Which requires more fuel

Engineering design technologies that make for a fuel efficient vehicle are so numerous that it would be virtually impossible to list them all and in any case for the majority of people they would be unintelligible and of little interest.

Thankfully for the vast majority of us there is a very useful and independent fuel efficiency tool that we can use to measure the relative fuel consumption and environment credentials of vehicles currently available in Australia: The Australian Government Green Vehicle Guide.

This tool uses a star rating system to categorise vehicles according to fuel consumption and environmental emissions data. It does not however categorise vehicles according to their design purpose i. The fact is that often a vehicle that is a top performer in terms of fuel consumption is often not the best in terms of performance and or load carrying capacity. It provides a level playing field upon which all vehicles regardless of make can be compared for their fuel consumption and their environmental performance.

However, it should be noted that whilst the driving cycle parameters are designed to try to replicate a real world driving cycle, the results only reflect the outcome achieved in respect of those specific parameters. Any departure from the parameters in the real world could mean very different results. NB: The top performers on the Green Vehicle Guide consist mainly of electric or hybrid vehicles and it should be understood that these ratings do not take into account things like the cost of electricity required to recharge, the high cost of replacement batteries or the initial purchase cost and resale value.

Total cost of ownership is a consideration that should also be taken into account when making decisions on fuel efficiency and vehicle purchases. Similarly, the heavier the load that the vehicle is required to carry, the higher the fuel consumption. Fuel economy is a measure of how far a vehicle will travel with a gallon of fuel; it is expressed in miles per gallon.

This is a popular measure used for a long time by consumers in the United States; it is used also by vehicle manufacturers and regulators, mostly to communicate with the public. As a metric, fuel economy actually measures distance traveled per unit of fuel. Fuel consumption is the inverse of fuel economy.

It is the amount of fuel consumed in driving a given distance. It is measured in the United States in gallons per miles, and in liters per kilometers in Europe and elsewhere throughout the world. Fuel consumption is a fundamental engineering measure that is directly related to fuel consumed per miles and is useful because it can be employed as a direct measure of volumetric fuel savings. It is actually fuel consumption.

The details of this calculation are shown in Appendix E. Fuel consumption is also the appropriate metric for determining the yearly fuel savings if one goes from a vehicle with a given fuel consumption to one with a lower fuel consumption. Because fuel economy and fuel consumption are reciprocal, each of the two metrics can be computed in a straight-forward manner if the other is known.

This relationship is not linear, as illustrated by Figure 2. Also shown in the figure is the decreasing influence on fuel savings that accompanies increasing the fuel economy of high-mpg vehicles.

Each bar represents an increase of fuel economy by percent or the corresponding decrease in fuel consumption by 50 percent.

The data on the graph show the resulting decrease in fuel consumption per miles and the total fuel saved in driving 10, miles. The dramatic decrease in the impact of increasing miles per gallon by percent for a high-mpg vehicle is most visible in the case of increasing the miles per gallon rating from 40 mpg to 80 mpg, where the total fuel saved in driving 10, miles is only gallons, compared to gallons for a change from 10 mpg to 20 mpg.

Likewise, it is instructive to compare the same absolute value of fuel economy changes—for example, mpg and mpg. The mpg fuel saved in driving 10, miles would be 50 gallons, as compared to the gallons in going from mpg. Appendix E discusses further implications of the relationship between fuel consumption and fuel economy for various fuel economy values, and particularly for those greater than 40 mpg.

Figure 2. Figures 2. Because of the nonlinear relationship in Figure 2. Larrick and Soll further conducted three experiments to test whether people reason in a linear but incorrect manner about fuel economy. These experimental studies demonstrated a systemic misunderstanding of fuel economy as a measure of fuel efficiency.

Using linear reasoning about fuel economy leads people to undervalue small improvements mpg in lower-fuel-economy mpg range vehicles where there are large decreases in fuel consumption Larrick and Soll, in this range, as shown in Figure 2. Fischer further discusses the potential benefits of utilizing a metric based on fuel consumption as a means to aid consumers in calculating fuel and cost savings resulting from improved vehicle fuel efficiency.

Throughout this report, fuel consumption is used as the metric owing to its fundamental characteristic and its suitability for judging fuel savings by consumers.

In cases where the committee has used fuel economy data from the. The width of each rectangle represents a 50 percent decrease in FC or a percent increase in FE.

The number within the rectangle is the decrease in FC per miles, and the number to the right of the rectangle is the total fuel saved over 10, miles by the corresponding 50 percent decrease in FC. Because of this, the committee recommends that the fuel economy information sticker on new cars and trucks should include fuel consumption data in addition to the fuel economy data so that consumers can be familiar with this fundamental metric since fuel consumption difference between two vehicles relates directly to fuel savings.

The fuel consumption metric is also more directly related to overall emissions of carbon dioxide than is the fuel economy metric. Motor vehicles have been powered by gasoline, diesel, steam, gas turbine, and Stirling engines as well as by electric and hydraulic motors. These internal combustion engines are of two types: gasoline spark-ignition and diesel compression-ignition.

