Legislation drive cycles are standard drive cycles that all mass produced cars are subjected to before being authorised for sale in a particular market. The total mass of emissions produced during a particular drive cycle must be below a set limit decided by the legislating authority. While many standardised drive cycles exist throughout the world (including special cycles used for research only), the most common ones are the cycles used by the U.S. Environmental Protection Agency (EPA) and the European ECE. In the U.S.A., federal emissions standards are set by the EPA whereas Californian standards are normally more stringent and are set by the Air Resources Board (ARB). European laws are developed and enforced by the following institutions:
  • European Parliament : elected by the peoples of the Member States.
  • Council : representing the governments of the Member States. The Council of Environment Ministers oversees the area of environmental regulations.
  • Commission : the executive and the body having the right to initiate legislation.

The EPA has a number of drive cycles used for various legislation purposes. The drive cycle used for emission certification testing of cars and light duty trucks is the Federal Test Procedures 75 (FTP75). This cycle is a compilation of various real-world driving routes performed on the streets of Los Angeles in California.

The FTP75 consists of 3 phases as can be seen in the figure. The cold phase is done to simulate starting your car in the morning and then driving almost immediately onto a highway. While this is feasible for a suburban home, for most city dwellers, one tends to drive out of his/her driveway and into a lot of stationary traffic. This is one of the problems of the FTP since it subjects the car to a relatively high load (thus exhaust gas temperatures) from the get-go and allows the catalyst to warm up in a time that is not representative of a lot real world driving.

The hot phase of the FTP is done 10 minutes after the end of the transient phase. This intends to simulate parking a car and then returning to it after a short period of time. It's probably clear that the hot phase is simply a repeat of the cold phase in terms of speed-time.

Even though the FTP75 seems to be representative of real-world driving, once the accelerations are calculated it is found that they are much lower than most of the driving being done in today's cars on todays road networks. One reason high accelerations were avoided back in 1975 when the FTP was designed was the fact that tire slip would occur on the rollers of the chassis dynamometers used for certification.

 

The emissions from each phase of the FTP75 are collected in a separate teflon bag, analysed and expressed in g/mile. The weighting factors are 0.43 for the cold start, 1.0 for the transient phase and 0.57 for the hot start phase. The total emissions during the FTP75 must meet a ceratin level as shown in the table below. The federal and Californian requirements are usually different since California has historically had smog problems and therefore took the early initiative and placed stringent requirements from car manufacturers. The U.S. emissions standards system is very complicated with tiers, service life levels and temporary bins for vehicle classification etc, but the most recent and stringent California standards are shown here.

California LEV II Emission Standards, Passenger Cars and LDVs < 8500 lbs, g/mi :

Category
50,000 miles / 5 years
120,000 miles/11 years
 
NMOG
CO
NOx
PM
HCHO
NMOG
CO
NOx
PM
HCHO
LEV
0.075
3.4
0.05
-
0.015
0.090
4.2
0.07
0.01
0.018
ULEV
0.040
1.7
0.05
-
0.008
0.055
2.1
0.07
0.01
0.011
SULEV
-
-
-
-
-
0.010
1.0
0.02
0.01
0.004

 


The European drive cycle is composed of the ECE (shown in the top figure on the left) and a recently introduced Extra Urban Drive Cycle (EUDC; bottom left figure). For emissions certification, the ECE is repeated 4 times and then the EUDC once. All this is done back to back on the chassis dyanomometer.

Compared the FTP, the ECE+EUDC drive cycle does not warm up the catalyst as fast since the engine operates at a low-load, low exhaust gas temperature condition for a lot of the time after startign up.

Previously, the vehicle was allowed to soak for at least 6 hours at a test temperature of 20-30°C. It was then started and allowed to idle for 40s. Effective year 2000, that idling period has been eliminated, i.e. engine starts at 0s and the emission sampling begins at the same time. This modified cold-start procedure is sometimes referred to as the “new European driving cycle” or NEDC.

The problem with the ECE+EUDC is that the cycle is very standardised and therfore not representative of real-world driving where rates of acceleration tend to be a lot higher and motorway speeds can exceed 120kph for extended periods of time.

Another commonly used drive cycle in the U.S.A. is the Inspection & Maintenance 240 (IM240). Enforced in many states, this is the EPA's way of checking that in-use vehicles are still meeting certain emissions standards even after they leave the showroom floor. Improper maintenance or illegal modifications to the engine will cause the car to fail the IM240 and subsequently be declared not legal for road use until it does meet IM240 standards. The IM240 drive cycle is basically the first 240 seconds of the FTP75. It is conducted on a chassis dynamometer at any number of privately run inspection centres around the U.S.A.

Emissions are sampled during the cycle according the the “Constant Volume Sampling” technique, analyzed, and expressed in g/km for each of the pollutants. The European system for certification is a lot simpler as can ben seend in the table below. Euro 1 through Euro 4 standards for petrol and diesel cars.

EU Emission Standards for Passenger Cars (Category M), g/km
Tier Year CO HC HC+NOx NOx PM
Diesel
Euro 1†
1992.07
2.72 (3.16)
-
0.97 (1.13)
-
0.14 (0.18)
Euro 2, IDI
1996.01
1.0
-
0.7
-
0.08
Euro 2, DI
1996.01a
1.0
-
0.9
-
0.10
Euro 3
2000.01
0.64
-
0.56
0.50
0.05
Euro 4
2005.01
0.50
-
0.30
0.25
0.025
Petrol
Euro 1†
1992.07
2.72 (3.16)
-
0.97 (1.13)
-
-
Euro 2
1996.01
2.2
-
0.5
-
-
Euro 3
2000.01
2.30
0.20
-
0.15
-
Euro 4
2005.01
1.0
0.10
-
0.08
-
† Values in brackets are conformity of production (COP) limits.
a - until 1999.09.30 (after that date DI engines must meet the IDI limits)

 

Many more drive cycles can be seen at this website, including the newly introduced SC03 and US06 supplemental FTP (SFTP) drive cycles. These cycles were added to overcome some shortcomings of the FTP. The SC03 simulates the use of air conditioning units and the US06 simulates aggressive highway driving.

