Karts are made of steel tubes with different compositions of Carbon, Nickel, Chrome and Vanadium. Such tubes have particular characteristics that determine the functioning of the chassis. Elasticity of the material, diameter of the tubes and thickness of tube walls determine the capacity of the chassis to bend, absorb bumps in the track and generate vertical forces on tyres as to create downward forces that are the base for grip generation between tyre tread and asphalt. All these situations are possible only because steel tubes are linked together in a certain way. This “way” is welding of the tubes one to the other.
What is a welding?
A welding is a volume of material that has been fused and solidified by the heating with different systems of a volume of material. This volume usually is composed by the sum of parts of two different elements that have to be joined one to the other and some additional material added and mixed with the two parts. In karting weldings are used for unifying together the tubes of the chassis and positioning parts such as rear axle bearing mounts. The way the weldings are realized determines much of the quality of the chassis in terms of resistance, flexibility and deformation.
Kart chassis are welded using a welding machine that creates a high tension between a cathode and an anode and, through the passage of energy between the two, generates very high temperatures. This high temperature melts the metal of an electrode and of the tubes of the chassis. These metals mix and solidify together creating a unique metallic element that links the tubes together.
These electrodes are made of iron or, for better quality, of steel. Finally the welded parts will show an area of melted and solidified material that, in ideal conditions and perfect welding procedure, will have almost the same strength and characteristics of the tubes themselves. In reality though, air and high temperature determine oxidation of melted metals and change in the microscopical structure of metals. This can bring to weakness of the welding.
Welding machine and process
A welding machine has the capacity of generating a tension between two electric lines that end one with a clamp, that has to be positioned on the chassis, and one with a changeable electrode that is mounted on a second clamp held by the welder. The welding machine has a scale that regulates the tension between the clamp, and the chassis to which the clamp is linked, and the electrode. When this element is positioned close to the clamp and/or the chassis the high tension between the two elements creates a passage of electricity and the creation of great heat between the parts even though they are not touching each other. The greater the tension regulated on the machine and the earlier and with greater distance between the two elements we will have a passage of electricity and the melting of both the electrode and the area of contact between the electrode and the chassis. This will lead to a fusion between the metal of the tubes in contact and the material from the electrode.
Welding is a complicated process that gives good results only if the welder has the capacity and experience to obtain the right fusion and melting of material from the electrode and the tubes. In fact if tension is regulated too weak no melting effect will be generated and we will only obtain a continuous and repetitive sticking of the electrode to the tubes with difficulty to take it off once it is stuck. On the other hand a too strong tension will determine an excessive fusion of material with holing of the tubes and weakening of the entire structure. The choice of tension must be done based on the thickness of the electrode and the thickness of the elements joint together (walls of the tubes).
The welding done by the welder manually usually is completed in more phases. After each weld, along the line of contact of the two tubes, the area, that has partially cooled down and from an orange-red colour given by incandescence has become grey, must be checked to see what material has really generated a good and strong link of the tubes, and which is instead only laid over the tubes and has no real joining effect. The first “good” weld shows a silver shiny area, the second is mainly a grey unsmooth surface. Best check is made by hitting hard the welded area with a metal hammer and a metallic brush (often given with the welding machine as kit, together with eye protector glasses). If the material is correctly hardened it will not come off even with the hardest hammering you can do, otherwise metal parts will come off, which means they would not anyway resist more then a few minutes when the kart is running and bending.
Practical welding hints
Let us see how to proceed when welding two parts of a chassis. First of all be sure to position and fix well the two parts that need to be joint together. The surfaces that are in touch must be in good contact and well blocked. Then position the clamp on one of the two parts. Be sure the clamp is in good contact with the metal. If the clamp is positioned on a chassis tube verify that paint does not isolate the chassis from the clamp, in fact there must be contact metal to metal to obtain tension transmission to the area that must be welded.
