IT’S ALL ABOUT TORQUE
Part IA: The Wood Knight Prowler silent belt drive system
by Donny Petersen
(continued from previous thread as it is too long to fint on one)
These gears are timed to piston position in the cylinders via the pinion shaft. The pinion shaft is cast as part of the right flywheel, which is rigidly fixed as one with the left flywheel -- well, at least it was until the 2007 problems with scissoring flywheels -- via the 0.007” pressed -in crankpin. The big ends of the connecting rods rotate on caged bearings on the crankpin. At the other end, the small ends of the connecting rods house the wristpin bushings, which attach the pistons to the rods via wristpins. Many also refer to the flywheels, shafts, crankpin, bearings and connecting rods as the crank or flywheel assembly.
The cam lobes (two on each cam) operate the engine’s four valves (two in each head). The elliptical lobes are timed to open the valves relative to piston location. The valve springs close the valves as the rotating cam lobes diminish in height. A magnetic sensor in the front, left side of the engine case (called the crank position sensor) reads notches cut into the outer edge of the left flywheel, also known as the sprocket or power flywheel. The crank position sensor sends magnetic pulses to the electronic control module (ECM), which is the brain of the engine management system. The crank position sensor’s pulses tell the ECM where the flywheel is relative to piston location in its cylinder. Flywheel speed, amongst other factors, will indicate when the ECM will fire the spark plugs to ignite the fuel mixture. Fuel mixed with the proper amount of air needs time (called combustion lag) to start burning. The pistons move in tandem with the flywheels. The faster the pistons are moving up and down their cylinders, the higher the engine’s rpm. The ECM includes this in its calculations concerning when to fire the spark plugs for the best combustion of the fuel/air mixture.
Truth is, the plugs will spark and begin ignition while the pistons are still on their way up their respective cylinders on the compression stroke. The goal is to have a full flame front traveling across the combustion chamber as the piston reaches its Top Dead Center position (TDC) on its compression stroke. This allows for the maximum amount of pressure to be generated to drive the piston back down the cylinder on its power stroke. The burning fuel creates heat. Heat expands to create pressure. Pressure drives the piston down on the compression stroke. Advancing the ignition timing ignites the fuel sooner, before the piston reaches TDC on its compression stroke.
As the engine’s rpm increase, ignition timing advance increases, thereby igniting the fuel/air mixture progressively sooner during the compression stroke. As rpm decrease, ignition timing retards because the fuel/air mixture will have more time to burn before the slower moving piston reaches TDC. Retarding ignition timing simply means the ECM starts the ignition process later with the piston progressively closer to its TDC point. When and at what points this occurs is called the ignition system’s ignition curves.
The point is, almost every moving part in a Harley-Davidson engine is timed to other parts for specific reasons. For example, the intake valves cannot deliver the fuel/air mixture into the engine at the proper time unless they open when the piston is at the proper point and stroke. The same holds true for when to ignite the fuel/air mixture, getting rid of spent gases out the exhaust valves, and many other processes that occur during the combustion cycle. This is what an engine is all about. How we design or play with the timing of this process will determine emissions and torque, either in opposition or support of each other. We also advance or retard other events in an engine, such as cam timing, cam lobe separation angles, and cam lobe centerlines, which we will examine in detail a little further on.
Conclusion
Form is about good looks while function is all about workability. The Knight Prowler silent belt system is the perfect marriage of both form and function, which is no mean feat. However, there are two other related ideas driving H-D design, primarily by the symbiotic aftermarket.
The first is the desire for form over function, whereby V-twins are individualized and personalized. This is all about cool! Well, the Knight Prowler’s belt and pulleys, visible through a clear Lexan glass and surrounded in a sea of lustrous chrome, is a definition of cool. It looks so nice on my bike that even seasoned riders do not comment, they just staring as if they don’t understand what is going on. In my neck of the woods, in and around Toronto, I’m the first person to have a silent belt system and other riders are in awe, wondering what Petersen is up to now.
With other products, many times cool will compromise functionality of the bike. Lowering a Harley so much that shocks and forks cannot work as originally designed is an example of form over function. Another is raking the frame and extending the forks to the point where they compromise safe riding. Both examples involve an extreme design.
