Potential of Expansion Chamber Exhaust Pipes for Two-Stroke Powered Tools
Gerhard Zsiga, Robert Kerres, Matthias Bach and Klaus Fuoss
Porsche Engineering Services
ABSTRACT
Due to reduced emission limits for handheld gasoline powered tools worldwide, hydrocarbon emissions of two-stroke engines used for those applications need to be reduced drastically. In this paper, the potential of expansion chamber exhaust pipes to reduce hydrocarbon emissions generated by scavenging losses of the two-stroke engine is investigated. On a series production backpack blower engine, a box silencer is compared to a baseline expansion chamber and a modified expansion chamber. Also, as this is a very cost-sensitive market, possibilities to achieve different power levels from the same engine using expansion chambers of different stages of tune are investigated. The results show that using expansion chambers, power of the series production engine can be raised by 40 percent while still offering an advantage in emissions of 34 percent. Tuning an expansion chamber for the same peak horsepower as the box silencer gives an advantage in emissions by 62 percent. With the shown possibilities to achieve different power levels from the same engine using expansion chambers, the number of engine displacements from an engine family can be reduced saving cost.
INTRODUCTION
Expansion chambers for two-stroke engines are well known for decades [1]. However, on small industrial engines such as mowers, blowers, cutters and so on, they are not yet in series production. With increasing requirements in terms of emissions and cost for these kinds of engines, expansion chambers might offer an opportunity to reduce both cost and emissions.
Previous investigations on this subject have been carried out by Blair, Gustafsson and Jonsson in 1999 [2], 2001 [3] and 2006 [4], using a tuned pipe which is parallel for the most part.
There appears to be an even greater potential in terms of horsepower and emissions in the use of expansion chambers as
typically used on mid to high BMEP two-stroke applications, but experience using expansion chambers on these kinds of industrial engines is limited. As the market demands for a wide variety of different power levels on these kinds of engines, typically 10 to 20 percent apart from each other and covered by a single displacement each, the possibilities to deduce several power levels from an identical engine are investigated also.
ENGINE
For the investigation, a backpack blower engine is chosen which is available on the market for some years now. The displacement of the engine is 51.7 cc. As the range of displacement of handheld powered tools reaches from about 20 cc to 120 cc, this engine is on the lower side of the midfield.
The engine specifications are listed in Table 1.
Copyright © 2010 SAE International and Copyright © 2010 SAE Japan
2010-32-0011 20109011
Published 09/28/2010
Prior to the investigation the following three modifications are made:
1. The air-box is removed
As the body of the air-box and the blower housing are one piece, the air-box is removed and instead the engine is allowed to breathe via the remaining inlet pipe.
2. The exhaust port outlet diameter at the cylinder flange is reduced
For the use of expansion chambers, a favourable ratio between the area of the exhaust port window and the area of the exhaust port flange is about one. To achieve this, the outlet diameter of the exhaust port at the cylinder flange is reduced from 22mm to 14mm. Fig. 1 shows the modified exhaust port.
Table 1. Engine parameters of the test engine
Fig.1. modified exhaust port of the test engine 3. The standard exhaust of the engine is replaced with a conventional box silencer.
To reduce noise and emissions, the engine uses an exhaust pipe configuration which incorporates a tuned pipe as described by Blair [5]. The goal of this investigation is to show the potential of expansion chambers compared to box silencers commonly
used by the majority of manufacturers. Therefore, the original exhaust of the engine is replaced with a simple box silencer.
The box silencer used for the tests consists of an empty box whose volume is ten times the volume of the displacement of the engine. It is connected to the exhaust port via a 50 mm long pipe of 22 mm diameter. A water cooled pressure transducer which is used to measure the dynamic exhaust pressure is added about 50 mm from the piston skirt. The peak power with this exhaust is the same as with the standard exhaust. Figure 2 shows a schematic diagram and the leading dimensions of the box-silencer.
