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PRV Induction 1
PRV Induction Technical Overview James Meyer Aspen Engineering Services (dba PRV Performance) 18990 W 54th Place, Golden, CO 80403 303-887-2032, [email protected]
July 23, 2009
PRV Induction 2 Table of Contents Abstract.......................................................................................................................................... 3 1.0 Identification and Significance of the Innovation ................................................................ 4 2.0 Background ............................................................................................................................. 5 2.1 The Conventional Intake Manifold ................................................................................... 5 2.2 The Pintle-Regulated Venturi (PRV) Concept............................................................. 5 2.21 Fuel Economy Improvement from Reduced Pumping Loss ..................................... 7 2.22 Improved Engine Torque ............................................................................................. 8 2.23 Fuel Economy Derived from Pre-Vaporized Fuel ..................................................... 9 2.24 Reduced Greenhouse Gas Emissions .......................................................................... 9 2.25 Ethanol-Fueled Engine Performance Expectation .................................................... 9 2.26 Initial Prototype Results............................................................................................. 10 References .................................................................................................................................... 11
PRV Induction 3 Abstract PRV PERFORMANCE has developed and patented an improved automotive fuel induction system to improve engine torque and reduce throttling losses for improved vehicle efficiency. Improved pre-mixing of fuel and air lowers tailpipe emissions. Pintle-regulated Venturi (PRV) induction, with fuel injection directly at the Venturi throat, effectively mixes fuel and air due to the high velocity and reduced pressure at the vaporliquid contact region, while pressure is recovered in the expansion section. Fuel vaporization in the confined Venturi volume facilitates air flow into the PRV entrance. PRV improves fuel economy and increases engine torque by 1. reducing piston pumping loss; 2. pre-vaporizing fuel upstream of the engine, thereby cooling the fuel/air mixture and lowering the mixture density; and 3. eliminating fuel stratification on cylinder wall.
PRV Induction
4
PRV Induction Technical Overview Aspen Engineering Services (dba PRV Performance) 18990 W 54th Place, Golden, CO 80403 James Meyer, 303-887-2032, [email protected] 1.0 Identification and Significance of the Innovation Pintle-regulated Venturi (PRV) induction is a cost effective, patented fuel/air induction system for improving the performance of spark-ignited automotive engines system (U.S. patent 6,868,830). The proposed induction system uses a (PRV) to create a convergent-divergent nozzle that can significantly recover the pressure drop across the pintle body and maintain a higher engine intake manifold pressure to boost engine torque. With a higher engine torque using the same amount of fuel, the fuel economy will be increased significantly. On the contrary, a conventional throttle plate causes unrecoverable pressure loss, reducing engine torque and efficiency. Specifically, engine torque is increased and fuel economy is improved while exhaust pollutants and greenhouse gases are significantly reduced by exchanging a conventional intake manifold with a pintle-regulated Venturi (PRV) induction system on a conventional spark-ignited engine. A schematic comparison of the two induction systems is shown in Figures 1 and 2.
Figure 1: Conventional manifold at partially open throttle that creates unrecoverable pressure loss such that the piston must pull against a vacuum, resulting in high pumping loss.
Figure 2: PRV manifold that is designed to recover pressure drop across the pintle in order to maintain a high intake pressure, boosting engine torque and fuel economy. A PRV is implemented for each cylinder.
