Secret #2: The Role of Rpm
There are other engine efficiencies as well, such as Mechanical Efficiency and Power Stroke Efficiency, and these are very important also. But you get the idea. There are a lot of leaks in the engine's processes, and each one affects the one following it because the inefficient outcome of each step ends up being the next step's input, and already there is a loss. Plugging all these leaks (as well as possible) is the art of the high performance engine builder, and helps explain why performance engine building is so much work. The next concept to be considered, and the second "secret" of the high performance engine, is the interesting role and effects of rpm.
There are other engine efficiencies as well, such as Mechanical Efficiency and Power Stroke Efficiency, and these are very important also. But you get the idea. There are a lot of leaks in the engine's processes, and each one affects the one following it because the inefficient outcome of each step ends up being the next step's input, and already there is a loss. Plugging all these leaks (as well as possible) is the art of the high performance engine builder, and helps explain why performance engine building is so much work. The next concept to be considered, and the second "secret" of the high performance engine, is the interesting role and effects of rpm.
MEP vs. Rpm
There are just two major ingredients in horsepower. These are combustion's push (we have just looked at this in our consideration of thermal efficiency), and revolutions per minute (rpm). Combustion's push has a fancy name -- Mean Effective Pressure (MEP). MEP is simply the single pressure which, acting on the piston, could theoretically do the same work as all the sucking and blowing that goes on inside the cylinder. Engineers use MEP as an indicator of the stress on the engine from the inside-out -- i.e. the constant strain inside the pressurized container. The beauty of MEP is that it is platform-independent. It doesn't matter whether the engine is a four-stroke or a two-stroke, single cylinder or a V-12. It is concerned only with cylinder stress. However, cylinder pressure isn't alone. Before you can have power, you must also have time, and in an engine, time is measured in rpm. MEP and time go together. They are the twin giants holding up the horsepower universe -- the bookends bracketing all that is important about how an engine makes power. They are that significant. And, there is an interesting relationship between them -- they are essentially opposites. Find for example a high performance engine that makes its power through a lot of rpm, and you will discover (through calculation) that it produces a relatively low MEP. Conversely, the engine that produces power mostly through MEP will be one that doesn't need much rpm. Of course, many engines benefit from a careful combination of both, but even then, every engine works on an emphasis on one or the other, due to its internal design. This fact, the relationship between MEP and rpm, quickly evaporates all the barstool arguments. It also produces some fascinating conclusions. For example, to make a high-rpm engine make more power, do you perform modifications that make even more use of rpm, or do you go after MEP? Oddly, such an engine will usually respond best to modifications that increase MEP, not those which make use of rpm. Clearly then, contrary to popular wisdom, rpm is not always the major player in the game of increasing engine performance, even in high rpm engines.
There are just two major ingredients in horsepower. These are combustion's push (we have just looked at this in our consideration of thermal efficiency), and revolutions per minute (rpm). Combustion's push has a fancy name -- Mean Effective Pressure (MEP). MEP is simply the single pressure which, acting on the piston, could theoretically do the same work as all the sucking and blowing that goes on inside the cylinder. Engineers use MEP as an indicator of the stress on the engine from the inside-out -- i.e. the constant strain inside the pressurized container. The beauty of MEP is that it is platform-independent. It doesn't matter whether the engine is a four-stroke or a two-stroke, single cylinder or a V-12. It is concerned only with cylinder stress. However, cylinder pressure isn't alone. Before you can have power, you must also have time, and in an engine, time is measured in rpm. MEP and time go together. They are the twin giants holding up the horsepower universe -- the bookends bracketing all that is important about how an engine makes power. They are that significant. And, there is an interesting relationship between them -- they are essentially opposites. Find for example a high performance engine that makes its power through a lot of rpm, and you will discover (through calculation) that it produces a relatively low MEP. Conversely, the engine that produces power mostly through MEP will be one that doesn't need much rpm. Of course, many engines benefit from a careful combination of both, but even then, every engine works on an emphasis on one or the other, due to its internal design. This fact, the relationship between MEP and rpm, quickly evaporates all the barstool arguments. It also produces some fascinating conclusions. For example, to make a high-rpm engine make more power, do you perform modifications that make even more use of rpm, or do you go after MEP? Oddly, such an engine will usually respond best to modifications that increase MEP, not those which make use of rpm. Clearly then, contrary to popular wisdom, rpm is not always the major player in the game of increasing engine performance, even in high rpm engines.
