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2006-07-16
Featured Article

Improve MPG: The Factors Affecting Fuel Efficiency

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Introduction

I own a 2006 Jeep Wrangler Unlimited. This vehicle is actually an upgrade from my previous 2004 Jeep Wrangler Sport that did not have any air conditioning or cruise control.1 After spending a year in the desert heat of Iraq, I vowed to myself that if it were in my control, then never again would I go without air conditioning.2 So, I figured if I was going to burn gas up in my 4×4, I may as well burn it up a little more comfortably.

However, there is nothing comfortable about rising gas prices. All jokes aside about Jeep Wranglers having the aerodynamics of a rolling post office box or a brick on wheels, I decided to figure out the most fuel efficient way to drive my car to save a little money. Granted, I could have traded in my old Wrangler for a more fuel efficient SUV like the Liberty CRD or the new hybrids, but none of those vehicles compare to the Wrangler when it comes to off-road performance. So, like it or not, I needed to figure out some new driving habits.


For increasing fuel efficiency, there are three basic courses of action. First, driving style affects fuel efficiency. Many people will cite "rules of thumb," but most cannot back these claims up with evidence. To learn the truth about how driving style impacts fuel efficiency, I conducted road tests with a computer hooked into my engine’s OBDII interface. The second way to improve fuel efficiency is to modify the way an engine performs. Modifications can take many forms, so I browsed the Internet to find the most common and analyzed the fact and fiction behind their influence on fuel economy. Lastly, the very fuel that goes into an engine plays a role in fuel efficiency. With all the talk of alternative fuels these days, I wanted to know what they are, whether they work in my engine and how they compare to regular gasoline. Ultimately, I take all the cards relating to fuel efficiency and lay them on the table for scrutiny.

Studying the Jeep’s Performance

I used the following equipment to collect performance data:

  • OBDII interface tool – The specific interface tool I used was the ElmScan5 – a multi-protocol OBDII reader offering improved performance over the previous versions.3 It is a hardware device based on the Elm327 chip (an upgrade over the Elm323) which converts the esoteric OBDII electric signal interface from the car’s computer into a human understandable, ASCII format.4,5 It is capable of reading approximately four data samples per second and connects via the computer’s serial port.
  • Software from DigiMoto – The software from DigiMoto interfaces with OBDII devices built upon the Elm32x chipset and supports selectively reading engine parameters, data logging and a ‘poor mans’ dynometer.6
  • Microsoft Excel – Once collected, I imported my OBDII data into Excel for later analysis.
  • Laptop Computer
  • My Trusty Jeep Wrangler Unlimited

Understanding OBDII

OBDII stands for On Board Diagnostics generation 2, which is a standard defined on paper describing how a car’s engine computer communicates emissions information.7 The OBDII standard is the product of the Society of Automotive Engineers (SAE) and International Organization for Standardization (ISO).8,9 The OBDI system originated from a mandate by the California Air Resources Board (CARB) demanding a means of emissions diagnostics be in place by 1988.10 As requirements grew more stringent, OBDI was replaced by the more standardized OBDII. The newer system was therein required by the Clean Air Act of 1990 to be in all American automobiles by 1999.11 OBDII requirements developed somewhat loosely in practice. At first, they were mostly a guideline to what automotive manufacturers should do, rather than how they should do it, which resulted in a variety of OBDII communications protocols and proprietary diagnostic codes.12 Ultimately, while the system is designed to detect emissions problems and illuminate the Malfunction Indicator Lamp (MIL) whenever problems occur, (e.g. an unpressurized fuel system, emissions at 150% federal limits, catalytic converter problems, improper combustion or sensor failures, etc) it can also serve as a handy interface for studying a vehicle’s performance characteristics.13

Determining which OBDII interface is supported by a given vehicle requires looking at the physical connector and studying the available electrical contacts.14 The standard requires the OBDII port to be between 300 and 750 millimeters from the vehicle’s center line in a location easily accessible to the driver. This typically results in an OBDII port location near the steering column underneath the dashboard. My Jeep Wrangler’s OBDII port is located at the bottom left edge of the dashboard, near the doorstrap. There are currently four basic OBDII interfaces: Pulse Width Modulation (PWM), Variable Pulse Width (VPW), ISO9141/14230 (ISO) and ISO15765 Controller Area Network (CAN).15 Looking at the connector, identify which pins have an electrical contact to determine which protocols are supported.

  • PWM: Pins 2, 4, 5, 10 & 16
  • VPW: Pins 2, 4, 5 & 16 but NOT 10
  • ISO: Pins 4, 5, 7 & 16 with OPTIONAL 15
  • CAN: Pins 4, 5, 6, 14 & 16

Traditional OBDII interfaces do not allow the user to reprogram the automobile’s engine computer. This requirement comes from the EPA (Environmental Protection Agency) so that owners may not tamper with the computer in such a way as to inadvertently violate emissions standards.16 Fortunately, the information needed for my experiments does not require altering the computer’s configuration, only reading ECU variables.

