How to Measure Electric Car Energy Consumption

As the world begins a move to electric vehicles, we are working out how to quote their fuel economy. Remember when GM announced their new electric Chevy Volt was going to get 230 MPG? What they did was consider a trip of a certain length (set by the EPA fuel economy driving cycle test) and only count the gas used. Because the first part of the trip could use electricity from the battery, the first miles were "free" from the point of view of the gas tank. Of course that is very misleading, because the battery is still providing energy.

It looks like the EPA is going to regulate how automakers must report electric car fuel efficiency. I see that we are going to need it ... the automakers have already shown they are not responsible enough to manage this on their own. They have let the marketing department take the driver's seat and publish information that is not useful to consumers.

It seems that the EPA is still thinking about the best way to present electric car fuel economy data. One suggestion is to report two numbers: one giving the gas (normal internal combustion engine) fuel economy, and the other the energy use of the electric engine. At the root, I think this shows us that we should move away from thinking about gallons and towards thinking about Joules, the physical unit for raw energy. The most useful number would be energy used per distance traveled. In modern international scientific units this probably would be expressed in Joules per meter. It could be given as millions of Joules per 100 miles or something else a little more familiar to the American public.

At the end of the day what is being spent to move the vehicle is energy. In most circumstances the distance we want to go is the fixed given information. The natural measure combining these two is energy per distance traveled. Simple and it works for all types of vehicles: electric, hydrogen fuel cell or regular gasoline.

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!

Human Powered Speed Record

A new Human Powered Vehicle speed record has been set. Sam Whittingham hit 82.4 miles per hour in a specially built racing bike in the Nevada desert. That is much faster than the 40 MPH or so records set by Olympic cyclists. The lesson here for saving on gas is to see how he reached such speeds. Of course Sam Whittingham is in great physical shape. He was a competitive cyclist himself. But how did he reach double the speed of the Olympic record?

The real secret is friction reduction. The bike has a very carefully streamlined shape and is small, so as to present less of a front to the wind and reduce weight. In fact, the bike is basically just big enough to hold his body. He can't move his head or arms when riding or maybe better wearing it. To reduce rolling resistance in the tires, they are very thin, less than an inch across. The bike also has very highly leveraged gears. The lowest gear has such a high velocity ratio that it is harder pedal than the highest gear on a racing bike. It takes 5 miles to get up to his top speed.

Although we are not going to be traveling around every day in body hugging, lightweight missile bikes we can see how friction reduction and gearing provide the key to massive increases in performance. Now what we need is for Detroit to seriously start applying the same lessons to their products.

Don't Speed For Nothing

A lot of people will speed and get very little advantage from it. But it costs them gas! The air resistance to your car's motion increases rapidly as speed increases. But the time you save does not. If you increase your speed from 30 to 40 MPH over a ten mile trip you would save 5 minutes. If you increase your speed from 55 to 60 MPH over a ten mile trip you save less than a minute, only 54 seconds. And if you increase your speed from 60 to 65 MPH over that ten miles, you save 46 seconds. That is if you can maintain a steady speed of 60 or 65. What often happens is that a driver tries to go at 65 but has to keep on slowing down and speeding up due to traffic. That acceleration / deceleration goes through gas like crazy.

So think twice about adding those extra 5 or 10 MPH. Over all but very long trips, you will save very little time and the time saved goes up very slowly with speed. On the other hand, you will be paying dearly in gas for that handful of seconds.

Do Pickup Trucks Get Better Gas Mileage With the Tailgate Down?

Some people drive their pickups with the tailgate down hoping their truck uses less gas per hundred miles. Does it work? First of all, the best solution is to get a truck bed cover. That will do the most to reduce air drag. But if getting a tonneau cover is not in the cards, should you put the tailgate down or up? According to a study done by the Society of Automotive Engineers, the answer is to keep it up.

