Showing posts with label APRILIA MOTORCYCLE Engine. Show all posts
Showing posts with label APRILIA MOTORCYCLE Engine. Show all posts

THE DITECH ENGINE OF APRILIA


New regulations will introduce even stricter emission controls and many fear that this could finally be the end of the two stroke engine. New technology is needed for the two stroke to survive. Luckily this is already with us in the form of the decidedly future proof DITECH engine.

Those who believe that the two-stroke has no future criticise its high fume and noise emission levels. Nevertheless the two-stroke engine still delivers more performance and is economically more valid than a four stroke of the same displacement. Even the world of motorcycle racing has turned its back on the once glorious two-stroke: in motocross and GP races four strokes are allowed to have nearly twice the displacement of two strokes, giving them a tremendous advantage.

Aprilia is one of the very few motorcycle manufacturers in the world to have made recent investments in two stroke technology. The effort that Aprilia has put into the DITECH project has nevertheless been vindicated by the incredible results achieved. These conclusively demonstrate that the much maligned two stroke still has a very long life in front of it thanks to its unquestionable performance, low manufacturing and operating costs, plus a new found environmental friendliness.

Aprilia has always been attentive to environmental issues. Back in 1992 Aprilia was the first manufacturer in the world to offer a two stroke scooter model with a catalytic exhaust. In 1993 Aprilia developed another innovative product, the world’s first four stroke, four valve scooter engine with an unbeatable emission/performance ratio (the engine hat was to equip the Leonardo scooter in 1996).

Since 1999, Aprilia’s efforts to prolong the life of two stroke technology have all been concentrated into the DITECH (Direct Injection Technology) engine. This revolutionary engine combines in a way never before possible the concepts of performance, record breaking economy and exceptionally low fume emissions. In the innovative DITECH fuel system, developed and manufactured entirely by Aprilia, fuel no longer enters the crankcase mixed with lubricant (as in conventional two stroke units), but is injected directly into the combustion chamber under the control of a system of sensors connected to an electronic control unit. The results achieved by this new technology are amazing to say the least. Fuel consumption is down 40% compared to a conventional two stroke engine, and can be even 50% less under constant speed riding conditions. Record distances of 50 kilometres have been achieved with just one litre. Compare this with an average consumption of 29 km/litre for conventional catalysed two-strokes. Even lubricant consumption is down by up to 50%.

But the most incredible news has yet to come. The real surprise from Aprilia’s DITECH two stroke is that emissions are down by an astonishing 80% compared to a traditional Euro 1 rated 50 cc two stroke It is therefore no surprise to learn that Aprilia’s DITECH engine is able to satisfy the new Euro 2 standards even without a catalytic converter in the exhaust. In greater detail, pollutant emissions are down by 80% for CO (carbon monoxide) and by 60% for HC + Nox (unburned hydrocarbons and nitrogen oxides). Also, because DITECH engines do not need catalysers, exhaust efficiency remains unaltered. The problem of exhaust clogging is eliminated, and DITECH exhausts do not require the same levels of maintenance as catalytic exhausts. Another significant benefit is that exhaust smoke is almost totally eliminated, something that only a miracle could have achieved on older two strokes.

Yet all this has been achieved without penalising overall engine performance. On the contrary, DITECH engines offer even better performance in terms of smooth power delivery and stable idling thanks to constant, optimised combustion at all engine speeds. Compared to a conventional 50 cc catalytic two stroke, the Aprilia DITECH engine gives 15% better acceleration over 100 metres. Initially applied to the liquid cooled engine of the SR 50 sports scooter, DITECH technology has recently been transferred to the air cooled engine of the Scarabeo 50, now available in a new DITECH version. This should put paid once and for all to any doubts about the wisdom of purchasing a two stroke 50 cc scooter instead of one of the four stroke alternatives which simply do not offer similar levels of performance. The saving graces and ace cards of the two stroke engine therefore remain its mechanical simplicity, high specific power, low weight and compact size, reliability and low manufacturing costs. Also, so far no really competitive alternative has emerged. Four stroke 50’s have just not caught on for their high costs and poor performance. Electric motors fare even worse: the problems are the same but far more serious, and limited autonomy practically rules them out. So once again, Aprilia has had the courage to leave the pack. And the future will prove us right.

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Posted by doudie Tuesday, December 4, 2007 0 comments

ANTI-BOUNCE CLUTCH



Canti-bounce clutches are the latest development that everybody is talking about in four stroke Moto GP racing. Here is how this rider-friendly technology works and why it is so much appreciated.

Basically, a pack of friction plates pressed together by springs is all that most bikes have to let us start off and change gear. Yet this simple component is assuming ever greater importance in high performance and racing motorcycles.

In Grand Prix racing clutch action is far more than just a subject of heated debate; it really can make all the difference between a great bike and a mediocre one. In performance road bikes the bounce-free clutch is still a luxury that only a handful of prestige models can afford to fit.

