Get to Know Your Valve Train

In operation, an internal combustion engine, such as the one you likely have in your car, needs to take in fuel and air. After burning them, it has to eject waste gases. It must do this while maintaining a sealed cylinder which can take advantage of the high pressure generated by the combustion. The solution is provided by the valves, which open and close as needed. The valves are the gatekeepers to the kingdom of the engine, controlling what goes in and out when. The valve train is the entire apparatus of rods, cams and springs dedicated to the operation of the valves. Understanding the valve train is absolutely fundamental to knowing how an engine works. This handy video gives a great introduction to the valve train, with nice animated cutaway views.

Compression Ratio

The compression ratio of an internal combustion engine is an important operating characteristic. It is the ratio of the volume inside a cylinder when the piston is at the very bottom (opening the most space) and the volume in the head when the piston is at the top of its range of motion. For typical automotive engines the higher the compression the more efficient the engine can be. This is fundamentally because a high compression ratio gives the hot combustion gases more opportunity to expand and do work. Imagine a compression ratio only a little above one. That would mean the energy filled gases, flush with heat from the combustion of the fuel would have barely any space to expand. They would thus do very little work. Then on the exhaust stroke they would be expelled to the air taking most of the energy with them. On the other hand, a very high compression ratio means that the hot expanding gas can do work over a much greater distance. That allows to extract more of the energy and waste less trapped in the exhaust gases. Miller cycle engines take advantage of this extra expansion without requiring a high compression ratio.

Secondarily to this is the fact that under compression, the fuel and air is tightly packed into a small space. This can help ensure complete combustion. It also helps control the combustion timing so that it is optimally phased relative to the piston position.

The limiting factor preventing high compression ratios is pre-ignition. This is commonly called knocking or pinging. The high temperatures generated during a high compression can cause the fuel to self ignite too early in the cycle. Such early ignition or detonation ruins the careful timing and reduces engine efficiency and power. Fuels that are more resistant to pre ignition are able to be run at higher compression ratios. This is the reason for the higher efficiency of Diesel engines. Typical automotive Otto cycle engines run at compression ratios of about 12 to 1 while Diesel engines with their different fuel run at around 25 to 1. Thus Diesel engines get more work out of their hot gases as they expand 25 times instead of just 12 times.

Regenerative Braking Video

In this video a Toyota representative explains how the regenerative braking system in a modern Camry sedan works. The brake system uses both electric and mechanical parts. They say that 80% of the braking is done by the recoverable electric system, with the last 20% provided by a standard mechanical pressure based system. Although when he says that the regenerative braking works by converting heat in the brakes to stored energy, I am almost certain he is oversimplifying. Normally regenerative braking systems work by using the rotational kinetic energy stored in the drivetrain to run a generator, which then charges the battery.

Regenerative Braking

Motion requires energy. To overcome inertia and add speed to a body, energy must be added. The energy a body has by virtue of its motion is called kinetic energy. The formula giving the amount of kinetic energy K that a body of mass m will have at a speed v is K = ½mv². Note the occurrence of the square of the speed. That means doubling the speed does not double the energy needed but rather quadruples it. Now energy cannot be created or destroyed. So to get your car up to speed the required kinetic energy has to come from somewhere. In a standard internal combustion engine driven vehicle, this energy comes from the chemical energy that was stored in the fuel, in the gasoline or diesel.

When you want to slow down or stop the kinetic energy will be lower at the new lower speed. Since it cannot just be destroyed, this energy must go somewhere. In a normal vehicle it is converted to frictional heat in the brakes. This heat then escapes and the energy is lost forever. Every time you slow down, you are throwing away all of the chemical energy that came from the fuel. Your brakes slow you down and in the process turn gasoline into heat.

What if it were possible capture the kinetic energy that is lost as you slow down and store it for reuse later? Obviously it cannot be converted back into chemical energy in the fuel. Internal combustion engines and standard brakes have no way of capturing the kinetic energy and it must be lost as heat. This is the reason that the Golden Rule of Hypermiling is "Maintain Momentum". Every time you slow down and speed up again, you have turned some fuel into heat.