PRV Performance has been developing the proposed system for five years, and a recent on-road test indicated that the gasoline engine vehicle could achieve 58 mpg for a 204-mile round trip with an average speed of 65 mph. Recently, the Obama Administration declared that vehicles must reach an average of 35.5 mpg before 2016. PRV technology will enable troubled US automakers to produce cars and trucks to meet the new fuel/emission standards with little or no expense to retool their factories, making it imperative to bring this proposed technology to market. The engine performance improvement using the proposed system is derived from four attributes of a PRV
PRV Induction
5
induction system: PRV induction improves fuel economy. First, PRV fuel induction substantially reduces piston pumping losses. With a conventional manifold, pumping losses account for 16 to 40% of an engine’s efficiency loss under normal cruising conditions because the air intake is at vacuum pressure (Heywood, 1988; Ferguson and Kirkpatrick, 2001). The air intake plenum of a PRV is never under vacuum and consequently, the piston pulls initially against full ambient (or turbo) pressure. Fuel efficiency is substantially improved because the wasted work of pulling against a vacuum is eliminated. Second, PRV induction vaporizes the fuel before the engine whereas a conventional manifold does not fully vaporized fuel until the compression stroke. The compression work used to vaporize fuel is eliminated with PRV induction. Third, the cooler, denser charge reduces the friction loss across the intake valves thereby further reducing pumping loss. Engine power output is increased. Engine power is directly proportional to air flow capacity. PRV induction vaporizes fuel upstream of the engine by injecting fuel directly into the highvelocity air stream at the low-pressure throat of the Venturi. By conservation of energy, the latent heat of the fuel is transferred into a cooled fuel/air mixture. Since a higher mixture flow rate is enabled with a cooled charge, the specific power output of the engine is increased. The cooler (higher density) charge allows the engine to produce more torque and horsepower. PRV induction reduces exhaust emission of CO2, CO and unburned hydrocarbons. A conventional fuel injection system sprays droplets into the cylinder. Some of the droplets impinge on the cylinder walls. Those impinged droplets become entrained into crevices in the cylinder wall, resulting in incomplete combustion of the fuel and the generation of CO and unburned hydrocarbons. CO2 reduction is accomplished by improving fuel economy. PRV induction will deliver an even greater benefit for ethanol-fueled engines than gasolinefueled engines. The power increase has been demonstrated on a dynamometer with a gasolinefueled engine. PRV will use the higher latent heat of ethanol to provide additional charge cooling and in turn, a greater percentage increase in power relative to a conventional manifold. 2.0 Background 2.1 The Conventional Intake Manifold The power of a spark ignited engine is directly related to the mass of air in the cylinder when the intake valve closes. The mass of air in the cylinder is directly proportional to the pressure inside the cylinder. The cylinder pressure, with the air fed from by throttle plate, is controlled by the pressure of a common manifold feeding all engine cylinders. While cruising, the throttle plate is rotated open only a few degrees, imparting a large frictional loss causing a vacuum in the intake manifold (Figure 1). 2.2 The Pintle-Regulated Venturi (PRV) Concept Figure 3 is a three-dimensional drawing of a PRV induction manifold for a four-cylinder engine. Cylinder one is cut away to show the detail of the Venturi and the pintle. A PRV induction system does not have a throttle plate. All pintles are withdrawn in parallel as the gas pedal is pressed. The pintle movement along the axis of the Venturi (Figure 4) changes the flow area to provide throttle control of the engine.
PRV Induction
Figure 3: PRV Induction Manifold, Venturi 1 Cut-away
6
Figure 4: PRV Cut-away View
The pressure profile of a PRV induction system is entirely different for two reasons. First, the PRV intake pressure is always ambient (or turbocharger discharge) pressure. Second, when the cylinder valve closes, air from the plenum can flow around the pintle, equalizing the Venturi discharge pressure with the plenum. There is plenty of time for this to happen because inlet valves spend at least twice as long closed as open. Third, in comparison with a butterfly throttle which is as obstructive as one could imagine, the PRV is as streamlined as possible. The throttle control fluid mechanics of a conventional throttle plate and PRV induction system are entirely different, contributing to the improved fuel economy performance derived from PRV. Throttle control with PRV is accomplished by the limitation of sonic velocity at the throat. The velocity at the throat of the Venturi is determined by the speed of the piston and the annular or area between the pintle and Venturi. Initially, the piston moves slowly and air easily flows through the streamlined Venturi. However, as the piston accelerates away, at some point the air flow through the Venturi will approach sonic velocity. At sonic velocity, the pressure in the intake duct can no longer influence the flow through the Venturi. The Venturi is said to be choked -- and in this way, the streamlined PRV controls the power of the engine. The throttle control of the conventional throttle plate is accomplished by non-recoverable frictional loss of the butterfly valve. Consequently, the piston is pulling against full vacuum for the entire down stroke, resulting in substantially higher pumping losses than PRV induction. The intake pressure from PRV induction pulsates to reduce pumping losses whereas a conventional manifold operates at a steady, but low, pressure (Figures 5 and 6). The streamlined convergent-divergent Venturi nozzle accelerates airflow as the throat is approached and decelerates airflow in the divergent section. Concurrently, as the air slows down, the pressure increases. The pressure recovery process is governed by the conservation of energy as described by Bernoulli’s principal (Perry & Chilton, 1973, pp. 5-18). The pressure recovery attribute of PRV improves the piston-pressure history and, consequently, reduces pumping loss.
PRV Induction
Figure 5: typical P-V diagram for a four-stroke engine with a conventional intake manifold.
7
Figure 6: Cylinder pressure comparison of a PRV and a conventional manifold while cruising; data from Honda D15B engine at 2300 rpm at 5700 feet of elevation.
