For vehicle and equipment OEMs, the cooling system is a major thermal consideration to support the performance, reliability, efficiency and correct operation of their products. For internal combustion engined (ICE) power machines, the first, and largest, area of consideration is the heat generated and rejected by the engine. Designing an effective cooling system starts with one key figure: heat rejection.
What Is Engine Heat Rejection?
Only a portion of the energy released during combustion becomes useful mechanical power. The rest leaves the engine as heat through the coolant, exhaust, charge air, oil and surrounding air.
Engine heat rejection is the amount of thermal energy that must be removed from an engine and the associated systems to keep temperatures within their optimal operating limits.
These thermal loads are usually expressed in kilowatts (kW) and form the basis of any cooling system or radiator sizing calculation.
For the cooling system designer, the important question is not simply how much power the engine produces, but the total heat the cooling system must reject under the most demanding operating conditions while also understanding the influence of the installation and engine bay design.
Which Heat Loads Need to Be Considered?
The engine radiator is often only one part of the overall thermal management system. Depending on the application, the complete cooling pack may need to manage heat from various sources:
Engine coolant
Charge air
Transmission
Hydraulic systems
Air-conditioning condenser
Fuel and other auxiliary circuits
The total cooling system heat load therefore needs to consider the whole machine rather than the engine in isolation.
What Information Does the Engine Manufacturer Provide?
Engine manufacturer data should normally be the starting point for the heat rejection calculation given it can have the biggest impact. Depending on the application, this may include maximum coolant heat rejection, charge air heat rejection, coolant flow rate, maximum coolant outlet temperature, air mass flow, pressure drop limits and engine speed and load data.
These figures should relate to the specific engine model, rating and emissions configuration being installed.
Where coolant flow and temperature data are known, heat transfer through a liquid circuit can be expressed as:
Q = ṁ × Cp × ΔT
Where Q is the heat transfer rate, ṁ is coolant mass flow, Cp is the coolant’s specific heat capacity and ΔT is the temperature rise across the system.
This can help validate heat transferred through a circuit, but it does not size the radiator on its own. Airflow, core design, pressure drop and installation conditions still matter.
Why Rated Engine Power Is Not Enough
Two engines with the same rated power will not necessarily reject the same amount of heat into the cooling system.
Differences in efficiency, turbocharging, emissions technology, operating speed and engine architecture can all affect how waste heat is distributed. Maximum heat rejection may also occur at a different operating point from maximum engine power.
Rated power highlights what the engine delivers mechanically. Heat rejection data indicates what the cooling system needs to manage.
How Ambient Temperature Changes Cooling Requirements
Ambient temperature has a major influence on cooling performance.
A system that performs well at 20°C may behave very differently at 40°C. As ambient temperature rises, the temperature difference between the coolant and outside air becomes smaller, reducing the system’s ability to reject heat.
The air reaching the radiator may also be hotter than ambient. If a condenser, charge air cooler or hydraulic cooler sits in front of it, the air may already have absorbed heat. Hot discharge air can also recirculate inside a restricted engine bay.
Addressing this may result for a larger heat exchanger, increased airflow, improved ducting, a revised cooling pack arrangement, or better sealing to stop hot air recirculating around the core.
Calculating the Total Cooling Pack Requirement
A simplified calculation can help establish the scale of the cooling requirement.
For example:
Heat Source | Heat rejection |
Engine coolant | 110 kW |
Charge air | 45 kW |
Transmission | 20 kW |
Hydraulics | 25 kW |
Air-conditioning condenser | 10 kW |
Combined heat load | 210 kW |
Although the simplest solution appears to be a 210kW cooling system, the components are not the only variables in the equation.
The vehicle’s overall heat load, operating conditions, peak load timing, cooler positioning, and operating temperatures must also be considered.
A heat load calculation is therefore only the starting point.
How Installation Affects Cooling Performance
A cooling pack can perform well in isolation but struggle once installed in the vehicle.
Limited space is a common challenge. Chassis rails, engines, bodywork and other components can restrict frontal area. Increasing core depth may improve heat transfer area, but it can also increase airflow resistance.
