Advanced Hydrostatic Drivetrain

Hydrostatic drives are in service for more than 70 years, mainly in heavy construction machinery where the infinite variable speed and torque range are very advantageous and the interruption of torque during shifting is not admissible. Because of their ease of use, they became recently popular in garden tractors and other small equipment, but efficiency and speed range are here of lesser importance.

Hydrostatic Drivetrain Concept

The drivetrain includes a Hydraulic Free-Piston Engine (HFPE) with pistons having a combustion piston with a hydraulic piston end for transmitting the combustion pressure directly into pressurized fluid for driving the wheel motors or charging the accumulator. The engine operates only at constant power and speed and will automatically be turned off when the accumulator is filled. This allows for an engine operating profile with lowest consumption and emissions. The wheel motors with variable displacement maximize the efficiency over a wide torque and speed range. The accumulator delivers and stores the power for acceleration and braking very fast, reducing the fuel consumption, emissions, and the size of the engine.

Current hydrostatic drives, as used in heavy equipment (excavators) are not suitable for road vehicles since their operating range is small and the energy losses, weight, and costs are high. Significantly improved technical data of the new drivetrain components (Free-Piston Engine, Axial-Piston Wheelmotor) are needed for vehicle concepts fulfilling the requirements regarding emissions, consumption, size and costs. In addition, hydraulics allow for simple and effective safety systems to protect passengers and other traffic participants.

Free-Piston Engines (FPE) with crank mechanism are, when compared with conventional engines, more compact and efficient due to lower combustion heat losses. Their production costs don't seem to be very different, but the shape of the FPE is not well suited for the engine bay in automobiles since they are too wide for horizontal and too high for vertical installation.

The HFPE without crank mechanism and piston side-forces operates with noticeably higher combustion pressure, increasing the efficiency and reducing the size and weight of the engine further. The Piston Impulse Charger & Compounder (PICC) is driven by the increased exhaust heat (lower combustion heat losses) to improve the efficiency. 

Only a significantly smaller engine (1/4 displacement) is required since the accumulator provides the power peaks for acceleration and absorbs those during braking. Other sources of energy (electrical, mechanical) can be applied to charge the accumulator. Due to the high internal pressure (450 bar / 6,530 psi), the CFRP accumulator is very stiff and serves in addition as load carrying backbone of the Ingocar. Its bending and torsional  stiffness is comparable to those of current car bodies. With support of the active bumpers, the new, lighter car body is not exposed to high crash forces.

The platform concept of the Ingocar is is based on previous tests of the hydraulic motor and early tests of the new HFPE. The technology includes a larger number of features described in Patents (6) or Patent applications (2). Their influence in the drivetrain components, and advanced hydrostatic drive systems is shown in the following sections.

Hydraulic Free-Piston Combustion Engine

The HFPE with the PICC for exhaust energy recuperation has few moving parts and is simple, small, and efficient. The combustion pressure is directly transferred into hydraulic pressure at the opposite end of the piston. Having no crank mechanism, the opposed pistons are free of side forces and the mass forces in axial direction are nearly fully balanced to provide low-vibration operation. The hydraulic forces at the pistons are controlled by fast acting electro-hydraulic valves. The engine is therefore less sensitive towards high pressures, temperatures, and velocities, and lubrication of the piston is not required. The conditions result in very high power density and efficiency by simultaneously reducing the emissions, size and costs significantly. The main factors are:

1. High pressure:
   Non-existing piston side forces allow for high combustion pressure, improving efficiency and power density. Proven materials and manufacturing processes. Multi-fuel capability, including Hydrogen. Lower costs through less material and fewer parts and manufacturing processes.
    
2. High combustion efficiency:  
   Efficient operation at constant speed and power with high compression ratio and charge pressure. Independent control of opposed pistons for variable Uniflow port scavenging for low charge losses. High free-piston acceleration at TDC and compact combustion chamber (D/H 10:1) fully ceramic coated for low heat losses. Peripheral Fuel Injection (PFI) with 24 micro-slots and 3.500 bar (50,700 psi) injection pressure for high atomization, distribution and minimization of fuel impingement for increased combustion efficiency and reduced emissions (CO2, NOx). Piston Impulse Charger & Compounder piston for exhaust energy recouperation.
    
3. Less friction:
   One cylinder. No piston side loads. No crankshaft and connection rods. No valve train. No oil pump.
    
4. Low emissions:
   Homogeneous Charge Compression Ignition (HCCI) through PFI with 24 micro-slots in the cylinder wall and 3.500 bar (50,700 psi) fuel injection pressure for highest atomization, distribution, and minimization of fuel impingement.
    