The fuel injector is aligned with the throat of the Venturi where the air velocity is at the maximum and the air pressure is at the minimum, thereby thoroughly vaporizing and mixing fuel into the air stream. The fact that the Venturi throat is the low pressure point is counter-intuitive, but consistent with the conservation of energy. The expansion of the vaporized fuel in the Venturi pushes the fuel/air mixture into the engine, thereby improving torque and fuel economy. 2.21 Fuel Economy Improvement from Reduced Pumping Loss The pressure equalization before each intake stroke, combined with the streamlined flow attribute of PRV induction, reduces pumping loss. How important is the pumping loss? John C. Hilliard et al. (1984) explains, Assuming that the exhaust manifold pressure is about 1 inch of mercury above atmospheric pressure, an intake and exhaust pumping loss of approximately 7 hp can be calculated, which equals 0.072 of the total fuel supply. While this number is not absolutely accurate it does illustrate the importance of the loss. At 50 mph road load the pumping losses almost 40% of the engine brake power. (Hilliard and Springer, 1984). Blackmore and Thomas (1977) write, "... pumping loss varies from 3.5% at wide-open throttle (WOT) to nearly 100% for a full throttled idling engine". Accordingly, the pumping loss for a compact U.S. 10:1 compression-ratio car cruising at 40 mph is about 16% (Blackmore & Thomas, 1977). Pumping loss is highly dependent on the throttle position in the specific engine design. In any case, most of the pumping loss is recovered by PRV induction. Figure 7 compares the in-cylinder pressure of a conventional manifold to a PRV induction system. When the intake valve opens on a conventional manifold, the cylinder pulls air from a plenum under vacuum. In contrast, the PRV plenum is constantly at ambient pressure. The bottomdead-center cylinder pressures are identical for both intake systems because the engine torque is dependent on the amount of mixture at the end of the intake cycle. Since the pressure trace is much higher for PRV induction, there is a cumulative saving of work by the piston. The work saving is reflected in the fuel economy improvement demonstrated by early PRV prototypes. The benefits of using PRV induction can be further expanded for a turbocharged engine. During cruise, the conventional manifold will exhibit the same pressure profile. The profile does not change because the pressure at bottom dead center must be the same. Consequently, the plenum pressure must also be the same. The pressure differential between the turbocharger and plenum pressure is regulated by the throttle. In contrast, the PRV plenum pressure is set directly by the
PRV Induction
8
turbocharger (Figure 7). The PRV induction system combined with the turbocharger will require even less work to be done by the piston to draw the charge into the engine, implying an even greater fuel efficiency gain. Individual throttle plates are much less effective for reducing pumping loss for two reasons. First, individual throttle plates preclude mixing and vaporizing fuel upstream of the engine and, consequently, the charge cooling effect is lost. Second, a throttle plate obstructs flow whereas the Venturi enhances flow. Although the manifold pressure will be the same as a PRV induction system, the in-cylinder pressure profile of PRV is much better than a throttle plate. Figure 7 illustrates the difference in pressure profile of a conventional manifold, individual throttle plate and PRV induction.
Figure 7: cylinder pressure of a conventional manifold versus individual throttle plates and PRV induction; normally aspirated engine (conventional manifold data from Taylor, 1985, p. 345, PRV data from prototype measurement). 2.22 Improved Engine Torque A cooler charge mixture improves the torque of the engine because a cooler charge mixture has a higher density. The higher density provides for more fuel/air mixture to be in the cylinder at the time the intake valve closes. A computer simulation shows that a pintle-regulated Venturi induction completely vaporizes the fuel before the mixture reaches the engine head (Meyer, 2009). Taylor states that if fuel evaporation is complete before inlet valve closing, the charge temperature decreases by 44° F, based on octane thermal properties. The temperature decreases in good agreement with the authors computer simulations. Results from the chassis dynamometer test on the initial prototype are shown in Figure 11 (section 2.8). Since PRV induction increases torque, automakers can produce smaller and lighter vehicles to comply with the 2009 fuel economy standards to be phased in by 2016.
PRV Induction
9
2.23 Fuel Economy Derived from Pre-Vaporized Fuel Blackmore and Thomas (1977) reported that vaporized fuel improves the fuel economy of a spark ignition engine. The percentage improvement is reflected in Figure 8.
