Reduction of Fuel Consumption and Exhaust Emissions
Chapter 7 Reduction of Fuel Consumption and Exhaust Emissions 7.2.1 Typical performance characteristics of simple engines
7.3 Optimizing the emissions and fuel economy of the simple two-stroke engine
of that capacity, and is employed in a snowmobile.
The measured data is given in Fig. 7.12. As would be expected, the higher the load or BMEP, the greater the peak cycle temperature and the level of the oxides of nitrogen. The values are shown as NO equivalent and measured as ppm on NDIR instrumentation. The highest value shown is at 820 ppm, the lowest is at 60 ppm, and the majority of the performance map is in the range from 100 to 200 ppm. This is much lower than that produced by the equivalent four-stroke engine, perhaps by as much as a factor of between 4 and 8. It is this inherent characteristic, introduced earlier in Sect. 7.1.1, that has attracted the automobile manufacturers to indulge in research and development of two-stroke engines; this will be discussed further in later sections of this chapter, as it will not be a "simple" two-stroke engine which is developed for such a market requirement.
7.3 Optimizing the emissions and fuel economy of the simple two-stroke engine
In Sect. 7.2, the problems inherent in the design of the simple two-stroke engine are introduced and typical performance characteristics are presented. Thus, the designer is now aware of the difficulty of the task which is faced, for even with the best technology the engine is not going to be competitive with a four-stroke engine in terms of hydrocarbon emission. In all otherrespects, be it specific power, specific bulk, specific weight, maneuverability, manufacturing cost, ease of maintenance, durability, fuel consumption, or CO and NO emissions, the simple two-stroke engine is equal, and in some respects superior, to its four-stroke competitor. There may be those who will be surprised to see fuel consumption in that list, but investigation shows that small capacity four-stroke engines are not particularly thermally efficient. The reason is that the friction loss of the valve gear begins to
316
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
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Fig. 7.12 Nitrogen oxides emission map for a 178 cc two-stroke engine.
assume considerable proportions as the cylinder size is reduced, and this deterio- rates the mechanical efficiency of the engine.
Fig. 7.2 lists options which are open to the designer, and the remainder of this section will be devoted to their closer examination. In particular, the engine computer model will be used to illustrate the relevance of some of those assertions.
This will reinforce much of the earlier discussion in Chapter 5.
7.3.1 The effect of scavenging behavior
In Chapter 5, and in this chapter, the QUB 400 single-cylinder research engine is used to illustrate much of the discussion. It is appropriate that it is used again to demonstrate the effects of design changes on engine performance, particularly as they relate to fuel economy and emissions. It is appropriate because the more complete the theoretical and experimental data given about a particular engine, it is hoped that the greater is the understanding gained by the reader of the design and development of any two-stroke engine.
In this section, the data for the physical geometry of the QUB 400 engine given in Figs. 5.3 and 5.4 are inserted into ENGINE MODEL NO. 1, Prog.5.1, and are run over a range of throttle openings at 3000 rpm for two differing types of loop scavenging. The variation of throttle opening area ratio used in the calculations is from 0.15 to 1.0, and the individual values inserted for the data are 0.15, 0.2, 0.3, 0.4,0.5, and 1.0. The scavenge systems tested are those listed as SCRE and YAM6, first introduced in Sect. 3.2.4. The SCRE scavenge type is a very good loop scavenged design, whereas the YAM6 type is shown to have rather indifferent scavenging qualities. The reader may be interested to note that the second model of the EXPAND function is used to describe the scavenging behavior within the computer program, this being introduced in Eq. 5.1.13; in the earlier exposition in Chapter 5, Eq. 5.1.11 is used for the analysis of behavior of the QUB 400 engine.
The results of the calculations are given in Figs. 7.13 and 7.14.
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DATA FROM Figs.5.3-5.4
0.4 0.6 THROTTLE AREA RATIO
Fig. 7.14 Predicted scavenging behavior ofQUB 400 engine as a function of throttle opening area ratio.
The Basic Design of Two-Stroke Engines
The principal variables being investigated are throttle opening, i.e., load vari- ation controlled by delivery ratio, and the quality of the scavenging system employed. In Fig. 7.13, the close relationship between delivery ratio and BMEP is evident, this point having been discussed before as being the typical effect one observes for an engine which does not have any exhaust pressure wave tuning (see Sect. 5.2.3). The delivery ratio for the two scavenging types is identical, but the superior retention of fresh charge by the SCRE system is very evident. This translates into superior trapping efficiency and BSFC over the entire load range. The trapping efficiency profile with respect to delivery ratio, but more importantly and theoretically with respect to scavenge ratio (by volume) if the discussion in Chapter 3 is recalled, shows a decrease with increasing load, BMEP, and scavenge ratio. Due to this effect, the best specific fuel consumption occurs at the highest trapping efficiency. It is interesting to note that the scavenge ratio (by volume) values for the two scavenge systems are identical, but their disposition into charging efficiency and scavenging efficiency describes clearly the effectiveness of the SCRE. and the ineffectiveness of the YAM6, scavenging design. The improvements in BMEP and BSFC are proportionately greater for the SCRE than for the YAM6 design, as the load is reduced by throttle opening.