The Experiment

To study the effects of driving habits on fuel efficiency, I chose a road that I could use for repeated tests. Fitting the bill nicely is a five mile span of Riverwatch Parkway leading into Augusta, GA, a road that is mostly flat and relatively light on traffic. To maintain consistency, each sample was taken along the same stretch of road while driving in the same direction. During the drive, there was little to no wind and an ambient temperature of 90° Fahrenheit. The Jeep was topped off with Shell 87 Octane fuel for the sampling. I selected engine parameters that would give a good indication of engine performance and proceeded to drive back and forth for two hours collecting information. I was delayed for a while because right before I tested the 75mph characteristics, a highway patrol car decided to take up residence on my test road. The speed limit on Riverwatch Parkway is only 55mph, so I had to wait for him to leave in order to finish collecting data.

Studying Driving Habits

Before differing driving habits can be analyzed, it is important to understand exactly what the engine is doing. Quantitative data may then be compared against this background. Once the engine basics are outlined, I will collect data and analyze the results for:

  • impact of driving at various speeds
  • effects of unnecessary loads on the engine
  • comparing cruise control to manual control
  • benefits of drafting
  • difference between fast and slow acceleration

Understanding the Engine

Internal combustion engines are often called aspirated engines because they are "air-breathing" machines. These engines burn a combination of air and fuel mixed at the stoichiometric ratio 14.7 to 1.17 In the past, fuel and air were mixed in the carburetor, relying on the venturi effect of the pipework to render the fuel into an aerosol.18 Modern systems rely on fuel injection, a process governed by the Jeep’s engine computer unit (ECU).19 A variety of sensors feed into the ECU providing real-time information so the computer can manipulate engine performance for optimal emissions. The ECU can regulate fuel flow precisely to match air intake or even adjust the spark timing for various RPM and engine loads.

A rough approximation of fuel use can be made through calculations on engine sensor data. Estimates on fuel consumption will be approximately equal to dividing the amount of airflow through the engine by 14.7, the stoichiometric ratio.20 Engine mass airflow may be determined either with an MAF (Mass Air Flow) sensor or through calculations on the MAP (Manifold Absolute Pressure) sensor.


The Effect of Speed on Fuel Efficiency

Using these equations, I can compare the fuel requirements for different driving speeds. To collect the necessary data, I configured the DigiMoto software to log data on the engine’s RPM, intake temperature and manifold pressure. For the tests, I drove the Jeep at 45, 55, 65 and 75mph along the same stretch of Riverwatch Parkway. To ensure consistency, logging only took place after the appropriate speed was reached and locked into with cruise control.21 During the drive, the Jeep’s windows were left up for consistent aerodynamics and the air conditioning was left off to eliminate extra engine variables. The engine intake added approximately 13° of heat to the ambient air temperature resulting in 103° Fahrenheit (312.5° Kelvin).


The graph above depicts an estimated quantity of fuel consumed for the calculated air mass inhaled by the engine at the four speeds. It is evident that increased speeds require more than a linear increase in fuel. There are many factors that play into increased fuel needs at higher speeds, but aerodynamics is the most significant. A basic equation to govern the amount of power a vehicle must exert is represented by:22


  • A – a constant resistance composed mostly from rolling resistance, brake drag and similar forces of linear friction.
  • B – a constant factor of resistance composed mostly from friction and resistance within the engine components.
  • C – a constant resistive element composed of aerodynamic forces, the coefficient of drag and the density of air.

Below, a graph represents the exponential growth in power a vehicle must exert versus increased speed (the coefficients are set to ‘1’ to isolate only the effect of speed.


I can conclude from these two graphs together that aerodynamics are likely the greatest cause for increased gas consumption at higher speeds. Experiments conducted by Edmunds indicate that driving slower can save up to 14% of wasted fuel.23 I hoped to calculate a "sweet spot" for my Jeep where fuel efficiency was at a peak. If you could graph miles per gallon across a complete range of speeds, there should be a curve of diminishing returns. Higher speeds burn gas excessively while lower speeds simply do not traverse great distances, e.g. idling the engine burns gas without traveling anywhere. However, as a Jeep Wrangler with a hard top has a 0.58 coefficient of drag, there was not an efficiency peaking speed of any usefulness (down around ~25mph).24 Typical cars have a coefficient of drag of approximately 0.30, while sports cars tend to be around 0.20; vehicles of this sort will probably have a calculable "sweet spot" in the 55mph range.25,26

OBDII and Oxygen Sensors

Without a doubt, the Jeep Wrangler suffers from its steep, flat windshield, the exposed surfaces of the undercarriage and both the aerodynamic losses of the wheels and rotational friction of large tires. The calculated findings above match records I had been keeping on the fuel efficiency of my various driving habits.