The study found that lowering the tailgate generated more aerodynamic drag. It also reduces vehicle handling by increasing aerodynamic lift. This leads to less traction with the road. Besides the extra lift having the tailgate in the down position makes it easier for air gusts and crosswinds to turn the pickup to the side of the direction it is driving. These effects were found to be small, but they add up to burn more gas and reduce handling. So if you have to choose between tailgate down and tailgate up, leave it up.



You do have another choice though. It could be the case that for given conditions of speed, crosswind and so on the best position for the tailgate is somewhere between up and down. You can install a system which automatically positions the tailgate dynamically according to current conditions to minimize drag. Watch the video to learn about how it works!

Wheel Alignment is Not an Elective Service

Out of alignment wheels will decrease the gas mileage of your vehicle. They will also reduce the handling performance of the car, which makes it less safe to drive. And of course the lifetime of your tires will be shortened resulting in you spending more on replacement tires. When should you have the wheels aligned? Align your wheels after installing new tires or rotating the old ones. Have them aligned after replacing or having work done on the steering or suspension. Also go for an alignment after any collision repairs.

Keep your wheels in good alignment and you will save a percent or two on gas!

Lower the Rolling Resistance of Your Tires

The two external sources of resistance to your car's motion are air drag and tire friction. Using low rolling resistance tires can improve your gas mileage by up to 5%. In the past tires with lower rolling resistance usually wore out really quickly or had poor traction, especially on wet roads. Having to replace tires often will defeat any savings on gas with the increased tire cost. However, modern day technology is now able to make low rolling resistance tires without compromising wear or traction. One of the key innovations has been to reduce the weight of the tires and increase the stiffness only in the areas that flex the most.

Tire Rack did a series of tests on new generation tires. I was interested to notice that they used the ScanGauge II in the tests. Tests covered fuel economy, ride comfort, noise comfort, road handling, wet and dry braking and wet and dry cornering. The conclusion? Modern low rolling resistance tires will save on gas without giving up on handling, wear or comfort.

That Hill Kills (Your Fuel Economy)

Going up hills costs a lot more fuel than you may think. After all, your car is doing all the work so the driver doesn't notice. The chart shows the power used by a 1995 Ford Taurus engine as it moves the vehicle along at a steady cruise speed on the level, as it climbs a 6% grade at constant speed, and as it accelerates on the level. You can see that depending on speed, going up the grade costs from triple to double the power. Power translates to fuel. To give an idea, a power output of 20 kW (the units shown on the chart) costs you about 2.1 gallons per hundred miles (GPHM). Remember that the chart shows the power output of the engine. After taking into account powertrain losses (in the transmission, differential, etc) it will cost you more than 2.1 GPHM.

The lesson is to avoid hills like the plague. If you can take a slightly longer route that detours around a hill instead of climbing over it and coming down the far side, do it. Figuring out these route adjustments is easy with an instrument like the scangauge. If you have to climb a hill, see if you can rearrange your trip so you do it after the engine is warm. Cold engines are less efficient. The last thing you want is to take a hill with an efficiency penalty. If you have to carry cargo during a series of trips, think about if it is possible to drop it off before taking a hill. If you are planning to move to a new house, think about picking a location where you can commute to work on the level. If you work in a valley, don't live up on the ridge. Climbing it every day could inflate your gas bill!

Avoid Long Idles

When you idle your car, you are burning gas and going nowhere. It is a waste of fuel. I recently heard from a Venezuelan that cars and trucks idle there all the time. Apparently taxis wait in line at their stations idling. The engine is never switched off all day! Venezuela is an OPEC oil producing country and the price of gas there is equivalent to cents per gallon.

If you are waiting for somebody for "just a sec" and more than 20 seconds go by, shut off your car. Instead of idling in a drive through line, park and walk in. It costs more gas to idle than to restart the engine. It is true that a cold engine uses more gas than a warm one, but your engine will not cool down in less than 15 minutes. It is almost always better to shut the engine down. When in doubt, turn it off! Your wallet will thank you.

Just remember, idling costs you money to get nowhere!

