Power Goal #3 - Decreasing Parasitic Losses
The concept behind this goal is very simple, but the means used to achieve it can be exceeding complex and nearly without limit. I'll cover some of the more common methods as well as the reasons behind them. Freeing up power through a reduction in parasitic losses is one of the more elegant approaches to the problem of building power and is one of the few areas where gains tend to be exponential as engine speed increases. Though this is often a long and difficult path, the rewards are many. Decreased fuel consumption in combination with better acceleration and improved engine response can't be found just anywhere.
First up, it's very important to understand that all mass has inertia and inertia is what causes momentum. Inertia is an object's resistance to changes in movement. This includes an object's resistance to acceleration AND deceleration. The more mass you have, the more inertia you have. Quite simply, something that weighs a lot is hard to move and hard to stop and this relationship is inversely proportionate. For every doubling in mass, you halve the acceleration. Or, if you wish to maintain the same acceleration while doubling the mass, you must double the force.
Know, also, that this applies to EVERY movement on and within your bike. The weight of the bike and rider both have an effect on acceleration. If you are able to cut the weight of both yourself and your bike in half, you have just doubled your acceleration.
A bit of a corollary, here, but don't confuse acceleration and top speed. You won't hit the ton by shedding weight, but your 0-60 times will be a lot better. In order to hit the ton, you need power and/or streamlining. It's important to know that acceleration is a function of power VS weight whereas top speed is a function of power VS friction (of the air). If you want to be faster, you need more power and less drag. If you want to be quicker, you need more power and less weight.
Anyway... back to the main point. Every part on your bike moves, takes energy to do it. The wheels need energy to rotate. The piston rings need energy to overcome the friction of rubbing on the cylinder walls. The cam and rockers need energy to be able to compress the valve springs and actuate the valves. Your transmission needs energy to spin the gears and your chain needs energy to bend each and every single link as the link rounds your sprockets. The vibration in your handlebars, when you blip the throttle, takes energy.
Now, you may notice from a few examples above, that not all parasitic losses have to do with inertia and so you need to understand how these different losses occur in order to be able to combat them. As teazer mentioned earlier, this is an excellent starting point for a lot of engine builds and is THE starting point for professional race builders. Your engine must be capable of getting as much power to the ground as possible, otherwise you're just throwing good money after bad. You're not going to invest money in a company that is inefficient, why would you invest money in an engine that is inefficient?
Blueprinting
Blueprinting is the process by which most race engines begin their life. While the concept is relatively simple, the process is tedious, difficult, and expensive. Most of us don't have the money, skills, and/or equipment to be able to tackle a full engine blueprint, but it's something nice to consider or, at least, be aware of.
The blueprinting process involves disassembly of most or all of the engine components. In an ideal world, brand new components are used for this process, often unfinished from the factory. All components are checked for clearance specifications and then adjusted, if necessary. Reciprocating and rotating components are also checked for balance. It's important to note that the blueprinting process doesn't usually involve taking an engine's running specifications out to a different measure, but rather making use of the factory specs and just decreasing the tolerances.
For instance, the top ring end gap on a CB360 should be between .15 and .35 millimeters when it comes from the factory. If I were building a drag racing bike from a 360 motor I may request that the ring end gap be between .150 and .185 millimeters. A race bike designed to cover 300 road racing miles may specify a ring end gap of .250 and .350 millimeters. In both cases, the specification falls within the manufacturer's allowances, but the precision is increased and a bias is given depending on the purpose of the engine. For a drag bike that sees short bursts of power and frequent rebuilds with only few miles, a smaller gap will help with cylinder sealing and provide a bit more power. For a road race bike that has to compete at high RPMs over a longer distance, the increased gap allows for more thermal expansion without a significant increase in friction on the cylinder walls.
Frictional Losses
Friction is the resistance faced when the surface of one object rubs up against the surface of another. These two surfaces can be made up of anything that has mass. So a bike traveling along the road encounters friction from the air as the air contacts all of the surfaces of the bike. There is also friction from the tires contacting the road surface. There is even friction from your metal parts rubbing up against oiled surfaces.
Inside your engine, two thirds of the friction will come from the piston assemblies, with the biggest majority of that being the rubbing of the piston rings against the cylinder walls. The amount of friction from the rings is great enough to the point where many racing engines often run with only with a single ring, in order to keep friction to a minimum. Unfortunately, this is not an option for most of us. A single ring will not only reduce compression (forcing you to make it up elsewhere), but it also requires the use of aluminum cylinders with special coatings. Single-ringed pistons also require frequent rebuilds; something else that most of us don't want to have to do.
