SEA High-Efficiency Engine
1. HEAT ENGINES AND LONG-TERM STORAGE OF ELECTRICAL ENERGY
Electrical energy can be stored by converting it to the chemical energy of a combustible fuel. For electricity generated from the solar resource, this energy conversion/storage process usually involves electrolysis of water to produce hydrogen, which can either be stored as a high pressure gas or converted to a more volumetrically energy-dense fuel such as liquid ammonia. When needed, a substantial portion of the electrical energy used in the electrolysis process can be reclaimed by burning the fuel in a heat engine that drives an electric generator.
The end-to-end efficiency (electricity to fuel to electricity) of an Electrolysis-Storage-Reconversion cycle (ESR Cycle) determines whether or not it can be used to bridge daily and seasonal variations of the solar resource. Each step in the cycle is important. During the past few years, water electrolysis units with efficiencies well above 80% have become commercially available. That’s to be compared with roughly 50 or 60% only a decade ago.
Also, large numbers of high-efficiency, low-cost ammonia generation units, complete with associated storage facilities, have been installed at locations spread across Europe and North America (Proton Ventures).
However, development of technologies that might be used in the third step, the reconversion step, have lagged behind. For many years, the favored candidate for the reconversion step has been the hydrogen fuel cell, but fuel cells have never reached their anticipated potential, even after several decades of intensive research and development efforts. Heat engines represent an alternative technology, but even though some types of heat engines—reciprocating piston engines in particular—are very reliable and relatively inexpensive, efficiencies have previously been so low that complete ESR cycles using heat engine have remained impractical. SEA has changed that.
Recently SEA has developed a design for a high-efficiency two-stroke internal combustion engine that makes ESR cycles economically viable. SEA’s engine is versatile in that it can be sized to meet the demands of large or small energy markets and, like other reciprocating piston engines, it can respond quickly to changing loads. Also, when an SEA engine is used in conjunction with an SEA cogeneration system (SEA Cogen), the low-grade thermal energy collected by the engine coolant can be carried to borehole heat exchangers and added—at a higher temperature—to thermal energy previously collected and stored by the SEA cogeneration system. When our high-efficiency engine is integrated with existing solar technologies, long-term storage of solar-derived electricity becomes economically viable; and even the valuable low-grade thermal energy that is rejected during operation of the engine can be collected and stored for later use.
2. EXISTING INTERNAL COMBUSTION ENGINE TECHNOLOGY
Every internal combustion engine that has been brought into service during the past century and a half has had an operating cycle that includes a compression process. In these engines, compression of a gaseous oxidant or a gaseous fuel/oxidant mixture prior to combustion provides the capability for large volumetric expansion ratios of the combustion products; and larger expansion ratios are directly associated with more efficient engine performance. However, the compression process, as it is performed in existing internal combustion engines, imposes severe mechanical and thermal design constraints on engine components. For example, in conventional reciprocating piston engines, expansion of combustion products occurs within the same physical volume as compression of the oxidant or the fuel/oxidant mixture. For these engines, the expansion ratio of the combustion products is constrained to be equal to, or very nearly equal to, the compression ratio of the oxidant or the fuel/oxidant mixture. Attempts to achieve higher compression ratios—and correspondingly higher efficiencies—require heavier engine components made of materials capable of withstanding the very high temperatures and pressures that result from the compression process. Ultimately, these design constraints limit the compression ratios and the corresponding expansion ratios that are practical, and that, in turn, limits the achievable efficiency of engines utilizing a compression process.
3. THE SEA INTERNAL COMBUSTION ENGINE
The SEA engine is a two-stroke internal combustion engine. The distinguishing feature of the engine design is that its operating cycle does not include a compression process. The engine achieves high efficiency through an innovatively timed sequence of injecting and igniting pressurized fuel and oxidant gases that are provided from external reservoirs. This makes the SEA engine ideal for the reconversion step in ESR cycles that are intended to store and then regenerate electricity derived from variable energy resources such as solar radiation.