Fig.2. schematic diagram of the box-silencer
The fuel consumption is measured by a fuel balance and the exhaust gas emissions are analyzed using a Horiba Exsa Exhaust Gas Analyzer. The values for delivery ratio, charging efficiency and trapping efficiency are calculated values. The full load curves of the engine are investigated maintaining a carbon monoxide value of 5%. This corresponds to an air to fuel ratio of about 0.80 on this engine. Throughout the investigation, no further modifications are made to the engine itself.
Figure 3 shows the baseline power curves of the engine with the standard tuned pipe and the box silencer.
Fig.3. baseline power curves of the engine
BASELINE EXPANSION CHAMBER EXHAUST PIPE
To reach the target engine speed, it is necessary for the expansion chamber to have the correct tuned length. The tuned length is the length from the piston skirt to the end of the rear cone. In order to get the correct tuned length of the expansion chamber, it is necessary to know the exhaust gas temperature of the engine. The exhaust gas temperature of the engine can be measured prior to the design of the expansion chamber, but, due to different surface areas and different levels of break mean effective pressure (BMEP) which can be achieved with the expansion chamber in contrast to a box silencer, exhaust gas temperature can differ from measured starting values.
For the baseline expansion chamber the target engine speed for peak power is 6000 rpm, which is the same speed as with the standard exhaust. Using the formulas of Blair [5] as a starting point, a three-stage diffuser expansion chamber is designed.
The exhaust gas temperature is assumed to be 385° C, which
Fig.4. Dimensions of the baseline expansion chamber
Despite the total length of the baseline expansion chamber of 1240 mm it is possible to integrate an expansion chamber inside the blower housing. This subject is covered later on.
RESULTS OF THE BASELINE EXPANSION CHAMBER EXHAUST PIPE
The measurements of the expansion chamber in comparison to the box silencer show major potential. Figure 5 shows the comparison between the baseline expansion chamber and the standard box silencer in power and torque, as well as specific fuel consumption and specific emissions.
Although the exhaust gas temperature turned out to be slightly lower than expected, with peak power at 5700 rpm, significant
Fig.5. Power, Torque, Fuel-consumption and Emissions of the baseline expansion chamber and the box silencer
gives a total tuned length of 1000 mm. Figure 4 shows the dimensions of the baseline expansion chamber.
Fig. 6 shows Delivery Ratio, Charging Efficiency, Trapping Efficiency and Trapping Efficiency as a function of Delivery Ratio of the expansion chamber and the box silencer. It can be seen that the increase in power is due to an increase in delivery ratio and thus an increase in charging efficiency. As the expansion chamber starts getting out of tune, which is at about 6000 rpm, the difference in charging efficiency compared to the box silencer is starting to become smaller. This is due to an increase in trapping efficiency of the engine with the expansion chamber below 6000 rpm. Trapping efficiency as a function of delivery ratio shows a potential for a general increase in trapping efficiency of about 5 percent for the baseline expansion chamber when it is in tune.
Fig.6. Delivery Ratio, Charging Efficiency, Trapping Efficiency and Trapping Efficiency as a function of Delivery Ratio of the expansion chamber and the box silencer
The pressure traces measured at the exhaust port are displayed in Fig. 7. The average level of the pressure curve of the engine benefits can be observed. Power rises from 2.1 kW to about 2.8 kW, which is a 33% increase whilst the specific emissions remain about the same. Break specific fuel consumption (BSFC) is reduced within the range where the expansion chamber is in tune. The torque of the engine increases about 20% in peak value, remaining above the torque output of the box silencer throughout the entire speed range.
with the box silencer is about 0.1 bar higher than atmospheric pressure. This is because the outlet of the box silencer is restricted to reach the power level of the standard exhaust that came with the engine. The pressure drop at about 130° crank angle is caused by a reflection of the outgoing pressure wave at the back wall of the box silencer. In comparison, the engine with the expansion chamber shows two major differences.
