AU584087B2 - Two-stroke engine variable tuned exhaust system - Google Patents
Two-stroke engine variable tuned exhaust systemInfo
- Publication number
- AU584087B2 AU584087B2 AU42305/85A AU4230585A AU584087B2 AU 584087 B2 AU584087 B2 AU 584087B2 AU 42305/85 A AU42305/85 A AU 42305/85A AU 4230585 A AU4230585 A AU 4230585A AU 584087 B2 AU584087 B2 AU 584087B2
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- exhaust
- exhaust port
- engine
- speed
- port
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- 239000002826 coolant Substances 0.000 claims description 41
- 238000002485 combustion reaction Methods 0.000 claims description 32
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- 230000001276 controlling effect Effects 0.000 claims description 2
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 63
- 239000007789 gas Substances 0.000 description 49
- 238000002347 injection Methods 0.000 description 26
- 239000007924 injection Substances 0.000 description 26
- 238000012546 transfer Methods 0.000 description 23
- 239000000446 fuel Substances 0.000 description 9
- 230000002000 scavenging effect Effects 0.000 description 9
- 230000008901 benefit Effects 0.000 description 7
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- 239000004215 Carbon black (E152) Substances 0.000 description 2
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- 229930195733 hydrocarbon Natural products 0.000 description 2
- 150000002430 hydrocarbons Chemical class 0.000 description 2
- 230000006872 improvement Effects 0.000 description 2
- 230000036316 preload Effects 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 239000007921 spray Substances 0.000 description 2
- 235000003930 Aegle marmelos Nutrition 0.000 description 1
- 244000058084 Aegle marmelos Species 0.000 description 1
- 235000014676 Phragmites communis Nutrition 0.000 description 1
- 230000002411 adverse Effects 0.000 description 1
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- 238000010276 construction Methods 0.000 description 1
- 239000000498 cooling water Substances 0.000 description 1
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Classifications
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F02—COMBUSTION ENGINES; HOT-GAS OR COMBUSTION-PRODUCT ENGINE PLANTS
- F02B—INTERNAL-COMBUSTION PISTON ENGINES; COMBUSTION ENGINES IN GENERAL
- F02B61/00—Adaptations of engines for driving vehicles or for driving propellers; Combinations of engines with gearing
- F02B61/04—Adaptations of engines for driving vehicles or for driving propellers; Combinations of engines with gearing for driving propellers
- F02B61/045—Adaptations of engines for driving vehicles or for driving propellers; Combinations of engines with gearing for driving propellers for marine engines
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- Engineering & Computer Science (AREA)
- Ocean & Marine Engineering (AREA)
- Chemical & Material Sciences (AREA)
- Combustion & Propulsion (AREA)
- Mechanical Engineering (AREA)
- General Engineering & Computer Science (AREA)
- Characterised By The Charging Evacuation (AREA)
Description
TWO- STROKE ENGINE VARIABLE TUNED EXHAUST SYSTEM
This invention relates to internal combustion engines, and particularly to exhaust systems of such engines. The invention is applicable to many engine types, be they of the spark ignition or compression ignition type, configured with reciprocating, rotary, or rotary oscillating pistons, operate on the 2-stroke cycle. However, the invention is considered to have special applicability to 2-stroke reciprocating engines. It is well known in the operation of inboard engines in pleasure boats to dispose to the engine's cooling water by dumping it in the exhaust pipe well downstream of the engine. This is a convenient route of disposal and also serves to cool the exhaust pipe as a fire safety measure.
It is also well known that two-stroke engines benefit greatly by the use of what is commonly known as a "tuned exhaust system".
The basic mechanical simplicity of the two- stroke crankcase-scavenged engine belies the complex tansient gas flow characteristics inherent in the design. Engine performance can be greatly improved by utilizing a tuned exhaust system to create a favourable pressuretime history at the engine exhaust port, albeit over a relatively discrete speed range. Output is compromised at other speeds, due to unfavourable pressure fluctuations created by the same exhaust at off-tune conditions. The tuned exhaust system only influences the gas flow characteristics during the period that the exhaust port of the engine is open.
