Feature: The Versatility of Yamaha 4-Strokes - Engine Innovation

Counter shaft for greater compactness and performance

The rpm of the crankshaft on a high power output, high-revving engine are too high to transmit directly to the primary sheave of a continuously variable transmission (CVT). Furthermore, the drive force at the track is a product of crankshaft torque multiplied by engine reduction ratio, the CVT gear ratio equivalents and the secondary reduction ratio divided by the track sprocket's pitch diameter, and rpm levels at the CVT's primary axis that are too high will result in horsepower loss (windage loss, etc.).
For these reasons, a separate output shaft from the crankshaft called a counter shaft is used on 2- and 4-cylinder engines to reduce rpm while also regulating torque and controlling torque / rpm variance at the primary clutch of the CVT. The critical element in the development of the counter shaft assembly for this engine was the hub damper that fits inside the shaft's bearing and functions to control rpm and torque variance.
This damper must have consistent hardness and elasticity to withstand strong external forces and temperatures as low as -30 degrees C. At first, the Yamaha engineers were hypothesizing a damper of about 30 mm in size, but when the conditions it would have to withstand were relayed to the development company involved in its joint development, the estimate that came back was for a unit at least 400 mm in size. That meant the development team faced the formidable task of creating a damper approximately one-tenth the size of what a conventional unit would be. They proceeded to search for ways to reduce the damper size and heighten its durability through changes in structure and repeated design improvements while also making efforts to increase space for the damper by redesigning the surrounding parts. It was a process plagued with difficulties, as early prototypes would soon crumple to pieces or chip away, but in the end an optimum damper was designed and fitted into the counter shaft assembly. This hub damper truly became a backbone element of the engine's low-vibration, low-shock character while also contributing to its very linear power development characteristics.
Later, when members of the development team would look back and say that the 4-cylinder layout would not have been possible for a snowmobile without this hub damper, it was evidence that this was more than just an innovation of the power unit. By reducing the size of the damper from 400 mm to approximately 40 mm, it was also an innovation that contributed significantly to reducing the weight and size of the 4-stroke engine. As a result of the efforts to reduce the weight and size of the engine, the designers were then able to envision ways to create new space that would expand freedom of design for the suspension and sub-assemblies. The end result was that they succeeded in making advances in areas like the ski stance and chassis dimensions that improved the "total performance" of the 4-stroke snowmobile.

Engineered to reduce pumping loss

Engine performance is usually spoken of in terms of the three areas of intake/exhaust efficiency, combustion efficiency and reducing horsepower loss (mechanical loss), and one of the elements of this third area of horsepower loss is pumping loss. It is the loss associated with the motion of the piston. When the piston is driven downward by the force of combustion in the combustion chamber, the power created at the top of the piston meets with resistance (pumping loss) created by the air, oil and other gaseous matter in the crankcase that the piston must compress as it descends.
Therefore, one way to reduce horsepower loss is to reduce the amount of air and oil in the crankcase. Theoretically, there would be no pumping resistance if there were a state of vacuum in the crankcase, although in actuality there is a constant churning of air and oil there when the engine is running. However, pumping loss can be reduced by reducing the air pressure in the crankcase and making it as close as possible to a state of vacuum.
All Yamaha 4-stroke snowmobile models adopt a dry sump type lubrication system for the sake of compactness, and inherently the dry sump system only requires a very small amount of oil in the crankcase. In short, the dry sump is a system that only applies oil at the points where it is necessary. Unlike a wet sump system, a dry sump system does not have an oil reservoir in the oil pan that is constantly churned by the crankshaft, and that itself helps reduce pumping loss. There is a large amount of research that has been conducted to find ways to develop on this advantage in the automotive industry, especially for race machines like those in F1, high-class sports cars and motorcycles as well. Yamaha engineers have also developed designs to reduce pumping loss by means of dry sump systems in the 2-cylinder and 3-cylinder engines of its snowmobiles.
These 2- and 3-cylinder engines incorporate a "breather box" to separate out the gaseous element in the lubricating oil. The lubricating oil is gathered by a large-capacity scavenge pump (for oil collection) along with the residual blow-by gas it contains and sent to the oil tank, where the gaseous element is separated out by means of a centrifugal separator. From there, the air (gas) is sent to the air cleaner and the oil is circulated under pressure by means of the feed pump. This minimizes the flow of excess oil in the crankcase and reduces its air pressure to as low as -40 kPa (for the 3-cylinder engine) at the point of max power output. This greatly reduces pumping loss and helps create an excellent feeling of power development up through the rpm range.