At about 160° crank angle before TDC, the expansion wave which travels along the diffuser creates pressure values of about 0.2 bar below ambient pressure in peak value at the exhaust port. This vacuum supports scavenging and thus the delivery ratio of the engine increases. Another difference is that there is an increase in pressure starting at about 30° crank angle before exhaust port closure. This is due to the reflection of the outgoing pressure wave at the end cone of the expansion chamber which returns to the exhaust port at about that time.
Fresh charge, which has already left the cylinder is pushed back into the cylinder. This is why trapping efficiency is improved using expansion chambers. As there is still a pressure of about 0.3 bar above ambient conditions at exhaust port closure, the full potential of the plugging pulse is not made use of at this engine speed. The expansion chamber is slightly out of tune.
Fig.7. Pressure history at the exhaust port of the expansion chamber and the Box silencer at 6000 rpm
OPTIMIZED EXPANSION CHAMBER EXHAUST PIPE
The operating speed of a blower is determined by the combination of the aerodynamic resistance of the blower fan in combination with the engine power. The engine speed where the power curve of the engine intercepts with the ventilator map of the blower fan will be the maximum speed of the engine. As an increase in power will allow the blower fan to rotate at a higher speed, the expansion chamber also has to be tuned to work at that higher speed. The new target speed for
peak power is about 1000 rpm higher than with the baseline expansion chamber. This gives a new tuned length of 810 mm in total. The maximum diameter at the middle section is kept the same as before. To enforce the negative pressure pulse at the exhaust port which supports scavenging, the diffuser mean angle is slightly increased. This is done by reducing the diameter of the parallel section at the beginning of the expansion chamber (the header section) from 19 mm to 16.5 mm. Fig. 8 shows the dimensions of the optimized expansion chamber. Fig. 9 shows a comparison between the baseline and the optimized expansion chamber.
Fig.8. Dimensions of the optimized expansion chamber
The tuned length of the optimized expansion chamber is 810 mm in total, which alters engine characteristics by a great extend. Fig. 10 shows Power, Torque, BSFC and specific emissions of the baseline and the modified expansion chamber.
Maximum power increases slightly. The maximum torque of the engine is reduced by about 12 percent, but stays within 85 percent of the maximum value throughout the whole speed range, now. BSFC drops to about 430 g / kWh, which is an improvement by about 12 percent. Specific emissions are reduced from 125 g / kWh to 82 g / kWh, an improvement of 34 percent.
Fig. 11 shows Delivery Ratio, Charging Efficiency, Trapping Efficiency and Trapping Efficiency as a function of Delivery Ratio of the baseline expansion chamber and the optimized expansion chamber. The delivery ratio achieved with the optimized expansion chamber is less compared to the values achieved with the baseline expansion chamber. This is due to the engine running into its time area limits for the higher speed
Fig.10. Power, Torque, Fuel-consumption and Emissions of the baseline and the optimized expansion chamber Fig.9. overlay of baseline expansion chamber (black) with optimized expansion chamber (red)
range. Nevertheless, maximum power increased, which is due to the higher charging efficiency at higher speed. The increase in charging efficiency is caused by the increase in trapping efficiency, which is about the same at engine speeds up to 6000 rpm, but rises to about 82,5 percent at engine speeds above 6000 rpm. A 10 percent increase in maximum values. Trapping efficiency as a function of delivery ratio shows that the gradient of the trapping curve is slightly steeper with the optimized exhaust pipe.
The pressure traces at the exhaust port at an engine speed of 6900 rpm are shown in Fig. 12. A lower amplitude after the exhaust port opening with the baseline expansion chamber can be seen. This is due to the lower BMEP value at this engine speed with the baseline expansion chamber. The depression created by the optimized expansion chamber is about 50%
higher than with the baseline expansion chamber, reaching a peak value of about 0.34 bar. This is caused by the modifications in diffuser mean angle and header diameter. Furthermore, the
Fig.11. Delivery Ratio, Charging Efficiency, Trapping Efficiency and Trapping Efficiency as a function of Delivery Ratio of the baseline and the optimized expansion chamber
plugging pulse of the baseline expansion chamber arrives too late to provide adequate plugging, whereas the plugging pulse of the optimized expansion chamber arrives earlier and thus makes plugging more effective. However, the pressure at the exhaust port at the exhaust port closure is still above ambient pressure, which means that there is still potential to further optimize the plugging pulse. Using a steeper end cone is one attempt to do so. Further information on the subject of optimizing expansion chambers are found in the papers by Kee et al. [7] and Cartwright and Fleck [8], for example.