An insight into the gas flow processes occurring can be gained by considering the operation of the engine during one scavenge cycle. During the period from exhaust port opening (EPO) to transfer port opening (TPO), the exhaust products in the engine cylinder, being at high temperature and pressure, are exhausted through the exhaust port. While the exhaust port is open, the
transfer ports begin to open, and the fresh charge which has been compressed in the crankcase discharges through the transfer ports into the relatively quiescent engine cylinder. Direct loss of charge out of the exhaust port, sometimes referred to as cross-over loss or short-circuiting, is kept to a minumium by the shape and direction of discharge of the transfer ports. In most modern high-output two-stroke engines, the scavenging process that takes place while both the exhaust and transfer parts are open result in some mixing of the fresh-charge and residual exhaust products, resulting in both dilution of the fresh charge with exhaust gas and also a lowering of charge density due to the high temperature of the residual exhaust gas. This transfer discharge continues until the pressures in the cylinder and crankcase have equalized.
As the piston begins its upwards stroke (after bottom dead centre), a number of factors combine to make it possible for the diluted fresh charge to escape through the exhaust port and to flow back through the transfer ports.
These factors include:
1. gas motion in the cylinder towards the exhaust port persists; and 2. continued movement of the piston upwards displaces fresh charge out the exhaust port.
After transfer port closure (TPC), charge loss is possible only via the exhaust port. The mass of fresh-charge trapped at exhaust port closure (EPC) is indicative of the torque output of that particular engine cycle, and is determined by the charge pressure, temperature, and charge purity of the gas present.
The pressure at EPC is determined, firstly, by the overall back pressure levels inherent in the exhaust manifold, secondly, by the magnitude of any fluid-dynamic pressure fluctuations present at the port, and thirdly (although to a lesser extent) any over-pressure caused by
the exhaust and transfer ports acting as a restricting orifice to out-flow. The temperature of the trapped charge is largely specified by the ambient temperature; the level of exhaust gas retention and heat transfer from the intake tract, crankcase, transfer-ports, cylinder liner, and piston etc. via forced convection and radiation.
As previously stated, the two-stroke engines can benefit from a tuned exhaust system, and a tuned, expansion chamber style exhaust system can be geometrically divided into five sections; as illustrated in Fig. 1, an exhaust header pipe 10, a divergent cone 11, an exhaust chamber 12, a convergent cone 13, and an exit pipe 14. The function of the tuned exhaust system is to establish a low pressure at the exhaust port while that port is open and before the transfer port closes, and to establish a higher pressure at the exhaust port after the transfer port is closed and before the exhaust port closes.
Upon the initial opening of the exhaust port, the sudden release of the pressure in the engine cylinder forms a wave front that travels at high speed through the exhaust port and along the exhaust system. Upon the wave front reaching the divergent cone 11, the expansion of the gas creates an inverted or negative wave front (subatmospheric) which travels back to the exhaust portion. The relevant portions of the exhaust system, namely the header pipe 10, and divergent cone 11 are designed so the inverted wave front will reach the exhaust port while it is open. In this way an increased pressure drop is provided through the port, to assist in scavenging of the exhaust gas from the cylinder. This negative wave front is commonly referred to as a "scavenging pulse". As the initial wave front, somewhat reduced in strength, continues to move along the exhaust system it encounters the converging cone 13, which functions
somewhat as a closed end and causes the wave front to rebound as a positive wave front. Upon this positive wave front reaching the exhaust port, it provides a high pressure to resist the outflow of the gas from the cylinder, and may even reverse the flow back into the cylinder. The formation of the rebound positive wave is dependent on thedesign of the expansion chamber 12 and converging cone 13, and this design is selected so the returning positive wave front will reach the exhaust port after the transfer port is closed but before the exhaust port closes. The returning positive wave is commonly referred to as a "plugging pulse".
A proper combination of these exhaust system parameters utilizes the scavenge pulse between EPO and TPC to assist a fresh charge of fuel/air mixture to be introduced into the cylinder, and a plugging pulse between TPC and EPC to retain this charge in the cylinder while producing a super-charging effect. A typical pressure-time history at the exhaust port created by an expansion chamber at the tunedspeed is shown in Figure 2 of the accompanying drawings.
The benefits available from this favourable exhaust port pressure-time history can be summarized as follows : 1. The transfer of fresh charge from the crankcase to the cylinder through the transfer port is assisted by the scavenging pulse.
2. Extra charge, in addition to that pumped through the crankcase by the piston displacement, can be drawn into the cylinder by the low-pressure scavenging pulse. An increase in delivery ratio invariably results in an increase in charge purity due to the mixing process which occurs in the cylinder.
3. Charge loss through the exhaust port in the period from TPC to EPC is minimized by the plugging pulse. Under certain conditions, charge which has moved out of the exhaust port into the exhaust system may be
forced back into the cylinder by this pulse. This super- charging effect (cylinder pressure at EPC above ambient) and subsequent increased trapping efficiency, results in both, improved fuel consumption and higher power output.