Low-pressure cast upper case for greater engine toughness

One of the exclusive design features of Yamaha's 3-cylinder engines is the upper case made by low-pressure casting and the closed-deck type cylinder structure. Low-pressure (LP) casting is a manufacturing process in which molten aluminum alloy is injected into the casting mold at relatively low pressure. The process takes more time than high-pressure (HP) die-casting, but the resulting cast piece has fewer air bubbles in the solidification structure and is thus stronger. Another merit of LP casting is that it can utilize sand cores. A sand core is a core piece made from a mixture of sand and oil in the shape of what will be the hollow part of the final cast. The sand core(s) are inserted into the main mold and then molten aluminum is injected into the spaces between the main mold and the core piece(s) to make the cast. After the aluminum solidifies, the mold is struck so the sand core crumbles away, leaving the completely shaped cast piece.
This process is used to cast the upper (cylinder) case of the 3-cylinder engine of the FXNytro series models. It enables a complex shape with water jacket passages twisting through it, finely shaped ports and oil passages that could not be cast with the HP die-casting method. Casting this complex design in a single piece also contributes to greater unit rigidity. This upper case is used with a closed deck type cylinder* head design that further boosts engine rigidity.
The high rigidity achieved in this cylinder head and case assembly makes a higher compression ratio possible that contributes to high power output and torque. The greater strength and rigidity also reduces heat-induced distortion in the cylinder, which makes for more stable piston ring motion and in turn reduces oil consumption. When the motion of the rings is more stable, it also reduces the amount of blow-by gas escaping into the crankcase through the slight gaps between the piston rings and the cylinder wall. The reduced amount of blow-by gas coming in contact with oil in the crankcase also slows oil deterioration.

*Closed deck cylinder: A type of cylinder head design in which only a select number of the coolant passages are left open. The type in which all the passages are fully open is called an open deck cylinder.

Fine-tuning the feeling of the acceleration

The main lineup of Yamaha snowmobiles use fuel injection systems. Unlike carburetor systems where negative pressure is used to atomize the fuel and suck the air-fuel mixture into the combustion chamber, a fuel injection system uses electronic control to inject finely atomized fuel into the chamber under high pressure. Furthermore, when increasing the amount of fuel supply to the engine (opening the throttle) with a carbureted system, the additional fuel has to be supplied all at once during a time slot that remains constant in length, whereas with fuel injection, rate of fuel flow through the injector remains constant, so the amount of fuel supplied (injected) is determined by the length of the injection.
The primary merit of fuel injection is the superior starting performance it provides. This is especially important with a snowmobile engine that is often started in extreme cold where a rich air-fuel mixture is needed instantaneously at the start and then a leaner mixture suffices as the engine warms up. The capability of a fuel injection system to perform this adjustment in the leanness or richness of the air-fuel mixture automatically ensures stable performance at all times, from starts through warm-up and then throughout the ride.
The second advantage of fuel injection is the superior fuel economy and cleaner emissions it can provide. The ability to control the air-fuel ratio in accordance with changes in riding conditions eliminates rich fuel conditions, which means greater fuel economy as well as fuller combustion resulting in cleaner emissions.
A third benefit of a fuel injection system is that it is affected very little by changes in machine attitude (lean and pitch) as you ride. This is because the fuel injection system uses an electric motor and pump to pressurize the fuel before sending it to the injector for injection into the engine. Since the fuel sent to the engine is always pressurized, it is very difficult for extremes such as hill climbs or going over bumps to cause stalling due to factors such as the fuel becoming suddenly diluted with air or for the fuel flow to the engine being interrupted.
A fourth advantage of a fuel injection system is its outstanding capacity to compensate for changes in running conditions or the riding environment. Because it can be mapped to automatically control the air-fuel mixture in accordance with changes in air temperature, air pressure and engine temperature, fuel injection is highly adaptable to changes in weather and altitude.
Yet a fifth benefit of fuel injection is that its digital control functions can be used to program the systems of the various models to achieve the type of drivability desired for each particular model. Thus, it is possible to set the system to provide quicker response on a sport model, while the systems on touring or utility models can be set for easier drivability in the practical use rpm range. What this means is that a fuel injection system is highly compatible with intake/exhaust systems like Yamaha's EXUP. It can be used quite effectively in the 12.5: 1 air-fuel ratio range that is especially advantageous for strong power development.
Yamaha places particular importance on using these qualities of the fuel injection system to fine-tune the feeling of the acceleration on its models. In addition to the feeling when full throttle is applied all at once, we also fine-tune the fuel injection system settings with special attention to the feeling when accelerating from partial throttle-for example from 2,000 rpm up to 5,000 rpm. Fine-tuning the settings in this way is actually a process of programming a number of parameters for variables in riding conditions (running environment) into the ignition timing advance maps and the fuel injection volume maps. It is in this process of programming the various combinations of maps that the engineers achieve the finely tuned performance characteristics they want the machine to have.

