Torque balanced opposed-piston engine

Details Torque balanced opposed-piston engine Torque balanced opposed-piston engine

1999-07-13

2001-02-20

McMahon, Marguerite (Department: 3747)

Internal-combustion engines

Multiple cylinder

Simultaneous compression, distinct pistons, restricted...

C123S052600, C123S059600

Reexamination Certificate

active

06189493

ABSTRACT:

FIELD OF THE INVENTION
The field of the invention is internal-combustion engines for motor vehicles. The invention is a means for small engines or engines of few cylinders to reduce or eliminate the effect of cyclic peak-to-valley torque variations characteristic of such engines, that would otherwise be transmitted to the engine mounts or, if in a vehicle, the vehicle frame, creating unwanted vibration and instability and discouraging the use of such engines in motor vehicles.
BACKGROUND OF THE INVENTION
The growing utilization of automobiles greatly adds to the atmospheric presence of various pollutants including oxides of nitrogen and greenhouse gases such as carbon dioxide.
Internal combustion engines create mechanical work from fuel energy by combusting fuel in a thermodynamic cycle consisting (in part) of compression, ignition, and expansion. The cycle results in the travel of one or more cylindrical pistons back and forth in a cylindrical combustion chamber. Each piston is typically connected to a crankshaft that converts the linear back-and-forth motion of the piston(s) into a unidirectional rotary motion that can be used to power a vehicle. Because torque is produced only during the expansion phase, and in fact torque is absorbed during the compression phase, there are large cyclic fluctuations in torque throughout each cycle.
The cyclic, fluctuating nature of the torque produced on the crankshaft tends to favor engines with many pistons operating at high speeds. During each cycle of each piston, the piston-crankshaft assembly and the cylinder walls bear the force of the expanding combustion products. The force on the connecting rod, which is converted to torque via the crankshaft, can be resolved into a force in the direction of piston travel and a side force acting on the cylinder wall and, hence, on the engine block. These piston and side forces vary greatly during successive portions of the cycle, resulting in large fluctuations that manifest themselves either as cyclic variations in crankshaft torque or as inertial engine movement especially when the torque is taken from the shaft. The inertial movement must be resisted by the engine mounts and is ultimately transmitted to the vehicle. The key concern is the peak-to-valley amplitude of the variation. To some extent, the peak-to-valley variation in crankshaft torque can be minimized by transmitting the power through a flywheel, but inertial engine vibration is still a problem. If multiple cylinders are present, the peak-to-valley variations in both crankshaft torque and inertial engine movement can be reduced by staging and timing the combustion cycle for each piston so that their relative torque production and relative motions in their respective portions of the cycle cancel out much of the variation. The more pistons involved, the smaller the peak-to-valley amplitude of the remaining variation. The problem is exacerbated when operating at low speeds, because any variation that remains has a longer period and is more noticeable. For these reasons, most internal combustion engines used in automobiles have from four to eight pistons and operate at high speeds, typically 800 to 4000 rev/min.
Minimizing the number of pistons in an engine and operation at low speed are very attractive from an efficiency standpoint. Few-cylinder engines are simpler in construction and therefore less expensive than many-cylinder engines. More importantly, they are lighter and smaller than many-cylinder engines, allowing reductions in engine weight and engine compartment size that translate into lower curb weight and better fuel economy. Many hybrid powertrain schemes call for unusually slow engine operation (perhaps 500 rpm or less). However, the prior art does not permit such engines to operate at a low speed and high load factor without invoking the problems discussed above.
Opposed-piston or “boxer” engines have existed for some time. They are mechanically balanced, characterized by pairs of opposed pistons in which each pair is arranged in linear opposition with a crankshaft inbetween, but are not torque balanced. Because the pair is connected, one piston head may be in the expansion stroke while the other is in compression, or both may be in the same phase, but their movement is always synchronized. As long as there is an even number of piston heads, the opposition of each pair theoretically cancels out an inertial vibration. However, because the conventional “boxer” engine is not torque balanced, when power is taken from the shaft there is still a tendency to spin the engine, which must be resisted by the engine mounts and vehicle frame, and any cyclic peak-to-valley torque variation must also be borne by the mounts.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide an engine design preventing cyclic engine-transmitted forces which produce torque variations and inertial variations from being borne by the engine mounts and ultimately transmitted to a vehicle frame.
The engine of the invention includes at least two engine subassemblies each, in turn, including a piston/cylinder set, crankshaft, and means for mechanically linking the crankshafts. The two engine subassemblies are physically connected, either simply bolted together or built together as a single entity. Each engine subassembly is independent except for a connection via a synchronization means, e.g. geared wheels which are connected to their respective crankshafts. The geared wheels of each subassembly are enmeshed together to synchronize the respective crankshafts in counter-rotation and in identical timing. In this manner, pairs of cylinders fire simultaneously.
The two counter-rotating crankshafts each receive torque from their respective piston/cylinder assembly. In the illustrated embodiment, each engine subassembly employs an arrangement of pistons such that the inertial effect of any piston is counteracted by the motion of a linked, identically timed twin piston traveling in the direction opposite that of the first piston.
The two crankshafts may power an electric generator, a fluid power device or other device which could be bolted or otherwise affixed entirely to the engine itself, thus eliminating the unwanted torque effect that could be transmitted to the engine mounts or other parts of the vehicle when taking rotary motion off a shaft. By these means the useful work of a few-cylinder engine (2, 4 or 6 piston/cylinder sets) may be conducted to the vehicle and the torque and inertial variation is dissipated within the engine assembly itself, rather than being transmitting through the engine mounts.


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