Appraising air-cooled and liquid-cooled engines

Your expensive mower has broken down again. Or your snow thrower's been sluggish. You know you've got to make some choices in repairing or replacing the unit, but how do you decide? How do you know whether, for example, to choose a liquid-cooled or an air-cooled engine? How do you know whether you should upgrade the current engine or opt for a new one? As you'd expect, no question on engines is ever simple. You must consider an array of factors, such as engine size or horsepower, weight, the environment in which you'll operate the engine and the cost of your options.

The question of liquid vs. air also depends on whether you are considering a replacement engine for a piece of equipment you currently own or you're looking to purchase a new piece of equipment. When you look at replacement engines, keep in mind whether you can adapt the replacement to the mounting frame. For example, does your shop have the capability to fabricate the needed brackets and bushings to complete the job? If so, you can probably make the task easier if you stick with the same engine manufacturer. Also consider whether you intend to stay with the same horsepower engine or upgrade to a higher-output engine.

Let's imagine--it shouldn't be too difficult--that you have a mower needing some extensive engine work. What should you do? First, consider whether the old engine had enough horsepower, or do you need to increase the power? Don't be fooled by the old engine's output in its tired, worn state when you last used it. If you were to choose a newer mower with the same-size engine as your old unit, try to figure out if the new model would be large enough. For our purposes, let's say it would be. Now consider that the current replacement model by this manufacturer now comes with a liquid-cooled engine rather than the air-cooled engine on your old unit. Can you upgrade the older unit to the same-size engine that is available on the new unit? Chances are the manufacturer doesn't offer an upgrade kit to a liquid-cooled model. To get this upgrade, then, you would have to hire the work to be done, and the cost would likely outweigh the benefits. Thus, it's easy to decide that you should stick with an equal replacement.

About air-cooled engines An air-cooled engine is just as its name implies: cooled with air. The unit forces air over the cooling fins that are cast to the cylinder walls.

These engines can be aluminum as well as cast-iron. Aluminum engines usually have cast-iron liners to increase cylinder life. Most also incorporate some type of counter-balancing system so they will run more smoothly.

The better-quality engines have a pressurized lubrication system. Some also have an overhead-cam or an overhead-valve arrangement.

Many incorporate an automatic-decompression system. This feature allows for easier starting, whether it's with a pull cord or an electric starter.

About liquid-cooled engines On a liquid-cooled counterpart, you have a cylinder with no fins and no fin shrouds for air flow. This in itself allows for a much tighter engine package--extra space you can use for placing larger engines in the same space as an air-cooled unit with smaller horsepower would occupy. Liquid-cooled engines have iron sleeves or liners to aid in longer life, along with pressurized-lubrication systems. As for other engine configurations when compared to air-cooled units, they remain the same (valves, cams and counter balancers). The one item that makes water-cooled units different from their air-cooled counterparts is the addition of a water pump and radiator. Manufacturers design these in all types of arrangements, from internal-mounted to cam-shaft driven to an externally mounted belt-driven unit much like the one on your car.

These motors also have a thermostat to enable them to maintain a given temperature through the wide range of loads you're likely to place on equipment with this engine.

Now that we have explored the differences between air and liquid cooling, we need to understand the advantages and disadvantages of each. (Keep in mind that both cooling methods are used on 2-stroke as well as 4-stroke engines.)

Cost comparisons of both types cooling systems On the basis of pure dollars and cents, you should consider several factors. First, when you talk about liquid cooling, you must realize these engines always cost more because they have more components. The tradeoff is that, with this added expense, you also get greater reliability, and--in many cases--an easier piece of equipment to maintain.

The reason these units are more reliable is that their running temperature is more consistent. This is accomplished via a thermostat that closes when the motor is cold so the coolant only circulates through the block. When the motor warms up, it opens to allow the coolant to circulate through the block and the radiator so heat can dissipate. The end result is that these engines can be built to more exact tolerances and run more efficiently. They do this regardless of the outside air temperature or the working environment in which you operate them.

The lower-cost air-cooled engine, however, may cost as much if, when you work the motor in the heat of the summer and under severe loads, the motor fails. The end result could be a major overhaul. Even so, we don't want to count out these little giants. They have been out there serving us well for a long time, and--for the most part--they have been doing a good job of it.

With the addition of a pressurized oil system, some air-cooled manufacturers spray small jets of oil at vital engine parts, and larger oil sumps also aid in cooling. But wait a minute. Isn't oil a liquid, so aren't they really liquid-cooled engines? Not really. They still technically remain air cooled. Even so, we are entering an era where some small, industrial-equipment companies are designing units with true oil-cooled engines. So what—if any—is the advantage of air-cooled motors? The biggest factor is the lower cost and lighter weight. After all, who wants to carry around a spin trimmer or a chain saw with a radiator?

