December 8, 2023

A new study published in Energy by the University of Eastern Finland, Tampere University, and the University of Eastern Finland found that preheating cars have minimal benefits for fuel economy and emissions. Researchers focused on fuel consumption and vehicle emissions in cold winter conditions. Cold start emissions and the relationship between preheating and cold starts were of particular interest.

Results show that cold starting is a challenge, particularly for diesel vehicles in cold winter conditions. The outdoor temperature dropped to -28 degrees Celsius during the Finland measurement campaign, which was reflected by vehicle emissions. The measurement campaign included a route that replicated typical commuter scenarios. This included both highway and urban driving as well as stopping at traffic lights and intersections.

The same route was driven in three different conditions: with the engine preheated and already warm from driving. After a cold start, it took nearly the entire journey (13.8 km or about 19 minutes) to get the engine coolant up to its optimal operating temperature. The vehicles studied were either equipped with electric or fuel-powered preheating systems.

“Efficient preheaters (at least one kW) warmed the engine coolant prior to starting but did not significantly accelerate reaching the optimal operating temperatures. The primary benefit of higher starting temperatures was improved vehicle comfort, as it provided a warmer interior and prevented window frost. According to auto manufacturers, preheating reduces engine wear when starting cold.

The study showed that fuel consumption was slightly lower (10-20%) after the vehicle had been warmed up compared to a cold start. Preheating was only effective in two of the six studied cars, which were both equipped with fuel-powered auxiliary heaters. Preheating only reduced fuel consumption in these vehicles by less than 4% when compared with cold starts.

The calculated fuel savings do not include the fuel and electricity consumed by the auxiliary heaters when preheating. The fuel consumption of the preheating drive was 26-37% more than the cold start. Santtu Mikkonen, Research Director at the University of Eastern Finland, points out that preheating affected emissions.

These results confirm that fuel-powered heaters are not justified in cold weather driving by fuel efficiency or emissions reduction. When considering the lifespan of a car, preheating can be beneficial for engine longevity and oil life. However, these factors weren’t examined in this research.

No evidence of the positive effect of preheating on particulate emissions

Assistant Professor Panu Karjalainen from Tampere University explains that as a result of the observations made on fuel consumption in general, preheating had no significant effect on particulate emission. The concentration of particulates exceeded the regulatory limits by as much as 100 times. The fact that the regulations only apply to emissions measured in warm conditions and do not take into consideration solid particles smaller than 23 nanometres may explain this. In cold winter conditions, measurements showed high concentrations in smaller particles. Some of these particles could have been liquid.

Although diesel particulate filter systems are designed to capture almost all emissions, particulate emissions from diesel vehicles with fuel-powered heaters were significant. The emissions from these heaters can be attributed because they provide extra heat to the cabin or engine while driving. This is because auxiliary heaters have a greater impact on emissions in use, as they do not require any emissions after treatment.

Contrary to particle numbers, particle mass, nitrogen oxides, and black carbon emissions varied in response to driving conditions. The emissions of black carbon and particle mass were lower with warm engines compared to cold ones, particularly in gasoline-powered vehicles. The biggest reductions were observed in particle mass at 85% for the entire route and 99% when the engine was first started.

Depending on the vehicle, nitrogen oxide emissions could be reduced by up to 90 percent when driving with an engine that is warm compared to a cold start. Preheating was found to minimize particle mass emissions in only one gasoline vehicle (72% reduction) and one diesel vehicle (24%). Preheating had minimal effects on black carbon emissions. Preheating had a significant impact on nitrogen oxide emissions in one gasoline vehicle. The reduction was 41%. When auxiliary heaters are included, nitrogen oxide emissions only decrease by 15%. Electric preheaters did not provide any significant benefit in terms of fuel usage or emissions reduction.

Diesel vehicles produce significantly more nitrogen oxides when in use than gasoline vehicles. Diesel vehicles were the most affected by nitrogen oxide emissions, with some exceeding the limits up to 21-fold. One diesel vehicle had nitrogen oxide emissions that exceeded the limit by 12 times despite having a selective catalyst reduction (SCR). It is possible that the SCR did not work properly at cold temperatures.

All benefits and downsides of preheating are not known

The use of preheating for fuel economy or to reduce emissions is not supported by actual driving in cold temperatures. In different driving situations and when considering the lifespan of vehicles, emission reductions may lead to different conclusions. According to the studied emissions (including particulate matter and nitrogen oxides), there were some reductions in cars that had fuel-powered auxiliary heating systems. These benefits are reduced or eliminated when the fuel consumption and emissions from the additional heaters during the pre-heating cycle are taken into account.

It’s also important to note that the preheating of auxiliary heaters may be beneficial in terms of emissions, which were not measured by this study. These include emissions-specific hydrocarbons. The emissions from fuel-powered additional heaters could be higher because of limited aftertreatment. This may offset any benefits in terms of these emissions.

The research was conducted by the AC Flagship of the Academy of Finland and funded by the Jane and Aatos Erkko Foundation.

Before the product can be sold, researchers must solve other problems. Experts will need to reduce the thickness of the solid electrolyte to a level similar to that found in lithium-ion battery electrolytes to commercialize the all-solid-state batteries. This will increase the energy density or the amount of Power that the battery can hold. The team also said that the high costs of basic materials are another problem.

Solid Power, a manufacturer of advanced batteries, plans to start testing the new technology in order to evaluate its commercialization potential. Researchers said that they will continue to research ways to boost energy density.


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