Purely electric machines, such as Reckless, have very high characteristics and can be converted into hybrid platforms of a sequential circuit with the integration of an internal combustion engine and a generator, allowing to increase the range
The need for electricity of a continuously increasing complexity of on-board electronic equipment of modern combat vehicles is an additional incentive when choosing solutions with a hybrid drive, while a number of mutually contradictory characteristics need further development.
The energy necessary for driving ground vehicles and the operation of their systems and assemblies is traditionally provided by diesel engines. Reducing fuel consumption not only increases the range, but also reduces the amount of material and technical support, determined by maintaining fuel reserves, and increases the security of rear service specialists in the process of servicing equipment.
In this regard, the armed forces are striving to find a solution in which a high efficiency coefficient and high specific heat of combustion of diesel fuel inherent in systems with an electric drive would work in one “harness”. New hybrid solutions and advanced internal combustion engines potentially promise great practical benefits along with silent movement on a single drive, silent monitoring (battery sensors during parking), and energy generation for external consumers.
Power train potential
The Canadian Research and Development Authority (DRDC), for example, is exploring the possibility of implementing hybrid diesel-electric power drives. The management published their research in 2018, focusing on light tactical platforms, for example, HMMWV, ultralight combat vehicles like DAGOR, on small single and multi-seat ATVs.
The report "The feasibility of hybrid diesel-electric power drives for light tactical vehicles" notes that in most driving modes in which speed and loads change significantly (typically when driving on the road), hybrids have 15% -20% better fuel economy Compared to traditional mechanically driven machines, especially when using regenerative braking. In addition, internal combustion engines, including diesel ones, are most effective when working at carefully selected constant speeds, which is typical for sequential hybrid circuits in which the engine only works as a generator.
As noted in the report, since engine power can be supplemented by batteries in short periods of peak power consumption, the engine can be configured to provide only the average required power, while smaller power plants, ceteris paribus, generally consume less fuel.
With sufficient battery capacity, hybrids can also remain in silent monitoring mode for a long time with the engine turned off and sensors, electronics and communication systems working. In addition, the system can power external equipment, recharge batteries and even power the military camp, reducing the need for towed generators.
While hybrid drives provide superior performance in terms of speed, acceleration and hill climbing ability, battery packs can be heavy and bulky, resulting in lower payloads, says the DRDC report. This can be a problem for ultralight vehicles and single-seat ATVs. Moreover, at low temperatures, the characteristics of the batteries themselves are reduced, they often have problems with charging and temperature control.
Although mechanical transmissions are eliminated in sequential-circuit hybrids, the need for an engine, generator, power electronics and battery inevitably makes them ultimately complex and expensive to purchase and maintain.
Most battery electrolytes can also pose a risk of damage, for example, lithium-ion cells are known for their tendency to ignite when damaged. Whether this poses a greater risk than the supply of diesel fuel is perhaps a contentious issue, the report says, but hybrids carry both of these risks.
The two main schemes for combining internal combustion engines with electrical devices are serial and parallel. As mentioned above, a serial hybrid platform is an electric machine with a generator, while in a parallel circuit there is an engine and a traction motor, which transmit power to the wheels through a mechanical transmission connected to them. This means that the engine or traction motor can drive the machine individually or they can work together.
In both types of hybrids, the electric component, as a rule, is the motor-generator set (MSU), which can convert electrical energy into motion and vice versa. It can set the machine in motion, charge the battery, start the engine and, if necessary, save energy due to regenerative braking.
Both serial and parallel hybrids rely on power electronics to control the battery charge and regulate its temperature. It also provides voltage and current, which the generator must supply to the batteries, and the batteries in turn to the electric motors.
This power electronics comes in the form of semiconductor inverters based on silicon carbide semiconductors, the disadvantages of which, as a rule, include the large size and cost, as well as heat loss. Power electronics also need control electronics, similar to that which ensures the operation of the internal combustion engine.
Until now история The electrically driven military vehicles consisted of experimental and ambitious development programs, which ultimately were all closed. In actual operation, there are still no hybrid military vehicles, in particular in the field of light tactical vehicles, several unresolved technological problems remain. These problems can be considered mainly resolved for civilian cars, since they work in much more favorable conditions.
Electric cars have shown themselves to be very fast. For example, the battery-powered experimental four-seater Reckless Utility Tactical Vehicle (UTV) from Nikola Motor can accelerate from 0 to 97 km / h in 4 seconds and has a range of 241 km.
“Layout, however, is one of those complex issues,” the DRDC report said. The dimensions, weight and heat dissipation of the battery pack are quite large, and it is also necessary to compromise between the total energy intensity and the instantaneous power that they can produce for mass and volume data. The allocation of volume for high-voltage cables, their reliability and safety are also bottlenecks along with the dimensions, weight, cooling, reliability and waterproofing of power electronics.
