Fuel Economy

A low-energy vehicle is any type of vehicle that uses less energy than a regular vehicle. The higher efficiency can be achieved by changing the vehicle's design, not only powertrain modifications. The biggest influence on the efficiency however is not the engineering quality but the vehicle specification such as top speed, safety reserves and load capacity.

Motivation

If we reduce energy demand, we reduce access conflicts to oil reserves and/or environmental damage when trying to produce fuel from natural or other fossil sources. Existing published consumption figures tend to underestimate the consumption seen in practice by 20 to 30% (Fuel economy in automobiles). The reason is that auto makers use every available trick to produce low looking fuel consumption figures to counter the motivation to abandon existing vehicle concepts. The promoters of LEnVs often exaggerate the likely range and performance envelope of their vehicles for much the same reason - marketing. Even the most fuel efficient two seater on the market - the Smart MHD consumes two times more energy per km than a cabin based ultralight two seater would. Pilot vehicles have proven that a feasible target may lie in the range of 1-2 l/100km, or lower, or 10 kWh/100 km electricity, for vehicles that do not meet current requirements for range, crashworthiness, passenger comfort or speed.

Available electric LEVs already use about ten times less energy than available cars, e.g. 4–8 kW·h/100 km (from 230 V AC grid) for the Twike.^[1] Here the challenges are increasing range and lifetime of batteries, crashworthiness, passenger comfort, and reducing the price.

Energy Efficiency in MJ per km: It is prefered to express energy efficiency in MJ (Mega-Joule) per km because terms like MPG (Miles Per Gallon) and Liters per km do not take into account what type of fuel is used and thus the numbers will be distorted for different fuel types. Diesel contains 38.7MJ per liter, Gasoline 34.6MJ per liter and Bio-Diesel 30.5MJ per liter, whereas LPG contains only 22.2MJ per liter which is why you will see the number of liters consumed go up drastically when coverting a gasoline car to LPG. This does not mean that the energy consumption goes up; it only means that there is less energy in a liter of LPG. Ethanol also contains much less energy per liter than gasoline.

Preconditions

Technological support for low energy operation may also come from driver assistance systems since driving style can be adapted to achieve lower energy consumption. Energy management becomes possible with hybrid vehicles with the possibility to recuperate braking energy and to operate the internal combustion engine (ICE) at higher efficiency on average. Hybrid power trains may also reduce the ICE-engine size thus increasing the average load factor and minimising the part load losses. Purely electric vehicles use up to 10 x less energy (0,3 to 0,5MJ/km)than those with combustion engines (3 to 5MJ/km and up to 10MJ/km for SUVs) because of the much higher motor and battery efficiencies. However, maximum ranges are much less because of the low energy density of electrochemical storage batteries compared to chemical fuels.

Facts

Some newer examples of efficient commercially available internal combustion-propelled vehicles:

Average data for vehicle types sold in the U.S.A.^[3] compared to an advanced vehicle concept, the Honda Insight:

Drag resistance for SUVs is at least (same drag coefficient) 30% higher and the acceleration force has to be 35% bigger compared to family sedans. This gives of 40% higher fuel consumptions (even when including parallel hybrid electric SUVs).

Reality

In practice we have not seen much on the market focusing on low propulsion energy demand. The last good examples were the GM EV-1 and the Honda Insight. There are however several relevant scientific competitions such as the Shell Eco-Marathon, Automotive X Prize and Solar car racing.

In the near future several low energy vehicles may be in production, including the Aptera Motors Typ-1 with three wheels and a claimed C_d of 0.11, due in late 2008, and the Loremo.

Triggered by congestion charging in London and other environmental incentives in Europe a few Battery electric vehicles have arrived on the market, such REVA G-Wiz and the MEGA City NICE.

Measures

The EU- sponsored RECODRIVE project^[5] has set up a quality circle to manage low energy consumption in fleets. This starts with energy aware procurement, and includes fuel management, driver information and training and incentives for all staff involved in the fleet management and maintenance process. Vehicle equipped with gear shift indicators, tire pressure monitoring systems and downsized internal combustion engines and for stop'n go operation also hybrid electric power trains will help to save fuel.