Now try to set voltage on the welding machine starting from a low value, usually indicated on the instruction sheet of the machine depending on the thickness of the electrode used. To warm up the electrode touch quickly the clamp that is surely the element with better tension transmission compared to the chassis of the kart. After the electrode has lit up producing some sparks it is ready for welding. Always cover eyes with welding mask or glasses!!! This secures from sparks in the eyes and even the strong light coming from the lit electrode must be absolutely avoided since it can harm seriously your eyes and anyway partly blinds you for some time.
Start touching quickly the area to be welded and see if sparks are produced. The quickness in doing this is motivated by the fact that if the movement is too slow the electrode sticks to the metal tubes and is very difficult to pull off. The real welding must be done positioning the electrode at a certain distance from the area that has to be treated. Also the electrode must not be vertical respect to the surface to be welded, but inclined. This should permit a continuous movement of the electrode on the surface with production of a strong light and no sticking of the electrode to the metal. If this does not occur and the electrode goes on sticking to the metal we can try to increase the tension on the welding machine and this will help avoid the problem. After each weld hit hard the area with a metal hummer so all the waist material comes off and leaves a good clean surface on which to work on.
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Pistons are elements that lay an important role in engine performance and reliability. They in fact act as valves on the gas produced by mixture burning in the combustion chamber. These gases at high temperature and pressure have a very high quantity of energy that can be transformed to mechanical energy through the con-rod and crank-shaft system. This system is moved thanks to the capacity of the piston in sealing the volumes over and under it in the cylinder and crank-case. Also in this passage mechanical energy of the piston expressed through a longitudinal movement is transformed in rotational mechanical energy. All this happens with great forces acting on the piston (mechanical stress) that are added to the high temperatures that the piston reaches in contact with the burned gases (thermal stress).
Pistons move longitudinally in the cylinder, but their speed changes constantly in a sinusoidal way. This means the piston starting for example at top dead corner (TDC) has a speed equal to zero. Then it accelerates quickly reaching top speed when the con-rod is at 90° respect to the crank-shaft arm. After that speed decreases again and is null at bottom dead corner (BDC). This quickly changing speed indicates strong accelerations (both negative, deceleration, and positive). Any acceleration generates a force called inertial force. Inertia is the capacity of every body to oppose itself to acceleration (negative and positive). The opposition is the inertial force. Inertia increases with weight of the element and the square value of otational speed of the crank-shaft. Maximum negative and positive inertial forces are at TDC and BDC.
In addition to the longitudinal movement along the cylinder axis he piston has orthogonal movements that are determined by the lateral forces generated by the inclination of the con-rod respect to the cylinder longitudinal axis. Since the diameter of the piston is slightly smaller than the one of the cylinder, the piston can move sideways inside the cylinder. When the piston is moving upwards it is pressing on one side of the cylinder liner. When it passes the TDC and comes down the piston presses on the opposite side of the cylinder liner. This is due to the fact that the con-rod is inclined on the opposite side.
Total forces on the piston
In addition to the inertial forces acting on the piston, the head of the piston has to deal with another mechanical stress, such as the high pressure coming from the burned gases in the combustion chamber, and a thermal stress as the temperatures transmitted from these same gases. In particular in two stroke engines a cycle is completed in 360° of the crank-shaft, which means the piston has less time to cool down respect to the situation of a four stroke engine. In two-stroke competition engines such as kart engines the highest temperatures are reached at the centre of the piston head and can reach 400°C (752°F) in air cooled engines and 360°C (680°F) in water cooled engines.
Heat transmitted from the burned gases in the combustion and expansion phases to the piston head is then given to the cylinder through the ring(s). From 30% to 60% of the total heat energy is transmitted this way and helps cooling the piston. Piston temperature increases around 3°C every 100 revs/min and also 15° every bar of increases of medium pressure in the combustion chamber. Also combustion timing and compression ratio influence the temperature of the piston.
All these factors stress strongly all the structure of the piston and this is why materials and shape are well designed to obtain a very light and strong/resistant component. Aluminium for control and limitation of deformation given by high temperatures and additional elements added to the piston are key for obtaining a good result. Shapes also have changed a lot during the years, but finally they seem to be today very similar to one another especially in two-stroke competition engines for karts.