Visualizing Airflow Around a Car

At higher speeds aerodynamic drag is the primary contribution to the resistance to motion suffered by your car. In the spirit of the old saying "know your enemy" here are a couple of videos showing experiments that visualize airflow. Both are based on the idea of putting a dye into the flow to make it visible.



In the first video, the fluid used is not air, but water. Models of a truck and a NASCAR race car are placed into a trough of water. A water current flows down the trough. By using a needle, dye is injected into the current. This lets you see the boundary layer around the vehicle and the wake pattern.



In the second video, a real vehicle (a 2009 Infiniti FX) is placed in a wind tunnel. So this time the fluid is really air. Green smoke is used to visualize the flow. The experiment clearly shows the boundary layer.

Knowing a little aerodynamics can be handy when you find yourself buying a car and want to get one that saves on gas!

Wind Powered Cars and Sail Trucks

Imagine running your car off of wind power. Like the sailing ships of yore that circled the Earth without using a drop of fuel, you would be able to go forever and spend nothing. Perfect freedom! Except of course that you are then forced to wait for the wind. While a sailing car may not be practical for everyday use it is an interesting engineering challenge to build one.

The Greenbird is an example of a wind powered (sail powered) racing vehicle. Looking at the picture, you can see that the "sail" resembles an airplane wing sticking up vertically. The rest of the craft is basically a needle with a blister at the back just large enough to hold the body of the pilot. There are outriggers on each side, necessary to keep the sail from simply toppling over sideways. Looking at the picture of the man seated on the outrigger gives a good idea of the scale. Like these record setting gas sippers the Greenbird is too small to carry cargo or passengers. Its small size is necessary to reduce the aerodynamic profile and increase the top speed. After all, the goal is to set a speed record, not bring home the groceries. It is made of carbon composites so it can be light yet strong.



It was designed to set a new world land sail powered speed record, which it successfully did. It reached a top speed of 126.2 miles per hour. The record was set on March 26th 2009, a high wind day. The top speed of the Greenbird can be 3 to 5 times faster than the wind speed. This depends on resistance coming from the ground. Smooth, hard surfaces are better than rough or loose packed ones. To understand how it can reach top speeds greater than the wind speeds, you have to take into account the apparent wind.

Although a wind powered car would not be practical for local trips around the neighborhood or city driving, we can imagine a future where they have a role. In the current world, gas mileage is not the most important factor. As a result we use the same vehicle for everything. In the future when gas mileage is more important, maybe even the most important factor, I think we will begin to use different types of vehicles for different purposes, choosing the most fuel efficient for each. For example, local trips could be taken in a personal Neighborhood Electrical Vehicle or NEV. Long haul trucking could be done by giant sail powered trucks, something like the sailing cargo vessels of old. Special highways or lanes could be set aside for them, and they could glide along at constant speeds set by the wind. Most long haul freight can be done without strict time limits, so the sail trucks could simply use whatever wind there was. Sail truck highways could be built along the windiest routes.


The picture to the side is an imaginary representation of the idea of a sail truck. A real sail truck would likely have the same general characteristics. A long, light needle shaped body with one or two airplane wing like sails on the back. The most important feature missing is that a real sail truck would need some kind of outriggers to keep it from being blown over by a crosswind. Another option would be to have a low, wide body which fills in the area covered by the outriggers with cargo space. That would have greater air drag and thus a lower top speed and lower threshold wind necessary to move at all but correspondingly greater cargo capacity.

Another option is to build long outrigger free needle like bodies and secure them to the highway with a special kind of runner. This would replace the keel of a sailboat with an attachment to the ground. It would of course require specially built highways. The highway could have a deep groove in the center of each lane and the sail trucks could have a "keel" which descended into the groove. The keel would have something like wheels which ran on low friction rails set into the side walls of the groove. This would require a bigger investment in highway infrastructure but would permit more aerodynamic sail trucks that could thus reach faster top speeds and operate at lower threshold wind speeds.

I think this kind of innovation will be necessary to keep us moving cheaply in a future where oil and gas prices will be very high and rising. Better to start early, because the sooner we start the sooner we save on gas!