Fuel economy-maximizing behaviors

Fuel economy-maximizing behaviors describe techniques that drivers can use to optimize their automobile fuel economy. The energy in fuel consumed in driving is lost in many ways, including engine inefficiency, aerodynamic drag, rolling friction, potential energy required to climb hills, and kinetic energy lost to braking (absent regenerative braking). Driver behavior can influence all of these. The city mileage of conventional cars is much lower than highway mileage due to: 1) a high proportion of idling time, 2) operation mostly at very inefficient low-output engine operating points, and 3) more frequent braking.

Terminology

Various terms describe drivers using unusual driving techniques to maximize fuel efficiency. A few of these are:

Techniques used to maximize fuel economy

Techniques used to improve fuel economy include general techniques that can be applied to most driving, and specialized techniques that are more narrowly applicable, but can be used to achieve extremely high mileage.

General techniques

One of the best ways to optimize mileage (both hybrid and non-hybrid) is to keep up with vehicle maintenance.^[1] Key parameters to maintain are tire pressure, tire balance and wheel alignment, and proper motor oil weight and level. Inflating tires to the maximum recommended air pressure ensures that less energy is required to move the car. Under-inflated tires can lower gas mileage by approximately 1.4 percent for every 1 psi drop in pressure of all four tires per gas tank. ^[5] Equally important is the proper maintenance of the Engine Control Module and all sensors it relies on to control engine operation such as oxygen sensors.

Beyond purchasing smaller vehicles, drivers can also increase fuel economy by minimizing the amount of luggage, tools, and equipment carried in the car, including such things as unneeded snow chains in the summer and outdoor sporting equipment in the winter.

Maintaining an efficient speed is also very effective in keeping mileage up.^[1] Optimal efficiency can be expected while cruising with no stops, at minimal throttle and with the transmission in the highest gear. Every car has a different optimum speed, although it is usually reported to be in the range of 35 to 55 mph^[6]^[7] ^[8]. For instance a 2004 Chevrolet Impala had an optimum at 70 kph, and was within 15% of that from 45 to 95 kph (roughly 25 to 55 mph)^[6]. Drivers of cars with fuel-economy displays can their own cars by cruising at different speeds (if safe and prudent to do so) and checking the readout.

Gentle acceleration is often recommended. This is primarily helpful in avoiding unnecessary acceleration, which might lead to inefficient high speeds or might lead to a need to brake a short time later. Brakes are designed to dissipate energy and should be avoided whenever possible, by anticipating what is happening ahead, not driving so fast, and slowing down by placing the gear in neutral, or hitting the clutch (this is illegal in many jurisdictions, and may annoy other road users). On the US standard highway cycle roughly 2% of the fuel's energy is wasted in braking, whereas on the urban cycle 6% is lost.^[9]

For a given target speed, more rapid acceleration can be more efficient because the engine is more efficient at larger throttle openings^[10] However, it is also important to keep the engine RPM in an efficient range, so rapid acceleration also requires rapid up-shifting. Rapid up-shifting is possible with a manual transmission, but some automatic transmissions avoid up-shifting when the throttle is wide open, negating the efficiency benefits of large throttle openings.

It is commonly believed that efficiency of a gasoline engine is related to the fuel's octane level; however, this is not true in most situations. Octane rating is only a measure of the fuel's propensity to cause an engine to "ping", this ping is due to pre-combustion, pre-combustion is caused by the fuel burning too rapidly (before the piston reaches top dead center). Higher octane fuels burn more uniformly at high pressures (instead of burning nearly instantaneously across the mass, which is equivalent to "exploding"), thus produce less ping. For the vast majority of vehicles (i.e. vehicles with "standard" compression ratios), standard octane fuel will work fine and not cause pinging. Using high octane fuel in a vehicle that does not need it is simply wasting money^[11], although Toyota have measured slight differences in efficiency due to octane number even when knock is not an issue ^[12]. Most cars fitted with emissions systems have sensors that will automatically adjust the timing, when ping is detected, so low octane fuel can be used even if the engine is designed for high octane, at some cost in efficiency. If the engine is designed for high octane then higher octane fuel will result in higher performance (with full-open throttle), but not necessarily fuel cost savings, since the high-octane is only needed with the throttle fully open. For other cars that have problems with ping, it may be due to a maintenance problem, such as carbon buildup inside the cylinder or incorrect spark plug tip length. In such cases, higher octane fuel may help, but this is an expensive fix, proper repair might make more long term sense. There is slightly less energy in a gallon of high octane fuel, than low octane^[13]. Ping is very bad for an engine and if this condition exists, it will decrease fuel economy and will damage the engine in the long term.