The U.S. Army is studying how to adjust the power drive to ensure that combat vehicles move over rough terrain at higher speeds
Heat and dust
The report said that the temperature differences encountered by military vehicles are perhaps the biggest problem, since lithium-ion batteries will not charge at temperatures below zero, and heating systems increase complexity and need energy. Batteries that overheat during a discharge are potentially dangerous, they must be cooled or reduced to a lower mode, while motors and generators can also overheat, finally, do not forget about permanent magnets, which are prone to demagnetization.
Similarly, at temperatures above about 65 ° C, the efficiency of such devices as, for example, inverters based on the technology of semiconductor bipolar transistors with insulated gate decreases, and therefore they need cooling, although the newer power electronics based on semiconductors made of silicon carbide or Gallium nitride, in addition to operating at high voltage, can withstand higher temperatures and, therefore, can be cooled by the engine cooling system.
As noted in the report, in addition, shock and vibration when driving over rough terrain, plus the potential damage that may result from shelling and explosions, also significantly complicate the integration of electric drive technologies into light military vehicles.
The report concludes that the DRDC should order a technology demonstrator. This is a relatively simple light tactical machine with a serial hybrid circuit, in which the electric motors are installed either in the wheel hubs or in the axles, the diesel engine is adjusted to the corresponding peak power, and a set of super- or ultracapacitors is installed to improve the acceleration process and overcome the slopes. Super- or ultracapacitors accumulate a very large charge for a short period of time and can give it out very quickly to receive power pulses. The machine will either not be installed at all, or a very small battery will be installed, electricity will be generated during regenerative braking, as a result, the modes of silent movement and silent monitoring are excluded.
Only the power cables laid to the wheels, replacing the mechanical transmission and drive shafts, will significantly reduce the weight of the machine and improve anti-explosion protection, since the expansion of secondary debris and fragments is excluded. Without a battery, the internal volume for the crew and payload will increase and become safer, problems associated with maintenance and thermal management of lithium-ion batteries will be eliminated.
In addition, when creating an experimental machine, the following goals are set: lower fuel consumption of a relatively small diesel engine operating at constant speeds, combined with energy recovery, increased power generation for operation of sensors or energy export, increased reliability and improved service.
The AGT 1500 engine of the M1 Abrams tank has a high power density compared to diesel engines and does not need a special cooling system
As explained by Bruce Brandl of the Research Center for Armored Vehicles (TARDEC) at a presentation on the development of engine building, the U.S. Army wants to get a power plant that will allow its combat vehicles to navigate more difficult terrain at higher speeds, which will significantly reduce the percentage of terrain in combat zones. on which current cars cannot move. The so-called impassable terrain is about 22% of these zones and the army wants to reduce this figure to 6%. They also want to increase the average speed over most of this area from today's 16 km / h to 24 km / h.
In addition, Brandl emphasized that it is planned to increase the energy requirements on board to at least 250 kW, that is, higher than what the machine’s generators can produce, since the load is added from new technologies, for example, electrified towers and protection systems, power electronics cooling , energy exports and armament of directed energy.
As calculated in the US Army, meeting these needs with current turbo-diesel technology will increase the engine occupied volume by 56% and vehicle weight by approximately 1400 kg. Therefore, when developing its promising Advanced Combat Engine (ACE) power plant, the main task was to double the specific power with 3 hp / cc. ft to 6 hp / cc foot.
Although higher power density and better fuel economy are very important for a new generation of army engines, it is equally important to reduce heat loss. This generated heat represents lost energy dissipated into the surrounding space, although it could be used to propel or generate electrical energy. But it is far from always possible to achieve a perfect balance of all these three parameters, for example, the AGT 1500 gas turbine engine of the M1 Abrams tank with the power of 1500 hp It has low heat dissipation and high power density, but very high fuel consumption compared to diesel engines.
In fact, gas turbine engines generate a large amount of heat, but most of it is removed through the exhaust pipe, due to the high intensity of the gas stream. As a result, gas turbines do not need the cooling systems that diesel engines require. High specific power of diesel engines can be achieved only by solving the problem of thermal control. Brandl emphasized that this is mainly due to the limited volume available for cooling equipment, such as piping, pumps, fans and radiators. In addition, protective structures, such as bulletproof grills also occupy volume and limit airflow, reducing the efficiency of the fans.