Peak Oil Means the End of Production Growth

Back in the 150th anniversary of oil drilling and peak whiskey posts we talked about the problem of Peak Oil. The problem is that we are going to run out of growth in supply not that we will run out of oil. The graph above shows that ever since about 2005 the total oil output of the world has stayed more or less constant. It is a flatline with some fluctuations above and below. This flatline happened even though the price skyrocketed to almost $150. There are a lot of reasons to think this limit to production is geological and cannot be removed.

There is going to be absolutely no flatline in demand. The global population is still growing. The population of the United States is growing. The billions of people living in Brazil, India and China are experiencing rising standards of living and will want the vehicles that come with it. What happens when the supply stops growing and the demand grows and grows? Prices rise. A lot. Looking at the graph of global oil production lets us understand why.

The solution is that we have to find ways to save on gas. The focus for the future must be on reducing gallons used per hundred miles driven. If that means we have to settle for smaller less powerful vehicles, so be it. Those are all we will be able to afford anyway. Detroit should make gas mileage a top priority to stay alive and relevant in the decades ahead.

Flex Fuel E85 Conversion Kit

Flex fuel vehicles can run on gasoline, ethanol or any mixture of the two. Using ethanol has its ups and downs which can be debated. But if you do want to try out a flex fuel vehicle and buying one is off the table, there are kits which you can install yourself to convert your existing vehicle. The White Lightning kit in the video and all the others I have heard of work by taking control over the electronics that run the fuel injection. Then the kit's computer adjusts the fuel air ratio according to the amount of ethanol present. They are easy to install. Go ahead and try it out if you have a mind to!

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.

Drive a NEV

Ok you need the gas-guzzling SUV to take the kids and the dogs and all your equipment to go camping. But do you need such speed and range to do errands around town?

You might want to consider buying an Neighborhood Electric Vehicle (NEV) as a second car for your family. Basically it's an electric golf cart that meets the basic requirements to be an on-road vehicle. They are much cheaper than cars, both in up-front costs and maintenance so you probably can afford one as a second vehicle even if a Prius is out of your price limit.

Electric golf carts use a bank of standard lead-acid batteries. They're designed to be used all day then recharged all night. That means that at the end of the day you plug them into a standard power outlet. If you won´t have such a steady use pattern you can look for a NEV that supports "opportunity charging," which allows you to simply plug in whenever you're not using it.

They may seem simple, but NEVs pack a lot of technology into a very small package. Stripped-down personnel carriers can be found for as little as $3,000, while fully configured burden carriers can reach $20,000. Pricing for new golf cars starts at around $4,000 for a bare-bones model. More typical are prices of $6,000 to $9,000. High-end models with all the extras can cost upwards of $10,000. These luxury models include upgraded upholstery, fancy wheels, lights, and details like radios and cell phone chargers.

The more options you add, the more you’ll pay. Some two-seater golf cars can be outfitted with a rear-facing bench seat that lets them carry four people. Some models are open-topped, some have a roof, and some have a roof and a windshield. Expect to pay $200 to $400 for a top and that much again for a windshield. A rain enclosure – usually roll-down plastic sides – can be a nice extra in wet climates.
Don’t feel compelled to purchase every accessory at the outset: in most cases, you can buy a basic model at first, then make the upgrades as you see fit.

An economical choice is buying a used golf cart. Many dealers sell refurbished carts with 30 day warranties. Used utility vehicles can be found for as little as $2,000. You’ll rarely find anything for much less than that – at least, not anything you’d want to depend on for transportation.

Modern NEVs have greatly increased range and power over models available five or ten years ago. They have features like regenerative braking, which helps recharge the batteries as the car slows or goes downhill. Imagine a gas burning car where the tank fills up every time you stop at a red light.

A distinct advantage of electric golf carts is their cost to operate: it can be five or 10 times cheaper than a gas model. Proof that the technology is good can be seen in golf courses with large fleets: gas-powered utility vehicles are no longer as popular as electric golf carts.

