- Email: [email protected]
Lean air-fuel mixtures supplemented with hydrogen for S.I. engines: A possible way to reduce specific fuel consumption?
hr. 1. Hydrogen
Energy,
Vol. 10.No. ‘I/s.pp. 491-495.
1985.
Printed in Great Bntain. @ 1985 hernational
03fx!-3199/85 13.00 + 0.00 Pergamon Press Ltd. Association for Hydtogen Energy.
LEAN AIR-FUEL MIXTURES SUPPLEMENTED WITH HYDROGEN FOR S.I. ENGINES: A POSSIBLE WAY TO REDUCE SPECIFIC FUEL CONSUMPTION? R. GENTILI Dipartimento
di Energetica,
Universiti
(Receioed for
degli Studi di Pisa, Via Diotisalvi 2. 56100 Pisa. Italy 15
publication
Ocrober
1984)
Abstract-From a theoretical viewpoint, a lean or ultra-lean mixture improves thermal efficiency of intepnal combustion engines when operating at partial loads. This is confirmed is current practice with compression-ignited motors. However, spark-ignited engines actually reach the lowest specific fuel consumptions at air-fuel ratios not very far from the stoichiometric when the fuel is gasoline. since with leaner mixtures combustion becomes too slow and erratic. Stratified-charge engines allow leaner mixtures, but they require combustion chamber designs that penalize efficiency. The presence of hydrogen in the mixture increases combustion speed and reduces misfiring. As a result, the lowest specific fuel consumptions are obtained with leaner mixtures as a whole and are reduced in amounts, as the ratio of hydrogen to the total amount of fuel supplied is increased. In this work. a new system of hydrogen enrichment is presented: a small amount of hydrogen is stratified around the spark-plug in an open combustion chamber. in order to combine stratified-charge and hydro$enenrichment benefits. An experimental prototype is described in detail and the first experimental results are reported.
INTRODUCTION One of the most important goals for automotive engine designers is the reduction of specific fuel consumption,
especially at partial loads. Unfortunately, four-stroke, spark-ignited engines have a poor efficiency at partial loads, also owing to the low pressure present in the intake pipes. In fact, the amount of air intaken is regulated by a butterfly valve. because it is necessary to keep the air-fuel ratio almost constant. Such a drawback could be reduced or nullified, if it were possible to operate with lean mixtures. By so doing, on the one hand it would be possible to reduce the depression in the inlet pipes and the consequent pumping losses and. on the other, owing to the lower temperatures reached, the indicated cycle would differ less from the theoretical one; besides, the emissions of NO, would decrease largely. However, in conventional spark-ignited, gasoline-fed engines, combustion becomes too slow and erratic with lean mixtZlres; thus the lowest specific fuel consumptions are reached at air-fuel ratios not far from the stoichiometric. Stratified-charge engines allow leaner overall mixtures, but they require direct fuel injection and/or two intake valves; moreover, in order to keep a quasistoichiometric mixture near the s$ark-plug, an auxiliary combustion chamber may, be necessary, with the same prejudices of engine efficiency which, in compression-
ignited motors with indirect injection, are caused by the presence of a pre-chamber. With a homogeneous charge, the presence of hydrogen in the mixture increases combustion speed and reduces misfiring. As a result, the lowest specific fuel consumptions are obtained with leaner mixtures as a whole, and are reduced in amounts as the ratio of hydrogen to the total amount of fuel supplied is increased. Experimental works [l] show the lean operating limit increases in a nearly linear way with the increase of the hydrogen fraction, i.e. the ratio of hydrogen mass to total fuel supplied mass, up to a value of 0.2; afterwards, the lean limit increase becomes less rapid. Such a mass ratio of 0.2, corresponding to an energy ratio of 0.45, may represent the maximum hydrogen supplementation level of practical interest. However, if hydrogen is extracted from gasoline, the optimum supplementation level from an economic viewpoint depends largely on the efficiency of the hydrogen generator which is the crucial element of the entire system. In any case, with this solution it is impossible to achieve a truly remarkable decrease of fuel consumption, as small percentages of hydrogen do not allow to use ultra-lean mixtures and, on the other hand, large percentages are penalized by the limited efficiency of hydrogen generators. The basic idea of the present work is stratification of a small amount of hydrogen around the spark-plug, in order to have a chemical primer which allows a regular ignition of lean air-fuel mixtures in a simple design, 491
492
R. GENTILI
open combustion chamber, thus combining hydrogen enrichment and stratified-charge benefits.