Yet thanks to the return of four stroke engines to GP racing this piece of technology is living a moment of glory. Nearly every team mechanic is working on them, because controlled clutch slip really does allow riders to make full use of the stopping power modern GP brakes can deliver. And not just GP bikes ... .

WHAT DOES IT DO?
During fierce braking the combined weights of the bike and rider compress the front fork and lift weight off the bike’s rear wheel. Fierce braking is also normally accompanied by one or more changes down the gearbox to add maximum engine braking to conventional brake action and slow down in the shortest possible distance. As engines get bigger and the number of cylinders smaller, engine torque increases dramatically. Along with torque, braking power increases too. In extreme cases (like large twins and competition machines) the braking torque applied to the rear wheel can prove excessive, causing the wheel to lock even without the rear brake being applied. With very little weight on it, the rear wheel starts to bounce, causing man and machine to lose their trajectory and preventing riders entering bends cleanly and down their chosen line. Anti-bounce systems limit the engine’s braking torque by allowing some or all of the clutch plates to slip.

This is a valuable aid indeed for the racing rider, so much so that anti-bounce clutches are advancing in leaps and bounds on competition machines.

Yet anti-bounce clutch technology is not limited to competition motorcycles. Aprilia has long been using it on its RSV Mille sports road bike, and with great success too. The RSV Mille might be a production machine, but it is equipped with technology advanced enough for any GP machine. Aprilia’s faith in anti-bounce technology for road bikes has been amply vindicated by the fact that nowadays many other manufacturers are following suite and fitting their own sports models with anti-bounce clutches. Aprilia’s RSV Mille has had one since it was first introduced back in 1998!

P.P.C.: MAGIC INITIALS!
Potentially, there are many ways of designing and building anti-bounce clutches. Aprilia uses its own patented, ingeniously simple but extremely effective PPC (Pneumatic Power Clutch) system. Transmission through the clutch is controlled by a number of springs that push the clutch plates into contact with each other to create the friction necessary to transmit drive.

When you pull in the clutch lever, you compress these springs, allowing the plates to turn at their own speeds. The RSV Mille’s clutch springs are equipped with a small pressure-sensitive chamber that expands or contracts depending on the air pressure in the intake manifolds. The clutch is connected to the intake manifolds by a simple rubber hose. Whenever you close the throttle, the vacuum created in the manifolds is transmitted to the clutch spring chamber.

This causes it to contract and reduce the load on the clutch springs, allowing the plates a controlled amount of slip. Under conditions of rapid deceleration, the system permits just enough clutch slip to ensure that the rear wheel remains in very firm contact with the asphalt and that the bike decelerates precisely down the line chosen by the rider.

Imagine that you could keep your clutch lever under precision control even during the fiercest braking, to achieve just the right amount of clutch slip for maximum deceleration without bouncing at any time. Not even top race riders can manage such a feat, of course. Luckily, they do not have to. Aprilia’s technical genius and advanced technology does the job for them, and for you too.

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45.V2 THE OFFROAD REVOLUTION FOR MOTORCYCLE


Aprilia makes its return to the off-road world and it does so with daring innovations never before seen in the sector. V twin design, ultra-compact, high specific power, record light weight. Noale represents the way ahead for the off-road bike.
You never forget your first love. And if Aprilia is now a motorbike constructor appreciated and famous throughout the world for its innovations, it owes this at least in part to the off-road world. This is where we took our first steps in racing and where we had our first satisfactions, our first wins. Motocross and trial are sectors where Aprilia has always stood out for its ability to innovate.
Innovation is also the watchword for our return to the off-road world. A comeback in grand style with an engine in every way innovative. The project is known as the 45.2, a 77° V twin engine with electronic injection and a total displacement of 450 cc (which can be increased to compete in a higher class).
The 45.2 project lays the foundations for a revolution in the sector. The bike has been entirely developed in-house in Aprilia's R&D department. Engine and frame mechanics have worked together in perfect symbiosis to follow a common road in designing the engine-frame assembly. Four months of intense work leading, with constant modifications and adjustments, to the first prototype and then the first track and bench tests, still underway today with excellent results.
The aim to obtain a product designed not just for the sports market, but also for all enthusiasts of a sector in constant ferment. Because the 45.2 engine will be mass-produced and mounted on standard bikes available on the market.
The size of the twin engine is no longer a problem. The 45.2 is in fact smaller and lighter than a compact latest generation single cylinder (despite the electric starter), so the engine can easily be mounted in cross or supermotard mechanics. To achieve these results, unstinting use was made of noble and ultra-light materials. The central casings are made from ALSi9 alloy, all covers are in magnesium, valves are in titanium and all the gears are considerably lighter.
The engine has been run for hours on the test bench, proving to have excellent performance characteristics and the ability to reach 14,000 rpm.
But why a twin engine? There are numerous advantages. Thanks to the low unitary displacement, the crank shaft can be extremely compact and light, making it possible to obtain both an engine highly responsive to throttle commands and a rapid and manoeuvrable bike, thanks to the limited gyroscopic effect. In addition, a low unitary displacement makes it easier to remain within the noise limits envisaged for the future, without considering that the 100% electronic engine control improves emissions, a factor which has become extremely important, even when talking of engines for off-road and racing use. The timing has a single overhead cam with four valves per cylinder.
That's all we can say for the moment – except that the bike will be ready during 2004 and will be no less surprising and innovative than the engine.