Figure 8: Economy Improvement from Vaporized Fuel, data from Blackmore & Thomas, 1977, p. 106
Taylor elaborates on fuel evaporation in his highly acclaimed book, The Internal Combustion Engine in Theory and Practice. Specifically, Taylor writes that although some of the fuel is vaporized from heat transfer with the cylinder wall, evaporation is seldom complete before the intake valve closes (Taylor, 1985, p. 185). Consequently, the efficiency is lost because work is required to evaporate fuel. 2.24 Reduced Greenhouse Gas Emissions CO2 emission is directly related to fuel consumption. Consequently, any improvement in fuel economy will decrease the CO2 emitted and the associated effect on global warming. Hydrocarbon and CO emissions are produced when the combustion process is not complete. Hilliard and Springer (1984) writes, "The major source of hydrocarbons found in the exhaust results from that portion of the fuel-air intake charge that is trapped during the compression stroke, and subsequent pressure rise during combustion, inside of crevices in the cylinder into which the main combustion flame cannot enter... In the exhaust port and exhaust manifold, a portion of the residual hydrocarbons are combusted to CO or CO2." (Hilliard and Springer, 1984, p. 61). PRV induction pre-vaporizes the fuel and, consequently, less fuel is trapped in the cylinder wall crevices thereby reducing hydrocarbon and CO emissions. 2.25 Ethanol-Fueled Engine Performance Expectation Ethanol is more difficult to disperse from a fuel injector because of the higher viscosity. Ethanol also has a lower vapor pressure and a higher heat of vaporization. Consequently, with a conventional fuel injection system, more of the fuel evaporation (in comparison to gasoline) will occur during the compression cycle. Conversely, the higher heat of vaporization of ethanol means that pre-mixing of ethanol by PRV lowers the charge mixture more than gasoline. The lower charge temperature and improved dispersion upstream of the engine, provided by PRV, is exactly what an ethanol-fueled engine needs for improved power. The physical properties of ethanol and gasoline are compared in Table 1.
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Table 1: Physical Properties of Gasoline and Ethanol (US Department of Energy) Heat of vaporization, Btu/lb @ Viscosity, Reid vapor pressure (100 degrees 60° F. mm2/s. F.), psi Gasoline 150 0.55 8-15 Ethanol 396 1.5 2.3 2.26 Initial Prototype Results Test results with the gasoline prototype have demonstrated a 21% increase in highway fuel economy with a stoichiometric air/fuel ratio and 35% increase in highway fuel economy within air/fuel ratio of 16.8. Torque is increased 15-20% on a normally aspirated engine fueled by gasoline (Figure 9). These results were obtained after a series of 10 prototypes were developed and tested over 5 years. Test results have been validated repeatedly over a 203 mile highway cycle is 65 mph, delivering 53 mpg at a stoichiometric tune to 58 mpg for a lean-burn tune.
Figure 9: Torque and Horsepower of a Gasolinefueled Normally Aspirated Engine with Conventional Manifold and PRV Manifold
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References Blackmore, D. R., & Thomas, A. (1977). Fuel economy of the gasoline engine. New York: John Wiley &, Inc. Energy efficiency and renewable energy news. (2008, March 12). US ethanol production totaled 6.48 billion gallons in 2007. Retrieved April 13, 2009, from http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=11633. Energy Information Administration. (2009, February 1). 1 (Petroleum Basic Statistics). Retrieved April 13, 2009, from http://www.eia.doe.gov/basics/quickoil.html. Environmental Protection Agency. Fuel economy of motor vehicles. 40 CFR 600. Ferguson, C. R., & Kirkpatrick, A. T. (2001). Internal combustion engines – applied thermosciences. New York: Wiley. Gold, A. What it is, how it works. Retrieved from about.com: http://cars.about.com/od/thingsyouneedtoknow/a/directinjection.htm. Heywood, J. B. (1988). Internal combustion engine fundamentals. McGraw-Hill. Hilliard, J. C., & Springer, G. S. (1984). Fuel economy and road vehicles powered by spark ignition engines. New York, New York: Plenum publishing. Kelly, K. M. (2009, April 1). Ricardo's ethanol boosted direct injection. Automotive Design & Production, 121(4), 13. Meyer, J. M. E. A. (2005, March 22). US patent 6,868,830 (U. S. Patent & T. Office, Eds.). 6.868,830. Perry & Chilton. (1973). Chemical engineer's handbook. New York: McGraw-Hill. Stress Engineering Services Inc. Venturi mixer. Retrieved April 28, 2009, from Process Technology Group: http://www.processinnovation.com/venturi.html. Taylor, C. F. (1985). The internal combustion engine in theory and practice. Cambridge MA: MIT press. US Department of Energy. Alternative fuels and advanced vehicles data center. Retrieved April 26, 2009, from US Department of Energy: http://www.afdc.energy.gov/afdc/.
