A CERTAIN RATIO: Achieving mobility in challenging environments is paramount for all military vehicles; however, assuring this in the armoured vehicle domain is far more difficult yet absolutely critical to allow platforms to achieve their missions.

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Although mobility is vital for armoured vehicles, this must compete with other essential characteristics such as the need to assure the survivability of the vehicle and crew; requirements which can easily find themselves in conflict with maintaining mobility. Yet understandably soldiers reliant on such vehicles demand increased cross-country capabilities, faster acceleration, and higher speed while enhancing survivability. Such needs have pushed the development of automotive systems to find solutions and a range of improvements have become available that begin to address these demands. Yet meeting these requires a combination and balancing of a number of aspects of automotive design. These include the capabilities and performance of a vehicle's suspension which directly influences the ride, the footprint of the tracks or wheels that determines the ground pressure, the vehicle's ground clearance and the engine's power output. The last could be considered both the most important, and the most difficult, to achieve. This is because even within the area of generating and distributing engine power there are traded-offs that must be made. Increasing power in an armoured vehicle is constrained by aspects such as the engine bay volume, the need to maintain the vehicle's operating range, weight constraints and the need to support the power demands of onboard systems like radio communications, navigation systems, sensors and countermeasures, in addition to automotive systems.It is the need for adequate protection against changing threats, in particular, that place the greatest demands on the automotive side of the vehicle. Protection almost inevitably means armour, and armour adds weight. This creates a paradox that offers uncomfortable compromises: As threats increase the required level of protection also increases. Greater protection typically translates into a requirement for more armour, and additional armour can create a heavier vehicle weight. To maintain, or improve, an armoured vehicle's mobility performance inevitably requires increasing the power provided by the engine, and the efficiency of the matching transmission and drivetrain. However, vehicle weight is also driven by vehicle size: The larger the vehicle, and surface area that must be armoured, the heavier the vehicle will become. Thus; not only must a vehicle's new power pack (its engine, plus transmission and drivetrain) be more powerful but it must, at least, fit the same allocated area, or preferably have less total volume. This is an absolute criterion especially for power packs destined for upgrading existing armoured vehicles, but also highly desirable in new designs.A commonly accepted measure of the level of mobility provided by an armoured vehicle is measuring the horsepower to weight ratio. Although not accounting for all the various factors that contribute to mobility this ratio is an appropriate rough indication and is useful both as a design objective and as a tool for comparing different vehicles. As a general rule the higher the horsepower-per-ton ratio the better the overall mobility characteristics that the vehicle will exhibit. Despite the fact that top speed is often considered in assessing a vehicle, for a combat vehicle responsive acceleration may actually be a far more important trait. Often overlooked in vehicle specifications, the ability to quickly accelerate and rapidly move to a covered position in response to an attack is invaluable. It directly contributes to the survivability of the vehicle and its occupants. Thus, available vehicle power contributes not only to mobility but also to survivability particularly when used in conjunction with self-protection measures including obscuring smoke, shot detection and laser warning sensors, alongside other passive and active countermeasures.POWER IN SMALL PACKAGESDespite forays into the use of turbine engines, as with General Dynamics' Ml Abrams Main Battle Tank (MBT) family the primary focus of engines for armoured vehicles continues to be diesel, or more accurately, multi-fuel power. Within the power pack industry MTU has established itself in a leading role. The company's integrated approach considers the entire "power package' not simply the engine, transmission and drivetrain, but also the vehicle's air supply and filtration, cooling, electrical power generation and other requirements. These are fully considered and accommodated to provide the most compact and efficient solution. MTU recognizes that for the armoured vehicle especially power-to-volume is a critically important factor for the combat vehicle developer and integrator. Giovanni Spadaro, head of MTU governmental sales, explained to Armada that for MTU "integration of all drive and power components into the vehicle system is of the highest importance, having developed a philosophy of symbiotic development of all parts of the power solution. For us this means that architecture, concept, design, software and all parameters are designed with the performance of the complete power pack in mind." The importance of this approach and achieving these objectives and reflecting their impact on the final vehicle is reinforced by the desires of the vehicle integrators like Krause-Maffei Wegmann (KMW), Nexter, BAE Systems and General Dynamics. As Scott Hall, manager of advanced systems at General Dynamics Land Systems explained: "From a power perspective, more is better, smaller is better and cheaper is better, while