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
ports.

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?

Engine Efficiency #2

The next efficiency that has an effect on the way an engine performs is thermal efficiency. As you can imagine this has something to do with heat, that is to say heat not temperature. In case you are not quite clear on this subject, heat is a reference to one of the basic forms of energy, and we typically measure this in Joules or British Thermal Units (BTU). Temperature is only a measure of the intensity of heat energy and we measure this in degrees Celsius or Fahrenheit depending on your unit persuasion.

One joule of heat is equal to the amount of heat given off by the human body at rest over a period of about 15 seconds. The temperature of the human body is 37° C, but if you were to take that 1 Joule and spread it out over the size of a medium sized living room, the intensity of the one Joule of heat would drop significantly so that the temperature of that room would be something very cold.

Gasoline is burned in the engine in a combustion process that converts the chemical energy of the gasoline into very concentrated heat energy. This concentration of heat causes a very rapid expansion of the compressed air in the engine’s cylinders. This expansion of air acts against the piston, pushing it down in the cylinder, which causes the crankshaft to turn. The more accurately the fuel and air are mixed, and the more precise the combustion process, the more efficient the engine will be.

The amount of energy that comes from the fuel that can be turned into force to move the piston down in the cylinder, rather than just turned into heat that will eventually be lost, the more thermally efficient the engine is said to be. Overall, internal combustion engines are very thermally inefficient because most of the energy in any engine is actually turned into heat and lost. Considering that super heating the air to create the expansion necessary to push the pistons, this excessive heat and the loss thereof is not really a surprise, because the heat cannot all be directed toward moving the piston. About half of the lost heat actually goes out the tailpipe, and the other half of the heat goes out through the cooling system and radiator.

The average gasoline engine has a thermal efficiency of only about 20%. This means that only 20% of the heat energy from combustion gets turned into work that moves the piston. This number is pretty low but it is much better than it used to be and will probably get better with time, however, most engineers theorize that that gasoline engine thermal efficiency can probably never get any better than about 35 or 40% due to the constraints of the laws of physics. A diesel engine has greater thermal efficiency than a gasoline engine which is the reason that diesels get better fuel economy. A diesel’s thermal efficiency can be as high as 30% for diesel engines found in cars and trucks and around 40% in many industrial equipment applications. Although this is an improvement over the efficiency of the gasoline engine it is still very low.

Diesel engines have higher thermal efficiency because they can operate with a much higher compression ratio than a gasoline engines. This is because the fuel is injected into a diesel engine just before the piston is at the top of the compression stroke. In most gasoline engines the fuel is injected onto the back of the intake valve and drawn into the cylinder on the intake stroke. The fuel then gets compressed with the air as the piston moves up in the cylinder. As this piston is moving up it is compressing the air which concentrates the heat energy in cylinder. When this heat becomes concentrated under compression, the intensity of the heat goes up. This increased intensity, or temperature, can ignite the fuel before the appropriate time arrives which will cause engine knocking. This is a terrible condition that causes the engine output to go down, and can potentially damage engine internals.

Direct injection has an injector that sprays fuel directly
 into the combustion chamber

Many engine manufacturers are building more and more direct injection gasoline engines. These engines inject the fuel in a very similar many to the way diesels inject fuel. This gives these gas engines greater thermal efficiency because they can run well with higher compression ratios. The compression ratio cannot be raised to high however, because the higher pressure that results causes a loss in the mechanical efficiency of the engine, not to mention the fact that the engine must be built to be more robust in order to withstand the higher pressure.

Next up, volumetric efficiency.

Engine Efficiency #1

Many things go into the design of an engine. Engineers must consider what kind of vehicle the engine will be used in, and this means everything from, how heavy the car will be, how many people it will haul, how much stuff it will haul, what will the overall volume of the vehicle be, and so on. Usually they will try and apply one engine to as many different vehicle platforms as possible in order to make the money spent on development go farther. This is why the V6 engine found in the Lexus ES350 bears a striking resemblance to the engine found in the Toyota Sienna, or the engine in the Pontiac Solstice seems just like the engine in the Chevy Cobalt.

When engineers adapt the same engine to multiple platforms they often make a few changes to the power plant that make it more suited to the platform. These changes affect the output of the engine because they usually change one of three efficiencies that ultimately determine how that engine will perform. These efficiencies are volumetric efficiency, mechanical efficiency, and thermal efficiency. These three efficiencies will be explained in time but for now let’s start with what might be the easiest to understand, and that would be mechanical efficiency.

Mechanical efficiency refers to how much energy the engine wastes because of the internal friction that is ever present. This friction loss is constant over the entire operating range of the engine. Metal parts rubbing together are going to cause friction that will of course cause resistance to the engine turning. Energy of course cannot be created nor destroyed; it can only be changed into something else. That something else in this case is heat. Anytime energy is lost to inefficiency in anything mechanical it is usually lost as heat. This heat from friction inside the engine is not the thing that makes the engine get as hot as it does, most of that heat comes from another source that we will discuss later.

