Over the last 10 years, more and more fire and rescue trucks have been specified with elevating area lights, electrically powered hydraulic rescue tool pumps and other electrical devices. As vehicle power requirements continue to increase, the alternator often gets taxed to its limit. In some cases, it's taxed beyond its limit, and an electrical load manager has to selectively shut down circuits to prevent the batteries from being drained.

One approach to this problem has been to shift some of the electrical load off of the vehicle's 12-volt system and onto a separate 120/240-volt system powered by a vehicle-mounted generator. The 120/240-volt power also allows the use of more powerful lights and motors for improved effectiveness.

Here are some of the features, advantages, disadvantages and typical applications of different types of vehicle-mounted generators.

Belt-driven Belt-driven generators are mounted in the vehicle's engine compartment and are powered by a belt off one of the engine's accessory drives. On some models, the generator is connected to an electrical control unit, which is mounted somewhere else on the vehicle. The control unit takes the generator's variable output and electronically converts it into a constant-voltage, constant-frequency 120-volt, 60-cycle, single-phase alternating current. Some units also produce 240-volt power. Typical power output for belt-driven generators is about 3.5kw-5kw.

The generator itself is about the size and weight of a standard vehicle alternator and can be mounted under the hood of many light- and medium-duty trucks. The electrical control unit usually weighs less than 20 pounds and is about 18 x 12 x 8 inches or less, depending on the manufacturer.

Some control units contain a cooling fan and must be mounted in a ventilated compartment or other location where there can be a free flow of air. One way to do this is to mount the control unit upside-down over the pump behind the pump panel or behind a roll-up door box at the top of a compartment.

Belt-driven generators have several advantages when compared to other types. They're compact and take little or no usable compartment space, and they're also lighter weight and lower cost than most generators. They require minimal maintenance, have no additional fuel tank to fill and contribute no noise above that of the vehicle engine itself. Electrical power is available whenever the vehicle engine is running, and the generator can be used even while the vehicle is in motion.

The biggest disadvantage to belt-driven generators is that they're limited to about 5kw output, which makes them unsuitable for vehicles with higher power requirements, such as larger pumpers, aerial ladders and heavy rescues. Another disadvantage is that although the voltage and frequency remain constant on most models, the current (and therefore the power output) is dependent on the engine speed. To maintain maximum power output, the engine must be held at a constant speed above idle. Finally, on some belt-driven generators, the voltage output is a square wave, rather than a pure sine wave, which may make it unsuitable for sensitive electronic equipment such as computers.

Applications for belt-driven generators include light and medium rescues, quick attack pumpers, light-duty command vehicles, and shop maintenance trucks. A belt-driven generator might not fit on certain ambulances that are equipped with dual alternators.

Portable engine-driven As the name implies, portable engine-driven generators are light enough to be carried by one or more people. For use in vehicles, the generator can be removed from a compartment and set on the ground before it's started, or it can be semi-permanently mounted in the compartment on a slide-out tray and pulled out for operation. The generator is usually powered by a gasoline engine and produces 120/240-volt, 60-cycle, single-phase ac. Power output is about 1kw-6kw, depending on the model.

Generator size and weight vary by power rating and manufacturer. For example, a 5kw unit may be 27 x 20 x 20 inches and weigh 200 pounds or less. The engine may have a manual pull starter or an electric starter powered by a separate battery. A fuel tank is provided with enough capacity for up to several hours of operation. All portable engine-driven generators are designed to operate in the free air and aren't intended for permanent mounting in enclosed compartments.

One of the advantages of portable engine-driven generators is that, depending on the mounting, they can be carried to a remote location to provide power. Another advantage is that they can operate separately from the vehicle engine, which means they can supply constant power in applications where the vehicle engine speed varies, such as pumpers. Finally, because portable engine-driven generators have so many other commercial applications, they're relatively inexpensive.

The disadvantages include a limited power output, which restricts their use. Because they have their own engine, they take more space and are generally heavier, more expensive and more difficult to maintain than some other generators. They also tend to be noisier, especially because they rely on air cooling and must operate outside of an enclosed compartment. Finally, they have a separate fuel tank, which must be monitored and refilled. Because most portable units use gasoline, some fire departments don't want to use them on vehicles that may be exposed to high heat or flame.

Applications for portable engine-driven generators include light and medium rescues, some pumpers, and shop maintenance trucks.

Mobile engine-driven Mobile engine-driven generators are like portable engine-driven generators, only larger and heavier. They're designed to be permanently mounted in ventilated vehicle compartments and can be powered by gasoline, diesel or lpg. They produce 120/240-volt, 60-cycle, single-phase ac with power ratings of about 4kw-20kw, though much higher ratings are also available.

