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.