4. The limitation of cross-over losses results in a large reduction in hydrocarbon emissions (generally the main source of hydro-carbon emissions in a homogenous-charge two-stroke engine is short-circuiting).
The timely arrival of these exhaust gas pressure pulses at the exhaust port is directly affected by: the speed of the engine, the geometric length of the various sections of the exhaust system, and the speed at which the pressure pulses propagate within the exhaust system. Only at the tuned speed of the exhaust, do the pulses arrive at the exhaust port at the correct timing. Engine performance is considerably compromised at offtune conditions. A characteristic of a two-stroke engine fitted with with a tuned exhaust system is that of a rather "peaky" torque curve, that is, with peak torque occurring at the tuned speed of the exhaust system and torque dropping off rapidly on either side of that speed. Two regimes of off-tune conditions of engine operation should be appraised; above, and below, the tuned speed.
At speeds above the tuned speed, the scavenging pulse extends into the period from TPC to EPC and the plugging pulse arrives back at the exhaust port after exhaust port closure, thus being ineffective. Thus both promote charge loss through the exhaust port, and also lowers charge density at EPC. This results in poor fuel consumption and a rapid drop off in power above the tuned speed.
Below the tuned speed, the plugging pulse arrives at the exhaust port before the transfer port close to exhaust port close period, giving rise to several detrimental effects :-
1. it interferes with the scavenging process; and
2. it promotes mixing of residual exhaust gas and fresh charge resulting in low charge purity. The design of a tuned exhaust system is by necessity a compromise between performance at the tuned speed, and of that at off-tune conditions. The rate at which the diverging and converging sections of the exhaust system change cross-section determines the magnitude and duration of any pressure fluctuations. In general there is a trade-off between magnitude and duration. A long duration wave tends to spread an engine's torque band; whilst a short duration wave with high amplitude is best for maximum torque at the tuned speed. In general, the spread of torque required of an engine is the limiting criteria for tuned exhaust system design, and hence some trade-off in maximum output is made to ensure an adequately broad spread of torque.
The tuned length of the exhaust system determines the range of speeds for which beneficial pressure fluctuations occur. The speed at which any pressure pulse propagates is determined directly by the velocity of the wave front. This velocity is the vector addition of the local fluid velocity and the acoustic velocity in the medium at the local fluid conditions. Typical full-load values for these velocities are 150 m/s and 600 m/s respectively.
The acoustic velocity is proportional to the square root of the fluid temperature. Modulation of the temperature of the gas can, therefore, provide a means for control over the time of arrival of the pressure pulses at the exhaust port.
It is the object of the present invention to provide control of the temperature of the gas in the exhaust system of a two stroke engine so as to increase the range of engine speeds over which desirable pressure conditions are created at the exhaust port.
With this object in view there is provided a method of operating an internal combustion two stroke
engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed comprising delivering the gaseous products of combustion from the cylinder to an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned to establish when the engine is operating at a selected speed a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a predetermined pressure pulse before the exhaust port closes, admitting a coolant to the exhaust passage at engine speeds below said selected engine speed, and regulating the quantity of coolant admitted in relation to the engine speed over a range of engine speeds to maintain a pressure pattern to create said predetermined pressure pulse over said speed range .
Conveniently the coolant is admitted over the engine speed range from approximately idle speed to the selected engine speed, and the regulating of the quantity of coolant is exercised over a lesser speed range with the coolant supply terminating at the selected engine speed.
The regulation of the coolant supply is preferably commenced at an engine speed determined by the formula: NL = NT (Ta/Tm)0.5 wherein NL = Engine spped at commencement of coolant regulation NT = Selected engine speed Ta = Temperature of coolant supply Tm = Buck exhaust gas temperature at maximum load and speed The rate of coolant supply is reduced at a
substantially steady rate so that the supply is terminated at the selected engine speed that the exhaust passage is designed to provide the desired pressure pattern. It is to be understood that without coolant addition the exhaust passage may provide the desired pressure pattern over a relatively narrow range of engine speed and the supply of coolant may be terminated within that range preferably in the lower end of the range.
The rate of supply of the coolant may be generally constant up to the speed at which the reduction in the coolant supply commences, since that speed is preferably the speed at which the exhaust passage will create the required pressure pattern with the exhaust gas at substantially the temperature of the coolant supply. Accordingly, below that speed further cooling of the exhaust gas cannot be conveniently achieved. The coolant is normally water that will in most situations be at an ambient temperature. It is to be understood that at lower speeds the rate of coolant supply may be progressively reduced since the quantity of exhaust gas reduces, and the coolant required to reduce the temperature thereof to ambient temperature also reduces .
