[ Pobierz całość w formacie PDF ]
.They tend to closeand then rebound from their seats due to inertia and/or resonance in the inductiontract.A dual reed assembly patented by Eyvind Boyesen reduces this compromise aconsiderable amount.The presence of a reed cage and petals in the induction tract stillreduces high speed air flow below that possible with rotary valve or piston portedinduction, but the difference is not so great as before.The Boyesen assemblycomprises a thin 0.25mm reed, riding on top of a thicker 0.7mm reed.The thin reedopens easily under a low pressure drop and the thicker one takes over at higher rpm.This gives the benefits of good low speed air flow, as well as an absence of high speedpetal flutter.As an added benefit, the ribs in the reed cage can be cut out when theBoyensen assembly is fitted, such is the design of the thick petal.This alone improvesair flow and crankcase filling at higher speeds.Over the years a lot has been said about the benefits of reed valve induction, but itseems that very few people realise the very high power outputs now being produced bymotocross and enduro engines are not a direct result of reed valve induction.A lotseem to think that, because the piston is cut away, or has windows in it, allowing insome engines up to 360° inlet open period, this automatically results in a high poweroutput.I can assure you that this is not so.In itself a reed valve improves low speed andmid-range power only, by preventing blow-back.To give you some proof of why I say this, we will have a look at the effect ofadding a reed valve to an old 250 Bultaco Matador.In standard tune the engine hadexhaust, transfer and inlet open periods of 170°, 126° and 150° respectively.As shownin TABLE 3.5, the engine has a gentle power curve.It pulls very well at low speed andproduces a maximum of 25.8 hp at 7000rpm.In test 2 a reed valve was added and four16mm holes were drilled in the piston skirt, increasing the inlet timing to 360°.As youcan see, there has been very little increase in power at the top end in spite of the factthat a 34mm Bing carburettor was fitted to replace the stock 32mm Amal.Note, too,that there has been only a marginal decrease in low speed power, due to the reed valveoffsetting to some extent the bad effect the larger carburettor would have had at lowrpm.In test 3, however, you can see that power right through the range has risen by anaverage of 1.5 hp below 5500rpm, and by up to 3.1 hp between 6000 and 7000rpm.What brought about such a sudden power increase? In this test, two boost ports wereadded in the rear of the cylinder.The ports were cut with a 13mm cutter tilted at 30°.Soit was the increase in transfer port area which picked up power significantly, not theaddition of a reed valve.In test 4, there was an increase in power above 6500rpm, but a decrease at lowerspeeds.For this test, a new piston was fitted which had 13mm removed from thebottom of the inlet skirt to give an inlet open period of 200°.This means that the pistonexercises control over which direction transfer flow will take.In test 3, the boost portsare always connected with the crankcase (i.e., for 360°) but in test 4 the boost ports areisolated from the crankcase (see FIGURE 3.20) once the piston skirt drops below thelevel of the inlet port floor.Thus, any flow through the reed valve will be diverted up 63 Two Stroke Performance TuningTABLE 3.5 Effect of reed valve inductionrprn Test 1 (hp) Test 2 (hp) Test 3 (hp) Test 4 (hp)3000 6.8 6.4 8.3 7.93500 7.9 8.1 10.9 10.44000 11.9 11.3 12.1 11.84500 14.2 13.6 14.8 14.65000 16.0 15.6 17.0 16.65500 18.1 18.0 19.7 19.36000 22.6 22.9 26.0 25.76500 23.3 24.9 27.2 27.17000 25.8 26.7 27.8 28.47500 25.6 25.1 26.3 27.68000 23.7 24.8 25.5 26.28500 18.1 20.6 22.1 22.8Test 1 standard Bultaco Matador 250ccTest 2 reed valve assembly and 34mm Bing carburettor added; piston modified to give 360°inlet open period.Test 3 as above, with the addition of two boost ports in rear of cylinder.Test 4 as above but with piston modified to give 200° inlet open period, i.e., 'powerported'.through the boost ports once the piston closes the inlet tract off from the crankcase.With this arrangement, low speed power falls away, because the boost ports flow onlyif the exhaust pulses create a depression low enough to open the reed valve and pullfuel/air mixture up through the boost ports.However, at higher speeds, peak power isincreased with this system, because the piston closes off the crankcase from the inlettract, preventing back flow out of the crankcase as the piston descends to BDC.Without the effects of reverse flow to fight against, mixture will continue on flowingthrough the reed valve and up through the boost ports until cylinder pressure equalspressure in the inlet tract, causing the reed valve to close.When this latter type of 'power porting' (i.e., test 4) is applied to more moderntwo-stroke engines, there is often little or no loss of low speed power because of largertransfer port areas being employed today.However, on some bikes the power curvecan become very peaky, making the bike difficult to ride.This is why you will seldomsee this arrangement employed on anything but small displacement motocross bikesand reed valve road race engines.Of course, with many engines there isn't much youcan do to convert from the conventional type of boost porting to power porting, unlessyou can find a suitable piston from another engine which doesn't have windows in theskirt.However, with some engines, such as the Honda CR125R, it is possible toconvert easily to power porting.These engines have two small passages, instead ofpiston skirt windows, which connect the inlet tract with the crankcase
[ Pobierz całość w formacie PDF ]