Carbon Dioxide

CO2 is a direct result of complete combustion as shown by the simplified combustion reaction in equation below:

HC + O2 --> CO2 + H2O

This implies that the more fuel is consumed, the more CO2 is produced, especially with today's cars that produce very little CO emissions. Previously there was little concern for the emission of CO2 since it was considered harmless to the atmosphere. In recent times with the increasing awareness of greenhouse gases and the fear of looming global warming, efforts are being made to reduce CO2 even though it is not as potent a GHG (Green House Gas) as some other compounds such as N2O. Europe has made a commitment in the Kyoto protocol to reduce GHG's by 8% by 2008/2012.

The UK has a legally binding commitment under the Kyoto protocol to reduce greenhouse gas emissions by 12.5% by 2008/2012 against the 1990 levels and a domestic goal of reducing CO2 emissions by 20% by 2010. Road transport is responsible for 22% of the UK greenhouse gas emissions. In 2002 the UK set a scheme to curb CO2 emissions by taxing company cars based on the amount of CO2 emissions they produce. Different tax rates are payable on different CO2 levels, ranging from 15% of list price for the cleaner cars all the way to 35% of list price for the dirtier models. The standards will get stricter every year and so cars will be taxed more as they age.

For the time being the U.S.A. is not too worried about CO2 emissions and their cars see to sprouting bigger more powerful engines every year. A bill was recently introduced in California which will possibly lead to some sort of CO2 control measures by 2008. In the meantime the only CO2 control measure in place is the Corporate Average Fuel Economy (CAFE). The Energy Policy and Conservation Act of 1975 required passenger car and light truck manufacturers to meet CAFE standards. The CAFE standards are applied on a fleet-wide basis for each manufacturer; i.e., the fuel economy ratings for a manufacturer's entire line of passenger cars must average at least 27.5 mpg for the manufacturer to comply with the standard. [If a manufacturer does not meet the standard, it is liable for a civil penalty of $5.00 for each 0.1 mpg its fleet falls below the standard, multiplied by the number of vehicles it produces. For example, if a manufacturer produces 2 million cars in a particular model year, and its CAFE falls 0.5 mpg below the standard, it would be liable for a civil penalty of $50 million.]. The problem is that the passenger car standard, currently at 27.5 mpg, has not been increased since the 1986 model. For light trucks (including vans and sport utility vehicles) the 1993 CAFE standard was 20.3 mpg while the 1994 and 1995 CAFE standards are 20.5 mpg and 20.6 mpg respectively. Manufacturers earn "credits" for exceeding CAFE standards, and these credits can be used to offset fuel economy shortfalls in the three previous and/or three subsequent model years. They can also be penalized for failure to meet the standard. Congressional efforts are aimed at increasing the standard to around 40 mpg.

 

A number of drive cycles have been used at the University of Leeds over the years. These cycles tend to be more representative of normal driving as can be seen when examinaing the table below. A good indication of the "aggressiveness" of a particular drive cycle is the Positive Kinetic Energy (PKE). This is calculated using the equation below:

where:
V final : Final velocity
V initial : Initial velocity
a>0: For positive acceleration only
Dist: Total distance travelled in trip or microtrip

Statistic
Leeds University cycle
FTP75
ECE+EUDC
Distance /km
1.68
17.77
10.93
Duration /sec
596
1874
1180
Avg. speed /kph
16.83
34.14
33.35
Avg. accel /kph
0.83
0.51
0.54
Avg. decel /kph
-0.70
-0.58
-0.79
Mean PKE /m/s/s
0.75
0.35
0.22
Max speed /kph
80.79
91.25
120.00
Max acceleration /m/s/s
2.50
1.48
1.06
Max deceleration /m/s/s
-2.46
-1.48
-1.39
Idle time /%
17.95
19.05
24.83
Cruise time /%
0.17
6.56
36.44
Acceleration time /%
19.30
39.43
22.97
Deceleration time/%
22.99
34.95
15.76
Number of stops
5
22
13

 

 

To get a visual picture of a drive cycle, a probability distribution plot (aka Watson plot) can be drawn. This 3D plot can make it clear if there the cycle is biased towards one or more speed or accelereation regions. The plots are shown for the FTP75 and the ECE+EUDC below:

For the FTP75, it seems that there is a bias around 30mph and another one at 55mph. This makes sense as most of the roads int he U.S.A. have speed limits of 30mph or 55mph. A lot of time is also spent idling as well. Accelerations as high as 3.5mph/s are present as low speeds. Decelerations are more spread out but there is an obvious peak at -3mph/s.
Compared to the FTP75, the ECE+EUDC is clearly a standardised cycle not representative of real-world driving. Obvious bias is present since there are 3 pronounced speed peaks. Accelerations seem to be a lot less than the FTP at about 2 mph/s.
For the sake of comparison, here is a real-world cycle driven around the block here at Leeds University. The most obvious difference is the much higher rates of acceleration and deceleration. The surface is also a lot smoother than the ECE+EUDC and looks more like the FTP surface. This is a characterisitc of real world driving.