I
E.
S. STARKMAN
University of California, Berkeley, Calif.
Four Hundred Horsepower for the Price of Two Hundred?
Nitroparaffins as Potential Engine Fuel
Nitroparaffins are proposed as a class of chemicals for use as fuels to increase the power from an engine without an attendant penalty of increasing engine size or mechanical complexity
R E S E A R C H on fuels for reciprocating engines has been confined mostly to hydrocarbons and usually has involved problems of knock, volatility, and fouling. However, in the work described here, combustion stoichiometry, oxidation, and decomposition of mononitroparaffins. from C1 to Cq. and 2,2dinitropropane were investigated. T h e nitroparaffins increase performance as predicted by considerations of product evolution and of thermal energy in the fuel-air mixture. Considering net power alone, it appears that the output of an engine can be more than doubled by using nitromethane rather than hydrocarbons as engine fuel.
Combustion Stoichiometry and Exhaust Gas Composition The most important consideration in the application of fuels such as nitroparaffins to engines is their mode of combustion and decomposition, and particularly the composition of the products. The fuel-air ratio for satisfactory combustion ranges from much leaner than hydrocarbon to, in some cases, a situation where no oxygen other
than that in the fuel is necessary for the heat release processes of the engine. T o take nitromethane as an example. two simple specific cases can be cited from which product composition will b r predicted. The first of these will be for a so-called chemically correct mixture ratio of fuel and air. Ignoring for the moment dissociation and the small amount of partially burned fuel and uncombined oxygen usually found ( 4 ) in engine exhaust, this reaction should be (70) 4C:HSNOz
+ 3 0 +2 +6H10 11.3N2 + 13.3h2 +
4COz
(1)
For mixtures leaner than chemically correct, excess oxygen appears as a component of the products. Enriching the mixture produces carbon monoxide and hydrogen. Carrying this enrichment to the extreme results in no atmospheric oxygen at all in the reactants. The reaction for such, which corresponds to a monopropellant (as in a rocket engine). would be ( 8 )
++3CO + 3Hz + 2N2
4CH3502 * COn 3H20 3H20
+
(2)
an exothermic reaction and one which presupposes an equilibrium yielding
equal volumes of carbon monoxide and hydrogen; a prediction appearing to be in the right direction from results, in this investigation, of limited mass spectrographic analyses, which indicate that the molal ratio of hydrogen to carbon monoxide ranges from 0.5 at equivalence ratios near 1, to 0.7 a t equivalence ratios near 2. (Equivalence ratio is defined as the fuel-air ratio bv weight divided by that fuel-air ratio to give complete oxidation of the carbon and hydrogen to carbon dioxide and water, respectively.) T h e products from a reciprocating engine cannot be accurately predicted because of the highly transient nature of the procejses. There is insufEcient time in compression. combustion, expansion. and blowdoivn for homogeneous mixing of the fuel and oxygen before combustion, for equilibrium to be established in thecombustion gases during expansion. or for relaxation to occur during expansion. Exhaust gas equilibrium, as measured, therefore, corresponds to a temperature much higher than that of the exhaust gas sample. Furthermore, small quantities of unburned and partly burned fuel are always found ( 4 ) . These same mass spectrographic VOL. 51, NO. 12
DECEMBER 1959
1477
analyses referred to above show approximately 0.1 molal % nitrous oxide from the combustion of a 60% weight blend of nitromethane in methanol at an equivalence ratio of 1. Other oxides of nitrogen were not detected, but nitromethane was found at a concentration of 0.05 molal %. By comparison, around 0.05 molal nitrous oxide was found in the products from methanol combustion. The oxides of nitrogen for all fuels are undoubtedly formed at the transient equilibrium existing during combustion. Because the temperatures during this period are even higher with nitroparaffins than with methanol, the slightly increased nitrous oxide accompanying use of the nitroparaffin may not be directly related to the presence of a nitro radical in the fuel. Oxygen in the fuel appeared in the exhaust as carbon monoxide, carbon dioxide, or water. Speciflc Energy of Mixture Oxygen bonded into nitroparaffin structures, which is available for recombination with carbon and hydrogen, increases the thermal energy which can be carried into the combustion process because it allows more fuel per unit of air charge. E (Figure 1) is defined as the net heat of combustion for any fuel in a unit mass of air divided by the net heat of combustion for a chemically correct mixture of methanol in the same amount of air. Figure 1 shows the nitroparaffins to be attractive from an energy density standpoint. T o present a more rigorous picture of
the energy relationship, it would be necessary to plot the specific energy released, rather than that carried into the combustion process. This would take into account dissociation energy as well as the energy in the carbon dioxide, hydrogen, and other products of what is normally considered incomplete combustion. There are not yet enough exhaust gas analysis data to allow such an assessment, nor have the equilibrium calculations been completed, but both of these are being obtained. Because information on product composition is lacking, Figure 1 is limited to equivalence ratios of 1 or less where combustion is practically complete and where the energy of combustion is substantially released during the engine processes.