Compromising function may not occur with minor lowering or conservative rake and fork length changes. In fact, such changes may assist handling and look good at the same time. A shorter rider will have more control with slightly lowered shocks, forks, and seat. I’m not a small guy, but before H-D began lowering all of its bikes to accommodate the average rider, there were many models I could not sit on flat-footed. A police department may extend forks a few inches on pursuit bikes for more high-speed stability. These are examples of form assisting function. The second idea is function over form. Pure functionality overriding form usually leads to an ugly bike that performs wonderfully. Harley riders, of which I am one, are forever seeking a look.
Many also seek performance and, hey, that’s me, too! A look is form whereas performance is function. Therefore, the question becomes how can we get rid of those bothersome stock Twin Cam timing chains, chain adjusters, and bearing problems? How do we control wandering camshaft timing caused by loose fitting sprockets, or timing chains further exacerbated by the pulls and pushes of the valve springs versus the timing chain adjusters? How about combining form and function to eliminate these aggravating situations, gaining performance and creating something wild in the process?
When Bob Wood designed the Knight Prowler, I just had to put it through its paces in aggressive city and highway riding last summer. Bob’s specialty is high-lift, high-torque cams producing oodles of power right out of the gate, and he is no slouch at designing something wild either. I’m looking good, I’m going fast, and, by the way, that is reliably fast.
EPA-Driven Design
The quest for less noise, compelled by EPA dictates, drives new methodology and design. After all, the Twin Cam’s dual cams design strategy was all about reducing gear-driven camshaft noise. However, it just keeps coming, as the strictest, exhaust emissions controls will be introduced in 2010. Water jackets and radiators will be one of the next factory responses to quelling noise even more, as the government mandates further restrictions.
I dislike being alarmist, especially with H-D traditionalists, but I view the Twin Cam as a transition engine from the past to the future. Water jackets and radiators, catalytic converters, overhead cams eliminating our beloved valvetrains, different angled cylinders (as found on the V-Rod and new 1125cc Buell) and other changes will continue until we reach alternative fuels.
The Lead Cylinder
Some of the following may sound confusing at first, so please be patient and think it through. Part of the confusion arises because Harley-Davidson and some in the automotive industry view the subject of which cylinder is best from opposing vantage points, but it’s all really the same thing. A four-stroke engine, like what’s used on a modern Harley-Davidson, requires 720 degrees of flywheel rotation to complete all four cycles or strokes. The four cycles are intake, compression, power, and exhaust; each lasting 180 degrees of flywheel rotation. One might think that each cylinder operates 360 degrees of rotation apart from the other. If the cylinders were 90 degrees opposed this would be true. However, we have 45-degree angled cylinders. Bobby Wood explained to me that one cylinder fires at 315 degrees, while the other fires 405 degrees. Therefore, the two cylinders fire awkwardly, but that’s life in the wacky world of Harley, where engineers in 1903 felt a 45-degree cylinder engine would not even run.
The confusion compounds because Harley-Davidson views the number one cylinder as the front one with the rear being the number two. The automotive industry, which is obviously huge compared with the Motor Company, views the rear cylinder as the master cylinder and, therefore, the number one cylinder. This is the logical way to look at it because the rest of the engine times off the rear cam lobes and the rear cylinder. Logic is not always the way with our beloved dinosaurs. So, the automotive sector calls the front cylinder number two, relegating it to slave-cylinder status because its activities take their timing from the master rear cylinder. We need to know who is doing the explaining to envision what exactly is happening. Therefore, from Harley-Davidson’s perspective, when the front (number one) cylinder fires, the rear (number two) cylinder will fire 315 degrees of crankshaft rotation later. The number one cylinder will fire again at 405 degrees of crankshaft rotation later. If the reader adds 315 and 405, the two figures will total the complete 720 degrees of crank rotation required to fulfill all four strokes of the combustion cycle.
The auto industry, along with Bob Wood, quite logically views the rear cylinder as the number one master cylinder because it drives the front cylinder cams. Therefore, when the rear (number one) master cylinder fires, the front (number two) slave cylinder will fire 405 degrees later. The number one cylinder fires again 315 degrees afterward, thus completing the required 720 degrees to fully complete the intake, compression, power, and exhaust cycles. If the reader is confused, the real problem is one of semantics. How confused will one become not knowing the above when an authority like H-D postulates its interpretation in literature and then other experts in the field postulate the other interpretation?
Donny Petersen
Tattoo Tony’s Heavy Duty Cycles
Toronto, Canada
www.HeavyDutyCycles.com 
May 2008 Wood Knight Prowler Part 2 
Linear Mode
