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
7.4 The more complex two-stroke engine
7.4.1 The stratified charging and homogeneous combustion engine This type of engine has been tested in various forms and by several research
groups and organizations. The most significant are discussed below in terms of their applicability for future use in production as power units which will rival the four- stroke engine in hydrocarbon emissions and fuel consumption levels, but which must retain the conventional advantage in carbon monoxide and nitrogen oxide emissions.
7.4.1.1 The QUB stratified charging engine
This early work is presented in technical papers published by Blair and Hill(7.9) and by Hill and Blair(7.10). The fundamental principle of operation of the engine is illustrated by Fig. 7.22. The overriding requirement is to introduce a rich mixture of air and fuel into the cylinder during the scavenge process at a position which is as remote as possible from the exhaust port. Ideally, the remaining transfer ports would supply air only into the cylinder. The engine has two entry ports for air. a main entry for 80% of the required air into the crankcase, and a subsidiary one for the remaining air and for all of the necessary fuel into a long storage transfer port. That port and transfer duct would pump the stored contents of air and fuel into the cylinder during the succeeding scavenge process so that no fuel migrated to the crankcase. In the meantime, during the induction and pumping period, the fuel would have some residence time within the air and on the walls of the long rear transfer port so that some evaporation of the fuel would take place. In this manner the cylinder could be supplied with a pre-mixed and partially evaporated fuel and air mixture in a stratified process. The resultant mixing with the trapped charge of cylinder air and retained exhaust gas would permit a homogeneous combustion process.
The test results for the engine, shown in Figs. 7.23 and 7.24 for fuel consumption and BMEP levels at several throttle openings, reveal significantly low levels of fuel consumption. Most of the BMEP range from 2 bar to 5.4 bar over a speed range of 1500 to 5500 rpm, but the BSFC levels are in the band from 0.36 to 0.26 kg/kWh.
These are particularly good fuel consumption characteristics, at least as good if not superior to an equivalent four-stroke cycle engine, and although the hydrocarbon emission levels are not recorded, they must be significantly low with such good trapping of the fuel within the cylinder. The power performance characteristics are unaffected by this stratified charging process, for the peak BMEP of this engine at 5.4 bar is quite conventional for a single-cylinder engine operating without a tuned exhaust system.
The mechanical nature of the engine design is relatively straightforward, and it is one eminently suitable for the conversion of a simple two-stroke cycle engine.
The disadvantages are the extra complication caused by the twin throttle linkages
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
side transfer ports
/ long bock transfer ports
Section at transfer port level
Fig. 7.22 Air and fuel flow paths in a QUB type stratified charging system.
and the accurate carburetion of a very rich mixture by a carburetor. The use of a low- pressure fuel injection system to replace the carburetor would simplify that element of the design at the further disadvantage of increasing the manufacturing costs.
7.4.1.2 The Piaggio stratified charging engine
The fundamental principle of operation of this power unit is shown in Fig. 7 25 and is described in much greater detail in the paper by Batoni(7.1). This engine takes the stratified charging approach to a logical conclusion by attaching two engines at the cylinder head level. The crankshafts of the two engines are coupled together in the Piaggio example by a toothed rubber belt. In the paper presented by Piaggio one of the engines, the "upper" engine of the sketch in Fig. 7.25, has 50 cc swept volume and the "lower" engine has 200 cc swept volume. The crankcase of both engines' ingest air and the upper one inhales all of the required fuel for combustion of an appropriate air-fuel mixture in a homogeneous process. The crankcase of the upper engine supplies a rich mixture in a rotating, swirling scavenge process giving the fuel as little forward momentum as possible towards the exhaust port. The lower
ines
0 45
0-40
0-35
0-30
0-25
o Full Throttle
* Half Throttle D Quarter Throttle
in =
^ 0 7 300-
£r0 - 6
250-
-0-5
1000 2000 . lQ-4
3000 4000 "5000~~6000 Engine Speed (rev/min)
Fig. 7.23 Optimized fuel consumption levels for the QUB stratified charging engine.
o Full Throttle
• Half Throttle a Quarter Throttle
-75
-65
55
200L__L_ _
35
3000 4000 5000 6000 Engine Speed (rev/min)
Fig. 7.24 BMEP levels at the optimized fuel consumption levels for the {JUb stratified charging engine.
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions cylinder conducts a conventional loop scavenge process with air only. Towards the end of compression the mixing of the rich air-fuel mixture and the remaining trapped cylinder charge takes place, leading to a homogeneous combustion process.
The results of the experimental testing of this 250 cc Piaggio engine are to be found in the paper by Batoni(7.1), but are reproduced here as Figs. 7.26-7.28. A direct comparison can be made between this stratified charging engine and the performance characteristics of the 200 cc engine which forms the base of this new power unit. Figs. 7.9-7.11, already discussed fully in Sect. 7.2.1.2, are for the 200 cc base engine. Fig. 7.9 gives the fuel consumption behavior of the 200 cc base engine, Fig. 7.10 the CO emission levels and Fig. 7.11 the HC emission characteristics.