Electric engines, such as used in hybrid vehicles or NEVs can capture some energy while slowing down. This is because electric engines run off of batteries, which can both provide and store electric energy. Compare that to liquid fuel, which can put energy out but it is impossible to pump energy into. Also an electric engine run in reverse is in fact an electric generator. Depending on which direction you run it, an electric engine can draw stored energy out of a battery to provide work or it can accept outside work, generate electricity and store it in the battery. Regenerative braking basically switches the electric engine to run in reverse, in generator mode. It makes the spinning wheels and drivetrain do work on it. As the drivetrain torques against the electric engine, it slows down, thus slowing your vehicle down. The drivetrain torquing the engine allows it to generate electricity, which is then stored in the battery.

Although regenerative braking is not capable of capturing all of the kinetic energy due to inevitable losses, any percentage it does recover is available to accelerate you back up to speed without needing to put in fresh energy. Unlike normal internal combustion engines and standard braking systems, not all of your motion energy is lost when you slow. And that means you save on energy.

Your Gas Cap

Gasoline, especially winter gasoline with plenty of butane, has a high vapor pressure and will evaporate quickly if exposed to the open air. A properly secured gas cap will keep the evaporated fuel under pressure in your gas tank and stop it from escaping into the atmosphere. This saves you money by reducing lost fuel and also helps air quality. Gasoline vapor is a heavy contributor to smog. A loose gas cap can easily emit more hydrocarbons due to evaporation from a parked vehicle than would be emitted by the running engine. This is because of the almost complete combustion of fuel.

So if you have a damaged gas cap or if you have lost it and been running without, think about replacing it today. You will save a lot of otherwise wasted gas and cut down on smog.

Winter Gasoline: What is the Difference?

Every year in the Fall we hear about how the gas stations are switching over to winter gasoline. The good news is that winter gasoline is a little cheaper. Have you ever wondered what the difference between winter and summer gasoline is and why one is cheaper than the other?

Gasoline is made from crude oil. Crude oil consists of a wide mix of chemicals. Refineries process crude oil and separate out the various constituents. These products are then blended into gasoline. The refinery does not convert crude oil directly into gasoline. What actually happens is the refinery uses the crude oil to make blending components which are then combined to make gasoline. Refineries have a multitude of operating units, some of which could be catalytic crackers, alkylate units and reformers. Each has a different cost to run.

When gasoline is made by blending these components, there are certain specifications that must be met. One of them is the octane rating. Another very important but less well known specification is the vapor pressure of the gas. Every fluid has a vapor pressure that depends on the temperature. Higher temperature gives more vapor pressure. When the vapor pressure is greater than the atmospheric pressure, the liquid will boil. For example, water at sea level boils at 100 degrees Celsius because water at that temperature has a vapor pressure of one atmosphere. If you climb a mountain, the water will boil at a lower temperature because the air pressure at altitude is lower. Thus the water vapor pressure reaches the air pressure at a lower temperature. The result of trying to cook in water that is boiling at a lower temperature is that you have to wait longer for your food to be done.

The vapor pressure of gasoline also rises with temperature. That means that in the baking heat of summer, the vapor pressure of gasoline in your car's fuel tank might rise up above atmospheric pressure. That would make it boil, filling your tank with gas fumes. The gas will escape into the atmosphere, causing pollution. To reduce that pollution, the gasoline must be changed so it has a lower vapor pressure at a given temperature.

The vapor pressure of gasoline depends on the components that are blended to make it. One of the cheapest gasoline blending components is butane. Butane also has a very high vapor pressure. So in the summer, gasoline has to be made with very little butane. That butane must be replaced with something more expensive. In the winter, when temperatures and thus vapor pressures are lower, cheap butane may be used in greater amounts.

So the big difference between summer and winter gasoline is the amount of butane blended in to make the gasoline. The cheap butane means winter gasoline can be a little cheaper too. Next time you hear about winter gasoline you will know what they mean. Winter gasoline means more butane!

Biodiesel Burning Demonstration

It is actually pretty easy to make your own biodiesel at home. We put up a video by the Mythbusters in which they did nothing more than filter used French fry oil. The result was a sort of "biodiesel" which they then used to fuel a normal diesel car. This video shows that if you do want to make biodiesel at home you don't have to worry much about safety. Biodiesel in liquid form is very difficult to get burning. But as you can see in the ending sequence of the video, when it is atomized into tiny droplets with a high surface area to volume ratio it does burn. That atomization coupled with the high pressures in the cylinders of a diesel engine is why it works to power your diesel vehicle.



