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.