  • 55mph = roughly 21 miles per gallon
  • 65mph = roughly 19.5 miles per gallon
  • 75mph = roughly 18 miles per gallon
  • 90mph = roughly 14 miles per gallon

I began to wonder, though, what was my Jeep doing with all the extra fuel? Was it being used to get enough power to go that fast? Or was the engine expelling it, unburned with the exhaust?

Remember, the OBDII protocol evolved as an emissions control standard. As part of the specification, automobiles include an array of sensors to measure unburned oxygen in the exhaust. Called the O2 sensors, they are placed before and after the catalytic converter.27 This allows the ECU to check oxygen levels immediately out of the engine and also to inspect the performance of the catalytic converter. A failure in the system will illuminate the Malfunction Indicator Lamp (MIL) on the dashboard. The sensors provide a voltage to indicate whether the remaining oxygen levels are lean or rich. A value of 0.1V indicates the engine fully consumed the fuel while a value of 0.9V is indicative of a rich, unburned level of fuel in the exhaust.28 The ideal point for engine performance is 0.45V which indicates the ratio of both air and fuel are burning optimally. The ECU will take this exhaust feedback into play when calculating the amount of fuel to inject into the cylinders and how to adjust the spark plug timing. A healthy system will automatically center itself around the stoichiometric value of 0.45V.29


The graph above represents the average O2 sensor readings for each of the various speeds. The Jeep Wrangler has two sensors in two banks. Bank 1 is located ahead of the catalytic converter and Bank 2 is behind it. In the graph, column 1 represents the four sensors at 45mph. Each successive column of sensors represents the advancing speeds from 55mph through 75mph. It is interesting to observe that there is little difference between the speeds in terms of exhaust elements. My Jeep appears to be running a little rich at the moment meaning it is wasting fuel. Since the ECU is an adaptive computer, these values will slowly creep towards the stoichiometric point to improve the burn. There are values in the computer referred to as the Long and Short Term Trim levels so the ECU can "remember" its calculated improvements.

The Impact of Air Conditioning on Fuel Efficiency

A study conducted by the SAE confirms many conclusions about the air conditioning vs. windows down debate.30 The study placed a Sport Utility Vehicle (SUV) and a sedan into wind tunnels at varying degrees of yaw to determine the aerodynamic effects of windows at varying speed.31 Ultimately, as many drivers have naturally inferred, driving with the windows down at high speed induces more resistive drag on the car than an air conditioner exerts on the engine. With windows down, sedans incur a 20% aerodynamic drag while SUVs are only affected by about 8%, mostly due to the SUVs naturally poor aerodynamics. Air conditioning on the other hand, imparts a 5-10% load on the vehicle’s engine. The rule of thumb goes that if owners need cooling, they must open their windows during city driving and use air conditioning at highway speeds.

Out of interest, I logged more data on the test strip to determine other factors that contribute to engine strain. For consistency, each sample was taken at 65mph with cruise control and the windows up. The same engine parameters were logged, to collect manifold pressure, intake temperature and RPM. This experiment was conducted to see the effect of air conditioning at highway speed. I also recorded the effects of driving in the wrong gears – 4th and 5th at highway speed. (Please note that starting in 2005, Jeep Wranglers featured the NSG 370 six speed transmission in lieu of the NV3550 five speed transmission. For drivers with a five speed transmission, these performance figures will correlate to driving at highway speed in your 3rd and 4th gear.)


The graph confirms common sense regarding not using the correct gears. Fifth and fourth gears incur an increased demand on fuel to operate the vehicle at the same speed. For vehicles equipped with an automatic transmission, this information corresponds to recommendations for using overdrive, the vehicle’s fourth gear. When the Jeep was driven in the correct gear, the engine’s vacuum (equal to atmospheric pressure minus manifold pressure) was high. In the wrong gear, the vacuum was lower because the engine was forced to ‘breathe’ and burn more air to maintain the test speed. According to the Haynes Techbook, high vacuum conditions result in the ECU leaning the combustion mix and advancing spark timing for extra efficiency while enriching the mix with more fuel under low vacuum conditions to boost power.32 Additionally, driving in the proper gear keeps the RPMs down to reduce excessive engine wear.

