Is Car Preheating Worth It? A Detailed Analysis of Fuel Economy and Emissions

Car heating, warming, warm up before startup


In a recent study published in Applied Energy, researchers from the University of Eastern Finland and Tampere University set out to examine the advantages of car preheating on both fuel economy and emissions in the unforgiving conditions of a frigid winter. The study’s primary focus was the role of preheating in cold-weather vehicle Performance. It assess how preheating affected vehicle fuel consumption, especially during cold starts, and its impact on emissions. The findings shed new light on the efficacy of preheating in these extreme conditions and its potential benefits.

Cold Starts and Emissions

The study began by delving into the difficulties posed by cold starts, particularly for diesel-powered vehicles, when temperatures plummeted to a bone-chilling -28 degrees Celsius during their measurements in Finland. To replicate real-life commuting scenarios, researchers designed a route that incorporated both urban and highway driving, as well as stops at intersections and traffic lights.

Vehicles were tested under three different conditions: after a cold start, after preheating, and with the engine already warmed up from prior driving. The results revealed that after a cold start, it took nearly the entire drive (13.8 km, approximately 19 minutes) for the engine coolant to reach its optimal operating temperature (>60°C). It’s noteworthy that the vehicles in the study were equipped with either electric or fuel-powered preheaters.

Efficient preheaters (with a minimum capacity of 1 kW) were effective in warming the engine coolant before starting, although they didn’t drastically accelerate the engine’s temperature rise. Instead, they primarily improved vehicle comfort by providing a warmer cabin and preventing window frost. Moreover, car manufacturers suggest that preheating can help reduce engine wear during cold starts.

Fuel Economy and Preheating

The study observed a marginal decrease in overall fuel consumption (around 10-20%) when the vehicle warmed up before driving compared to a cold start. However, this reduction significantly manifested in only two out of the six tested vehicles, which had fuel-powered auxiliary heaters. Even in these cases, preheating before starting yielded less than a 4% decrease in fuel consumption compared to cold starts.

It’s essential to consider that these calculated fuel savings didn’t account for the fuel or electricity consumption of the auxiliary heaters during the preheating process. When factoring in the fuel consumption during preheating, the post-preheating drive led to a substantial 26-37% increase in overall fuel consumption compared to a cold start. Additionally, preheating had an impact on overall emissions.

Emissions and Preheating

This study emphasized that the use of fuel-powered auxiliary heaters couldn’t be justified based on improved fuel economy or reduced emissions during cold-temperature driving. However, when considering the entire lifespan of a vehicle, the advantages of preheating might become evident through extended engine oil life and prolonged engine durability, even though these factors weren’t directly examined in this research.

Particulate Emissions

Assistant Professor Panu Karjalainen of Tampere University underscored that preheating didn’t significantly affect particulate emissions, mirroring the findings regarding total fuel consumption. The measurements under cold winter conditions unveiled particle number concentrations exceeding regulatory limits for new vehicles by up to a hundredfold. This discrepancy can be attributed to the regulations, which only consider solid particles larger than 23 nanometers in size and apply to emissions measured under warm conditions. Under cold conditions, high concentrations of smaller particles were observed, some of which could be liquid.

Even though diesel particulate filters are designed to capture nearly all emissions, this study observed substantial particulate emissions from diesel vehicles equipped with fuel-powered auxiliary heaters. The emissions emanated from these heaters during operation, as they automatically provided additional heat to the engine or cabin while driving. The effect of auxiliary heaters on in-use emissions is more pronounced because there is no emissions aftertreatment for heaters, as there is for engine emissions.

Variations in Emissions

Regarding particle mass, black carbon, and nitrogen oxides, variations in emissions were observed in different driving scenarios. Preheating yielded the most significant reductions in particle mass emissions, with up to 85% lower emissions over the entire route and a staggering 99% reduction during the initial idle and early route sections.

Nitrogen oxide emissions also displayed substantial variations, with up to a 90% decrease when driving with a warm engine compared to a cold start, depending on the vehicle. However, the benefits of preheating in reducing particle mass emissions were observed in only one gasoline vehicle (a 72% reduction) and one diesel vehicle (a 24% reduction). For black carbon emissions, preheating exhibited only minimal advantages. Notably, when considering emissions from auxiliary heaters, the reduction in nitrogen oxide emissions dwindled to 15%.

It’s important to mention that electric preheaters didn’t offer significant benefits in terms of fuel consumption or emissions reduction.

Diesel Vehicles and Nitrogen Oxide Emissions

In diesel vehicles, the role of auxiliary heaters in total nitrogen oxide emissions was not as pronounced as in gasoline vehicles. Diesel vehicles exhibited significantly higher in-use nitrogen oxide emissions. Despite all the vehicles studied exceeding nitrogen oxide emission limits, diesel vehicles, in particular, exceeded these limits, sometimes up to 21 times. The study additionally revealed that one diesel vehicle exceeded the regulatory limit for nitrogen oxide emissions by a factor of 12, despite being equipped with a selective catalytic reduction (SCR) system. This indicates potential issues with the SCR system in cold temperatures.


In conclusion, the use of preheating as a means to enhance fuel economy and reduce emissions doesn’t find substantial support in actual cold-temperature driving conditions. However, different driving scenarios and considering the full lifespan of vehicles might lead to somewhat different conclusions. Although this study revealed some emissions reductions for vehicles with fuel-powered auxiliary heaters, these benefits decrease significantly when factoring in the fuel consumption and emissions of the auxiliary heater during the preheating cycle.

Furthermore, it’s crucial to acknowledge that preheating may have advantages in terms of emissions that weren’t investigated in this study, such as emissions of specific hydrocarbons. Additionally, emissions from fuel-operated auxiliary heaters may be higher due to limited aftertreatment, potentially counteracting any advantages in terms of these emissions as well.

This study was supported by the Jane and Aatos Erkko Foundation and was part of the research conducted within the Academy of Finland’s ACCC Flagship.

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