For the SEA engine, the desired compression of fuel and oxidant gases is done by equipment that is not involved with the engine’s operating cycle. Pressurized fuel and oxidant are delivered to the engine from external reservoirs, either separately or as a mixture. Since compression is done independently of the engine’s internal operations, the gases can be compressed in a manner that closely approximates a reversible isothermal process. This minimizes the compression work that has to be done on the fuel and oxidant gases to create the desired conditions of specific volume and gas pressure in the engine’s cylinders. No internal compression work is done by SEA engines.
Since the operating cycle of an SEA engine does not have a compression process, the mechanical interconnections of the engine’s components can be designed to achieve other engine design goals. We have used this design flexibility to make the internal volume of each of the engine’s cylinders at piston top-dead-center (clearance volume) as small as possible without allowing the piston head to actually contact the closed end of the cylinder under any set of operating conditions. This means that, to a good approximation, the expansion ratio for combustion products is determined solely by the choice of the engine’s crank ignition angle. (The crank ignition angle is the angle through which the crankshaft rotates in moving the piston from its top-dead-center position to the position at which ignition of the fuel and oxidant gases occurs.) The crank ignition angle can be set independently of other design concerns to achieve a high expansion ratio—and high engine efficiency. In addition to providing high efficiencies, this simple control of the expansion ratio gives SEA engines high torque values, low thermal loading of engine components, and lower vibration and noise levels.
4. OPERATIONAL WORKINGS OF THE SEA ENGINE
The two strokes of a piston in an SEA engine are (1) a power stroke, wherein the engine performs useful work as the piston is driven from top-dead-center to bottom-dead-center, and (2) an exhaust stroke, wherein the engine expels combustion products as the piston moves from bottom-dead-center to top-dead-center.
When an SEA engine is operating, fuel and oxidant gases are introduced at high pressure into each cylinder during the initial portion of the associated piston’s power stroke, that is, as the piston is beginning to move away from its top-dead-center position. After the required quantities of fuel and oxidant have entered the cylinder, the fuel/oxidant inlet valves close. Then, later in the power stroke, an electric spark ignites the fuel/oxidant mixture and the subsequent expansion of hot combustion products drives the piston down towards its bottom-dead-center position. The piston’s downward movement forces the rotation of the engine’s crankshaft.
As the piston approaches its bottom-dead-center position, its associated cylinder contains relatively low-pressure combustion products that have undergone expansion-induced cooling during the preceding portion of the power stroke. When the piston reaches its bottom-dead-center position, an exhaust valve in the closed end of the associated cylinder opens, thereby terminating the piston’s power stroke and initiating its exhaust stroke.
When the exhaust valve opens, the pressure inside the cylinder drops rapidly to near atmospheric pressure as combustion products flow through the exhaust valve and into an exhaust manifold. The piston then moves upward towards its top-dead-center position, expelling combustion products from the cylinder. It should be noted that the piston does negligible work during this exhaust stroke because the pressure inside the cylinder is never appreciably above atmospheric while the exhaust valve is open.
As mentioned above, an important feature of the SEA engine design is that the internal volume of the individual cylinders at piston top-dead-center is very small. The mechanical design of the SEA engine is such that, when a piston reaches its top-dead-center position, the front surface of the piston engages the closed end of the associated cylinder as closely as fabrication techniques and normal mechanical tolerances will allow, without the piston actually making contact with the cylinder’s closed end under any set of operating conditions. With a small clearance volume, essentially all of the combustion products are forced into the exhaust manifold during the exhaust stroke and, as will be explained later, very high expansion ratios can be achieved during the power stroke that follows.
When a piston reaches its top-dead-center position, the exhaust valve in the closed end of the associated cylinder closes, marking the termination of the piston’s exhaust stroke. Then the fuel/oxidant inlet valves open, fuel and oxidant enter the cylinder, and the cycle begins again. It should be noted that an engine with a compression stroke cannot be designed to have the small clearance volume that characterizes an SEA engine. This is because a very small clearance volume in a compression-type engine would result in extremely high gas pressures and temperatures inside the cylinder.
