EC TDC 1500 DELIVERY RATI0=0.829

5.1.7) The value of CGtl is the superposition particle velocity at the first mesh point in

TC 10 EC TDC 1500 DELIVERY RATI0=0.829

BSFC,kg/kWh=0.422 BMEP, bar= 7.10 PMEP, bar= 0.35 FMEP, bar= 0.62

TEFF=0.585 CEFF=0.513 , bar= 57.4 and TEMP., K=2934. at deg.ATDC= 13.4

Fig. 5.21 The 125 cc Grand Prix engine on the third calculation cycle.

power which which will place it in a race-competitive category. Fig. 5.23 illustrates the eighth cycle where the power maximizes. The BMEP is at 11.29 bar, the power at 26.8 kW (35.9 bhp) and the delivery ratio is up to 1.1. This is a specific power output of 215 kW/liter (287 bhp/liter), more than double that of the Husqvarna MK4 power unit. It will be noticed that the secondary exhaust reflection around the tdc period has been reduced and broadened.

On subsequent calculation cycles, the program shows performance oscillations about this power output level, rising and falling between 26 and 28 kW. Of general interest are the very good (for a racing engine, that is) specific fuel consumption characteristics.

The internal pressure wave motion within the pipe, due to the disposition and

shape of the diffusers and nozzles, is such that secondary pressure waves are being

combined with the next outgoing exhaust pulse, making it more extensive in both

amplitude and time. This produces suction reflections from diffuser and nozzle

which are also larger in both amplitude and time. The combination of lesser

reflections with major pressure waves is referred to as "resonance." The end effect

is a theoretical prediction of high specific power output by Prog.5.2.

CRANK-ANGLE from EXHAUST PORT OPENING f o r one cycle 2.0 J L I I i l l i

EO IC TO BDC TC 10 EC TDC EO ENGINE SPEED, rpm= 11500 DELIVERV RATI0=0.9O9

POWER, kW=21.0 BSFC,kg/kWh=0.370 BMEP, bar= 8.87 IMEP, bar= 9.88 PMEP, bar= 0.39 FMEP, bar= 0.62 SEFF=0.916 TEFF=0.717 CEFF=0.638 PEAK CYLINDER PRESS., bar= 69.4 and TEMP., K=3032. at deg.ATDCr 14.3

Fig. 5.22 The 125 cc Grand Prix engine on the sixth calculation cycle.

Experimentally, a similar pressure wave resonance behavior is observable in raang engines at the highest engine speed test points. The engine will be held by the

iyaimometer at an engine speed just beyond the point of normal maximum power, anitthe power recorded will be as disappointing as that in Fig. 5.21, until the IvB'.mometer load record will suddenly swing up to a level such as that seen in Fig.

5.2, and remain there. This experimental phenomenon, which is slightly different froii the theoretical example in that it is associated with a corresponding elevation inathaust gas temperature as the BMEP rises on succeeding cycles, is referred to cofcquially as "uptune." The similarity to the theoretical example is that the prasure waves recombine as the exhaust gas temperature rises with each succeed- ingicycle to reform a pressure wave resonance pattern in the manner of the pr«f ression of Figs. 5.21-5.23. It is also thought, but not known for certain, that so*- of the short-circuited air-fuel mixture, pulled into the exhaust pipe by the ejtffDtionally strong suction reflections from the diffuser, actually burns in the exfc ust pipe, and that this initiates the temperature rise which ultimately results in thet jptune effect." This effect is particularly important in racing engines, as it can ex»id the usable engine speed power band of the machine by as much as 10%. To ther-ader, this may not appear to be an impressive margin, but it will be so to the c o v e n t o r in second place.

Chapter 5 - Computer Modelling of Engines CRANK-ANGLE from EXHAUST PORT OPENING f o r one cycle

I I I I I I I l i_ (j 3

E0 IC TO BDC TC 10 EC TDC E0 ENGINE SPEED, rpm= 11500 DELIVERY RATI0=1.095

POWER, kWr26.8 BSFC,kg/kWh=0.350 BMEP, bar=1 1.29 IMEP, bar= 12.39 PMEP, bar= 0.47 FMEP, bam 0.62 SEFF:0.972 TEFF=0.865 CEFF=0.799 PEAK CYLINDER PRESS., bar= 84.5 and TEMP., K=3 1 4 4 at deg.ATDC-14.5

Fig. 5.23 The 125 cc Grand Prix engine on the eighth calculation cycle.

