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Energy & Fuels 1998, 12, 72-77
Advanced Hydrogen Fueled Internal Combustion Engines Peter Van Blarigan Sandia National Laboratory, 7011 East Avenue, Livermore, California 94550 Received July 7, 1997. Revised Manuscript Received September 29, 1997X
The Hydrogen Program at Sandia National Laboratories is developing internal combustion engine generators for application in series hybrid vehicles and stationary power units. The program consists of two approaches: investigating the utilization of hydrogen in a conventional crankshaft driven engine and in an advanced free piston configuration. The conventional engine program has taken the direction of utilizing the unique ability to spark ignite homogeneous fuel/ air mixtures of hydrogen at low equivalence ratios (f ≈ 0.4) to achieve low NOx emissions and high thermal efficiency. The goal is to translate the indicated thermal efficiency of single-cylinder engines into multicylinder configurations achieving at least 40% brake thermal efficiency. When coupled to an electrical generator, the fuel to electricity conversion efficiency would be approximately 37%. A modified Perkins 3.152 Diesel engine (850 cm3 per cylinder) is currently being tested and has achieved an indicated thermal efficiency of 45% in preliminary operations. The advanced free piston design utilizes a new approach to IC engine combustion. A doubleended free piston is used to compression ignite homogeneous fuel/air mixtures as it oscillates inside a closed cylinder. Through this oscillation, electrical energy is generated in a linear alternator which acts also to control the piston motion by actively adjusting the electromagnetic forces felt by the piston. Electricity is the output of this engine, and electronic control of the compression ratio is achieved with extreme mechanical simplicity. Fresh charge is introduced to the engine in a modern two-stroke cycle fashion. Initial testing in a single-cycle experiment has validated the potential of this alternative combustion system, as it has produced an indicated thermal efficiency of 56% with essentially zero NOx emissions. Development of these advanced generators continues.
Introduction The Hydrogen Fueled Internal Combustion (IC) Engine Program at Sandia National Laboratories is developing engine generators for application in series hybrid vehicles and stationary power units. This application allows the engine to operate at a constant power and speed, achieving maximum thermal efficiencies while complying with the strictest emission requirements. It is believed that this type of operation will compete realistically with potential fuel cell performances, benefiting from lower costs and proven technologies that are available in the near term. The program to develop such engines is based on first understanding and optimizing the combustion parameters and then developing techniques to maximize the production of useful work. Both evolutionary and revolutionary directions are being pursued. Results Conventional IC Engines. The conventional crankshaft engine program has taken the direction of utilizing the unique ability of homogeneous fuel/air mixtures of hydrogen to spark ignite at low equivalence ratio (f ≈ 0.4) to achieve low NOx emissions and high thermal efficiency. Here equivalence ratio is defined as the ratio X
Abstract published in Advance ACS Abstracts, December 1, 1997.
of the actual fuel/air ratio to the stoichiometric ratio. NOx is the principal emission from hydrogen-fueled engines, but when a sufficient quantity of excess air is present, the combustion temperatures remain low enough that NOx formation is drastically reduced, to a level that complies with the California Air Resources Board’s (CARB) proposed Equivalent Zero Emissions Vehicle (EZEV) standard.1 Low equivalence ratio operation has the added benefit of increasing the ratio of specific heats of the combustion products, resulting in improved thermal efficiency, as can be seen in Figure 1. The approach has been to use a fundamental understanding of ideal Otto cycle performance (the highest efficiency cycle for a piston IC engine cycle) to maximize efficiency within these constraints. The experimental program is thus far investigating the dependency of thermal efficiency on incylinder fluid motion, compression ratio and fuel/air reaction rates. Initial investigation was conducted in a modified for spark ignition, Onan Diesel single-cylinder research engine of 490 cm3 displacement. Details for this engine can be found in Table 1. A quiescent combustion chamber design of 14:1 compression ratio was employed (1) California Air Resources Board (CARB). Proposed equivalent zero emission vehicle standards, in IEEE Spectrum, 1995, Sept 72.
S0887-0624(97)00110-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/12/1998
Advanced Hydrogen Fueled Internal Combustion Engines
Figure 1. Ideal Otto cycle thermal efficiency vs compression ratio.