duced for 4.57 moles of reactant, a ratio of 1.27. Nitromethane in the limiting case, as a monopropellant, yields 4 moles of product for every mole of gaseous reactant (Equation 2 ) . If the fuel is considered as a liquid, an assumption which may be open to debate in a carbureted engine, octane produces 64 moles of product for each 59.5 moles of air-a ratio of 1.07-while nitromethane produces 5.82 moles of product per 3.57 moles of air-a ratio of 1.63. For simplicity, the influence of dissociation has been ignored in the above comparisons, even though computations show from 10 to 15% of the products are dissociated for the mixture of nitromethane and air at an equivalence ratio of 1. T h e influence of dissociation would enhance the product-reactant ratio for some nonhydrocarbons, even though dissociation energy would somewhat offset the resulting pressures and temperatures in a process under consideration. The net result from disregarding dissociation is to yield a conservative result in comparing nitroparaffins to hydrocarbons. A reciprocating engine is in reality a fixed volume machine for any piston position. The relative number of moles of product determines the resulting pressures and temperatures produced by the combustion reaction. A search of the literature shows that little attention has been paid to this factor. Therefore, computations were made to serve as a comparison basis between various fuels. The basis for Figure 2, a unit mass of air (or atmospheric oxygen), was chosen because a given engine will assimilate
Relation between Products of Fuels of Differing Composition
A second chemical factor favoring certain nonhydrocarbon fuels relates to the composition of the products of combustion. Even if compared on the basis of gaseous reactant, the number of moles of product for most other fuels are greater per mole of mixture than for hydrocarbons. Taking octane as an example, the reaction CsHia 12.502 47Nz + 8 C 0 2 9H?O 47Na ( 3 )
+
+
+ +
yields 64 moles of product per 60.5 moles of reactant, a ratio of 1.06. A similar mixture for nitromethane or
shows 5.82 moles of product are pro20
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- EOUIVALENCE
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Figure 1. Thermal density of mixture based on a unit charge o f air and negligible fuel volume (liquid) Nitroparaffins a r e attractive from an energy density standpoint
1 478
INDUSTRIAL AND ENGINEERING CHEMISTRY
0.75 0.75
1.00
I .25
1.50
# - EOUIVALENCE
RATIO
Figure 2. Comparative production of combustion products based on a unit charge of air and negligible fuel volume (liquid) The relative abundance of products of combustion per unit charge of the lower nitroparaffins and poly nitroparaffins improves engine performance
NITROPARAFFINS air at the same rate regardless of the composition of the fuel if the fuel quantity variation is negligible, or if the fuel is considered as a liquid. Such an assumption corresponds to laboratory results. T h e largest variation in air flow rate caused by a change in fuels was order of magnitude of &2y0at very rich mixtures and negligible at lean. As there was such a minor change in air flow rate, it was interpreted that the fuel remained substantially in the liquid phase until the inlet valve closed. ,I(Figure ’ 2) is defined as the number of moles of product per unit of air at any 4 compared to that for methanol at 4 = 1. The conclusion from Figure 2 is that the lower nitroparaffins and the polynitroparaffins should contribute to engine performance by their characteristic relative abundance of products of combustion per unit charge. Figure 2 also indicates that iso-octane, as representative of hydrocarbon fuels, is poor by comparison to other fuels in this respect. Taking nitroethane as an example, a 470 increase over methanol is predicted at $ = 1. Compounding this with the 29% from the thermal density computation (see Figure 1) results in the prediction of an over-all increase in engine performance corresponding to about 33% for nitroethane at an equivalence ratio of 1, when compared to methanol. Engine Thermodynamical Considerations T h e energy delivered to the piston is a function of pressure in the combustion space and position of the piston in the cycle. The work delivered per cycle by the gases to the engine can be most simply expressed as an integral of pressure and volume over the processes composing the engine cycle. I t is obvious that the cylinder volume at any position is a mechanical function and is fixed by engine dimensions. (It is also apparent that power, or the rate of doing work, is a function of the number of cycles completed per unit of time.) The work per cycle will therefore be used for comparison purposes. T h e pressure at any volume is determined by- charge density and temperature before combustion and by the specific thermal energy in the charge. Important also is the time dependency of p on V (a function of ignition timing and reaction rate) and, in addition, the relationship between moles of product and moles of reactant. Recent advances in automotive engine output, while they have been primarily of a mechanical nature, may be used as an example to illustrate the practical significance of some of the important factors in determining engine output.