The passion of technicians hidden in the bead

In a well-tailored suit, you will see the craftsmanship of the tailor in the type of thread, the needlework and the direction of the stitch. You will find the same type of craftsmanship in the weld made by a skilled welder in manufacturing. In the welds of the titanium exhaust pipes of the Apex model's exhaust system, you will also get a glimpse of that artful craftsmanship in the bead of the weld.
The exhaust system of the Apex uses a 4-into-1 format that helps optimize the exhaust pulse effect for each cylinder with great efficiency to contribute to outstanding performance. A continuously variable valve (EXUP valve) is situated in the section where the four exhaust pipes come together with a mechanism that improves intake and exhaust efficiency by opening and closing in accordance with engine rpm. The titanium tubes used in the exhaust pipes are chosen for their light weight, which also contributes to mass centralization so that the exhaust system is not only improving engine performance, but also contributing to the machine's handling performance.
Titanium is not a metal that can simply be substituted for the conventional stainless steel used in other exhaust pipes. It has to be used in a design that not only provides basic performance functions, but also takes into account the inherent qualities of the titanium itself. The basic functions include the necessary pipe length, shape and having the strength and rigidity to limit the reverberations and vibration resulting from the exhaust pulse. As a material, titanium is light, provides excellent strength, corrosion resistance and heat resistance. However, it is not an especially strong material when heated to high temperatures. Additionally, motorcycle exhaust pipes are subjected to vibration from the engine and from the road that produce various forms of vibration in the pipe itself. To accommodate all these factors, the system design involves welding the pipes to brackets.
Based on extensive simulations and analysis, it was discovered that the various forms of vibration could best be suppressed by positioning the bracket relatively close to the engine-side ends of the exhaust pipes. Compared to aluminum or steel, welding titanium is technically difficult. Due to the possibility of interaction with a number of elements, certain conditions need to be prepared such as blocking oxygen and nitrogen to some degree when welding at high temperatures. Furthermore, if there are any gaps between the bracket and the pipe, the strength of the joint depends on the weld alone and the possibility of breakage increases. So, it is necessary to make sure the pipes fit into the bracket as snugly as possible before welding.
Then there is the weld bead. At first glance there may not appear to be anything special about this bead, but in fact it is an important element developed with painstaking care by Yamaha technicians. Currently as you will see, the bead is a "U" shape. At the beginning of the development process however, the welds were made in a straight line along the edge where the pipe met the bracket or other shapes. But when those original welds were strength-tested, they easily cracked and broke. Working from that point, the technicians developed specialized know-how in the process by creating simulations, conducting analysis and strength-testing prototypes until they found the optimum bead shape, amount of welding wire to use, level of voltage to weld at and the thickness [volume] and direction of the bead. Thereby they arrived at the best combination of welding factors to reduce stress and produce the strongest weld. The product of all these efforts is the shining bluish bead you see on the exhaust system today, and what it represents is one more result of Yamaha technicians and their passion to reduce machine weight without compromising strength.