Fig. 7.26 shows the fuel consumption levels of the experimental engine (note that 1 g/kWh=0.746 g/bhp.hr). The lowest contour in the center of the "oyster" map is 240 g/hp.hr or 322 g/kWh. The units of BMEP on this graph are in kg/cm2, which is almost exactly equal to a bar (lkg/cm2=0.981 bar). These are quite good fuel consumption figures, especially when one considers that this engine is one of the first examples of stratified charging presented; the paper was published in 1978. The reduction of fuel consumption due to stratified charging is very clear when one compares Figs. 7.26 and 7.9. The minimum contour is lowered from 300 g/bhp.hr to 240 g/bhp.hr, a reduction of 33%. At light load, around 1 bar BMEP and 1500 rpm, the fuel consumption is reduced from 500 to 400 g/bhp.hr, or 20%. This condition is particularly important for power units destined for automotive applica- tions as so many of the test cycles for automobiles or motorcycles are formulated to simulate urban driving conditions where the machine is accelerated and driven in the 15-50 km/h zone. The proposed European ECE-R40 cycle is such a driving cycle(7.21).
The reduction of hydrocarbon emissions is particularly impressive, as can be seen from a direct comparison of the original engine in Fig. 7.11 with the stratified charging engine in Fig.7.28. The standard engine, already discussed in Sect. 7.2.1.2, showed a minimum contour of 1500 ppm HC (C6, NDIR), at a light load but high speed point. In the center of the load-speed map in Fig. 7.11 the figures are in the 2500 ppm region, and at the light load point of 1 bar and 1500 rpm, the figure is somewhat problematic but 5000 ppm would be typical. For the stratified charging engine the minimum contour is reduced to 200 ppm HC, the center of the load-speed picture is about 500 ppm, and the all-important light load and speed level is somewhat in excess of 1000 ppm. This is a very significant reduction and is the level of diminution required for a successful automotive engine before the application of catalytic after-treatment.
A comparison of the carbon monoxide emission levels of the standard engine in Fig. 7.10 and of the stratified charging engine in Fig. 7.27 shows significant improvements in the two areas where it really matters, i.e., at light loads and speeds and at high loads and speeds. In both cases the CO emission is reduced from 2-3%
to 0.2-0.3%, i.e., a factor of 10. The absolute value of the best CO emission at 0.2%
is quite good, remembering that this experimental data was acquired in 1978.
INLET FOR AIR AND F U E L " ^
INLET (AIR ONLY)
Fig. 7.25 The operating principle of the Piaggio stratified charging engine.
l.S.F.C. , G/BHP
itfoo itsr°° 3tfOO ~ 4-O"O0 5 0 0 0 6 0"0 0
ENGINE SPEED , R PM
Fig. 7.26 Fuel consumption levels from the Piaggio stratified charging engine.
5000 6000 ENGINE SPEED , DpH
Fig. 7.27 Carbon monoxide emission from the Piaggio stratified charging engine.
Fig. 7.28 Hydrocarbon emission levels from the Piaggio stratified charging engine.
It should also be noted that the peak BMEP of the engine is slightly reduced from 4.8 bar to 4.1 bar due to the stratified charging process, and there is some evidence that there may be some diminution in the air utilization rate of the engine. This is supplied by the high oxygen emission levels at full load published by Batoni(7.1, Fig. 8) where the value at 4 bar and 3000 rpm is shown as 7%. In other words, at that point it is almost certain that some stratified combustion is occurring.
This engine provides an excellent example of the benefits of stratified charging.
It also provides a good example of the mechanical disadvantages which may accrue from its implementation. This design, shown in Fig. 7.25, is obviously somewhat bulky, indeed it would be bulkier than the equivalent four-stroke cycle engine. One of the profound advantages of the two-stroke engine is lost by this particular mechanical layout. An advantage of this mechanical configuration, particularly in a single-cylinder format, is the improved primary vibration balancing of the engine due to the opposed piston layout.
Nevertheless, a fundamental thermodynamic and gas-dynamic postulation is verified from this experimental data: stratified charging of a two-stroke engine is a viable and sound approach to the elimination of much of the excessive fuel consumption and raw hydrocarbon emission from a two-stroke engine.
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions 7.4.1.3 An alternative mechanical option for stratified charging
The fundamental principle of stratified charging has been detailed above, but other researchers have striven to emulate the process with either less physical bulk or less mechanical complication than that exhibited by the Piaggio device.