These recently increased engine ratings can be attributed to at least four primary causes. I n descending order of their importance, these are: increased charge density at high engine speed through a reduction of inlet and exhaust flow resistances, increased total volume (displacement) , higher average pressures through greater compression ratio, and reduced engine friction through mechanical design improvements. T h e latter two, though significant, are relatively minor. By contrast, themajor resulting increase in engine power, using nitroparaffins and other fuels of like character, is due entirely to the effect of their differing chemical composition on cyclic pressure at any engine speed, on the relative moles of products to reactants, and on a change in the pressure-volume dependency which may be brought about by their reactivity. Such increased output, attributed to increased thermal energy in the charge and to increased moles of
complexities of a bipropellant fuel and control system. T h e diesel engine fuel application of 2,2-dinitropropane reported by Albright, Xelson, and Raymond was limited to modest quantities, up to 170 by weight. T h e reported results were: reduced combustion shock, reduced exhaust smoke, reduced exhaust odor, and improved cold starting. All these factors are related to the results of the present investigation, but the concentrations used by Albright, Nelson, and Raymond were a small fraction of the concentrations used here. Because quantities of material needed for engine testing are relatively large, the selection of compounds for laboratory investigation is limited to the most readily obtainable. Fortunately, most of the lower nitroparaffins are being produced in commercial quantities. The table shows the relevant physical properties of the nitroparaffins investigated (3, 5-7).
Fuel Properties Used in Calculating Specific Energy Ratio and Products Molal Ratio
Purity,
% Nitromethane Nitroethane 1-Nitropropane 2-nitro propane
95 90 94 94 95 95
Mol. W t . 61.04 75.07 89.09 89.09 103.12 134.09 114.2 32.0
2-Nitrobutane 2,2-Dinitropropane a Iso-octane Methanol 99.85 a ASTM knock test reference fuel.
+
product, in reality is manifested as an increased cylinder pressure in the working stroke of the engine cycle (modified, of course, by increasing dissociation at the resulting higher temperatures of the cycle). Engine Applications of Nitroparaffins Two early reports in the literature on engine application of the nitroparaffins are noteworthy. T h e report of Zwicky and Ross (73)relates to the use of nitromethane as a rocket engine monopropellant. Albright, Nelson, and Raymond ( 7 ) detail the use of 2,2-dinitropropane as a diesel fuel additive or cetane number improver. T h e reasons for these applications are different from that for which this investigation was performed, but the results are related. Zwicky and Ross used nitromethane as a monopropellant engine fuel to obviate the use of separate oxidizer and the
Density, 200 c. 200 c.
Heating Value (Liquid), B.t.u./Lb.
Corrert Fuel-=lir Ratio by Weight
1.139 1.052 1.003 0.992 0.969 (Solid) 0.698 0.793
5,000 7,720 9,650 9,650 11,120 5,730 20,580 9,770
0.588 0.244 0.173 0.173 0.143 0.390 0.066 0.154
Other Considerations Kitroparaffins, and particularly nitromethane, being thermally sensitive, exhibit a tendency to induce preignition in an engine. While this may be a desirable characteristic for diesel fuel, it is not normally desirable in spark ignition engines and can lead to engine damage if sufficiently pronounced and prolonged. This consideration made it necessary to utilize dilution of the n i t r o p a r a h s with less sensitive fuels in early work ( 7 7, 72). Methanol was used as the base fuel and diluent when blending was found necessary, because large concentrations of nitroparaffins are not readily soluble in hydrocarbons, and thus research results are related to methanol as a datum fuel. However, iso-octane has also been included in computations and in engine tests for further comparison purposes. Methanol yields greater net output in an engine by about 10% than a hydrocarbon of gasoline range (9). Some VOL. 51, NO. 12
DECEMBER 1959
1479
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Figure 4.