Belt developed to deliver highly linear drive force

A CVT unit for a snowmobile has to have a specially designed structure. Unlike the CVT unit of a vehicle like a scooter, where it functions as a single "clutch + CVT" unit with a centrifugal type clutch and separate transmission, a snowmobile uses a "belt clutch" in order to reduce weight and enable a more compact unit design. In the absence of a separate centrifugal clutch, the clutch function is achieved with a design in which drive force is initially created when rising engine rpm causes a given level of movement in the primary-side (drive) pulley and the sheave takes hold of [pinches] the belt. After the rpm reaches a certain level, the sheave has fully engaged the V-belt, which up until that point was slipping against the sheave. Because the clutch function involves this slippage while engaging, the V-belt of a snowmobile must have good heat-resistance and strength. Yamaha's approach to achieving the required system durability is by developing both the V-belt and the CVT mechanism to Yamaha standards.
The primary material used in the V-belts is rubber, and it must have a high level of heat resistance. Fortunately, a snowmobile is used in low air temperatures, and that advantage is maximized by thorough air management (the efficient induction of outside air) to achieve optimum cooling effect. Other design elements are also used to inhibit heat generation, including the "thrust balance" (the force applied to the belt), the angle of the contact surfaces of the belt and sheave (how they make contact) and the shape of the back surface of the belt. Note that the material and shape of the belt is based on joint research with Japan's leading belt manufacturer, Mitsuboshi Belting Ltd., with a constant exchange of data concerning belt length, width, thickness, cog shape and match with the sheave, etc., during the development process.
This obsession with perfecting the belts used in the CVT units of Yamaha snowmobiles comes from the belief that it will be a big waste if we can't turn the awesome power of the engines we create into maximum drive force with as little loss as possible through a mechanism that will continually perform at top capacity for long periods of use. That's why this 1,133 mm V-belt is a major feature that supports the very linear and stable drive train performance of Yamaha snowmobiles.

The composition of Yamaha's engine oil

Having their machine start right up at the turn of a key in cold weather is a pleasing moment for all snowmobilers. One of the important factors that enables this is the viscosity of the engine oil. If it is a high-viscosity oil, it will thicken at below-freezing temperatures and inhibit starting performance. Overcoming this problem by means of a higher amperage battery will add weight to the machine and detract from the light feeling ride and handling riders want. Since it affects both starting and machine weight, engine oil has an important role and that is why Yamaha developers have worked hard to offer quality oil specially tailored for snowmobiles.
Engine oil performs a wide range of functions as a lubricant, a cleaning agent, a sealant, a cooling fluid, an anti-corrosion agent, an oxidation stabilizing agent, and more. To fulfill these roles, engine oil is made up of a base oil that accounts for between 80% and 90% of the mix, while the rest is additives. These additives increase the performance of the base oil by boosting its anti-abrasion function and its resistance to oxidation while also regulating its viscosity and increasing its capacity for reducing friction.
Of the common types of base oils used for engine oils, the first is mineral oil refined from crude oil. This type of oil is relatively inexpensive to produce, but its viscosity changes considerably when the temperature rises or falls. The other common type of base oil is synthetic oil that is chemically synthesized as a composite of raw materials made from crude oil and other substances. Because the synthesizing process can bring together various elements in ideal combinations, it is possible to develop synthetic oils that maintain good fluidity at low temperatures and also maintain good lubricating membrane performance at high temperatures.
Because of the many processes involved in manufacturing this type of synthetic oil, it is more expensive. This led to the creation of a new category of oils called semi-synthetic oils that are a blend of mineral oil and synthetic oil.
YAMALUBE / 0W-30, our Yamaha Genuine factory installed engine oil is a semi-synthetic oil developed especially for snowmobiles through a research project aimed at achieving the functions and ideal composition a snowmobile oil needs. It features an optimum blend of highly-refined mineral oil and selected synthetic oils that have passed the stringent demands of snowmobile use to the highest Yamaha standards so that we can offer it with pride under our YAMALUBE brand name. This is an oil that maintains excellent fluidity at low temperatures and provides reduced churning resistance and friction in the engine to help ensure good starting performance. Easier starting means less load on the battery, which in turn helps provide a high level of reliability.