ACHIEVEMENT OF DIFFERENT POWER LEVELS
On engine families, which are common on handheld powered tools, the difference between the engines in terms of horsepower and displacement is usually in the range of 10 to 20 percent. If different power levels are achieved with an expansion chamber, this can reduce the number of different displacements needed to meet demanded power levels for the engine family, which saves cost. There are different possibilities in order to achieve different power levels using expansion chambers, of which two have are investigated further and can also be combined.
RESTRICTING THE EXPANSION CHAMBER HEADER DIAMETER
Restricting the header diameter prevents the engine from making peak power. Fig. 13 shows the effect of this modification performed on the optimized expansion chamber.
The header diameter is reduced from 16.5 mm to 14 mm and 15 mm, respectively. At lower speeds, where the header is not restrictive yet, the power curve is about the same as with a non-restrictive header. Depending on the grade of restriction, the peak power speed is reduced also. Emissions and fuel consumption increase slightly with this modification.
RESTRICTING THE EXPANSION CHAMBER OUTLET PIPE
DIAMETER
Restricting the outlet pipe diameter of an expansion chamber and thus increasing the expansion chamber back-pressure reduces the engine’s delivery ratio. This effect has also been described briefly by Fleck et al. [6]. This way, power curves of different levels can be displayed. The shape of the power curves will be similar, only their level will be lower. Fig. 14
Fig.12. Pressure history at the exhaust port of the baseline and optimized expansion chamber at 6900 rpm
shows the effect of this modification performed on the optimized expansion chamber at an engine speed of 6900 rpm.
The engine parameters are shown as functions of the exhaust back pressure. As power and torque decrease, BSFC remains within 5 percent of the starting value of 440 g / kWh. Specific emission values are reduced by about 50 percent in total.
At a backpressure value of 0.32 bar, power is about 2 kW, which is the same power as with the baseline box silencer at that engine speed (Fig. 4). However, emissions at this operating point are 125 g / kWh with the box silencer and 48 g / kWh with the expansion chamber. A 61 percent benefit. Furthermore, there is a reduction in BSFC of 18 percent with the expansion chamber at this operating point, compared to the box silencer.
Fig. 15 shows delivery ratio, charging efficiency and trapping efficiency as a function of exhaust backpressure, as well as trapping efficiency as a function of delivery ratio. As delivery ratio and charging efficiency decrease, trapping efficiency rises above 90 percent in value.
Compared to the box silencer from the baseline measurements (Fig.5), trapping efficiency increased by 12 percent from 77.5 percent to 89.5 percent. A 15 percent benefit. The increase in trapping efficiency is the reason for the lower emissions and lower BSFC which are measured with the expansion chamber setup. Also due to the increase in trapping efficiency, delivery ratio is less with the expansion chamber setup whereas charging efficiency is about the same at 6900 rpm.
The reason for the increase in trapping efficiency is shown in Fig. 16, in which the pressure traces of the optimized expansion chamber with and without restriction are displayed. As exhaust backpressure increases, the average value of the pressure traces is also moved to a higher level. This enhances the plugging pulse and thus increases trapping efficiency. But it also hinders supercharging by the vacuum created by the diffuser of the expansion chamber, which is why delivery ratio decreases.