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CONCENTRATION IN METHANOL
Comparative engine performance of representa-
tive fuels Figure 3. At an equivalence ratio of 1 nitroethane gives 26% more power than methanol
sources credit this effect to latent heat of the alcohol and its effect on charge density. T h e present author differs in opinion with this theory. I n the early work ( 7 7 , 72), the concentrations of nitroparaffins in methanol were small, because of preignition problems. Lowering air and jacket temperatures and smoothly machining the combustion envelope eliminated these limitations and all of the available mononitroparaffins have now been run undiluted, some for extended periods of time. Although undiluted nitromethane has been run for short periods, it still has not been possible to operate long enough to collect reliable data. Engine performance tests with dinitropropane have been limited to a concentration of 16y0weight thus far. Being a solid a t room temperature, residue problems have been encountered, but there have been no other noteworthy difficulties. Research Engine T h e engine used as a tool for this investigation was a Cooperative Fuel Research (2) unit. For the work reported there, the significant engine conditions which were held constant were: speed, 1800 r.p.m.; spark timing, 45Obefore top dead center; inlet temperature, 150’ F. ; compression ratio, 6 to 1 ; coolant temperature, 150’ F. Manifold pressure was alternately 20, 25, and 30 inches of mercury absolute for various tests. Comparisons were always made with data a t the same manifold pressure.
1480
Experimental Data The primary dependent variable in this work was the engine power output, expressed as the usual characteristic “mean pressure” exerted on the piston. This was measured externally with a dynamometer. The friction used internally was also determined and the sum expressed as the gross or “indicated mean effective pressure,” with dimensions of pounds per square inch. All data plotted are as “indicated mean effective pressure.” A typical set of results is plotted in Figure 3. Performance curves for other nitroparaffins were similar. The displacement to the right of each successively higher concentration from an equivalence ratio of 1.2 for methanol to one of 1.6 for nitroethane in Figure 3 is noteworthy. T h e reason for this shift of peak is related to the change in product composition with increased nitroparaffin concentration. At an equivalence ratio of 1, Figure 3 shows that nitroethane gives 26y0 more power than methanol. This compares to the 33y0 previously predicted on the basis of product composition and thermal density. Relative Performance of Nitroparaffins I n Figure 4 are plotted all the reduced nitroparaffin data available to date. T h e upward inflection in the nitromethane curve with increasing concentration is felt to reflect the factor N: products molal ratio. As can be seen from Figure 2, such might be expected because of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
disproportionate influence of the so-called products molal ratio of nitromethane compared to other fuels. Acknowledgment The contributions of Thomas Cline, Joseph Gileau, Louis Gibbs, Vernon Kask, Fredric Strange, and Joseph DeCosta are recognized, and gratitude is expressed for the aid of the Commercial Solvents Corp. literature Cited (1) Albright, R. E., Nelson, F. L., Raymond, L., IND. ENG. CHEM.41, 929 (1949). (2) Am. SOC. Testing Materials, Philadelphia, Pa., “ASTM Manual of Engine Test Methods for Rating Fuels.” (3) Bogin, C., C h i . Revs. 31, (1 942). (4) Gerrish, H. C., Meem, J. L., Jr.,
Natl. Advisory Comm. Aeronaut., Ann. Rept. 757 (1943). (5) Hass, H. B., IND. ENG. CHEM. 35, 1146 (1943). (6) H a s , H. B., Riley, E. F., Chern. Rerv. 32, 373 (1943). (7) Kharasch, M. S., B u r . Standards J . Research 2, 359 (1929). (8) Klein, F. L., S.A.E. Journal 5 5 , 12 (1947). (9) Lichty, L. C., Phelps, C. W., IND. ENG.CHEW30, 222 (1938). (10) Powell, R. E., University of California, private communication. (11) Starkman, E. S., publication b y Commercial Solvents Corp., Symposium. March 1956. (12) Starkman, E. S., Moulic, E. S., Dunn, M., Automotrue Znds. 114, 8 (1956). (13) Zwicky, F., Ross, C. C., S.A.E. Journal 57, 22 (June 1949). RECEIVED for review December 29, 1958 ACCEPTED June 12, 1959 Division of Petroleum Chemistry, Symposium on Chemistry and Technology of Petroleum Fuels of the Future, 133rd Meeting, ACS, San Francisco, Calif., .4pril 1958.
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