All-electric vehicles propulsion based solely on battery or fuel cell technologies shows good promise for commercial vehicles. When coupled with a renewable energy sources the vehicle emissions can be dramatically reduced. However, development and research are still needed for all-electric propulsion to approach all the functional requirements that are currently made possible with an Internal Combustion Engine (ICE). For heavy commercial and combat vehicles, it is estimated that batteries will need to achieve a much higher power density to be on par with the functional capabilities of an ICE based propulsion.
Hybrid-electric vehicles for both military and off-road commercial applications will likely show a significant market share gain in the next decade. From plug-in electric parallel, to series hybrid, the solutions are diverse and complicated. The reclassification of series hybrid-electric vehicles as “new energy” has already started to shift hundreds of millions of dollars of technology investments after the realization that the billions invested in all-electric propulsion might not achieve the full desired impact. For the military, hybridization offers some very compelling product attributes. The ability to run solely on batteries, even for a short time, could enable a run-silent mode. Heat signatures can be minimized or in some cases, eliminated. A challenge will be how to enable these technological advances without compromising payload, package, cost, or the burst speed that comes along with a powertrain sized for the vehicle speed necessary to get out rapidly of the harm’s way.
For the foreseeable future, the ICE will continue to dominate the propulsion landscape. And because the global market is growing and will continue to grow, the share gains by all-electric electric propulsion will be offset by the increase of the number of engines still required globally.
If investments in full battery electric propulsion do not achieve the desired impact, the propulsion industry could face the fact that is falling behind in new ICE technology and may need to dramatically shift and accelerate the investments in that category.
While a range extender powered by Wankel engine remains a good choice for the hybrid-electric propulsion for both ground and aircraft vehicles due to its attractive form factor and high-power density other ICE technology should be considered.
Advanced ICE technology used in car racing have already managed to reach over 50% total effective efficiency by recovering the energy contained in the exhaust gases. However, in order to achieve this level of efficiency the engines have to use an external recovery system attached to the engine block.
Also, recent new ICE developments showed the great promise of the Opposite Piston Opposite Cylinder (OPOC) engines with respect to power density, low vibrations and effective efficiency.
Micor Technologies’ new prime mover concept, RECOVER, aims at obtaining even higher specific power and efficiency while preserving the cost-effective technology gains obtain to date in the continue quest for the conversion of chemical energy into electric power for use in electrified ground and aircraft vehicle powertrains.
RECOVER: A high efficiency range-extender engine.
RECOVER is a four-stroke double crankshaft engine with internal heat recovery. The engine uses a tandem piston system, the main piston hosted by the main cylinder and the auxiliary piston hosted by the auxiliary cylinder. The bore of the auxiliary cylinder is substantially larger than the bore of the main cylinder. The main piston and the auxiliary piston are each positioned at the extremity of a rigid rod. The rod has a balancer fitted with a piston pin. The balancer has on each side a joint with a connecting rod as shown in figure. Each connecting rod rotates a crankshaft. The two crankshafts are synchronized by two gears. The main cylinder is closed by a cylinder head having the standard function. The cylinder head contains the intake and exhaust valves, activated mechanically or in any other manner. The intake valve controls an intake pipe. The exhaust valve controls a re-circulation pipe. The auxiliary cylinder is closed by a cylinder head. The auxiliary cylinder head contains an intake valve for the recirculated exhaust gas from the main cylinder, an exhaust valve for the exhaust gas recirculated in the auxiliary cylinder, an intake valve for the fresh air and a transfer valve for pressured air. All valves can be activated mechanically, hydraulically or electronically.
The cycle developed in the main cylinder can be considered as being a conventional supercharged cycle (Otto or Diesel). The auxiliary piston/cylinder assembly works, in a first phase, as positive displacement supercharger (360°- a complete crank rotation) which supplies with pressurized air the main cylinder and in a second phase ( 360°-the subsequent complete crank rotation) as an expander which recover the energy existent in the exhaust gases supplied from the main cylinder’s combustion chamber through a recirculation pipe. As a result, the auxiliary piston/cylinder assembly achieves a secondary expansion having almost a constant pressure, which produces a supplementary work. This increases the effective efficiency by recovering an important part of the exhaust gas energy. With two expansions phases at each two crankshaft rotations the output power density will be increased.