While modern hybrids come with built-in trip computers which display real-time MPG, which helps the driver adjust driving habits, most gasoline powered vehicles do not have this as a standard option (although some luxury cars do). However, this information is available with an add-on device that connects to the car's onboard computer (post 1996 for most vehicles), such as the ScanGauge.^[14]. This information is helpful in allowing the driver to see in real-time how driving techniques affect gas mileage.^[1]

Specialized techniques

This method consists of accelerating to a given speed (the "pulse"), followed by a period of coasting (the "glide"), and then repeating the process. The glide is most efficient when the engine is not running. Because some cars inject extra fuel when the starter is activated, this was originally best accomplished with a manual transmission.^[15] Hybrid vehicles, such as the Toyota Prius, are ideally suited to performing this technique as well: the internal combustion engine as well as the charging system can, be shut off for the glide by simply manipulating the accelerator.^[16]

Some hybrids must keep the engine running whenever the vehicle is in motion and the transmission engaged, although they still have an "auto-stop" feature which engages when the vehicle stops, avoiding waste. Maximizing use of autostop is critical on these vehicles because idling causes a severe drop in accumulated mileage (0 miles per gallon). In addition, many hypermilers will actually turn off their cars entirely (a "forced stop") or put them into neutral when going down hills or in other situations when momentum will carry the car on its own.^[1]

Draft-assisted forced stop, a variation of the forced (auto)stop (sometimes abbreviated as D-FAS), involves turning off the engine and gliding in neutral while tailgating a larger vehicle, in order to take advantage of the reduced wind resistance in its immediate wake (This practice is illegal in some areas due to its danger); while tailgating itself is inherently risky, the danger of collision is increased with D-FAS as power for power brakes can be lost after a few applications of the brake pedal and the pressure that causes power steering to function can be lost as well.^[17]

Hybrid and electric engines

The most effective commonly available hybrid vehicles in the hypermilage marathons are the Honda Insight Hybrid, the Toyota Prius Hybrid, and the Honda Civic Hybrid. Other hybrids have also done very well. Some historical non-hybrid vehicles such as the Honda Civic CR-X HF and the Smart Fortwo have also done remarkably well on mileage. The Toyota and Ford hybrids use two motor generators called a series-parallel hybrid with unique characteristics different from the single motor generators of the Honda and GM hybrids (as of January 2007). The Honda motor generator is integrated with the engine, the Integrated Motor Assist (IMA) that enhances the low-end torque of the engine. The current GM hybrids turn-off the engine at a stop and restart it when ready to leave.

The Toyota and Ford hybrids have a threshold speed—around 42 mph in the case of the Prius—above which the engine must run to protect the transmission system. Below this model-dependent speed, the car will automatically switch between either battery-powered mode or engine power with battery recharge. These hybrids typically get their best fuel efficiency below this model dependent threshold speed. Coasting can be achieved by using Neutral transmission range. The Honda IMA vehicles have a limited, battery-only, powered capability, although after-market modding has made the Insight capable of running in electric only-mode [7]. They achieve higher fuel economy [8]. Another way to save fuel includes turning off the engine on manual transmission vehicles when coasting.

The GM hybrids have an engine auto-stop when halted. As of January 2007, they have no battery-only, powered capability. In late 2007, GM will introduce two two-mode hybrid, full-size SUVs, which can be powered by electric motors, V8 engines, or a combination of both.

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