If anyone has the inclination to try making their own biodiesel, I want to encourage you. There was a time when people where much more do it yourself oriented. Today a big percentage of the population depends on buying premade stuff in department stores. If you know how to make things yourself it gives you a better understanding of how the world and our economy works. You know that products don't just appear on shelves because you have seen what it takes to make them. Also you are much more independent. If you can do it yourself you can also find a lot of chances to save too. Doing it yourself will also develop skills that you will keep forever, possibly offering you opportunities to sell your services in the future. Spending a few hours trying out biodiesel production at home will benefit you way more than sitting in front of Today's Reality TV Show for that same time.

Where Does the Energy in Your Tank Go?

Energy cannot be created or destroyed, only converted among its various possible forms. This physical fact is known as the First Law of Thermodynamics. So what happens to all of the energy in a tank of gas? How much of it performs useful work and how much of it is wasted? Where are the losses? Of course for a vehicle, the useful end purpose is to move passengers and cargo. Anything else is waste. This excellent paper has a nice breakdown of where the energy goes. It considers a composite driving cycle including both highway and city driving. The results go as follows.

We start with 100% of the energy in the fuel, and the table shows where it goes.

Fuel Energy Destination

Destination	Percent
Irreversible Combustion	30
Cooling and Exhaust	32
Engine Friction	18
Accessories	2
Transmission	3
Air Resistance	5
Tire Rolling Resistance	5
Brakes	5

Irreversible combustion refers to the fact that during combustion, a portion of the energy is necessarily converted to forms not available to do work. That is a basic result of thermodynamics. No heat engine can escape this. A heat engine is one that generates work by using energy to heat a working fluid, and then allowing this hot working fluid to expand. The pressure generated during the expansion then does the work. By directly converting energy to work, for example in a fuel cell or electric engine it is possible to avoid this loss. However, automobiles are still overwhelmingly using the internal combustion engine. Car engines are definitely heat engines. They use the energy in the gasoline to generate heat by burning it with air as oxidizer. Then the air is heated up. The same hot air + combustion products serves as the working fluid.

The 30% lost to the cooling system and the exhaust is partly recoverable. Saving some of this energy is the basis for turbo compounding engines and six stroke engines.

Engine friction refers to losses in the moving parts of the engine itself. There are engine designs, like the Brickley engine that focus on reducing these losses. In particular, modern high precision machining techniques are allowing cheap production of complicated friction reducing designs. Machining tolerances have decreased as well, also allowing for new lower friction designs.

The numbers are representative of a typical vehicle averaged over a typical driving cycle. Under specific conditions, say going 60 MPH up a 3% grade, the values will break down slightly differently. Only 20% of the energy in your gas makes it out of the engine. That 20% is where you have control. You can't do much about thermodynamics or engine friction. But the 2% typically diverted to accessories represents 10% of the out of the engine energy. Reducing use of the air conditioner is an example of exerting control. The 5% of the total typically going to air resistance represents 25% of the past the engine energy and you can control that by reducing your speed.

The table shows that it is the engine designers of Detroit that will have to bear the largest part of the load on the way to better fuel economy. And if they won't do it, then there are plenty of smart engineers in the rest of the world who will do it and are doing it.

Variable Valve Timing Reloaded

Variable valve timing is a simple idea that took time to be implemented due to practical engineering difficulties and cheap fuel prices. The idea is to adjust the opening and closing cycles of the valves to change according to engine speed. When the engine is operating at low RPM, at speeds slow compared to the movement speed of air, the intake and exhaust cycles can have almost no overlap. At high RPM when the air has trouble moving fast enough to keep up with the engine there has to be a large overlap.

The video gives an overview of the ways to achieve variable timing. Watch and learn!

Torque vs RPM

On a dynamometer chart you always see the torque and power produced by an engine plotted versus RPM. That shows you how the engine performance depends on speed. The engine speed is related to the vehicle speed but not directly because of the gear ratio in the transmission. Have you ever wondered why the torque curve has a peak? What causes the torque to be lower at low and high RPMs?

First of all, why is there a peak torque? Peak torque occurs when the engine receives the maximum amount of fuel air mixture in the cylinders to burn. The torque is produced by the pressure of the explosion on the pistons. This pressure is transmitted to the wheel rims as rotational force against the road. More cylinder pressure gives more torque, and to get more cylinder pressure you need to burn more fuel. So peak torque is when the cylinders are getting the most fuel with air sufficient to burn it all.