Maintenance is an important consideration for any vehicle. Electric vehicles need to have the water level in their batteries checked regularly, which is a simple but critical task. More significantly, the batteries in electric vehicles need to be replaced every couple of years at a cost of $400 to $500. The overall maintenance costs work out to be fairly similar to gasoline engines - combustion engines spread the cost out more, but require more frequent servicing.

Having basic maintenance done regularly – on the schedule recommended by the manufacturer – can drastically extend your vehicle’s lifespan and improve its performance. Something as simple as making sure there’s enough air in the tires can help safeguard your investment. With proper maintenance, you can expect about 10 years out of an electric motor – but manufacturers report vehicles lasting 15 or even 30 years in some cases.

One thing you need to be careful about: Laws governing the use of NEVs are different to those applying to golf carts. You should check with your local authority for information that is relevant to where you intend to be driving your golf cart/NEV. There are differences regarding use of windshields, headlights, brakes etc. There are also differences regarding where exactly you are allowed drive them.

If you think you might want to upgrade your golf cart to a NEV, contact a licensed dealer, as it makes the registration process simpler for you. A dealer can do the paperwork for you, but only if she or he is licensed.

What Happened to Carburetors?

Remember when every vehicle had a carburetor? Now very few have them. What happened and where have all the carburetors gone? Nowadays engines are all fuel injected and the engine management computer is responsible for deciding how much fuel each cylinder gets. And that is a good thing.

Fuel injection has a long history. Aircraft in WWII were already using it for example. However, until the late 1980s or so the simplicity of carburetors meant that they were preferred. But as emissions requirements came on the scene and to get better power and fuel economy the carburetors started getting more and more complex. There arrived a point where the fuel injection systems were not much more complex than the carbs. When this point arrived, the switchover en masse began.

Fuel injection systems are generally more efficient that carb based ones. They can provide exactly the right air fuel ratio to each cylinder. The holy grail of combustion is to get exactly matching numbers of oxygen and fuel molecules into each cylinder, and fuel injection is the way to reach it.

Amphicar: Think of the Shortcuts!

If you can shorten the route you take by finding a detour or shortcut it can save on gas. Imagine cutting across a lake with the Amphicar! Of course as a boat it will get poor mileage due to the enormous drag involved in pushing through the water. Being an old car it probably also gets poor gas mileage on land. But the idea of shortcutting across the narrow waist of a lake instead of commuting all the way around it and saving gas as a result is fun to think about.

Monitor Your Gas Mileage

Driving habits are among the biggest contributors to saving on gas. But it is difficult to improve your driving without knowing how you are doing. Instantaneous feedback lets you know if a particular tip or trick actually works for example. The scangauge II is a little computer than plugs into the standardized diagnostics port under your dash. It reports the instantaneous and average gas mileage. Unfortunately it reports in MPG, not in GPHM which would be better. The video has an interview with an executive from the company that makes the ScanGauge II. Like he says, with this device saving gas while you drive becomes like a video game.

Lightweight Electric Fans

Does your vehicle have an engine-driven fan? Many trucks, vans and SUVs have a heavy metal cooling fan that is run off the engine crankshaft by a belt. These fans are heavy and have a lot of inertia. It takes a lot of rotational kinetic energy to get them going which is just lost when they slow down. That translates to more gallons spent per hundred miles of driving. Think about replacing it with a much lighter, plastic electric driven fan. The weight savings will give a fuel savings. And many modern electric fans are designed to be more efficient than engine driven ones because they have a computer which can control their speed independently of the engine speed. You will get a fuel economy boost and a litle extra horsepower too!

Resistance to Motion

Your car has to fight against friction to maintain its motion. There are two categories of friction: internal friction in the engine, the transmission and every other component where moving parts are found and external friction. Here is a breakdown of engine friction by subsystem. The external friction comes from rolling resistance in the tires and air drag.

