Making EVAP work for full hybrid EVs

By Laurent Otte, Melexis 

The latest summers saw record temperatures measured in many regions of the world, ice melting in Greenland and degradation of the permafrost in Alaska at a pace never registered before. And 2020 is on course to be the hottest year since measurements began, according to meteorologists. These are just a few examples that show that we face a significant climate change.

We face a significant climate change 

To limit the increase in the planet’s average temperature, people are attempting to reduce global greenhouse gas emissions. Many studies have been carried out to identify the human contributing factors in increasing levels of greenhouse gases. The results vary by location, but in general the ballpark figures are that the transportation industry contributes 20% of emissions (the automotive industry represents 15%, aviation and marine about 5%), electricity/energy production 30%, manufacturing/industry/construction 20%, residential/commercial buildings 15% and agriculture/forestry 10%.

Focusing on transportation, legislation has pushed the automotive industry to expend effort on developing propulsion technologies that are more environmentally friendly when it comes to CO2 emissions (one of the major greenhouse gases). The increasing population in urban areas means that more and more people are directly exposed to pollution from cars – and this, coupled with incidents such as the diesel emissions scandal (Dieselgate), has put the car in the spotlight. Limiting global warming has to be looked at holistically, however, including all sources of CO2 emissions, and the production of electricity has a clear link to the automotive industry.

Car electrification is being widely adopted as a means to address the tank-to-wheel aspect of CO2 emissions. Many regions of the world are not ready to power pure electric cars with green sources of energy to the extent that the well-to-wheel CO2 emissions result would be beneficial. Fully converting the worldwide automotive fleet to BEV also raises the question of managing the mineral resources necessary to build batteries. Hybrid technologies therefore represent an interesting compromise in the well-to-wheel CO2 emissions and mineral resources preservation discussion.

Levels of electrification 

Many different flavors of hybrid technologies exist, from the mild hybrid, which uses electric power during specific transitions such as acceleration, up to the full hybrid, which depends on the electric motor to entirely drive the car over short urban daily commuting distances.

With new technologies come new challenges, and the full hybrid is a good example of this. Despite the known advantages of diesel when compared to petrol for CO2 emissions, government penalties imposed on diesel following the Dieselgate scandal will steer hybrids to favor petrol. However, petrol is a much more volatile fuel than diesel. Petrol vapors build up in the tank, and it is important to avoid releasing them into the atmosphere. This challenge is already known in pure internal combustion cars, and is addressed in some regions of the world by mandating the Evaporation Emission Control – or EVAP – system.

The EVAP system consists of storing the vapors in a filter called a canister, and regularly purging the canister to prevent its saturation bringing stored fuel into the combustion chamber. Pressure sensors are used in such systems to ensure that it remains sealed, and to provide an alert in the case of any vapor leakage. With full hybrids, however, the combustion engine remains unused for long periods. With daily commuting distances being short and the number of hot days increasing every year, cases of excessive fuel vapor accumulation in the tank will be common, and the canister will no longer be able to cope with it. The option to isolate the tank from the canister therefore becomes critical in full hybrids. Having pressure sensors that are not only able to detect tiny leakages of fuel vapor but are capable of measuring and coping with increasing pressure levels in sealed tanks is key to the success of EVAP in full hybrids.

The MLX90821 has been developed for this purpose. It can be used in classic EVAP of pure internal combustion engines and, following recalibration, can also monitor the increasing fuel vapor pressures in sealed tanks of full hybrid cars. Standardizing the sensor implementation on a single concept independent of hybridization levels helps OEMs meet legislation deadlines. The 2019 release of the China ‘national 6’ emissions norm is an example. 

Air intake and exhaust plus fuel delivery management

Multiple efforts will be necessary to address global warming, starting from habits such as reducing the impact of the agriculture sector by consuming less meat, reducing marine transportation by consuming local products, or traveling in a more environmentally friendly way than by plane, for example. 

A full solution can only come about with strong political will to prioritize global-scale issues, looking for a common way to address them, including by tackling all sources of greenhouse gases. The recent political landscape is moving in the direction of governments focusing mainly on local and national topics instead of adopting climate change-related solutions.

Looking on the bright side, though, technology can make a difference. Humans are creative, and are always developing solutions in response to tough challenges. One recent example, among others, was a team of engineers bringing to the market sensors that could help with the deployment of full hybrid cars, necessary to reduce the automotive contribution to global warming. This is inspired engineering, bringing new solutions with a purpose, helping with the challenges that lie ahead.