SCAC, CAC, AATAC, what does this all mean?

New emissions regulations have led to massive changes in the way diesel engines are cooled. Older engines had a single jacket water circuit (using a mixture of ethylene glycol and water) that was responsible for removing heat from the engine and transporting it to a radiator. Simple enough, right?

Fast forward to today! The engines are now equipped with separate cooling circuits, aftercoolers, charge air cooled circuits, and even oil cooling circuits. This article describes some of these systems and the radiators that are used to cool them.

Let’s start with the most basic, the single jacket water circuit. Here, the cooling fluid circulates through internal passages in the engine block to extract and carry part of the heat produced by the combustion cycle. It is a unique circuit because all the cooling water circulates through a single water pump mounted on the engine.

SCAC stands for “Separate Aftercooled Circuit”. This technology circulates the jacket water in the normal way, but a separate water circuit is added to cool the engine intake air after it is pressurized by the turbocharger. By circulating cooling water through the aftercooler, the engine can run more efficiently and produce more horsepower. This system uses two engine mounted water pumps and each circuit is pumped to an individual radiator core for cooling.

ATAAC stands for air-to-air-aftercooled, also known as CAC (charge-air-cooled). This system uses forced air (instead of water) to cool the turbocharged air before it enters the engine’s combustion chamber. Similar to the SCAC design, the goal is to lower engine intake temperatures to improve emissions and power output efficiency.

So how do these systems affect external heat exchangers (radiators)?

Engines equipped with a combined water jacket and aftercooler circuit are fairly straightforward. There are two water circuits, each connected separately to a split-core or dual-core radiator. When using a remote radiator (not engine mounted), the limitations of the engine water pump and aftercooler pump head must be considered. In the worst case, shell and tube heat exchangers are used to circulate the engine circuits, and auxiliary pumps are used to circulate the cooling fluid to the remote radiator. This is known as a dual circuit remote radiator cooling package.

Engines equipped with charge air-cooled circuits are a bit more complicated. The charge air circuit is highly susceptible to even the smallest pressure drop in your piping system. This requires a minimum distance between the engine and the charge air cooler. While kit-mounted radiators are the most common, remote radiator packs are sometimes required. In these cases, the engine installs a liquid-to-air charge air cooler air box assembly. This assembly is then connected to a remote radiator, with auxiliary pumps that provide the necessary coolant circulation.

Engines equipped with SCAC designs employ a stacked core radiator (a single radiator assembly with two stacked cores) or a split core radiator (a single radiator assembly with cores side by side). Each core is connected to the corresponding circulation pump (JW pump for jacket water core and AC pump for aftercooler core). In remote radiator applications, pump head limitations may require the use of auxiliary pumps and shell and tube heat exchangers.

Here are some ideas to consider when selecting a generator set for a particular application:

• If the radiator will be mounted to the engine, be sure to account for the temperature rise throughout the engine (i.e .: radiator operating temperature = ambient air plus ambient temperature rise observed as air flows over the engine.
• Calculate the airflow requirements and maximum external static resistance limitations of the radiator. Communicate this information to the radiator duct manufacturer and louver supplier to ensure that the air flow from the radiator is not excessively restricted.
• For remote radiator applications, a mechanical engineer must calculate the pressure drops associated with the pipes leading from the engine to the radiator. These values ​​should be compared with the engine water pump data sheets.
• When it comes to cooling systems, different engine manufacturers often apply substantially different technologies for the same engine rating. A manufacturer that uses a SCAC design for a given classification may compete with another manufacturer that uses an AATAC design. If your project will present a competitive bid, your design must take into account the worst case scenario.
• Consult with several engine manufacturers or an independent radiator manufacturer so that you can be aware of the cooling system limitations that are critical to your design.

How much exposure have you had with these new refrigeration systems? Have you found a shortage of information on this topic? Share your comments or questions.