Hypermiling Bumper Sticker

***Disclaimer: Real hypermiling is pretty dangerous and some aspects of it are illegal, so don’t break the law. Also, I do not really care about the status of the environment; I only hypermile for fun.

These days it seems that fuel economy is the selling point of every new car. Most people just look at these overused numbers (like, 32 mpg Highway!) and don’t really understand what it means. If they are comparing two similar vehicles, though, then value might actually come down to the best fuel-economy (most of the time, vehicle comparisons are not fair). But chances are that person will not actually achieve the EPA-rated numbers for numerous reasons. To hypermile is to drive in a way that minimizes fuel consumption, and there are many ways to try it. A lot of American drivers like to accelerate quickly for no apparent reason. If you are driving in the city, you should drag out the 0-30 acceleration as long as you can (but it’ll probably depends on the general attitude of other drivers in your city).

During my week of total dedication to hypermiling, I don’t think I pressed the gas pedal past about 10%, which is tough on the mind if you live for turbocharged acceleration as I do. If you see a light going yellow in your path, instantly let off the gas. If you know the light pattern well, shut off your engine if you’ll be idling at a red light for a little while. (I’ve found this helps a lot at some rural lights that seem to never change.) Also try not to slow down as much into turns, but keep it safe. Carrying speed, i.e. momentum, is the best way to achieve better mileage. Run your engine at lower RPMs, if possible, just not if you are breaking in a new car. I don’t think I used anything past 2500 and I shifted to the top gear as soon as I could. My friend was telling me that he hardly sees anything more than 25 mpg out of his 2.5 Subaru engine, which is unusual for someone that cares about mileage, but when I rode around with him I discovered that he ran the car at about 3500 RPM on normal roads when he could be shifting. Since the engine spins only about 72% as much at 2500 than at 3500, this can be a significant problem. I also like to coast in neutral up to stop lights and skip shifts (3^rd to 6^th) to keep engine speeds down.

Hypermiling is a fun challenge and I find myself trying to beat my record each time I make the 100 mile (mostly 45-55 mph roads) commute back home throughout the summer. With a record of 38.8 mpg out of my 2.0T (and without angry drivers or Amish buggies stuck behind me), I’m somewhat above the rated value of 21 city/31 highway for my model. Almost any car can perform well above its rating if driven correctly. The regular checks such as keeping your tires properly inflated and removing roof racks for extended trips will also help out.

Good Ways to Get Started

If you’re looking to buy a new car, try something small, a family of four should have no problem fitting in a mid-size sedan orwagon. Try to find engines with small displacements that put out as much power as larger engines. Used diesels are probably the best you can get these days from a miles per dollar standpoint. Older VW diesel models are known to get 50+ mpg without any hypermiling techniques, and when coupled with some basic strategies, they can deliver better numbers than almost any hybrid for a fraction of the cost.

Car Skidding

You drivers out there have probably heard of the ABS in your car (at least, if you are driving a car, you had better know about it!). ABS stands for “Anti-lock Braking System,” which should never be written as the “ABS system.” This is not a post about how ABS works, but rather one about the underlying concept behind it.

Generally what ABS does is to prevent wheels from locking up. To understand this, consider what would conventionally occur (i.e. in a vehicle without ABS). If someone were to slam on the brakes in a moving car without ABS, the braking discs or drums would cause the wheel to stop turning. Typically, that is the whole purpose of brakes in the first place, but usually one would gradually brake the car to stop by applying more and more brake pressure until the vehicle stops. If the brakes lock up the wheels suddenly (which is unhealthy for the brakes anyway), wheels lock up. Remember that the car is still traveling at considerable speed. Simply preventing the wheels from turning does not stop a car. The momentum of the car keeps the vehicle moving forward, so we get tire skidding, and we lose traction. That’s a bad thing; we would be sliding and have little to no control of steering.