In accordance with another aspect of the present invention, there is provided a method of operating an internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed; comprising delivering the exhaust gas from the exhaust port into an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned relative to the exhaust port to establish, when the engine is operating in a first selected speed range, a pressure pattern in
the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a low pressure pulse before the inlet port closes and a high pressure pulse after the inlet port has closed and before the exhaust port has closed, and admitting coolant to the exhaust passage to control the temperature of the gas in the exhaust passage over a second selected speed range below said first selected speed range to thereby maintain said pressure pattern over said second selected speed range .
There is also provided by this invention an internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open and closes before the exhaust port is closed, an exhaust passage communicating with the exhaust port. The exhaust passage is dimensionally proportioned to establish, when the engine is operating at a selected speed, a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a predetermined pressure pulse before the exhaust port closes. Means are provided to supply a coolant to the gas in the exhaust passage at engine speeds below said selected engine speed, and to regulate the quantity of coolant admitted in relation to engine speed over a range of engine speeds to maintain said pressure pattern over said range of engine speeds.
Conveniently, the coolant is supplied over a speed range from idle speed to said selected engine speed.
The invention will be more readily understood from the following detailed description with reference to the accompanying drawings wherein :
Fig. 1 diagrammatically shows simplified a typical 2-stroke engine with a tuned exhaust system
fitted thereto.
Fig. 2 shows the pressure-time history at the exhaust port while open when a tuned exhaust system fitted with the engine operating at the tuned engine speed. Fig. 3 is a plot showing the amount of water injected into the exhaust system as a function of engine speed.
Fig. 4 shows plots of exhaust pressure just downstream of the exhaust port against engine crank angle, at an engine speed of 3000 R.P.M. and under part load, for the engine with and without water injection into the exhaust system.
Fig. 5 shows plots of engine torque at wide open throttle against engine speed for the engine with and without water injection into the exhaust system.
Fig. 6 shows torque and BSFC results from operating the engine at wide open throttle over a range of speeds with and without water injection into the exhaust system. Fig. 7 shows plots of BSFC against torque for the engine when run on an approximate boat load power requirement curve.
Fig. 8 is a map of percentage change in fuel consumption between water injected and non-injected standard marine engine.
Fig. 9 is a map similar to Fig. 8 with a redesigned water injected exhaust system.
Fig. 10 is a section view of a leg of an outboard marine engine including portion of the exhaust system illustrating one embodiment of the invention.
Fig. 11 is a diagramatic representation of a water flow control device suitable for use in the controlling the mass flow of water to the exhaust system.
Referring to Fig. 1 the conventional two stroke engine 5 has a cylinder 6 in which the piston 7 reciprocates. The wall of the cylinder 6 has formed therein an exhaust port 8 and a transfer port 9. The transfer port
9 communicates through the passage 15 with the crankcase 16. Induction ports 17 are provided with reed valves 18. The engine operates on the conventional crankcase compression two stroke cycle and shall not be described in detail as it is common knowledge to those skilled in the art.
The transfer port and exhaust port are opened and closed by the piston as it reciprocates in the cylinder, and a typical port timing is Crank Angle Degrees
Bottom Dead Centre 0
Transfer Port Closed 60
Exhaust Port Closed 90
Top Dead Centre 180 Exhaust Port Opens 270
Transfer Port Opens 300
Extending from the exhaust port 8 is an expansion chamber type exhaust system designed to provide tuned performance as previously described. The designing of such an exhaust system to provide tuned performance at a selected engine speed is well known and will not be further described herein.
Typical dimensions of such an exhaust system having a tuned speed of 4000 R.P.M. is Length Dia. Angle
Header Pipe 385 mμ 54.7 mμ 0°
Diverging Cone 505 3° Half
Angle
Expansion Chamber 117 107.7 0°
Converging Cone 316 -6°
Half Angle
Exit Pipe 262 33.9
An exhaust system of this design would provide enhanced torque output over the engine speed range of
3500 to 4600 R.P.M.
As previously explained, changes in the exhaust
gas temperature will vary the acoustic velocity of the exhaust gases, and thus the pressure-time pattern of the gas may be controlled by varying the gas temperature. Thus by varying the gas temperature by the introduction of a coolant, such as water, to the exhaust system the range of engine speed over which the exhaust system will perform as a tuned system may be extended downwardly.
