RM Online - What you need to know about power sources

The majority of rental tools require a power source - LPG, propane, gasoline or diesel-fueled engine, compressed air or electricity - to function. So it's important to know the different power sources, how and why they are used, and how to specify them.

Engines

Conventional piston engines are either two-cycle or four-cycle. Compact, lightweight two-cycle engines usually air-cooled, are used on many hand-held tools (trimmers, saws, vibrators, rammers) and require a special oil/fuel mixture.

"It's important that your customers follow the oil-to-gas mixture specifications when using a two-cycle engine," says Jim Layton, director of sales promotions for Wacker Corp, Menomonee Falls, Wis. "Modern engineering has led to some recent advancements in two-cycle engine design, making them more fuel-efficient and cleaner burning while maintaining the power-on-every-stroke feature and longevity that they are well-known for."

The majority of engines in rental use today are four-cycle, either gasoline- or diesel-fueled. Up to 25 hp are generally considered small engines, 25 to 100 hp medium, 100 hp and above, large. Medium and large engines tend to be liquid-cooled rather than air-cooled. Four-cycle engines are lubricated by oil from a bottom pan, or sump. Maintaining the proper level of oil is critical; most modern engines have an auto shutoff feature when the level dips too low. Other than keeping an eye on oil and coolant levels, and replacing filters when recommended, four-cycle engines are relatively trouble-free and long-lived.

"No matter what type of engine goes out the door, it's important that you and your customer know how to start and maintain the engine in the field," cautions Brad Murphy, director of information systems for Robin America, Wood Dale, Ill. "How to check the oil and fuel levels, how to make sure the fuel supply is on, operate the choke and ignition switch beforehand can save you a lot of calls from the job site - familiarity is key."

Today many engines 50 hp and above are diesel, especially in applications in which the engine has to run at a constant rpm (usually under 3,000 rpm for peak efficiency) for long periods, such as on a pump or generator. Diesels are more robustly designed, for durability, and are about 40 percent more fuel-efficient than comparable gasoline engines. In the United States, engines are rated in SAE or brake horsepower by testing them to strict standards on a dynamo-meter, to produce a measure of horsepower at a given rpm. Torque - the ability of an engine to produce rotation with a given force, rated in foot pounds - is another spec to be aware of. Engines with high torque are better able to handle a heavier load at lower speed; those with less torque have to wait until higher rpm is reached before producing full power. Because of their design, diesel engines are typically used in high-torque applications.

"Engineers carefully match the twin forces of horsepower and torque when fitting an engine to a specific application," Murphy says. "In some instances, too much torque could produce too much power at the wrong speed and damage the engine and whatever it is powering." On the other hand, being "over-engined," with too much horsepower and a high rpm, can wear out clutches and take a toll on equipment.

In the 1998 engine article, Rental Management asked industry sources to comment on the relative merits of gasoline or diesel. Paul Blum of Kohler Co., Kohler, Wis., and Walter Krieger of Deutz Corp., Norcross, Ga., staked out the debate with these observations:

'"Gasoline engines typically are less expensive than diesel engines," Blum says. "Gasoline engines start better in colder weather because gasoline does not gel as diesel fuel has a tendency to do. Gasoline engines run much quieter and cleaner. Not all service stations carry diesel fuel. And gasoline engines are lighter than diesel engines, a factor in [the] overall weight of equipment. "

But, said Krieger, look at durability and overall cost throughout the life of the engine - what is sometimes referred to as "cost per horsepower" or "overall cost of ownership."

"Diesel engines last longer, require less maintenance, are more fuel efficient, and in some cases, even approach the light weight typical of gasoline engines," Krieger told RM. "Because diesel engines rely on compression ignition, they are in most cases built with stronger components and have higher torque backup."

"A diesel engine usually has a higher original purchase price, but has longer life in the field and can survive under more rigorous conditions," agreed Marsha Dutkowski of Deere Power Systems Group in Waterloo, Iowa.

With the introduction of new, compact diesels, medium-sized and small machines with diesel power are becoming more popular.

Generators

Generators rated 2 through 10 kW (10,000 watts) are the backbone of the portable electric power rental market. Larger units may power an entire job site, run a particularly large piece of equipment or supply standby power. Smaller units, 600 watts to 1 kilowatt, are typically for homeowners, do-it-yourselfers, campers or emergency use.

Besides the given horsepower of the engine, it's important to know the maximum (or surge) output and rated output. While most have gasoline engines, diesel-powered generators are preferred by contractors who have a lot of other diesel equipment on-site and a ready supply of diesel fuel.

Generator rentals begin with the simple question, "What are you going to run?"

"Keep in mind that it's always better to oversize than undersize when matching the generator to the intended use," says Terry Rode, sales manager for NAC Construction Equipment, Elk Grove Village, Ill.

"For example, if the customer needs to run two circular saws and a drill, and the math tells you the total consumption will be around 3,500 watts then it's best to recommend a 4-kW generator because it has a built-in cushion factor. Larger generators will also have more plug-ins."

To determine the right size generator, check the nameplates or manuals to determine wattage of the equipment (lights, tools, heaters, etc.) to be connected to it. Put all these factors together and select the proper unit allowing for some reserve power.

"Another reason for slightly oversizing the generator has to do with how a typical electric motor ramps up to speed," Rode continues. "Depending upon its efficiency, an electric motor can consume two to three times its rated power draw until it reaches its proper rpm. Most generators can handle this under surge-load conditions. On the other hand, resistive loads (e.g., lights, heaters) will consume only their rated power throughout when they are in use."

