Hydrogen Fuel Cells

A hydrogen fuel cell is able to combine hydrogen with oxygen from the air, producing electricity in the process. This electricity can be used to run an electric motor. Fuel cell technology allows to run vehicles off of hydrogen as a fuel instead of gasoline. As an important benefit, they produce water as the only byproduct of combustion between the oxygen and the hydrogen, which means no air pollution and no global warming boosting carbon dioxide.

Spherical Engine

I doubt it would save on gas, and it is probably a manufacturing nightmare to mass produce, but it is interesting to see a spherical engine design!

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.

Engine Emissions

Operating engines consumes fuel, delivers useful work and produces emissions. Left uncontrolled, those emissions can produce blankets of smog that nobody wants. Using less gas will also produce less pollution. Pollution levels are also controlled by use of emission reduction technologies and fuels. What is in the emissions coming out of your tailpipe? You will find that almost all of them are one of the following five things.

1. 
   Hydrocarbons C_nH_m: these are the fuel itself. If the fuel is not completely burned up in the combustion process, whatever is left over must come out the tailpipe. Engines running rich (with excess fuel relative to the amount of air in the cylinder) will emit hydrocarbons. It is also possible to find hydrocarbons in the exhaust if the engine is running very lean (excess air relative to fuel) because sometimes under lean conditions the combustion does not get going at all or does not complete, leaving fuel in the exhaust.
   
   These hydrocarbons are the dominant component in ground hugging smog. Mixed with nitrogen oxides
   in the sunlight they combine to form ozone. Low level ozone is a health hazard.
   


2. 
   Carbon Dioxide CO₂: completely burned fuel. Thermodynamics shows us that the most stable combination of Carbon and Oxygen is as carbon dioxide. Thus whenever possible combustion continues to this stage.
   
   Carbon dioxide has no adverse health effects but famously contributes to global climate change. One of the big villains on an international scale.
   


3. 
   Carbon Monoxide CO: incompletely burned fuel. Combustion favors the attachment of two Oxygen atoms to each Carbon atom. In rich conditions there might not be enough Oxygen to go around. Thus CO will tend to be produced in rich burn and very little or none will be produced in lean conditions.
   
   Carbon Monoxide is poisonous. Not a good thing.
   


4. 
   Oxygen O₂ : from incompletely burned air. Just as running rich can leave some fuel unburned in the combustion process, running lean can leave unburned Oxygen.
   
   This oxygen was in the air anyway before it got ingested into your engine. So this one is no problem.
   


5. 
   Nitrogen Oxides NO_x : from the combustion of the Nitrogen in the air. Air is a mixture of gases containing about 70% N₂. At high temperatures some of this reacts with the Oxygen in the cylinders producing NO_x.
   
   NO_x is a health hazard. They cause lung damage in low concentrations and can be outright fatal at high concentrations. They also corrode metals, eat away at fabrics and kill plants.
   

Just think: when you reduce gas consumption, you also reduce production of all of these emissions. Yet more reasons to save on gas! Better gas mileage helps a lot more than just your wallet.

One Person Commuter Cars

Here is another vision of a gas sipping future. According to Rick Woodbury, inventor and developer of the Commuter Car 88% of all cars doing the daily haul in to work carry one person. That means that most of the road space in the morning and evening commutes is being taken up by empty seats. His solution is a small, one person electric vehicle that is not much larger than a big motorcycle. It has an energy usage equivalent to using 1.0 gallons per hundred miles (GPHM) which is good. It also has a price tag of $120,000 which is bad.

Although a one person vehicle selling at more than a hundred thousand is never going to become the commuting choice of the masses, the concept gives us a glimpse of what a fuel economy conscious future could look like. I believe that to get high fuel economy we will turn to a range of cheap vehicles each designed to perform one and only one function very efficiently. It is possible that we will see something like these single occupant electric cars widely used to get to and from work in the not so distant future.

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.

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.

Variable Valve Timing

This is the second in our series of seven fuel economy technologies Detroit could have pursued but did not. Although almost all of the world's automakers have made at least one engine with variable valve timing within the last 10 years, before that time they were rare. Even today the majority of engines have fixed timings.