We will see in the next issue exactly how pistons are built and how the selection of the materials is made of a series of elements that help make a resistant component with also good thermal characteristics that help reduce friction, increase performance, reliability and durability of the piston and the entire engine.
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The MAX range of ROTAX engines has remained largely unchanged for the past fifteen years. This is the first time in the history of karting that a single engine model or type has continued to be successful and grow on the market to become the World leader. After this period of time the technology in production has improved to the point where latest electronics and mechanical design can be introduced without de-stabilising the existing product in use today. In short, the Rotax Max EVO engine is the natural progression of an already successful theme, to enhance and protect the market for the forthcoming years.
2. What are the changes to performance and power?
The EVO engine remains fundamentally unchanged internally, the crankshaft has improved tolerances in the area of the main bearing and oil seal journals. This means that all new products from the factory will have an end float on the crank when delivered, the oil seals will also be free running. This should reduce the need for pre-race preparation and therefore significantly reduce the price of the new engine to the end user. There is no difference in performance between the EVO engine and a current race-prepared unit. The Piston skirt has a slightly different taper, this allows quicker running-in and reduced wear in use. Again, this should lengthen piston life and be less costly for the end user. The Con-rod has been altered slightly, the big end has been strengthened and now has two lubricating slots. The external component changes include ignition system and carburettor.
3. What do the dyno charts look like?
At the time of writing, there are no official Dyno charts available. In simple terms the ease of use shows through there is no specific gain in maximum horsepower. The mis-understood press release from the factory where gains in performance were reported, is as a result of BRP-ROTAX redefining their testing procedure and having improved Dyno technology for more consistent results. If anything, the Dyno charts for the Rotax Max EVO engine offer a smoother power curve which is less affected by changing meteorological conditions.
4. Will the engine be more reliable?
The detail changes inside the engine will all enhance reliability. The BRP-ROTAX warranty scheme is in place in order for the factory to closely monitor any recurring faults in the engine or its ancillaries. The Rotax Max is the most reliable kart engine that the sport has ever known, this is due in no small part to the engines built in component strength and the warranty scheme. The new Piston, Con-rod and refined crankshaft tolerances will all enhance component life and reliability.
5. Will the EVO engine be eligible for MSA competition?
Yes, but the question is when? The MSA are helping to arrive at the best possible solution for the introduction of the EVO specification. The good news is that there will be a number of offers in place to help the end user financially. There is no reason why the internal components cannot be installed as soon as they become available. The Piston is effectively the same, the crankshaft is the same component as before with improved tolerances. The con-rod is the only question mark as it is clearly not the same as the outgoing component.
The fact that the Rod does not affect performance but does improve reliability should help its introduction. All external accessories will be available complete with new engines throughout 2015 and as a subsidised upgrade kit to be used with existing engines from January 1st 2016.
6. How much will it cost to upgrade?
The engine unit will not need to be upgraded until it is due for a service, then the new replacement components may be fitted as standard parts. There is no specific price increase for these parts over existing components. Upgrade kits of external parts will depend on which class. MiniMax is expected to stay exactly as it is for the foreseeable future. Junior upgrades will include Carburettor, Ignition, wiring loom and Exhaust. Seniors will include all the junior parts plus the electronic power valve and its associated components. We believe that these will be at a very low introductory price of up to 50% discount.
7. Will servicing costs increase?
Definitely not. All new component prices will match their predecessors or be subject to introductory offers that could reduce servicing costs. New components are expected to have a significantly longer service life, thereby reducing running costs to the end user.
8. Can I convert my current MAX to a Rotax Max EVO?
Yes. The engine unit itself will not require upgrading, the new internal components may be fitted as and when the engine is due for service. There will be upgrade kits available to convert existing engine accessories to the new specification. These are expected to be available soon, but will not be in use in the UK for MSA racing until 2016. Click here to ensure you get the right Rotax Max jetting.
9. Can I buy individual parts to fit to my MAX?
Yes all components will be available as individual spare parts. It is not yet decided whether the EVO must be used as a complete set of accessories or if it will be acceptable to use some of the upgrade parts and not others. There will be plenty of time to test and decide on this before the official introduction of the accessories for MSA racing in 2016.