SPARK PLUG ~....
INTAKE VALVE
;!
SOLUTION PROPOSED Because of the remarkable difference in absolute gravity between hydrogen and air (their ratio is 1/14.4), a very simple way to obtain a stratification of hydrogen in an open combustion chamber is to exploit the centrifugal force, by giving a swirling motion to the incoming air during the intake stroke. Such a motion can be produced either by an appropriate port design, or by a shrouded intake valve. By so doing, hydrogen swirls in a logarithmic spiral towards the axis of the cylinder. This kind of solution was proposed and tested in natural-gas engines some years ago with satisfactory results [2], in spite of the limited difference in absolute gravity between natural gas and air. Obviously, a good result can be achieved more easily with hydrogen and some parameters, like introduction timing and introduction system design, are less critical. Besides, the small amounts of hydrogen necessary make it possible to use the same port and valve for intaking both hydrogen and the air-gasoline mixture, thanks to a solution which will be explained in the following. EXPERIMENTAL ENGINE A Gilera 200 cm~ four-stroke, single cylinder, aircooled motor-cycle engine was selected as a basis to carry out the experimental prototype. In order to place the spark-plug tip on the axis of the combustion chamber, the original engine head was modified: the intake port was reduced to the same size of the exhaust port and the axes of both ports were displaced towards the periphery of the head (Figs 1 and 2). Thus the space for the introduction of a spark-plug core was obtained. The mass electrode was fastened directly on the head surface. An intake valve with a shroud of 120" was adopted; the valve was fitted with a special arrangement allowing
................ I\
i
)"~;)'\
REFERENCE !. ~'.,' )/ 'PO$1~O'-N F'0R sHRouD
SHROUD POSITION FOR VALVE . OROGE
HOLE
Fig. 2. Modified head.
the adjustment of the angular position of the shroud even while the engine is operating. A hole for the introduction of hydrogen was made on the internal edge of the intake port seal, in such an angular position that it is always fronted by the shroud, however rotated within logical limits. So the unwanted mixing of hydrogen with the air-gasoline mixture before they enter the cylinder is avoided. (If such a mixing took place, the desired concentration gradient of hydrogen would not be obtainable by means of the swirling motion of the charge.) For the sake of comparison, a pipe, debouching just before the intake port, was arranged for hydrogen introduction, too. A special fiat-head piston was adopted, in order to avoid squish effects which could break the stratification.
[ AKE VALVE
VALVE
Fig. 1. Original head.
Fig. 3. Flathead piston."
LEAN AIR-FUEL MIXTURES WITH HYDROGEN SUPPLEMENT
493
Table 1. Efficiency-optimizingoperation; tests at 5800 r.p.m, and 2.9 Nm at the crankshaft (max torque at 5800 r.p.m.: 11.8 Nm) with the shroud at 80° from its reference position Hydrogen flow (g s -l) 4.33 x 8.66 x 13.0 x 17.3 x 21.6 x 26.0 x
10-3 10-3 10-3 10-3 10-3 10-3
Gasoline flow (g s -1)
Hydrogen energy flow (KJ s -l)
Gasoline energy flow (KJ s -1)
Total energy flow (energetic consumption) (KJ S-I)
Efficiency
0.270 0.257 0.240 0.221 0.201 0.181 0.167
-0.517 1.03 1.55 2.06 2.58 3.10
11.9 11.3 10.5 9.71 8.83 7.95 7.34
11.9 11.8 11.5 11.3 10.9 10.5 10.4
0.148 0.149 0.153 0.156 0.162 0.168 0.169
Finally, the engine was equipped with a manually controlled fuel injection, with the sole purpose of freeing gasoline metering from the intake air flow. E X P E R I M E N T A L TESTS A N D R E S U L T S To begin with, the engine head was set on a transparent cylinder, in order to verify whether the swirl took place and to establish the most favourable position of the shroud. This test was made under steady-flow conditions: a metallic mesh was placed in the transparent cylinder orthogonally to its axis; air was aspirated from the end of the cylinder, while small balls of polystyrene foam were introduced from the intake port. The test, repeated with various mesh and shroud positions
and with different intake valve lifts, showed a perfect swirl occurred whatever mesh position and valve lift were, provided the shroud was at about 80 ° from its reference position (see Fig. 2). Next, the engine was assembled and set on a dynamometric bench. A i r and hydrogen flow-meters were provided. Hydrogen was supplied at a pressure slightly lower than atmospheric by means of a pressm'e reducer normally used for gas engines. Tests were made both to find out how the lean operating limit varied when varying the percentage of hydrogen, and tol verify the effect of hydrogen enrichment on specific fuel consumption. As a first result, both with gasoline only i and with gasoline and hydrogen, the best shroud rotation was
to
Z O
E =E
12
0.150
¢/) z O
o~,,
¢J Ill n" I.U Z g.J
0.160
11
°~o~ °
I'-
01.1
012
0L3
HYDROGEN ENERGY FLOW TOTAL ENERGY FLOW
Fig. 4. Energetic consumption and efficiencycurve (data from Table I).