As with their smaller portable cousins, the size and weight of mobile generators varies with the power rating and manufacturer. In general, units with diesel engines are significantly heavier than those powered by gasoline. For example, a 6kw air-cooled diesel unit could weigh 500 pounds or more. Water-cooled units may weigh more or less, depending on the manufacturer.

Some models have their own fuel tank and battery, while others draw their fuel and electrical power from the vehicle. Maintenance points are usually located on the outboard side for ease of access.

The advantages of mobile engine-driven generators include the ability to operate separately from the vehicle engine for independent power control. Mobile units are designed to run inside a ventilated compartment, which means the noise levels are muffled by the compartment enclosure. Many of them use diesel fuel to minimize concerns about flammability.

The main disadvantage of mobile generators is that they have a separate engine. This means they take more space and are generally heavier, more expensive and more difficult to maintain than other generators. Although they're not as noisy as portable generators, the separate engine still contributes to the overall noise level.

Applications for mobile generators include pumpers, aerials, medium and heavy rescues, hazmats, larger command posts, and some ambulances.

PTO-driven pto generators are driven directly off the transmission power takeoff with a driveshaft. They generate 120/240-volt, 60-cycle, single-phase ac with power ratings of about 5kw-40kw.

Because they use the vehicle engine for power, pto generators are relatively small and lightweight. For example, a 40kw pto unit weighs only 500 pounds, compared to about 2,000 pounds for a diesel-powered, water-cooled mobile unit of the same rating. The generator is usually mounted under the vehicle frame close to the transmission.

The biggest advantage of these generators is that they don't need a separate engine, and therefore take less space, are lighter and less expensive, and require almost no maintenance. They're also quieter than other generators. Like belt-driven generators, they can generate power while the vehicle is moving. On the basis of cost per kilowatt, they represent an excellent buy for higher power ratings.

The main disadvantage is that the power output is dependent on the vehicle engine speed. Thus pto generators are generally not suited for pumpers, where the engine speed varies depending on the required water flowrate. The mounting location of the generator is also somewhat limited by the length and angle restrictions of the driveshaft.

Applications for pto generators include aerials, medium and heavy rescues, hazmats, and some ambulances.

Hydraulic-driven Hydraulic-driven generators incorporate many of the advantages of pto-driven generators without the disadvantages. Instead of driving the generator with a driveshaft off the transmission pto, the pto drives a hydraulic pump. In turn, the pump powers a hydraulic motor through two flexible hoses, and the motor turns the generator. This eliminates the driveshaft and allows the generator to be mounted anywhere on the vehicle.

By using a variable-displacement pump with a simple feedback control system, the pump can maintain a constant flow output under varying engine speed inputs. A constant flow output means a constant generator speed and therefore a constant electrical power output, no matter how fast the engine is running. This system can generate 120/240-volt, 60-cycle, single-phase ac with power ratings of about 3kw-30kw. Some models also produce direct current for welding.

Hydraulic-driven generators have all of the advantages of pto-driven generators - less space, less weight, less noise, less maintenance - plus they eliminate the engine speed dependency problem. The disadvantage is that they cost significantly more than pto-driven units. When compared to mobile engine-driven units, however, hydraulic-driven generators are estimated to cost only slightly more, with far fewer disadvantages.

Applications include pumpers, aerials, medium and heavy rescues, haz-mats, and mechanics trucks.

It's your choice If you're considering a vehicle-mounted generator for your next apparatus purchase, you need to add up all your electrical power needs.

In addition to elevated light towers - which can consume 6kw-12kw by themselves - remember to add the power requirements for portable lights, smoke ejectors, hydraulic rescue tool pumps and any other 120/240-volt devices. As a rough rule of thumb, allow about 5kw for light rescues, 10kw for aerials, 15kw for medium rescues and pumpers, and 25kw for heavy rescues. Your requirements may vary significantly.

After you've added up your power needs, review the advantages, disadvantages and limitations of the various generator systems. Then contact some of the manufacturers listed in this article and get their recommendations. Because many of these systems require installation at the time the apparatus is built, you may want to review your specs with your apparatus builder as well. You'll probably also want to have them install one or more power cord reels at the same time.

Adding a generator to your next vehicle can improve its performance and give you added versatility at an incident scene. The additional electrical capacity can also help your personnel work more quickly and safely by providing better illumination and more powerful tools.

nfpa standards for vehicle-mounted line voltage generators and related equipment such as cable reels and light towers are covered in Chapter 21 and Appendix A of nfpa 1901, Automotive Fire Apparatus (1999). These standards apply to engine-, pto-, hydraulic- and belt-driven and portable generators, as well as low-voltage inverter types of line voltage systems.

To gain iso credit in the loose equipment category, each service company and ladder company must carry a generator with a 2.5kw or greater rating.