Fundamentally, ABS adds some electronics (like computers and sensors) in between the brake pedal and brakes and modulates the braking pressure so that tires don’t skid and traction is maintained. We’re going to focus on why it’s so important not to skid by taking a look a friction. If you want to learn about how ABS actually works, you can visit HowStuffWorks’s “How Anti-Lock Brakes Work“ article or About.com’s “How Does ABS Work” article.

There are two basic types of kinematic friction: static and kinetic, but both are a function of the normal force. (Do you remember what the normal force is? “Friction at the Tip of the Finger” talked a little about it a few weeks ago.) The friction force can be loosely described by this relation:

Friction Force

Coefficient of Friction

If you recall from “Friction at the Tip of the Finger“:

The coefficient of friction is a dimensionless constant that takes into account the two surfaces. The higher this coefficient, the greater the friction force. Using everyday experience, for example, the coefficient of friction between leather shoes on ice is much smaller than between sneakers on the concrete sidewalk. (There are tables we can use to looks these coefficients if we really wanted to. And it’s important to note that these coefficients are generally higher at higher temperatures for the same pair of surfaces.)

Note: the coefficient of friction is basically depends on the two types of materials in use, and static coefficient of friction is usually greater than the kinetic coefficient of friction. We’ll see this in a bit.

Kinetic Friction

Sliding Dictionary Example

Let’s take a look at these two cases. Kinetic friction (or sliding friction) is simple. It only comes into play when something is physically rubbing against something else. It is the resistive force that goes in the opposite direction of motion. So, imagine you are successfully pushing a heavy dictionary across your desk. You can clearly feel the resistance, which you’d usually attribute to its relatively heavy weight. In fact, what you are feeling is the kinetic friction that resists the motion (remember, it has to be moving for the concept of kinetic friction to apply). This friction force is a function of the normal force, which, in turn, varies directly with the object’s weight. So, it is generally correct to assume that heavier objects are harder to push (assuming the same coefficient of friction, of course). Does that make sense?

Kinetic Friction Force

Static Friction

Static friction is pretty awesome, but it takes some explaining. Firstly, this friction is present whenever there is no motion (or static). The magnitude (or value) of this force, however, depends on what is acting on it. This can go from 0 to a maximum value that is dictated by the friction formula above. So, for the case of static friction, we have:

Static Friction Force

Let’s go back to our dictionary example. Once the pushing force exceeds the maximum static friction force, the book begins to slide, and we move into the kinetic friction regime. But before that point, it takes some increasing amount of force to get the dictionary to slide. The magnitude of the static friction force is only to the extent that it is needed. Consider the following series of diagrams. (Remember that the Newton is the metric unit for force.) Let’s assume for sake of example, that the maximum static friction force is 10 N. Note that the kinetic coefficient of friction is less than the static one, which we see because the kinetic friction force is less than the maximum static friction force.

Now, how does it apply to ABS and car tires? Keep in mind that as a car tire rolls, there is a patch of rubber that is actually stationary and static friction is in effect. We want to maintain this static friction for as long as possible (for grip, traction, and control). If the rubber starts to slide across the asphalt (when the wheel locks up), we lose the benefit of the static friction force and the stopping force exerted by the tires on the car. That’ll be a bad thing, especially since we usually slam on the brakes when we really, really need to stop (to avoid hitting the ducks crossing the road, let’s say). In order to stop in the shortest time possible, we prefer the greater static force over the kinetic force. In this case, our first instinct of abruptly pressing on the brakes fails us. Thank goodness for ABS. (Or, we can all just pay more attention to the road!)

Car image from: http://www.aa1car.com/library/stability_control.htm

Source: http://en.wikipedia.org/wiki/Friction#Static_friction

The new standards, covering model years 2012-2016, and ultimately requiring an average fuel economy standard of 35.5 mpg in 2016, are projected to save 1.8 billion barrels of oil over the life of the program with a fuel economy gain averaging more than 5 percent per year and a reduction of approximately 900 million metric tons in greenhouse gas emissions.