0.170
>(J Z _ rj i.k
494
R. GENTILI Table 2. Lean limit operation; tests at 6000r.p.m. and 5.8Nm at the crankshaft with the shroud at 80° from its reference position Air mass flow
Air mass flow Hydrogen mass flow
Gasoline mass flow 17.4 19.7 21.4 23.0 24.9 26.3 27.2
1231 793
535 402 325 299
confirmed to be about 80° from the reference position. Therefore this rotation was kept during all the following tests. (Note that, when the engine head was removed after a number of tests, a suggestive spiral appeared designed in the coking on the top of the piston (Fig. 3).) An other important preliminary result was that no remarkable energetic economy could be obtained with small percentages of hydrogen when it was introduced from the pipe debouching before the intake port; however, this disposition proved to be interesting because no flashback occurred even when the engine was operating with hydrogen only. Results in regard to fuel consumption are presented in Table 1 and diagrammed in Fig. 4. In Table 2 results are reported regarding the lean
Actual air flow Stoichiometric air flow (4)
Hydrogen energy flow
1.17 1.23 1.25 1.27 1.28 1.26 1.26
0 0.0417 0.0632 0.105 0.144 0.180 0.198
limit of the mixture, and they are diagrammed in Fig. 5. (This lean limit was defined as the air-fuel ratio beyond which a further leaning causes such irregular operating conditions that the engine risks stopping.) All the tests were made maintaining the original inductive (not electronic) ignition system and timing. Test results show that the stratification of hydrogen occurred; in fact the lean limit was considerably improved by small hydrogen supplementations but overcoming a ratio of hydrogen energy to total energy supplied of about 0.07 only, the improvement became less sensible to the increase of the hydrogen fraction. The rather limited energetic economy obtained was probably caused by a remarkable quenching effect of the peripheral lean charge. This effect can be related
/
0.2-
oo/O/
o ..J U. . >-
o ILl Z t4J
..j .<
Total energy flow
0.1-
I,-
o
I-,
1.2
1.3
1,4
ACTUAL AIR FLOW STOICHIOMETRIC AIR
Fig. 5. Lean limit curve (data from Fig. 6).
1.5
,t
FLOW
LEAN AIR-FUEL MIXTURES WITH HYDROGEN SUPPLEMENT to shape of the combustion chamber, designed with the main purpose of avoiding turbulences which could interfere with the stratification of hydrogen. To improve engine efficiency, it was decided to modify the form of the combustion chamber and for this purpose new pistons have been designed and they will be tested as soon as possible. CONCLUSIONS An experimental prototype of an engine operating with lean air-gasoline mixtures enriched with stratified hydrogen has been presented together with the first experimental results. These results proved hydrogen stratification occurred
495
and suggested an engine modification in order to obtain further improvements in fuel economy.
AcknowledgementsnThe author wishes to acknowledge the intelligent and continuous help of Mr S. Bimbi and the valuable work of Mr G. Lastrucci. Piaggio & C. S.p.A. is especially acknowledgedforiproviding the Gilera 200 engine. REFERENCES 1. R. F. Stehar and F. B. Parks, Emission control with lean operating using hydrogen-supplemented fuel. SAE Paper 740187. 2. J. E. Witzky and R. W. Hull, The development of the pumpless gas engine concept. SAE Paper 700073.