At engine speeds below the tuned speed of the exhaust system, water introduced to the exhaust near the cylinder exhaust port enables the temperature of the exhaust gas to be lowered through evaporation of the water droplets and heat transfer from the gas. Modulation of the exhaust gas temperature to achieve a commensurate modulation of the acoustic velocity is effected by modulation of the mass flow of water, the latter being controlled in response to engine speed such that the exhaust temperature at the relevant speed will give an acoustic velocity so the exhaust system is in tune. With the pressure-time pattern of the exhaust gas controlled in this matter, the scavenging pulse and plugging pulse as previously discussed will arrive at the exhaust port in the desired relation to the time of the opening and closing of the transfer and exhaust ports of the engine. To achieve this, the mass flow of water is gradually reduced and then stopped as the engine speed approaches that of the tuned speed of the exhaust system.
A typical water mass flow to engine speed relationship is shown in Fig. 3 for an engine having an exhaust system designed to a natural tuned speed of 4000 R.P.M.
Over the engine speed range of 1000 R.P.M. to 2500 R.P.M. water was injected at a slightly increasing rate up to 18g/sec. Between engine speeds 2500 R.P.M. and about 3800 R.P.M. the injection rate varied from
18g/sec to zero along the line shown. Above 3500 R.P.M. there was no water addition. Calculations suggest that
below 2500 R.P.M. the water rate required to reduce the exhaust temperature to approximately ambient decreases slightly with speed as the graph indicates. At 2500 R.P.M. the water addition of 18g/sec was calculated as optimum. Although the actual tuned speed of the exhaust was 4000 R.P.M. it was considered that 3800 R.P.M. was sufficiently close to the tuned speed that no modification of the sonic characteristics of the exhaust gas was required beyond that speed and hence water injection was ceased. At and above 4000 R.P.M. water injection would have a detrimental effect.
The rate of decrease of the supply of water to the exhaust system is a straight line if engine speed is the only factor considered, however in practice other factors should be taken into consideration. In practice the exhaust gas temperature may rise with engine speed, and also with increase engine load. Also changes in ignition timing will influence exhaust gas temperature. The efficiency of the engine cooling system, particularly any external exhaust system cooling, and variations therein with engine speed are also relevant.
These factors account for the somewhat exponential form of the plot of water rate against speed in Fig. 3, which was developed for the particular engine from test results such as those later referred to in respect of Figs. 8 and 9.
Fig. 4 shows comparative exhaust system conditions of a water injected and non injected system. Plot A is for operation without water injection whereas plot B is the same exhaust system with water injection as described above. The pressure measurements were taken from a pressure transducer placed in the exhaust system slightly upstream of the water spray. The improved exhaust pressure characteristics with water injection are apparent from the curves when considered in relation to the engine port timing, this timing being as stated in respect of the engine shown in Fig. 1. These plots were
taken with the water injection history as illustrated in Fig. 4 with five water spray nozzles each of 0.9 mm diameter located approximately 30 cm from the exhaust port. Fig. 5 shows comparative torque figures for water injection and non injection exhaust systems, the two plots were obtained by taking respective readings at 1000, 2000, 3000 and 4000 R.P.M. Curve C1, C2, C3, C4 shows the results for the engine without water injection. Curve D1, D2, D3, D4 shows the results with water injection when water flows rates are used of 18g/sec at 1000 R.P.M., 18g/ sec at 2000 R.P.M., 9g/sec at 3000 R.P.M. and 18g/sec at 4000 R.P.M. It should be appreciated that D4 is not meant to be a point in real life operation of an engine, as at 4000 R.P.M. the tuned speed of the exhaust system, there would preferably be no water injection. Point D4 is shown to demonstrate the adverse effect of water injection at the tuned speed of the exhaust expansion chamber. The operation of an engine correctly operated with the required variation of water injection to the exhaust would be generally in accordance with curve C1, F, D2, D3, E, C4.
Figs. 6 and 7 are each self-explanatory and indicate the improved fuel economy obtained with the controlled injection of water to the tuned exhaust system over the normal operating speed range of the engine.
It is to be appreciated that as the controlled injection of the water into the exhaust system improves the torque of the engine over a wide speed range, it is possible to further increase the peak power output of the engine by other changes to the engine. Firstly, the expansion chamber design can be optimized to produce a higher torque, although at the cost of a peakier torque curve, and the torque curve can be flattened, without lowering the highest value, by water injection to the exhaust. Secondly, a more radical port timing, which could not previously be used because of poor low speed
performance, can be incorporated to allow peak output to be increased.
A manufacturer of engines can thus use the present invention to do one of three things :- 1. Incorporated onto an existing engine with its existing exhaust system, the invention offers a broader power band with the same specific output, and exhibits considerable gains in fuel economy in the previous off-tune speed range, with no change at the tuned speed of the exhaust.