still being safe, reliable, and maintainable and quiet."MTU has found that the adaptation and militarization of commercial power packs can be adequate for light and medium armoured vehicles like the ARTEC Boxer eight-wheel drive armoured fighting vehicle which uses a MTU 8V199 TE20 diesel engine. However, for heavier armoured vehicles and MBTs engines like the company's Series 880 and 890 developed uniquely for use in military platforms are necessary. The possibilities in today's power packs are demonstrated in the KMW/Rheinmetall Puma tracked Infantry Fighting Vehicle (IFV). Mr. Spadaro shared that "the MTU power pack for the Puma includes the gearbox, the starter/generator and the cooling and air-filter systems. Its MTU 10V 890 diesel engine is distinguished by its very high power density and compact size. Compared with other military engines in the same power class, weight and volume have been reduced by around 60 percent." Jurgen Schimmels, director of MTU's special purpose engines and propulsion systems further stated: "This drive unit is more compact than any other previous defence drive system." The benefits of the MTU designs are especially evident when its power packs outfit older vehicles. It's EuroPowerPack engine family was used by GIAT (now Nexter) in the re-engined Leclerc-EAU MBT for the United Arab Emirates. It has also been fitted to the BAE Systems' Challenger-2E MBT where it both provided significant space savings while also improving range through reduced fuel consumption.Caterpillar, renown worldwide for its heavy construction equipment, has become a major supplier of power for defence tactical and armoured vehicles. Its military offerings are based on its commercial off-the-shelf range used worldwide, thus, gaining the benefits of reduced costs associated with volume manufacturing together with the availability of global support. The firm's developments for military applications have included its C9.3 engine platform with an increased power density of 600 horsepower (hp) unregulated emissions maximum output. Yet its true innovation comes from the C9-3's ability to switch the rating into a second mode aimed at meeting the demanding Euro-Ill European Union emission requirements while maintaining 525hp. Caterpillar indicated that: "The benefit is that the user can choose the mode of operation. It is possible to provide maximum performance during active field duty but then be able to operate within emission control during training or in civilian populated areas/ times." This 'switchable' capability actually derives from technologies invested in by Caterpillar for commercial needs.The company has been selected repeatedly for replacing and upgrading existing armoured vehicles. Its CV8 engine is equipping the British Army's BAE Systems Warrior tracked IFV. This effort is being conducted under Lockheed Martin's contract to upgrade the vehicle to the WCSP (Warrior Capability Sustainment Programme) standard which extend the vehicle's service life to 2040. Caterpillar is also replacing the 350hp engine in the US Army's General Dynamics Stryker family eight-wheel drive armoured vehicles with its C9 450hp engine. The upgrade was accomplished within the existing space. This is part of General Dynamic's ECP-1 (Engineering Change Proposal) which includes a 910 amp alternator, suspension upgrades and other improvements.ELECTRIC DRIVESTraditionally power from the vehicle engine is transferred to the wheels or tracks mechanically. The electric drive replaces this physical linkage with electric motors located at the driving wheels or sprockets. The power to run these electric motors can be drawn from batteries, a combustion engine or a combination of these. The 'hybrid' approach uses either a diesel or turbine power source which, now free from needing a mechanical connection can be positioned anywhere within the chassis offering greater design flexibility. It is also possible to utilise two engines rather than one, as BAE Systems does in its Hybrid Electric Drive (HED) automotive test rig. Deepak Bazaz, programme director for BAE Systems' new and amphibious vehicles division shared that the HED's two engines are coupled to generators and batteries giving it the options of operating on a single engine to save fuel as when idling, using both when high power is needed, and just running off the batteries for 'silent watch'. The HED concept utilised the firm's tracked AMPV (Armoured Multipurpose Vehicle) platform but the concept is intended to be sized and applied to any vehicle weight class, and both tracked and wheeled vehicles. The HED was prepared by BAE Systems as a testbed for the hybrid concept with Northrop Grumman as part of its US Army Ground Combat Vehicle (GCV) proposal.A study by the North Atlantic Treaty Organisation's Research and Technology Organisation found that: "The automotive performance of hybrid electric vehicles in terms of speed, acceleration, gradeability and stealthy operations is superior to the performance of mechanically driven vehicles ... while their fuel economy could offer a predicted 20 percent to 30 percent improvement." Electric drive motors also provide near instant acceleration responsiveness and higher traction efficiently. The latter is directly due to the improved torque inherent in the electric motor drive. For a combat vehicle this translates to faster reaction times when moving to cover, being more difficult to hit and having greater mobility in marginal ground conditions. The HED uses two six cylinder engines, a specially designed transmission from QinetiQ and 600 volt lithium-ion batteries.Another attractive aspect of the electric drive is its potential to provide more efficient and higher levels of electrical power. The Northrop-Grumman/BAE