Smart Car in America

A lot of naysayers and critics say positive results are unlikely and the new policy is detrimental to passenger safety, among other things. The argument goes as follows: to comply with these restrictions, automakers may turn to reducing the amount of protective metal in their vehicles. A lighter car, after all, would require less energy to run, thereby increasing gas mileage. Smaller cars, too. Conservatives would very much like to choose the type of car they would like to drive, even if it is an inappropriate gas-guzzling SUV. (I’m all for freedom of choice, but when those choices are clearly selfish and wrong, something needs to be done, perferably not through government regulation, but instead through personal responsibility. But, oh well.)

In an article from Salon, titled “Auto safety for dummies,”

For years, physicists, road safety experts and auto designers have shown that big vehicles are not necessarily safer than smaller ones. Of course, nobody would debate that a head-on collision between a Chevy Tahoe and a Smart Car is going to favor the Tahoe. But that hypothetical crackup does not represent the ecology of the roads, where the truth of auto safety resides.

The article goes on to say that there are more real-world factors like road conditions and driver behavior that affect the safety of a vehicle. The informal conclusion? “The correlation to safety is with the cost of the vehicle, much more than size or weight.”

In the end, larger vehicles may better protect passengers but they are also more of a threat to everyone else on the road, whether drivers in other cars, cyclists, pedestrians, or even animals. Let’s not be selfish here.

But don’t worry, car makers will be turning to hybrid technology, innovative fuels, supercharging and turbocharging to improve gas mileage rather than shave metal from the body. Plus, improvements in materials can let us hope for stronger and lighter composites for car parts.

Read the article. It is a good one.

Smart Car image from Smart USA.  (This is not an endorsement.)

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Turbocharger Illustration

When categorizing car engines there are two groups that describe how the air/fuel mixture is sent into the cylinders. Most common is the naturally-aspirated engine (with no forced induction system). This engine brings air into the cylinder at atmospheric pressure and temperature to be mixed with fuel and combusted (or burned). This would work fine for all engines, but more power can be developed if more air is brought in because more air means more oxygen. At higher pressures, air will be denser and can be mixed with more fuel to deliver more power per stroke of the engine. To create this higher pressure, a forced induction system, such as a turbocharger or supercharger, is added.

A turbocharger (or turbo, for short) system has a compressor that is used to compress the intake air for the engine and a turbine that is used to drive the compressor. The turbine, like a wind turbine, is driven by the exhaust gases leaving the engine, which contain a fair amount of kinetic energy that would otherwise be wasted to the atmosphere. This is like recycling the exhaust energy. As the exhaust passes through the turbine, it spins up and gains rotational inertia, which is transmitted to the compressor through a mechanical linkage. Air from outside the car is sucked into the compressor where it will rise in temperature as well as become denser. This rise in temperature, however, is undesirable. Air at higher temperatures will have lower density than air at atmospheric conditions.

The density through the compressor will still increase a good amount though, but it could be made better if the output temperature were lower. To help this situation an intercooler is introduced. The intercooler is basically a heat exchanger that runs the hot air from the compressor through it. Its main goal is to cool the air before it goes into the engine without losing any of the pressure from the compressor. It is usually placed at the very front of the car so that air coming from the atmosphere flows through the intercooler and cools the hot air from the compressor (which is flowing inside the intercooler in a series of little tubes). These tubes even have little ‘fins’ to increase the surface area of the intercooler so that maximum heat transfer between outside air and the pressurized air can occur.

So air comes from the atmosphere, into the compressor (driven by the exhaust turbine), exits the compressor into the intercooler, then the cooled and pressurized air enters the cylinder where it is ignited with some fuel. It then leaves the cylinder as exhaust where it will spin the turbine for in order to compress more intake air. After passing through the turbine the air leaves the car through the exhaust pipe into the atmosphere.

Turbo System Diagram

A supercharger is similar because its function is to compress the intake air for the engine. It only differs in that its compressor is driven by the engine itself instead of an exhaust turbine. This robs some engine power, but the pressurized air advantage (more oxygen) more than makes up for the power consumed to drive the compressor.