2. For an existing engine, with a redesigned exhaust system the invention can achieve a higher specific output with torque curves of the same nature as the original engine. This would exhibit benefits in fuel economy throughout the load speed range.
3. The invention enables the construction of engines with port timings not otherwise considered viable.
Experimental data demonstrating the improvement in engine performance possible with the injection of water to an exhaust system shown in Figures 8 and 9.
In general, the propeller characteristics are chosen so that at Wide Open Throttle, under steady state conditions, the motor operates at or near the max. operating speed specified by the manufacturers. These load-speed characteristics are commonly known as the boat-load curve. The steady state load that the engine sees for any speed is specified by the following equation. Torque = C(N)2.5 where C - a constant;
N - the speed of the engine; The constant C can be determined by the following equation:
C = (Tm/Nm)0.4 where Tm = Wide open throttle torque at maximum operating speed Nm = Maximum operating speed
Fig. 8 shows the percentage change in brake specific fuel consumption, between a standard 60 HP outboard marine engine and the same engine fitted with water injection into the manufacturer's exhaust system on a water mass flow verseas engine speed plot.
Fig. 9 shows the further improvement that may be obtained in fuel consumption with the same engine fitted with a redesigned exhaust system to take full advantage of the wide band of high torque obtained with the water injection.
Fig. 10 is a sectional view of the exhaust passage of a conventional outboard marine engine unit, as usually incorporated in the leg that extends between the motor (power head) and the propeller support. The exhaust gas from the motor enter the tuned length expansion chamber (21). at the top of the leg. The expansion chamber undergoes a gradual increase in cross section along its length and doubles back on itself at (22), near the bottom of the leg, in order to achieve acceptable compactness. The expansion chamber exhausts at the exhaust outlet (23) from which the gases flow around the propeller shaft bearing (not shown), and are channelled out through the propeller (not shown). Water injection unit (24) is positioned close to the inlet of the exhaust gases to the expansion chamber (21), the water being supplied from the motor cooling system's water pump. The amount of water injected is controlled by regulator unit (25).
Modulation of mass flow rate of water into the expansion chamber (21) with speed can be achieved in a number of ways: The pressure versus speed characteristic of the existing water supply pump for the engine may be used in conjunction with a diaphragm or plunger actuated valve. A typical piston type diaphragm regulator unit is illustrated in Fig. 11 which is coupled into the pressure side of the engine water circulating
pump. A speed below say 2500 R.P.M. pressure developed by the pump in the control volume (26), and acting on the piston (28) is insufficient to overcome preload of the spring (29). Hence maximum flow of water through the nozzle (30) into the expansion chamber (21) is achieved. At speeds above 2500 R.P.M. and below about 3800 R.P.M. increasing pressure in the water supply overcomes the spring preload and gradually moves the valve (27) towards its seat (31) against spring force. At a speed of about 3800 R.P.M., the pressure is sufficient to close the valve (27) completely and water flow to the nozzle is stopped. If the rate of decrease in water flow is to be non-linear, the spring 29 may be a variable rate spring or may operate in conjunction with one or more other springs to give the required characteristic. An alternative form of control may be a simple needle valve in seat arrangement is worked off an existing cam plate attached to the throttle linkage of the conventional engine carburettor, to achieve a similar speed versus mass flow characteristic. Another form may be a centrifugal device driven at a speed proportional to engine speed and operating a valve in a similar manner to the piston in Fig. 11.
Because of the rate of use of water, the invention is most applicable to marine engines, inboard and outboard, and particularly water cooled marine engines as there is then a readily available supply of water at an appropriate pressure. However, the invention is also applicable elsewhere such as on an engine driving an agricultural irrigation or fire water pump. Also, there are an increasing number of light and ultralight aircraft using air cooled 2-stroke engines which would benefit from a power boost during take-off and initial climb. Sufficient water could be carried for this purpose, the supply being exhausted during the first few minutes of operation.
It is preferred to reduce exhaust gas
temperatures by water injection into the exhaust gas, but it will be clear that any means of reducing the exhaust gas temperature will provide the desired control of acoustic velocity. While it is believed easier and more practical to control the acoustic velocity of the exhaust gases by cooling, such as by the water injection described above, it is also possible to use a tuned length exhaust system having a greater time interval before a given return pressure fluctuation reaches a given point, such as an exhaust port. The exhaust gases in that instance can be heated to increase acoustic velocity and thus the time of arrival of the return pressure fluctuation, with the amount of exhaust gas heating varying with the engine speed so as to optimize the characteristics of the return pressure fluctuation. The heating could be accomplished by various methods ranging from external heating coils or control of water flow for water-cooled jacketed exhaust systems to the use of a flame in the exhaust gases. The important feature of this invention is thus to control the temperature of the exhaust gases, and thus the acoustic velocity of the exhuast gases, so as to control the time of arrival, magnitude and/or duration of returning pressure fluctuations at a given point, such as the exhaust port . Any means of achieving such temperature control should result in the desirable and beneficial features of the present invention.
