Charge Air Heating

Hannu Jääskeläinen

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Abstract: Heating charge air is an important measure to ensure reliable cold starting, and to reduce white smoke and unburned hydrocarbon emissions. Intake air heating can be provided in-cylinder with glow plugs or in the intake system with electrical heaters or flame-type heaters.


Managing low charge air temperature has been a concern since the early days of the diesel engine. Unlike spark ignition engines, conventional diesel engines do not use a positive ignition source but rather rely on the temperature of the intake charge at the time of fuel injection to initiate the combustion process. If the temperature is too low, the engine may not start or may run poorly until it is warmed up. The problems associated with improper air temperature management during cold start and initial warm-up extend beyond customer satisfaction; the emissions certification process may be affected by excessive unburned THC emissions. THC emissions from unburned fuel can have a significant impact on the certified emissions level for vehicles that include a cold start in the emissions certification procedure.

While there are numerous engine design factors—such as compression ratio, combustion chamber shape and charge air bulk flow—that can be used to affect the unaided cold starting ability of a diesel engine, starting aids are commonly used to increase the air temperature either in the manifold or in the combustion chamber to further lower the temperature at which the engine can be started. Also, managing the air temperature for several minutes after a cold start is a very effective way to reduce white smoke emissions and provide some valuable reductions in unburned hydrocarbon emissions.

The primary reason that many diesel engines use high compression ratios—higher than that which provides optimum efficiency—is to aid cold startability. The trend in modern diesel engine is to utilize a reduced compression ratio that is much closer to values that provide optimum efficiency. Unfortunately, this exacerbates the cold start problem and increases the potential to generate white smoke. On top of this trend is the customer demand for gasoline engine-like starts (i.e., engine start within a few seconds of inserting and turning the key) for light-duty diesel vehicles even at very low winter temperatures. These factors have combined to increase the need for cold starting aids and have forced manufactures to adopt various technologies to manage cold start and engine warm-up air temperature. It should be noted that other measures such as variable valve timing can also be used to overcome the cold starting challenges associated with low compression ratio.