A two-stroke engine’s cylinder block has cut into its walls, ports or openings which perform the duty of evacuating burnt or exhaust gases. Another set of ports transfer the pre-compressed fuel air charge from the crankcase into the combustion space via two ports. Usually when three ports are present, the third port is a boost port located at the rear opposite the exhaust port. This port can start flowing slightly before the main ports, with the main ports or marginally after the main ports start to flow. A five port engine has its two main ports divided by a wall. Each main port therefore becomes two. So the two main ports are aided by two auxiliary ports and along with the boost port the total number of ports are five. The transfer ports dictate power output, along with certain key features of course; they are also responsible for power spread or power band. When the main transfer ports are divided, the total area, even if not increased can be controlled in a much better fashion. The auxiliary ports can be aimed differently and with staggered timings. The combination of five ports can create a well directioned and controlled turbulence in the combustion chamber. So in the case of two-stroke cylinder ports, more is better.
The valve intake system which works in tune with the crankcase pulsations prevents precious blowback through the carburettor and improves low end torque. The five port cylinder layout accounts for pronounced control over the entire transfer phase resulting in improved combustion.
More power is available, through the entire range but emphasised in the lower rpm range where the engine spends most of its active life. When an engine’s operating efficiency increases, the fuel consumption always decreases. The engine has been revamped in another critical area as well. Piston ring thickness in the earlier engines was 2 mm. This engine carries rings which at 1.5 mm are 0.5 mm thinner. Now this, to anyone other than engineering students, would seem almost like an insignificant decrease. A piston ring is the most important component in the piston cylinder combination and its healthy working depends on quite a number of factors. Not the least of which is its thickness. The most obvious benefit of this reduction in thickness is the contact area which being less makes for less friction. Reducing friction by any means in an engine can bring about large gains overall. A thinner piston ring obviously weighs less and therefore requires less gas pressure to keep the engine expanded and provide adequate sealing. In the normal course of events piston rings do form an effective seal because they get a lot of help from the cylinder pressure itself; gas pressure above the ring forces it down in its groove against the bottom of the groove and acting behind the ring in the back of the groove shoves it hard against the cylinder wall. Thus the gas pressure holds the ring in place.
When piston acceleration exceeds the sum total of gas pressures holding the ring outwards, the ring flutters; a situation where the ring will lift upwards and gas pressures are relieved from behind the ring, leaving it to fend for itself and as its own feeble pressure outwards is much less and no match for the pressures in the upper cylinder, the ring fails to seal.
Although the move from 2 mm thick piston rings to 1.5 mm rings is certainly in the right direction, the 2 mm rings will show no signs of disturbance or power loss below 7000 rpm as compared to the thinner ring. The 2 mm ring will start to flutter at something like 60,000 ft/sec2 piston acceleration. The engine stays well below that figure and doesn’t seem to be in danger of suffering from ring flutter even with the thicker ring. Having said that it is still a move in the right direction, even considering that the thicker rings scores in terms of reliability right up to the point where piston acceleration causes it to flutter.
With the reduction in piston ring thickness, the top ring land has been reduced, the claim being that the HC emissions are reduced. This is effected because of better and more complete burning. An added benefit and a very real one is that with the top ring higher up on the piston, though it is subject to flame heat, much better control of exhaust port opening and closing is managed and engine efficiency increases.
In keeping with all the changes to the induction and porting layout, the location of the spark plug has been moved to a spot where the redirected charge would, after following its swirl pattern, be trapped to aid complete combustion. Many other factors determine the best locations for spark plugs, like proximity to piston crown, ease of replacement and short flame travel following ignition.
The exhaust system will also be suitably modified to blend with all the radical changes made to the engine and the total package, power plant, along with the exhaust system succeed in making the engine a truly modern two-stroke engine.