The discussion also addresses alternative power trains, including hybrid electrics. Gasoline engines, which operate on a relatively volatile fuel, also go by the name Otto cycle engines after the person who is credited with building the first working four-stroke internal combustion engine.

Over the years, variations of the conventional operating cycle of gasoline engines have been proposed. A recently popular variation is the Atkinson cycle, which relies on changes in valve timing to improve efficiency at the expense of lower peak power capability.

This report uses the generic term compression-ignition engines to refer to diesel engines. The distinction between these two types of engines is changing with the development of engines having some of the characteristics of both the Otto and the diesel cycles.

Although technologies to implement homogeneous charge compression ignition HCCI will most likely not be available until beyond the time horizon of this report, the use of a homogeneous mixture in a diesel cycle confers the characteristic of the Otto cycle. Likewise the present widespread use of direct injection in gasoline engines confers some of the characteristics of the diesel cycle. In a conventional vehicle propelled by an internal combustion engine, either SI or CI, most of the energy in the fuel goes to the exhaust and to the coolant radiator , with about a quarter of the energy doing mechanical work to propel the vehicle.

This is partially due to the fact that both engine types have thermodynamic limitations, but it is also because in a given drive schedule the engine has to provide power over a range of speeds and loads; it rarely operates at its most efficient point.

This is illustrated by Figure 2. It plots the engine efficiency as functions of torque and speed. The plot in Figure 2. In conventional vehicles, however, the engine needs to cover. One way to improve efficiency is to use a smaller engine and to use a turbocharger to increase its power output back to its original level. This reduces friction in both SI and CI engines as well as pumping losses. Other methods to expand the high-efficiency operating region of the engine, particularly in the lower torque region, are discussed in Chapters 4 and 5.

As discussed in Chapter 6 , part of the reason that hybrid electric vehicles show lower fuel consumption is that they permit the internal combustion engine to operate at more efficient speed-load points. The monitoring of engine and emission control parameters by the onboard diagnostic system identifies emission control system malfunctions.

A more recent development in propulsion systems is to add one or two electrical machines and a battery to create a hybrid vehicle. Such vehicles can permit the internal combustion engine to shut down when the vehicle is stopped and allow brake energy to be recovered and stored for later use. Hybrid systems also enable the engine to be downsized and to operate at more efficient operating points. Although there were hybrid vehicles in production in the s, they could not compete with conventional internal combustion engines.

What has changed is the greater need to reduce fuel consumption and the developments in controls, batteries, and electric drives. Hybrids are discussed in Chapter 6 , but it is safe to say that the long-term future of motor vehicle propulsion may likely include advanced combustion engines, combustion engine-electric hybrids, electric plug-in hybrids, hydrogen fuel cell electric hybrids, battery electrics, and more.

The challenge of the next generation of propulsion systems depends not only on the development of the propulsion technology but also on the associated fuel or energy infrastructure. The large capital investment in manufacturing capacity, the motor vehicle fleet, and the associated fuel infrastructure all constrain the rate of transition to new technologies. The combustion process within internal combustion engines is critical for understanding the performance of SI versus CI engines.

SI-engine combustion occurs mainly by turbulent flame propagation, and as turbulence intensity. A more detailed explanation is provided in Chapter 4 of this report. Thus, combustion characteristics have little effect on the ability of this type of engine to operate successfully at high speeds. Therefore, this type of engine tends to have high power density e. CI engine combustion is governed largely by means of the processes of spray atomization, vaporization, turbulent diffusion, and molecular diffusion.

Therefore, CI combustion, in comparison with SI combustion, is less impacted by engine speed. As engine speed increases, the combustion interval in the crank-angle domain also increases and thus delays the end of combustion. This late end of combustion delays burnout of the particulates that are the last to form, subjecting these particulates to thermal quenching.

The consequence of this quenching process is that particulate emissions become problematic at engine speeds well below those associated with peak power in SI engines. This ultimately limits the power density i. While power density gets much attention, torque density in many ways is more relevant.

Thermal auto ignition in SI engines is the process that limits torque density and fuel efficiency potential. This type of combustion is typically referred to as engine knock, or simply knock. If this process occurs prior to spark ignition, it is referred to as pre-ignition. This is typically observed at high power settings. Knock and pre-ignition are to be avoided, as they both lead to very high rates of combustion pressure and ultimately to component failure.

While approaches such as turbocharging and direct injection of SI engines alter this picture somewhat, the fundamentals remain. CI diesel engines, however, are not knock limited and have excellent torque characteristics at low engine speed. That is, at equal engine displacement, the turbocharged diesel tends to deliver superior vehicle launch performance as compared with that of its naturally aspirated SI engine counterpart.

The fuels and the SI and CI engines that use them have co-evolved in the past years in response to improved technology and customer demands. Engine efficiencies have improved due to better fuels, and refineries are able to provide the fuels demanded by modern engines at a lower cost. Thus, the potential for fuel economy improvement may depend on fuel attributes as well as on engine technology.

