The inherent advantages of the two-cycle engine over the four-cycle (its favorable power to weight and volume ratio) are undisputed within the engine-making community, yet two-stroke is not popular. In fact, it's being phased out. The main reason? Its failure to meet stiffening emission laws.
Up to now, problems of combustion control and cylinder scavenging have remained unresolved in the two-cycle engine. and have not been able to meet emission regulations. The research work under this proposal will show the feasibility of solving these problems by bringing the two-cycle combustion process under control through advanced CFD and an opposed piston, opposed cylinder (OPOC) engine. At the same time, the research work will show that the OPOC engine will further enhance the existing advantages in power to weight and volume ratio, engine efficiency, and driving comfort.
The OPOC engine will have numerous advantages over current mass-produced four-cycle engines. The research work will focus on the advancement of OPOC engine technology with emphasis on precise combustion control, proper cylinder scavenging, high-efficiency, high-EGR dilution, good driving comfort (low vibration), low weight, small packaging volume, and manufacturing cost reduction. The work will also show the OPOC engine as a favorable alternative to other new engine designs, currently pursued under several government-funded programs.
The results of the research will be applicable if the OPOC engine is used as a stand-alone vehicle power plant or if the OPOC engine is used as the power plant in hybrid electric vehicles. The engine could also be used as a source of power in a wide range of other applications, like aircraft, generators, lawn movers and pumps.
The OPOC engine is a two-stroke CIDI engine, comprising opposed cylinders, each having two which are opposed. All pistons are connected to a single crankshaft, located between the two opposed cylinders, by unconventional connecting rods. The volume formed between the two opposed pistons is the combustion chamber, and located near the top dead center (TDC) is a position fuel injector. Intake and exhaust pons are located at the ends of the combustion chamber near the bottom dead center (BDC) positions. These positions, in conjunction with an electrically assisted turbocharger, provide the means for cylinder scavenging (there are no valves or camshafts).
The crankshaft journals, which control the position of the pistons, are arranged asymmetrically, such that for each cylinder, the exhaust ports open before the intake ports and close before the intake ports. The asymmetric timing provides for proper exhaust blow-down and allows intake supercharging.
The dynamics of the OPOC engine provide a virtually vibration free operation if the masses and geometric configurations of the moving parts are selected intelligently. To preserve this free mass force balance, and the asymmetric timing, the intake ports are located inboard (adjacent to the crankshaft) on one cylinder and outboard (remote from the crankshaft) on the opposed cylinder.
The success or failure of the two-stroke design is determined primarily by its ability to scavenge. Optimal scavenging is needed over the entire engine map to achieve near perfect combustion. Modern requirements for NOx-reducing EGR further strengthen the need for near-complete scavenging. The OPOC engine aims to solve the scavenging issues of two-stroke engines by uti-lizing uniflow scavenging. controlled boost pressure, pulse turbocharging, and asymmetric intake and exhaust timing. With innovative boost pressure control technology, the achievable EGR rates are anticipated to be much higher in the OPOC engine than the rates achieved in current two- and four-cycle engines.
Proper high-efficiency cylinder scavenging requires a well-formed front between the intake air and the exhaust gas. With the widely used loop scavenge or reverse flow scavenge, the present and future demands of vehicle engines cannot be accomplished because the exhaust gas and intake air mix together. Of the possible uniflow scavenging methods - poppet exhaust valves, opposed pistons, and split single designs - the opposed pistons is the most promising because the port configuration allows for the highest level of volumetric efficiency and the least mixing of exhaust gasses with fresh intake of air.
The OPOC engine will utilize an electrically assisted turbocharger to satisfy its supercharging needs. Since the electrically assisted turbocharger will be both exhaustgas driven and electrically driven, boost pressure becomes independent of engine operation, and high-pressure boost becomes available for acceleration at low engine loads and low rpm without any associated mechanical drag. Additionally, high boost pressure is available for forcing extremely high EGR rates during partial bad conditions.
In summary, therefore, asymmetric timing will take place:
The OPOC engine achieves this asymmetric timing configuration by splitting the crankshaft throws for each cylinder.
Therefore, the throw that is associated with the piston that controls the exhaust ports is angularly advanced to the throw that is associated with the piston that controls the intake ports of the same cylinder.
The electrically assisted turbocharger is an integral subcomponent of the OPOC engine, without this device, scavenging the two-cycle and reduction of emissions, along with other engine benefits are unachievable.
In addition to providing precise scavenging, the electrically assisted turbocharger allows for higher than normal EGR rates, which will help to reduce NOx emissions. With a proper control mechanism, it will also be capable of monitoring and maintaining a constant air fuel ratio.