Although still fairly rare, turbocharged cars are making a comeback in recent years due in part to rising fuel costs and demand for technologically advanced engines. Maybe you’ve seen some surviving sport-compact cars from the 1980’s with their prominent ‘TURBO’ stickers still intact. Although stickers are sometimes used to increase visual appeal (think racing stripes), these labels generally indicate that the car is indeed equipped with a turbo. Turbo advantages include decreased fuel consumption because they allow smaller engines to produce the power of much larger ones. They also allow for a lighter engine so they are popular for high-performance cars trying to cut weight everywhere possible. It’s common to see turbocharged engines that are about 2.0 liters (in total piston stroke displacement, which is a unit of engine volume) providing the same power as naturally aspirated engines with almost twice the volume. Some disadvantages are that fuel consumption is increased if the driver always drives the car as hard as possible because more fuel than usual is used when running the engine under ‘boost’, or with a pressurized intake. It also takes higher engine speeds (RPM) to generate enough exhaust to effectively drive the turbine, so a turbo does not provide ample boost until the driver has the car about midway through the engine’s RPM range, this delay is also known as “turbo lag”. Overall though, a turbo is an effective way to give more power to a small engine by simply forcing more oxygen into the cylinder, which yields more power.

Amazing illustration from clfloyddesign.com. Diagram from BTN Turbo.

Wheel Imbalance Can Easily Be Detected When Driving on the Highway

Have you ever driven a car that vibrated uncontrollably at certain driving speeds, especially on the highway? If you have, you probably noticed that these large vibrations occur when you drive at one specific speed, as shown on your speedometer, and the vibrations die out when you travel a little lower or higher than that specific speed. Personally, I’ve driven a car that undergoes unusually large vibrations when I’m driving it at around 70 miles per hour. This phenomenon arises from an imbalanced rotor.

Rotors are objects that rotate, like wheels on a car. An imbalanced rotor is one whose center of mass is not in line with its axis of rotation. For a wheel, the axis of rotation would be the axle of the car. Ideally, if a wheel is perfectly circular and uniform, meaning its center of mass is exactly in the center of the wheel, and the axle for the wheel goes through the center of the wheel, then the wheel would be “balanced” because its center of mass and axis of rotation are in line (both at the center of the wheel). However, in the real world, these ideal cases are few and far between. Imbalances on a wheel, if they are drastic, can cause undesired vibrations.

Now, why does an imbalanced rotor tend to have large vibrations at certain speeds? The answer comes from the principles of resonance and resonant frequency. Every object has a resonant frequency at which the object experiences large vibrations or large-amplitude oscillations (resonance). Consider a small wine glass. If you drive the wine glass at its resonance frequency long enough (perhaps by directing sound waves at it), it will shatter due to the resulting resonance. Similarly, for an imbalanced rotor (such as a wheel), there are certain speeds of rotation (called “critical speeds”) that cause large-amplitude vibrations in the rotor (up-and-down and side-to-side). If one changes the speed of rotation away from this critical speed (either higher or lower), then the resonance will die out and the rotor will rotate more smoothly. Interestingly, at very high speeds, an imbalanced rotor will tend to balance itself out and rotate smoothly as if it was balanced to begin with.

When driving a car with imbalanced wheels at its critical speed, you might notice that the resonant vibrations feel uncomfortable to you, especially since you can feel the vibrations  from the steering wheel, which you are probably holding onto if you are moving on the road. Similarly, resonant vibrations are uncomfortable for your car as well (think of the wine glass analogy again). Prolonged periods of driving a car with an imbalanced wheel at its critical speed will subject the car to increased stress and cause it to wear out faster (and potentially break down as a whole due to failure under stress…just like people sometimes do). So, if you ever experience unusually large vibrations from your car when you are driving at particular speeds, you will know what the problem most likely is, and the strategic play would be to get the wheel imbalance fixed as soon as possible to avoid excessively damaging your car.