Great Debates: Air Versus Liquid Cooling
A Matter of Society over Science
by Jason Giacchino
When we think of the internal combustion engine, we usually think of horsepower numbers and zero to 60mph stats, but from an engineering perspective, the very process of pistons sliding up and down cylinder bores is one rife with friction. Where there’s friction, there is heat, and heat can have some unwanted side effects. Fortunately since we do not exist in a vacuum, the means of dissipating heat can be as simple as allowing the air around us to absorb temperature increases.
In the world of motorcycling, the populous typically finds itself in one of two camps, as far as the process of engine-heat removal goes: Liquid versus air. While a lot of the desire to represent one over the other has more to do with implied lifestyle association over genuine performance gains or losses of late, there are some serious scientific pros and cons of each.
Let’s take a look at the differences between allowing O2 versus H2O as the medium with which to carry engine heat away from the block.
AIR
Air-cooling is the oldest means of engine heat dissipation. Actually, the process of heat being carried off by the ambient air surrounding a motor’s surface is prevalent in every engine ever created.
As such, air-cooling is typically facilitated with a variety of metal fins covering the outside of the hottest surfaces of an engine (the cylinders) so as to increase the surface area that air can access.
The automobile has virtually done away with air cooling entirely on account of the fact that moving air is a requisite to proper heat dispersion, and most car engine bays simply do not allow for adequate air flow, like that of a motorcycle.
The beauty of the open-air design of most motorcycles is that the faster the bike travels, the more air passes across the engine’s surface. Hence, the system literally makes the most of the abundant resource all around.
Another major advantage is simplicity. The lack of a dedicated cooling circuit means less componentry that could potentially fail. It means one less fluid level to have to be concerned about/ maintain and of course a slight savings in overall weight.
The disadvantages of air-cooling include engines that typically operate at higher temperatures, potential of overheating at prolonged idle, and the simple fact that cooling efficiency is conditional (meaning it will by nature work better in cooler conditions than it will in higher temps).
WATER
In the realm of physics, it’s proven that water boasts higher specific heat capacity, density, and thermal conductivity when compared to air. In other words, water is more efficient at dissipating heat than air. In fact, even with all of our modern chemistry methods of today, nothing beats good old H2O at collecting and distributing heat. Additives such as antifreeze are used simply to increase the boiling point and lower the freezing point of the water in the system.
In the realm of motorcycling, liquid cooling has typically become associated with performance models for its ability to maintain a consistent engine operating temperature, regardless of the ambient conditions. Studies have also proven increased engine life as a result of fewer internal temperature swings over prolonged use.
As far as disadvantages to liquid cooling are concerned, a water circuit (which typically includes a heat exchanger, radiator) adds a bit of mechanical complexity, weight and an increased risk of areas that can fail or become damaged in the event of a crash.
A Matter of Taste
In the modern motorcycling industry, the choice of air versus liquid cooling has become analogous with national pride. The reason for this boils down (no pun intended) to the core methodology witnessed in domestic versus foreign engineering. Americans (thanks in no small part to Harley-Davidson, big-bore customs and recently companies like Victory) have become known for engine designs that favor large displacement and relative mechanical simplicity to achieve performance while engines developed overseas (traditionally from Japan) have gained notoriety for widespread acceptance of complex technology to maximize output of smaller configurations.
As such there is no right or wrong when it comes to supporting a method of engine heat displacement so much as it has become an extension of individual expression. Almost like the body styling or color of the paint selected, many riders view the presence of radiator lines or finned cylinders a part of their motorcycle’s personality. And that’s an area that science cannot dissect in a lab.