Air conditioning’s effect on fuel efficiency was particularly interesting. In sixth gear with the air conditioning operating, the vehicle consumed more fuel than if it were driving at the equivalent speed (without air conditioning) in fourth gear! To understand the results, it helps to know how the air conditioning system works.33 An air conditioner relies on a pump, sometimes referred to as a condenser, compressing a refrigerant liquid which, according to Boyle’s Law, causes it to heat up.34 The heated liquid is transferred to a heat exchanger for cooling. Then the pressure is released, again by Boyle’s Law, making the refrigerant colder than when it entered the compressor. It is the air conditioning’s compressor that affects fuel efficiency. The power needed to repeatedly compress the refrigerant comes from attaching the pump directly to the engine via a clutch and pulley belts.35 When the air conditioning clutch is engaged, the pump action imparts extra resistance on the engine. As the findings demonstrate, the extra resistance from air conditioning is handled by drawing more fuel into the engine, thus decreasing fuel efficiency.

The Impact of Cruise Control on Fuel Efficiency

When a driver controls the throttle, the pedal regulates air flow into the throttle body by varying a throttle valve which is generally controlled by a cable.36 A cruise control works by enabling a vacuum pump to control the throttle valve’s cable in place of the driver’s foot.37 The system maintains speed in one of two ways. The older method adjusted vacuum pressure based on the input from a solenoid, linked to a magnetic sensor that detects variances in axle revolutions. Modern controllers use an electronic throttle control that relies on direct input from the ECU’s analysis of various sensors.38 These new solid state electronics make more informed choices at a rapid rate to avoid disruptive speed adjustments.

To analyze the effects of cruise control on fuel efficiency, I conducted a series of drives at 65mph on my test strip. To ensure consistency, data was only logged between two speed limit signs so comparisons could be made on the same terrain. Furthermore, to preserve vehicle parameter consistency, the windows were kept up for aerodynamics and the air conditioning left off. I made the assumption that between a driver and a cruise control, the only variables of interest would be those related to variances in throttle so I selected Intake Manifold PSI and engine RPM. If a cruise control provides a fuel efficiency bonus, than I should find that over the same stretch of road, the resulting data will be smoother.



Looking at the graphs, I can see that both the cruise control and manual control resulted in similar trends over time regarding general rises and falls. This behavior is undoubtedly indicative of matching requirements for changing PSI and RPM to maintain speed over the two small humps in Riverwatch Parkway at the beginning of the test strip. Overall, the cruise control’s graph is smoother than when I controlled the Jeep manually. The PSI graph provides a reasonable picture of varying input by the throttle valve. The cruise control made only minor adjustments for ‘like’ terrain segments unlike the manual control which bounced above and below the computer’s line. Of interest are the three areas where the cruise control raised the engine’s RPM at time intervals 30, 60 and 100. Each time I controlled the Jeep, there was a sharp drop in RPM with a subsequent RPM spike that likely correlates to the human inconsistency of working the pedals while shifting.

The PSI graph provides evidence of the minute variations introduced by human control. Human input varies over approximately a 2psi range while those from the cruise control varied by less than 1psi. While the PSI input difference is small, it can have detrimental effects on ignition advance timing and the fuel ratios, which over the course of a fuel tank results in lost fuel efficiency. I can conclude from the RPM graph that a cruise control unit saves fuel by adapting quickly to engine needs and providing incremental steps for speed maintenance. Human senses, on the other hand, do not realize minute changes are needed until after the fact and must compensate with more drastic changes which consume more fuel. Edmunds performed experiments showing cruise control saving fuel up to 14%, while typically averaging 7% fuel savings.39

To caveat the findings, cruise controls are best suited for flat terrain. Hills, especially rolling hills, introduce varying needs from the engine. Because the ECU has no means of detecting an upcoming incline or decline, it may make adjustments that are not suitable for a sudden shift in grade. This is why many vehicles struggle on hills with their engine RPMs rising and falling dramatically.

The Impact of Drafting Vehicles on Fuel Efficiency

Another technique for improving fuel efficiency involves reducing the air density through which the vehicle moves. It is a practice called drafting, one found in both nature (bird migration), athletics (road cycling) and sports (NASCAR).40 The following equation describes the drag force a body in motion experiences in a fluid state:41


  • ρ – fluid density (for atmospheric air, this value corresponds to 1.29 kg/m3)42
  • Cd – coefficient of drag
  • A – surface area of the drag inducing plane
  • v – velocity of motion is squared

The graphs below illustrate the effects of reduced air density that would be experienced while drafting. Without an instrument to measure air density, I made the assumption that in the proper slipstream, the air density would be one half that normally encountered. I decreased the C value in the power equation by one half and decreased rho to 0.65 kg/m3. Mathematically, the force equation shows that the decreased air density from drafting results in an immediate decrease in resistance by a factor equivalent to the coefficient. Still, velocity is the primary factor of resistance resulting in increases governed by the law of squares. The power, and thus fuel, required to muscle through that air density is also exponential.