The insertion of fuel and oxidant gases into a cylinder creates an overpressure (pressure above atmospheric). Since the internal volume of the cylinder is increasing at this point, the overpressure does useful work by driving the piston away from its top-dead-center position. This gas-expansion work recovers a portion of the compression energy originally used to pressurize the fuel/oxidant gases. The overpressure exerts steady, relatively low-magnitude forces (soft forces) on the slowly moving piston, and the resulting engine vibrations are significantly less than in conventional engines. In conventional engines that utilize a compression process, combustion is initiated when the piston is at or near its top-dead-center position. This produces sudden, high-amplitude forces on the slowly moving piston, thus creating significant noise and engine vibration.
Finally and most importantly, it should be noted that, for the SEA engine, when fuel and oxidant are injected into a cylinder in the manner described above, it is possible to produce conditions of pressure and molar density (moles per unit volume) for the fuel/oxidant gases which are not achievable by the compression processes that are carried out prior to combustion in conventional engines. In an SEA engine, the expansion ratio for the combustion products—and the corresponding engine efficiency—can be selected independently of other engine design concerns.
5. DETAILED OPERATION OF THE SEA ENGINE
The schematic diagram below shows the interconnections and geometric relationships of some of the basic components of an internal combustion engine. The diagram shows the crank angle, θ, and the rod angle, Φ. The crank ignition angle, θi, is the crank angle at which ignition of the fuel and oxidant gases occurs. The instantaneous torque provided by a piston at any particular time during its power stroke is proportional to sin(θ + Φ) at that time. In particular, very little torque is provided by a piston when it is near its top-dead-center position, that is, when both θ and Φ are close to zero. In SEA engines, the ignition event is delayed by the time required for the crankshaft to rotate through the crank ignition angle, θi, from the top-dead-center position of the piston. This allows the piston to provide high torque as the combustion products are expanding. Also, it should be noted that the crank ignition angle can be electronically delayed by any desired amount from piston top-dead-center, thereby providing a means to balance engine torque and engine efficiency for any specific application.
With the assumption of zero clearance volume, it can be seen from the diagram that the expansion ratio of the combustion products, ψ, is dependent only on the ignition angle, θi, and the rod ratio, Ω, where Ω = L/R:
The graph below shows the relationship between the expansion ratio, ψ, and the crank ignition angle, θi, for two different values of rod ratio, Ω. As θi decreases, ψ increases rapidly. Greater values of ψ are associated with higher engine efficiency, but their is a point of diminishing returns. The value of θi should usually be chosen to be between 12 and 15 degrees because (1) this allows several degrees of crankshaft rotation during which fuel and oxidant gases may be injected into the cylinder, and (2) high expansion ratios—and high engine efficiency—are still achieved.
The graph shows that once the connecting rod length, L, and the crank arm length, R, have been chosen, the expansion ratio of the combustion products—as well as the engine’s achievable efficiency—is determined solely by selection of the crank ignition angle, and the crank ignition angle can be selected independently of any other design considerations. This is an extremely important design option that is only possible in SEA-type engines, that is, in engines that (1) do not have a compression process in the operating cycle, and (2) have near-zero clearance volume. For engines utilizing a compression process, the compression ratio and the corresponding expansion ratio are limited by the engine’s ability to withstand the high pressures and high temperatures produced by the compression process. Since the SEA engine does not utilize a compression process, those design limitations are removed. SEA engines can be designed to achieve very high expansion ratios without creating excessive mechanical or thermal stresses on engine components.
