Engine Efficiency #3
The third and perhaps the most important efficiency related to engine performance and affecting overall engine output is volumetric efficiency. This is essentially a measure of how easily the engine breathes air in and out. The more air that can be moved into the engine, the more oxygen there will be to mix with more fuel. This creates a more powerful combustion event whenever more power is needed. The power is in the fuel. If you can burn more fuel you can create more power. The actual rating of volumetric efficiency is the measurement of air that actually makes it into the cylinder while the intake valve is open, and the piston is moving down, expressed as a percentage of the theoretical potential volume of the cylinder.
To understand volumetric efficiency it must first be understood that the piston moving down on the intake stroke, with the intake valve open, only creates a negative pressure within the cylinder; it does not suck the air in as you might believe. Once negative pressure, or vacuum, is created in the cylinder, a greater force can push the air into the cylinder. That force that is atmospheric pressure. This is the primary reason that if you drive your car over a high mountain pass, where atmospheric pressure is low, the car always seems to have less power than it does when you are driving around town. Oxygen density at high elevation is also diminished. Sorry, Denver and Salt Lake, but cars are always faster in Los Angeles or Houston.
So if atmospheric pressure pushes the air into the cylinders, it will have to push past obstacles that are in the intake manifold, to get as much air in as possible before the intake valve closes. Not too many engines can get a full dose of air into the cylinder, but the more air that can get in there, the greater the volumetric efficiency.
Equally important as breathing air in is the ability of the engine to easily push the exhaust out. If all of the exhaust is not evacuated from the cylinder then it will displace some of the incoming oxygen. A well designed exhaust system will not only allow easy removal of exhaust gases from the combustion chamber but it can actually aid in drawing the air/fuel mixture into the cylinder. After the piston moves to the top to push exhaust out of the cylinder, and before the exhaust valve closes completely, the intake valve will start to open. The air/fuel mixture will then begin to move into the combustion chamber. Ideally this burst of air from the opening intake valve will help to move the last bit of exhaust out of the combustion chamber. This is a condition known as scavenging, and the better the scavenging the more the inert, exhaust gasses can be removed and the more space there will be for the fresh air/fuel mixture.
|Exhaust manifold with tuned|
Other than more precise fuel control, improvements in volumetric efficiency represent the biggest difference between old engine designs and those found in the modern day automobile. Better intake manifold and exhaust manifold designs help the air to flow in and out much more effectively. These manifolds use what is called tuned ports in order to make this happen. A tuned port intake manifold is designed so that all of the tubes or passages that carry intake air to ports in the cylinder head are exactly the same length. The length will also be designed according to the way pressure waves in the manifold resonate back and forth as intake valves open and close. These tuned ports take advantage of a phenomenon known as Hemholtz resonance.
As the air is being pushed into the engine through one of the passages in the intake manifold, it will suddenly stop every time the intake valve at the end of that runner slams shut. The momentum of this air will actually cause the air to reverse direction as it now kind of bounces off the closed intake valve. This pressure wave will move back into the plenum where is serves to help push air into the other runners in the manifold. If the ports are all the same length, then just when the intake valve in the first port opens again, another pressure wave coming from another port in the manifold will be timed just right to help push the next charge of air through the open intake valve into the combustion chamber. This is where that scavenging effect comes into play.
|A dramatic example of an engine with a tuned port intake manifold.|
Because Hemholtz resonance only occurs at specific engine speeds and loads, many engines also have what is called a variable length intake manifold (VLIM). This is a manifold that can change the length of the intake runners while the engine is running, in order maximize air flow for various operating conditions. At low speeds a longer narrower intake helps to increase the velocity of the incoming air, this increased velocity helps to fill the cylinder with air when the engine is running slowly, and the negative pressure in the cylinder doesn't build as quickly. This gives better throttle response off the line because of the increased torque that can be provided.
|The inside of a variable intake manifold. Notice the butterfly |
valves that can close to redirect intake air to other passages.
At high engine speeds a series of valves within the intake manifold will simultaneously switch the intake air to a much short runner that is wider. At high speeds the negative pressure in the cylinder builds quickly and in order to fill the cylinder we just need a short, wide path for the intake air. Race cars and other high performance vehicles have very short wide intake runners in the manifold because they are meant to run almost exclusively at high speeds.
Another very significant improvement in engine design that increases volumetric efficiency is the size and number of valves. Since the valves are the gateways in and out of the combustion chamber this of course makes sense. In the old days the combustion chamber had one intake valve and one exhaust valve. These valves were mounted in the cylinder head right next to each other, facing the top of the piston. In the old old old days they were mounted in the block but we won’t go back that far. These valves and the ports that they seal, can be made bigger which allows more air in and out. The problem is that they can only be made so big before the ports start to touch in the center of the combustion chamber. In order to provide even more flow, extra valves are added to the cylinder.
|A newer valve arrangement referred to as a pent-roof hemi.|
Intake and exhaust valves are set opposite of each other. The
spark plug hole being right in the middle of the combustion
chamber also provides benefits.
|An old style wedge combustion chamber. The air does not flow|
in and out efficeintly, and there is no room to make the valves any bigger.
