This an article by Mike Nixon that I found very interesting. It is a three part article that I will make three different posts about. The article is called "Three Secrets of High Performance Engine Building".
Part 1 deals with engine efficiency.
"We begin our study with efficiency. The word means simply "what you get compared with what you wanted." Mathematically, efficiency is expressed as output divided by input. For example, 1000 BTU of energy in the form of fuel might be fed into an engine, and the result might well be 200 BTU of power output. That's 1000 in and 200 out, or 200 divided by 1000. This would indicate an efficiency of 20 percent. Let's look at engine efficiency in three steps, in the same order in which the engine deals with them.
Volumetric Efficiency
Efficiency begins in an engine with Volumetric Efficiency. Volumetric efficiency refers to how well the cylinder is filled. Most production powersports engines fill their cylinders less than 100 percent full. After air enters the cylinder and the intake valve closes, the cylinder typically contains less air volume than if you took the cylinder off the engine and set it out on your driveway! In other words, instead of having 14.7 psi of sea level air pressure in it, the cylinder has 13 psi or so. That is, a slight vacuum. The cylinder has been "charged" with less than its full capacity. And the engine must work with this amount, never getting any more than that due to design limitations. However, the engine can be altered to fill its cylinder more, and even over-fill it. If through one of several means we manage to get slightly more than normal capacity into the cylinder, ending up with say 16 psi instead of 14.7, we call the cylinder "supercharged." That's where the legendary supercharger gets its name -- it's simply a device that overfills the cylinder. The supercharger is mechanically driven from the engine and results in positive displacement of air -- the faster the engine turns, the more extra air that is pumped in. Another mechanical supercharging method is the use of the turbocharger, a supercharger that is driven by exhaust gases and therefore does not result in positive displacement. That is, the turbocharger's movement of air is not linearly proportional to engine revolution. However, supercharging does not have to be done mechanically. There are also non-mechanical means of supercharging the cylinder. A common one is through chemical supercharging. The use of alcohol, nitrous oxide and other oxygen bearing substances force added oxygen into the cylinder, with the result cylinder over-filling and over-pressurization. Finally, methods that are neither mechanical nor chemical are also used to coax more air into the cylinder. Pulse and inertia tuning are good examples of this kind of supercharging. Pulse tuning, for its part, capitalizes on the lengths and angles of the intake and exhaust tracts to harness the inevitable resonances within these tracts to performance advantage. Carefully calculated lengths and angles control pulses in the intake and exhaust tracts to persuade the charge to move faster or slower as needed to increase cylinder filling. Inertia tuning is very similar, but a little different. Inertia tuning juggles tract gas speed and density with valve timing for increased cylinder filling. Both pulse and inertia tuning are used to significant effect in today's high performance engines. Without them, the engine's cylinder would not be as completely filled, and power would be wasted.
Efficiency begins in an engine with Volumetric Efficiency. Volumetric efficiency refers to how well the cylinder is filled. Most production powersports engines fill their cylinders less than 100 percent full. After air enters the cylinder and the intake valve closes, the cylinder typically contains less air volume than if you took the cylinder off the engine and set it out on your driveway! In other words, instead of having 14.7 psi of sea level air pressure in it, the cylinder has 13 psi or so. That is, a slight vacuum. The cylinder has been "charged" with less than its full capacity. And the engine must work with this amount, never getting any more than that due to design limitations. However, the engine can be altered to fill its cylinder more, and even over-fill it. If through one of several means we manage to get slightly more than normal capacity into the cylinder, ending up with say 16 psi instead of 14.7, we call the cylinder "supercharged." That's where the legendary supercharger gets its name -- it's simply a device that overfills the cylinder. The supercharger is mechanically driven from the engine and results in positive displacement of air -- the faster the engine turns, the more extra air that is pumped in. Another mechanical supercharging method is the use of the turbocharger, a supercharger that is driven by exhaust gases and therefore does not result in positive displacement. That is, the turbocharger's movement of air is not linearly proportional to engine revolution. However, supercharging does not have to be done mechanically. There are also non-mechanical means of supercharging the cylinder. A common one is through chemical supercharging. The use of alcohol, nitrous oxide and other oxygen bearing substances force added oxygen into the cylinder, with the result cylinder over-filling and over-pressurization. Finally, methods that are neither mechanical nor chemical are also used to coax more air into the cylinder. Pulse and inertia tuning are good examples of this kind of supercharging. Pulse tuning, for its part, capitalizes on the lengths and angles of the intake and exhaust tracts to harness the inevitable resonances within these tracts to performance advantage. Carefully calculated lengths and angles control pulses in the intake and exhaust tracts to persuade the charge to move faster or slower as needed to increase cylinder filling. Inertia tuning is very similar, but a little different. Inertia tuning juggles tract gas speed and density with valve timing for increased cylinder filling. Both pulse and inertia tuning are used to significant effect in today's high performance engines. Without them, the engine's cylinder would not be as completely filled, and power would be wasted.