The resistance of the tires depends primarily on the contact area with the road. Minimizing this area minimizes friction. In the picture you can see than underinflated tires have flat, square contact areas. The outer edges of the tires touch the road. Correctly inflated tires will not touch the road with the outer edges. Overinflated tires will have even less contact area and less friction but they will give you a bad, bumpy ride. It is even possible to damage the undercarriage of your vehicle with overinflated tires because they will not be helping to soak up shock and vibration.

The air resistance depends on the shape of your vehicle and the speed you are going. At low speeds the air resistance is low and increases slowly. At higher speeds the air resistance is large and increases rapidly. Low speed driving is dominated by tire resistance and high speed driving by air resistance. The tire resistance is constant, basically independent of speed. The chart above compares the tire and air resistance and shows their total.

Knowing the relative contributions of tires and aerodynamics helps understand what is happening. For example, there is a debate about whether running the air conditioner or opening the windows (and thus increasing air drag) is better for gas mileage. Since the open windows add to air resistance which is small at low speeds, we can tell that for low speeds windows down and air conditioner off saves on gas.

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!

Road Rage: Just Say No!

Smooth driving that minimizes start and stop acceleration and braking and slower driving that minimizes air resistance are the keys to save on gas. Driving aggressively means you will wind up speeding up to the light and then slamming on the brakes, speeding and accelerating and braking constantly while weaving through traffic. It will reduce your gas mileage a lot, especially on the highway where you normally have a good chance of being able to maintain a steady, moderate speed. Aggressive driving is also less safe.

If you often find yourself pressured and stressed while driving think about how to calm down. Try leaving earlier to reduce "deadline pressure". Put soft relaxing music on. Avoid music that pumps you up. If being hungry makes you edgy, then always eat something before a long commute or trip. Have your morning coffee before getting behind the wheel. Don't let other drivers get to you. The competition is not against them but rather against the gas pump. Remember who the enemy really is. Slow down, calm down and you can be safer while saving on gas!

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.

Going By Bike Gives Best Gas Mileage

You get the best gas mileage when you don't drive! By changing your habits and reducing the miles you drive you automatically save on gas. For many of us one of the biggest sources of miles is the commute to and from work. If you can make the commute by bicycle you will save on gas mileage and improve your fitness at the same time. Depending on the distance, the type of road in your area, traffic density and factors such as bike storage or showers at work bicycle commuting can be more or less attractive. The video gives some tips and hints on how to deal with issues such as storage of the bicycle, how to carry cargo, how to dress for the weather and where to park.

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!

Could Nitromethane Save on Gas?

You have probably heard about how racers will mix nitromethane or nitroethane with their fuel for a power boost. Could mixing it with your street car fuel give a gas mileage boost? The answer is no, but let us see why.

Your car's engine is fundamentally an air breather, just like people. It gets energy from the chemical reaction of fuel with oxygen (from the air) and both of those ingredients have to be present. The proportions matter. Thinking simplistically, imagine that one part oxygen reacts with one part fuel. The end products of the reaction are carbon dioxide (the greenhouse gas) and water. Gasoline is mainly made of short chain alkanes, such as octane. The complete combustion of octane with oxygen is described by the chemical equation

2 C₈H₁₈ + 25 O₂ --> 16 CO₂ + 18 H₂O

We can see that we need 25 oxygen molecules from the air to burn two octane molecules. This oxygen is what makes your engine an air breather. Without air there would be no oxygen and the engine cannot run. Air is also the limiting factor. When the intake stroke fills the cylinder with fuel air mixture there will be a certain amount of octane and oxygen. Reacting all of it gives the maximum possible energy output of that explosion. If the balance is not perfect, the component in excess goes to waste. A rich mixture has too much fuel and a lean mixture has too little fuel for the amount of oxygen.

Air is very thin, much less dense than fuel. That means the oxygen is the limiting factor. It is very easy to produce a cylinder full of fuel, but without oxygen it can't be burned. This is where turbochargers and superchargers come in. They pressurize the air to increase the density and pack more of it into the cylinder. This allows more fuel to be burned per cycle. Looking at the picture of a nitromethane molecule above, you can see two red atoms. Those are oxygen. Nitromethane has the chemical formula

CH₃NO₂

Those two oxygen atoms mean that the nitromethane is bringing oxygen into the cylinder. And because nitromethane is a liquid and not a gas it has a density a thousand times greater than air. Nitromethane allows you to greatly increase the amount of oxygen in the cylinder, much more than a turbocharger or supercharger could. And that in turn means you can now add more fuel and thus get a bigger energy output on the power stroke of the cycle.

