Efficiency (I-Fish-En-Se) Noun [Origin: Latin] The Ratio Of The Useful Output To The Otal Input Into A System. Also Known As Effectiveness, Efficiency Ratio, Loss Ratio.
In the classic sci-fi novella "The Moon is a Harsh Mistress," Robert A. Heinlein's lunar characters proclaim "TANSTAAFL," or There Ain't No Such Thing As A Free Lunch. In other words, Mother Nature always exacts a toll when you try to make progress in life. Nothing is effortless.
Well, what's valid on Heinlein's moon is also true here on nonfiction earth-especially in the world of power production and automotive engines. There ain't no free energy lunch in that strange and wonderful process called combustion. There's also no free lunch in the rest of the drivetrain either, including a car's transmission, differential and wheel bearings. It takes energy to make and transfer energy, and usually a lot of it gets wasted or diverted during the process.
We engineers love to get geeky and talk about efficiency ratios and energy balances. We're describing the fundamental idea that energy cannot be created or destroyed, but it can be transferred from one form to another. For example, a roller coaster converts what is called "potential" or stored energy (due to its height above the ground) into "kinetic" or motion energy as it accelerates down the track.
There are many different forms of energy, from sonic and vibrational, to chemical, heat, electrical and magnetic types. Given the right kind of mechanical device, any one of these forms of energy can be converted into one of the others. In the process of conversion, however, some of the energy usually escapes from the system.
In the case of the roller coaster, for example, a small but significant fraction of the stored energy at the top of the track is lost to friction (heat energy) as the coaster accelerates downward. When the roller coaster reaches the bottom of the track, it reaches its fastest speed, and consequently has its highest kinetic energy. This kinetic energy, however, is less than the original potential amount that existed at the top of the run. The difference is heat that went into warming the wheel bearings and track, and also into the air itself, due to aerodynamic drag.
This is why the second hill at an amusement park is always shorter than the first one. There simply isn't enough energy left in the system to push the coaster cars up to their original height again. Friction paid the bill for the energy transfer lunch, and it will continue to do so until the cars come to a complete stop at the end of the ride.
With energy in mind, automotive powerplants operate in a similar manner to the roller coaster. In other words, one form of energy is transferred into another type, but at a cost. In the case of an internal combustion engine, stored energy (contained chemically in the fuel) is converted into kinetic energy (the pistons moving up and down). The cost of converting this energy, however, comes in the form of friction and heat. The same goes for the entire drivetrain, including everything from cam and crank friction, to oil windage, gear train, constant velocity joints and wheel bearing losses. In other words, there ain't no free energy lunch in your car's engine.
This is one of the reasons why some cars make more horsepower than other similarly equipped vehicles. It's also one of the reasons why some get better mileage than others. The same gasoline goes into the tanks of both cars, but how efficiently it gets used and converted into motion depends on many factors. These include: how much air actually gets drawn into the cylinders (Volumetric Efficiency); how much energy from the burning gasoline ends up moving the piston up and down (Thermal Efficiency); how much of the piston kinetic energy finds its way into crankshaft rotation (Mechanical Efficiency); and how much of the crankshaft energy actually arrives at the drive wheels (Drivetrain Efficiency).
Let's take a look at each of these terms, starting with the basic charge of air drawn into the cylinders on the intake stroke. This, of course, is the oft-misunderstood Volumetric Efficiency.
Volumetric EfficiencyGasoline can be considered a stored, chemical form of energy, and scientists will tell you a typical pound of it theoretically contains about 20,000 BTUs of energy. Just like 60 seconds equals one minute, or 16 ounces is a pound, 2,545 BTUs burned in one hour is equivalent to 1 hp. In other words, the more fuel burned every minute inside an engine, the more horsepower that engine produces.
The important thing to remember, however, is that for every one part of gasoline that gets burned, 14.7 parts of air need to be present and accounted for. If significantly more (or less) than this amount of air is mixed with the fuel, the gasoline won't be able to convert its full amount of chemical energy into heat energy. This 14.7:1 ratio of air to gas is called stoichiometry, and it's the key to understanding why horsepower production is tied so closely to the amount of air that can be physically drawn into the cylinders.
Imagine we have a 1.6-liter engine. On the intake stroke, each downward-traveling piston temporarily increases the volume of its cylinder by 1,600cc / 4 = 400cc. In other words, 400cc of air should get drawn into the cylinder through the intake valve, and then (if all goes according to plan) the corresponding stoichiometric amount of gasoline would be injected and burned with it.
The problem, however, is that 400cc of air doesn't get drawn into the cylinders of the typical street engine. Instead, somewhere between 100 and 350cc of air gets pulled in, varying strongly with engine design and rpm.
The ratio of actual intake air to the theoretically correct amount is called the Volumetric Efficiency, or VE, of the engine. Sometimes VE is simply referred to as engine-breathing ability, but whatever you call it, it's directly related to the amount of horsepower the engine can produce. If only 350cc of air get drawn into a 400cc cylinder, the VE is 350 /400, or about 87 percent. This is a fairly typical VE for a hot street engine; and, the higher the VE, the more power that can be produced.
There are a number reasons why VEs are less than 100 percent for most normally aspirated engines. Chief among these is the partial vacuum present in the intake manifold plenum and runners. The suction created by the downward traveling piston has to fight against this partial vacuum, and the result is that less than 100 percent cylinder fill. At idle and low engine speeds, the vacuum in the plenum can be quite high, and therefore, the VE is correspondingly low. As the throttle body butterfly opens and engine rpm increases, VE also increases. In fact, the VE typically follows the shape of the torque curve, peaking at the engine's maximum torque output.
The other things that affect engine VE are the restrictions, bends and obstructions in the intake path the air has to travel through. Intake runner length can also affect VE, as does cam timing, porting and valve sizes. Ordinary low-performance, grocery-getting econoboxes typically have VEs of around 80 percent at their torque peak. High-performance street engines come in around 85 to 90 percent, and all-out race engines can be as high as 95 to 100 percent.
In fact, with clever selection of the intake runner lengths and port work, some all-out, normally aspirated, high-performance engines can do better than 100 percent VE. Big-motor Pro Stock V8s, for example, often achieve around 125 percent VE with their highly optimized intake systems and sky-high, narrow powerbands. These big VE values are in the realm of the volumetric efficiencies found in turbocharged or supercharged cars, which operate with the benefit of excess pressure in their intake plenums.