Systems GCV design would have provided 1100 kilowatts of electricity in a notably smaller and lighter power package than a conventional power pack. However, as energy storage is an essential part of the hybrid electric drive the inadequacies of current batteries is a problem. As a result a number of advanced battery types, including Lithium-Ion (LIION), Nickel Metal Hydride (NIMH), Sodium Nickel Chloride (ZEBRATM) and Lithium-Metal Polymer (LMP) with higher energy densities are being considered for hybrid vehicle applications. Yet all are in the development stage and present some challenges that must be resolved before they can be considered suitable for military use. Another aspect that needs further development for the hybrid to be practically applied to armoured vehicles is the design limitations of current traction motors. Although successful demonstrators, like the HED, have integrated them these units have size, weight, and cooling limitations. Until these are resolved all-electric power, despite its benefits, remains practically illusive for the armoured vehicle.Still advanced research and development agencies retain interest in the electric power concept. For example, QinetiQ's 2014 and 2016 contracts from the US Defence Advanced Research Projects Agency (DARPA) will take its hub-drive concept to test hardware. The hub-drive replaces multiple gearboxes, differentials, and drivetrain with compact, high-powered electric motors contained within a vehicle's wheels and could even be retro-fitted to existing wheeled armoured vehicles. In fact, in early June 2017 BAE Systems signed a teaming agreement with QinetiQ to incorporate new technology for electric drive mobility systems into combat vehicles. Dean Medland, vice president of combat vehicle programmes at BAE Systems stated that this allows the company to "offer our customers a mature, low-cost technology that can enhance the performance of current and future combat vehicles."FUTURE POWER CHALLENGESDemands in combat vehicles for more electrical power has increased several fold in just the last decade. Mark Signorelli, vice president and general manager for combat vehicles at BAE Systems reflected to Armada that "in future armoured vehicles could be stressed to meet electrical power demands." In response the CE Niehoff 300 amp alternator is being considered for the United Defence/BAE Systems' M2 Bradley family tracked IFV and the new AMPV uses two 150 amp alternators. MTU's Mr. Spadaro stated that "key factors that have been influencing the development of power and drive solutions in the past years and are continuing to do so are the constantly growing weight of MBTs and wheeled vehicles (mostly due to requirements for higher protection) and at the same time the need for more electrical power for all kinds of onboard systems such as electronics, self-defence suites and comforts for the crew, such as advanced air conditioning)." MTU views "these being addressed by a growing integration of electrical components into the drive system. A good example for this is again the MTU Puma power pack (see above) which contains a starter/generator with a nominal power of 170 kilowatts driving two electrical cooling fans as well as the coolant compressor of the air conditioning."Armoured vehicle power directly contributes to combat capability and survivability. The basic keys to battlefield survival remain: "to make every effort to not be seen, if seen to not be hit, and if engaged to not be killed." The first is facilitated by having the ability to move where the opponent does not expect you. The second demands responsive acceleration and battlefield agility to seek cover and complicate the enemy gunner's ability to engage effectively, while the last entails the ability to accommodate adequate passive protection, and passive and active countermeasures. Each of these objectives can run at odds with the others. For example, additional armour can increase weight thereby compromising mobility.Improvements in armoured vehicle power pack design; new engine and drive technologies; and innovative integration and packaging by industry have given vehicle developers additional options to respond to these diverse demands. Many improvements have drawn directly from commercial advances. Onboard engine and vehicle computers, digital electronic controls, automatic health monitoring, forced induction and much of the practical ground work on the hybrid approach have seen commercial advances adopted and applied to military power systems. Yet challenges to this delicate balance continue to push industry innovation and solutions.Please Note: Illustration(s) are not available due to copyright restrictions.Caption: The German Army's Puma required a power pack that would offer exceptional horsepower yet fit into a limited space. The MTU 10V 890 met this requirement providing an exceptional power-to-weigh ratio.Caption: Achieving the lowest volume while achieving the required power output is critical in a power pack for an armoured vehicle. The greatest factor contributing to the vehicle weight is the amount of area the armour must cover.Caterpillar's military engines are based on its commercial heavy equipment products modified to specific applications. This offers price, support and availability benefits. The C9 engine used in the Stryker upgrade is also widely used in construction equipment.Caption: Armoured vehicles, such as this Indonesian Leopard-2A MBT place special challenges on engine and drive train developers. They require high power levels yet must have the lowest possible size and volume.Caption: BAE Systems in collaboration with QinetiQ designed and built the HED; a demonstrator for the hybrid power solution for an armoured vehicle, using the chassis of the AMPV.