As Brandl noted, the ACE program focuses on two-stroke diesel / multi-fuel engines with opposed pistons, which is associated with their inherent low heat dissipation. For such engines, two pistons are placed in each cylinder, which form a combustion chamber between themselves, as a result, the cylinder head is excluded, but two crankshafts and an inlet and an outlet port in the cylinder walls are required. Boxer engines appeared in the 30 of the last century and have been constantly improved for decades. Achates Power, which in collaboration with Cummins revitalized and modernized this engine, did not pass by this old idea.
Achates Power spokesman said their opposed technology is characterized by increased thermal efficiency, which is determined by lower heat losses, improved combustion and reduced pumping losses. The exception of the cylinder head made it possible to significantly reduce the ratio of surface area to volume in the combustion chamber and thereby the transfer and transfer of heat in the engine. In contrast, in a traditional four-stroke engine, the cylinder head includes many of the hottest components and is the main source of heat transfer to the coolant and the surrounding atmosphere.
The Achates combustion system uses twin fuel injectors diametrically located in each cylinder and a patented piston shape to optimize the air-fuel mixture, resulting in low soot combustion and reduced heat transfer to the walls of the combustion chamber. A fresh charge of the mixture is injected into the cylinder, and the exhaust gases exit through the ports, which is facilitated by a supercharger pumping air through the engine. Achates points out that this direct-flow purge has a positive effect on fuel economy and emissions.
The US Army wants the ACE family of modular scalable power plants to include engines with the same cylinder bore and stroke and different cylinder numbers: 600-750 hp (3 cylinder); 300-1000 HP (4); and 1200-1500 hp (Xnumx) Each power plant will occupy a volume - height 6 m and width 0,53 m and, accordingly, length 1,1 m, 1,04 m and 1,25 m.
The boxed two-stroke engine from Achates Power and Cummins should become the basis of the ACE family of the American army
An internal army study conducted in 2010 confirmed the benefits of boxer engines, as a result of which the Next-Generation Combat Engine (NGCE) project was launched, in which industrial enterprises presented their developments in this area. The task was set to achieve the power of 71 hp per cylinder and total power 225 hp By 2015, both of these numbers were easily exceeded by an experimental engine that was tested at the Armored Research Center.
In February of the same year, the army awarded AVL Powertrain Engineering and Achates Power companies contracts for experimental ACE single-cylinder engines under a two-year program, in which the goal was to achieve the following characteristics: power 250 hp, torque 678 Nm, specific fuel consumption 0,14 kg / hp / h and heat transfer less than 0,45 kW / kW. All indicators were exceeded, except for heat transfer, here it was not possible to drop below 0,506 kW / kW.
In the summer of 2017, Cummins and Achates began work under the ACE Multi-Cylinder Engine (MCE) contract to demonstrate the four-cylinder 1000 engine. 2700 Nm and the same requirements for specific fuel consumption and heat transfer. The first engine was manufactured in July 2018, and initial operational tests were completed by the end of that year. In August 2019, the engine was delivered to the TARDEC Office for installation and testing.
The combination of the boxer engine and hybrid electric drive would improve the efficiency of vehicles of various types and sizes, both military and civilian. Aware of this, the Office of Advanced Research and Development has given Achates two million dollars to develop an advanced opposed single cylinder engine for advanced hybrid cars; in this project, the company collaborates with the University of Michigan and Nissan.
In accordance with the concept, in this engine for the first time the electrical subsystem and the internal combustion engine are so closely integrated, each of the two crankshafts rotates and can be driven into rotation by its own motor-generator set; there is no mechanical connection between the shafts.
Achates confirmed that the engine was designed only for serial hybrid systems, since all the power it generates is transmitted electronically, and the motor-generator sets charge the battery to increase the range. Without mechanical coupling between the shafts, the moment is not transmitted, which leads to lower loads. As a result, they can be made lighter, reduce the total mass and dimensions, friction and noise, and also reduce the cost.
Perhaps most importantly, the disconnected crankshafts allow independent control of each piston through the use of power electronics. "This is an important part of our project, it is important to determine how the development of electric motors and controls could increase the efficiency of the internal combustion engine." Achates spokesman confirmed that this configuration allows you to control the timing of the crankshaft, which opens up new possibilities. “We are striving to increase the efficiency of piston control, which is not available in the case of traditional mechanical coupling.”
At the moment, there is little information available on how independent piston control can be used, but theoretically it is possible to make the stroke longer than the compression stroke, for example, and thereby extract more energy from the charge of the air-fuel mixture. A similar scheme is implemented in Atkinson four-stroke engines installed in hybrid cars. In the Toyota Prius, for example, this is achieved through the control of the valve timing.
For a long time it was obvious that big improvements in proven technologies, for example, in internal combustion engines, were not so easy to achieve, but advanced opposed engines could become what would provide real advantages to military vehicles, especially in combination with electric power plants .