10. Summing up
1 The EVO engine is not being introduced to step up performance.
2 The EVO internal parts are introduced to improve reliability & component life.
3 The EVO accessories will not be eligible in MSA racing until 2016 to help class stability.
4 EVO engine accessories will not be more expensive
than current components.
Combustion in the combustion chamber acts on the piston with a pressure (force when multiplied by the piston head area) which generates a linear movement of the piston. This movement is then transformed into a rotating one since the piston is mounted on a reciprocating mechanism consisting of the crankshaft and conrod. The crankshaft rotates with the support of spherical bearings mounted in the crankcase referred to as the main bearings.
The force acting on the piston is transformed and transmitted through the conrod and crankshaft and must then be absorbed by the crankcase. The crankcase not only has to absorb these forces, it also has many other functions that make it an essential element of the engine. A fundamental role is to act as a pre-compression chamber. When the piston moves downwards in the cylinder, mixture is compressed and pushed into the transfer ducts, through the transfer ports and into the combustion chamber. In addition to this, the crankcase also has the function of being the bond between the engine’s different components such as the cylinder, crankshaft, carburettor, engine-chassis mount, ignition system and coil.
The material used to build the crankcases of modern 2-stroke competition engines, such as those used for karting purposes, is aluminium alloy, which reduces considerably the weight of the engine. Aluminium though has some weaknesses linked to the fact that it heats up more easily than cast iron and consequently deforms more. Its heating up generates a rise in temperature of the crankcase walls which consequently heats up the mixture in the crankcase. A fluid (mixture) that heats up increases in pressure and expands, reducing its density and the volumetric efficiency of the crankcase pump.
Temperature increases also generate expansion of the aluminium alloy and therefore the walls of the crankcase (they reach around 100°C). This expansion will be different in different areas of the crankcase since the component’s walls vary in thickness and also fresh mixture can act as more of a coolant in some areas than others. Consequently, the crankcase will deform and generate negative effects. For example, alignment between the two main bearings that carry the crankshaft can be lost. Gas sealing between the crankcase and cylinder can also become critical. Another important effect is that the perpendicularity of the cylinder’s axis to the crankshaft is at risk. Finally, main bearings can change the tightness of assembly of the roller balls inside the cages which can produce seizures and breaking of the cages.
The limited mechanical strength of light aluminium alloys from which crankcases are constructed also needs to be taken into account with regard to the threads of the holes for the holding down bolts and the cylinder head studs. To avoid rapid damage to the threads it is advisable for the effective threaded length to be not less than 2.5 times the diameter. Assistance can also be given by the insertion of steel threads that are much more resistant, especially when studs and bolts are frequently screwed and unscrewed. Since the crankcase working as a pump needs reduced internal volume to be effi cient, it is constructed of two halves symmetrical with respect to a vertical plane perpendicular to the axis of the crankshaft. When joined, the two halves must be completely gas tight and to do this a gasket is also positioned between the parts. Mixture loss would reduce the efficiency and performance of the engine.
To have good matching of the two symmetrical parts, production must be very precise. Cylindrical dowel pins also help couple the two elements. Sealing must also be obtained where the crankcase opens to permit the exit of the ends of the crankshaft. Oil seals are used on both sides and are rubber rings that have one or two ‘lips’ that seal the area around the crankshaft. The aim is to have good sealing but low friction loss generated by the ‘lips’ and the crankshaft that are in contact with one another. As already mentioned, the crankcase also has the role of absorbing all the forces transmitted by the crankshaft. To limit deformation of the crankcase it is built with ribbing all over its external surface. These ribs help both to strengthen the structure and to cool down the surfaces
Iam sure that everyone who has raced seriously in karting will have heard of IAME. If you have raced 100cc machinery then you will have either used one of their products or been beaten by one of their products at some time in your career. The IAME story starts a long time ago when only a handful of those still involved today were karting back in 1959. The Parilla motorcycle factory decided to put their toes in the water of the burgeoning kart market by producing a bespoke engine for this growing sport. The engine was a fan cooled 2-stroke of 100cc capacity with a 48mm bore and a 54mm stroke running up to 11,000rpm. The engine probably produced about 11 horsepower and was suitably called the V11. This engine was designed by Cesare Bossaglia, a name that was to become synonymous with famous models of kart engine for many years. At that time Bruno Grana was export manager for Moto Parilla.
