Coolant Surge Tanks

A (transit bus) coolant surge tank, also known as an expansion tank or overflow tank, is a crucial component in the bus’s cooling system. It plays a vital role in maintaining optimal engine temperature and ensuring the longevity and efficiency of the engine.

Function and Purpose of Coolant Surge Tanks

The primary function of the coolant surge tank is to manage the expansion and contraction of coolant as it heats up and cools down during engine operation. It helps maintain proper coolant levels and prevents overheating, which can lead to engine damage.

Key Features

• Pressure Regulation: The surge tank is designed to accommodate the expansion of coolant when it gets hot and contracts as it cools. This helps maintain the correct pressure within the cooling system, ensuring efficient operation.
• Overflow Management: During periods of high engine temperature, excess coolant flows into the surge tank, preventing overflow and potential spillage. When the engine cools down, the coolant is drawn back into the main cooling system, maintaining a consistent level.
• Air Separation: The tank helps to remove air bubbles from the coolant, which can cause inefficient cooling and potential hotspots within the engine. This ensures the coolant circulates smoothly and effectively.
• Durability: Surge tanks are constructed from robust materials, often high-density plastic or metal, designed to withstand extreme temperatures and pressures. This durability is essential for the demanding conditions in which transit buses operate.
• Integrated Sensors: Modern coolant surge tanks are equipped with sensors that monitor coolant levels and temperature. These sensors provide real-time data to the bus’s onboard diagnostic system, alerting the driver or maintenance personnel of any issues.
• Maintenance and Easy Access: The design of the surge tank typically allows for easy access and inspection, facilitating regular maintenance checks and refilling when necessary. This helps in promptly addressing any cooling system issues.

Testimonials with our Coolant Surge Tanks

After a wonderful referral by an employee of our largest customer, we were approached by the Bus Maintenance Engineering dept. at Chicago Transit Authority to develop a replacement for failing coolant surge tanks on one brand of their transit buses. After another OEM had installed 400 of our stainless steel coolant level sensors on their warranted fleet at CTA, the agency had a high level of confidence in our engineering capabilities and quality of our components.

A complete Finite Element Analysis (FEA) was performed on the original OEM tank which demonstrated that there were serious flaws in the design that led to the very high percentage of failures in the fleet. 

In order to keep buses on the road, the agency had to do welding repairs on the tanks constantly, often times repairing the same tank more than once. The agency asked us for a 3X safety factor in our design so our new tank design needed to withstand a maximum testing pressure of 30 Psi and an operating pressure of 15 Psi. With our seriously improved design, which moved the stress points away from the tank edges to welded seams on the body and employing machined end caps, all of the five prototypes tested above 33 Psi. Our destructive maximum pressure test topped 160 Psi !

The tanks have undergone an evaluation of over 20 months now on buses at CTA with no issues and we have delivered 40 tanks so far with another 200 on order.  We are expecting a positive outcome for implementation on a significant number of additional buses at this agency and other agencies with similar problems.  We have added a second part number, VT440020 for a different version of the CTA tank which repersents about 35% of the affected fleet.