Although the present invention is most applicable to conventional 2-storke reciprocating or rotary engines where valves are not fitted to the exhaust ports (the valves' function being performed by the pistons) the invention could also be advantageously applied to 4-strokes or 2-strokes with exhaust valves . This is because of the invention's ability to control the scavenging effect of the low pressure pulse at the exhaust .
Although the invention is most applicable to
engines with one or two combustion chambers exhausting into a tuned length expansion chamber, an advantage (although diminished) may also be obtained by its use with an engine with a pulse charged exhaust system such as is found on some multicylinder 2-stroke engines, especially those with a 3 or more cylinders.
Claims (22)
1. A method of operating an internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed comprising delivering the gaseous products of combustion from the cylinder to an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned to establish when the engine is operating at a selected speed a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a predetermined pressure pulse before the exhaust port closes, admitting a coolant to the exhaust passage at engine speeds below said selected engine speed, and regulating the quantity of coolant admitted in relation to the engine speed over a range of engine speeds to maintain a pressure pattern to create said predetermined pressure pulse over said speed range.
2. A method of operating an internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed; an exhaust passage communicating with the exhaust port comprising delivering the gaseous products of combustion from the cylinder to an exhaust passage communicating with said exhaust port, said exhaust passage being dimensionally proportioned to establish when the engine is operating at a selected speed a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a predetermined pressure pulse before the exhaust port closes, admitting a coolant to the exhaust passage while the engine is operating in a speed range from idle speed to said selected engine speed, and regulating the quantity of coolant admitted in relation to engine speed from a speed in said range up to said selected engine speed to maintain the pressure pattern to create said predetermined pressure pulse while the engine is operating between said speeds.
3. A method of operating an engine as claimed in claim 2 where said speed range is approximately 1000 R.P.M. to said selected engine speed.
4. A method as claimed in claim 2 or 3 wherein the speed in said range is between approximately 2000 and 3000 R.P.M.
5. A method as claimed in claim 2 or 3 wherein the speed in said range is approximately 2500 R.P.M.
6. A method of operating an internal combustion engine as claimed in claim 1 or 2 wherein regulation of the quantity of coolant admitted is commenced at an engine speed of:
NL = NT (Ta/Tm)0.5
wherein NL = Engine speed at commencement of coolant regulation in R.P.M. NT = Selected engine speed in R.P.M. Ta = Temperature of coolant supply Tm = Back exhaust gas temperature at maximum load and speed
7. A method as claimed in claim 1 or 2 wherein the exhaust passage is dimensionally proportioned so said predetermined pressure pulse is at a pressure below the pressure existing in the combustion chamber at the time the pulse is created at the exhaust port.
8. A method as claimed in claim 7 wherein said pressure of the predetermined pressure pulse is below atmospheric pressure.
9. A method as claimed in claim 7 or 8 wherein the exhaust passage is dimensionally proportioned to create said pressure pulse before the inlet port closes.
10. A method as claimed in claim 1 or 2 wherein the exhaust passage is dimensionally proportioned so said pressure pulse is above the pressure existing in the combustion chamber at the time the pulse is created at the exhaust port.
11. A method as claimed in claim 9 wherein the pressure pulse is created after the inlet port closes and before the exhaust port closes.
12. A method as claimed in claim 1 or 2 wherein the exhaust passage is dimensioτially proportioned to create said predetermined pressure pulse over a tuned range of engine speed, and the coolant is admitted to the exhaust passage at speeds below the maximum of said tuned range.
13. A method as claimed in claim 12 wherein the coolant is admitted to the exhaust passage at speeds below approximately the minimum of said tuned range.
14. A method of operating an internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed; comprising delivery the exhaust gas from the exhaust port into an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned relative to the exhaust port to establish when the engine is operating in a first selected speed range a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a low pressure pulse before the inlet port closes and a high pressure pulse after the inlet port has closed and before the exhaust port has closed, and admitting coolant to the exhaust passage to control the temperature of the gas in the exhaust passage over a second selected speed range below said first selected speed range to thereby maintain said pressure pattern over said second selected speed range.