Energy,
Vol. 10.No. ‘I/s.pp. 491-495.
1985.
Printed in Great Bntain. @ 1985 hernational
03fx!-3199/85 13.00 + 0.00 Pergamon Press Ltd. Association for Hydtogen Energy.
LEAN AIR-FUEL MIXTURES SUPPLEMENTED WITH HYDROGEN FOR S.I. ENGINES: A POSSIBLE WAY TO REDUCE SPECIFIC FUEL CONSUMPTION? R. GENTILI Dipartimento
di Energetica,
Universiti
(Receioed for
degli Studi di Pisa, Via Diotisalvi 2. 56100 Pisa. Italy 15
publication
Ocrober
1984)
Abstract-From a theoretical viewpoint, a lean or ultra-lean mixture improves thermal efficiency of intepnal combustion engines when operating at partial loads. This is confirmed is current practice with compression-ignited motors. However, spark-ignited engines actually reach the lowest specific fuel consumptions at air-fuel ratios not very far from the stoichiometric when the fuel is gasoline. since with leaner mixtures combustion becomes too slow and erratic. Stratified-charge engines allow leaner mixtures, but they require combustion chamber designs that penalize efficiency. The presence of hydrogen in the mixture increases combustion speed and reduces misfiring. As a result, the lowest specific fuel consumptions are obtained with leaner mixtures as a whole and are reduced in amounts, as the ratio of hydrogen to the total amount of fuel supplied is increased. In this work. a new system of hydrogen enrichment is presented: a small amount of hydrogen is stratified around the spark-plug in an open combustion chamber. in order to combine stratified-charge and hydro$enenrichment benefits. An experimental prototype is described in detail and the first experimental results are reported.
INTRODUCTION One of the most important goals for automotive engine designers is the reduction of specific fuel consumption,
especially at partial loads. Unfortunately, four-stroke, spark-ignited engines have a poor efficiency at partial loads, also owing to the low pressure present in the intake pipes. In fact, the amount of air intaken is regulated by a butterfly valve. because it is necessary to keep the air-fuel ratio almost constant. Such a drawback could be reduced or nullified, if it were possible to operate with lean mixtures. By so doing, on the one hand it would be possible to reduce the depression in the inlet pipes and the consequent pumping losses and. on the other, owing to the lower temperatures reached, the indicated cycle would differ less from the theoretical one; besides, the emissions of NO, would decrease largely. However, in conventional spark-ignited, gasoline-fed engines, combustion becomes too slow and erratic with lean mixtZlres; thus the lowest specific fuel consumptions are reached at air-fuel ratios not far from the stoichiometric. Stratified-charge engines allow leaner overall mixtures, but they require direct fuel injection and/or two intake valves; moreover, in order to keep a quasistoichiometric mixture near the s$ark-plug, an auxiliary combustion chamber may, be necessary, with the same prejudices of engine efficiency which, in compression-
ignited motors with indirect injection, are caused by the presence of a pre-chamber. With a homogeneous charge, the presence of hydrogen in the mixture increases combustion speed and reduces misfiring. As a result, the lowest specific fuel consumptions are obtained with leaner mixtures as a whole, and are reduced in amounts as the ratio of hydrogen to the total amount of fuel supplied is increased. Experimental works [l] show the lean operating limit increases in a nearly linear way with the increase of the hydrogen fraction, i.e. the ratio of hydrogen mass to total fuel supplied mass, up to a value of 0.2; afterwards, the lean limit increase becomes less rapid. Such a mass ratio of 0.2, corresponding to an energy ratio of 0.45, may represent the maximum hydrogen supplementation level of practical interest. However, if hydrogen is extracted from gasoline, the optimum supplementation level from an economic viewpoint depends largely on the efficiency of the hydrogen generator which is the crucial element of the entire system. In any case, with this solution it is impossible to achieve a truly remarkable decrease of fuel consumption, as small percentages of hydrogen do not allow to use ultra-lean mixtures and, on the other hand, large percentages are penalized by the limited efficiency of hydrogen generators. The basic idea of the present work is stratification of a small amount of hydrogen around the spark-plug, in order to have a chemical primer which allows a regular ignition of lean air-fuel mixtures in a simple design, 491
492
R. GENTILI
open combustion chamber, thus combining hydrogen enrichment and stratified-charge benefits.