Combustion Efficiency
But cylinder filling is just the first area of potential waste in the engine. Whether normally charged ("aspirated") or supercharged, once the charge is in the cylinder, it must be burned to unlock the fuel's BTU energy. And of course there is loss here too. Combustion Efficiency simply considers how much of this charge gets completely burned. The piston is moving all the time combustion takes place. It is still coming up as combustion starts, which actually promotes combustion but may lead to detonation, the instantaneous and massive explosion of the whole charge due to excessive heat and pressure, with the result a waste of BTU energy as the piston is hammered instead of pushed. Combustion is also ongoing as the piston strokes downward again. This downward movement rapidly changes the cylinder's pressure, discouraging steady flame movement across the cylinder. Other things that affect combustion efficiency are combustion chamber shape, intake tract air speed, and spark plug location. Flatter chambers burn more of their contents than do arched ones, which is why modern engines have such chambers. High intake charge speed promotes good combustion because the air and fuel stay more thoroughly mixed as they enter the cylinder. A centrally-located spark plug causes combustion's flame to reach deeper into the remote edges of the cylinder, turning more BTU into useful work, and eliminating the end-gases that often initiate detonation. During combustion then, some of the power we're trying to produce is lost, and we haven't even got to the crankshaft yet!
But cylinder filling is just the first area of potential waste in the engine. Whether normally charged ("aspirated") or supercharged, once the charge is in the cylinder, it must be burned to unlock the fuel's BTU energy. And of course there is loss here too. Combustion Efficiency simply considers how much of this charge gets completely burned. The piston is moving all the time combustion takes place. It is still coming up as combustion starts, which actually promotes combustion but may lead to detonation, the instantaneous and massive explosion of the whole charge due to excessive heat and pressure, with the result a waste of BTU energy as the piston is hammered instead of pushed. Combustion is also ongoing as the piston strokes downward again. This downward movement rapidly changes the cylinder's pressure, discouraging steady flame movement across the cylinder. Other things that affect combustion efficiency are combustion chamber shape, intake tract air speed, and spark plug location. Flatter chambers burn more of their contents than do arched ones, which is why modern engines have such chambers. High intake charge speed promotes good combustion because the air and fuel stay more thoroughly mixed as they enter the cylinder. A centrally-located spark plug causes combustion's flame to reach deeper into the remote edges of the cylinder, turning more BTU into useful work, and eliminating the end-gases that often initiate detonation. During combustion then, some of the power we're trying to produce is lost, and we haven't even got to the crankshaft yet!
Thermal Efficiency
And the game still isn't over. The charge has entered the cylinder, and the combustion chamber has burned it, and each of these two has resulted in some inevitable waste in the power-producing process. However, there is more waste yet. There is still the conversion of that thermal energy into pressure, that is, "piston push". And there is loss here also. Thermal Efficiency therefore concerns itself with how much of the heat of combustion ends up actually pushing the piston downward. Usually, as much as 60 percent of combustion's heat is wasted heating up the engine or going out the exhaust pipe, leaving very little to drive the piston. Maximizing thermal efficiency is difficult, but not impossible. Well-shaped combustion chambers that finish their job quickly waste less of combustion's heat because there is less time for that heat to radiate away from the combustion process into the engine castings. Smaller chambers offer less surface area that will soak up combustion's heat, improving thermal efficiency even more. Coating the piston top, which is really the combustion chamber's floor, with a ceramic layer, also prevents heat absorption, this time into the piston, with the result greater piston pressure. Even higher cylinder compression can help, because it makes the charge more ready to burn, speeding up flame travel and resulting in more charge consumption. Combustion chamber design is so critical that engineers often use the engine's torque per cylinder to rate a combustion chamber's effectiveness. How much pressure the engine produces in a given size cylinder directly reflects the combustion chamber's efficiency. Even before the crankshaft turns, the first three steps in the power-producing transaction each waste some of the precious power that will ultimately be outputted."
And the game still isn't over. The charge has entered the cylinder, and the combustion chamber has burned it, and each of these two has resulted in some inevitable waste in the power-producing process. However, there is more waste yet. There is still the conversion of that thermal energy into pressure, that is, "piston push". And there is loss here also. Thermal Efficiency therefore concerns itself with how much of the heat of combustion ends up actually pushing the piston downward. Usually, as much as 60 percent of combustion's heat is wasted heating up the engine or going out the exhaust pipe, leaving very little to drive the piston. Maximizing thermal efficiency is difficult, but not impossible. Well-shaped combustion chambers that finish their job quickly waste less of combustion's heat because there is less time for that heat to radiate away from the combustion process into the engine castings. Smaller chambers offer less surface area that will soak up combustion's heat, improving thermal efficiency even more. Coating the piston top, which is really the combustion chamber's floor, with a ceramic layer, also prevents heat absorption, this time into the piston, with the result greater piston pressure. Even higher cylinder compression can help, because it makes the charge more ready to burn, speeding up flame travel and resulting in more charge consumption. Combustion chamber design is so critical that engineers often use the engine's torque per cylinder to rate a combustion chamber's effectiveness. How much pressure the engine produces in a given size cylinder directly reflects the combustion chamber's efficiency. Even before the crankshaft turns, the first three steps in the power-producing transaction each waste some of the precious power that will ultimately be outputted."
That is only part one of this three part blog. Let me know what you think! Do you disagree or agree or have something to add?
No comments:
Post a Comment