Another parameter that may have a significant impact on pollutant emissions is the technical state of the vehicle. Ageing, poor maintenance, low-quality replacement parts or tampering, can result in substantially higher emissions levels. The bandwidths of the type approval do not cover all these real-world conditions.
In the existing type approval legislation, modern vehicles are required to meet emission levels up to a mileage of 160.000 km. However, in reality, the average European car is driven for much longer. Because there is no legal obligation for vehicles to meet any emission standards above 160.000 km, their emissions may deteriorate rapidly above this threshold, as illustrated in the following picture.
NOx emissions over the mileage lifetime of diesel and petrol vehicles
Ageing in combination with poor maintenance can have a significant impact on emissions. However, regular maintenance and constant awareness concerning vehicle’s requirements, may result in significant improvements on emissions, as well as on fuel economy.
Some indicative actions and measures are:
- Regularly check of tyre pressure: It is suggested not to inflate tyres above manufacturer specifications because when tyres get hot from driving, the pressure will increase even more. However, an under-inflated tyre increases more the rolling resistance and therefore the fuel consumption and CO2 emissions (Sikarwar and Baghel, 2017). Tyre pressure should be in line with manufacturer’s recommendations and be checked regularly.
- Use of cleaning agent: As the car gets older, harmful deposits can build up in the engine, reducing its efficiency. Adding a cleaning agent into the fuel system, now and then, will help remove deposits and lower emissions.
- Regular oil changes: The engine oil, the fluid that lubricates, cools and prevents wear, is the lifeblood of the engine. The regular change of the engine oil, as well as the appropriate use of it, when necessary, will keep vehicle run at optimum efficiency.
- Replacement of air filter: Clogged air filters lead to reduced airflow to the engine and to accumulation of harmful deposits reducing vehicle efficiency and deteriorating emissions. Additionally, it was found that fuel consumption can increase by 2% to 6% in older cars, due to clogged air filters (Fontaras et al., 2017).
Moreover, when exhaust aftertreatment systems require replacement or repair, vehicle owners may be tempted to replace them with low-quality replacement parts or even totally remove them to save costs. The use of inferior replacement parts or the complete removal – commonly known as tampering – may lead to increased emissions. It is estimated that a small share of tampered vehicles in a diesel fleet can cause a disproportionally high share of the total particle emissions. The following picture illustrates the PM emissions of a diesel fleet, when they are all equipped with a DPF and in the case that a share of vehicles is tampered, leading to a significant increase of PM emissions.
PM emissions of a diesel fleet under different shares of DPF removals (vehicle tampering)
Poor maintenance, durability and tampering lead to elevated emissions. Their overall effect on emissions is however different. The impact of tampering is generally higher than poor maintenance and durability, as illustrated in the schematic of the picture below. The level responsibility of the vehicle manufacturer is also shown in the same graph. It generally decreases with vehicle age, especially close to the 160.000 km threshold and beyond.
Evolution of vehicle emissions through its lifetime and effect of different parameters[1]
[1] The size of each area is not indicative of the real contribution of each risk, but only an illustration