PRV Induction Technical Overview James Meyer Aspen Engineering Services (dba PRV Performance) 18990 W 54th Place, Golden, CO 80403 303-887-2032, [email protected]
July 23, 2009
PRV Induction 2 Table of Contents Abstract.......................................................................................................................................... 3 1.0 Identification and Significance of the Innovation ................................................................ 4 2.0 Background ............................................................................................................................. 5 2.1 The Conventional Intake Manifold ................................................................................... 5 2.2 The Pintle-Regulated Venturi (PRV) Concept............................................................. 5 2.21 Fuel Economy Improvement from Reduced Pumping Loss ..................................... 7 2.22 Improved Engine Torque ............................................................................................. 8 2.23 Fuel Economy Derived from Pre-Vaporized Fuel ..................................................... 9 2.24 Reduced Greenhouse Gas Emissions .......................................................................... 9 2.25 Ethanol-Fueled Engine Performance Expectation .................................................... 9 2.26 Initial Prototype Results............................................................................................. 10 References .................................................................................................................................... 11
PRV Induction 3 Abstract PRV PERFORMANCE has developed and patented an improved automotive fuel induction system to improve engine torque and reduce throttling losses for improved vehicle efficiency. Improved pre-mixing of fuel and air lowers tailpipe emissions. Pintle-regulated Venturi (PRV) induction, with fuel injection directly at the Venturi throat, effectively mixes fuel and air due to the high velocity and reduced pressure at the vaporliquid contact region, while pressure is recovered in the expansion section. Fuel vaporization in the confined Venturi volume facilitates air flow into the PRV entrance. PRV improves fuel economy and increases engine torque by 1. reducing piston pumping loss; 2. pre-vaporizing fuel upstream of the engine, thereby cooling the fuel/air mixture and lowering the mixture density; and 3. eliminating fuel stratification on cylinder wall.
PRV Induction
4
PRV Induction Technical Overview Aspen Engineering Services (dba PRV Performance) 18990 W 54th Place, Golden, CO 80403 James Meyer, 303-887-2032, [email protected] 1.0 Identification and Significance of the Innovation Pintle-regulated Venturi (PRV) induction is a cost effective, patented fuel/air induction system for improving the performance of spark-ignited automotive engines system (U.S. patent 6,868,830). The proposed induction system uses a (PRV) to create a convergent-divergent nozzle that can significantly recover the pressure drop across the pintle body and maintain a higher engine intake manifold pressure to boost engine torque. With a higher engine torque using the same amount of fuel, the fuel economy will be increased significantly. On the contrary, a conventional throttle plate causes unrecoverable pressure loss, reducing engine torque and efficiency. Specifically, engine torque is increased and fuel economy is improved while exhaust pollutants and greenhouse gases are significantly reduced by exchanging a conventional intake manifold with a pintle-regulated Venturi (PRV) induction system on a conventional spark-ignited engine. A schematic comparison of the two induction systems is shown in Figures 1 and 2.
Figure 1: Conventional manifold at partially open throttle that creates unrecoverable pressure loss such that the piston must pull against a vacuum, resulting in high pumping loss.
Figure 2: PRV manifold that is designed to recover pressure drop across the pintle in order to maintain a high intake pressure, boosting engine torque and fuel economy. A PRV is implemented for each cylinder.
