Diesel generator: What's the difference between air and liquid cooling systems?

Why do diesel generators need cooling?

Heat can build up quickly within the diesel generator during operation, creating additional heat that must be properly removed to reduce the risk of damage. If the heat is not properly removed from the generator system, the windings can quickly fail. Then there will be many problems affecting the normal operation. However, diesel generator cooling system can significantly reduce this heat that keep the generator cool and minimize the risk of damage to the generator itself.

Most diesel generators are air-cooled or liquid-cooled cooling method, and the cooling method is one of the basic design elements of generator manufacturing, is usually determined by the size and type of the generator. Air cooling systems are typically used on smaller diesel generators, while larger diesel generators require liquid cooling. For this reason, industrial large generator manufacturers are more liquid-cooled. Now, in this article, BISON diesel generator manufacturers take you to learn more about difference between air-cooled and liquid-cooled generators.

Air cooling system

This cooling system uses air circulation to reduce the temperature. In an air-cooled system, the BISON engine takes cool air from the atmosphere and blows it through cooling fins and different parts of the generator set. This prevents the generator from overheating.

Air-cooled systems are commonly used for open frame or enclosed diesel generators up to 22 kW. In an open ventilation system, air is drawn in, circulated through the generator, and then released back into the atmosphere. In a closed system, air can be kept in place. What it does is recirculate the air. At the same time, the air is cooled, thus keeping the BISON generator cool.

However, the air-cooled system may still overheat the generator when it is used for a long time, which means that the machine needs to stop working, that is, stop power supply, and wait for the temperature of the generator to drop.

Liquid cooling system

Liquid-cooled (water-cooled) systems use oil or coolant to cool internal engine parts via radiators and water pumps. Coolant circulates through hoses to the engine block, absorbs heat, flows to the radiator for cooling, and recirculates in a closed-loop system. This enables better heat dissipation and supports higher kW ratings for industrial applications. Generators above 22 kW typically use liquid cooling. These systems are more expensive to produce but more durable and powerful than air-cooled systems, making them the standard for commercial and industrial use.

The difference bewteen air cooling and liquid cooling diesel generator

Usage

Air-cooled diesel generatorfeatures an engine that uses a fan to force air through the engine to cool the engine. Therefore, generators with air-cooled engines can often be used for short-term power supply applications, they can still power your entire home during a power outage, so they're an ideal backup system.

On the other hand, a liquid-cooled engine contains a closed radiator system for cooling, and tend to be used in higher kilowatt generators as efficient reduce the amount of heat generated by larger generators.

Larger homes can take advantage of these liquid-cooled engines, and other common usage of these generators include convenience stores, restaurants, offices, and many more industrial, large buildings.

Generator size

Air-cooled generators are smaller, they sit directly on soil or concrete blocks. Due to their large size, liquid-cooled generators weigh four times more and require powered equipment for transport; they must be placed on a prepared bed or pedestal.

Cost comparison

Speaking of cost, liquid cooling systems are more complex, with precision-engineered components and higher upfront cost, but offer greater durability. Air-cooled systems cost less and are more portable, so they are often an affordable option for importers looking for a simple power supply system for a generator.

Efficiency and service life

Liquid-cooled generators are usually bulky and consist of complex mechanisms capable of generating large amounts of power, with cooling mechanisms that allow them to efficient function even in extremely hot weather. The air-cooled generator is not the same, the volume is lighter, the structure is simple, and the efficiency is low.

Efficiency differences between the two systems trace back to how the engines themselves are built and run, which directly shapes how long they last.

Air-cooled diesel engines are typically designed as high-speed units, running at around 3,600 RPM. This higher rotational speed lets manufacturers use lighter internal components and a more compact block, which keeps the generator's overall weight and footprint down. But parts moving at this speed under continuous load wear out faster. Most air-cooled diesel engines are rated for roughly 800 to 1,000 hours of service life before major overhaul or replacement becomes necessary.

Liquid-cooled diesel engines run at a slower 1,800 RPM. This lower speed reduces mechanical stress on pistons, bearings, and crankshafts with every cycle. Combined with the more stable operating temperatures that coolant circulation provides, this allows liquid-cooled units to reach service lives of 8,000 hours or more — roughly eight to ten times longer than a comparable air-cooled engine. For anyone relying on a generator across multi-day outages at home, liquid cooling the more dependable option when reliability really matters.

Maintenance requirements 

Air cooled generators involve maintenance but are simpler and less time consuming than liquid cooled generators, without the complicated work of a liquid-cooled generators and the mess of a cooling process, an air-cooled process is easy to maintain.  for a business or shop all it requires is checking the oil level from time to time and making sure there is voltage and everything is fine. It's simple and saves time, which is what every workplace wants because time is money.

However, the cooling process for a liquid-cooled engine on a generator is a little more complicated and therefore requires some monitoring by the generator owner. In addition to checking the antifreeze level, you must also ensure that the coolant is functioning properly, which may mean checking lines and connections, as well as checking for potential leaks. Liquid-cooled engines also require more frequent maintenance, as it's important to catch any potential problems promptly. This may be a disadvantage to some, but the extra maintenance is worth it for the efficiency and power of the liquid-cooled engine. Maintenance kits for these generator types are readily available and should be used on a regular basis.

Noise level

Another key consideration is noise levels. Depending on usage, air-cooled engines do produce a fair amount of noise, as the fan can be heard blowing air across the engine. Also, engine noise cannot be further reduced with an attenuating enclosure due to the airflow required to keep the engine cool. This can be annoying, especially when the air-cooled engine is powering the house 24/7.

On the other hand, liquid-cooled engines are very quiet compared to air-cooled generators because the sound they produce can be contained within an attenuated enclosure. This is often a very attractive factor for many users.

Which system do you need?

Each system has advantages and disadvantages. Air-cooled systems are simpler, cheaper, and require less maintenance but are less efficient for large-scale use. Liquid-cooled systems are more powerful, efficient, and suitable for extended runtimes but cost more and demand ongoing maintenance. Choose based on your power needs, operating environment, expected run-time, noise tolerance, portability, and budget. Liquid-cooled generators are recommended for extended run hours under continuous high loads; their sustained performance over longer service life often justifies higher upfront cost. Air-cooled generators are very capable in the range they are used and are suitable for most household use.

BISON manufacturers hopes this information will help you learn more about air and liquid cooled generators and help you get the best deal for your needs.

Faq

Generator units with a capacity greater than 15 kilowatts generally use which cooling system?

There's no single universal cutoff; thresholds vary by manufacturer and design. As a general guideline, units above roughly 15-20 kW increasingly shift toward liquid cooling because higher power output generates more heat than airflow alone can safely dissipate. Many manufacturers, including BISON, set the practical air-cooled ceiling closer to 22 kW, so a 15 kW unit could still be air-cooled or liquid-cooled depending on the manufacturer's engineering choices and intended application.