Perhaps the most important aspect of the foregoing demonstration is that Prog.5.2, "ENGINE MODEL No. 2," is shown to be capable of analyzing engines of the highest specific power output, of providing understanding of the complex pressure wave motion in the exhaust systems of such engines, and of being a theoretical tool which can be used for the design and development of every aspect of the geometry of these power units.

5.5 Computer modelling of multi-cylinder engines

Up to this point in the discussion, all of the engines which have been modelled have been single-cylinder engines, or those multi-cylinder engine units which have independent intake and exhaust systems. For example, all of the discussion in the previous section on the racing 125 Grand Prix engine is equally applicable to 250 cc twin-cylinder and 500 cc four-cylinder power units, for such motorcycle engines have completely separate intake and exhaust systems. The same separation of exhaust and intake ducting is also to be observed in engines for racing snowmobiles, hydroplanes, go-karts or remotely piloted vehicles (RPV). There is room in such vehicles for the bulk which is inherent in the expansion chamber design, although later discussion in Chapter 6 will demonstrate that some contraction of that bulk is

The Basic Design of Two-Stroke Engines

possible. For certain applications, this exhaust system bulk on a multi-cylinder engine is either unacceptable, or the narrow speed band providing high power output is unusable in the particular application. A particular example would be a multi-cylinder outboard motor, either in two, three, in-line four, V4, V6 or V8 cylinder configurations, all of which are to be found in the marketplace( 1.12)(4.10).

A further example would be of an automobile, where past history has provided some case histories(3.5)(5.6) and the future may provide some more.

At some expense in terms of programming complexity and of computer run- times, there is nothing inherently difficult in programming any multi-cylinder configuration, for the fundamental theory of unsteady gas flow at a three-way branch has already been presented in Sect. 2.3.5. The extension of that theory to cover any number of branched pipes at a common junction is straightforward and is covered by Bingham(2.18) or Benson(1.4).

The main reason for discussing it further at this point in the text, is to illustrate that the main design information which hasheen disseminated thus far for single- cylinder engines, namely that exhaust pressure action which assists good trapping behavior without deteriorating the scavenge and intake flow will provide high power output and good specific fuel consumption, is also applicable and achievable for compact exhaust systems in certain multi-cylinder engine configurations. More importantly, such characteristics are predictable by a computer model, which permits the analysis of all possible combinations of engine cylinders and exhaust and intake manifolding.

Fig. 5.24 illustrates the result of a computer model of a three-cylinder in-line two- stroke engine(5.14). The engine data is exactly as that for the QUB 400 engine in Fig. 5.3. The exhaust manifold and pipe system is sketched in Fig. 5.24 and shows a pipe system which has parallel pipes, all of which are of a uniform diameter of 44 mm. The exhaust pipe length from the piston face at the exhaust port to the center of the first junction is 120 mm for each cylinder and the length of the common exhaust from that junction to the open pipe end in the silencer box is 240 mm. The outlet tail pipe is 90 mm long. The box volume is 10 liters, which is 8.3 times larger than the total swept volume of the engine at 1200 cc. The engine which is being modelled is a three-cylinder in-line outboard motor with a common compact manifold and downpipe to a volume above the gearcase followed by a short underwater outlet exhaust pipe(1.12).

The first part of the computer output from the model is in the same format as for Prog.5.1, and shows the open cycle in Fig. 5.24 from exhaust port opening to closure on the fifth calculation cycle. The presence of the plugging pulse is clearly seen, arriving along the manifold from another cylinder firing 120

s

later. The cylinder trapping pressure is elevated to 1.75 atm. Even though the box volume is only 8.3 times larger than the total swept volume, the presence of a suction reflection around bdc is evident and is contributing to the extraction of exhaust gas and giving assistance to the scavenge flow. The only difference between this pressure diagram and that from a good expansion chamber design is the magnitude of the suction pressure available around bdc, for the plugging pulse at trapping is equally as strong.