The RECOVER engine can match the fuel cell efficiency of 40-50% without using for its manufacturing expensive, strategic metals competing as such with the fuel cell which is considered until now the technology of the future with respect to efficiency.
RECOVER can be an attractive energy source in a hybrid-electric powertrain as a range extender for hybrid-electrical ground vehicles and CTOL or VTOL aircrafts. In addition, RECOVER could use Carbon Neutral Fuels, including Hydrogen in order to significantly reduce the global emission level.
In its application to hybrid-electric propulsion the two-crankshaft configuration allows the use of two electric generators in the simplest manner and with increased redundancy level of the propulsion system.
The advantages of the RECOVER range extender:
A short video clip showing the RECOVER engine cycle can be viewed HERE.
THERMAX: A quasi-free piston engine for range extenders.
THERMAX has a similar configuration with a conventional free piston engine but modified to transmit its power to a rotating electric generator in order to create an attractive new type of range extender.
THERMAX employs two axially disposed stepped pistons, connected between them by a stiff rod and forming as such an innovative type of two-headed piston. The solid piston transmits its reciprocating motion by a single conventional connecting rod to a conventional crankshaft located in the space formed between the two stepped pistons, respectively in the middle zone of the engine. The crankshaft has its axis perpendicular on the axis of the two stepped pistons. The small portion of every stepped piston, named motor piston, works in a conventional way as the movable wall of the combustion chamber. The stepped or enlarged diameter portion named pumping piston, operates in an axially aligned cylinder, named pumping cylinder. The fresh air enters the pumping cylinder on the “down-stroke” of the piston using some flexible inlet valves and is compressed on the “up-stroke”, being delivered through a transfer pipe. The transfer pipe makes the connection with the combustion chamber of the opposite cylinder.
The rotating masses of the engine are balanced by means of counterweights which oppose the centrifugal force. The inertia forces, caused by the solid piston (or the oscillating masses), are mostly balanced by the pressure existent in the opposite combustion chamber and by the compression pressure existent in the opposite pumping cylinder. These forces are directly transmitted from one stepped piston to the other using the stiff rod(s). Consequently the connecting rod and the crankshaft are forced to transmit the useful work of the engine and can be sized properly with this considerable diminished stress (the connecting rod and the crankshaft are solicited by the inertia forces only in the starting conditions when the engine speed is very low and these forces are proportionally lower).
Due to the special balancing solution, the mass of the solid piston does not limit the engine speed and consequently it can obtain a high level of power density. It is a very simple and cost-effective solution because every two stepped piston uses only one connecting rod and the crankshaft has only one crankpin.
The stroke to bore ratio can be achieved in the range of 0.7-2 because the pistons are interconnected in the most convenient zone, respectively using the stepped portion which has a large diameter. The connecting rod and the crankshaft transmit the useful work of the engine and can be sized properly.
Having an efficient scavenging process this engine can operate with “lean” mixture and the fuel consumption can be significantly lower. The scavenging pump and the crankshaft mechanism are separated and consequently the lubrication problem disappears. There is no oil burned in the cylinder, lowering the HC emission and the soot deposits. Slide bearings for crankshaft and connecting rod can be used. Also using the stepped pistons and the control of the exhaust timing a supercharger effect is obtained and consequently the power density is increased. The foot print of the engine becomes much smaller and the weight, the engine’s cylinder displacement and combustion chamber volume can be reduced. The size of the engine is greatly reduced relative to the size of the vehicle in order to minimize the effect of the engine friction losses. This in turn improves vehicle fuel economy. The number of the main parts in motion (only 3 for each double cylinder) are reduced which improves the durability and reliability of the engine and respectively that of the range extender.
A short video clip showing the THERMAX kinematics can be viewed HERE.