PRV Performance has been developing the proposed system for five years, and a recent on-road test indicated that the gasoline engine vehicle could achieve 58 mpg for a 204-mile round trip with an average speed of 65 mph. Recently, the Obama Administration declared that vehicles must reach an average of 35.5 mpg before 2016. PRV technology will enable troubled US automakers to produce cars and trucks to meet the new fuel/emission standards with little or no expense to retool their factories, making it imperative to bring this proposed technology to market. The engine performance improvement using the proposed system is derived from four attributes of a PRV
PRV Induction
5
induction system: PRV induction improves fuel economy. First, PRV fuel induction substantially reduces piston pumping losses. With a conventional manifold, pumping losses account for 16 to 40% of an engine’s efficiency loss under normal cruising conditions because the air intake is at vacuum pressure (Heywood, 1988; Ferguson and Kirkpatrick, 2001). The air intake plenum of a PRV is never under vacuum and consequently, the piston pulls initially against full ambient (or turbo) pressure. Fuel efficiency is substantially improved because the wasted work of pulling against a vacuum is eliminated. Second, PRV induction vaporizes the fuel before the engine whereas a conventional manifold does not fully vaporized fuel until the compression stroke. The compression work used to vaporize fuel is eliminated with PRV induction. Third, the cooler, denser charge reduces the friction loss across the intake valves thereby further reducing pumping loss. Engine power output is increased. Engine power is directly proportional to air flow capacity. PRV induction vaporizes fuel upstream of the engine by injecting fuel directly into the highvelocity air stream at the low-pressure throat of the Venturi. By conservation of energy, the latent heat of the fuel is transferred into a cooled fuel/air mixture. Since a higher mixture flow rate is enabled with a cooled charge, the specific power output of the engine is increased. The cooler (higher density) charge allows the engine to produce more torque and horsepower. PRV induction reduces exhaust emission of CO2, CO and unburned hydrocarbons. A conventional fuel injection system sprays droplets into the cylinder. Some of the droplets impinge on the cylinder walls. Those impinged droplets become entrained into crevices in the cylinder wall, resulting in incomplete combustion of the fuel and the generation of CO and unburned hydrocarbons. CO2 reduction is accomplished by improving fuel economy. PRV induction will deliver an even greater benefit for ethanol-fueled engines than gasolinefueled engines. The power increase has been demonstrated on a dynamometer with a gasolinefueled engine. PRV will use the higher latent heat of ethanol to provide additional charge cooling and in turn, a greater percentage increase in power relative to a conventional manifold. 2.0 Background 2.1 The Conventional Intake Manifold The power of a spark ignited engine is directly related to the mass of air in the cylinder when the intake valve closes. The mass of air in the cylinder is directly proportional to the pressure inside the cylinder. The cylinder pressure, with the air fed from by throttle plate, is controlled by the pressure of a common manifold feeding all engine cylinders. While cruising, the throttle plate is rotated open only a few degrees, imparting a large frictional loss causing a vacuum in the intake manifold (Figure 1). 2.2 The Pintle-Regulated Venturi (PRV) Concept Figure 3 is a three-dimensional drawing of a PRV induction manifold for a four-cylinder engine. Cylinder one is cut away to show the detail of the Venturi and the pintle. A PRV induction system does not have a throttle plate. All pintles are withdrawn in parallel as the gas pedal is pressed. The pintle movement along the axis of the Venturi (Figure 4) changes the flow area to provide throttle control of the engine.
PRV Induction
Figure 3: PRV Induction Manifold, Venturi 1 Cut-away
6
Figure 4: PRV Cut-away View
The pressure profile of a PRV induction system is entirely different for two reasons. First, the PRV intake pressure is always ambient (or turbocharger discharge) pressure. Second, when the cylinder valve closes, air from the plenum can flow around the pintle, equalizing the Venturi discharge pressure with the plenum. There is plenty of time for this to happen because inlet valves spend at least twice as long closed as open. Third, in comparison with a butterfly throttle which is as obstructive as one could imagine, the PRV is as streamlined as possible. The throttle control fluid mechanics of a conventional throttle plate and PRV induction system are entirely different, contributing to the improved fuel economy performance derived from PRV. Throttle control with PRV is accomplished by the limitation of sonic velocity at the throat. The velocity at the throat of the Venturi is determined by the speed of the piston and the annular or area between the pintle and Venturi. Initially, the piston moves slowly and air easily flows through the streamlined Venturi. However, as the piston accelerates away, at some point the air flow through the Venturi will approach sonic velocity. At sonic velocity, the pressure in the intake duct can no longer influence the flow through the Venturi. The Venturi is said to be choked -- and in this way, the streamlined PRV controls the power of the engine. The throttle control of the conventional throttle plate is accomplished by non-recoverable frictional loss of the butterfly valve. Consequently, the piston is pulling against full vacuum for the entire down stroke, resulting in substantially higher pumping losses than PRV induction. The intake pressure from PRV induction pulsates to reduce pumping losses whereas a conventional manifold operates at a steady, but low, pressure (Figures 5 and 6). The streamlined convergent-divergent Venturi nozzle accelerates airflow as the throat is approached and decelerates airflow in the divergent section. Concurrently, as the air slows down, the pressure increases. The pressure recovery process is governed by the conservation of energy as described by Bernoulli’s principal (Perry & Chilton, 1973, pp. 5-18). The pressure recovery attribute of PRV improves the piston-pressure history and, consequently, reduces pumping loss.
PRV Induction
Figure 5: typical P-V diagram for a four-stroke engine with a conventional intake manifold.
7
Figure 6: Cylinder pressure comparison of a PRV and a conventional manifold while cruising; data from Honda D15B engine at 2300 rpm at 5700 feet of elevation.