There are modifications that can be done to further reduce the friction from the piston assembly, however. One of the more common methods is to reduce the length of the piston skirts. This is a great option because it also reduces the weight of the pistons. Two birds with one stone. In order to be able to remove metal from the pistons skirts, tight tolerances are needed. The piston skirts give a mechanical advantage to the cylinder walls when it comes to keeping your piston from rocking back and forth. You must use tighter tolerances to prevent this rocking motion if the skirts are to be shortened. It is definitely possible to go too short on the skirts. The ideal skirt length on any given bike will be different, based not only on the make and model, but the tolerances which are being employed. Consult the professionals before making any changes.
In addition to skirt changes, is it not terribly uncommon to slightly offset the wrist pin of the piston to reduce side loading on the thrust side. As I mentioned the previous post on displacement, the pistons are being pushed up against the side of the cylinder walls by the force of the crankshaft resisting the downward motion of the pistons during the power stroke. By placing the wrist pin further away from the thrust side, it helps to reduce the mechanical advantage of the conn rod against the cylinder wall. This directly translates into a savings in friction as the two surfaces are no longer being pushed together with the same force.
Finally, further friction can be saved in the piston assembly by shortening the stroke. Less distance for the rings and skirts to move against the cylinders means less friction. This option requires careful consideration, however, as it will decrease displacement and compression, both. It may also lead to an increased likelihood of detonation. To help illustrate (side-loading, especially), I jacked this picture off the Internet. Pay special attention to the upwards pointing arrow coming through the conn rod. This is the force that is pushing your piston against the cylinder wall.
The next largest source of friction within the engine is the valve train. Most of this friction comes from the followers on the cam, but a good deal also comes from the valves and valve guides. Reduction of friction between the cam and the follower usually isn't a primary goal because this is one of the very few areas in your engine where friction actually decreases as RPMs increase. The inertia of the rocker arms and valves tends to work in our favor with this component. As the speed of the camshaft increases, the rocker arms' resistance to movement means they don't press quite as hard against the camshaft.
The most common solution to the valve friction problem is to ensure everything is well within specifications. Too little clearance and the increased interference causes friction. Too much clearance and the valve stem will wobble within the guide and this causes friction, too. It is also quite common to make materials changes as well. Bronze offers less friction than aluminum or iron and so it is common for use in valve guides and bushings. Most aftermarket valves will come with some sort of coating that aids in heat dissipation and/or friction.
Tackling friction in other areas of the engine starts to take some imagination. Oil viscosity is one prime example. Using a lower viscosity oil will usually result in less frictional loss because the oil isn't quite as thick. However, at high RPM operations, a lower viscosity oil may not provide enough lubrication and friction will increase (not to mention wear on engine components). Replacement of journals with bearings (often of the roller variety) will reduce friction in other areas as well. Perhaps going to a dry sump and dry clutch are an option for your motor? This will enable the use synthetic oils, which are generally slipperier. It also keeps friction down because engine parts don't need to be drug through an oil bath.
When dealing with friction, the goal is to reduce contact as much as possible in as many places as possible. Where contact is necessary, ensure the surfaces areas are well lubricated with quality oils.
Losses due to Inertia
Inertial losses aren't really losses, per se, but inertia does have an effect on your engine and so I'll discuss it, but briefly. First off all, let me clarify what I mean by a "loss". Yes, it takes energy to accelerate your pistons to TDC, stop them, and the reverse their direction. But this isn't a loss in energy, it's merely changing where the energy is stored.
For instance, as your piston reaches the end of the exhaust stoke and approaches TDC, it must slow down and as it leaves TDC and approaches 72°, it accelerates. "Ah ha, that takes energy to accelerate the piston and energy to decelerate the piston!", I can hear many of you say. You are correct. But your assumption on where that energy is coming from may need some revision. As the piston is decelerating, it pulls on the crank and that causes the crank to accelerate and store the energy from the decelerating piston. Next, as the piston passes TDC and begins to accelerate, the energy (for the intake stroke, at least) is coming from the, now decelerating, crankshaft. The energy to move the pistons up and down isn't lost, it's merely transferred. Now bear in mind that this energy transfer is not without losses, but they are minor. The majority of losses during this process come back to our old friend friction.
This same concept applies to your valve train. Yes, the valve springs take a lot of energy to compress. But that energy is largely returned to the system as the springs decompress and apply pressure back to the camshaft through the rocker arms.
The biggest concern about inertia is acceleration. While much of the energy that goes into creating inertia within your engine is reclaimed every or every other rotation, there still must be the initial expenditure of energy. The energy has to go in before it can come out. This creates a direction relationship between the inertia in your engine and the rate at which it accelerates.
The more mass and inertia your engine has, the more energy that is required to accelerate it. But also, more energy is required to decelerate it. Many production engines that have have areas at which great initial investment of energy is required will make use of heavier flywheels in order to preserve inertia. Diesel engines, with their compression ratios approaching 20:1, will often make use of heavier flywheels. This allows for smoother engine operations because the energy stored within the flywheel helps overcome the resistance of the mixture to compression and so keeps the engine turning at a more uniform rate throughout its rotation.