Some manufacturers started building engines with two intake valve and one exhaust valve, or two intake valves and two exhaust valves. Some even made engines with three intake valves and two exhaust valves. In order to fit all of these valves in the combustion chamber the arrangement had to be changed. Instead of the valves being arranged next to each other, facing the same direction in the combustion chamber, the valves are mounted on opposite sides of the combustion chamber with the valve faces being angled towards each other. This angled arrangement also increases volumetric efficiency because it provides more of a straight path for the air, through the combustion chamber.
All of these valves crammed into the cylinder head would be difficult to operate with a cam-in-block design that was the standard. For this reason and others related to mechanical efficiency, the cam was moved out of the block and put in the top of the head. In some instances two cams are used to allow the valves to be bigger and angled more towards each other in the combustion chamber. These designs that use two cams in each head are referred to as dual overhead cam, and they are used on all of the most modern high-performance engines. A DOHC engine with a straight cylinder arrangement has two cams, an engine with a V arrangement, or a boxer engine with a flat arrangement has 4 cams.
Cam timing or phasing, and systems that can manipulate this during engine operation, have become a major contributor to increased volumetric efficiency. One of the challenges to making an engine operate efficiently and return good fuel economy, while maximizing power, and burning the fuel as cleanly as possible, is the fact that the engines must operate under a wide range of speeds and loads. An engine can very easily be made to run well at a specific load or RPM range. This is why stationary engines used in industrial equipment such as generators and pumps are usually among the most efficient. They really only operate at one speed and under a very similar load condition each time they are used.
|This cam sprocket has five vanes that can move when|
acted upon by pressurized oil to change the phasing
between the hub of the sprocket and the teeth
Changing the cam timing will change the timing of the opening and closing of the intake and exhaust valves. Considering that atmospheric pressure is ultimately responsible for pushing the air into an engine, in order to maximize volumetric efficiency at high RPMs, the intake valves must open sooner than they would at low RPMs. This can be done by changing the cam phase. The cam is essentially rotated a few degrees one way or the other in accordance to where the crankshaft is in its normal rotation. This feature is usually referred to as variable cam timing. Ford calls there system TiVCT, Toyota calls their system VVTi, Honda calls their system iVTEC, and so on (What is the deal with the lower case “i” anyway; marketing people everywhere like using it). All of these systems do essentially the same thing. The VTEC system from Honda also incorporates a system that can change how much a valve opens. This doesn’t necessarily change cam phasing but it does cause a sizable increase in volumetric efficiency.
The most fun and perhaps most dramatic way to increase volumetric efficiency is through forced induction. Using a device such as a turbocharger or a supercharger, volumetric efficiency can be literally pushed to over 100%. In a naturally aspirated engine some form of vacuum nearly always exists in the intake manifold and atmospheric pressure rushes in to fill the void. In an engine that uses forced induction the intake manifold is pressurized to anywhere from just a few psi to a few dozen psi. This causes even more air or oxygen to move into the cylinder which allows the engine to burn even more fuel. This might seem like it would cause a decrease in fuel economy and in some extreme applications it does. In reality however, it allows a vehicle to use a much smaller more fuel efficient engine because the forced induction can make the engine more powerful only when more power is need such as during acceleration or passing. The rest of the time all that extra power is not needed. On a normal flat road a Geo Metro can go 65 mph just as easily as a Corvette.
|A turbo charger is just an air compressor that pushes air|
into the intake manifold.
Turbochargers and superchargers have a drawback in that they increase the load on the engine. Superchargers affect the mechanical resistance that the engine has to deal with because they are driven by a belt off of the crankshaft. Turbochargers are driven by the exiting exhaust gases in the exhaust pipe. This makes it more difficult for exhaust to leave the engine. Both of these drawbacks are minor compared to what is gained in the overall breathability of the induction system.
So what does all of this have to do with engine design that was referenced in the first efficiency article? An engine that is to be used in a mid-size sedan may be the same engine that is used in a mid-size SUV. Because the SUV weighs more or because it might have to tow a trailer once in a while, the way the engine reacts under these different situations is going to make a difference in the overall drivability of the vehicle. The larger vehicle will need more torque at lower RPM than the smaller vehicle. Changing the arrangement of the intake manifold runners, or the way the variable cam timing functions will change the way the engine responds. The engine in the sedan may need to rev higher so things that cause internal resistance affecting mechanical efficiency may need to be considered.
The one thing that is constant is that the engines that perform the best are the ones that rate well for mechanical efficiency, thermal efficiency, and volumetric efficiency. This doesn’t mean that an engine with good ratings in all three of these categories is going to have the highest output; it may get the best fuel economy or perhaps the lowest emissions instead. Ideally it would have some combination of the three. Beyond gas mileage, power, and emissions, what else matters?