Comparing special features and serviceability In most cases, repair technicians prefer the liquid-cooled engine when it comes to routine maintenance tasks. They know they will have fewer shrouds to remove to access the valves for adjustment or the fuel system for service. Radiators also are usually easier to clean and to flush out the dirt and debris from than cooling fins.

On heavy-duty commercial equipment that you intend to work hard for 8 or more hours a day, creature comfort also comes into play. You often need to operate this equipment in cold climates, and--with the addition of a cab--you can run the engine's warm water through a cab heater. Some units blow warm air from an air-cooled engine into the cab, but you run the risk of picking up exhaust or gas fumes with the warmed air. Also, engines with this feature have more air shrouds to remove when service is necessary.

Let's do a comparison between liquid- and air-cooled engines to see how other maintenance tasks come into play. For example, let's consider a job in which you must remove the head or even the entire engine to compare what you might have to do differently to service each type of engine. On a liquid-cooled engine, the mechanic first must drain the cooling system. On an air-cooled unit, you simply remove the shrouds and then you can access most cylinder heads.

If the mechanic must remove a cylinder head for valve work, he or she must be careful to clean all gasket surfaces before reassembly to ensure coolant doesn't leak from the combustion chamber. If the coolant leaked out--with no finning on the motor to cool the engine--the engine would overheat and experience a major failure.

Repair parts, for the most part, are similar in price between both types of engines. You do, however, have to compare apples with apples. Don't try to compare a Briggs & Stratton air-cooled piston with a Kawasaki liquid-cooled piston. Try to compare prices and models within the same manufacturer's line.

Down time for repairs No matter which manufacturer's product you purchase, you're going to eventually experience down time for a repair job. Generally speaking, however, down time on equipment with liquid cooling occurs far less. This is due, in part, to more consistent running temperatures. When you power up a liquid-cooled piece of equipment and bring it up to operating temperature, it stays within a given temperature range far longer than an air-cooled unit. For example, if you shut this engine down for a work break--or some other brief period--the coolant temperature remains warm, and you won't do the same damage to internal engine parts that occurs with cold starts.

On air-cooled units, however, metal parts expand and contract at vast rates. For example, these parts contract any time the motor comes in contact with water (such as water puddles or a rinse from a garden hose) or cold air (such as taking the equipment out of a warm shop and putting it into service blowing snow or loading a hot motor on a truck and hauling it down the road in the middle of winter). This is what normally causes cracks or gasket failures on these engines.

As another example, consider when a head gasket fails. On an air-cooled motor, you could drive the equipment back to the shop for repairs. In some cases, in fact, you won't even know the air-cooled engine is having a problem. With a water-cooled unit, however, a bad head gasket releases compression into the cooling system. This results in pressure blowing out the coolant, leading to overheating and putting the engine completely out of commission.

Environmental issues Environmentally speaking, both engines have some issues about which you should be concerned. The liquid-cooled engine is usually much quieter because its water jacket absorbs its sounds.

However, the environmental negative to this positive is that, if it overheats, these units discharge coolant into the environment. Not only is this coolant a hazardous waste, it also can kill expensive turf. Of course, these units do have overflow tanks, and they can typically handle simple expansions. But when these engines experience a serious failure, expansion tanks can't control the entire volume in their systems.

With an air-cooled motor, you don't have to deal with coolant, but you do have more engine noise. Thus, manufacturers must add more insulated covers and shrouds to keep noise to an acceptable level (see illustration at right). Oftentimes, owners remove these shrouds and never replace them. While doing so makes the units louder, you decrease the possibility of fires. This is because the shrouds often absorb oil and gasoline, as well as attracting dirt, making them a dangerous fire source at the slightest spark.

So what's the best choice? So, what's the best motor: air-cooled or liquid-cooled? For my money, given the choice, I typically choose liquid-cooled. The repair costs are only a little more if you even experience them, which is less often. In fact, normal, routine maintenance costs are no more per cylinder of operation on liquid-cooled vs. air-cooled. While it's true that most liquid-cooled engines are twin cylinder, this is also what makes them run smoother.

I also prefer liquid-cooled units because of their more-consistent operating temperatures. Plus, most of today's advanced technology--such as electronic fuel injection and digital electronic ignition--is being added to liquid-cooled units rather than air-cooled units.

For those who believe that simpler is better--and thus advocate air-cooled units--consider this: If our industry believes in taking advantage of technology such as computer-controlled irrigation and global-positioning satellites, as well as improved seed and fertilizers, why not step up to the plate where our engines are concerned as well? Don't be afraid of the finer things in life. Choose the equipment that will really perform best for you.

Larry Van Deusen is technical adviser for the College of Agriculture and Technology at the State University of New York (Cobleskill, N.Y.).

Technical source for illustrations: Deere and Co. (Moline, Ill.)

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