Generally speaking, the more inertia within your engine, the harder it is to start and the slower it will be to accelerate or decelerate. Furthermore, the increased weight of the components will generally cause an increase in friction in all connecting assemblies. One possible benefit to increased inertia is a lower idling speed, but this usually translates into a lower redline as well.
In a bike with a sport pedigree, the goal should be to lower inertia as much as possible. In any moving part, your goal should be lighter without sacrificing strength. The energy required to get these parts moving is energy that would otherwise go toward accelerating your bike. On decel, an engine with lower inertia can also make better use of engine braking and provided your rubber holds, your bike will stop better as well.
For those of you that have been following crazypj's 360 build(s), you can see this in effect in the modifications he's made to his rotor and gears. This is done to reduce inertia.
Also, think outside the box (engine) for combating inertia. The chances are, anything that moves on your bike got the energy from your engine. Chain, wheel hubs, wheels, rubber. All these things have inertia.
Engine Balance
The one final area of parasitic losses I will address is engine balance. Mainly, this deals with the balance of the crankshaft and that's the area of most concern. Other rotating components will benefit from being balanced, as well, but all to a lesser degree than the crank and piston assemblies.
So why is it important to ensure your is balanced? Well, primarily this has to do with wear. If an engine is out of balance it will wear more rapidly and many of the components will be placed under greater friction and greater stress. As a more minor problem, the vibrations of an unbalanced engine can be pretty damn annoying.
There are two types of crankshaft balance. The first, and most important, type is called the primary balance. Primary balance is pretty simple to understand as it is basically just ensuring that the counterweights on the crankshaft properly balance out the weight of the pistons and conn rods. You want to ensure that the center of mass for rotation along the crankshaft is as central to the crankshaft as possible. It is isn't always necessary to change the primary balance of the crankshaft when you change pistons or conn rods, but it certainly is desirable. This change in balance usually comes in two forms.
In order to "overbalance" a crankshaft, or add counterweight, holes are drilled into the existing counterweights and tungsten plugs are then inserted into the holes. To "underbalance", or remove counterweight, holes are drilled and then left empty. It is usually much easier and cheaper to remove weight from the counterweights on a crankshaft and so many aftermarket crank options (where they exist) will be intentionally overbalanced from their maker.
Just about any crankshaft in any configuration can achieve a perfect or near-perfect primary balance, at a given RPM.
Secondary balance has to do with how the assembly balances when rotating under load. This starts to take into account the kinetic energy of the pistons (which increases as the rotational speeds increase), sideways motions of the counterweights, and any changes in balance due to offset crank pins (in the case of some stroked engines) that would cause the pistons to operate outside of a normal sine wave-like pattern (called "sinusoidal").
The primary means of secondary balancing are the phase of the pistons along the crank (it is very common to rephase 360° twins such as the XS650) and the use of balancing shafts. These shafts rotate at twice the speed of the crankshaft and work to negate the harmonics of the secondary forces.
As mentioned earlier, nearly any engine can be balanced for primary forces, but it can be very difficult to balance secondary forces. Rephasing is never a complete solution and at higher speeds, the balancing shafts may need balancing shafts of their own (this is not actually done, to the best of my knowledge, but that is what would be required to remedy the situation). The configuration of the engine will have a lot to do with how it is to balanced. Opposed cylinders like you see on many BMWs are naturally balanced for secondary forces and have no need of balancing shafts or rephasing.
This next bit is purely academic, but the best configuration for a balanced engine is a flat eight design (or any number of opposed cylinders evenly divisible by eight). This is because each bank of pistons is opposed by the piston opposite and the outside pistons of one bank are opposed by the inside pistons on the same bank. This cancels all major secondary forces.
Conclusion
So... now that you know the idea behind parasitic forces, the solutions should be fairly straight forward.
The primary means of reducing parasitic forces is the reduction in weight of all components. Anywhere you can shed weight without affecting performance, do it. Unless you are well aware of the consequences of doing so, a reduction in strength of these components is a bad idea. Ensure all bearings are in good working order, all clearances are within spec, and your oil is changed regularly. Keep your chain lubed and your your wheel bearings greased. Don't over-tighten anything.
For balancing, my advice is to leave this to the pros. It takes special equipment and special know-how to not make things worse. The addition or reduction of weight in a crankshaft is a precision operation. Getting it wrong by just an ounce can add a couple of hundred of pounds in unbalanced forces as crank speeds approach redline. Rephasing can be undertaken by the garage mechanic, but requires special tools and a custom camshaft. A daunting task for the first time, but a rewarding experience when done correctly.
