Filling and Burning
Performance engine building has as metrics a daisy-chain of three efficiencies. These are guidelines that help keep things scientific in the quest for improved performance. The first of these is concerned with cylinder filling. Volumetric efficiency is its name. Air enters the cylinder, but the usual result is less air volume than if you took the cylinder off the engine and set it out on your driveway! The cylinder has been "charged" with less than its capacity, and it must work with this amount, never getting more. However, the engine can be altered to fill its cylinder more. If we manage to get slightly more air “charge” into the cylinder we call the cylinder "supercharged." Everyone knows about superchargers and turbochargers, that’s what they do. But there are also non-mechanical means, and one of these is chemical supercharging. The use of alcohol, nitrous oxide and other oxygen bearing substances force added oxygen into the cylinder. And there are also ways neither mechanical nor chemical. Wave tuning for example uses the lengths and angles of the intake and exhaust tracts to harness the resonances that can persuade the charge to move, though this approach is more subtle and is not blessed with as much concrete understanding of how it all works.

Once the cylinder is charged, that fuel/air glob must be burned to unlock the fuel's BTU energy. Our second metric, combustion efficiency, gauges how much of the charge gets burned. Because not all of it does. The piston is moving all the time combustion takes place; it is still coming up as combustion starts, and is going downward again while combustion is underway. This 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; high intake charge speed promotes good combustion because the air and fuel stay more thoroughly mixed as they enter the cylinder; and a centrally-located spark plug causes combustion's flame to reach deeper into the remote edges of the cylinder in less time, resulting in more of the charge being consumed.

And the game still isn't over. The charge has entered the cylinder and been combusted. However, what we’re ultimately trying to make is pressure. Thermal efficiency describes how much of combustion’s heat actually ends up pushing the piston downward. As much as 60 percent of this heat is wasted heating up the engine and going out the exhaust pipe. Maximizing thermal efficiency takes the form of well-shaped combustion chambers that finish their job quickly and waste less of combustion's heat—there is less time for that heat to radiate away from the combustion process into the engine castings, and smaller chambers that offer less surface area that will soak up combustion's heat, improving thermal efficiency even more. Coating the piston top, which is actually 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 burn more readily and thoroughly.

So this is the efficiency picture. There are other engine efficienciy measurements as as well, such as mechanical efficiency and power stroke efficiency. But you get the idea. Even before the crankshaft turns, the first few stages of the power-producing transaction each waste some of the precious power that will ultimately be outputted. 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 much as possible—is the art of the high performance engine builder, and helps explain why performance engine building is so much work. Not magic, not mystery, but just plain old work.