From experience, I have noticed the effects of drafting to be an enormous boon to fuel efficiency – especially in an aerodynamically challenged vehicle like a Wrangler. Once, when carefully following a truck at 65mph from Augusta to Florence, I was able to exceed 30 miles per gallon. The benefits of drafting trucks were even noticeable in a Toyota Camry driving from the Atlanta airport to Augusta, although not as pronounced since the coefficient of drag is not nearly as high. Ultimately, however, the practice of drafting vehicles on the highway can be dangerous. Vehicles are required to maintain a close following distance which does not provide a driver much reaction time in the event of emergency. Furthermore, as vehicles draft behind tractor trailers, the close proximity limits the visibility for the trail driver and the truck drivers themselves may not be comfortable with close followers due to safety and insurance liabilities.

The Impact of Acceleration on Fuel Efficiency

Multiple studies show that accelerating slower will dramatically improve fuel efficiency. On a closed fifty-five mile track circuit, Edmund’s test drivers accelerated cars at various throttle positions to study the effect of ‘jack rabbit’ starts.43 They found fuel savings up to 37% with an average of 31% simply by accelerating a car slowly. Similar findings by the Florida Solar Energy Center (FSEC) show slower acceleration saving between 14% and 21% of fuel use over aggresive acceleration.44

Curious to find similar tendencies with my own Jeep, I devised another driving experiment to record data directly from the car computer. Basically, I would need to record engine parameters while performing different, controlled accelerations so that I could compare the spent fuel. Since testing these varieties of driving styles would be dangerous to other drivers, I found a remote location in the training area on Fort Gordon. This ensured no vehicle accidents would take place. Again, I configured my software to record OBDII parameters that would allow me to analyze fuel efficiency. I conducted the following tests:

  • 1st gear: 0 to redline RPM
  • 2nd gear: 0 to redline RPM
  • 1st gear: 0 to 15mph @ 2000 RPM
  • 2nd gear: 0 to 25mph @ 2000 RPM
  • 0-60 mph: 1st-3rd gear @ Full Throttle
  • 0-60 mph: 2nd-3rd gear @ Full Throttle
  • 0-60 mph: 1st-6th gear @ 2000 RPM
  • 0-60 mph: 2nd-6th gear @ 2000 RPM


The fuel mass consumption graphs were plotted using the same equation based on intake temperature, manifold pressure and RPM. Each dip in the curve represent a point at which I changed gears, a drop in fuel consumption as the engine was not under load with the transmission disengaged. With the throttle ‘floored,’ an average fuel mass of 132 was consumed to accelerate to 60 mph. While redlining the engine, it was only necessary to shift into third gear to achieve this speed in approximately 13 seconds. On the other end of the spectrum, to keep the engine below 2000 RPM, it was necessary to utilize all six gears. 60 mph was achieved in a much slower 40 seconds, but the fuel mass consumption equation yielded an average value of 5.

The ECU calculates its ‘engine load’ based on the ratio of intake manifold pressure to atmospheric pressure.45 A 100% load, therefore, means the manifold pressure is equal to the atmospheric pressure. Short of a turbocharger, supercharger or carefully tuned ram intake, it is impossible to have a higher manifold pressure than atmospheric pressure. During the acceleration tests, the engine load held consistently between 85% and 95% for both low and high RPM. The difference between the two scenarios for fuel consumption lies in the RPMs. Looking back at the equation for engine mass airflow, both situations have roughly the same intake pressure and intake temperature. Therefore, only RPM can discriminate the fuel consumption between different styles of acceleration. Higher RPM will consume more air in the same amount of time. Without a doubt, stressing the engine results in a higher volume of fuel to perform the same task.

The Impact of Performance Modifications on Fuel Efficiency

Throttle Body Spacers

One of the most commonly advertised devices for improved horsepower and fuel efficiency is a throttle body spacer.46 They are relatively inexpensive devices and simple to install.47 In theory, the devices work by creating turbulence in the air intake stream. Fuel becomes more volatile when rendered aerosol and turbulent air flow allegedly makes this transition easier, but the claims are met with a great deal of skepticism.48

The rationale behind this skepticism is the engine computer itself. An ECU is designed to continually monitor engine performance and emissions output. While the device may initially increase engine performance, the O2 sensors will detect the increased emissions from an improved burn and adjust accordingly to reduce emissions. This is after all, the primary purpose of the ECU, to reduce emissions for environmental protection. Therefore, after the ECU adapts to the presence of the throttle body spacer, a vehicle’s performance will normalize. Older engines using a carburetor in lieu of an ECU may actually benefit from a throttle body spacer, but not a computer controlled engine.