In order to make the engine more mechanically efficient, friction must be reduced. Not only does reducing friction make the engine more efficient, it also makes the engine last much longer because friction and heat destroy all things mechanical. Obviously, the one thing that reduces friction inside the engine more than anything else is plain old motor oil. Everyone knows that without oil the engine would not last long and would seize up into one hot flaming chunk of cast iron and aluminum. Of course many people will still blow their engines up because they will forget to check the oil or something like that, even though they should know better. In the old days the oil was just about the only thing used to reduce friction and increase mechanical efficiency.

An old push-rod valve train.

Another thing that has been used to reduce friction on modern engines is more efficient valve train designs. The valve train begins with the camshaft. This shaft uses teardrop shaped lobes that spin on the shaft to push on some kind of a follower, or rod, or lifter, in order to open a valve. The valves allow the air into the cylinder, and let the exhaust out. On older less efficient engines the valve train was actually more complex and used more moving parts to transmit this force from the cam lobes to the valve itself. Every time another device is added for transmitting force, friction will be present.

Most manufacturers use what is called an “overhead cam” design in the valve trains of their engines. This places the camshaft, or camshafts, on top of or right next to the valves, instead of being in an area down near the crankshaft, alongside the cylinders. Placing the camshaft above the valves means the force travels a shorter distant from the cam to the valves, so less energy is lost to friction. Not all modern engines use this design, but most do and the few left out there that don’t, are likely holding on to the old designs because of the expense associated with R&D of new engine architecture. The overhead cam design also helps to increase volumetric efficiency which we will discuss later.

The mechanical efficiency is further increased by using rollers against the cam instead of just a regular lifter or rocker, which pretty much rubs against the cam. Rollers of course, roll instead of rub, and the rollers can use tiny roller bearings within themselves to eliminate friction even more. These rollers can be used on any kind of engine no matter where the camshaft is located.

A modern over-head cam design.

Modern engines employ many more things to further reduce friction and increase mechanical efficiency. Manufacturing and machining processes have gone a long way to reduce friction. More exact tolerances within the engine help oil to work more effectively. Parts that fit together the way they should will not bind in a way that increases friction, but will fit together well enough to not be constantly pounding each other apart.

While not always internal to the engine, better design of engine accessories and things that are driven by belts from the engine help to increase the mechanical efficiency. Many of the accessories on the engine that used to be belt driven are being eliminated by the use of more electrical and electronic devices. Smog pumps have disappeared, power steering pumps are being eliminated, and even A/C compressors on some vehicles are no longer driven directly by a belt or by the engine. All of this relates to improved mechanical efficiency. If you aren’t using power to power your peripherals under the hood, then that means there will be more power going to the wheels.

Next time we will discuss thermal efficiency.

Good Explosions

Tiny explosions occur inside the engine and that’s what makes it go. Squirt some gas into the engine at just the right time, light it off, and you’re ready to go. This is true enough but the best way to describe the combustion that takes place inside an engine is as a process of energy conversion. Convert energy from a form that is cheap, portable and easy to store, into a form that will move us down the road. That is what the combustion process is all about.

A few simple things are needed to make this energy conversion take place smoothly: spark, fuel, and compression. That’s it; nothing else is required, except for maybe the proper timing of these three things. All of these must happen at the right time, or at least close to the right time and the engine will run. In order to have compression, the intake and exhaust valves must be closed, and the piston must be moving in an upward direction within the cylinder. This is easy enough, what about spark and fuel?

Fuel

The fuel is where the energy is. This energy is in a chemical form and in order to make this fuel propel the vehicle down the road it must be converted into heat energy. Over the years the fuel distribution mechanism has evolved, and in the process has grown more and more efficient. For decades the fuel going into the engine was blended with the intake air stream in the carburetor. These old, inefficient devices were last found on new cars in the early 90’s, since that time all cars sold have had some form of a fuel injection system. The carburetors use what’s called a venture to create a low pressure area that relies on atmospheric pressure to push the fuel into the intake manifold where it vaporizes as it gets sucked into the cylinders. This is very inaccurate and leads to poor fuel economy, dirty exhaust emissions, and less power output.

Fuel injection systems will either spray the fuel into the intake onto the back of the intake valves, or spray the fuel directly into the combustion chamber. This is very accurate and each individual cylinder will get the exact amount of fuel that it will need to make the most of the capacity of the cylinder and the amount of energy that is in the fuel.

When the air and fuel are mixed, and the mixture has entered the combustion chamber, both the intake valve and the exhaust valve will be closed and the piston will move be moving up in the cylinder. The piston moving up will compress the air fuel mixture and squeeze it into a very small space at a ratio of about 10:1. Squeezing the mixture like this concentrates the oxygen in the fuel and the heat in the cylinder. Both of these things help to make the combustion much more powerful and the consumption of the fuel much more thorough.