The fuel injector is aligned with the throat of the Venturi where the air velocity is at the maximum and the air pressure is at the minimum, thereby thoroughly vaporizing and mixing fuel into the air stream. The fact that the Venturi throat is the low pressure point is counter-intuitive, but consistent with the conservation of energy. The expansion of the vaporized fuel in the Venturi pushes the fuel/air mixture into the engine, thereby improving torque and fuel economy. 2.21 Fuel Economy Improvement from Reduced Pumping Loss The pressure equalization before each intake stroke, combined with the streamlined flow attribute of PRV induction, reduces pumping loss. How important is the pumping loss? John C. Hilliard et al. (1984) explains, Assuming that the exhaust manifold pressure is about 1 inch of mercury above atmospheric pressure, an intake and exhaust pumping loss of approximately 7 hp can be calculated, which equals 0.072 of the total fuel supply. While this number is not absolutely accurate it does illustrate the importance of the loss. At 50 mph road load the pumping losses almost 40% of the engine brake power. (Hilliard and Springer, 1984). Blackmore and Thomas (1977) write, "... pumping loss varies from 3.5% at wide-open throttle (WOT) to nearly 100% for a full throttled idling engine". Accordingly, the pumping loss for a compact U.S. 10:1 compression-ratio car cruising at 40 mph is about 16% (Blackmore & Thomas, 1977). Pumping loss is highly dependent on the throttle position in the specific engine design. In any case, most of the pumping loss is recovered by PRV induction. Figure 7 compares the in-cylinder pressure of a conventional manifold to a PRV induction system. When the intake valve opens on a conventional manifold, the cylinder pulls air from a plenum under vacuum. In contrast, the PRV plenum is constantly at ambient pressure. The bottomdead-center cylinder pressures are identical for both intake systems because the engine torque is dependent on the amount of mixture at the end of the intake cycle. Since the pressure trace is much higher for PRV induction, there is a cumulative saving of work by the piston. The work saving is reflected in the fuel economy improvement demonstrated by early PRV prototypes. The benefits of using PRV induction can be further expanded for a turbocharged engine. During cruise, the conventional manifold will exhibit the same pressure profile. The profile does not change because the pressure at bottom dead center must be the same. Consequently, the plenum pressure must also be the same. The pressure differential between the turbocharger and plenum pressure is regulated by the throttle. In contrast, the PRV plenum pressure is set directly by the
PRV Induction
8
turbocharger (Figure 7). The PRV induction system combined with the turbocharger will require even less work to be done by the piston to draw the charge into the engine, implying an even greater fuel efficiency gain. Individual throttle plates are much less effective for reducing pumping loss for two reasons. First, individual throttle plates preclude mixing and vaporizing fuel upstream of the engine and, consequently, the charge cooling effect is lost. Second, a throttle plate obstructs flow whereas the Venturi enhances flow. Although the manifold pressure will be the same as a PRV induction system, the in-cylinder pressure profile of PRV is much better than a throttle plate. Figure 7 illustrates the difference in pressure profile of a conventional manifold, individual throttle plate and PRV induction.
Figure 7: cylinder pressure of a conventional manifold versus individual throttle plates and PRV induction; normally aspirated engine (conventional manifold data from Taylor, 1985, p. 345, PRV data from prototype measurement). 2.22 Improved Engine Torque A cooler charge mixture improves the torque of the engine because a cooler charge mixture has a higher density. The higher density provides for more fuel/air mixture to be in the cylinder at the time the intake valve closes. A computer simulation shows that a pintle-regulated Venturi induction completely vaporizes the fuel before the mixture reaches the engine head (Meyer, 2009). Taylor states that if fuel evaporation is complete before inlet valve closing, the charge temperature decreases by 44° F, based on octane thermal properties. The temperature decreases in good agreement with the authors computer simulations. Results from the chassis dynamometer test on the initial prototype are shown in Figure 11 (section 2.8). Since PRV induction increases torque, automakers can produce smaller and lighter vehicles to comply with the 2009 fuel economy standards to be phased in by 2016.
PRV Induction
9
2.23 Fuel Economy Derived from Pre-Vaporized Fuel Blackmore and Thomas (1977) reported that vaporized fuel improves the fuel economy of a spark ignition engine. The percentage improvement is reflected in Figure 8.