AS the RPM goes above the point of peak torque, torque decreases mainly due to difficulty getting air in and out of the cylinders. Basically the engine can't breathe fast enough to keep up with the pistons. The air will only move around so fast. The maximum external static pressure to drive air into the cylinders for example is one atmosphere. If the pistons begin to move up and down faster than the air can move into the spaces they leave behind then the air fuel charge amount will start to drop. As the exhaust gases are forced into the exhaust manifold faster than they can flow out through it, the back pressure in the manifold rises and rises. This rising back pressure robs output from the engine as it must now divert some effort to fighting it. Also mechanical efficiency drops at high RPM due to generally increased frictional resistance at high relative speeds between moving parts.

Now what happens at lower RPM? First of all, the pistons are now moving slowly. This gives time for heat to flow. During the compression stroke and power stroke at slow piston speeds, there is time for some of the heat in the gas mixture in the cylinder to escape through the cylinder wall. This loss of heat leads to a drop in temperature and pressure. This pressure loss directly reduces torque. Remember that the torque is being generated by the pressure on the pistons. There will always be an imperfect seal around the piston rings and the valves. At low RPM there is time for the pressure to bleed off through leaks. In addition to these factors the timing of the ignition and valve open/closing for normal engines is optimized for midrange RPM. Engines with variable timing systems do not have to suffer low RPM torque losses due to this factor, but the majority of today's engines do not have such systems.

The Dyno Test or Where Do Engine Stats Come From

Ever wondered how they get the fuel economy (MPG) numbers that you see for vehicles and the engine performance stats like torque? The numbers are produced by an instrument called a dynamometer (or dyno for short). It basically consists of a platform with rollers that the wheels of a car will sit on. The dyno has sensors that allow it to measure the torque applied to the rollers by the car and the rotational speed of the rollers. Knowing both torque and rotational speed allows the calculation of the power output as the product of the torque and rotational speed. The video above shows several dynos in action. Now you know where fuel economy statistics come from!

Conventional Continuously Variable Transmissions

A few posts ago we talked about the NuVinci continuously variable transmission during our series on gas mileage technologies. The NuVinci design is an innovative type of continuously variable transmission that has not yet been used by a major automaker. However, there are other types of conventional continuously variable transmissions that have been used in cars. These are conventional in comparison with the NuVinci, but advanced compared to the common manual or automatic transmissions in almost all of our cars.

Everyday transmissions adjust the ratio between the rotational speed or RPM of the engine and the wheels by choosing from four or five fixed metal gears. Current continuously variable transmissions use a pair of variable diameter pulleys instead of the gears. Each pulley is formed out of two cones with their tips or apexes pointing towards each other. These cones can move closer together or farther apart. A hydraulic or spring system is used to control the spacing between the cones.

A belt runs between the pulleys, fitting in the groove between the two cones. Older designs used rubber belts that were shaped like a V to better contact the sides of the cones. Now with newer metal alloys there are metal belts. The metal belts are stronger and allow for transmissions that can support much higher torques. The idea is that as the cones move farther apart, the belt can slip farther down between them and get closer to the pulley axis. The lower down the groove the belt is the smaller the effective diameter of the pulley.

It is necessary to have variable diameter pulleys in pairs. If one pulley pushed the cones closer together forcing the belt up the groove and increasing the belt diameter the belt would have to stretch if the other pulley did not simultaneously decrease its diameter the same amount. One pulley of the pair is connected to the engine crankshaft and the other to the vehicle driveshaft. As they change their diameters in lockstep the ratio of crankshaft diameter to driveshaft diameter can vary continuously. The ratio of these diameters is in inverse proportion to the ratio of their rotational speeds or RPM. If for example the largest diameter (when the cones are close together) is double the smallest diameter (when the cones are far apart) then the output (driveshaft) RPM can vary continuously between one half the input (crankshaft) RPM and double the input RPM.

The result is better gas mileage due to two main factors. One is the fact that the engine can run closer to its most efficient RPM more of the time. The second is that the transmission is very simple so the losses due to friction within the mechanisms are reduced. These factors give a 6% increase in fuel economy.