15. A method of operating an internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed comprising delivering the exhaust gas from the exhaust port into an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned relative to the exhaust port to establish when the engine is operating at a selected speed a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a high pressure pulse after the inlet port has closed and before the exhaust port has closed, and controlling the temperature of the gas in the exhaust passage over a range of speeds below said selected engine speed to maintain said pressure pattern over said range of speeds.
16. A method of operating an internal combustion engine as claimed in claim 14 or 15 wherein the lower limit of the range of speeds over which the gas temperature is controlled is:
NL = NT(Ta/Tm)0.5 wherein NL = Engine speed at commencement of coolant regulation in R.P.M. NT = Selected engine speed in R.P.M. Ta = Temperature of coolant supply Tm = Back exhaust gas temperature at maximum load and speed
17. An internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed, an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned to establish when the engine is operating at a selected speed a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create the exhaust port a predetermined pressure pulse before the exhaust port closes, means to supply coolant to the gas in the exhaust passage at engine speeds below said selected engine speed, and means to regulate the quantity of coolant admitted in relation to engine speed over a range of engine speeds to maintain said pressure pattern over said range of engine speeds.
18. An internal combustion engine as claimed in claim 17 wherein said means to supply coolant is adapted to supply coolant over a speed range from idle speed to said selected engine speed.
19. An internal combustion engine as claimed in claim 17 or 18 wherein said exhaust passage is dimensionally proportioned to create said pressure pulse after the inlet port has closed and before the exhaust port has closed.
20. An internal combustion two stroke engine as claimed in claim 17, 18 or 19 wherein said means to regulate the supply of coolant gas in the exhaust passage is arranged so said pressure pattern is maintained over said range of speeds of NL to said selected engine speed (NT) wherein:
NL = (Ta/Tm)0.5 NT wherein NL = Engine speed at commencement of coolant regulation NT = Selected engine speed Ta = Temperature of coolant supply Tm = Back exhaust gas temperature at maximum load and speed
21. An internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movement of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed, an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned relative to the exhaust port to establish when the engine is operating at a selected speed a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a low pressure pulse before the inlet port closes and a high pressure pulse after the inlet port has closed and before the exhaust port has closed, and means to control the temperature of the gas in the exhaust passage over a range of speeds below said selected engine speed to maintain said pressure pattern over said range of speeds.
22. An internal combustion two stroke engine having a cylinder with a piston movable therein defining a combustion chamber, a charge inlet port and an exhaust port in said cylinder, said ports being arranged to be opened and closed in a timed relation by the movemnet of the piston in the cylinder so the inlet port is open during at least part of the time the exhaust port is open and closes before the exhaust port is closed, an exhaust passage communicating with the exhaust port, said exhaust passage being dimensionally proportioned relative to the exhaust port to establish when the engine is operating in a first selected speed range a pressure pattern in the gas in the exhaust passage upon opening of the exhaust port that will create at the exhaust port a low pressure pulse before the inlet port closes and a high pressure pulse after the inlet port has closed and before the exhaust port has closed, and means to control the temperature of the gas in the exhaust passage over a second selected speed range below said first selected speed range to maintain said pressure pattern over said second selected speed range.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AU42305/85A AU584087B2 (en) | 1984-04-09 | 1985-04-04 | Two-stroke engine variable tuned exhaust system |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| AUPG4492 | 1984-04-09 | ||
| AU449284 | 1984-04-09 | ||
| AU42305/85A AU584087B2 (en) | 1984-04-09 | 1985-04-04 | Two-stroke engine variable tuned exhaust system |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| AU4230585A AU4230585A (en) | 1985-11-01 |
| AU584087B2 true AU584087B2 (en) | 1989-05-18 |
Family
ID=25610768
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| AU42305/85A Ceased AU584087B2 (en) | 1984-04-09 | 1985-04-04 | Two-stroke engine variable tuned exhaust system |
Country Status (1)
| Country | Link |
|---|---|
| AU (1) | AU584087B2 (en) |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4350010A (en) * | 1979-04-17 | 1982-09-21 | Yamaha Hatsudoki Kabushiki Kaisha | Exhaust system for an outboard engine |
-
1985
- 1985-04-04 AU AU42305/85A patent/AU584087B2/en not_active Ceased
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4350010A (en) * | 1979-04-17 | 1982-09-21 | Yamaha Hatsudoki Kabushiki Kaisha | Exhaust system for an outboard engine |
Also Published As
| Publication number | Publication date |
|---|---|
| AU4230585A (en) | 1985-11-01 |
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