Chapter 5 - Computer Modelling of Engines

2.0 P.

A T M

1.5

1.0

120 mm, 044

^ V — \ 10000 co CYLINDER TRAPPING PRESSURE RAISED

EP0 TP0 " " BDC TPC T P C CRANK-ANGLE f r o m EXHAUST PORT OPENING to EXHAUST PORT CLOSING ENGINE SPEED, rpm= 3000 DELIVERY RATIO=0.886

POWER, kW=51.3 BSFC,kg/kWh=0.344 BMEP, bar= 8.61 IMEP,bar= 9.22 PMEP, bar= 0.40 FMEP, bar= 0.21 PEAK CYLINDER PRESS., bar= 49.6 and TEMP., K=2951. at deg.ATDC=16.1

Fig. 5.24 The QUB 400 data used in a three-cylinder engine modelling program.

This form of exhaust port plugging, by utilizing an exhaust pulse from another cylinder blowing down at a convenient phasing in a compact manifold of a multi- cylinder engine, is referred to as "cross-charging" or "cross-stuffing."

As the engine speed is identical with that calculated for the single-cylinder QUB 400 engine data, it is informative to compare both the pressure diagrams and the performance characteristics from Fig. 5.9. The same engine data as a single-cylinder engine programmed in Prog.5.1 had power and fuel consumption characteristics of 6.22 bar and 0.462 kg/kWh. For a single-cylinder engine scavenged with air and fuel and an untuned exhaust system, that is a good full-throttle characteristic for both power and fuel economy. In three-cylinder form, the BMEP is increased to 8.61 bar and the specific fuel consumption is reduced to 0.344 kg/kWh. In other words, the power is up by 38% and BSFC is reduced by 26% from the same design of engine cylinder but incorporated into a three-cylinder formation. The air flow through the engine is also increased slightly, for the delivery ratio has gone up by 3%.

The three-cylinder in-line engine is ideal for this form of cross charging, for the

cylinders fire at 120

⁹

intervals and exhaust port durations are usually between 160

^Q

and 180

s

in such multi-cylinder outboard motor or automobile engine designs. Thus,

the cross-stuffing pulse arrives at the exhaust port perfectly phased to trap the

cylinder charge. Furthermore, this tuning action is almost independent of speed so

that the range of speed over which good torque and fuel economy are available is much wider than would be the case if the expansion chamber exhaust was attached to a single-cylinder engine, or to each cylinder of a multi-cylinder power unit.

As Flaig and Broughton( 1.12) indicate for the OMC V8 300 hp outboard motor, it is possible to have good cross-stuffing characteristics within a four-cylinder engine or a V8 layout, where each cylinder fires at 90° intervals. In this case, as shown in Plate 5.2 for this very engine, it is necessary to have some path length available for the exhaust pulse between the relevant cylinders so that the phasing of the cross-charging process is optimized. This implies that the usable speed range would not be as wide as that of a three-cylinder, or V6, two-stroke engine. The short extension exhaust pipe, leading from the branched junction to the four cylinders of each bank of the Vee into the generous volume of exhaust box above the gearcase of the outboard, is clearly visible in the photograph.

Plate 5.2 A cut-away view of a 300 hp V8 outboard motor (courtesy of Outboard Marine Corporation).

Chapter 5 - Computer Modelling of Engines NOTATION FOR CHAPTER 5

NAME Air-Fuel Ratio Charging Efficiency

Crankcase Compression Ratio Delivery Ratio

Geometric Compression Ratio Index of compression Index of expansion Pressure Ratio

Ratio of Specific Heats Scavenge Ratio

Scavenge Ratio by volume Scavenging Efficiency Trapped Air-Fuel Ratio Trapped Compression Ratio Trapping Efficiency Area

Brake Mean Effective Pressure Brake Specific Fuel Consumption Clearance Volume

Cylinder Bore Cylinder Stroke Density

Engine Rotation Rate

Friction Mean Effective Pressure Gas Constant

Indicated Mean Effective Pressure Particle Velocity

Power Pressure

Pumping Mean Effective Pressure Swept Volume

Temperature Torque

Trapped Swept Volume Trapped Charge Mass Trapped Air Mass Trapped Fuel Mass Volume

SYMBOL