Warm Up & Cool Down Periods

Warm up Considerations

Excessive idling and warm up periods are widely controversial as a result of the phenomenon in compression ignition engines known as "wet stacking". Wet stacking is the process by which relatively cool combustion temperatures results in an incomplete combustion event. When this occurs, partially and unburnt fuel has a tendency to stick to the cylinder walls, through which a portion of the contaminants will enter the engine crankcase. Fuel in the crankcase is typically known as fuel dilution, and since diesel fuel has nowhere near the lubricity of engine oil it can lead to excessive wear in extreme cases. It's worth noting that fuel dilution is invertible in a diesel engine - since it's impossible to avoid entirely, it's necessary to take certain steps in order to reduce the degree to which it occurs.

As a result of wet stacking and fuel dilution of the engine oil, manufacturers typically categorize engines that idle excessively under "Severe Duty" with regard to recommended maintenance intervals. Under this service category, vehicles are subject to more frequent oil changes, amongst other interval alterations. While wet stacking concerns are greatest when an engine is dead cold, wet stacking occurs in vehicles at normal operating temperature while idling - a diesel engine consumes a relatively low amount of fuel while idling, and as such combustion temperatures rapidly drop while there is no load placed on an engine.

And since diesel engines consume little fuel at idle, they tend to take a long time to reach operating temperature without driving. In fact, a diesel engine that is started and left idle may not reach operating temperature at all until it is driven. Driving places a load on the engine, essentially making it work harder and therefore creating more heat. The combustion process is much more efficient at normal operating temperature and fuel dilution concerns are significantly reduced.

On the contrary, driving while the engine is dead cold can have its own set of consequences. The primary concern with placing a load on a cold engine is related to the oil viscosity. As the temperature of oil increases it becomes less viscous - likewise, oil becomes more viscous (thicker) as temperature decreases. The viscosity of oil is related to both its flow and lubrication properties. After starting a cold engine ("cold" referring to an engine whose temperature has reached equilibrium with its surroundings), oil pressure typically takes time to stabilize and it may not be of best interest to drive a vehicle during this period (which typically lasts no more than a minute in most instances).

Cool Down Considerations

Allowing an engine to cool before turning it off is important to turbodiesels, although mostly irrelevant in naturally aspirated engines. A turbocharger is manufactured with low tolerances and relatively high precision, as its operating conditions are somewhat severe. Modern turbochargers can experience compressor/turbine speeds in excess of 100,000 rpm, while even older models will see rotational speeds in the 40,000 to 60,000 rpm range. At these speeds, lubrication and the condition of the turbocharger bearings are of the utmost importance to longevity.

All turbochargers are lubricated via the engine's pressurized oil system, meaning that engine oil is constantly circulated through passages entering and exiting the bearing cartridge. While a vehicle is driving and the turbocharger is functioning, it becomes hot - the temperature of the turbocharger is relative to load. When a vehicle has been driven and is abruptly shut off (and the oil flow to the turbocharger ceases), engine oil contained in the turbocharger absorbs heat from its surroundings. If the temperature of the turbocharger prior to shut down is great enough, the oil risks burning and will have a tendency to create deposits in and around the turbo bearings in addition to contaminating the engine oil supply.

Recommended Warm up & Cool Down Times

Warm Up Recommendations

Diesel engines are not "start-and-go" machines and therefore a warm up period, however brief, is recommended after starting a cold engine. The length of the warm up period depends significantly on ambient temperature. If the OE manufacturer has provided recommended warm up procedures, follow them accordingly. However, most manufacturers don't provide this information as it is not an exact science.

The warmer the ambient temperature, the lower the recommended warm up idling period. However, the opposite is not necessarily true; there is a point of diminishing return and, therefore, extended idling (>7 minutes) is typically ill advised and provides no benefit regardless of climate conditions. A diesel will almost always require driving to reach full operating temperature. The warm up process is more oriented toward increasing the combustion chamber temperature and not placing any excess load on an engine while its components soak in heat and begin to slowly expand. You may also consider the following in determining an appropriate warm up procedure based on your needs:

• All diesel engines have block heaters and you should be using yours per the manufacturer recommendations. Block heaters reduce warm up idle times and significantly aid starting in cold weather. When using a block heater, the aforementioned acceptable warm up periods can be notably reduced.
• Glow plug and heater grid systems typically "post cycle" in extremely cold temperatures, which reduces engine warm up times and increases the efficiency of combustion while an engine is cold in addition to providing sufficient heat for starting. In cold climates, consider cycling the glow plug/heater grid twice prior to starting, which may allow for less cranking and easier starts.
• 15W-40 is a common oil weight for diesel applications. However, this viscosity is not always recommended for temperatures below 0° F. Mind your engine manufacturers recommendations with regards to oil viscosity and ambient temperature.
• If you plan on towing, consider a slightly extended warm up period as it is not ideal to place a great load on a cold engine.
• Keep the engine speed low and drive with only light throttle input until an engine reaches its normal operating temperature.
• If cold climate has you idling extensively, make sure to follow the manufacturer's recommended service intervals accordingly.

Cool Down Recommendations

An appropriate cool down period depends on the situation. Following light/normal driving, for example, a cool down period should not typically exceed 30 seconds. After towing, hauling, or other high load condition, the cool down period should be slightly longer.

This short cool down period allows for circulating engine oil to remove heat from the turbocharger so that the engine oil does not "cook" in the turbocharger upon shutdown. While not always necessary, it's considered good practice and will promote longevity and reliability of a turbo. Allowing the turbo to cool down is, however, extremely important following a regeneration cycle in DPF equipped pickups, as the turbocharger tends to reach relatively high temperatures following this process.