Grilles, guards and ducting create additional restriction, so fan performance needs to be assessed against the complete installed system.
Cooling pack orientation matters too. Position and angle can influence airflow distribution, debris build-up and service access. Air sealing is equally important because hot discharge air returning to the front of the pack raises inlet temperature and reduces performance.
Dynamic Versus Steady-State Duty Cycles
Cooling systems should reflect how the machine is actually used.
A generator at steady load presents a different thermal challenge from a wheel loader repeatedly accelerating and operating hydraulics.
Steady-state cooling assesses whether the system can continuously reject a specified heat load. Dynamic or transient cooling considers the resulting behaviour as loads rise and fall.
Short peaks can sometimes be absorbed by the thermal mass of the coolant, oil and engine components. Repeated peaks, however, can gradually raise temperatures if the system does not have enough time to recover.
For demanding applications, real duty-cycle data can therefore provide a more accurate picture than a single maximum engine operating point.
Designing for Arduous Conditions
Construction, quarrying, agriculture and waste handling applications can expose cooling packs to dust, straw, chaff, mud, fibres and other debris.
This can influence heat exchanger design:
A dense fin pitch may provide strong cooling performance when clean, but it may also block more quickly in contaminated environments.
A more open fin pitch can improve resistance to fouling and make the cooler easier to clean, although this must be balanced against heat transfer performance and package size.
Material choice should also reflect vibration, corrosion and physical damage. Service access matters too, particularly where several coolers are stacked together and need regular cleaning.
Combining Cooling for Multiple Systems
Modern ICE vehicles often use one cooling pack for several thermal systems, including the radiator, charge air cooler, hydraulic cooler, transmission cooler, condenser and fuel cooler.
The order, depth and position of each cooler can therefore have a significant effect on overall performance.
A condenser positioned ahead of the radiator, for example, adds airflow restriction and increases the temperature of the air reaching the radiator.
These components should be designed as one system. The important behaviours do not live within a single component. They live within the interactions between them.
Why A Larger Radiator Isn’t Always the Answer
If an engine is running hot, fitting a larger radiator may seem like the obvious solution.
In reality, poor cooling performance can also be caused by low airflow, excessive restriction, poor fan positioning, hot air recirculation, air bypass, poor ducting, blocked fins or incorrect coolant flow.
A deeper radiator can even make matters worse if it increases resistance and reduces airflow.
Effective thermal management is about balancing heat exchanger performance with airflow performance.
Balancing Cost Against Performance
OEMs rarely design around cooling performance alone; the final system also needs to balance package size, weight, fan power, noise, durability, serviceability and cost.
A larger core may improve cooling but increase cost and weight. A higher fan speed may increase airflow but also raise noise and parasitic power consumption.
The objective is to achieve the required thermal performance without over-engineering the system.
Using Thermal Simulation to Validate Heat Rejection
Initial heat rejection calculations establish the load, but simulation and testing help determine whether the complete system will work in practice.
Thermal modelling can assess heat exchanger sizing, airflow, fan selection, pressure drop, ambient capability, cooler stacking and installation restrictions. It can also compare different layouts before physical prototypes are produced.
Testing can then validate the design under representative operating conditions.
Supporting OEMs With ICE Thermal Management
Calculating heat rejection is an essential part of engine cooling system design, but the final solution also needs to account for auxiliary heat loads, ambient temperature, duty cycle, airflow, packaging, contamination and the interaction between components.
At Calatherm, we work in partnership with vehicle and equipment manufacturers to develop thermal management solutions around the requirements of the complete application.
From individual heat exchangers to integrated cooling packs, considering all the points raised in the above, the aim is to achieve the right balance between performance, packaging, durability, serviceability and cost.
We can support this development at every stage, from desktop study, rapid prototyping, real world testing in our hot cell, through to on-time production and delivery.
Can your supplier do all this? Why not give us the challenge of your next mobile vehicle application and let us prove what we can do.
Contact us today.