Fuel economy is measured on a driving cycle for testing compliance of the vehicles with regulations. The US, Europe and Japan have developed their own test procedures to measure vehicle emissions and fuel economy. Other countries have adopted these procedures, sometimes with modifications to suit their driving conditions. The driving cycle used is designed to represent the actual driving pattern on road. The driving cycles generally, give appropriate weightage to the city and highway type of driving patterns in their own country or region.
Fuel Economy Test Cycles used in the USA, Europe and Japan
The US and European cycles are composed of a driving schedule that represents city driving pattern and another representing the highway driving. In the US CAFE standards, 55% weightage is given to the US city driving cycle and 45% to the highway driving cycle when determining fuel economy. Europe uses the same cycle as the one used for emission measurement. The cycle is called New European Driving Cycle (NEDC). In Japan for the current standards, 10-15 mode test cycle would be used. From the model year 2015 when new regulations would come in force a new cycle JC08 cycle would be employed.
As the vehicle emission and fuel consumption levels are sensitive to the driving pattern, the fuel economy levels for a given vehicle would be different when measured using different driving cycles specified in different countries. The Japan test cycles are the most stringent tests of all the three. The CAFE test is least stringent as it has higher component of driving at moderately high speeds where the engines generally have higher fuel efficiency.
Test Cycle FE
Arithmetic Average *
Arithmetic Average of 6 cars**
Arithmetic of 5 vehicles**
The NEDC is nearly 12 % more severe than the CAFE test procedure.
Major fuel economy improvement technologies being pursued and implemented on modern gasoline vehicles, but not every manufacturer uses these technologies.
Vehicle when moving on road has to overcome rolling resistance caused by the friction between wheels and road, and air drag besides the electric power required by a variety of vehicle accessories that are essential as well as those required for comfort. The rolling resistance depends on vehicle weight and friction coefficient between tires and road surface. The rolling resistance depends on the quality of road surface and, the tire design and material. The rolling resistance increases with vehicle speed, but weakly. The air drag however, increases in proportion to the square of vehicle speed, and engine power required to overcome air resistance increases in proportion to the cube of vehicle speed. Therefore, reduction in the vehicle weight and air drag is important to reduce the vehicle power demand.
The vehicle weight can be reduced by use of light weight engineering materials like aluminum and magnesium alloys, and plastics wherever possible. It is estimated that a reduction of 100 kg in weight of the average European car would reduce fuel consumption by 0.2 liters/100 km. For a typical midsize of present day European car, it works out to be around 0.45 km/l or 3% improvement in fuel economy. For the North American SUVs, a reduction of 20 to 30 kg (40 to 70 pounds) in vehicle weight is expected to improve vehicle fuel economy by 0.10 mpg. The aerodynamic styling of the body and the resulting Air Drag Coefficient is also very important. A reduction of 10% in drag coefficient can decrease fuel consumption by 2 to 3.5 % depending upon optimization of the transmission train. The next factor is the Vehicle rolling resistance. It depends on vehicle weight and the coefficient of friction between tires and road at a given speed. In larger SUV type vehicles use of crankshaft mounted starter cum alternator is an emerging technology. Also, a 24 V system has been found to be more efficient than the conventional 12/14 V electrical system, improving the fuel economy by 1 to 2%. In the larger vehicles, power steering need more power hence 24 V system becomes necessary.
Power consumption, kW
Power Consumption, kW
Now let’s talk about engine technologies. In the USA, the passenger cars and light duty vehicles like SUVs and MUVs are powered mostly by the gasoline engines. In Europe on the other Hand it is about 50/50 Gasoline to Diesel. The diesel vehicles have a higher fuel economy than the gasoline vehicles. The diesel engines operate unthrottled, have a higher compression ratio and operate lean. These factors result in a much lower specific fuel consumption (fuel consumed per kWh of work) of diesel engines compared to the gasoline engines. The diesel engine combustion is also best suited for supercharging and turbo charging, which further reduces its brake specific fuel consumption. The gasoline engine has more potential for improvement than the diesel engine and this is being exploited through several technological improvements.
Both Engine types can use a 4-valve configuration, which improves the fuel economy 2% to 5% compared to a 2 valves per cylinder engine. Variable valve timings and valve lift makes it possible to improve efficiency of charging engine with fresh mixture and reduce pumping work at part loads while retaining good high speed engine performance. Variable valve timings (VVT) and valve lift makes it possible to improve efficiency of charging engine with fresh mixture and reduce pumping work at part loads while retaining good high speed engine performance. With this technology, the fuel economy gains of the order of 3 to 5% are obtained. The Gasoline Direct Injection should be a standard today, but it is not. The benefits are between 6% to 17%. Using an engine with smaller swept volume is another way to improve fuel economy. The smaller engine has lower friction losses and during city driving it would operate closer to best efficiency point resulting in improvement in fuel economy. The engine power is boosted by turbo charging /super charging when it is required at high speed driving. If Downsizing is not implementable the car should at least have a cylinder deactivation system (6 and 8 or more cylinders). Also the replacement of crankshaft mounted or belt driven parts such as water pumps or oil pumps with electric pumps has a potential. Stop-Start Systems combined with a Starter-Generator System (starter and generator is the same part) reduces fuel consumption by 5% to 8%. These things are all already developed and ready to implement, but not every manufacturer does it consequent. The next possible improvement is at the power train. Manual Transmissions who are shifted hydraulically (they act like an automatic transmission) eliminate the high volume of transmission fluid and are more fuel efficient.
Hybrid electric vehicles (HEV) employ two propulsion systems; an IC engine and a battery powered electric motor. Presently, two types of HEV considered are
Plug-in Hybrids: These HEVs during city driving operate on storage batteries. The batteries have a limited range of operation of about 80 to 100 km sufficient for driving in a single day. The batteries are recharged by plugging-in them to the main household electricity supply system. On highways or if the batteries are discharged during city operation the vehicle is powered by the IC engine. In the city, vehicle operation involves a large fraction of idling, stop, start and low load operation when the efficiency of the IC engine is very low. The Plug-in Hybrids insulate IC engine from operation in city, an operation regime of low fuel economy thus overall fuel economy of vehicle is improved.
Full Hybrids: In full hybrids the IC engine is normally cut-off from the wheels. The propulsion batteries on board are continuously charged by the IC engine. Vehicle propulsion in series hybrids is entirely from the battery powered motor. In parallel hybrids engine provides traction simultaneously when power demands are higher like during acceleration mode.toyota.PNG
Modern turbo diesel powertrains already match petrol-electric hybrids in fuel-economy and often outmatches them when it comes to drivability and performance. A combination of the two separate technologies creates an
could provide a perfect blend of performance and economy. French auto giant PSA Peugeot-Citroen group is at the forefront of the technology, already previewing a number of concept vehicles packing diesel engines and electric motors under its Hymotion4 moniker.