Implementing certain engine technologies may require changes in fuel properties, and vice versa. Although the committee charge is not to assess alternative liquid fuels such as ethanol or coal-derived liquids that might replace gasoline or diesel fuels, it is within the committee charge to consider fuels and the properties of fuels as they pertain to implementing the fuel economy technologies discussed within this report.

Early engines burned coal and vegetable oils, but their use was very limited until the discovery and exploitation of inexpensive petroleum. The lighter, more volatile fraction of petroleum, called gasoline, was relatively easy to burn and met the early needs of the SI engine.

A heavier, less volatile fraction, called distillate, which was slower to burn, met the early needs of the CI engine. The power and efficiency of early SI engines were limited by the low compression ratios required for resistance to pre-ignition or knocking. This limitation had been addressed by adding a lead additive commonly known as tetraethyl lead.

With the need to remove lead because of its detrimental effect on catalytic aftertreatment and the negative environmental and human impacts of lead , knock resistance was provided by further changing the organic composition of the fuel and initially by reducing the compression ratio and hence the octane requirement of the engine. Subsequently, a better understanding of engine combustion and better engine design and control allowed increasing the compression ratios back to and eventually higher than the pre-lead-removal levels.

The recent reduction of fuel sulfur levels to less than 15 parts per million ppm levels enabled more effective and durable exhaust aftertreatment devices on both SI and CI engines. The main properties that affect fuel consumption in engines are shown in Table 2. The table shows that, on a volume basis, diesel has a higher energy content, called heat of combustion, and higher carbon content than gasoline; thus, on a per gallon basis diesel produces almost 15 percent more CO 2.

However, on a weight basis the heat of combustion of diesel and gasoline is about the same, and so is the carbon content. One needs to keep in mind that this difference in energy content is one of the reasons why CI engines have lower fuel consumption when measured in terms of gallons rather than in terms of weight. Processing crude oil into fuels for vehicles is a complex process that uses hydrogen to break. This is commonly called cracking.

So if one wants to minimize the barrels of crude oil used per miles, diesel would be a better choice than gasoline. Ethanol as a fuel for SI engines is receiving much attention as a means of reducing dependence on imported petroleum and also of producing less greenhouse gas GHG. Today ethanol is blended with gasoline at about 10 percent. Proponents of ethanol would like to see the greater availability of a fuel called E85, which is a blend of 85 percent ethanol and 15 percent gasoline.

The use of percent ethanol is widespread in Brazil, but it is unlikely to be used in the United States because engines have difficulty starting in cold weather with this fuel. The effectiveness of ethanol in reducing GHG is a controversial subject that is not addressed here, since it generally does not affect the technologies discussed in this report. It is interesting to note that in a very early period of gasoline shortage, it was touted as a fuel of the future Foljambe, Ethanol has about 65 percent of the heat of combustion of gasoline, so the fuel consumption is roughly 50 percent higher as measured in gallons per miles.

Ethanol has a higher octane rating than that of gasoline, and this is often cited as an advantage. Normally high octane enables increases in the compression ratio and hence efficiency. To take advantage of this form of efficiency increase, the engine would need to be redesigned to accommodate an increased combustion ratio. For technical reasons the improvement with ethanol is very small.

Also, during any transition period, vehicles that run on 85 to percent ethanol must also run on gasoline, and since the compression ratio cannot be changed after the engine is built, the higher octane rating of ethanol fuel has not led to gains in efficiency.

A new clutch has minimal slippage tendency but this increases as the clutch begin wearing out with time. This results in a loss in the overall power that the wheels receive from the engine and thereby decreases the transmission efficiency which leads to car consuming more fuel and delivering low fuel mileage. Overloading Last on the list, but this one can also lead your car to consume more fuel and result in delivering poor fuel mileage.

Do you know that some of the cars that have racks installed on their roofs also deliver poor fuel mileage? Yes, all the cars having load-carrying roof racks are not meant to have those! The roof racks badly affect the airflow when the car is running and result in poor fuel mileage.

So this was all for the factors which make your car consume more fuel and result in low fuel efficiency or mileage. Some of these can be improved by changing our daily practices but some need serious attention from a trained technician.

Make sure you keep a check on your car and notice the decreased fuel efficiency at early stages so that it can be sorted out easily. The main question now is how will you know that your car is consuming more fuel? The answer to this is really simple. All the modern-day cars come with an advanced MID that displays the mileage of the car. This system installed in your car is p[rogrammed through an ECU and is extremely trustworthy.

This figure changes according to how when and where you are driving. On highways, you can extract better mileage from your car as you need to shift less and the overall momentum of the vehicle remains conserved. Talking about the city driving conditions, it is quite normal to get fuel efficiency lower than what you got on the highway. Your car will consume more fuel in city drives than it does on the highway for sure.

But if this figure drops significantly, you surely need to get your vehicle checked. For all the people out there having cars like these, calculating the efficiency is really simple. Just get a tank full of fuel and before you set off, switch the metre to a separate trip and start from 0. This way you will know how many kilometres your car travelled with a tank full of fuel.

The calculation is really simple after this. This site uses Akismet to reduce spam. Learn how your comment data is processed. Sign in. Log into your account. Forgot your password? Create an account. Sign up.