A TASTE OF THE 45.2 ENGINE
- Expected displacement: 450 cc.
- Architecture: 77° V twin
- Timing: single overhead cam with silent chain drive, four valves per cylinder
- Expected maximum revs: about 14,000 rpm
- Starter: electric
- Gear change: front couplings, 4/5 gears with different spacings available.
- Clutch: multiple disk in oil bath with hydraulic or, on demand, mechanical action.
- Powerful and compact “Alle Terre Alte” generator able to guarantee all the energy required for the injection system, electric starter, battery charger and to operate the auxiliary services.
- Lubrication: separate double circuit. The engine oil is kept separate from the transmission oil. The pressurised oil circuit is fitted with a paper cartridge filter, return reservoir and check valve. All oil circuits are inside the engine.
- Cooling: forced liquid with centrifugal pump.

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SUSPENSIONS: THE CLICKS



Bikes are becoming faster and more sophisticated by the day, and their chassis ever more complicated. Suspension systems have become a real headache not only for racers but road riders too. Here is a concise explanation of the various adjustments you are likely to meet.

Spring loading, hydraulic adjustments, compression and rebound are no longer terms confined to the specialist press. We now read and hear them every day even in the context of normal road-going bikes. It is obvious that increased engine performance and stickier tyres need better suspensions to cope, even on so-called “entry level” machines. The result is that even if you are not a serious racer, even if you never go anywhere near a race track, you are still likely to have to cope with adjustments that are more complex than ever before. All you can do is take one step at a time and always apply common sense.


When they first leave the dealers, all bikes have their suspensions adjusted to the manufacturer’s default settings. These normally give a good compromise between comfort and control under typical road-going conditions. Adjustments let you adapt the performance of your suspensions to suit your own needs and preferences. If you want adjust your suspensions, always write down what the initial settings are before you start and record every change you make. Above all, only make one adjustment at a time. These simple precautions allow you to return to the original settings if your adjustment does not have the desired effect. Making more than one adjustment at the same time combines different effects and makes it impossible for you to know exactly which adjustment does what – a perfect recipe for disaster!

PRELOAD
Spring preload is the simplest of all adjustments. You will need to adjust spring preload to adapt the bike to variations in load. If you intend to travel two up with luggage (as when touring in summer) you may need to increase preload to make the spring harder and prevent bottoming out. Spring preload is the most common adjustment as well as the simplest. That is why many touring bikes now feature a special knob to adjust preload quickly and easily.

HYDRAULIC SUSPENSION SYSTEMS
Hydraulic suspensions can be more or less sophisticated but always provide three basic types of adjustment, spring preload, compression damping, and rebound damping. On technically advanced machines, the front fork and the rear shock both offer all three adjustments. Each of these adjustments has an effect on the dynamics of the bike and on riding comfort. The adjustments themselves are normally easy to make and require only a few simple tools (typically a screwdriver and the shock adjuster wrench) plus, of course, a minimum of mechanical experience.
You need to know exactly what you are doing when you make these adjustments. The same criterion normally applies to all of them: screw in to increase effect; screw out to decrease effect. Hydraulic spring preload adjustments are particularly simple: Screw the adjuster in to increase spring preload and obtain a harder suspension, or screw it out to make the suspension softer. Unfortunately, undamped springs bounce, making control difficult. That is why the front fork and the rear shock contain oil. Forced through specially calibrated holes, this oil damps the spring action to a greater or lesser extent. In simple systems the holes are of a fixed size. In adjustable systems the size of the holes can be varied to obtain greater or lesser damping action. By adjusting compression damping you adjust the action of the suspension during compression. In practical terms, screw in the adjuster to make the suspension slower to compress.
By adjusting rebound damping you adjust the action of the suspension during rebound (the stroke that returns it to rest position). The ideal suspension set-up is always a compromise between firmness and comfort. Suspensions that are too soft tend to bottom out. Suspensions that are too hard reduce comfort and even reduce control on uneven road surfaces. Hydraulic adjustments can be made individually, but normally if you adjust preload you will also need to fine tune the damping adjustments to avoid too much bounce.

The following is a brief glossary of terms to help you follow technical talk about the difficult subject of suspension adjustments.

SPRING PRELOAD
Spring preload is the static load imparted to the spring. In simple terms, it is the force with which the spring is compressed when the bike is stationary with no load on it.

REBOUND
The rebound stroke is the suspension return stroke, i.e. the stroke that returns a compressed suspension to its rest position.

COMPRESSION
The compression stroke is the stroke that compresses or pushes in the suspension as the result of load or shock.

COMPRESSION DAMPING
Compression damping is the adjustment that allows you to control how quickly/easily the suspension can be compressed under load. Screwing the adjuster in increases compression damping, screwing it out reduces compression damping.

