Evaporative Emission Regulations and EVAP Systems
While much of the focus in the automotive industry is on reducing exhaust emissions in order to meet carbon reduction targets, there is another cause of emissions which must not be overlooked. The fuel in a vehicle’s tank and fuel lines is subject to evaporation over time, releasing volatile organic compounds into the atmosphere. These emissions are now regulated in the majority of automotive markets around the world, and evaporative emission control (EVAP) systems have become commonplace in all new vehicles.
Although this is not a new situation – evaporative emission control systems have been used since the early 1970’s - the targets set out by regulations now require manufacturers to develop new, innovative ways to reduce these emissions. The regulations are intended to control running losses and permeation losses from fuel systems, and have an impact on the entire fuel system; from fuel hoses and fuel caps to the materials used to manufacture the fuel tank itself.
Approximately 20% of all hydrocarbon emissions from vehicles originate from evaporative sources, so it is clear to see why effective evaporative emission control systems are essential in today’s modern vehicles, and why innovative new systems must be developed as we move towards a carbon zero society.
Evaporative emission control systems
Before we look at worldwide regulations governing evaporative emissions, it is worth a brief look at how current EVAP systems work to put into context some the challenges that the automotive industry is faced with.
A typical EVAP system is a fully closed system designed to maintain stable fuel tank pressures without allowing fuel vapours to escape into the atmosphere. A system would usually consist of the fuel tank, the fuel tank cap with a vacuum check valve, a charcoal canister with vacuum and pressure check valves, a thermo vacuum valve, and a ported vacuum purge port. Fuel vapours are naturally created in the fuel tank as a result of evaporation, and when tank pressure becomes excessive, are then transferred to the charcoal canister. The vapours are stored in this canister until operating conditions can tolerate additional enrichment, when they are purged into the intake manifold and added to the incoming air/fuel mixture. This typically happens when the vehicle is in motion rather than at idle.
There are several ways in which evaporative emissions can escape a vehicle, including permeation through the walls of a fuel tank and fuel hoses, losses through valves and fuel caps, as well as diurnal losses when the vehicle is at standstill caused by temperature changes.
All global regulations for evaporative emissions follow the same basic certification test flow:
• The carbon vapour canister is prepared and loaded
• Fresh fuel is added to the fuel system
• The vehicle is preconditioned through a driving cycle
• Running loss test
• Hot soak test
• Diurnal test
Testing procedures and protocols vary significantly however, and limits on emissions vary throughout the world, as the map below shows. Euro 5/6 regulations continue the Euro 4 limits of 2 grams of evaporative emissions per day, but require a more demanding test fuel with 5% ethanol and imposing durability requirements.
Japanese emission standards for vehicles are jointly developed by two government ministries, the Ministry of the Environment (MOE) and the Ministry of Land, Infrastructure and Transport (MLIT). Evaporative emission limits in Japan are roughly in line with Euro 4 standards of 2 grams of emissions per day.
U.S. regulations (EPA stage II enhanced/CARB LEVII) limit evaporative emissions to 0.5 grams per day over a three day diurnal temperature profile. Although state regulations cannot normally exceed federal regulations, the California Air Resources Board (CARB) has an exception allowing more stringent emission standards. Additional legislation includes PZEV regulations, which were introduced to encourage the development of zero emission vehicles in California. Evaporative emissions for these types of vehicle are limited to 0.35 grams per day.
Source: Inergy Automotive
European testing and regulations
The evaporative emission test is designed to determine hydrocarbon evaporative emissions as a consequence of diurnal temperature fluctuations and hot soaks during parking after urban driving. Hot soak emissions are usually attributed to the evaporation of the petrol in the fuel system immediately after the vehicle is shut off, while diurnal emissions are the evaporative emissions occurring from a vehicle while it is not being operated. Evaporative emissions are measured using a gas tight chamber (VT SHED) able to contain the vehicle under test, and the VOC concentration inside the chamber is monitored via a FID analyser. The mass emissions of hydrocarbons from the hot soak and the diurnal loss phases are added up to provide an overall result for the test.
Before starting the measurement of the evaporative emissions, both the vehicle and the carbon canister must be properly prepared according to a specific conditioning procedure prescribed by the legislative procedure. As far as the vehicle is concerned, the following conditioning steps must also be carried out:
• Fuel drain and refill - The fuel tank of the vehicle must be emptied taking care not to abnormally purge or abnormally load the evaporative control canister fitted to the tank.
• Preconditioning drive – Within one hour of completing the canister loading, the vehicle must be placed on the chassis dynamometer and driven through one Part 1 and two Part 2 driving cycles of Type 1 test (NEDC cycle).
• Soaking (before hot soak test) – Within five minutes of completing the preconditioning drive the vehicle must be driven off the dynamometer and parked in the soak area. The vehicle has to be left in the soak area at a temperature of 20-30°C for a minimum of 12 hours and a maximum of 36 hours.
• Conditioning drive – After completion of the soak period the vehicle should be driven through a complete Type 1 test drive (cold start urban and extra urban test). Within two minutes of completing the Type 1 test, the vehicle must undergo a further conditioning drive consisting of one urban test cycle (hot start) of a Type 1 test. The engine is shut off at the end of this conditioning drive.
The hot soak test simulates the condition of a vehicle parked after having been driven for a certain distance. Within seven minutes of the end of the conditioning drive the vehicle is pushed into the measuring chamber. The engine must be turned off before any part of the vehicle enters the chamber, and the test lasts 60 minutes at temperatures not less than 23°C and not more than 31°C.
The diurnal test lasts 24 hours and simulates the situation of a vehicle parked for one full day in the summer period. The temperature in the VT SHED is varied according to a profile defined by the directive to reflect temperature fluctuations during day and night time. The starting temperature is 20°C which rises to 35°C after 12 hours before gradually reducing back to 20°C.
The final result of the test is given by the sum of the emissions measured during the hot soak and the diurnal tests, with the current limit set at 2 grams per test.
Comparison with U.S. regulations
Although similar, it is fair to say that U.S. legislative requirements on evaporative emissions are more stringent than European regulations, and generally more complete – covering all the critical factors affecting evaporative emissions. One of the main differences is the duration of the diurnal test. In Europe the test lasts 24 hours, but in the U.S. there are two different diurnal tests lasting 48 and 72 hours. The two-day test is designed to cover driving conditions corresponding to short distance driving and two-day parking, while the three-day test is designed to cover the worst parking conditions of a saturated canister and three days parking.
U.S. regulations also have additional stipulations for the durability of EVAP systems, and an in-use verification programme which requires manufacturers to provide in-use exhaust and evaporative data for 1 year in service in low and high mileage vehicles, and 4 years in service in low and high mileage vehicles.
Evaporative emissions are becoming subject to more stringent regulations worldwide, with the U.S. and California in particular, leading the way in terms of legislation. With up to 20% of hydrocarbon emissions from vehicles caused by evaporative emissions, there is a clear challenge for manufacturers to develop more efficient EVAP systems, and develop new materials and manufacturing processes for fuel tanks and fuel lines to prevent permeation.
It is likely that the U.S. will continue to tighten regulations with a view to encouraging the development of zero emission vehicles, while we can also expect European legislation to catch up in the coming decades as we continue on the path to a carbon zero society by 2050. Of course, at some point in the future the electrification of vehicles may be the preferred route to zero emissions; but while there is still a demand for traditional combustion engines, the onus is on the automotive industry as a whole to develop systems to mitigate emissions more and more effectively.