The fundamental messages to the designer regarding the options in this area ate:
(a) Optimize the scavenging system to the highest level possible using the best theoretical and experimental tools available. It is recommended, as in Chapter 3, that a combination of experiment, using a single cycle gas scavenging rig, and theory, using CFD techniques, is employed in this regard.
(b) At the design stage, consider seriously the option of using an engine with a large swept volume and designing the entire porting and inlet system to operate with a low delivery ratio to attain a more modest BMEP at the design speed, while achieving the target power by dint of the larger engine swept volume. In this manner, with an optimized scavenging and air-flow characteristic, the lowest BSFC and exhaust emissions will be attained at the design point. The engine durability will also be improved by this methodology as the thermal loading on the piston will be reduced. There is a limit to the extent to which this design approach may be taken, as the larger engine will be operating closer to the misfire limit from a scavenging efficiency standpoint.
7.3.2 The effect of air-fuel ratio
Fig. 7.15 shows the result of employing the engine model, Prog.5.1, to predict the behavior of an engine, in this case the QUB 400 engine, at a throttle opening area ratio of 0.15 and an engine speed of 3000 rpm, over the range of air-fuel ratios from
10.5 to 17.5. The stoichiometric value is at 15. On the same figure, as a means of showing the relevance of the calculation, is plotted the same experimental data for a throttle opening of 0.1 which was previously presented in Fig. 7.6 for that same engine. This relatively simple theoretical computer model is seen to predict the variation with respect to air-fuel ratio quite well, and should give the designer confidence in its employment in this regard. The most important message to the
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
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designer is the vital importance of having the fuel metered to the engine in the correct proportions with the air at every speed and load. There is at least as large variations of BSFC and BMEP with inaccurate fuel metering as there is in allowing the engine to be designed and manufactured with bad scavenging.
There is a tendency in the industry for management to insist that a cheap carburetor be installed on a simple two-stroke engine, simply because it is a cheap engine to manufacture. It is ironic that the same management will often take an opposite view for a four-stroke model within their product range, and for the reverse reason!
7.3.3 The effect of exhaust port timing and area
One of the more obvious methods of increasing the trapping efficiency of an engine is to lower the exhaust port timing, i.e., reduce both the period for scavenging flow and/or the area through which it passes, thereby lowering the delivery ratio, the BMEP attained, and the BSFC and exhaust emissions. Perhaps this obvious solution would accompany the suggestion in Sect. 7.3.1(b) of using a larger, more lightly loaded engine. Needless to add, if one applied this approach to the QUB 400 engine, it would no longer produce the desirable BMEP level of 6.24 bar from a high delivery ratio of 0.86 which is well trapped by a good scavenging system. However, it is informative to use the engine simulation model to examine the effect of a change to the exhaust port timing of the QUB 400 engine when the air throttle opening area ratio is at 0.15 and the engine speed is 3000 rpm. In other words, the data being used is exactly the same as in Sect. 7.3.2, but the exhaust port timing will be varied successively from the original standard value of 962 atdc opening, in 4° steps, until the exhaust port opening is at 116B atdc. At this point the blowdown period would be just 2°, for the transfer ports open at 118- atdc. The result of this calculation is shown in Fig. 7.16 for the variations of BMEP, BSFC, delivery ratio and trapping efficiency.
Rather surprisingly, the model predicts that the delivery ratio, at this low level of 0.33, would not be reduced further by lowering the exhaust port timing edge and, by inference, reducing its area. The trapping efficiency rises sharply, as does the BMEP produced, and the fuel economy is improved dramatically. From this calculation it is evident that the designer has to ensure that the lowest possible exhaust port timing is employed on any particular engine, consistent with attaining the peak power and speed required from the powerplant. This is a particularly subtle area for optimization, and one where it is vital to remember that neither one's experience, nor a computer simulation, nor any form of theoretical assistance will completely supplant a well organized test program conducted under the most realistic of experimental conditions. The reason for this is that the simple engine is always on the verge of four-stroking at light load and speed, and the first priority to attain good fuel economy and exhaust emissions is to ensure that the engine charges itself and fires evenly on each cycle. This can only be done effectively under experimental conditions.
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The Basic Design of Two-Stroke Engines
Nevertheless, the fundamental message to the designer is that control over the exhaust port timing and area has a dramatic influence on power output, fuel economy and exhaust emissions at light load and low engine speeds. This approach has been employed in several production engines, and the publication by Tsuchiya et al(7.3) discusses the subject in some detail.
There are two basic mechanical techniques to accomplish the design requirement for an exhaust port restriction to improve the light load behavior of the engine. The two methodologies are illustrated in Fig. 7.17 and discussed below.