Fuel Magnets

Another technique for improving fuel efficiency is the use of fuel magnets or fuel line cleaners.49 The concept behind the technology is that magnets on the fuel lines will trap impurities or alter the nature of the fuel as it passes through the magnetic fields. With cleaner fuel comes a cleaner and more efficient burn. However, an EPA analysis of fuel magnet devices determined they have no effect on the fuel efficiency of automobiles.50

ECU Override

Thus far, the ECU itself is often the culprit behind why engine modifications do not have a lasting effect on fuel efficiency. Like the analysis of the O2 sensors demonstrated, the engine will ‘recenter’ itself to maintain a level of emissions in accordance with EPA guidelines. To circumvent this behavior, the ECU can be modified, overridden or completely replaced. Hypertech features a product that rewrites the mappings of the ECU through the vehicle’s OBDII port, altering the manner in which it treats sensor input.51 Remapping the ECU is a common technique used by performance tuners to make competitive racers from inexpensive cars or to give older cars a refresh to handle modern fuels.52 Another means of overriding the ECU’s default behavior is plug-in modules.53 It is less invasive to the ECU as it utilizes external modules. Lastly, the ECU can be replaced completely.54,55 When enough changes to an engine have been made, complete ECU replacement is often necessary for the emissions management to make proper use of upgraded components. Direct manipulation of the ECU will likely void any warranty by the manufacturer.

O2 Sensor Falsification

One trick to evading the tendency for the ECU to self-calibrate is to provide false sensor data. If the ECU is tricked into believing a different condition exists, it will make fuel and timing assumptions from that data. The easiest sensors to manipulate are the oxygen sensors.

Oxygen sensors operate after being heated to 600° Fahrenheit.56 During the warm-up phase, the ECU operates in what is called Open Loop conditions. The ECU does not interpret any emissions data from the sensors until they have sufficiently warmed up. Once heated, the ECU switches to Closed Loop conditions, where O2 inputs affect the fuel injection and spark timing.

Two techniques are available to bypass the Closed Loop emissions management system. The first, and most simple, involves disconnecting the sensors altogether. Essentially, the ECU will operate in Open Loop mode continuously. This will allow the engine to operate without the emissions data to full potential. Another technique is to create a bypass circuit on the O2 wiring. By directly manipulating the O2 value with a potentiometer, the ECU can be fooled into believing the emissions are 0.1V or 0.9V. In turn, the ECU will enrich or lean the air fuel mixture which will have an impact on fuel efficiency. Ultimately, the practice of tampering with the O2 sensors will void the warranty on the engine because of abnormal wear in operating in a potentially extreme rich or lean state. Furthermore, the vehicle will likely fail state emissions inspections.

Cold Air Intake

Another common technique for improving engine performance is the introduction of colder air. Looking at the engine mass airflow equation, it can be concluded that colder air will increase the amount of air available to the engine at the manifold. Although each vehicle is different, generic dynometer results from a 2003 Chevy Tahoe match the conclusion that power is increased from the colder air.57 Cold air can be achieved by using heat shield barriers, ram air, turbochargers or snorkels.58,59,60



My Jeep Wrangler is equipped with an ARB Safari Snorkel. To test the theories of cold air intake and ram air, I configured the OBDII software to measure intake temperature and manifold absolute pressure. For the experiment, I measured information at 65mph and at idle. The ambient temperature during the test was 92° Fahrenheit. To simulate a ‘stock’ airbox, I wedged open the Jeep’s air filter compartment and sealed the snorkel to ensure air came only from inside the engine compartment. I discovered the snorkel does in fact serve as a cold air intake, averaging only about 12° Fahrenheit warmer than ambient temperature while driving. While both stock and snorkel were significantly hotter while the vehicle was not moving, the snorkel was significantly cooler than the stock airbox. Ultimately, however, recorded data reveals the snorkel does not serve as a ram air intake for a Jeep. At both idle and forward speed, the manifold air pressure was virtually identical. On the other hand, snorkels provide that handy, emergency ability to drive a Jeep through deep water while other cars remain trapped or hydrolocked.