5. Efficient cooling:
   The power is transmitted by hydraulic fluid which serves also as coolant for the engine and wheel motors. The hydraulic circuit and separate coolers radiate the heat very efficiently.   

Piston Impulse Charger & Compounder

The piston of the PICC is driven by the exhaust gases of the engine and extract thermal and kinetic energy from the exhaust. The distribution in charge air and hydraulic power is infinitely adjustable to maximize the extraction of energy. The radiation of noise is reduced through a stepwise reduction of gas pressure from the combustion chamber -  to the charger chamber  -  to the environment.

Free-Piston Engine (POC Model)    Vh  =  0.27 Liter.    Projected: 30 kW (40 hp) @ 2,600 1/min,  Dimensions: 53 x 30 x 23 cm (21 x 12  x 9”)

The hydraulically driven fuel injection pump transmits the hydraulic pressure of the accumulator directly into very high fuel pressure (3.500 bar / 50,800 psi). The injection through a large number of very small slots at the circumference of the combustion chamber, results in a homogeneous distribution of highly atomized fuel in the combustion chamber and nearly prevents the impingement of fuel at the cylinder wall, as shown in the graph Peripheral Fuel Injection, below. This arrangement provides significantly improved preconditions for a Homogeneous Charge Compression Ignition (HCCI) operation, including the emissions of soot and NOx. The HC and CO emissions are generally relatively low in Diesel engines, and the CO2 emissions are reduced proportionally to the fuel consumption.

The improvements in the Quality of Combustion are based on a comparison with current systems where fuel injectors have a small tip with 6 to 8 closely spaced micro holes for injecting the fuel, resulting in a high concentration of fuel beams with insufficient access to oxygen, creating a delayed, incomplete combustion with lower efficiency and higher emissions.

Peripheral Fuel Injection

It is expected, that the high efficiency (155 g/kWh) of large Ship Diesel engines (low cylinder wall heat losses, no impingement) are compensated through the higher combustion efficiency, quality of fuel and reduced mechanical friction of the HFPE. The higher rpm. of automotive 4-stroke engines is compensated through the doubling of power strokes of the less heat sensitive 2-stroke HFPE, noticeably higher brake mean effective pressure (BMEP) of (35-40 bar) and reduced losses of combustion heat and friction.

Achieving the high expected thermal efficiency of 63% is based on improved dynamic behavior of the free-piston principle (closer to constant volume combustion), smaller combustion chamber surface area (-30%) with ceramic coating to reduce heat losses, and efficient recuperation of exhaust energy through the piston impulse charger & compounder. Considering the low friction losses of the FPE and additional hydraulic losses (7%) for controlling the pistons and driving the fuel injection pump, a reduction of 30% in specific fuel consumption to 140 gr/kW•h (0.230 lb./hp.h) is expected.

Display at the 12. International Engine Congress, Febr. 25. – 26. 2025, Baden-Baden, Germany.

Hydraulic Axial-Piston Motor

Hydraulic motors for road vehicles must be very efficient, small, light, and their displacement adjustable to adapt to the driving conditions regarding speed and torque. The required high operating pressures can reliably obtained only with piston units. Axial-piston motor types provide, in comparison with radial-piston units, a wider range of adjustment, and higher rotational speed, and power density. Of those, bent-axis type axial-piston motors have currently a wider range of adjustment and speed and less mechanical losses, but are noticeably larger and heavier, more expensive, and have a shorter theoretical lifetime expectancy.

Operating ranges, new and current hydraulic motors

The new motor is a swashplate-type unit. Several patented and new features result in a reduction in size and weight (-60%), an increase in maximum speed (+75%) and a larger Tilt angle (+57%). The wider range of the displacement adjustment, and signifi-cant decrease in fluid, compression losses and friction, especially in areas of low powers increases the operating range by a factor of more than 5.

The concept is scalable over a wide area, from about 10 to 10.000 kW (14 to 14,000 hp). The specific weight (kW/kg / hp/lbs.) is increased by a factor of 6. The Diagram shows the  o operating ranges of the best current motors in comparison to those of the new motor.

Compared with electric motors for automotive drivetrains, the new hydraulic motor has a 7 times higher specific power density (kW/kg / hp/lbs.) and a high reduction in specific volume kW/Liter (hp/cu.in.) In addition, control and cooling are noticeably simpler.

Based on the new motor geometry, the critical mechanical loads are lower and the mechanical friction losses reduced through Diamond-Like-Coating (DLC) by more than 50%.

The smaller size and shorter sealing areas reduce the external leakage by more than 30%. A significant reduction of internal leakage, generally the largest portion of losses, is reached through new design features.

U.S. Patents, International Patents pending.
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