HYDROSTATIC TRANSMISSION ATTRIBUTES
HYDROSTATIC TRANSMISSION ATTRIBUTES …
HYBRID EXAMPLE WITH INFINITELY VARIABLE TRANSMISSION
TWO PARTS
For this Compound Electric Hybrid application, The Hydrostatic Transmission (HST) would consist of two integral components: a Hydraulic Infinitely Variable Displacement Motor, and an Infinitely Variable Displacement Pump. These should be high-efficiency rotary actuators with fully balanced moving parts. Notably, the Pump would also be equipped with integrated reverse flow capability.
SPIN FROM INFINITY
The HST Motor is in a direct link with the vehicle axles by connection through the final drive gearing. Its operation is always synchronized with the wheel axle’s RPM, turning in precise relationship with these axles. This means that if the vehicle attains a speed of 60mph, then the HST motor will correspondingly spin at a rate proportional to that – and change in direct proportion as the vehicle goes faster or slower.
Conversely, the HST Pump is directly connected to the Prime Mover power source, so it will always rotate at the same RPM as the Prime Mover (the fuel engine & electric motor combination). However, within the framework of this Hydrostatic Transmission, the Motor and the Pump do not maintain, or are in any way required to have, an absolute fixed RPM relationship between them. Instead, their essential synchronization revolves around aligning their flow rates (e.g., Gallons Per Minute), for the hydraulic fluid circulating between them.
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This synchronization of flow rate is achieved by independently varying the displacement of these two units, just as their RPMs also vary independently. This capability ensures that the requisite flow rate matches, regardless of RPM at either end of the transmission. (Matching even if there is no rotation at either end.) Under pressure this approach makes possible the transmission of power and the associated Torque in an infinitely variable range of ratios between rpms of the Pump’s input shaft and the Motor’s output shaft.
And all that infinite & continuous flexibility can be started from a neutral zero displacement (or undefined infinity).
THE FORCE IS WITH US
Any Hydraulic Motor with a capacity of 334cc possesses the remarkable capability to generate 1,460 lb/ft. of Torque while operating at a working pressure of 5,400 psi. Furthermore, when the output shaft is also turning 1,800 RPM it seamlessly transmits 500 HP at full displacement. This output is similar to that of large Diesel Truck engines, except for one crucial distinction: the Hydraulic Motor delivers the full 1,460 lb/ft of Torque instantly at a standstill of zero RPM’s. This is a most important functional capability - eliminating the need to rev up to 900+ RPM for the start of maximum torque, as is typically required by a Diesel Engine. In this practice, Diesel engines not only waste fuel but also overheat and wear out the clutch, eventually leading to its failure.
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This one of its many features truly distinguishes the HST. Its ability to sustain exceptional peak Torque even when the vehicle remains at an ongoing standstill or moves at a slow crawl, is a testament to the inherent smoothness of hydraulic systems. This function surpasses the performance of typical vehicle electric motors, which are prone to overheating and potential burnout under such demanding low speed conditions (e.g., Locked Rotor).
WIDE FULL POWER RATIO
A truly viable “Infinitely Variable Hydrostatic Motor” has the capability to increase RPM while proportionally decreasing displacement. This means that, under a constant psi working pressure, it can maintain the maximum power output consistently as described in the following example box below.
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This remarkable feature ingeniously enables continuous transmission of a consistent power level while rpms are fluctuating. So, the power output can remain steadfast even as the vehicle’s speed undergoes fluctuations affecting HST motor rpm - simply by corresponding displacement changes in both the HST pump & motor.
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The ultimate objective is to attain the highest possible duty cycle horsepower output whenever needed, across a very broad range of RPM’s. This pursuit attains the realization of a “Wide Full Power Ratio” unique to this system’s versatility and effectiveness. And thereby delivering exceptional economy and efficiency for any Heavy Truck application.
EXAMPLE USING THE PREVIOUS HYDRAULIC MOTOR
Reaching 1,800 RPM’s and operating at full displacement, while maintaining 5,400 psi working pressure, the motor throughput is at 500 horsepower. And as the vehicle accelerates to its top highway speed, the HST motor also increases to 9,000 RPM, while simultaneously decreasing its displacement in direct proportion.
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This is an 80% displacement reduction above – but since pressure stays the same and so does the fluid flow due to increased RPM’s – then the power throughput also stays the same at 500HP.
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This dynamic displacement adjustment ensures that the power going out to the axles remains consistent throughout the transition to higher speeds, effectively modulating Torque and RPM in the same proportions. Seamlessly without steps or variations.
In this example, it has resulted in a remarkable 5:1 Full Power ratio, showcasing an exceptionally “Wide Full Power Ratio” for an infinitely variable output. This method is used so the system can consistently apply the full needed horsepower while navigating tough highways, surmounting hills, or in any difficult scenario, regardless of vehicle speed, exemplifying its adaptability and performance.
(See also below in “Heavy Duty” about incredible Torque abilities)
THE BALANCING GAME
With the current realm of hydrostatic actuators having high inertia rotating groups, achieving the wide power ratio described above becomes an insurmountable challenge, necessitating innovation on these moving parts. Analogous to a combustion engine, where the crankshaft incorporates lobes to mitigate forces during rotation and enable higher RPMs, the quest for a higher RPM range to achieve a "Wide Full Power Ratio" for the HST motor and HST pump - calls for both infinitely variable and harmoniously balanced rotating groups.
With such a configuration in place, it becomes possible to maintain consistent hydraulic pressure and fluid flow over this "Wide Full Power Ratio," thereby preserving the desired high-power output dispersed over this broad range of RPM fluctuations.