The Impact of Alternative Fuel on Fuel Efficiency

While understanding how driving style and performance modifications play a significant role in squeezing extra miles from fuel, ultimately, the fuel itself needs to be understood as well. A British Thermal Unit (BTU) is a unit of measurement defining the amount of heat necessary to raise one pound of water by one degree Fahrenheit.61 Alternatively, BTUs are used to define the amount of energy potential stored in a chemical compound. With automotive fuel, the contained BTUs offer a means by which to compare the output potential of alternative fuels.62,63


Petroleum

Gasoline is an aliphatic hydrocarbon, which is a chain of hydrogen and carbon molecules between 7 and 11 links in length.64 Oil refineries heat crude oil in a distillation process to separate the varying hydrocarbon chains. The mixtures of hydrocarbon chains result in fuel combinations that differ in combustive properties. These properties represent the various octanes found at pump stations, lower octanes are fuels that do not handle compression well and are prone to early combustion (engine knock).65

Petroleum fuels face two major problems today, the prospect of achieving a declining availability known as Peak Oil and the environmental problems associated with hydrocarbon emissions.66 Aside from continued oil field exploration, improved engine efficiency and alternative fuels, there is not much that can be done about diminishing oil supplies. Emissions on the other hand have been the subject of automotive and oil engineering for several decades to comply with the Federal Clean Air Act imposed on the nation’s most polluted cities.

Part of the solution is a process called Reformulation to oxygenate the gasoline. With the addition of Methyl Tertiary Butyl Ether (MTBE) and ethanol, gasolines will burn cleaner to reduce smog and ozone emissions.67 MTBE is added into the fuel mixture as both an oxygenate octane booster, to allow the use of lower octane fuels. Ethanol is typically mixed in a 10% ratio with standard gasoline producing, resulting in what is sometimes called "gasohol." This process is done for two reasons. First, to comply with a requirement for winterized, oxygenated gasolines. The second, to extend the fuel supply by diluting fuel with a combustible for distribution to remote locations.68 Each oil company manipulates the reformulation process in different ways, resulting in differing performance aspects by both brand and location. Reformulated fuels do not significantly impact a vehicle’s typical fuel efficiency, usually by only a 2% drop in performance.69

Ethanol

Ethanol is produced through a fermentation process of plant sugars. For usefulness as a fuel source, it must be further distilled to remove water and achieve a purity of at least 99.5%.70 The fuel itself has an octane value of 105, meaning that it is very stable for high compression demands. As a product of fermentation, it is clean burning, biodegradeable and a non-contaminant in water.71

Several manufacturers have already began making automobiles that will run on both regular gasoline and E85 blends, gasoline with a mixture of 85% ethanol.72 These "Flexible Fuel" vehicles differ from normal vehicles in only a few components. Ethanol fuels in heavy concentration are severely corrosive and require the replacement of fuel related rubber and plastic components – tank, seals, pump and lines.73 With the higher octane of ethanol, the ECU must be able to detect the presence of E85 fuel and adjust the spark timing advance accordingly. This is not to say that a regular automobile will not function on E85. Continued use on a non-prepared vehicle, however, may damage components through corrosion and premature engine detonation from improper spark timing. Ethanol fuels provide nearly 30% less BTUs than standard gasoline. Regardless of whether the vehicle was designed for E85 or using it accidentally, it will not obtain the fuel efficiency of standard gasoline.74

It is not unheard of to perform a conversion to utilize E85 in standard vehicles.75 However, home grown kits will likely require maintenance from the same, home mechanic. With the proper component replacements, in theory, any vehicle can operate on E85.76 The problem with home grown conversions and even kits comes from an EPA finding that converted vehicles emitted grossly worse pollutants afterwards.77 Manufacturers are reluctant to re-engineer existing products because they have already tweaked performance for the intended application and for profitability. Even kit producers face consequences for products to convert automobiles. EPA and CARB regulations require modifications to vehicles not render them worse for emissions.78,79 Basically, an improperly converted vehicle will fail state emissions inspections.

Liquefied Petroleum

Liquefied Petroleum (LP) is a fossil fuel and is processed in refineries from crude oil or more typically from natural gas.80 Natural gas, usually methane, is often found in conjunction with petroleum, but does exist in gas fields without crude oil. LP can therefore be derived from synthetically produced, biologic methane or that extracted from the Earth. When burned as an automotive fuel, LP is clean – 90% reduced carbon monoxide, 33% reduced nitrous oxides and 50% reduced hydrocarbons.81

One of its advantages is easy portability. Under pressure, LP will remain liquefied in a stable form. With a regulated release, the fuel will be rendered aerosol easily for use in combustive applications. Regular cars can be converted to operate on LP. Essentially, an LP car requires a compression tank, fuel lines, LP regulators and a modification to the ECU for proper spark timing. What is especially interesting is the conversion design is for dual-fuel use so that standard gasoline can be used when an LP filling station is not available.82

Diesel

Diesel engines are fundamentally different than their gasoline counterparts.83 Whereas the internal combustion engine compresses both fuel and air simultaneously, the diesel compresses only the air. A direct fuel injector renders diesel fuel aerosol into the compressed cylinder where it burns. By not compressing the fuel simultaneously, a diesel engine is able to obtain a higher compression ratio than gasoline engines, sometimes a twofold increase.84 Diesel engine designs emerged about fifteen years after gasoline engines because consumers sought a means of obtaining improved fuel efficiency. Not only did the higher compression ratio offer improved power, but diesel fuel itself contains more energy per unit resulting in even more efficiency.