A very good relationship developed between Grana and Bossaglia and, following the success of the V11, many other Parilla kart engines were produced such as the S12, S13, BA13, TG14 and the GP15. All these models were Bossaglia creations, culminating in the first World Championship win for the GP15 in the hands of Guido Sala in 1964. In 1961 Grana founded a new company named Komet Italiana and contracted Bossaglia as his designer. The product of this relationship was the Komet K12 with an ‘over square’ bore and stroke of 51 x 48.5mm, this was also a fan cooled engine and its success was guaranteed when a single order for 750 units was received from ItalKart. Incidentally, Tal-Ko took their name from these two manufacturers, ItalKart and Komet, for whom they were the UK concessionaires. Cesare Bossaglia continued to work for Grana on a freelance basis and through the following few years Komet and Parilla continued to compete against one another with definite divisions between their supporters. All the popular Komets were short strokes and the Parillas long strokes. Komet users would always have bigger rear sprockets and tended to feel superior in that era.
The Komet K77 and later the K88 in particular, were very successful indeed. In 1968 Bruno Grana was approached by three American industrialists and together they formed the new company of Italian American Motor Engineering (IAME). After a while the American interest died away leaving Bruno Grana with total ownership and control of IAME, a position he was to retain until he died in August 2005. In 1975 IAME took over the Sirio company that belonged to the Rovelli family, and in 1976 they also took over the BM company. The four big names of Parilla, Komet, Sirio and BM then formed the nucleus of IAME with these four names still branded products today. Bossaglia was a vital part of IAME until he died in 1985. While the company has always concentrated on kart engine production they did produce engine for light aircraft under the KFM brand between 1981 and 1991. These engines were produced in both 2 and 4-stroke models. At that time they became the reference point in their field for quality and performance. I believe it is true to say that Bruno Grana was IAME. He ran the company with a passion for the sport he loved. While he was ultimately competitive by nature, he also believed in the future of the sport as a whole and he would often sell vital components to his arch-rivals. The quality of IAME components is well known and many winning engines of other makes will be found with an IAME connecting rod.
IAME manufacture their own crankshafts and conrods, generally respected as being the best. Some years ago, pistons were the Achilles heel of 100cc engines, Grana set about finding where the stress came from that was causing piston failure and the IAME piston developed almost 15 years ago solved the problem. This piston has been copied but never bettered and remains the preferred choice to this day. Anther important figure joined IAME in 1978, Paul Conde. Well versed in international business, Conde became Grana’s right hand man. As IAME expanded Conde gave the company extra flexibility that meant that they could be represented in more than one place at a time. Paul also took care of the international distributors. Cedi Nap in France is a wholly owned subsidiary of IAME managed by Thierry Seminger. France is an important market for IAME, the infrastructure for karting and their unrivalled circuits make it an ideal testing ground for new products. France was among the first to run TaG engines in any quantity and they now have strong sales for the Leopard, X30 and 80cc Gazelle models. IAME currently produce in excess of 6,500 engines a year in about a dozen different model types with the factory employing 55 people in total. It was a real pleasure to have the opportunity recently to tour the factory and see everyone quietly going about their business. There is no rush or panic here, just well ordered, quality engineering taking place.