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Figure 3. Indicated thermal efficiency and NOx vs spark advance. Table 2. Perkins Engine Specifications
Table 1. Onan Engine Specifications bore (mm) stroke (mm) displacement (L) compression ratio valve timing spark plugs ignition system inlet system
82.55 92.08 0.4928 14.04:1 stock Champion 53R Mallory HyFire 667 CDI (2 systems) Mallory ProMaster 28880 coils pressurized, unthrottled
Figure 2. Indicated thermal efficiency vs valve shroud height.
with two spark plugs for reduced burn duration, and the level of intake generated swirl varied by 180° shrouding of the intake valve. The performance of the Onan engine at one particular operating condition with various height valve shrouds is shown in Figure 2. It appears that an optimal value exists. The criterion for this assessment is the indicated thermal efficiency where this is defined as the ratio of net work on the piston to the fuel’s lower heating value. (Here the net work is computed by integrating the pressure-displacement data over the complete fourstroke cycle, and the thermal energy input is determined by measuring the fuel/air flow rates. The error in this efficiency calculation is less than (2%.) The advantage of using the indicated efficiency as a metric is that the mechanical losses from the engine can
bore (mm) stroke (mm) displacement (L) compression ratio valve timing spark plug ignition system inlet system
91.44 127.0 0.8340 14.04:1 stock Champion A49R Mallory HyFire 667 CDI (1 system) Mallory ProMaster 28880 coil pressurized, unthrottled
be excluded, and various operating points can be compared relative to changes in combustion parameters. Figure 3 illustrates the efficiency and emissions performance of the Onan engine as a function of spark advance and equivalence ratio, where a 0.3175 cm high valve shroud has been used. Included in Figure 3 as well is the NOx emissions level below which compliance with CARB’s proposed EZEV standard is assured. Inherent in this level is the assumption of a vehicle efficiency of 60 miles per gallon gasoline equivalent.2 As a result of the Onan experiments, a larger displacement Perkins 3.152 Diesel engine was selected for investigation. This engine, with a single-cylinder displacement of 850 cm3 (other details given in Table 2), possesses a smaller surface area-to-volume ratio for the combustion chamber at top dead center. This seems to improve the indicated thermal efficiency through lower heat transfer losses. Figure 4 compares some of the results from the Perkins with the optimized Onan data. More details regarding the Perkins and Onan experiments can be found in ref 2. New IC Engine Approach. To improve the engine’s thermal efficiency significantly, yet maintain the low NOx emissions required, a new combustion approach will be required which can utilize higher compression ratios in conjunction with low equivalence ratios. The spark ignition crankshaft engine is limited in compression ratio by autoignition of the end gases. Diesel engines can utilize high compression ratios, but suffer from fuel injection rate limitations. Both engines are limited by the finite amount of time required for the fuel/air mixture to burn. (2) Van Blarigan, P. “Development of a Hydrogen Fueled Internal Combustion Engine Designed for Single Speed/Power Operation”, SAE Paper 961690, 1996.
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Van Blarigan Table 3. Single-Cycle Combustion Experiment Specifications bore (mm) stroke (mm) displacement (L) piston and cylinder head cylinder maximum piston speed (cm/s) natural oscillation frequency (Hz)
Figure 4. Indicated thermal efficiency and NOx vs spark advance.
76.2 254 max, 236 typical 1.076 typical 304 stainless steel 6061 aluminum, hard anodized 1100 typical 40 typical
engine. The alternator efficiently converts the kinetic energy of the piston into electrical energy by means of moving permanent magnets through fixed electrical stator windings. Since the compression of the fuel/air mixture is accomplished inertially, the electrical conversion process must be done precisely leaving the piston with enough kinetic energy to autoignite the gas in the opposing cylinder on the next stroke. Electronic control of the compression ratio is thus achieved with extreme mechanical simplicity. Three new concepts must be demonstrated in order to prove the performance advantage of this system. First, the high efficiency and low emissions potential of the combustion system must be established. Second, the linear alternator must be proven to be efficient, and capable of starting and controlling the engine. Third, the two-stroke cycle process must be shown to support the high efficiency and low emissions of the basic combustion system. Presented here are the initial results from the combustion system analysis. Experimental Section
Figure 5. Thermal efficiency vs compression ratio (ref 3).
Caris and Nelson3 effectively showed that in their spark ignition engine geometry (production V8 engines), increasing the compression ratio above 17:1 no longer improves thermal efficiency due to the finite burn duration problem. This is shown in Figure 5. In ideal Otto cycles, however, this is not the case, as is illustrated in Figure 1 and in ref 4. What is needed therefore is a combination of high compression ratio, short burn duration, and lean homogeneous fuel/air mixtures. The utilization of these optimizing characteristics thus requires a new combustion system. The advanced electrical generator concept illustrated in Figure 6 possesses these characteristics. A doubleended free piston oscillates inside a closed cylinder. Fuel and air are introduced in a modern two-stroke cycle fashion on each end, where the cylinder charge is compressed to the point of autoignition. The piston is driven in an oscillating motion as combustion occurs successively on each end. This approach leads to rapid combustion (i.e., almost constant volume) for any fuel/ air equivalence ratio mixture at very high compression ratios. To control the process and generate electrical power, a linear alternator is built into the center section of the (3) Caris, D. F.; Nelson, E. E. SAE Trans. 1959, 67, 112-124. (4) Edson, M. H. SAE Progr. Technol. 1964, 7, 49-64.