Check Your O2 Sensor

We talked before about oxygen sensors going bad. A bad oxygen sensor will think there is not enough fuel in the air-fuel mixture and add more. The result will be too much fuel for the amount of oxygen and not all of it can be burned. The extra unburned fuel will be wasted with the exhaust. Remember that your engine is an air breather.

This video compares a bad and a good oxygen sensor to show the difference. The two sensors are given time to warm up. Then a rag is wet with brake cleaner. Because the brake cleaner is combustible, the sensor treats it as fuel. The good sensor indicates the presence of fuel (full rich) as long as there is still brake cleaner in the rag. The bad sensor after a short time shows full lean. The bad sensor thinks there is far too much oxygen relative to fuel even though the situation is the opposite. You do not want this to happen in your car!

Turbo-Compounding

We have come to number four in our series. Turbo compounding is a method for recovering otherwise lost energy from the exhaust of a normal internal combustion engine (ICE). The design puts a turbine in the exhaust manifold which collects the kinetic energy (energy of motion) of the escaping exhaust gas. This turbine then transfers the power it generates to the crankshaft. The transfer is usually made by a hydrodynamic linkage, like in a transmission.

There are two basic types of turbines that operate by extracting energy from either the velocity (kinetic energy) of the working fluid or the pressure of the working fluid. In the case of pressure turbines there must be a large pressure drop across the rotor blades. This type is not used in turbo compound engines because the pressure drop restricts exhaust outflow, smothering the engine. Instead of pushing exhaust out against atmospheric pressure, the engine has to push it out against atmospheric pressure plus the turbine pressure drop. Using kinetic turbines avoids this problem.

Note that this is different from a turbocharger. In turbocharged engines there is a turbine powered by the flow of exhaust gases, but instead of adding this power to the driveshaft of the engine directly it is used to run a compressor which pressurizes the intake air. This results in a density boost, filling the cylinders with more air (and thus more oxygen) per charge. Since the ultimate limit on the energy you can get out of the combustion is set by the amount of oxygen present, turbochargers also increase power output. The mechanism is different though.

Turbo compounding allows for more power output given the same fuel input because it captures energy that would otherwise escape as exhaust gas velocity. However, the power per weight ratio is lower due to the turbine. The engine is also bulkier. But it is possible to greatly increase either the power available or the fuel economy or a mixture of both.



Although some World War II era aircraft before the development of turboprops used turbo compounding the technology has not been used by automakers. That is now changing. For example, the Daimler Trucks Detroit Diesel DD15 uses turbo compounding. The video above talks about the turbo compounding at about the 3:10 minute mark. Note there is also a turbocharger on this engine. Once again, turbo compounding and turbocharging are two different methods for recovering energy from the exhaust gas.

Perhaps someday soon car engines will also feature turbo compounding.

Variable Displacement Engines

Here is number three in our series of posts. The displacement of an engine refers to the total volume covered by the piston stroke inside the cylinders. Note that it does not include the heads. This is because the thermodynamic work done by the engine happens when the piston is forced down under the pressure of the hot combustion products.

Variable displacement technologies use mechanisms that can change this active piston swept volume according to the power demanded of the engine. When the engine needs less power the displacement is reduced and when the engine needs more power it is increased. It is more efficient to run a smaller engine at normal power output than to run a big engine at a bare idle. This is because the big engine has to be throttled way back and it suffers heavy frictional losses trying to suck in air. The energy lost while sucking air into the engine and pushing it back out on the intake and exhaust strokes is known as pumping loss.

The conventional way to reduce the displacement is to shut off some of the cylinders. For example, the 2008 Honda Accord V6 three, four or all six cylinders depending on the load. A management computer directs the switchover between different numbers of cylinders in use.

More advanced non-conventional techniques also exist. The Hefley engine controls the displacement by moving the average position of the pistons up and down the cylinder. To be able to do this required a complete redesign of the engine layout.

According to Wikipedia the first variable displacement engine was built over a hundred years ago (although it was a stationary engine). The first try at commercial use in cars was by Cadillac in the 1980s but failed due to mechanical breakdown being too common. Only as recently as 2004 was there mass commercial deployment of this technology. One cannot help but wonder if this fuel saving tech might have been developed and deployed a decade or two earlier if Detroit had made it a priority.