The factory is on one floor with plenty of space between workstations and machinery. There is a wide range of lathes and milling machines with both manually operated and modern, multi-axis, numerically controlled examples. There are production build areas and a specialist build shop for race team equipment. No one seems to be in a hurry but the work gets done quickly and efficiently. While we were walking through, I saw one man assemble a JICA engine in a matter of minutes. True, all the correct tools and equipment were to hand, but it was easy to see that this bloke knew what he was doing and getting it right was second nature to him. There is a separate design and development department where I saw a piston being measured on a shadowgraph. Next to the R&D offices were three dyno rooms. In fact this was the only area that our happy snapper Chris Walker was not allowed to photograph in detail. There was a new model TaG engine revving away on one dyno and another on the bench being examined. I have to say they look the business! Probably 50 or 60 race engines all prepared, labelled and ready for action also really looked the part. There is no pretension at IAME, just solid purposeful manufacturing of a world-leading product. The best quality raw materials are a prerequisite and quality is controlled right through the factory until the finished article is produced. IAME have a reputation for best quality products and many other manufacturers aspire to equal them. It is a matter of opinion whether any of them achieve it. Since the death of Bruno Grana, Mr. Fagnani has been appointed as Managing Director. With no disrespect to Grana, Mr. Fagnani has a modern attitude to business and management.
While the industry has unlimited respect for the memory of Bruno Grana, it has to be accepted that things will change and these changes will be for the better. While Grana had a very successful, if autonomous management style, Fagnani has a much more open and modern approach. He believes that every member of staff has their special skill and that they should have a voice. In this way he has opened up the lines of communication within the factory to unprecedented levels. Every member of staff is now able to put their point of view to improve any aspect of the IAME product. A board of directors has been formed including Paul Conde and the production manager Guiseppe Mioso. Other important names in the management structure include Mr. Molinari, head of design, Mr. Pelizzoli the purchasing manager and Pinuccia Donatelli who keeps them all in order as head of administration. The final jewel in the crown came last December when Mr. Fagnani managed to woo Cesare Bossaglia’s son Andrea on board. While there are still strong ties with the past and sound foundations are essential to the success of any business, IAME are dedicated to the future. They are a stand alone kart engine manufacturer and do not make any other products.
While their race engines for Formula A, ICA and JICA are of great importance to them, they do have a very good feel for the commercial needs of the industry. At the moment their top selling model is the 125 TaG Leopard, followed by the newer model TaG X30. This has a more modern appearance with a very neat crankcase housing a balance shaft and starter. While the engine still has an external water pump at present, the Cedi Nap team in France have mounted the pump on the engine with a belt drive from the crankshaft. One of the advantages of the IAME product is the cast iron cylinder liner, allowing a wide range of piston sizes to be retro fitted, giving the engine a very long life expectancy. IAME also make all their own clutches, a quality example is the clutch drum that is machined from cast iron.
The JICA engine is third in their popularity poll which is not surprising really. JICA has become a Parilla benefit in the last couple of years and the PV100 Swift is the reference point in the class. It is a short stroke ‘over square’ engine. Perhaps it should be called a Komet! Chris Walker and I very much enjoyed our visit to IAME. We both thank Mr. Fagnani for changing his busy schedule in order to see us. We would further like to thank Mr. Bossaglia for his very informative factory tour. It has been a pleasure to write this article, I hope I have done IAME justice, at the very least it is an insight into the production of kart engines. IAME have been at the top of their game for many years and they look set to stay there for the foreseeable future.
One of the main aspects of two-stroke engine performance is scavenging. The way fresh mixture enters the combustion chamber and burned gases exit through the exhaust ports determines the quantity of mixture burned in each cycle of the engine (each rev for two-stroke engines) and the way it is burned.
Quantity of ports
A modern two-stroke competition engine for 100cc direct drive category is made of an inlet port, which differs for piston-port, reed valves and rotary valves engines. Transfer ports are then positioned at the end of the transfer ducts on the walls of the cylinder, these are usually two, symmetrically positioned (generally four on 125cc gearbox engines), and one called “TT” centrally positioned on the opposite side of the exhaust port. Exhaust ports can be of two different shapes, rectangular with curved angles and a central separating wall of 2-3mm thickness, or elliptical with two additional holes on the sides on the upper part of the central hole. These two additional ports are called “boosts”.