Demonstration of the homogeneous charge compression ignition system is well underway. It is important to quantify the performance of such a system in the free piston geometry, idealized to indicate the ultimate achievable efficiencies and emissions. Initial testing in a single-cycle experiment has validated the attractiveness of this concept. A schematic of the free piston combustion experiment used in these tests is illustrated in Figure 7. Briefly, the device uses high-pressure helium to drive a piston from one end of a closed cylinder to the other. This driving motion compresses a fuel/air charge on the other side of the piston to the point of autoignition. The piston returns to the back end of the cylinder as the combusted gases expand, compressing the helium charge. The piston bounces back and forth several times before stopping. The oscillating piston motion is shown in Figure 8. Table 3 lists relevant specifications for the experiment. As in the conventional engine study, the indicated thermal efficiency is used as the metric by which different experimental points are compared. This efficiency is calculated by measuring both the pressure in the combustion chamber and the displacement of the piston. The net work is integrated over the compression and expansion strokes. However, since the piston does not fully return to the starting position on the expansion stroke (due to the trapped helium driver gas), as shown in Figure 8, in order to calculate the full potential for this cycle, the piston position has to be extrapolated to the initial position. The method utilized for this extrapolation is to match the expansion line slope on a log pressure/log volume plot. This is illustrated in Figure 9. The data presented in Figure 9, and throughout, is smoothed using a binomial method to remove noise. This smoothing, however, did not affect the efficiency calculations, which are accurate to within (1.5%. Pressure Measurement. The static magnitude of all lowpressure gases was measured using two 10 000 Torr absolute
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Figure 6. Advanced electrical generator concept.
Figure 7. Schematic of free piston combustion experiment. pressure range MKS Baratrons, Type 390 HA heads with Type 270B display units achieving an accuracy of 0.05% of reading. High-pressure gases (i.e., helium) were measured using Teledyne-Taber strain gauge transducers with an accuracy of 0.25% of full scale. Dynamic pressures were measured using piezoelectric effect transducers. Pressure on the driver end was quantified with a Kistler Type 607 transducer coupled to a Kistler Type 5004 charge amplifier. The combustion end utilized two transducers. Kistler Type 7061A, 7063A, and 7061B as well as AVL Type QC42D-X transducers have been employed along with Kistler Type 5010 and 5026 charge amplifiers. The best results, as determined by agreement of the initial and expansion pressures with absolute transducers, were obtained from the AVL unit. For this experiment, coating of the transducer faces with Silastic J (0.5 mm thick) also improved the precision. All of the presented data was recorded with the coated AVL unit. Pressure data was recorded on three Nicolet 4094 digital oscilloscopes through 12-bit Type 4570 plug-ins, at a rate of 500 000 samples per second.
Figure 8. Piston position vs time.
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Figure 9. Log pressure vs log volume. Displacement Measurement. Piston displacement was measured with a Data Instruments FASTAR Model FS5000HP inductive transducer. Data were recorded on a fourth Nicolet 4094 digital oscilloscope at a rate of 200 000 samples per second. NOx Measurement. A Rosemont Analytical Model 951A NOx analyzer was employed for post-test analysis of the combustion gases. Due to the generation of only one cylinder’s worth of combustion products the instrument was modified for utilization of a smaller than usual sample. Here the bypass flow was eliminated, and the burned gases were pumped directly through the analyzer. The NOx analyzer was calibrated by loading a 6.7 ( 0.01 ppm span gas into the evacuated combustion chamber in its starting position and then pumping it through the analyzer. Thus, a quantity of gas equivalent to the post-test combustion gases was utilized. Comparison with standard calibrating techniques verified this process. Experimental Procedure. The combustible gases were premixed in a 10 L tank at 10 000 Torr. Mass spectrometer analysis has verified the precision of the mixture. The singlecycle combustion experiment was performed in a high-pressure test cell, and transfer of the gases from the 10 L fuel tank to the experiment cylinder was done remotely, as was the firing of the device. To fuel the experiment the piston was moved to the starting position utilizing vacuum and atmospheric pressures. The entire assembly was then evacuated to less than 200 mTorr, and the premixed gas was sent into the cylinder’s combustion end to the desired pressure. The helium driver gas was then pumped up to the firing pressure (based on the desired compression ratio; typically 362 000-490 000 Torr). To function, a specially modified Nupro bellows valve was actuated with an Autoclave Engineers air operator (using helium) to open the tube connecting the helium supply to the driver end of the piston. Data acquisition was triggered by the rising pressure in the driver end. Combustion Gas Analysis. Following the stroking and eventual stopping of the piston, the combustion products were quantified by pressure-volume-temperature (PVT) analysis. In order to quantify leakage of gases from the cylinder during the test sequence, a post-test quantification was conducted. The combustion products were expanded into a larger measurement tank to a pressure of approximately 80 Torr, ensuring that all the water was vaporized. For NOx analysis, the gases were pumped to 1000 Torr utilizing a Danielson Tribodyn TD-100/38 oilless pump. Pump output was then sent to the NOx analyzer at a constant flow rate using an Omega Engineering Inc. Model FMA 7305 mass
Van Blarigan
Figure 10. Log pressure vs log volume.