REBOUND DAMPING
Rebound damping is the adjustment that allows you to control how quickly/easily the suspension rebounds (returns to rest position) when load is removed. Screwing the adjuster in increases rebound damping, screwing it out reduces rebound damping.

CLICKS
Some suspension adjustment systems click as they are turned to help you identify the setting position.

RISING RATE SUSPENSIONS
These are special suspensions that use a system of levers or linkages to become harder to compress the more they are compressed.

END OF STROKE / WHEEL TRAVEL
This is the full distance through which a suspension system can move. These physical limits should never be reached.

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BACK TO THE SINGLE CYLINDER


Among street bikes, the four-speed single-cylinder seemed a thing of the past, but now it’s reaffirming itself as the motor of the highest quality, and its use is being expanded to include bikes that aren’t exclusively for professionals. The single-cylinder, for its compact dimensions and for its mechanic simplicity, has always been one of the favorite motors used to equip a motorcycle.
Furthermore, with the original design of motorcycles, this type of propulsion was practically the only one thinkable. Then, over the years, it was always present in all gear ranges, widening its sphere of application.

It’s a light motor, substantially simple, with few components, which with the passing of time has been able to benefit many of the new technologies allowing for highly elevated levels of comfort and performance.
After years of only multi-cylinder, it was only in the 80s that we learned to recognize the single-cylinder under its modern definition. Its recognition from the media came as a result of such races as the Paris Dakar, and the consequent arrival of the first single-cylinder endurance races. It was reported then as being en vogue, and from that point onward has always been consistently developed, thus bettering the presentation and allowing it to overcome the chronic defects generally associated with this type of configuration, for example, vibrations. Research of the maximum performances in lightness and power of specialized motorcycles, such as cross and supermotards, resulted in extremely sophisticated motors (of which the single-cylinder 5 valve Aprilia of the ‘90s can be considered the precursor). That technology, however, backfired even for the most simple of motors, such as sanctioned ones, which are slowly overstepping their principle use (that of off-road) and ending up in motorcycles principally dedicated to the asphalt. A return to its roots was necessary for this type of motor, unique to motorcycles. Over the years there were many different trespasses of the single-cylinder motor into the street sector, but all of the experiments done by the Houses were never really appreciated by the public. Instead, the year 2005, also thanks to the arrival of the Pegaso 650 Strada, marked a change in tendencies that seems to be continuing, and also increasing, in recent years.

And this is really a novelty: in a moment in which the power of propulsion is greatly increasing, there’s also the return of this motorization that satisfies an emerging desire for simplicity. A simple technology, like that enclosed in the Pegaso, the winged horses of Aprilia are a testimonial to this type of motorization. The success of the Pegaso 650 Strada is furthermore a testimony to the desire for street motorcycles that are light and fun. Utilizing all of the best technology available, the modern single-cylinder is fun, very reasonable, light and fast. The use of distribution over four valves and the arrival of electronic injection has allowed them to reach the threshold of the 50 cv, which is by now absolutely at the portal of motors regularly filtered and sanctioned to satisfy the most sever regulations against pollution. The use of innovative materials has allowed the obtaining of much lighter and also more balanced motors such that the vibrations have been drastically reduced. And in this way, the lightness and fun of the new single-cylinders is appreciated and fashionable like never before.

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Posted by doudie Sunday, December 2, 2007 0 comments

THE COUNTERSHAFT



To make an engine function well it is not enough to make it ride strongly, also necessary is the limiting the vibrations to the least possible so that the rider doesn’t excessively tire himself out, also avoiding fractures in the motorcycle. In fact, nothing compares to vibrations when it comes to the fastest way to deteriorate the engine of any vehicle. If they aren’t eliminated (or at least reduced to a minimum), the effects can have disastrous long-term consequences. Such examples include fractures not only in the interior of the engine, but throughout the entire vehicle.

Those who are familiar with vintage engines can certainly understand what we are talking about. Welding that let up and frames that split were all in a day’s work and were all caused by vibrations. In particular, the problem was with the propellers’ most simple architectural operations of single and bi-cylindrical engines, especially those with high cylinders and sporty impositions (therefore destined to run highly).