Piston Speed
Some engines of course will need to be modified for higher rpm to get more power out of them. There are a number of problems that arise with high rpm however, and the first to be considered is the valve train. The four-stroke engine's valve train must be rigid enough and accurate enough to avoid "floating" its parts. Valve float is when the valve (and/or its operating parts) has accumulated so much inertia (through high speed) that the valve spring doesn't immediately return the valve after the cam lobe has passed. It's a dangerous thing because other parts intermittently share the same space, and interference is therefore inevitable. Along with detonation, valve float is every four-stroke engine builder's nightmare. But the valve train can be made to cope. Stronger springs, lighter valves, and less flexible (and even fewer) valve train components all result in more faithful adherence to the dictates of the camshaft and are therefore more high rpm compatible. The minimalist shim and bucket valve system for example evolved as a result of this quest for the ultimate valve train integrity. However, even with the valve train taken care of, rpm can still be a problem. The other reciprocating parts in the engine are also susceptible to being moved too quickly for their good. These are of course the piston and connecting rod. There is a limit to how fast these two relatively heavy parts can be yanked around. At some point, their inertia will just be too great, and the failure of the rod and even the crankshaft bearings (which absorb much of the rod's loading) will result. For this reason, an engine's Piston Speed has traditionally been a measure of its maximum safe rpm. Piston speed as a measurement was conceived back in the days of steel pistons that couldn't be thrown around too quickly without "fragging". Today however, though still referring to the piston by name, piston speed more than anything indicates the reciprocal loading on the connecting rod, not the piston. The piston's (and rod's) speed increases with rpm because the distance of its stroke never changes, though the crankshaft turns faster and faster and there is less time for the piston to traverse its stroke. The piston's speed is also anything but constant, since it stops at the top and bottom of its stroke. The piston's maximum speed then is actually near the middle of its stroke, and so calculated piston speed is really an average. The piston's speed also increases with stroke. That is, for a given rpm, an engine having a longer stroke pushes and pulls its piston faster. This is one big reason modern high rpm engines have relatively short strokes. What all this means is that when modifying an engine, piston speed is often the first consideration, even if it has little to do with the durability of the piston itself. It is rather a general rule of thumb for rpm.
Some engines of course will need to be modified for higher rpm to get more power out of them. There are a number of problems that arise with high rpm however, and the first to be considered is the valve train. The four-stroke engine's valve train must be rigid enough and accurate enough to avoid "floating" its parts. Valve float is when the valve (and/or its operating parts) has accumulated so much inertia (through high speed) that the valve spring doesn't immediately return the valve after the cam lobe has passed. It's a dangerous thing because other parts intermittently share the same space, and interference is therefore inevitable. Along with detonation, valve float is every four-stroke engine builder's nightmare. But the valve train can be made to cope. Stronger springs, lighter valves, and less flexible (and even fewer) valve train components all result in more faithful adherence to the dictates of the camshaft and are therefore more high rpm compatible. The minimalist shim and bucket valve system for example evolved as a result of this quest for the ultimate valve train integrity. However, even with the valve train taken care of, rpm can still be a problem. The other reciprocating parts in the engine are also susceptible to being moved too quickly for their good. These are of course the piston and connecting rod. There is a limit to how fast these two relatively heavy parts can be yanked around. At some point, their inertia will just be too great, and the failure of the rod and even the crankshaft bearings (which absorb much of the rod's loading) will result. For this reason, an engine's Piston Speed has traditionally been a measure of its maximum safe rpm. Piston speed as a measurement was conceived back in the days of steel pistons that couldn't be thrown around too quickly without "fragging". Today however, though still referring to the piston by name, piston speed more than anything indicates the reciprocal loading on the connecting rod, not the piston. The piston's (and rod's) speed increases with rpm because the distance of its stroke never changes, though the crankshaft turns faster and faster and there is less time for the piston to traverse its stroke. The piston's speed is also anything but constant, since it stops at the top and bottom of its stroke. The piston's maximum speed then is actually near the middle of its stroke, and so calculated piston speed is really an average. The piston's speed also increases with stroke. That is, for a given rpm, an engine having a longer stroke pushes and pulls its piston faster. This is one big reason modern high rpm engines have relatively short strokes. What all this means is that when modifying an engine, piston speed is often the first consideration, even if it has little to do with the durability of the piston itself. It is rather a general rule of thumb for rpm.