Miller Cycle

Today is the first of our seven fuel economy technologies that might have been deployed by Detroit on a mass scale but were not. The standard 4 stroke engines we have in our cars today use the Otto cycle. The Miller cycle was developed by Ralph Miller in the '40s and is also a 4 stroke cycle. It has been used commercially: the Mazda Millenia S had an engine using it. This Mazda engine was a 2.3 liter V6 that generated 210 horsepower and got 3.57 GPHM (Gallons Per Hundred Miles) highway driving.

The difference between a Miller cycle and an Otto cycle is in the compression stroke. The other 3 strokes are the same. In the Miller cycle, the intake valve is left open during the first 20% or so of the piston's rise up the cylinder. That means during the first part of the compression stroke, there is actually no compression. The fuel-air charge is forced back out of the intake valve instead of compressing. Then the intake valve closes and the remainder of the compression stroke does compress the charge. So that is the difference in the Miller cycle. The Miller cycle has unequal expansion and compression factors. The expansion phase uses the whole cylinder, while the compression phase uses only 80% or so of it.

If we build a Miller cycle engine so that it has the same size compression stroke as an Otto engine, it will be bigger. This is because the 80% of the cylinder that is used for compression in the Miller engine will have the same size as the whole 100% of the Otto cylinder. Looking at the diagram, the shaded area on the right shows the extra work that can be extracted from the Miller engine. The basic idea is that by lengthening the expansion stroke we give the engine extra time to extract useful work from the explosion that drove the piston down. By maintaining the same compression ratio, we do not have to worry about higher temperatures or pressures. But there is the problem of increased cylinder length. Miller cycle engines have the intrinsic disadvantage of lower power to mass ratios.

In practice, what is done is to build a Miller engine that is the same overall size as an Otto engine which means that for the same compression ratio the volume of cylinder that holds fuel air charge will be smaller. So for the same size, a Miller engine will be more efficient, but have less power, because there is less fuel-air mix to burn on each cycle. To compensate for this, it is common to add a supercharger to the Miller engine.

A supercharger precompresses the input fuel-air mixture, meaning it is denser. Although only 80% of the cylinder volume is useful for holding the fuel-air charge (because 20% got blown out) the denser charge means that the total amount of fuel is the same. Superchargers use some of the engine output power to run themselves, but even so a supercharged Miller engine can provide the same power output as a equal sized Otto engine while remaining about 15% more efficient.

So the question now is why has Detroit not invested in the Miller cycle? I think it is because of the costs and technical complications involved with the supercharging system. If Detroit wants to build Miller engines of the same size as their Otto engines, they need superchargers to get the same power output. The supercharger is the solution to the power to weight penalty Miller engines have. Until recently, the 15% gain on fuel economy was not worth the trouble and expense of including the superchargers. We will see if that begins to change in the future.

Brickley Engine - Less Friction, Better Mileage

Friction in the engine itself lowers your car's gas mileage. Instead of begin converted to useful work, some of the energy in the fuel is wasted in the form of heat or noise. Mainly heat. The Brickley engine design aims to rearrange the cylinders and crankshaft arms to reduce this friction. A Brickley engine is an internal combustion engine with specially connected pistons that move along paths to a very high tolerance. Because the piston stroke is defined to a couple thousands of an inch, the piston skirts can be eliminated or reduced. So far this engine exists only as a patent. I doubt there are working models. Not to say I doubt they will work, just that there is still no prototype. Apparently the Brickley design can eliminate 35% of the engine friction. This could give a 15% to 20% increase in vehicle mileage.

One other interesting bit of information was a list of components and their contribution to friction in a typical engine. Here is the breakdown of engine friction by part according to Mike Brickley, the engine designer:

Research attributes the following approximate amounts to the various components: crankshaft 18%, connecting rods 15%, accessories 10%, camshaft 15%, piston rings 21%, piston skirts 21%.

Engine Friction Breakdown

Component	Friction
Piston Rings	21%
Piston Skirts	21%
Crankshaft	18%
Camshaft	15%
Connecting Rods	15%
Accessories	10%

Piston Rings

The piston rings in your car's engine are what keeps the compression build up in the cylinders. Anything off kilter with them will lead to loss of performance and efficiency in your engine. Here is a little video explaining how they work, and what the difference is between gasoline and diesel engine piston rings.