But why are vibrations generated? Let’s take as example the most essential architecture: that of the single cylinder engine. During rotation, the engine shaft clearly has to be equilibrated and in contrary cases, the creation of vibrations is inevitable. However, on one side of the shaft, both components of opposite movement (the piston and plug) and the connecting rod are attached. The movement of which is “intermediated” between alternative rectilinear motion and the motion of the rotation. You could think that it is sufficient to well equip the shaft from the opposite part of the handle’s hinge with a pair of adequate counterweights to obtain good equilibrium. At this point, you need to identify what is an “adequate” mass for the counterweights. To start off with, the counterweights have to equilibrate the rotating masses of the opposite side of the shaft. In other words, those of the handle’s hinge and part of the connecting rod. To balance out the forces created from a rotating mass, it is sufficient to arrange another identical mass in a diametrical opposite position the same distance from the rotating beam.
Unfortunately, beyond the forces created by the mass in rotation, you need to equilibrate the inertial forces created by the parts moving in alternate motion. Yet these forces, contrary to centrifugal ones, are not constant during the rotation of the shaft as they vary depending on the position of the piston. Instead, the action of the counterweight is constant and continually moves in the opposite direction of the handle. For this reason, as we have already stated, the possibility of equilibrating the forces created by the rotating mass isn’t possible to balance out those caused by inertia. The adoption of counterweights determines the creation of exuberant forces for certain positions of the connecting rod’s hinge which remain insufficient for others. If the counterweight equilibrates 100% of existing forces when the piston is at the Superior Dead Point, then in all other positions during the shaft’s rotation space is given to the forces of decisively superior entities in respect to those which it should balance, for this reason, vibrations of relevant entities are created. For this reason, you always end up a compromise, a solution that balances the shaft.

If up until a few years ago this could achieve satisfactory results, today it is no longer acceptable. To eliminate irritating vibrations, auxiliary equilibrating shafts were adopted and standardized in road motorcycles. They normally consist of a single shaft equipped with an outlying mass that, opportunely segmented, rotates in the direction opposite of the crankshaft with the exact same velocity. Normally, it is commanded by a train of gears or by a short chain. The solution with an equilibrated shaft is usually sufficient to bring the vibrations to an acceptable level but to reach perfection you can also use two countershafts, as the Aprilia V2 engine with a patented AVDC solution (Anti-Vibration Double Countershaft) does. The AVDC uses two countershafts (one in the carter engine and another on one of the four camshafts) to annihilate the vibrations. It’s not by chance that the V60 Magnesium of the Rsv is one of the less-vibrating engines in existence among two-cylinders.

Let’s look at the methods used to diminish the vibrations according to the type of engine:

ELASTIC ANCHORAGE
This system simply serves to isolate the engine from the frame so that the vibrations don’t transfer over to the hull. This way, at the points of anchorage between the engine and the frame, blocks of rubber elastic (silent blocks) alternate that allows for a certain oscillation of the propeller in respect to the frame. This system is especially applied to single, bi, and in-line cylinder engines, but also to the V-shaped engines with an inferior angle of 90°. The system has two defects: it doesn’t diminish all vibrations and it still solicits rigidity of the engine.

SINGLE CYLINDERS
These are the most difficult engines to balance because the action of the cylinder cannot be balanced out by another cylinder. The ideal solution would be to install four balancing shafts, that is, two for the inertial forces and another two for the second order forces (which depend upon the double value of the angle occupied in that instant by the handle). Normally they use one, two at most. This solution limits the vibrations without absorbing much power.

BI-CYLINDERS
For engines with two in-line cylinders, with the handles offset by 360°, the mechanical problem is the same as that of the single-cylinders. If the handles are instead offset by 180°, the first order forces are balanced. For the second order forces, a couple of balancing shafts are necessary just like for single-cylinders.
The offsetting of the handles is one of the possible solutions for engines with tight V-cylinders and consists in the attaining two different handles. The sum of the offsetting angle should be that of the value of the angle between the two cylinders added to half the value of the offsetting angle of the handles, resulting in 90°. The V-architecture of 90° is in fact the best among two-cylinders when seeking to cancel out all vibrations. The second solution is to adopt one or two balancing shafts.

THREE-CYLINDERS
If with a segmenting of 120° you end up with a nearly perfectly equilibrated engine, for engines with a high number of rotations it is typical to use a balancing shaft in the usual place in the carter, in front of the engine shaft.

FOUR-CYLINDERS
If the architecture of in-line are engines with perfectly balanced first order forces, those of second order are not. Yet for motorcycle engines these forces are often small units where it is preferred to not use countershafts that continue to provoke an absorbing of power. The countershaft can be adopted for extremely sophisticated engines with high cylinders. The V four-cylinder engines and those with only two handles behave similarly to two-cylinder engines.

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APRILIA ENGINE PERFORMANCE



Designers are using the latest calculation and simulation software combined with advanced materials and production techniques to achieve a level of engine performance that would have been inconceivable only a few years ago. The latest techniques allow parts to be made with extraordinary precision, conferring on them amazing strength and functionality. This rapid evolution in motorcycle design has obviously revolutionised the way in which we look at new models and the criteria that form the basis for any objective evaluation of overall performance. By way of example, nowadays it would be absurd to judge a new engine on the basis of its power and torque characteristics alone. Just as with chassis and frame technology, we have to take a number of aspects of engine design into consideration if we are to understand just how good a new engine is.