Ring Flutter
Piston speed is an archaic design limit that has been violently stretched in recent years thanks mostly to advances in metallurgy. A modern sportbike has a piston speed in the 4,000 feet-per-minute range. Twenty years ago this was a racing-only level, yet now it's covered by a three-year factory warranty! Therefore, piston speed is no longer the rpm gauge that it once was, but more of a general limit depending on crankshaft and connecting rod technology. Everything else being equal (we have already dealt with the valve train), today's performance engine's rpm limit is indicated by something else entirely, and that is Ring Flutter. Ring flutter is a piston ring problem. The piston ring of course seals compression and combustion pressure inside the cylinder. But it doesn't do this job by itself, at least not during combustion. During combustion, high pressure gases get behind the ring and push it downward in its groove and outward against the cylinder wall. This is how a piston ring with only a few pounds of radial tension can resist over 1000 psi of peak combustion pressure. It borrows from that pressure. But this works only if the ring stays on the bottom of its groove. As rpm increase, the tendancy is for the ring to float upward, especially on the piston's downstroke. That's when it reaches maximum acceleration (near the middle of the piston's stroke) and at the same time almost completely loses effective combustion pressure. If the engine rpm is also very high, these three occurances (maximum acceleration point, loss of cylinder pressure, and very high rpm) combine to cause the piston to literally out-accelerate its own piston ring, leaving the ring momentarily "floating" in its groove. The ring drifts upward, off the bottom of the groove, and touches the top of the groove. This cuts off the path of whatever gases are present, and the ring then collapses inward on itself against combustion's forces, failing to seal the cylinder. The result is blowby of the residual gases past the piston, which reduces lubrication and overheats the piston and cylinder. Also, since the piston gets up to 80 percent of its cooling through its rings, and the ring isn't against the cylinder wall any more, the piston heats up even further, often to the point of seizure. There are a number of ways to prevent ring flutter. Obviously, keeping rpm down is one way. However, for a high rpm high performance engine, this is not an option. Another is for the engine to be built with a very short stroke. Even at high rpm, the short stroke moves its piston more slowly. This has commonly been the tactic used by modern engine designers. However, along with this is simply the use of thinner piston rings. Thinner rings are less massive. With their reduced weight, they build inertia more slowly and can withstand high acceleration without floating. All modern high performance engines use very thin piston rings. One of the advantages of using today's forged high performance pistons is that most of them are designed for very thin, high performance rings.
Piston speed is an archaic design limit that has been violently stretched in recent years thanks mostly to advances in metallurgy. A modern sportbike has a piston speed in the 4,000 feet-per-minute range. Twenty years ago this was a racing-only level, yet now it's covered by a three-year factory warranty! Therefore, piston speed is no longer the rpm gauge that it once was, but more of a general limit depending on crankshaft and connecting rod technology. Everything else being equal (we have already dealt with the valve train), today's performance engine's rpm limit is indicated by something else entirely, and that is Ring Flutter. Ring flutter is a piston ring problem. The piston ring of course seals compression and combustion pressure inside the cylinder. But it doesn't do this job by itself, at least not during combustion. During combustion, high pressure gases get behind the ring and push it downward in its groove and outward against the cylinder wall. This is how a piston ring with only a few pounds of radial tension can resist over 1000 psi of peak combustion pressure. It borrows from that pressure. But this works only if the ring stays on the bottom of its groove. As rpm increase, the tendancy is for the ring to float upward, especially on the piston's downstroke. That's when it reaches maximum acceleration (near the middle of the piston's stroke) and at the same time almost completely loses effective combustion pressure. If the engine rpm is also very high, these three occurances (maximum acceleration point, loss of cylinder pressure, and very high rpm) combine to cause the piston to literally out-accelerate its own piston ring, leaving the ring momentarily "floating" in its groove. The ring drifts upward, off the bottom of the groove, and touches the top of the groove. This cuts off the path of whatever gases are present, and the ring then collapses inward on itself against combustion's forces, failing to seal the cylinder. The result is blowby of the residual gases past the piston, which reduces lubrication and overheats the piston and cylinder. Also, since the piston gets up to 80 percent of its cooling through its rings, and the ring isn't against the cylinder wall any more, the piston heats up even further, often to the point of seizure. There are a number of ways to prevent ring flutter. Obviously, keeping rpm down is one way. However, for a high rpm high performance engine, this is not an option. Another is for the engine to be built with a very short stroke. Even at high rpm, the short stroke moves its piston more slowly. This has commonly been the tactic used by modern engine designers. However, along with this is simply the use of thinner piston rings. Thinner rings are less massive. With their reduced weight, they build inertia more slowly and can withstand high acceleration without floating. All modern high performance engines use very thin piston rings. One of the advantages of using today's forged high performance pistons is that most of them are designed for very thin, high performance rings.
Rolling Element Bearing Failure
Older engine designs that still use rolling element bearings at their crankshafts and connecting rods can also develop rpm-related problems. These bearings aren't as high rpm compatible as their plain shell counterparts. Especially when they are undersized, as they are on Harley Evolution engines, a design rooted in the 1930s, yet one that favors high-rpm tuning. This engine's rolling element main and rod bearings can be spun so fast that their rollers skip across their races, ultimately damaging them. Such is the problem the many 150 hp big inch Harleys and clones are experiencing. These are not long-distance machines, with their connecting rod bearings becoming trash in just 5,000 miles.
Older engine designs that still use rolling element bearings at their crankshafts and connecting rods can also develop rpm-related problems. These bearings aren't as high rpm compatible as their plain shell counterparts. Especially when they are undersized, as they are on Harley Evolution engines, a design rooted in the 1930s, yet one that favors high-rpm tuning. This engine's rolling element main and rod bearings can be spun so fast that their rollers skip across their races, ultimately damaging them. Such is the problem the many 150 hp big inch Harleys and clones are experiencing. These are not long-distance machines, with their connecting rod bearings becoming trash in just 5,000 miles.
Any questions so far???
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