When the air/fuel mixture is fully compressed a spark will be fired to the spark plug where it jumps the air gap between the two electrodes of the plug. This spark introduces a small source of heat that lights the air/fuel mixture. When the air/fuel mixture burns it causes a tremendous increase in temperature, and when temperature goes up, pressure goes up. As this pressure wave propagates within the combustion chamber, the piston must be in the right position to take the brunt of this expanding force. The piston takes the force of this expansion and moves down in the cylinder exerting tremendous force on the connecting rod, which connects the piston to the crankshaft. The crankshaft turns the reciprocating motion of the piston to the rotational motion that goes to the wheels.

Much is happening within the engine and considering how fast all of this takes place, it’s a wonder that the engine runs as well as it does. Not only does it run well and produce gobs of power but it can do it for hours on end, day in and day out with very little trouble.

Spark

In the old days the spark would originate in the ignition coil which essentially works like an electrical transformer. The coil takes a small amount of voltage with a high amount of current, and produces a high amount of voltage with a small amount of amperage. This spark is produced in accordance with the physical position of the pistons in the cylinders. When a piston is on the compression stroke and nearing the top of the run, on older engines, the coil would fire a spark to the distributor which would then send the spark to the appropriate cylinder at the appropriate time. The distributor was mechanically timed to the crankshaft and the camshaft so that the rotor that was spinning in the distributor would be lined up with the spark plug in the cylinder that had a piston nearing the top of the cylinder.

On the most modern ignition systems found on today’s engines, each cylinder has its very own coil. No mechanical connection is needed between the engine and the coil controls. A computer looks at piston position via crankshaft and camshaft position sensors, and when the time is right it will fire each individual coil for each cylinder. This is extremely accurate and very efficient. This way of firing the spark requires fewer moving parts, and fewer parts in total. This system allows the computer complete timing control. Eliminating moving parts and turning all control over to the computer makes the engine more efficient.

In order to have good, strong combustion, the spark that lights off the air fuel mixture must be introduced at different times depending on how the engine is operating. The amount of time required for the air/fuel mixture to burn is usually about the same no matter how the engine is running. Combustion occurs very quickly, so much so that it seems like an explosion, but in reality it is a very controlled process. In order for the piston to be in the right position to accept the rapidly expanding air, the spark must be introduced at just the right time.

On nearly all engines the spark must hit the air gap of the spark plug before the piston is actually all the way at the top of the cylinder. When the engine is running at high RPM’s the spark must be introduced even sooner because it will take just as much time for the air/fuel mixture to burn. This early timing of the spark is referred to as timing advance. The faster the engine is running the more advance is needed. This timing must be precise in order to maximize power output and efficiency. This early introduction of the spark is measured in degrees of crank shaft rotation before the piston is at the very top of the cylinder. When the piston is at the top of the cylinder it is said to be at top-dead-center or TDC.

Timing Advance

If the spark is introduced too late, then by the time the air/fuel mixture burns thoroughly, the piston will be so far past top-dead-center that the expansion of the air will not exert as much force on the top of the piston. If the spark is introduced too early then the combustion process will push on the piston when it is still in a position before TDC, this not only does not produce very much power but it can also be very damaging to the engine. The piston essentially slams into a rapidly burning and expanding air/fuel mixture. When this happens it is known as knocking or pinging. This knocking usually produces a sound deep in the engine that sounds like a rattle. This is a very bad thing, but usually only happens when something within the engine control systems is not working properly. Knocking can also be a problem if the fuel that is used in the combustion process has too low of an octane rating.

The ideal position for the piston to accept the power of the combustion in this example is 23° after TDC. At 1200
RPM the spark must fire at 18° BTDC and at 3600 RPM the spark must fire at 40° BTDC.

In order to make sure that spark timing stays exactly where it needs to be for the varying engine operation, a computer receives information from several different sensors. A cam sensor and a crank sensor look at piston position, and engine speed. A throttle position sensor looks at throttle position to see what the driver wants the engine to do. A coolant temp sensor looks at how hot or cold the engine is because this too has an effect. The computer can even look at a sensor called a knock sensor to see if the combustion process is happening to soon. This early combustion is the engine knocking that was explained above.

When the knock sensor detects engine knock, the circuits in the engine control computer that adjust ignition timing will back off the timing of the spark so that no engine damage will occur. This means that the spark will be fired at the spark plug closer to TDC. The ability of the computer to rapidly adjust this timing advance is a drastic improvement over the way timing advance used to work. On old engines, timing could never be advanced as much as would be considered ideal, because the mechanisms that controlled timing advance where crude mechanical devices that were slow to react and imprecise.

Burn It Good

Despite the fact that combustion controls are far more efficient then they used to be, the gasoline powered internal combustion engine is still only about 25% percent efficient on average. This is much better than the 15% efficiency that was common in the old days. Most likely the internal combustion engine will continue to become more and more efficient but the likelihood that it could become as efficient as an electric motor is not very high. Diesel engines are more efficient than gasoline engines but they still waste a tremendous amount of energy compared to electric motors. As long as the internal combustion engine keeps getting better and better, and the cost of electric cars stays high, we will keep on driving the cars that we know best.