If the assumption of adiabatic expansion of the combustion products is made, the efficiency of the SEA engine may be approximated as the ratio of the work done by the expanding combustion products to the internal energy of the combustion products before they expand. This latter quantity, the internal energy of the combustion products before expansion, is approximately equal to the chemical energy released during the formation of the combustion products. If the combustion products are assumed to be ideal gases, the adiabatic efficiency, Ea, of an SEA engine is given by:
where again, ψ is the expansion ratio of the combustion products, and γ is the ratio of the specific heats of the combustion products. (This equation is based on the assumption that the effective compression ratio produced by injection of high-pressure fuel and oxidant gases is high enough that work done in pushing against the pressure of the atmosphere is negligible.) Two graphs of the engine’s adiabatic efficiency, Ea, as a function of ignition angle are shown below, with each graph having two curves. In each graph, one curve is for the case of Ω = 3 and one for the case of Ω = 6. The first graph assumes that the combustion products are primarily diatomic gases, wherein the value of Cv, the specific heat at constant volume, is taken as 2.5R, with R being the ideal gas constant. This means that the ratio of the specific heats, γ, is equal to 1.4. The second graph assumes water vapor as one of the combustion products, with the value of Cv being taken as 3R, with the result that γ is equal to 1.33.
The plots show that higher expansion ratios and higher efficiencies correspond to small ignition angles, but it should be remembered that a piston cannot do useful work if the pressure within its associated cylinder drops below atmospheric, which can occur if the expansion ratio is too large for any particular set of engine operating conditions. Also, if the temperature of the combustion products inside the cylinder drops to the point where water vapor begins to condense, the pressure will drop considerably and the amount of useful work done by the piston will also drop. Expansion ratios which would result in moisture condensation within the cylinder are unacceptable for several reasons. However, it is also noted that the adiabatic efficiency curve flattens out at high expansion ratios, and there is very little advantage to be gained by going to expansion ratios that would involve either moisture condensation or sub-atmospheric pressures inside the cylinder.
A few examples illustrate the advantages offered by SEA engines. For an SEA engine, if the value of Ω is equal to 3—as in smaller engines—and the crank ignition angle is 15 degrees, then the expansion ratio for the combustion products is 44:1. If the value of Ω = 6the ratio of connecting rod length to crank arm length is equal to 6—as in larger engines—and the crank ignition angle is 15 degrees, then the expansion ratio is 50:1. Again, these numbers are based on the assumption that the clearance volume of the engine’s cylinders is insignificant relative to their cylinders’ internal volume at ignition. The ratios above (44 and 50) should be compared with typical compression and expansion ratios of high-performance diesel engines, which are generally in a range of roughly 17:1 to 20:1. Clearly, much higher expansion ratios and correspondingly higher efficiencies are achievable with SEA-type engines. Also, assuming an adiabatic compression process, it should be noted that in order to provide a compression/expansion ratio of 50:1, a conventional diesel engine would create a pre-combustion pressure within the cylinders of more than 3500 pounds per square inch and the pre-combustion temperature would be more than 1100°C. (The fact that actual temperatures and pressures are less than these values means that, in real engines, the adiabatic assumption is violated and a large fraction of the internal compression work done by the engine is converted to heat and lost through the cylinder walls.) With quantities of fuel and oxidant that are equivalent to the previous example, achieving an expansion ratio of 50:1 in a hydrogen/oxygen engine of the SEA design would require pre-combustion pressure and temperature of roughly 450 pounds per square inch and 30°C, respectively. It can be seen that SEA engines operate at much lower pressures and temperatures than conventional engines that utilizes a compression process.
In summary, the SEA two-stroke internal combustion engine has innovative design features which translate into important performance capabilities. The SEA engine does not use a compression process as part of its operating cycle. Instead, as the engine operates, it is supplied with high-pressure fuel and oxidant gases from external reservoirs. The mechanical design of the engine is such that the clearance volume of the cylinders is very small. The result of this design choice is that the expansion ratio of the engine’s combustion products can be set to a very high value by choosing an appropriate crank ignition angle. Reasonable choices for the crank ignition angle provide high efficiency, high torque, low thermal loading of critical engine components, and low noise and vibration levels.