Figure 8: Economy Improvement from Vaporized Fuel, data from Blackmore & Thomas, 1977, p. 106
Taylor elaborates on fuel evaporation in his highly acclaimed book, The Internal Combustion Engine in Theory and Practice. Specifically, Taylor writes that although some of the fuel is vaporized from heat transfer with the cylinder wall, evaporation is seldom complete before the intake valve closes (Taylor, 1985, p. 185). Consequently, the efficiency is lost because work is required to evaporate fuel. 2.24 Reduced Greenhouse Gas Emissions CO2 emission is directly related to fuel consumption. Consequently, any improvement in fuel economy will decrease the CO2 emitted and the associated effect on global warming. Hydrocarbon and CO emissions are produced when the combustion process is not complete. Hilliard and Springer (1984) writes, "The major source of hydrocarbons found in the exhaust results from that portion of the fuel-air intake charge that is trapped during the compression stroke, and subsequent pressure rise during combustion, inside of crevices in the cylinder into which the main combustion flame cannot enter... In the exhaust port and exhaust manifold, a portion of the residual hydrocarbons are combusted to CO or CO2." (Hilliard and Springer, 1984, p. 61). PRV induction pre-vaporizes the fuel and, consequently, less fuel is trapped in the cylinder wall crevices thereby reducing hydrocarbon and CO emissions. 2.25 Ethanol-Fueled Engine Performance Expectation Ethanol is more difficult to disperse from a fuel injector because of the higher viscosity. Ethanol also has a lower vapor pressure and a higher heat of vaporization. Consequently, with a conventional fuel injection system, more of the fuel evaporation (in comparison to gasoline) will occur during the compression cycle. Conversely, the higher heat of vaporization of ethanol means that pre-mixing of ethanol by PRV lowers the charge mixture more than gasoline. The lower charge temperature and improved dispersion upstream of the engine, provided by PRV, is exactly what an ethanol-fueled engine needs for improved power. The physical properties of ethanol and gasoline are compared in Table 1.
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Table 1: Physical Properties of Gasoline and Ethanol (US Department of Energy) Heat of vaporization, Btu/lb @ Viscosity, Reid vapor pressure (100 degrees 60° F. mm2/s. F.), psi Gasoline 150 0.55 8-15 Ethanol 396 1.5 2.3 2.26 Initial Prototype Results Test results with the gasoline prototype have demonstrated a 21% increase in highway fuel economy with a stoichiometric air/fuel ratio and 35% increase in highway fuel economy within air/fuel ratio of 16.8. Torque is increased 15-20% on a normally aspirated engine fueled by gasoline (Figure 9). These results were obtained after a series of 10 prototypes were developed and tested over 5 years. Test results have been validated repeatedly over a 203 mile highway cycle is 65 mph, delivering 53 mpg at a stoichiometric tune to 58 mpg for a lean-burn tune.
Figure 9: Torque and Horsepower of a Gasolinefueled Normally Aspirated Engine with Conventional Manifold and PRV Manifold
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References Blackmore, D. R., & Thomas, A. (1977). Fuel economy of the gasoline engine. New York: John Wiley &, Inc. Energy efficiency and renewable energy news. (2008, March 12). US ethanol production totaled 6.48 billion gallons in 2007. Retrieved April 13, 2009, from http://apps1.eere.energy.gov/news/news_detail.cfm/news_id=11633. Energy Information Administration. (2009, February 1). 1 (Petroleum Basic Statistics). Retrieved April 13, 2009, from http://www.eia.doe.gov/basics/quickoil.html. Environmental Protection Agency. Fuel economy of motor vehicles. 40 CFR 600. Ferguson, C. R., & Kirkpatrick, A. T. (2001). Internal combustion engines – applied thermosciences. New York: Wiley. Gold, A. What it is, how it works. Retrieved from about.com: http://cars.about.com/od/thingsyouneedtoknow/a/directinjection.htm. Heywood, J. B. (1988). Internal combustion engine fundamentals. McGraw-Hill. Hilliard, J. C., & Springer, G. S. (1984). Fuel economy and road vehicles powered by spark ignition engines. New York, New York: Plenum publishing. Kelly, K. M. (2009, April 1). Ricardo's ethanol boosted direct injection. Automotive Design & Production, 121(4), 13. Meyer, J. M. E. A. (2005, March 22). US patent 6,868,830 (U. S. Patent & T. Office, Eds.). 6.868,830. Perry & Chilton. (1973). Chemical engineer's handbook. New York: McGraw-Hill. Stress Engineering Services Inc. Venturi mixer. Retrieved April 28, 2009, from Process Technology Group: http://www.processinnovation.com/venturi.html. Taylor, C. F. (1985). The internal combustion engine in theory and practice. Cambridge MA: MIT press. US Department of Energy. Alternative fuels and advanced vehicles data center. Retrieved April 26, 2009, from US Department of Energy: http://www.afdc.energy.gov/afdc/.
