Can a coolant heater have an effect on the emissions performance of a diesel engine?

With stricter regulations and the constant struggle to stretch the maintenance budget, fleet maintenance directors are always looking for ways to reduce emissions and maintenance costs. One approach is to employ an auxiliary coolant heater.

A recent study shows that using a coolant heater to pre-heat an engine will improve diesel particulate filter (DPF) performance.

A DPF’s regeneration occurs when exhaust gases reach a high enough temperature to oxidize and burn the trapped particulate matter (PM). In order for the exhaust gases to be hot enough, the engine has to be warmed to the optimum operating temperature.

Enter the auxiliary coolant heater, also known as an engine pre-heater or fuel operated heater. This device, which has been around for more than 60 years, essentially pre-heats the engine before it is turned on. It draws “cold” coolant into a heat exchanger where it is warmed then pumped back to the engine.

Fleets have been using coolant heaters with great success for years in cold climates, and in some cases, they are essential to the engine even being able to start in extreme cold conditions.



This helps the performance of the DPF in two ways. First, it reduces emissions during engine start-up and brings the engine to optimum operating temperature. The amount of PM that the DPF has to trap and burn is reduced.

Secondly, it helps the exhaust gases reach the temperature needed for the DPF to perform the regeneration process. This is especially crucial for shorter routes where the vehicle is not driven long enough for exhaust temperatures to reach that high temperature.

The previously reference study showed that PM emissions from an engine equipped with a 5 kW Webasto coolant heater were reduced by 66 percent by pre-heating the engine during cold weather conditions and 27 percent during normal ambient conditions.

NOx emissions were reduced by around 40 percent by pre-heating the engine in both conditions. CO (carbon monoxide) was reduced by 29 percent during winter conditions and 62 percent during normal ambient conditions.

What does that mean? It means that a coolant heater is effective in more than just cold climates.

The “normal ambient testing conditions” in the study were 75 degree F. That means a coolant heater is effective in reducing emissions in any part of the country.

The use of a coolant heater can help extend the longevity of a DPF’s life and avoid more frequent regenerations and cleaning intervals. Any time a fleet maintenance manager can reduce taking a truck out of service for maintenance is critical to reducing costs and stretching the budget.


Kurt Van Portfliet is director of North America aftermarket sales for the commercial vehicle division of Webasto Thermo & Comfort North America ( Webasto is one of the top 100 automotive suppliers worldwide.