SPARK PLUG ~....
INTAKE VALVE
;!
SOLUTION PROPOSED Because of the remarkable difference in absolute gravity between hydrogen and air (their ratio is 1/14.4), a very simple way to obtain a stratification of hydrogen in an open combustion chamber is to exploit the centrifugal force, by giving a swirling motion to the incoming air during the intake stroke. Such a motion can be produced either by an appropriate port design, or by a shrouded intake valve. By so doing, hydrogen swirls in a logarithmic spiral towards the axis of the cylinder. This kind of solution was proposed and tested in natural-gas engines some years ago with satisfactory results [2], in spite of the limited difference in absolute gravity between natural gas and air. Obviously, a good result can be achieved more easily with hydrogen and some parameters, like introduction timing and introduction system design, are less critical. Besides, the small amounts of hydrogen necessary make it possible to use the same port and valve for intaking both hydrogen and the air-gasoline mixture, thanks to a solution which will be explained in the following. EXPERIMENTAL ENGINE A Gilera 200 cm~ four-stroke, single cylinder, aircooled motor-cycle engine was selected as a basis to carry out the experimental prototype. In order to place the spark-plug tip on the axis of the combustion chamber, the original engine head was modified: the intake port was reduced to the same size of the exhaust port and the axes of both ports were displaced towards the periphery of the head (Figs 1 and 2). Thus the space for the introduction of a spark-plug core was obtained. The mass electrode was fastened directly on the head surface. An intake valve with a shroud of 120" was adopted; the valve was fitted with a special arrangement allowing
................ I\
i
)"~;)'\
REFERENCE !. ~'.,' )/ 'PO$1~O'-N F'0R sHRouD
SHROUD POSITION FOR VALVE . OROGE
HOLE
Fig. 2. Modified head.
the adjustment of the angular position of the shroud even while the engine is operating. A hole for the introduction of hydrogen was made on the internal edge of the intake port seal, in such an angular position that it is always fronted by the shroud, however rotated within logical limits. So the unwanted mixing of hydrogen with the air-gasoline mixture before they enter the cylinder is avoided. (If such a mixing took place, the desired concentration gradient of hydrogen would not be obtainable by means of the swirling motion of the charge.) For the sake of comparison, a pipe, debouching just before the intake port, was arranged for hydrogen introduction, too. A special fiat-head piston was adopted, in order to avoid squish effects which could break the stratification.
[ AKE VALVE
VALVE
Fig. 1. Original head.
Fig. 3. Flathead piston."
LEAN AIR-FUEL MIXTURES WITH HYDROGEN SUPPLEMENT
493
Table 1. Efficiency-optimizingoperation; tests at 5800 r.p.m, and 2.9 Nm at the crankshaft (max torque at 5800 r.p.m.: 11.8 Nm) with the shroud at 80° from its reference position Hydrogen flow (g s -l) 4.33 x 8.66 x 13.0 x 17.3 x 21.6 x 26.0 x
10-3 10-3 10-3 10-3 10-3 10-3
Gasoline flow (g s -1)
Hydrogen energy flow (KJ s -l)
Gasoline energy flow (KJ s -1)
Total energy flow (energetic consumption) (KJ S-I)
Efficiency
0.270 0.257 0.240 0.221 0.201 0.181 0.167
-0.517 1.03 1.55 2.06 2.58 3.10
11.9 11.3 10.5 9.71 8.83 7.95 7.34
11.9 11.8 11.5 11.3 10.9 10.5 10.4
0.148 0.149 0.153 0.156 0.162 0.168 0.169
Finally, the engine was equipped with a manually controlled fuel injection, with the sole purpose of freeing gasoline metering from the intake air flow. E X P E R I M E N T A L TESTS A N D R E S U L T S To begin with, the engine head was set on a transparent cylinder, in order to verify whether the swirl took place and to establish the most favourable position of the shroud. This test was made under steady-flow conditions: a metallic mesh was placed in the transparent cylinder orthogonally to its axis; air was aspirated from the end of the cylinder, while small balls of polystyrene foam were introduced from the intake port. The test, repeated with various mesh and shroud positions
and with different intake valve lifts, showed a perfect swirl occurred whatever mesh position and valve lift were, provided the shroud was at about 80 ° from its reference position (see Fig. 2). Next, the engine was assembled and set on a dynamometric bench. A i r and hydrogen flow-meters were provided. Hydrogen was supplied at a pressure slightly lower than atmospheric by means of a pressm'e reducer normally used for gas engines. Tests were made both to find out how the lean operating limit varied when varying the percentage of hydrogen, and tol verify the effect of hydrogen enrichment on specific fuel consumption. As a first result, both with gasoline only i and with gasoline and hydrogen, the best shroud rotation was
to
Z O
E =E
12
0.150
¢/) z O
o~,,
¢J Ill n" I.U Z g.J
0.160
11
°~o~ °
I'-
01.1
012
0L3
HYDROGEN ENERGY FLOW TOTAL ENERGY FLOW
Fig. 4. Energetic consumption and efficiencycurve (data from Table I).