Functioning and aim of the exhaust ports
The importance and function of the ports is to define the inlet or outlet of fluid (mixture or burned gases) and the direction of the flow. Ports are shaped so that timing of their opening and closing is precise, in particular this works on transfer and exhaust ports, whilst inlet ports differ as said from the kind of valve used. The piston determines the opening and closing of such ports: the opening occurs when the piston is moving downwards and the closing when the piston is moving upwards. The “lines” that determine the timing are the piston ring (on “L” shaped section ones as on kart 100cc engines) or the piston crown (on piston with rectangular section rings as on 125cc gearbox engines), and the upper edge of the ports. This means that if the upper profile of the port is perfectly horizontal the opening of the port will be extremely immediate and precise, which is something that helps functioning of the engine and port timing. In fact one of the main aims of port timing is to obtain the inlet (in the combustion chamber) and outlet (in the exhaust duct) of the fresh and burned mixture as quickly as possible, but with the right timing. When the piston moves from top dead corner downwards and then opens the exhaust ports we want as much burned gases to exit through the exhaust duct as soon as possible, which means we want also a good total area of the exhaust ports so that more gases exit the combustion chamber in less time. This helps total evacuation of the combustion chamber from burned gases, which brings to depression of such volume and better inlet of fresh mixture from the transfer ports.
On the other hand though we must regulate the exhaust phase so that fresh mixture does not also exit the combustion chamber during exhaust phase, or if it does, it does so in limited quantity. This means a good area of the exhaust port sections, but not excessive.
Also another limit to exhaust port dimensions is that a perfectly straight and horizontal upper profile of the ports can determine, especially when ports have a good width, that the piston ring might get stuck inside the port with consequent ring breaking or bending, and possible final engine seizure.
For this reason we have the two solutions anticipated previously. A rectangular section port with central separation wall, or a more or less elliptical port with boosts (these can also be sometimes found together with the rectangular solution). The rectangular shaped exhaust port has surely a greater area of its section, but the central wall, used to avoid piston ring getting stuck inside the port, creates some interference with the gas flow exiting the port. On the other hand the elliptical exhaust port has a smaller area, but no obstruction to gas flow. This is because it needs no central separation wall, since the curved upper and lower edges of the port avoid ring blocking.
Upper area of the exhaust ports
The area of ports, and of the exhaust ports is extremely important for the exiting of the burned gases from the combustion chamber, but not “all the area” has the same importance. In fact the upper part of the exhaust ports is the area that is uncovered first by the piston. This means that when the pressure inside the combustion chamber is extremely high the exhaust port opens and gases exit through the opening of the first upper part of the exhaust port. The speed of the burned gases exiting the port is maximum since pressure is high and the open area of the port is still not complete. To have maximum area open as soon as possible the port must have a great area of its section on the superior part. This is why the best solution is the rectangular shape. But, as already said, such shape gives problems with reliability and piston ring damage, so a central separating wall is used. The great speed of gas flow in the final part of the opening phase of the exhaust port determines great turbulence of the flow especially because of the central separating wall. Such turbulence is also created by the sharp edges of the port. Turbulence means that the flow does not follow straight lines, but creates swirls. Such swirls reduce the flow of gas so that the result is an area of the port smaller than the real one, which means that the swirl and turbulence creates a block to the flow just around the central separating wall and the edges of the port.
How to reduce turbulence
To reduce turbulence we can work with sandpaper or machine the edges of both port and central separating wall. This must be done to eliminate sharp edges, but we shall not exceed because the edges of the port must always maintain a certain sharpness to give the right timing of port opening and closing and the right sealing. In fact if port edges are too much “rounded” gases will pass by piston and cylinder in the area of the port edge before the port is really open.
Also the central wall can be machined to reduce its thickness, but do not exaggerate this work as well since a reduced thickness of the central wall of the exhaust port can determine expansion of such area and a pressure between such wall and the piston, with possible engine seizure.
The same treatment can be done to the lateral boosts of the exhaust port that also work in the initial moments of the opening of the exhaust port, so gas flow is fast and easily produces turbulence. You will always find sharp edges in new engines since honing of the cylinder produces such sharp edges. Also from time to time repeat the machining and rounding of sharp edges after honing is done to the cylinder for example after engine seizure.
Reed valves are surely an important element of the engine and must be regulated and chosen to obtain a good tuning of reed valve engine. Performance but also reliability are determined by the use of a reed valve or another.