According to Rode, "heat is the biggest cause of generator breakdown." Voltage drops, the result of overloading from excessive power demands, can cause motor burnouts as well as wiring and insulation damage. Voltage drops are also harmful to the electric tools being used. Better generators have thermal or electromagnetic breakers to prevent this from happening.

Troubleshooting includes checking gas and oil levels. First, check to be sure the unit is on a level surface and check for a tripped breaker, overloaded receptacles, ground fault interruptions, incorrect use of extension cords and low oil shutoff.

Add-on rentals include electric tools, lights and components required by the tools themselves, such as saw blades.

Most generators are of the newer brushless design, especially in smaller units. These are typically low-maintenance items and need little more than changing oil at recommended intervals. Better generators are 120/240v switchable. Most contractor equipment is 120v, but there is the occasional demand for 240v to power tools such as core saws and breaker hammers. Fuel run times are rated in no-load, half-load and full-load conditions and by tank size. You need to know how long the job is likely to take and the typical load on the generator to advise the customer properly about fuel.

Larger electric power-delivery devices include gen-sets, fully integrated matched or coupled engine/generator units that deliver large amounts of power. These range from 8 kW to 200 mW and are used for light towers, load banks (multiple plug-ins) at special events, chillers during power interruptions and many more applications in a huge variety of markets. They may be pad- or trailer-mounted and can be specified for prime or standby power.

Long-term use includes load-shedding and peak-sharing when the local utility grid can't meet the needs of the user reliably or efficiently.

Air compressors can range in size from small, extremely portable units for roof-top use up to tow-behind, diesel-driven behemoths that are as big as a compact car. Whether the unit is direct- or belt-driven, uses an electric motor or an engine, relies on an air tank or is tank-less, the cfm rating - cubic feet per minute produced at operating pressure, not horsepower or tank capacity - is the most important specification.

Compressors are ranked as small, medium and heavy duty, depending upon cfm rating and intended use.

Small (4 cfm or below), diaphragm-type as opposed to piston (reciprocating style) compressors are light-duty units designed for intermittent use by the homeowner or handyman.

Medium-size compressors range from 6 to 20 cfm. These would normally be twin- or four-cylinder units, depending on rating and efficiency of the piston/valve design, and are electric powered. Because of efficiency increases in electric motors, many of these units require standard 112v, 15 to 20 amp power found on most job sites. Gasoline engine units are ideal for use on remote locations or where power hasn't been connected.

Smaller units are direct-driven; medium-size units are belt-driven off a pulley on the engine or motor. Units above 20 cfm are needed for use on portable lifts, on fastener applications and for using multiple ceiling-texture sprayers.

A compressor rated at least 80 cfm is required for powering a jackhammer. Most rental stores will have up to a 180-cfm compressor for rent. Compressors 100 to 800 are often diesel-powered, tow-behind units needed to run multiple jackhammers, pumps, air tools and other tools at a construction site.

According to Harold Wagner, National Mobile Air Sales Manager, Kaesar Compressors, the 185-cfm compressors account for about half of all compressor rentals and sales: "This unit can handle two concrete breakers, plus a smaller chipper hammer or rock drill and is still compact enough so it can be towed easily. It's the all-around workhorse of the industry. These are usually diesel driven with engines rated 45 to 60 hp."

Contractors may rent a large unit when one of their units is down, when there are extra subcontractors who need power, or when a larger compressor can help them finish a job quicker.

"In determining the proper size of compressor to handle the job, the number and types of tools the user plans to connect to it are the most important variables," notes Dale Mishler, rental manager for Emglo Air Compressors, Johnstown, Pa. "If a homeowner is roofing and he and a helper are both using air-powered roofing tackers drawing 2 cfm each, then a tankless 4 cfm unit is ideal. If two subcontractors are using framing nailers that draw 3 cfm each, then a 5-hp gasoline engine or 112-hp electric motor delivering 6 cfm will do. At the other end of the scale, you'll need a 100-cfm unit for one air hammer, a 200-cfm for two, etc.

"Keep in mind that horsepower doesn't make air - the compressor does. Tanks don't make air - the volume is a matter of piston stroke and cylinder bore. The tool manufacturer may specify that a tool needs 80 psi and 2.4 cfm. Size the compressor accordingly."

As with generators, it's always better to err somewhat on the side of overcapacity, Mishler stresses. "Allowing a bit of wiggle room to compensate for coupling leaks or for a tool with weak O-rings is recommended. A tool with blown O-rings can require up to two and a half times its specified rating in order to function properly."

Ideally the compressor should run 60/40 - 60 percent off-time to 40 percent run time - although compressors made for the contractor trade, unlike home handyman units, are designed to run continuously, if needed.

The cycle time - the time needed to produce a full tank of air - should be on the unit. If it takes too long to recycle, or is constantly recycling, worn tools or leaks may be the culprits.

Or if your air tool requires 4 cfm and your compressor is producing 2 cfm, you're going to continually wait for it to build tank pressure to reach operating pressure - not a good scenario. Your customer will be unhappy. Advise customers properly and thoroughly in the first place and avoid this problem. Don't wait for the guy down the street to do it, the next time the customer rents something.

Air tool oil, extra air hoses and couplers may be add-on sales. The unit should go out with the proper amount of oil in the sump and the user should be instructed to check it - usually through a sight glass - throughout the day.

Keeping abreast of advancements in engine technology and familiarizing yourself with all the power units, of whatever type as machines arrive at the store, will make you a more informed authority and a counselor customers trust.