Figure 11. Indicated thermal efficiency and NOx vs compression ratio. flow controller. In this way approximately 90% of the gases could be utilized. Experimental Results. Figure 10 shows a log pressure versus log volume plot of a typical experiment, demonstrating the similarity in shape to the ideal constant-volume cycle. In fact, with a hydrogen fuel/air mixture at an equivalence ratio of 0.32, and initial temperature of at 20 °C, indicated thermal efficiency is 56% with essentially zero NOx emissions. This compares with the 45% thermal efficiency of conventionally configured and optimized engines. Figure 11 gives the efficiency and NOx levels as a function of compression ratio for this mixture. Under these conditions, autoignition first occurs at a compression ratio of about 32:1, where the pressure versus time trace shown in Figure 12 reveals a late ignition point. After this point, efficiency is only a weak function of increasing compression ratio, an indication that heat loss near top dead center is not a critical performance factor. This characteristic of the combustion system is also attractive for engine control, since a limited variation of compression ratio is not detrimental.
Discussion The conventional crankshaft engine work presented demonstrates the potential of lean burn hydrogen combustion systems to produce an indicated efficiency of 45% with essentially zero NOx emissions. With continuing optimization this value may increase several percentage points, but this has yet to be accomplished.
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Figure 12. Pressure vs time.
Figure 13. Pressure and piston position vs time at top dead center.
What also must be developed is the machinery necessary to convert this piston work to shaft power with minimal losses. Utilization of friction-reducing components on each moving part will be more costly than in typical production engines; however, this will still be inexpensive compared with the projected cost of fuel cells. The goal of this program is to translate the indicated thermal efficiency results into a multicylinder engine of greater than 40% brake thermal efficiency. When coupled to a generator the fuel to electricity conversion efficiency would be approximately 37%. The free piston concept is an attempt to more closely achieve Otto cycle performance. While the idea of homogeneous charge compression ignition has been explored by several researchers5-7in conventional engines, control of the process has been achieved by exhaust gas recirculation and intake heating. The desired goal has been a smooth release of energy, without end gas autoignition (knock). Conventional compression ratios of approximately 8:1 have been utilized. The concept explored here is quite different. Rapid reaction rates are desired, and the compression ratio is to be adjusted to the operating conditions with no recirculated exhaust. The intent is to generate constantvolume combustion regardless of the violence of combustion, and at as high a compression ratio as is possible. For example, in the experiments thus far, the achievable reaction rate is so rapid, as shown in Figure 13, that considerable ringing is generated in the pressure and displacement records, (this accounts for the sawtooth pattern and other irregularities in Figure 10). The free piston geometry is ideal for such a combustion system. Since compression is developed inertially, there are no bearings or support structures to be damaged by high pressures or shocks. In addition, this geometry dramatically reduces the time spent at top dead center over the crankshaft driven piston. This is illustrated in Figure 14 where the position profile is
given for a free piston and crankshaft-driven piston of identical stroke and maximum velocity.
(5) Thring, R. H. “Homogeneous-Charge Compression-Ignition Engines”, SAE Paper 892068, 1989. (6) Najt, P. M.; Foster, D. E. “Compression-Ignited Homogeneous Charge Combustion”, SAE Paper 830264, 1983. (7) Karim, G. A.; Watson, H. C. “Experimental and Computational Considerations of the Compression Ignition of Homogeneous FuelOxidant Mixtures”, SAE Paper 710133, 1971.
Figure 14. Piston position vs time.
Conclusions The results presented for the advanced free piston design are for an idealized combustion system. The cylinder gases are at 20 °C and completely quiescent at the start of combustion. A stroke-to-bore ratio of 3.1:1 is utilized to ensure no piston to cylinder head contact at high compression ratio. The intent is to demonstrate the potential that such a system possesses before the details of power removal, inlet/exhaust processes and higher operating temperatures are included. While many engineering challenges must be met to develop the free piston generator, no new technologies are required and the payoff could be significant. On the basis of our initial experiments and model predictions, a fuel to electricity conversion efficiency of 50% with essentially zero NOx emissions seems possible. Acknowledgment. This work was supported by the U.S. Department of Energy, Office of Solar Thermal, Biomass Power and Hydrogen Technologies and the Laboratory Directed Research and Development program at Sandia National Laboratory. The author gratefully acknowledges the technical assistance of Scott Goldsborough, Nicholas Paradiso, and David Zanini. EF9701100