Before starting the engine, the electrically assisted turbocharger can rapidly compress and recycle air in order to heat it to 100°C in less than one second. This would ensure an easy start in cold weather, without the need for: glow plugs and their related costs and complexity; excessive fuel and resultant black smoke; and unnecessary high compression ratios in the range of 19-22. Instead, compression ratios would be in the range of 15-16, resulting in reduced fuel consumption and reduced NOx emissions.
The goals for the combustion system are:
For fuel consumption, the cyclical combustion process is superior to the continuous combustion process (gas turbines and Stirling engines) in an internal combustion engine, since the working gas temperature can be much higher than the wall temperature. This leads to a much higher thermodynamic efficiency. Of internal cyclical combustion engines, the CIDI has the highest potential because it offers the opportunity for an optimal heat release by controlling the injection rate over the crank angle. Creating the desired combustion process (which delivers the optimal heat release) requires the combination of the correct injection rate and mixture swirl characteristic.
For reduction of pollutants, the OPOC engine offers promising possibilities. Complete freedom exists for designing the shape of the combustion chamber since there are no valves in this engine (Figure 1a). The combustion chamber is formed by the exhaust piston, which bas a torroidal shape matching the intake piston with a reverse profile. The pistons form a broad area squish-band that create a swirl of high intensity near TDC. The Injector projects through the torroidal chamber. This combustion system, offered by the opposed piston design, has the potential to improve exhaust emissions and also fuel consumption, power output and comfort.
In addition to the features found in conventional combustion, the OPOC engine provides the opportunity for unconventional new combustion systems (Figure 1b). By splitting the cylinder volume into a combustion chamber and cylinder, it is possible to install a NOx-reducing heat sink or a catalytic converter between the combustion chamber and cylinder. For kinetic reaction reasons, and to maintain the optimum configuration for scavenging, the converter will be fixed to the exhaust piston. These unconventional combustion systems might offer a breakthrough in extreme low emission combustion without sacrificing fuel consumption, power output and comfort of the engine.
Approximately 50 per cent of all friction losses in engines come from lateral forces produced by the rotating connoting rod acting on the piston (i.e. pushing the piston against the cylinder wall). A short connecting rod produces high lateral forces. while a long connoting rod produces low lateral forces (an infinitely long connecting rod would produce no lateral forces on the piston at all, but it would also have to be infinitely large and infinitely heavy). lt is desirable to reduce these lateral forces and therefore friction losses without an increase in the connecting rod size or weight.
By their nature, two-stroke engines have lower friction losses than four-stroke engines. However, the unique design of the OPOC engine offers even further reductions in friction. The inner piston connecting rod on the OPOC engine is subject only to compression loading, eliminating the need for a wrist pin. This is replaced by a concave radius ( of large diameter) on which a sliding crosshead slipper impinges, and on which the connecting rod slides (Figure 2). For this design to work, the forces at the end of the crosshead slipper must be greater than zero. This is the case as long as the friction between the crosshead slipper and the slide of the connecting rod is held at a predetermined minimum. With this configuration the theoretical rod length is increased by over 67 per cent, thereby decreasing the lateral forces acting on the piston and the friction losses in the engine.
The outer pistons transfer their reciprocating motion to the crankshaft via two connecting rods outside the cylinder. The connecting rods are predominantly subject only to tension loads, and are called pull rods. Again, there is a reduction in friction due to the long length of the pull rods. The pull rods are kept light by taking advantage of a constant tension, no buckling bad condition and designing them to be long and thin.
Additionally, in a standard four-cylinder in-line engine, five crankshaft main bearings must support the many "tons" of force produced by the downward thrust of the pistons on their power strokes. The friction losses in these main bearings are appreciable. In the OPOC engine, however, the reactive forces of the inner and outer pistons cancel each other out. The loads in the main bearings are very small, which eliminates the need for any center main bearings and results in much lower friction losses.
In many engine configurations, balance depends on having four, six, eight, or more cylinders arranged in such a way that the free mass forces contributed by the individual pistons cancel. Counter-rotating weights are also often employed, adding complexity to the engines. One advantage of the OPOC engine is that total balance may be achieved in a compact engine with only two cylinders. Larger engines will then be made by placing multiple small engines side-by-side and coupling their crankshafts together, allowing pairs of cylinders to be uncoupled when not needed at low loads.
Engines do exist that do not use all of their cylinders when run at partial load, but their cylinders remain connected to the crankshaft and their pistons continue to move within the cylinders, resulting in friction and pumping losses. ETI
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