As an intriguing aside, even extremely diminutive Hydraulic motors, like those with a 19cc capacity, can achieve 14,000 RPM, despite their inherent imbalance—although their tiny size plays a crucial role in this exceptional achievement.
HEAVY DUTY
The HST's "Infinitely Variable Displacement Motor" has its connection with the drive axles through a final drive gear set meticulously chosen to suit the specific truck application type, payload, differential gearing, top speed, and duty cycle requirements.
For all these heavy trucks and demanding vocational applications there is a 2-speed final drive. This final drive lower speed is the included underdrive gear, which proves ideally pivotal for leveraging the “Wide Full Power Ratio”. In this setup it will elevate Torque output by over fourfold, resulting in a Torque multiplying Ratio that can exceed 22:1. This 22:1 Ratio, in turn, empowers the HST to match the high-level working torque output typically demanded by super-duty manual transmissions essential for heavy truck hauling—which now by switching to HST would no longer be burdened with having to navigate shifting through 18 gears. (And still be able to obtain 115,000+ lb/ft of start-up torque at the axles – even in the Electric only mode!)
Incredibly, the HST Compound Electric Hybrid unlocks the same potential for highest torque Electric-only mode operation (e.g., Zero Emissions), and most importantly, the HST electric hybrid system possesses the capability to simultaneously generate electricity on the fly, recharging batteries, while also putting out the energy for propelling a truck down the road, combining any one or all engine components of the Prime Mover—an embodiment of unparalleled versatility and efficiency.
All the while keeping the Hybrid ethos, as this system also captures brake energy, channeling it for regeneration into batteries or accumulators. Amazingly, even a Flywheel for regeneration use can be seamlessly accomplished.
PUMPING IT OUT
This HST would also feature an "Infinitely Variable Displacement Hydraulic Pump," characterized by a balanced rotation and a 223cc displacement, so it exhibits the impressive capability to transmit 582HP. This remarkable output is achieved while utilizing 57% of its displacement, maintaining a constant 5,400 PSI working pressure, and the shaft is rotating at 5,500 RPM. It is directly connected to a featured Prime Mover power source operating at the same rpm’s, which is the “Sweet Spot” for this engine. This implies that more efficient fuel engines and electrical motors, operating at higher RPMs, can be employed as the Prime Mover for use as the force creator behind a truck’s drivetrain.
The pump as typical would have an integral reverse flow for backing up, unless for certain vocational applications the reverse function is in the 2 speed final drive as a selection off of the lower range.
Furthermore, since the HST easily adjusts and manages the production of all necessary torque, there's no requisite for a specific Torque curve from the Prime Mover. This further underscores the HST system's adaptability and efficacy to work with high efficiency, higher rpm, lower torque Prime Movers.
SWEET SPOT
Most Importantly, the Prime Mover engine combination can maintain continuous operation within its optimal RPM range, known as the efficiency Sweet Spot*. Any surplus power generated by the engine while operating within this Sweet Spot can seamlessly flow into the Electric Motor for battery charging. This ingenious setup allows the engine to remain within its efficiency Sweet Spot while concurrently making a dual contribution to the loads as computed at any moment of time.
Moreover, this Infinitely Variable HST, when integrated into a Compound Electric Hybrid arrangement, affords the remarkable capability to operate the Prime Mover engine at the lowest possible RPMs for maximum cruising efficiency, regardless of how low that RPM might be. This flexibility ensures peak efficiency in diverse operational scenarios when operating outside the Electric-Only mode.
* “Sweet Spot” is that Engine RPM point where the engine is operating at its highest efficiency also known as the Best Efficiency Point on a fuel consumption map.
Picture harnessing the electricity stored in batteries to assist in ascending a hill, then in turn, replenishing those batteries while descending the other side, only to repeat this cycle with each subsequent incline. So, when seamlessly integrated with advanced Topography-aware Smart GPS Route planning technology, this synergy will significantly enhance long-haul efficiency (i.e., fuel savings!).
ADVANTAGES OF HYDROSTATICS FOR TRANSMISSION MANUFACTURERS
Heavy Truck Transmission Manufacturers have developed a wide array of sizes and options for the geared transmissions they offer OEMs to use in a multitude of truck applications. This requires substantial resources to provide numerous gear ratio combinations and power take-off choices – to meet diverse customer requirements. In many cases, the options and variables extend into hundreds of choices, and with add-ons as well as controls it can be a thousand variations to stock parts for.
However, by including Hydrostatic Infinitely Variable transmissions in their repertoire, then a great many of these geared variations can be eliminated. Since by electronic controls, a multitude of application requirements, for a mix of power and torque ratios, can be accommodated by one HST unit. And any group of applications can use the same final drive this way - and so there are far less parts to make or stock.
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Virtually a “one size fits almost all concept”. Greatly enhancing sustainability!
And most importantly, these transmission manufacturers would have a Zero Emissions option for their customers in heavy trucking and long-haul applications to fulfill that need.
(Bonus: without waiting for infrastructure changes)
The top 10 heavy truck transmission manufacturers:
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Allison Transmission - Indianapolis, Indiana, USA
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Eaton Corporation - Dublin, Ireland
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ZF Friedrichshafen AG - Friedrichshafen, Germany
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Dana Incorporated - Maumee, Ohio, USA
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Volvo Group - Gothenburg, Sweden
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Voith Turbo GmbH & Co. KG - Heidenheim an der Brenz, Germany
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Meritor, Inc. - Troy, Michigan, USA
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MAN SE - Munich, Germany
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Scania AB - Södertälje, Sweden
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PACCAR, Inc. - Bellevue, Washington, US