Engine architecture
Engine architecture obviously plays a major role in performance. Compare in-line units with V engines for example.
The in-line configuration, in the form of the transversely mounted straight four, is without a doubt the most common engine in today’s motorcycle industry. On the one hand, unlike the longitudinal twin, it does not concentrate mass around the bike’s axis of roll. On the other, it is far cheaper to produce because only one set of valve timing gear (camshafts and timing drive) is needed for the single integrated cylinder head. Furthermore, the fuel supply (the battery of carburettors or injectors) can be arranged in the most rational way possible, with the throttle bodies lying along the same axis, so that there is no need for complex linkages.
Even though the latest design techniques have led to a reduction in the overall dimensions of in-line engines, especially transversely mounted units, they nevertheless remain extremely bulky. A great deal of effort has been put into reducing the gap between the cylinders by adopting special casting techniques. The alternator, that used to be keyed on to the end of the crankshaft, is now generally installed behind the cylinders for the same purpose.
The reason why in-line four-cylinder engine architecture is still so popular in the world of motorcycling is that a larger number of cylinders do indeed offer a number of advantages. The fact that the crank, the valve gear, and the timing drive are of limited mass makes it possible for these engines to reach higher rotational speeds without generating excessive mechanical stress. Another benefit of these engines is that they can easily be fitted with devices that divide up the total cross-section of the inlet and exhaust inlets, not only improving power delivery but reducing noise emissions too.
‘V’ engine architecture is inevitably more complex and therefore more costly to produce. Despite this, in many ways it is far more suitable for motorcycle applications. The transverse dimensions of the engine are much smaller so that the bike as a whole can be narrower, even with frame designed to enclose the entire engine. V twin engines in particular are renowned for their lightness, and those with a V angle of less than 90° (like the Magnesium engine that powers the Aprilia RSV 1000, which has a 60° V angle) are relatively easy to install inside the frame. This leaves frame designers greater freedom to optimise their designs in terms of rake angle, yoke offset and trail; and better frames mean better handling.
In the case of Aprilia’s Magnesium engine, dry sump lubrication has enabled the crankcase to be made a lot smaller, reducing the unit’s overall height. Inevitably with a 60° V, the engine is not naturally balanced; it would vibrate badly if this problem were not prevented by countershafts that cancel out the unwanted momentum and inertia. (The RSV 1000 uses Aprilia’s patented AVDC - Anti Vibration Double Countershaft system).
The complexity and consequently the relatively high production costs of these engines derive from the fact that by definition they consist of two separate cylinder groups, whether the engine is a V twin or a V four. The valve timing drive, fuel inlets and exhaust systems are therefore twice as difficult to manufacture.
If it is a requirement for the crankshaft to be arranged longitudinally, V architecture is almost a necessity: the only viable alternatives are boxer architecture (and even this is best defined as a 180° V) or flat in-line engine architecture. These configurations are ideal for use with air cooling (since the cylinder heads are freely exposed to the air flow) and with shaft drive (since the final drive shaft lies parallel to the engine crankshaft and gearbox shafts).
Since V twin engines by definition only have two cylinders, they cannot achieve very high revolutions, and maximum performance is therefore limited compared to a four cylinder engine of similar displacement. However, since maximum torque is produced at relatively low engine speeds, V twins are far more pleasant to ride.
In brief, thanks to its reduced overall dimensions, limited weight and excellent torque curve, the V twin is, without a doubt, the best suited engine for motorcycle applications.

MEP (Mean Effective Pressure)
An important parameter in the evaluation of any engine, and one that can be calculated quite easily using a simple mathematical formula, is MEP, or mean effective pressure. MEP is one of the main parameters in calculating engine efficiency. It refers to the average pressure applied to the crown of the piston during the power stroke, but of course also defines the total work produced by the engine in a complete cycle. Combustion causes a sudden increase in pressure on the crown of the piston, pushing it down towards its BDC (bottom dead centre) position. This is the only stroke in the four stroke cycle that actually generates any useful power. The value of MEP also takes into consideration the various phenomena that contribute to the generation and dispersion of pressure throughout the cycle.
To express it simply, the higher the MEP, the more efficient the burn of the fuel mix induced into the cylinder. MEP therefore provides an excellent overall evaluation of engine efficiency in terms of the combustion process (thermal efficiency), the aspiration process constituted by the expulsion of spent exhaust gases and the induction of a fresh charge of fuel-air mix (volumetric efficiency), and mechanical losses through friction between the piston skirt and rings and the cylinder walls (mechanical efficiency).
In order to achieve a high MEP under all engine operating conditions, the designer needs to take account of a large number of factors. For example, the design of the aspiration system (particularly the air ducts and air box) needs to be optimised to ensure that the cylinder is efficiently filled with air-fuel mix. The mix itself must also be as homogeneous as possible to ensure that the fuel burns completely in the available air during the combustion process. The fuel injection and ignition systems play a fundamental role in this; electronic control is essential and engine management has to be fully understood and optimised. Valve timing and overlap likewise have to be calculated precisely to optimise engine aspiration: the valves must open and close at exactly the right times to achieve the best possible exchange of gases inside the cylinder. The combustion chamber itself also needs to have exactly the right shape for optimum combustion. Precision ignition timing needs to be provided too. (The spark must jump across the plug’s electrodes at the instant that ensures that pressure on the piston crown reaches its maximum value just after the piston has passed through TDC and has begun moving downwards on its useful power stroke. Last but not least, friction losses need to be kept to the bare minimum (something that can be done by using advanced materials and specially designed parts). To sum up, if an engine has a good MEP value, it has certainly been designed with great care and attention and will certainly prove both efficient and powerful. Obviously, MEP does not remain constant across the rev range, but varies with engine operating conditions. The highest MEP value will coincide with the engine’s optimum operating conditions, i.e. those conditions under which the engine can aspirate the largest volume of fuel-air mix and therefore combine optimum combustion and minimum mechanical losses.