Fig.13. the effect of restricting the header diameter on the optimized exhaust pipe
Fig.14. the effect of restricting the outlet pipe diameter on the optimized exhaust pipe at 6900 rpm
Fig.15. the effect of restricting the outlet pipe diameter on the optimized exhaust pipe at 6900 rpm
Fig.16. pressure history at the exhaust port with and without restriction
INTEGRATION OF THE EXPANSION CHAMBER INTO THE BLOWER
The expansion chamber can be fit into the blower housing nicely by curling it in one plane. Other types of two-stroke powered tools usually operate at a higher speed, so an expansion chamber for them would be shorter. Also, the length of the tailpipe showed virtually no effect on engine performance which allows for another reduction in total length of about 24 percent. The weight of the expansion chamber is less than the standard exhaust which leaves potential to include a silencer. Fig. 17 shows the expansion chamber integrated into the blower.
Fig.17. the expansion chamber integrated into the blower
CONCLUSIONS
In this study, the potential of expansion chamber exhaust systems for two-stroke powered tools was investigated. Two expansion chambers, a baseline version and an optimized version were applied to a series production engine and compared to a box silencer as commonly used on this kind of engines. The following results were achieved.
Performance of the engine was increased up to 40 percent, lowering specific emissions by 34 percent. A maximum reduction in specific emissions by 62 percent was achieved.
Trapping efficiency was raised up to 15 percent in maximum value. BSFC was reduced up to 18 percent.
Furthermore, two possibilities to achieve lower power levels, which can also be combined, were shown. This way, the number of engine displacements needed to display the demanded power levels of an engine family can be reduced and thus cost can be reduced also. As a side effect, using this method will reduce vibration and weight of the engines because bigger displacement engines would be replaced by smaller displacement engines with expansion chambers.
REFERENCES
1. Huelsse, W.A., “Investigation and Tuning of the Exhaust System of Small Two-Stroke Cycle Engines,” SAE Technical Paper 680469, 1968.
2. Blair, G.P., “Design and Simulation of Engines: A Century of Progress,” SAE Technical Paper 1999-01-3346, 1999.
3. Gustafsson, R.U.K., Blair, G.P., and Jonsson, B.I.R.,
“Reducing Exhaust Emissions and Increasing Power Output Using a Tuned Exhaust Pipe on a Two-Stroke Engine,” SAE Technical Paper 2001-01-1853, 2001.
4. Gustafsson, R.U.K., “A Practical Application to Reduce Exhausts Emissions on a Two-Stroke Engine with a Tuned Exhaust Pipe,” SAE Technical Paper 2006-32-0054, 2006.
5. Blair, G.P., “Design and Simulation of Two-Stroke Engines,” SAE International, Warrendale, PA, ISBN 978-1-56091-685-7, 1996.
6. Fleck, B.J., Fleck, R., Kee, R.J., Chatfield, G.F. et al.,
“Validation of a Computer Simulation of a High Performance Two-Stroke Motorcycle Racing Engine,” SAE Technical Paper 2004-01-3561, 2004.
7. Kee, R.J., O’Reilly, P.G., Fleck, R., and McEntee, P.T.,
“Measurement of Exhaust Gas Temperatures in a High Performance Two-Stroke Engine,” SAE Technical Paper 983072, 1998.
8. Cartwright, A. and Fleck, R., “A Detailed Investigation of Exhaust System Design in High-Performance Two-Stroke Engines,” SAE Technical Paper 942515, 1994.
CONTACT INFORMATION
Gerhard Zsiga
Porsche Engineering Services GmbH [email protected]
DEFINITIONS / ABBREVIATIONS
CO carbon monoxide
EO Exhaust opens
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ISSN 0148-7191 doi: 10.4271/2010-32-0011
Positions and opinions advanced in this paper are those of the author(s) and not necessarily those of SAE. The author is solely responsible for the content of the paper.
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EC Exhaust closes
TDC Top Dead Center
ATDCAfter Top Dead Center
BTDCBefore Top Dead Center
BSFCBreak Specific Fuel Consumption
doi: 10.4271/2010-32-0011