The fuel is derived from a similar refining process to gasoline by high temperature distillation. Of interest, is that diesel fuel can be obtained from a variety of sources beyond crude oil; diesel can be synthesized from bio-mass, natural gas, sewage and even coal.85,86 These new diesel derivatives and associated improvements to diesel engines are being considered as a possible saving throw against the oil crisis.87 Common Rail Diesel (CRD) systems are an improvement to the traditional diesel. The CRD make better use of fuel by increasing the delivery pressure and reducing even further the number of necessary engine components.88 Combined with innovative emissions filters like advanced ceramics, urea screens and particulate traps, diesel engines are making a cleaner burn of their fuel.

Bio-Diesel

Bio-diesel fuel is the result of a chemical reaction, called transesterification, between vegetable oil, a catalyst and either a methyl alcohol or ethyl alcohol (typically methyl alcohol) which produced fuel and glycerine. It can also be directly blended with standard diesel fuel as needed without penalty. Blends and additives are often necessary to overcome bio-diesel’s few limitations. The fuel congeals in low temperatures, requiring additives to maintain a usable viscosity. Interestingly, each gallon of vegetable oil will convert directly into one gallon of bio-diesel. Overall, for each BTU of energy used to grow the crop and perform the conversions, an energy surplus of 3.3 BTUs of energy is available from the resultant bio-diesel.89

There are many property improvements over conventional diesel fuel. One of the most advantageous properties of bio-diesel is that it can be used in contemporary diesel engines without modification. Occasionally, older diesel engines will require a retrofitting of rubber gaskets and hoses to prevent degradation. On the other hand, bio-diesel exhibits a peculiar property over conventional diesel in that it actually provides more engine lubrication.90 Bio-diesel also burns cleaner than standard diesel; emission decreases have been recorded in carbon monoxide (30% lower), polyaromatic hydrocarbons (74% lower) and nitrous oxides (3% lower).91 Despite these improvements over conventional diesel, the vegetable oil derivative suffers a slight decrease in BTUs, requiring approximately 1.1 gallons of bio-diesel for a comparative burn to 1 gallon of diesel.92

Bio-diesel appears to be the alternative fuel of the future. With more BTUs than even gasoline, bio-diesel provides a clean burning source of energy that can alleviate a nation’s oil independence. Switching to bio-diesel also requires zero changes to infrastructure as it can be put into existing diesel pump stations and run on existing engines. It is unlikely, however, to take a lead in the United States because of historical beliefs about the noise, dirtiness and potential cancerous nature of nitrous oxide emission in regular diesel.93 Only about 2% of American vehicles are diesel powered, unlike nearly 30% of those in Europe and abroad.

Summary

Fuel efficiency can be achieved in a number of ways. The purchase of a fuel efficient automobile, of course, leads the way in effectiveness. But for those owners already hampered by a gas guzzler or those with a functional need for the vehicle, simply buying a new car is not a viable option. Instead, owners can achieve greater fuel efficiency by adjusting driving habits, installing appropriate performance upgrades or selecting improved fuels.

Changing driving habits introduces the most profound effect on fuel efficiency for any vehicle. In brief, the following tips collectively save gas in the long run.

  • slower acceleration
  • reduced top speed
  • proper tire inflation
  • using cruise control
  • proper vehicle lubrication
  • correct transmission gears
  • using air conditioning only when necessary
  • reducing aerodynamic drag
  • removing excess weight94

Some aftermarket accessories will also allow owners to improve the fuel efficiency of their existing vehicles while others prove to be nothing more than snake-oil. For non-carbureted engines, only the products that target the ECU itself will assist with fuel efficiency. Otherwise the closed loop feedback from the O2 sensors will negate any performance improvements over time. Owners with a carburetor may find benefit from a greater pool of engine modifications because there is not an ECU constantly working to stabilize emissions.

In the near future, fuel efficiency will become increasingly dependent on which alternative fuels are chosen. Bio-diesel provides the most promise for continuing the status quo of contemporary vehicle performance, although it requires an engine swap or the purchase of a diesel engine. Ethanol seems to provide the cleanest emissions at the cost of energy output. Although ethanol also requires conversion kits for existing cars, such a process will not likely be as costly as converting to diesel. The choice of fuel is one that consumers will not always have control over, but simply must understand.

For off-road enthusiasts, these tips demonstrate with evidence how to get more fuel efficiency from your vehicle when it is being used as a daily driver. Plus, by improving on-road fuel efficiency, there will be more gas in the tank once your 4×4 gets to its off road destination.



Footnotes

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