Steering is now a step in key technology in the fight against emissions. What’s more, it typically allows different steering assistance levels for the driver depending on the area in which the bus is travelling.
The solution leverages the development of a new range of Parker GVM permanent magnet AC (PMAC) motors dedicated to mobile applications. Importantly, these high efficiency, compact motors can be installed at the front of the bus. A major issue on conventional diesel buses is the distance between the rear of the vehicle (where the hydraulic power is generated) and the front of the bus (where the steering system is located). This long distance equals losses which have to be offset with additional power. The only way to generate more power is to have bigger engines/power plants; but these of course create greater emissions.
In contrast, using a GVM motor provides the option of locating the power and the steering control at the front of the bus, exactly where it’s needed. As a result, system efficiency is improved through shorter hydraulic connections from the pump to the steering box. Other mobile markets can also benefit from this type of system, such as construction and off-highway vehicles, for example.
Cost and safety benefits
The low losses associated with PMAC motor technology makes it possible to utilise a low-voltage EHP (Electro-Hydraulic Pump), which is less expensive and better suited to this application than high-voltage alternatives. It also delivers a higher safety level in the event of high-voltage energy storage failure.
Solutions even exist for when passing over crossroads in trolley bus applications, where, due to the lack of high-voltage energy availability, there needs to be a way to ensure that the steering function remains operational. Here, the low-voltage functionality allows the electrical drive to be connected to a standard 24 VDC battery that is typically used for all bus auxiliaries, to maintain the steering capability.
Electric future
Ultimately, all urban transport is heading towards non-diesel engine approaches such as electric power. This trend means that E-Steering systems can be configured for PTO from the motor as opposed to traditional PTO from the diesel engine. Here, GVM motors are proving an increasingly popular solution in support of E-Steering systems.
GVM PMAC motors offer excellent power density (compact design) and high efficiency in comparison with solutions such as induction motors. The range offers impressive power ratings, peak torque and rotational speeds. Furthermore, they are designed to be shock-proof, vibration-proof and salt spray resistant.
To summarise, the world’s urban roads are being increasingly populated with electric, hybrid or smaller diesel-based buses and coaches with fewer PTOs. In turn, the preference for E-Steering solutions is certain to continue. Sure enough, as time passes, these systems will continue to evolve. A first step for future development could be for OEMs to design a more ergonomic system that provides hydraulic power for steering. In the longer term, OEMs could consider using a four-quadrant pump to allow the suppression of the steering box system and provide hydraulic flow only when required, lessening the need for electric power while driving. For now, however, the introduction of EHPs is a significant step in the vehicle hybridisation process that will soon lead to the complete electrification of city transport systems.
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