The video illustrates the idea. Basically, the fuel-air intake valves and the exhaust valves open a certain distance and stay open for a certain time. Also the location in the piston cycle at which they are open is important. At a given RPM, the engine has to open the valves different amounts at different positions for different times to get maximum efficiency. If the valves have fixed timings, the engine will only be at its top efficiency at one narrow RPM band. However, if the valves can modify their timing, the engine can reach high efficiency over a wider band of RPM.

In normal operation, there is a moment near the end of the exhaust stroke when the exhaust valve and the intake valve are both open. Also the exhaust valve stays open a little while into the intake stroke. This time when both valves are open is known as the overlap, and is one of the most important variables to control. At low RPM, the overlap should be low. This is because at low RPM the airflow is fast relative to the engine speed. At high RPM, the engine is moving so fast relative to the air that it is better to open the intake valve early so that the air has a better chance to enter.

The first vehicles with variable valve timing technology were not introduced to the US market by Detroit. Instead Alfa Romeo, Nissan and Honda blazed the trail. One more chance missed by Detroit.

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.

What Does Engine Oil Do?

The picture shows that sometimes we need friction so we don't go sliding out of control. One place where we do not want friction is in our engines. Engine oil is there to lubricate the metal surfaces and get rid of friction. In a properly working engine, a thin layer of oil separates all metal parts so that there is no metal-metal contact. Instead we find metal-oil-metal contacts. But reduce friction is not all that oil does.

Oil also helps to cool the engine. There are places where the water cooling system just can't reach, like down in the crankcase. Oil gets in these areas and removes the heat. Another role of oil is to help the piston ring seal the combustion chamber or head off from the crankcase. Oil also scavenges tiny metal particles which are worn off when engine surfaces work against each other. These particles are then removed from the oil by the oil filter. Acids can be formed by chemical processes occurring in the combustion of fuel. All gasoline has at least a small amount of sulfur in it. This sulfur can react with water (brought in with the air) to produce sulfuric acid. The sulfuric acid is dissolved in the oil, which has acid neutralizers in it.

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%

Velozeta Six-Stroke Engine

Six stroke engines add another two piston motions to each fuel injection cycle to those of the common four stroke engines. Everyday cars and trucks use four stroke engines. These are called four stroke because for each time that a new shot of fuel is burnt, the pistons sweep up twice and down twice, for a total of 4 sweeps or strokes. The four stroke sequence goes like this:

Intake Stroke

This stroke begins with the piston at the top of the cylinder. The piston moves down, opening up space in the cylinder. As it does so, the valves in the head above open and fuel-air mixture gets sucked into the cylinder.

Compression Stroke

When the piston reaches the bottom of the cylinder, the valves close off. As the piston rises up the cylinder, it compresses the fuel air mixture, which now has nowhere to go. This compression primes the mixture for detonation. The stroke ends with the piston at the top of the cylinder and the mixture compressed into the small space in the head above.

Power Stroke

Now the fuel air mixture is detonated by the spark plug. The resulting explosion forces the piston down the cylinder and rotates the crankshaft against any attached load.

Exhaust Stroke

Exhaust valves in the heads above open up, and the piston rises up the cylinder, forcing out the exhaust gases, which are the ashes resulting from the combustion of the fuel air mixture.

Students from the College of Engineering at Trivandrum in India have developed an engine which adds two more strokes after the exhaust stroke. It uses air to scavenge heat from the cylinder and convert it to motive power. This can improve efficiency, because normally that heat is just wasted. The engine is a modified Honda four-stroke engine. After the exhaust stroke, valves open and allow cool air to flow in as the piston descends. The air gains heat from the very hot cylinder which causes it to expand. This heat-driven expansion occurs forcefully enough to actually power the piston down. In other words, there is a secondary, weaker power stroke after the exhaust stroke. On the sixth stroke (which is a secondary exhaust stroke), the rising piston forces the now warmer air out the exhaust.

This engine uses 40% less fuel and can run on normal gasoline.