Mean piston speed
MEP (mean piston speed) is another fundamental parameter in proper engine evaluation. It is relatively easy to calculate, since the formula for obtaining it is based only on piston stroke and engine revolutions. At TDC and BDC (top dead centre and bottom dead centre), the points at which alternating motion is inverted, the piston is momentarily stationary. In other words it has zero speed at either end of its stroke. The piston accelerates and decelerates between TDC and BDC but always returns momentarily to rest at them. MEP represents the average speed of the piston over its entire stroke.
Mean piston speed should not be allowed to become too high since it is directly proportional to the friction generated by contact between the piston rings and the cylinder walls. Unless adequately controlled, this friction can provoke a significant reduction in the engine’s mechanical efficiency. Mean piston speed also has a direct influence on the inertia of the connecting rod, a source of potentially dangerous mechanical stress.
The best way of reducing mean piston speed is to reduce piston stroke rather than reduce the engine’s red-line speed, since reducing the latter would have a negative influence on power output. If stroke is reduced, however, bore has to be increased correspondingly to avoid reducing displacement, but this in turn has the negative effect of increasing the circumference of the piston rings and therefore the friction generated between them and he cylinder walls.
This dilemma is not an easy one to solve. The best chance lies in the use of advanced materials: if these can minimise friction and reduce the mass of engine parts in alternating motion, they can achieve a significant reduction in mechanical stress.
To prevent mean piston speed from becoming excessive as the result of an uncontrolled increase in engine revolutions, some sort of rev limiting device has been incorporated in all modern electronic injection and ignition systems. Usually, the limiter cuts in just above the engine’s maximum power speed. After this speed, engine performance drops off dramatically in any case, leaving little or no reason to continue increasing the stress on mechanical parts. In reality, if you examine the power curves of some engines, you will see that the rev limiter actually cuts in while the power curve is still rising, before it flattens out and reaches the maximum power value after which it begins to decrease naturally. This means that the engine manufacturer has preferred to err on the side of safety, and prevent engine speed from rising any further even though more power would in theory be available. The obvious reason for doing so is to avoid compromising the mechanical reliability of the engine.

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THE SECRETS BEHIND APRILIA’S MOTORCYCLE OFF-ROAD TWIN


Aprilia’s off-road twin caused quite a stir when it was first presented to the public and motorcycle media. The design was characterised by solutions in complete contrast with conventional technology, and was totally unlike the single cylinder machines that then dominated motocross, enduro and supermotard racing. The idea that drove the development of Aprilia’s new off-roader was the desire to create a twin cylinder machine of similar size and weight to a single, but with far better performance. The new bike was also to have excellent ground clearance and be as narrow as possible. To cut a long story short, anybody sitting on the new Aprilia should not be able to feel any difference between it and a classical single, apart from its performance. The need to keep overall dimensions down therefore played a fundamental role in the design process. The V angle between the cylinders, for example, was set at 77° to make room for the engine’s auxiliary equipment. In theory, this means that the crank cannot be perfectly balanced, but on a machine like the SXV or RXV, designed and made exclusively for competition use, this was not a primary consideration. Another important fact is that the 4.5 engine (450 cc) and the 5.5 engine (550 cc) both use the same crankcases. The crankshaft of the higher displacement unit therefore requires extremely compact flyweights. Tiny pads of mallory (a tungsten alloy of extremely high density, used to balance rotating parts when there is no room to fit large counterweights) were used to provide the necessary balancing.


The step up from 450 to 550 cc has been achieved by increasing stroke (from 49.5 to 55 mm) and also increasing bore (from 76 to 80 mm). This has meant completely new pistons, cylinders, con-rods and, of course, completely different valve timing. Only the heads themselves remain identical. Ultra-short con-rods have been used to limit the height of the cylinders, even though this creates a high thrust between the piston skirt and the cylinder. This factor too can be overlooked given the fact that these engines are made specifically for competition use. After all, the SXV and RXV are thoroughbred racing machines and are built without compromise. The models homologated for road use were derived from them at a later date. Owners of Aprilia’s new off-road twins therefore benefit from machines that are ‘tame’ enough to ride on the public highways but still deliver fantastic performance thanks to the very latest motorcycle technology (Aprilia also supplies a range of dedicated accessories to modify road-going models for competition use). Also to keep overall dimensions down, the valve gear is driven by two chains, one for the front and one for the rear cylinder. These chains are driven by a layshaft in the V between the two cylinders, which is itself driven from the crank. This solution has made it possible to keep the diameter of the camshaft pulleys down, and this in turn has meant that the valve covers can be compact too. The necessary reduction in revolutions is achieved instead by the gear that drives the layshaft off the crank. Both cylinders have just one cam each, even though they are four valve heads. The two inlet valves are operated directly by spring cups while the two exhaust valves are operated by a single rocker arm with two fingers, driven in turn by the central lobe of the cam and a small intermediate roller. Because the valves are made from titanium, the seats and guides have to be made from a special material to resist wear.