One such engine is the double piston device, an extension of the original split- single Puch engine of the 1950's. Such an engine has been investigated by Ishihara(7.7). Most of these engines are designed in the same fashion as shown in Fig. 7.29. Instead of the cylinders being placed in opposition as in the Piaggio design, they are configured in parallel. This has the advantage of having the same bulk as a conventional twin-cylinder engine, but the disadvantage of having the same (or worse!) vibration characteristics as a single-cylinder engine of the same total swept volume. The stratified charging is at least as effective as in the Piaggio design, but the combustion chamber being split over two cylinder bores lends itself more to stratified burning than homogeneous burning. This is not necessarily a criticism.
However, it is clear that it is essential to have the cylinders as close together as possible, and this introduces the weak point of all similar designs or devices. The thermal loading between the cylinder bores is somewhat excessive if a reasonably high specific power output is to be attained.
Another design worthy of mention and study, which has considerable applica- bility for such designs where the cost and complexity increase cannot be excessive due to marketing and packaging requirements, is that published in the technical paper by Kuntscher(7.23). This design for a stratified charging system has the ability to reduce the raw hydrocarbon emission and fuel consumption from such engines as those fitted in chainsaws, mopeds, and small motorcycles.
7.4.1.4 The stratified charging engine proposed by Institut Francois du Pet role
This suggestion for stratified charging which emanates from IFP is probably the most significant yet proposed. The performance results are superior in most regards to four-stroke cycle engines, as is evident from the technical paper presented by Duret et al(7.18). The fundamental principle of operation is described in detail in that publication, a sketch of the engine operating principle is given in Fig. 7.30, and a photograph of their engine is shown in Plate 7.1. The engine in the photograph is a multi-cylinder unit and, in a small light car operating on the EEC fuel consumption cycle at 90 and 120 km/h, had an average fuel consumption of 30.8 km/liter (86.8 miles/Imperial gallon or 73.2 miles/US gallon).
The crankcase of the engine fills a storage tank with compressed air through a reed valve. This stored air is blown into the cylinder through a poppet valve in the cylinder head. At an appropriate point in the cycle, a low-pressure fuel injector sprays gasoline onto the back of the poppet valve and the fuel has some residence time in that vicinity for evaporation before the poppet valve is opened. The quality of the air-fuel spray past the poppet valve is further enhanced by a venturi
The Basic Design of Two-Stroke Engines
Fig. 7.29 Alternative stratified charging system of the double piston genre.
surrounding the valve seat. It is claimed that any remaining fuel droplets have sufficient time to evaporate and mix with the trapped charge before the onset of a homogeneous combustion process.
The performance characteristics for the single-cylinder test engine are of considerable significance, and are presented here as Figs. 7.31-7.33 for fuel consumption, hydrocarbons, and nitrogen oxides. The test engine is of 250 cc swept volume and produces a peak power of 11 kW at 4500 rpm, which realizes a BMEP of 5.9 bar. Thus, the engine has a reasonably high specific power output for automotive application, i.e., 44 kW/liter. In Fig. 7.31, the best BSFC contour is at 0.26 kg/kWh, which is an excellent result and superior to most four-stroke cycle engines. More important, the BSFC value at 1.5 bar BMEP at 1500 rpm, a light load and speed point, is at 0.4 kg/kWh and this too is a significantly low value.
The unburned hydrocarbon emission levels are shown in Fig. 7.32, and they are also impressively low. Much of the important legislated driving cycle would be below 20 g/kWh. When an oxidation catalyst is applied to the exhaust system, considerable further reductions are recorded, and this data is presented in Fig. 7.34.
Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions
ENGINE SPEED CAMSHAFT POPPET VALVE
Fig. 7.30 Stratified charging system proposed by the Institut Franqais du Petrole.
The conversion rate exceeds 91% over the entire range of BMEP at 2000 rpm, leaving the unburned hydrocarbon emission levels below 1.5 g/kWh in the worst situation.
Of the greatest importance is the nitrogen oxides emissions, and they remain conventionally low in this stratified charging engine. The test results are shown in Fig. 7.33. The highest level recorded is at 15 g/kWh, but they are less than 2 g/kWh in the legislated driving cycle zone.
The conclusions drawn by IFP are that an automobile engine designed and
developed in this manner would satisfy the most stringent exhaust emissions
Plate 7.1 Stratified charging engine from the Institut Francois du Petrole (courtesy of Institut Francois du Petrole).
legislation for cars. More important, the overall fuel economy of the vehicle would be enhanced considerably over an equivalent automobile fitted with the most sophisticated four-stroke cycle spark-ignition engine.
The bulk of the engine is increased somewhat over that of a conventional two- stroke engine, particularly in terms of engine height. The complexity and manufac- turing cost is also greater, but no more so than that of today's four-stroke engine equipped cars, or even some of the larger capacity motorcycles or outboard motors.
7.4.2 The stratified charging and stratified combustion engine
At QUB there is an investigation into an engine which has both stratified charging and stratified combustion. The basic principle of operation is shown in Fig.
7.35, and a photograph of the test engine is illustrated in Plate 7.2. The engine is of the two-piston type with the cylinder axes at 90", and the geometrical shape has led
1
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