Pictured is the DEF fill point on a Thomas Built Saf-T-Liner HDX school bus.
Photo courtesy of Thomas Built Buses
If your operation’s shop routinely services diesel-equipped school buses and other diesel-powered vehicles, you may be aware of the use of DEF (diesel exhaust fluid). DEF consists of a mixture of high-purity urea and deionized water. Controlled by a “dosing” module, DEF is injected into the diesel engine’s exhaust stream. DEF consists of a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water.
DEF dosing is controlled by the engine’s ECU. When heated, DEF splits into ammonia and carbon dioxide, which is then atomized and vaporized. Once DEF enters the exhaust, the water in the DEF vaporizes, leaving ammonia molecules to travel to the catalytic converter, where it neutralizes NOx molecules. This reaction converts NOx to harmless nitrogen and water, substantially cleaning up diesel emissions.
The EPA has mandated that all on-road diesel-equipped vehicles manufactured after January 2010 must reduce NOx emissions. The most common method to achieve this reduction is by injecting DEF into the exhaust path. All diesel-equipped vehicles that feature an SCR (selective catalytic reduction) system utilize DEF injection.
The use of SCR systems reportedly provides an additional benefit of a 3% to 5% increase in diesel fuel economy. This is a result of enhanced combustion as exhaust temperature rises, especially in the heavy-duty applications. Cummins reportedly has gained substantial mileage improvements with its SCR system.
DEF usage rate
When the DEF onboard tank is approaching empty, the vehicle’s onboard warning system will alert the driver with enough lead time to replenish the DEF tank.
Instead of (or in addition to) refilling DEF tanks when an early warning is provided, it’s wise to top off a DEF tank at the same time that the engine oil is changed (for example, every 3,000 to 4,000 miles). In other words, you need to keep on top of the DEF level status. Running the system dry will result in severe driver inconvenience, in terms of entering a limp mode (either loss of power or severely limited vehicle speed).
DEF tank fill location
All DEF fill caps, regardless of make or model, should be light blue in color and should be labeled “DIESEL EXHAUST FLUID.” When filling the DEF tank, do not overfill. In freezing temperatures, the DEF mixture will expand. DEF tanks are designed to accommodate this expansion, but only if you fill to the advised level. If a DEF tank does become damaged and leaks, there’s no real hazard since it’s not toxic or flammable. It’ll just make a stinky ammonia-smelling mess.
Once DEF temperature reaches approximately 12 F, it will freeze. However, this won’t leave the vehicle stranded. The engine is able to start, and the vehicle may be driven normally. As engine coolant warms, the DEF will thaw and will once again flow properly. While there have been claims that urea will become toxic at 118 F, this isn’t so. Actually, urea isn’t toxic at any temperature. If it becomes too hot, the shelf life may simply decrease (as ammonia begins to form).
Do not add any additives, such as antigel, to any DEF tank. DEF will expand by about 7% when frozen and will revert to a liquid state when heated during engine operation. Don’t treat DEF as you would diesel fuel. You don’t need, nor should you ever add, any modifiers/additives. Keep the DEF pure!