Even the lubrication system is unusual. The engine features a dry sump with separate gearbox lubrication. A double stage oil pump draws oil from the bottom of the crankcase and pumps it up to an external oil tank located behind the steering head. From here, the pump feeds oil under pressure into the crank and to the top of the cylinder heads where it lubricates the camshaft, valve cups and rockers. The oil lines between the pump and the tank are embedded in the left hand side crankcase and flywheel cover. This not only provides a high degree of component integration but also contributes to a reduction in overall dimensions.
The gearbox is lubricated in such way that the oil level is kept high in the gearbox itself but low in the clutch casing. This limits the power loss that occurs when a clutch runs in a deep oil bath. Having two separate lubrication systems for engine and gearbox is of fundamental importance. Because bushes are used for the crankshaft journals and big end, it is essential for the oil supply to remain perfectly clean. Gearbox oil can easily become contaminated with particles of clutch plate that can cause rapid wear in bushes. The new Aprilia off-road engine also features quite a complex oil fume recovery circuit. Fumes escape from the crankcase through a small hole. This is shielded by the starter motor drive gear which separates out the bigger oil droplets. The remaining fumes then pass along a pipe into the oil tank behind the steering head. The oil tank itself is specially designed, with internal walls to prevent the oil sloshing around (as it otherwise would on an off-road bike), and to ensure a constant supply of oil to the pump. Once the oil fumes have released all remaining droplets back into the oil, they are fed into the filter box, from where they are sucked back into the engine via a small sponge filter which removes all remaining traces of lubricant.

The new Aprilia twin is fuelled by an electronic injection system. The throttle bodies, made by Dell’Orto, are miniature works of art. The ECU for the fuel injection, which also controls the ignition, is made by Walbro. Their website offers a selection of mappings, that can be downloaded into the ECU using a special diagnostic instrument. The fuel supply to the two injectors is a return-less system, so a fuel pressure regulator is installed on the fuel pump delivery line inside the fuel tank. It goes without saying that Aprilia’s twin cylinder off-road engine far excels alternative single cylinder powerplants in terms of performance.

Because it can rev higher and more easily, a twin develops more power than a single of equivalent displacement. It therefore stands to reason that the new Aprilia engine is at a significant advantage in supermotard racing. A lot of development remains to be done to perfect it for motocross and enduro use, where more low down grunt is needed, but the Aprilia twin is still a young project. Design work only began in October 2002 and the engine ran for the first time in August 2003. So there is still a lot of scope for further development. The Aprilia design team are currently working on specific valve timing diagrams, pistons (to change the compression ratio), and exhaust and inlet systems for the various speciality competitions in which the new bikes are likely to compete. We can be confident that the right level of performance will soon be achieved, with either more top end power or more low end muscle. Neither the 4.5 nor the 5.5 engine is equipped with a kick start, but an automatic (centrifugal) valve lift device is nevertheless provided to reduce compression on start-up. This has allowed Aprilia to fit a small, lighter starter motor that does not need to produce a high torque output. Finally, another factor that we cannot afford to ignore is that a twin cylinder engine produces far less exhaust noise than a single, because the shock waves from the two cylinders tend to balance each other out in the silencer expansion chamber, dramatically cutting noise emissions. Noise reduction is becoming an increasingly important issue, since noisy bikes can be a major source of annoyance in the countryside, and the authorities are actively cracking down on vehicle noise in general.

Just like the engine, the frames of the RXV and SXV also feature unique designs. A mixed tubular trellis frame with aluminium side plates was adopted for a specific purpose. Maximum rigidity is achieved at the side plates, while in the remaining areas of the frame, rigidity can be varied quite easily and at little cost, simply by modifying the thickness of the trellis tubes. It is therefore a simple matter to adapt the frame and to obtain different dynamics without having to interfere with any of the major parts. The holes in the aluminium side plates that hold the steel trellis tubes are also highly unusual. The first section of the hole is conical to stop the aluminium plate from coming into direct contact with the steel tube. This prevents flexing in the steel tube from causing cracks in the aluminium. The tubes are then held in two other sections of the hole with an increasingly tight interference fit. The frame is first hot-assembled, using a special glue, then the ends of the steel tube are rectified and secured to the plates with screws. The frames produced with this advanced manufacturing technique have proved extremely reliable under competition conditions, when the fixing screws are actually omitted as a further weight-saving measure. Even the swingarms of the RXV and SXV are impressive: the side members are made from an aluminium box section (formed by welding together pressed profiles), while the pivot section is made in cast aluminium.

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