7.3.3.1 The butterfly exhaust valve
The first of these, shown in Fig. 7.17(a) and (b) on the left of each diagram, is a butterfly valve and the concept is much like that described by Tsuchiya et al(7.3).
This is a relatively simple device to manufacture and install, and has a good record of reliability in service. The ability of such a device to reduce exhaust emissions of unburned hydrocarbons is presented by Tsuchiya(7.3), and Fig. 7.18 is from that paper. Fig. 7.18 shows the reduction of hydrocarbon emissions, either as mass emissions in the top half of the figure or as a volumetric concentration in the bottom half, from a Yamaha 400 cc twin-cylinder road motorcycle at 2000 rpm at light load.
The notation on the figure is for CR which is the exhaust port area restriction posed by the exhaust butterfly valve situated close to the exhaust port. The CR values range from 1, i.e., open as in Fig. 7.17(b), to 0.075, i.e., virtually closed as in Fig. 7.17(a).
It is seen that the hydrocarbons are reduced by as much as 40% over a wide load variation at this low engine speed, emphasizing the theoretical indications discussed above.
Tsuchiya(7.3) reports that the engine behaved in a much more stable manner when the exhaust valve was employed at light load driving conditions in an urban situation.
7.3.3.2 The exhaust timing edge control valve
It is clear from Fig. 7.17 that the butterfly valve controls only the area at the exhaust port rather than the port opening and closing timing edges as well. On the right side of Fig. 7.17 is sketched a valve which fits closely around the exhaust port and can simultaneously change both the port timing and the port area, exactly as in the mathematical simulation discussed in Sect. 7.3.3. There are many innovative designs of exhaust timing edge control valve ranging from the oscillating barrel type to the oscillating shutter shown in Fig. 7.17. The word "oscillating" may be somewhat confusing, so it needs to be explained that it is stationary at any one load or speed condition but it can be changed to another setting to optimize an alternative engine load or speed condition. While the net effect on engine performance of the butterfly valve and the timing edge control valve is somewhat similar, the timing edge control valve carries out the function more accurately and effectively. Of course, the butterfly valve is a device which is cheaper to manufacture and install than the timing edge control device.
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Fig. 7.17 Variable exhaust port area and port timing control devices.
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Fig. 7.18 Emissions reduction using a restriction at the exhaust port by a butterfly valve.
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions Hata and Iio(7.4) describe the use of such an exhaust timing control valve, although it is clear from the paper that the emphasis is more on the improvement of the power performance of a sports motorcycle than on reduction of fuel consump- tion and exhaust emissions. Indeed, informed readers will have observed these exhaust control valves on racing motorcycle engines, where their employment is intended to retune the expansion chamber exhaust system over a wider speed range.
If Eq. 6.2.2 is re-examined, it is clear that if the exhaust period, EP, is reduced by a timing edge control valve, then for a fixed tuned length, LT, the engine speed for peak tuning effect is reset to a new and lower level. Reproducing Eq. 6.2.2:
LT=83.3*EP*Ao/RPM (6.2.2) Rearranging this to find the tuned speed, RPM, if the other terms are variables
or constants:
RPM=83.3*EP*Ao/LT (7.3.1) Thus, for any fixed pipe length, if the exhaust control valve is in the position
shown in Fig. 7.17(a) and the exhaust period, EP, is less than at the peak tuning speed, the pipe will provide well-phased plugging reflections at a lower speed. This raises the BMEP and power at that lower engine rate of rotation.
Nevertheless, in their paper Hata(7.3) shows a reduction of as much as 40% in BSFC at the lower end of the speed and load range when the exhaust control valve is employed.
7.3.4 Conclusions regarding the simple two-stroke engine
The main emphasis in the discussion above is that the simple two-stroke engine is capable of a considerable level of optimization by design attention to scavenging, carburetion, lubricants and lubrication, and exhaust timing and area control.
However, the very best design will still have an unacceptably high emission of unbumed hydrocarbons even though the carbon monoxide and nitrogen oxide levels are acceptably low, perhaps even very low. Indeed, it is possible that the contribution of internal combustion engines to the atmospheric pollution by nitrous oxide, N20 , may be a very important factor in the future(7.24). There is every indication that any two-stroke engine produces this particular nitrogen oxide component in very small quantities by comparison with its four-stroke engine counterpart.
The simple two-stroke engine, optimized at best, has a low CO and NOx exhaust pollutant level, but a high HC and 02 exhaust emission output. This leaves the engine with the possibility of utilizing an oxidation catalyst in the exhaust to remove the hydrocarbons and further lower the carbon monoxide levels.
An early paper on this subject by Uchiyama et al(7.11) showed that a small Suzuki car engine could have the hydrocarbon emission reduced quite significantly by exhaust gas after-treatment in this manner. They reported an 80% reduction of the hydrocarbon exhaust emission by an oxidizing catalyst.
The Basic Design of Two-Stroke Engines