Materials, thicknesses and shapes
Reed valves seem to be a simple element, but are not. In fact they differ in shape, material and thickness. So many parameters are used to obtain the best of an element that “decides” the timing and quantity of mixture entering the engine. The more mixture entering at the right time and the better combustion and greater performance we will obtain from our engine.
Reed valves have rectangular shapes of the petals linked by a common base. The petals are usually four (two per reed valve) or six (three per reed valve), respectively in 100cc direct drive and 125cc geared engines. The reed valves are positioned on a metallic cage. The materials usually used are glass fiber and carbon fiber, but also metallic reed valves are used for less performing engines. Also the thickness of the petals varies determining a different rigidity of the reeds. Finally some reed valves have been produced with different shapes, such as double petals, to obtain best performance at different revs of the engine.
Functioning of reed valves
Reed valves are mounted inside the reed block that is usually positioned in kart engines on the front side of the crank-case. The depression or pressure that is generated by the piston in the volume of the crank-case determines the opening and closing of the reed valves. When the piston moves upwards a depression in generated inside the crank-case and the petals of the reed valves tend to lift and open the passage so that the external air passes through the Venturi of the carburettor and enters the crank-case as mixture. When the piston starts its way downwards the pressure under it increases and the reed valves tend to close initially because of the elasticity of the petals tends to reposition them in the initial shape and because the pressure in the crank-case is higher then the external pressure. The closure of the reed valves determines the compression of the mixture in the crank-case and the movement of the fluid through the transfer passages to the combustion chamber.
Limits to reed valves
The above situation is the ideal one. Unfortunately the high revs of competition engines used in kart racing, and also the wide range of revs (from 7000 to 20000) determine some malfunctioning of the reed valves that reduce performance.
The first aspect is the weight and inertia of the petals. The ideal situation is when the petals follow the pressure changes in the crank-case exactly with no delay. Unfortunately the weight of the petals determines inertia, which is the characteristic of every body to oppose itself to movement. This inertia generates a delay of the movement of the petals respect to the indications given by the pressure changes. This inertia is directly proportional to the weight of the body that has to be moved, in this case the weight of the petals. The heavier the material and the thicker the petals (which means more weight) and the more delay we will have in the movement of the petals. This will mean that the opening of the reed valve will happen later then in the ideal situation and less mixture will enter the crank-case. Also the reed valve will close later when pressure in the crank-case will increase. This will determine a loss of mixture from crank-case through the carburetor to the outside. This two “losses” will reduce the mixture in the combustion chamber, and less mixture burned means less performance of the engine.
A second limit felt much more at high revs is the deformation of the petals dew to the “resonance effect”. This phenomena happens when the movement of the reed valve is so fast that deformation of the petal happens not as a simple curve of the petal, but deforms with a sinusoidal shape, like a horizontal “S”. Closing and opening of the petals will not be precise anymore. Sealing by the reed valve will not be sufficient anymore and we will have mixture loss both in the intake phase and in the transfer phase (when the piston presses down in the crank-case and the mixture enters the combustion chamber through the transfer ports).
Choice of reed valves
In kart engines today the most frequent choice of reed valves is done to obtain the minimum weights with the maximum rigidity of the petals. The limited weight helps to avoid inertia, while rigidity eliminates resonance effects (shifting them to revs superior to 20000). Carbon fiber is usually the best choice, because it has the best rigidity and reduced weight. Some engine tuners have tried to produce carbon fiber reed valves with very little resin as to reduce even more the weight (carbon fiber is very light, the resin that keeps it together is heavier) and keep a good rigidity. This solution has given great advantages in performance, I was astonished by how much torque output I gained in some occasions. Unfortunately such reed valves often break and really do not last a complete race. They can be a good choice for qualifying, even though some bad lots break in the very first accelerations of the engine. Glass fiber reed valves are not as performing as carbon fiber, but cost less and generally last longer.
Even within the carbon fiber reed valves thickness must be chosen. A thicker valve usually helps at high revs, whilst a thinner valve is a better choice for low and medium revs. Make your choice by trying different solutions for each track.
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