0.170
>(J Z _ rj i.k
494
R. GENTILI Table 2. Lean limit operation; tests at 6000r.p.m. and 5.8Nm at the crankshaft with the shroud at 80° from its reference position Air mass flow
Air mass flow Hydrogen mass flow
Gasoline mass flow 17.4 19.7 21.4 23.0 24.9 26.3 27.2
1231 793
535 402 325 299
confirmed to be about 80° from the reference position. Therefore this rotation was kept during all the following tests. (Note that, when the engine head was removed after a number of tests, a suggestive spiral appeared designed in the coking on the top of the piston (Fig. 3).) An other important preliminary result was that no remarkable energetic economy could be obtained with small percentages of hydrogen when it was introduced from the pipe debouching before the intake port; however, this disposition proved to be interesting because no flashback occurred even when the engine was operating with hydrogen only. Results in regard to fuel consumption are presented in Table 1 and diagrammed in Fig. 4. In Table 2 results are reported regarding the lean
Actual air flow Stoichiometric air flow (4)
Hydrogen energy flow
1.17 1.23 1.25 1.27 1.28 1.26 1.26
0 0.0417 0.0632 0.105 0.144 0.180 0.198
limit of the mixture, and they are diagrammed in Fig. 5. (This lean limit was defined as the air-fuel ratio beyond which a further leaning causes such irregular operating conditions that the engine risks stopping.) All the tests were made maintaining the original inductive (not electronic) ignition system and timing. Test results show that the stratification of hydrogen occurred; in fact the lean limit was considerably improved by small hydrogen supplementations but overcoming a ratio of hydrogen energy to total energy supplied of about 0.07 only, the improvement became less sensible to the increase of the hydrogen fraction. The rather limited energetic economy obtained was probably caused by a remarkable quenching effect of the peripheral lean charge. This effect can be related
/
0.2-
oo/O/
o ..J U. . >-
o ILl Z t4J
..j .<
Total energy flow
0.1-
I,-
o
I-,
1.2
1.3
1,4
ACTUAL AIR FLOW STOICHIOMETRIC AIR
Fig. 5. Lean limit curve (data from Fig. 6).
1.5
,t
FLOW
LEAN AIR-FUEL MIXTURES WITH HYDROGEN SUPPLEMENT to shape of the combustion chamber, designed with the main purpose of avoiding turbulences which could interfere with the stratification of hydrogen. To improve engine efficiency, it was decided to modify the form of the combustion chamber and for this purpose new pistons have been designed and they will be tested as soon as possible. CONCLUSIONS An experimental prototype of an engine operating with lean air-gasoline mixtures enriched with stratified hydrogen has been presented together with the first experimental results. These results proved hydrogen stratification occurred
495
and suggested an engine modification in order to obtain further improvements in fuel economy.
AcknowledgementsnThe author wishes to acknowledge the intelligent and continuous help of Mr S. Bimbi and the valuable work of Mr G. Lastrucci. Piaggio & C. S.p.A. is especially acknowledgedforiproviding the Gilera 200 engine. REFERENCES 1. R. F. Stehar and F. B. Parks, Emission control with lean operating using hydrogen-supplemented fuel. SAE Paper 740187. 2. J. E. Witzky and R. W. Hull, The development of the pumpless gas engine concept. SAE Paper 700073.