So nitromethane does not increase the efficiency at which an engine burns fuel. It allows the engine to burn more fuel per cycle which increases the power output. Running at 2000 RPM a six cylinder engine will experience 12000 combustion events in its cylinders. By combusting more fuel each of those 12000 times we have a larger power output. But we also burned more fuel, so no increase in efficiency.

Nitromethane lets you burn more fuel in less time for better power but does not get you more mileage. Good for racers, but not for saving on gas.

Think You Have a Gas Guzzler? NASA Probably Doesn't Agree!

NASA uses two enormous tracked vehicles called Crawler-Transporters to carry the space shuttles out to their launch pads from the hangers where they are prepared. The Crawler-Transporters use diesel fuel. Their mileage? They use 150 gallons of diesel per mile. That is per mile, not per hundred miles. Expressing that in gallons per hundred miles or GPHM we have 15,000 GPHM. Next to that outrageous consumption the puny 10 GPHM a guzzling SUV uses is a mere nothing. Of course an SUV would be crushed flat if you sat the space shuttle on top of it!

Carpool When You Can

Carpooling can be a simple change in your habits that might lead to a big savings on gas. A vehicle with four people is much lighter than four vehicles with four people. This alone means a large gain in fuel economy. If you have a regular commute look around to see if any neighbours are heading in the same direction and split the gas bill. Also if you find yourself driving all over the map to visit friends, meet clients or take care of other chores consider offering rides to friends and associates. Then split the gas bill. Both of you will save something on gas. Carpooling can easily be a win win situation!

Do You Need a Bigger Car to be Safe?

One reason for choosing a big vehicle in spite of the gas savings hit is safety. Most people feel that a big vehicle will be overall safer to drive. Is that true? The Monash University Accident Research Centre tried to find out and the result was this report.

They looked at two measures of vehicle "bigness": the mass and a measure of the physical size such as volume or wheelbase length. The Monash study was one of those "study of studies" where they reviewed many other studies in the literature looking for common factors and data. The aim was to find how size and mass are related to occupant safety. There were problems getting definitive results due to the variability of the data. For example, some studies used interior cabin volume to measure size and others used wheelbase. But overall?

Overall they found that in multi-vehicle crashes (things like head on collisions, rear ending, sideswipes, anything with a least two cars involved) the bigger the vehicle mass, the safer the occupant. On the other hand, in single vehicle crashes (rollovers, hitting a tree, crash into a wall) the bigger the vehicle size the safer the occupant.

In multi-vehicle crashes it makes sense that bigger mass will help more. Physics says that the bigger car will always be least effected in a 2 way crash. It also makes sense that mass doesn't help much in single vehicle crashes. If you hit a wall or embankment, it doesn't matter how much mass you have in your car: the obstacle will always outgun you. On the other hand, if there is a lot of interior space, there is room for the vehicle to crumple, compress and slow down gradually without the driver's body being impacted by something.

How can we relate this to saving on gas? Well, careful design of interior cabin space can give us vehicle inner volumes that are as close as possible to the outer volume (no wasted space). Careful aerodynamics will let us make larger volume, light mass vehicles with little air drag. So in that respect, we can have safety and gas savings at the same time. But we need Detroit to play along and start designing vehicles accordingly.

With regard to mass, it is the direct enemy of fuel economy and they can't really be made to live together. But if all the vehicles on the road are lighter, being made of composites and modern light alloys, then what counts as a relatively "massive" vehicle in a two-way collision will also be lighter. We might see that over time, the "heavyweights" of the road become quite light compared to our heavyweights of today. That would open up the choice of driving a "heavy" car for safety and still saving on gas.